Department of Civil Engineering
Concrete Technology
Portland Cement and Its Manufacturing
Introduction
Portland cement (PC) is a fine powder which is produced by heating limestone and clay minerals in a kiln and forms clinker. The clinker is grinded to fine powder with addition of small amount of other materials. It is the most commonly used types of cement in the world. It is used as basic ingredients of concrete, mortar, stucco etc. It is named as Portland cement because of its similarity to Portland stone which was found on the Isle of Portland in Dorset, England.
It is invented by the Joseph Aspdin in 1824. But the credit for the invention of the modern Portland cement goes to his son William Aspdin in 1840s.
Raw Materials for PC
Calcareous rocks (CaCO3 > 75% such as limestone, marl, chalk) Argillaceous rocks (CaCO3 < 40% such as clay and shale) Agillocalcareous rocks (40-75%CaCO3 such as clayey limestone, clayey marl)
For the manufacturing of PC, materials from any two of these groups may be used but they must contain in proper form and proportions of lime, silica and alumina.
Chemical Composition of Portland Cements
Department of Civil Engineering
Concrete Technology The raw material that is used in the manufacturing of Portland cement mainly consists of lime, silica, alumina and iron oxide.
In the cement, the oxide content is about 90%. The oxide composition of ordinary Portland cement is given below:
During the heating of raw material in the kiln, the oxides interact with each other and forms more complex compounds. The Portland cement consists of 4 basic chemical compounds. These four compounds are given in the table below:
1. Tri calcium silicate
It hardens rapidly on addition of water and responsible for initial set and early strength.
2. Dicalcium silicate
It hardens slowly. The effect of C3S on strength increases with ages beyond one week.
3. Tri calcium aluminate
In first few weeks, it contributes in strength development. It hydrates first among all four compounds of the PC. Since it produces high heat and also reacts with soils and water that contains moderate to high sulfate concentrations, it is considered as the least desirable component of the PC.
4. Tetra calcium alumino ferrite
It hydrates very rapidly but contributes very little to the strength of the concrete. It lowers clinkering temperature i.e. temperature at which clinker is formed.
Manufacturing of Portland cement (PC)
First the raw materials obtained from the source are reduced to fine particle size.
Department of Civil Engineering
Concrete Technology The raw materials are blended and mixed in such a manner that it produces uniform chemical composition containing Calcium carbonate (CaCO3), silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3) etc. Now the blended raw mix is heated to a temperature at which all the moisture from it escapes out as steam or water vapor. The dried raw mix is heated to 8000C. At this temperature what happens, the calcium carbonate gets dissociated into calcium oxide (known as free lime) and carbon dioxide which escapes out as gas. As the temperature rises above 8000C, the formation of principal active compounds of Portland cement starts. And the completion of this process takes place at 1400-16000C. Final product that we get is PC clinker. Usually 1 ton of clinker is produced from 1.8 tons of raw material. The cement clinker so produced is cooled to a temperature of 60 to 1500C and then stockpiled. After that the clinker is ground to specified fineness by adding small amount of gypsum. The gypsum added controls the setting time of finished cement. Finally the finished cement is stored in silos for a short period of time before it is being sent to the customers.
The various production steps of the Portland cement are-
Crushing, screening and stockpiling of raw materials. Proportion calculation of raw materials. Preparation of raw mix by blending Feeding of raw mix into rotary kiln. 100 C: Evaporation of free water. 150-300 degree C: Evaporation of more firmly bounded water. 600 degree C: MgCO3 —–> MgO + CO2 900 degree C: CaCO3 —–> CaO + CO2 Reaction between lime and clay starts. 1300 degree C: Initiation of major compound formation. 1400-1600 degree C: completion temperature (clinker forms having C3A, C2S, C3S, C4AF) Cooling and storage of clinker. Clinker is ground with gypsum. Storing and marketing of cement.
Reference:
- https://youtu.be/aMkoHaoHTek
SETTING
Setting refers to a change from a fluid to a rigid stage
Cement + water → cement paste → lose its plasticity gradually → when it lose its plasticity completely → setting occurs.
Department of Civil Engineering
Concrete Technology The stages of setting include:
Initial setting
Final setting: It is important to distinguish setting from hardening-which refers to the gain of strength of a set cement paste.
The two first to react are C3A and C3S. The setting time of cement decreases with a rise in temperature. The importance of setting in concrete works comes from the importance to keep the fresh concrete in the plastic stage for enough time necessary to complete its mixing and placing under practical conditions (this is the purpose from initial setting time). But, from the economical side, it is important that the concrete hardens at convenient period after casting (this the purpose of final setting time).
Vicat apparatus - use to measure the setting time for cement paste.
Initial setting time - refers to the beginning of the cement paste setting.
Final setting time - refers to the beginning of hardening and gain of strength.
Iraqi Standard Specification No. 5 limits:
- Initial setting time not less than 45 minutes.
- Final setting time not more than 10 hours.
Factors affecting the setting
1- Water/cement (w/c) ratio - The setting time of cement increase with the increase of w/c ratio. 2- Temperature and relative humidity - The setting time of cement decreases with a rise in temperature and decrease of relative humidity. 3- Fineness of cement - The setting time of cement decreases with a rise in fineness of cement. 4- Chemical composition
Flash setting
It is abnorma1 premature stiffening of cement within a few minutes of mixing with water. - It differs from flash set in that: - No appreciable heat is evolved.
- Remixing the cement paste without addition of water restores plasticity of the paste until it sets in the normal manner and without a loss of strength.
Department of Civil Engineering
Concrete Technology Occurs when there is no gypsum added or exhausting the gypsum (added with little amount). so
C3A reacts with water causing liberation high amount of heat causing rapid setting of cement, and leading to form porous microstructure that the product of hydration of the other compounds precipitate through unlike the normal (ordinary) setting that have much lower porosity microstructure.
Causes of false setting
1- Dehydration of gypsum - when interground with too hot a clinker formed: hemihydrates o (CaSO4.0 .5H2O) - when temperature between 100- 190 C - or anhydrite (CaSO4) - when temperature > 190 o C -. And when the cement is mixed with water these hydrate to form gypsum, with a result stiffening of the paste. 2- Reaction of alkalis of the cement
During bad storage - alkalis in the cement react with CO2 (in the atmosphere) to form alkali
carbonates, which they react with Ca(OH)2 liberated by the hydrolysis of C3S to form CaCO3. This precipitates and induces a rigidity of the paste.
K2O or Na2O + CO2 → K2O3 or Na2CO3 K2CO3 or
Na2CO3+ Ca(OH)2 → CaCO3
3- Activation of C3S subjected to wet atmosphere
During bad storage - water is adsorbed on the grains of cement (the water stick on their surfaces) and activates them. and these activated surfaces can combine very rapidly with more water during mixing: this rapid hydration would produce false set.
FINENESS OF CEMENT
The last steps in the manufacture of cement is → the grinding of clinker mixed with gypsum.
Hydration → starts at the surface of the cement particle (it is the total surface area of cement)→ Represent the material available for hydration → The rate of hydration depends on the fineness of the cement particles. Higher fineness is necessary → For a rapid development of the strength as shown in Figure below. It reduce the water layer separate one the mixture surface due to bleeding.
Department of Civil Engineering
Concrete Technology
Relation between strength of concrete at different ages and fineness of cement.
On the other hand the fineness of cement has disadvantages:
Increasing the cost of grinding with increase fineness
Storage difficulties, due to the finer the cement the more rapidly deteriorates on exposure to the atmosphere. Because the increasing of surface area that exposed to atmosphere. Increasing the cement fineness means increasing in drying shrinkage.
Fineness of cement is tested in two ways:
1- sieve Method
It is the classical method to measure the cement fineness, in which the residue percent of cement on sieve No.170 (90 µm) according to BS (British Standard) shall not exceed 10% for ordinary Portland cement.
According to American Standards ASTM the residue percentage on sieve No.200 (74µm) shall not exceed 22% .
2- By determination of specific surface (total surface area of all the particles in one gram of cement by air-permeability apparatus).cm2/gm or m2/kg.
Department of Civil Engineering
Concrete Technology a- Wagner Turbidimeter Method, ASTM C115-10 b- air- permeability Method, BS 12:1971 c- Blaine Method, ASTM C204-07-, BS EN 196-6:2010.
This method used by Iraqi Standards No.5.
According British Standard BS 12:1971, the minimum specific surface measured by air-permeability Method is 2250 cm2/gm for ordinary Portland cement.
CONSISTENCY OF STANDARD PASTE (NORMAL CONSISTENCY)
Its defined as the percentage by mass of water to cement required to produce cement paste of desired consistency. It is used in the determination of the initial and final setting times and soundness of cement. The consistency is measured by the Vicat apparatus, and it is defined as that consistency which will permit a Vicat plunger having 10 mm diameter to penetrate the paste to a point (5±1 mm) from the bottom of the mould.
SOUNDNESS TEST
The testing of soundness of cement to ensure that the cement does not show any appreciable subsequent expansion is of prime importance which could result in a disruption of the hardened cement paste (namely the cement paste, once it has set, does not undergo a large change in volume).
The unsoundness in cement is due to
The delayed or slow hydration.
The presence of excess of lime than that could be combined in kiln.
Excessive proportion of magnesium. Excessive proportion of sulphates.
Because unsoundness of cement is not apparent until after a period of months of years, therefore accelerated tests are required to detect the unsoundness of cement. The cement soundness could be tested by two methods:
1- Autoclave Test, ASTM C 151-09
2- Le Chatelier Test, BS EN 196-3:2005
Department of Civil Engineering
Concrete Technology 1- Autoclave Test, ASTM C 151-09
The autoclave test is sensitive to both free magnesia and free lime. In this test, prescribed by ASTM C151-09, a neat cement bar, 25 mm (or 1 in.) square in cross-section and with 250 mm (or 10 in.) gauge length, is cured in humid air for 24 hours. The bar then placed in an autoclave (a high- pressure steam boiler), which is raised to temperature of 216 °C in 60 min, and maintained at this temperature for 3 hours. The high steam pressure accelerates the hydration of both magnesia and lime. The autoclave is cooled and the length measured again. The expansion of the bar due to autoclaving must not exceed 0.8 per cent.
2- Le Chatelier Test, BS EN 196-3:2005
Le Chatelier apparatus consists of a small brass cylinder split along its generatrix. Two indicators with pointed ends are attached to the cylinder on either side of the split; in this manner, the widening of the split, caused by the expansion of cement, is greatly magnified and can easily be measured. The cylinder is placed on a glass plate, filled with cement paste of standard consistency, and covered with another glass plate. The whole assembly is then placed in a cabinet at 20 ± 1 °C and a relative humidity of not less than 98 percent. At the end of that period, the distance between the indicators is measured and the mould is immersed in water and gradually brought to the boil in 30 minutes. After boiling for 3 hours, the assembly is taken out and, after cooling, the distance between the indicators is again measured. The increase in this distance represents the expansion of the cement, and for Portland cements is limited to 10 mm by BS EN 197-1 : 2000.
REFERENCE
https://www.youtube.com/watch?v=CHs83nfkTy8
STRUCTURE OF HYDRATED CEMENT PASTE
Hydrated cement paste is composed of capillary pores and the hydration product. The pores within the structure of the hydration product are termed ‘gel’ pores. This hydration product includes C-S-H,
CH, AFt, AFm, etc. Gel pores are included within the structure of hydrated cement. According to Powers, 1/3 of the pore space is comprised of gel pores, and the rest are capillary pores. The pores inside cement paste contain water (or pore solution), which can be classified into:
1. Capillary water: Present in voids larger than 50 Ao. Further classified into: (a) free water, the removal of which does not cause any shrinkage strains, and (b) water held by capillary tension in small pores, which causes shrinkage strains on drying. 2. Adsorbed water: Water adsorbed on the surface of hydration products, primarily C-S-H. Water can be physically adsorbed in many layers, but the drying of farther surfaces can occur at about 30 % relative humidity. Drying of this water is responsible for a lot of shrinkage. 3. Interlayer water: Water held in between layers of C-S-H. The drying of this water leads to a lot of shrinkage due to the collapse of the C-S-H structure. 4. Bound water: This is chemically bound to the hydration product, and can only be removed on ignition. Also called ‘non-evaporable’ water.
Department of Civil Engineering
Concrete Technology 2 and 3 are together called ‘gel’ water.
Calculation of the structure of hydrated cement
Theoretically, 0.23 g of bound water is required to completely hydrate 1 g of cement. The remaining water fills up the pores within the structure of the hydrated cement paste (hcp), called the gel pores, as well as the pores external to the hcp, called the capillary pores.
Upon hydration, a volume decrease in the amount of 25.4% of the bound water occurs in the solid hydration product. The characteristic porosity of the hydrated gel is 28%.
Structure of cement hydration products
The structure of C-S-H is best described by the Feldman-Sereda model, shown in Figure 8. It consists of randomly oriented sheets of C-S-H, with water adsorbed on the surface of the sheets (adsorbed water) , as well as in between the layers (interlayer water), and in the spaces inside (capillary water). Such a model implies a very high surface area for the gel. This is indeed found to be true. Using 2 water sorption and N2 sorption measurements, a surface area of 200000 m /kg is reported (ordinary PC has a fineness in the order of 225 – 325 m2/kg). Small angle X-ray scattering measurements show results in the range of 600000 m2/kg. The corresponding figure for high pressure steam-cured cement paste is 7000 m2/kg, which suggests that hydration at different temperatures leads to different gel structures. The structure of C-S-H is compared to the crystal structure of Jennite and Tobermorite. A combination of the two minerals is supposed to be the closest to C-S-H.
Figure 8: Feldman-Sereda model for CSH
Calcium hydroxide deposits as hexagonal crystals. These crystals are typically aligned in the long direction inside pores and around aggregate surfaces.
The structure of ettringite consists of tubular columns with channels in between the columns. The imbibing of water in these channels can lead to substantial expansions. Ettringite demonstrates a trigonal structure, while monosulfate is monoclinic.
Department of Civil Engineering
Concrete Technology Figure 9 depicts the relative sizes of pores in concrete. At one end of the scale are entrapped air voids, while on the lower extreme are the interparticle spaces between sheets of CSH.
Figure 9. Ranges of pore sizes in concrete
REFERENCE https://freevideolectures.com/course/86/building-materials-and-construction/6
HYDRATION OF CEMENT
It is the reaction (series of chemical reactions) of cement with water to form the binding material. In other words, in the presence of water, the silicates (C3S and C2S) and aluminates (C3A and C4AF) form products of hydration which in time produce a firm and hard mass - the hydrated cement paste.
There are two ways in which compounds of the type present in cement can react with water: In the first, a direct addition of some molecules of water takes place, this being a true reaction of hydration.
The second type of reaction with water is hydrolysis, in which its nature can be illustrated using the C3S hydration equation:
3CaO.SiO2 + H2O → Ca(OH)2 + xCaO.ySiO2.aq. (calcium silicate hydrate)
The reaction of C3S with water continue even when the solution is saturated with lime and the resulted amounts of lime precipitate in crystals form Ca(OH)2. Calcium silicate hydrate → remains stable when it is in contact with the solution saturated with lime.
Calcium silicate hydrate → hydrolyzed when being in water - some of lime form, and the process continues until the water saturate with lime. If the calcium silicate hydrate remains in contact with water → it will leave the hardened compound only as hydrated silica due to the hydrolysis of all of the lime.
The rates of the chemical reactions of the main compounds are different:
Department of Civil Engineering
Concrete Technology Aluminates
React with the water in the beginning
Affect the route of the chemical reactions at early periods of hydration. Silicates - Affect the later stage reactions.
The main hydrates of the hydration process are:
- Calcium silicates hydrate, including hydrated products of C3S, and C2S.
2 C3S + 6H → C3S2H3 + 3 Ca(OH)2
2 C2S + 4H → C3S2H3 + Ca(OH)2
- Tricalcium aluminate hydrate.
C3A + 6H → C3AH6
- C4AF hydrates to tricalcium aluminate hydrate and calcium ferrite CaO.Fe2O3 in amorphous form.
Since calcium silicates (C3S and C2S) - are the main cement compounds (occupies about 75% of cement weight) - they are responsible for the final strength of the hardened cement paste.
With time:
- The rate of hydration decreases continuously.
- The size of unhydrated cement particles decrease. For instance, after 28 days in contact with water, grains of cement have been found to have hydrated to a depth of only 4 µm, and 8 µm after a year.
This is due to:
Accumulation of hydration products around the unhydrated cement grains which lead to prevent water from channeling to them. Reduction of the amount of water either due to chemical reaction or evaporation. Reduction of the amount of cement due to reaction.
The process of hydration of cement can be determined by different means:
The measurement of the amount of Ca(OH)2 in the paste resulted from the hydration of the silicates. The heat evolved by hydration.
Department of Civil Engineering
Concrete Technology The specific gravity of the paste. The amount of chemically combined water. The amount of unhydrated cement present (using X-ray quantitative analysis). Also indirectly from the strength of the hydrated paste.
Tricalcium Aluminate Hydrate and the Action Of Gypsum
The amount of C3A present in most cements is comparatively small but its behavior and structural relationship with the other phases in cement make it of interest. The tricalcium aluminate hydrate forms a prismatic dark interstitial material in the form of flat plates individually surrounded by the calcium silicate hydrate.
The reaction of pure C3A with water is very violent with evolution of large amount of heat, forming calcium aluminates hydrate in the form of leaf hexagonal crystals. In Portland cement, this reaction leads to immediate stiffening known as "flash setting".
Gypsum, added to the clinker through grinding process cause delaying the reaction of C3A with water by its reaction with C3A to form insoluble calcium sulfoaluminate (3CaO.A12O3.3CaSO4.30-
32H2O) - ettringite - around C3A particles, which permits enough time for the hydration of C3S that its reaction is slower than C3A and permits the occurring of natural setting. But eventually tricalcium aluminate hydrate is formed, although this is preceded by a metastable 3CaO.A12O3.
CaSO4.12H2O, produced at the expense of the original high-sulfate calcium sulfoaluminate.
The reaction of gypsum with C3A continues until one of them exhausted, while C3S continue in hydration.
If C3A exhausted before gypsum The surplus gypsum → expand → become an agent assist the disruption and deterioration of cement paste.
If gypsum exhausted before C3A
The remaining C3A begins in hydration:
C3A + 6H → C3AH6
C3AH6 is stable - cubical crystals- with high sulfate resistance.
Calcium aluminate hydrate - Be at many forms before transforming to the stable state (C3AH6). It is probably forming hexagonal crystals (C4AH8, C4AH10. C4AH12) before the cubical crystals. When the hexagonal crystals expose to sulfates (inside concrete from sand or external from soil or ground water) → react with it forming calcium sulfoaluminate → with increase in volume, depending on the amount of remaining aluminates and the concentration' of sulfates → crack and deteriorate of the hardened concrete.
The transformation of calcium aluminates hydrate from the metastable exagonal form to the stable cubical form is accompanied with - change in the density and size of the crystals - leading to decrease in the late ages strength of the cement paste due to:
Department of Civil Engineering
Concrete Technology lose the adhesion and cohesion in the microstructure
increase the porosity of the hardened cement paste.
The presence of C3A in cement is undesirable: it contributes little to the strength of cement except at early ages (1- 3 days) and, when hardened cement paste is attacked by sulfates, expansion due
to the formation of calcium sulfoaluminate from C3A may result in a disruption of the hardened paste. But it is useful in the cement industry - work as flux material - reduce the temperature needed to form the clinker. Also it facilitates the combination of lime with silica.
C4AF compound
Gypsum reacts with C4AF to form calcium sulfoaluminates and calcium sulfoferrite. C4AF - work as flux material and also it accelerates the hydration of silicates.
Using The Optimum Percentage of Gypsum is Very Important Because:
- It regulates the speed of the chemical reactions in the early ages.
- Prevent the local concentration of the hydration products. The necessary gypsum content increase with the increase of:
- C3A content in the cement.
- Alkalis content in the cement.
- Fineness of cement.
Iraqi specification No.5 limits the maximum gypsum content (expressed as the mass of SO3 present)
to be not more than 2.5% when C3A ≤ 5% and 2.8% when C3A >5 % for ordinary cement ,for sulfate
resistance cement, SO3 ≤ 2.5%.
Calcium Silicates Hydrate C3S
C3S+ water →
- lime and silica ions in the solution with molecular weight of 3: 1
- Ca(OH)2 crystals
- Calcium silicate hydrate gel (tobermorite)
Hydration of C3S - take about one year or more
This initial gel form an external layer over C3S causing the delay of the reaction. After few hours, this initial C-S-H undergo hydrolysis to form the second product of the gel CSH. The full hydration of
C3S can be expressed approximately following equation
2 (3CaO.SiO2) + 6H2O → 3CaO.2SiO2.3H2O + 3 Ca(OH)2
Department of Civil Engineering
Concrete Technology
There are three main crystal forms of C2S (α, β, ) but the β-form is the only one occurred in the Portland cement and it react slowly with water.
- its reaction is slower than C3S
- The amount of Ca(OH)2 from its hydration is less.
Its formed gel is similar to that produced from C3S, but there is difference in the route of the
chemical reactions between the two components - the lime: silica during the hydration of C2S
differs than that formed during the hydration of C3S. Hydration of C2S - take more than 4 years.
The gel formed after the completion of hydration of the two components is →
C3S2H3 - Tobermorite. Hydration of Cement can be presented by following schematic diagram
Influence of the Compound Composition on Properties of Cement Main Compounds
C3S and C2S - are the most important compounds - responsible for strength.
C3S - contributes most t the strength development during the first four weeks.
C2S - influences the gain in strength from 4 weeks onwards. At the age of about one year, the two compounds, contribute approximately equally to ultimate strength.
C3A contributes to the strength of the cement paste at one to three days, and possibly longer, but
causes retrogression at an advanced age, particularly in cements with a high C 3A or (C3A+C4AF)
content. The role of C4AF in the development of strength of cement is not clear till now, but there certainly is no appreciable positive contribution.
Department of Civil Engineering
Concrete Technology REFERENCE
https://www.youtube.com/watch?v=jJbuAAJAdpc
TESTS FOR PHYSICAL PROPERTIES OF CEMENT
Cement is the major raw material used in any construction. Therefore quality of cement must be checked before using it as a building material. Field and laboratory tests can be performed on cement to check its quality.
Field Tests of Cement
Date of Manufacturing: As the strength of cement reduces with age, the date of manufacturing of cement bags should be checked.
1. Cement Colour: The colour of cement should be uniform. It should be typical cement color i.e. gray colour with a light greenish shade. 2. Whether Hard Lumps are formed: Cement should be free from hard lumps. Such lumps are formed by the absorption of moisture from the atmosphere. 3. Temperature inside Cement Bag: If the hand is plunged into a bag of cement, it should be cool inside the cement bag. If hydration reaction takes place inside the bag, it will become warm. 4. Smoothness Test: When cement is touched or rubbed in between fingers, it should give a smooth feeling. If it felt rough, it indicates adulteration with sand. 5. Water Sinking Test: If a small quantity of cement is thrown into the water, it should float some time before finally sinking. 6. The smell of Cement Paste: A thin paste of cement with water should feel sticky between the fingers. If the cement contains too much-pounded clay and silt as an adulterant, the paste will give an earthy smell. 7. Glass Plate Test: A thick paste of cement with water is made on a piece of a glass plate and it is kept under water for 24 hours. It should set and not crack. 8. Block Test: A 25mm × 25mm × 200mm (1”×1”×8”) block of cement with water is made. The block is then immersed in water for three days. After removing, it is supported 150mm apart and a weight of 15kg uniformly placed over it. If it shows no sign of failure the cement is good.
Laboratory Tests of Cement
CONSISTENCY TEST
This is a test to estimate the quantity of mixing water to form a paste of normal consistency defined as that percentage water requirement of the cement paste, the viscosity of which will be such that the Vicat’s plunger penetrates up to a point 5 to 7 mm from the bottom of the Vicat’s mould. The water requirement for various tests of cement depends on the normal consistency of the cement, which itself depends upon the compound composition and fineness of the cement.
Department of Civil Engineering
Concrete Technology Test Procedure:
300 g of cement is mixed with 25 per cent water. The paste is filled in the mould of Vicat’s apparatus and the surface of the filled paste is smoothed and levelled. A square needle 10 mm x 10 mm attached to the plunger is then lowered gently over the cement paste surface and is released quickly. The plunger pierces the cement paste. The reading on the attached scale is recorded. When the reading is 5-7 mm from the bottom of the mould, the amount of water added is considered to be the correct percentage of water for normal consistency.
INITIAL AND FINAL SETTING TIME
When water is added to cement, the resulting paste starts to stiffen and gain strength and lose the consistency simultaneously. The term setting implies solidification of the plastic cement paste. Initial and final setting times may be regarded as the two stiffening states of the cement. The beginning of solidification, called the initial set, marks the point in time when the paste has become unworkable. The time taken to solidify completely marks the final set, which should not be too long in order to resume construction activity within a reasonable time after the placement of concrete. The initial setting time may be defined as the time taken by the paste to stiffen to such an extent that the Vicat’s needle is not permitted to move down through the paste to within 5 ± 0.5 mm measured from the bottom of the mould. The final setting time is the time after which the paste becomes so hard that the angular attachment to the needle, under standard weight, fails to leave any mark on the hardened concrete. Initial and final setting times are the rheological properties of cement.
Vicat's Apparatus Test Procedure:
A neat cement paste is prepared by gauging cement with 0.85 times the water required to give a paste of standard consistency. The stop watch is started at the instant water is added to the cement. The mould resting on a nonporous plate is filled completely with cement paste and the surface of filled paste is levelled smooth with the top of the mould. The test is conducted at room temperature of 27± 2°C. The mould with the cement paste is placed in the Vicat’s apparatus and the needle is lowered gently in contact with the test block and is then quickly released. The needle thus
Department of Civil Engineering
Concrete Technology penetrates the test block and the reading on the Vicat’s apparatus graduated scale is recorded. The procedure is repeated until the needle fails to pierce the block by about 5 mm measured from the bottom of the mould. The stop watch is pushed off and the time is recorded which gives the initial setting time. The cement is considered to be finally set when upon applying the needle gently to the surface of test block, the needle makes an impression, but the attachment fails to do so.
SOUNDNESS TEST
It is essential that the cement concrete does not undergo large change in volume after setting. This is ensured by limiting the quantities of free lime and magnesia which slake slowly causing change in volume of cement (known as unsound). Soundness of cement may be tested by LeChatelier method or by autoclave method. For OPC, RHC, LHC and PPC it is limited to 10 mm, whereas for HAC and SSC it should not exceed 5 mm. It is a very important test to assure the quality of cement since an unsound cement produces cracks, distortion and disintegration, ultimately leading to failure.
Test Procedure
The Le Chatelier apparatus is used. The mould is placed on a glass sheet and is filled with neat cement paste formed by gauging 100 g cement with 0.78 times the water required to give a paste of standard consistency. The mould is covered with a glass sheet and a small weight is placed on the covering glass sheet. The mould is then submerged in the water at temperature of 27°-32°C. After 24 hours, the mould is taken out and the distance separating the indicator points is measured. The mould is again submerged in water. The water is now boiled for 3 hours. The mould is removed from water and is cooled down. The distance between the indicator points is measured again. The difference between the two measurements represents the unsoundness of cement.
Le Chatelier Apparatus COMPRESSIVE STRENGTH: Compressive strength is the basic data required for mix design. By this test, the quality and the quantity of concrete can be controlled and the degree of adulteration cabe checked.
Test Procedure
Department of Civil Engineering
Concrete Technology The test specimens are 70.6 mm cubes having face area of about 5000 sq. mm. Large size specimen cubes cannot be made since cement shrinks and cracks may develop. The temperature of water and test room should be 27°± 2°C. A mixture of cement and standard sand in the proportion 1:3 by weight is mixed dry with a trowel for one minute and then with water until the mixture is of uniform colour. Three specimen cubes are prepared. The material for each cube is mixed separately. The quantities of cement, standard sand and water are 185 g, 555 g and (P/4) + 3.5, respectively where P = percentage of water required to produce a paste of standard consistency. The mould is filled completely with the cement paste and is placed on the vibration table. Vibrations are imparted for about 2 minutes at a speed of 12000±400 per minute. The cubes are then removed from the moulds and submerged in clean fresh water and are taken out just prior to testing in a compression testing machine. Compressive strength is taken to be the average of the results of the three cubes. The load is applied starting from zero at a rate of 35 N/sq mm/minute. The compressive strength is calculated from the crushing load divided by the average area over which the load is applied.The result is expressed in N/mm2.
TENSILE STRENGTH
The tensile strength may be determined by Briquette test method or by split tensile strength test. Importance: The tensile strength of cement affords quicker indications of defects in the cement than any other test. Also, the test is more conveniently made than the compressive strength test. Moreover, since the flexural strength, is directly related to the tensile strength this test is ideally fitted to give information both with regard to tensile and compressive strengths when the supply for material testing is small.
BRIQUETTE TEST METHOD
A mixture of cement and sand is gauged in the proportion of 1:3 by weight. The percentage of water to be used is calculated from the formula (P/5) + 2.5, where P = percentage of water required to produce a paste of standard consistency. The temperature of the water and the test room should be 27° ± 2°C. The mix is filled in the moulds of the shape shown in Figure. After filling the mould, an additional heap of mix is placed on the mould and is pushed down with the standard spatula, until the mixture is level with the top of the mould. This operation is repeated on the other side of the mould also. The briquettes in the mould are finished by smoothing the surface with the blade of a trowel. They are then kept for 24 hours at a temperature of 27° ± 2°C and in an atmosphere having 90 per cent humidity. The briquettes are then kept in clean fresh water and are taken out before testing. Six briquettes are tested and the average tensile strength is calculated. Load is applied steadily and uniformly, starting from zero and increasing at the rate of 0.7 N/sq mm of section in 12 seconds.
REFERENCE https://www.youtube.com/watch?v=CHs83nfkTy8
Department of Civil Engineering
Concrete Technology GRADES OF CEMENT
Cement is one of the oldest binding materials used for construction. Be it buildings, bridges, skyscrapers, subways, or pipelines, good quality cement is paramount for world-class infrastructure. But how do you measure the quality of cement? Are there set parameters that distinguish high quality cement from low quality cement? Well, the answer is yes. You may have heard of terms like 33 grade cement or 53 grade cement. These are terms used for describing different grades of cement. A grade of cement indicates the strength of cement that is measured in Mega pascal (Mpa) or N/mm2. Cement is usually measured after 28 days of curing for a standard cube.
There are Three Main Grades of Cement.
33 Grade Ordinary Portland Cement
33 grade cement refers to cement that has a compressive strength of 33 N/mm2 at the end of 28 days of curing. As per the Cement Association of Canada, all cement used in concrete construction in Canada is manufactured to meet the requirements of the CSA A3000 compendium of cement standards. This type of cement is used for general construction work under normal environmental condition.
43 Grade Ordinary Portland Cement
43 grade cement refers to cement that has a comprehensive strength of 43 N/mm2 at the end of 28 days of curing. It gets this grade only after it meets the requirements of the Cement Association of Canada. This type of cement is used for plain concrete work and plastering works.
53 Grade Ordinary Portland Cement
53 grade cement refers to cement that has a compressive strength of 53 N/mm2 at the end of 28 days of curing. As per the Cement Association of Canada, all cement used in concrete construction in Canada is manufactured to meet the requirements of the CSA A3000 compendium of cement standards. Unlike other grades of cement, 53 grade ordinary portland cement is used for structural purposes as in reinforced cement concrete.
Every grade of cement is suitable for different set of tasks as each of them have varying levels of strength. Using the wrong strength levels for a particular job can affect your overall structural design. In the world of construction, most professionals opt for 53 grade cement.
REFERENCE https://www.youtube.com/watch?v=xqb5x4gqTBA
Department of Civil Engineering
Concrete Technology CONCRETE ADMIXTURES (ADDITIVES)
Concrete admixtures (additives) enhances the properties of concrete for applications in construction with special requirements. Concrete additives are used to achieve desired workability in case of low water cement ratio, and to enhance setting time of concrete for long distance transportation of concrete.
So, it is of much importance for a civil site engineer to know about the types of admixtures (additives) and their properties for better selection and application in concrete works.
Definition of Concrete Admixtures As per BIS (IS – 9103: 1999) Page No.1, Concrete Admixture is defined as a material other than water, aggregates and hydraulic cement and additives like Pozzolana or slag and fiber reinforcement, used as on ingredient of concrete or mortar and added to the batch immediately before or during its mixing to modify one or more of the properties of concrete in the plastic or hardened state.
Reasons for Using Admixtures Admixtures are used to modify the properties of concrete or mortar to make them more suitable for the work at hand or for economy or for such other purposes as saving energy.
Some of the important purposes for which admixtures are used are:
To modify properties of fresh concrete, mortar and grout to:
Increase workability without increasing water content or decrease water content at the same workability. Retard or accelerate time of initial setting. Reduce or prevent settlement. Modify the rate or capacity for bleedings. Reduce segregation. Improve pumpability. Reduce the rate of slump loss. To modify the properties of hardened concrete, mortar and grout to:
Retard or reduce heat evaluation during early hardening. Accelerate the rate of strength development at early ages. Increase strength (compressive, tensile or flexural). Increase durability or resistance to severe condition of exposure. Decrease permeability of concrete. Control expansion caused by the reaction of alkalies with certain aggregate constituents. Increase bond of concrete to steel reinforcement. Increase bond between existing and new concrete. Improve impact resistance and abrasion resistance.
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Concrete Technology Inhibit corrosion of embedded metal. Produce colored concrete or mortar When Concrete Admixtures Used
When properties cannot be made by varying the composition of basic material. To produce desired effects more economically. Unlikely to make a poor concrete better. Not a substitute for good concrete practice. Required dose must be carefully determined and administered How to Use Concrete Admixtures
Check job specification Use the correct admixture Never use one from an unmarked container. Keep containers closed to avoid accidental contamination. Add the correct dosage. Avoid adding ‘a little bit extra Use a dispenser Wash thoroughly at the end the day Best if added to the mixing water Manufacturer’s recommended dosage is usually adequate Trial mixes are important to determine most effective dosage
Types of Concrete Admixtures (Additives)
1. Accelerating admixtures 2. Retarding admixtures 3. Water-reducing and set controlling admixtures 4. Air-entraining admixtures 5. Super plasticizing admixtures 6. Admixtures for flowing concrete 7. Miscellaneous admixtures Classification of admixtures according to the book of “Concrete Admixtures: Use and Applications” edited by M. R. Rixom are given in the forward pages.
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Concrete Technology
1. Air Entraining Admixture These are generally used to improve workability, ease of placing, increased durability, better resistance to frost action and reduction in bleeding. The common Air-Entraining agents are natural wood resins, neutralized vinsol resins, polyethylene oxide polymers and sulfonated compounds.
Mechanism of Air Entraining Concrete Admixtures These are anionic, because the hydrocarbon structures contain negatively charged hydrophilic groups, such as COO, SO3 and OSO so that large anions are released in water. Conversely, if the hydrocarbon ion is positively charged, the compound is cation active or cationic.
In other words, anionic surface active agents produce bubbles that are negatively charged, cationic charged cause bubbles to be positively charged, surface active agents of all classes can cause air entrainment in concrete, but their efficiency and characteristics of air-void system vary widely.
Properties of Air entraining Admixtures
These are foaming agents, gas producing chemicals. It introduces millions of tiny, stable bubbles of uniform size that are uniformly distributed throughout the mix (usually about 5% of the volume). Improves properties of fresh concrete such as workability, cohesion and reduces segregation and bleeding. Improves properties of hardened concrete – For every 1% of air there is a 4% loss in strength which is minimized by the reduction in water content. It improves durability of hardened concrete. 2. Accelerating Admixtures Accelerating admixtures are used for quicker setting times of concrete. It provides higher early strength development in freshly cast concrete.
Main uses of Accelerating Concrete Admixtures
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Concrete Technology These admixtures are suitable for concreting in winter conditions During any emergency repair work In case of early removal of formwork Disadvantages of Accelerating Concrete Admixtures
It has increased drying shrinkage It offers reduced resistance to sulphate attack CaCl2 high risk of corrosion of steel – not permitted in reinforced concrete It is more expensive and less effective 3. Water Reducing Admixtures Chemical Types for Water Reducing Admixtures
1. Calcium or sodium salt of lignosulfonic acid 2. Poly carboxylic acid Mode of Action The principal role on mechanism of water reductions and set retardation of admixtures are usually composed of long-chain organic molecules and that are hydrophobic (not wetting) at one end and hydrophilic (readily wet) at the other.
Such molecules tend to become concentrated and form a film at the interface between two immiscible phases such as cement and water, and alter the physio-chemical forces acting at this interface.
The mechanism by which water reducing admixture operate is to deflocculated or to disperse the cement agglomerates into primary particles or atleast into much smaller fragments.
This deflocculating is believing to be a physio chemical effect whereby the admixture is first of all adsorbed on to the surface of the hydrating cement, forming a hydration “sheath”, reduces the antiparticle separated from one another.
The presence of water reducing admixture in a fresh concrete results in:
1. a reduction of the interfacial tension. 2. an increase in the electro kinetic potentials and 3. protection sheath of water dipoles around each particle i.e. mobility of fresh mix becomes greater, partly because of reduction in inter-particle forces and partly because of water freed from the restraining influence of the highly flocculated system which is now available to lubricate the mixture. Hence less water is required to achieve given consistency. Why Water Reducing Admixtures are used? a) Concrete having greater workability be made without the need for more water and so strength losses are not encountered
b) By maintaining some workability, but at a lower water content, concrete strengths may be increased without the need for further cement addition
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Concrete Technology c) While maintaining the same w/c ratio and workability concrete can be made to a given strength as in the reference concrete at lower cement content.
Effect on durability The straight addition of admixtures of this type does not came any increase in permeability and indeed where the admixture is used to reduce the w/c, then permeability is considerably reduced.
Effect on shrinkage Admixture of this type when used as workability aids on water reducers do not adversely effect the shrinkage.
Effect on creep Materials of this type of admixture have no deleterious effect on the creep of concrete.
Detrimental effect a) While using water reducing agent. Care must be taken in controlling the air content in the mix. Most water-reducing agent entrain air due to their surfactant properties. b) At high dosages of lignosulphonate material, retardation of the mix occurs.
Applications of Water Reducing Concrete Admixtures The application of the type of admixtures are as follows — a) When concrete pours are restricted due to either congested reinforcement or this sections. b) When harsh mixes are experienced such as those produced with aggregates (crushed). Then considerable improvement in the plastic properties of concrete can be obtained. c) When required strengths are difficult to obtain within specified maximum cement content and where early lifting strengths are required. d) By addition of this admixture in concrete cement economics of about 10% can be obtained.
4. Retarding Concrete Admixtures The function of retarding concrete admixture is to delay or extend the setting time of cement paste in concrete. These are helpful for concrete that has to be transported to long distance in transit mixers and helpful in placing the concrete at high temperatures, specially used as grouting admixture and water reducers results in increase of strength and durability.
Chemical type for Retarding Concrete Admixture a) Unrefined lignosulphonates containing sugar, which of course the component responsible for retardation. b) Hydroxyl carboxylic acid and their salts c) Carbohydrates including sugar d) Soluble zinc
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Concrete Technology e) Soluble borates etc.
Mode of action
It is thought that retarding admixtures are absorbed on to the C3A phase in cement forming a film around the cement grains and presenting or reducing the reaction with water. After a while thus film breaks down and normal hydration proceeds. This a simple mixture and there is a reason to believe that retards also interact with C3S since retardation can be extended to a period of many days. Why Retarding Concrete Admixtures are used? To delay in the setting time of concrete without adversely effecting the subsequent strength development.
Advantage of Retarding Concrete Admixture a) The hydroxyl carboxylic acid type admixture normally produces concrete having a slightly lower aim content them that of a control mix. b) Materials of this class (lignosulphonate containing sugar and derivatives of hydroxyl carboxylic acid) in some cases have a much higher dispersing effect and hence water reducing capacity. c) Durability increases.
Detrimental effect a) When lignosulphonate based material used, then the air content might be 0.2 to 0.3% higher unless materials of the tributyle phosphate type are added. b) As the water content increases, so there is a tendency for drying shrinkage.
Applications of Retarding Concrete Admixture Retarding admixtures are used a) Where long transportation of ready mixed concrete is required then premature setting can be usefully avoided by this type admixture. b) When concrete is being placed or transported under conditions of high ambient temperature. c) In case of large concrete pours d) Concrete construction involving sliding formwork
5. Super Plasticizers or High Range Water-Reducing Admixtures in Concrete These are the second generation admixture and also called as Superplasticizers. These are synthetic chemical products made from organic sulphonates of type RSO3, where R is complex organic group of higher molecular weight produced under carefully controlled condition.
The commonly used superplasticizer are as follows: i) Sulphonated melamine formaldehyde condensate (S M F C)
Department of Civil Engineering
Concrete Technology ii) ii) Sulphonated napthalene formaldehyde condensate (S N F C) iii) iii) Modified ligno-sulphonates and other sulphonic esters, acids etc.,
Chemical type a) formaldehyde derivatives such as melamine formaldehyde and napthalene sulphonate formaldehyde.
Mode of action of Super Plasticizer Admixtures This admixture acts as the same way as that of a water reducing admixture acts. It disperses the cement agglomerates when cement is suspended in water and adsorbed on to the surface of cement, causing them mutually repulsive as a result of the anionic nature of super plasticizers.
Why Super Plasticizer Admixtures are used? a) At a given w/c ratio, this admixture increases the workability, typically by raising the slump from 75 mm to 200 mm. b) The second use of this admixtures is in the production of concrete of normal workability but with an extreme high strength (super plasticizer can reduce the water content for a given workability by 25 – 35 percent compared with half that value in the case of conventional water reducing admixtures).
Advantages of Super Plasticizer Admixtures a) The concrete using this admixture can be placed with little or no compaction and is not subject to excessive bleeding or segregation. b) They can be used as high dosages became they do not markedly change the surface tension of water. c) It does not significantly affect the setting of concrete except that when used the cements having a very low C3A content. d) They do not influence shrinkage, creep modulus of elasticity or resistance to freeing to thawing.
Disadvantage The only real disadvantage of superplasticizer is their relatively high cost.
Applications of Super Plasticizer Admixtures a) In very heavily reinforced sections, in inaccessible areas in floor or road slabs. b) Where very rapid placing is desired.
6. Mineral Admixtures for Concrete Mineral admixtures are finely divided materials which are added to the concrete in relatively large amounts, usually of the order of 20 to 100 percent by weight of Portland cement.
Source of Mineral Admixtures a) Raw or calcined natural minerals
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Concrete Technology b) Industrial by products
Reasons for using mineral admixtures a) In recent years’ considerable efforts have been made by the cement industry world wide to reduce energy consumption in the manufacture of Portland cement. Therefore, a partial replacement of Portland cement by mineral admixtures which can be of the order of 50 – 60% by weight of total cementitious material, represents considerable energy savings. b) The ability of cement and concrete industries to consume mithions of tons of industrial byproducts containing toxic metal would qualify these industries to be classified as environmentally friendly. c) Since natural Pozzolana and industrial by products are generally available substantially lower costs than Portland cement, the exploitation of the Pozzolanic and cementitious properties of mineral admixtures are used as a partial replacement of cement can lead to a considerable economic benefit. d) Possible technological benefits from the use of mineral admixtures in concrete include entrancement of impermeability and chemical durability, improved resistance to thermal cracking and increase in ultimate strength.
Classification of Mineral Admixture Mineral admixtures may be classified as follows — a) Pozzolanic — Siliceon or siliceons and admixtures material which itself possesses little or no cementitious value but is the presence of moisture chemically react with CalOH2 at ordinary temperature to form compounds possessing cementitious properties. b) Pozzolanic & Cementitious — The materials which have some cementitious properties in itself.
ASIM specification C618 recognizes the following three classes of mineral admixtures. a) Class N — Raw or calcined natural pozzolanic such as diatomaceous earths, clay and shales, tuffs and volcanic ashes. b) Class F — Fly ash produced from burning anthracite or bituminous coal. c) Class C — Ash normally produced from lignite or sub-bituminous coal which may contain analytical CaO higher than 10%.
7. Silica Fume as Concrete Admixture Although the use of silica fume (SF) in concrete has increased significantly in the past few years, its beneficial properties were not well realized until comprehensive research was undertaken in the late 70’s and early 80’s at the Norwegian Ins. of technology to study the influence of SF on concrete properties.
Production of Silica Fume
Department of Civil Engineering
Concrete Technology Silicon, ferrosilicon or other alloys are produced in so-called “submerged are electric furnaces”. There are two types of electric furnaces one is with heat recovery system and the other is without heat recovery system.
Types of Alloys Produced in Submerged Arc Electric Furnaces a) Ferrosilicon of various Si contents
– FeSi – 50% with a 43 to 50% Si content
– FeSi – 75% with a 72 to 78% Si content
– FeSi – 90% with a 87 to 96% Si content b) Calcium silicon c) Ferrochromium Silicon d) Silicomanganese
Specific Gravity and Specific Surface Area of SF The specific gravity of SF is generally equal to that of amorphous silica which is about 2.20. However, depending on its chemical composition, the specific gravity of SF particle can be as high as 2.40 and 2.55, as in the case of FeGSi.
The specific surface area of SF is measured by nitrogen absorption is given below.
Calculated Surface Measured by Nitrogen Mean Diameter SF Area (m2/kg) Adsorption (mm)
Si 20000 18500 0.18
FeCrSi 16000 – 0.18
FeSi – 50% 15000 – 0.21
FeSi – 75% 13000 15000 0.26
However, regardless of the differences in chemical composition, color and carbon content, all types of SF share a certain number of common, yet important physio chemical characteristics, which make them effective supplementary cementitious materials to cement concrete. these properties are as follows — a) SF originates from the condensed SiO vapors and generally has a high content of silica of 35 to 98% b) SF is an amorphous material c) SF is composed mostly of fine spheres with a mean diameter of 0.1 to 0.2 mm.
Department of Civil Engineering
Concrete Technology Filler and Pozzolanic effects of Silica Fume The unique characteristics of SF that make it suitable for use as supplementary cementitious materials are its fineness, highly amorphous in nature and elevated content of SiO2. The small SF spheres act as fillers since they occupy some of the space between the relatively coarser cement grains which can be otherwise occupied by water. This results also in a denser matrix with a better gradation of fine particles.
Bache stated that in a super plasticized, low w/c ratio concrete, small SF spheres can displace water entrapped between the flocculated cement grains, thus increasing the amount of free water in the paste which enhances fluidity.
Several researchers have studied the pozzolanic properties of SF. The resulting reactions between SF and Ca(OH)2 increases the volume of CSH and reduces the total volume of capillary pores in the cement paste. The pozzolanic reactions of SF with Ca(OH)2 reduces the amount of Portlandite in the hydrate cement paste. Mehta explained that the absence of large Portlandite crystals in a SF mixture can be due to the fact that each SF particles can act as a “nucleation site” for precipitation of Ca(OH)2. As a result, numerous small crystals of Ca(OH)2 can form rather than a few large ones. This absence of large and week crystals of Portlandite enhance the mechanical properties of concrete.
The beneficial action of SF has also been attributed to the reduction of the porosity of the transition zone between the cement paste and aggregate which increases the strength and impermeability of the concrete. In or conventional concrete, the transition zone can have large and oriented Portlandite crystals which form weak zones in the concrete.
The thickness of the transition zone can be drastically produced by adding SF to the concrete since SF reduces bleeding and the amount of water accumulation under aggregate. As a result, it decreases the porosity of the transition zone and it also reduces the concentration of oriented
Ca(OH)2 crystals. Selection of Concrete Admixtures Concrete admixtures shall be selected carefully as per the specifications and shall be used as recommended by the manufacturer or by lab testing report. The quantity of admixtures to be used for specific application of admixtures are recommended by the manufacturers.
For use in large construction projects, the quantity of the admixture to be used shall be obtained from tests reports for concrete mixed with admixtures at various percentage admixtures use. These tests are conducted to understand the behaviour of admixtures on the desired quality and strength of concrete at different quantity of admixtures used. Thus, the optimum quantity of admixtures can be selected for specific application based on results.
The selection of specific admixtures for use in concrete to alter properties of concrete should be selected carefully as per requirement of concrete works. Concrete admixtures should be used judiciously according to specification and method of application to avoid adverse effect on concrete properties at fresh and hardened state.
Department of Civil Engineering
Concrete Technology After selecting the admixtures product, one should carefully choose the supplier with quality product, timely service and at competitive price. The admixture supplier should be with good history and should possess the staff with efficient and professional experience to guide on effective application/use of admixture in right way.
REFERENCE https://www.youtube.com/watch?v=bgEUI9HvkZ4
Department of Civil Engineering
Concrete Technology
Department of Civil Engineering
Concrete Technology Classification of Aggregates as per Size and Shape -Coarse and Fine Aggregates
Aggregates can be classified in many ways. Classification of aggregates based on shape and size such as coarse and fine aggregates are discussed here,
What is an Aggregate Aggregates are the important constituents of the concrete which give body to the concrete and also reduce shrinkage. Aggregates occupy 70 to 80 % of total volume of concrete. So, we can say that one should know definitely about the aggregates in depth to study more about concrete.
Classification of Aggregates as per Size and Shape Aggregates are classified based on so many considerations, but here we are going to discuss about their shape and size classifications in detail.
Classification of Aggregates Based on Shape We know that aggregate is derived from naturally occurring rocks by blasting or crushing etc., so, it is difficult to attain required shape of aggregate. But, the shape of aggregate will affect the workability of concrete. So, we should take care about the shape of aggregate. This care is not only applicable to parent rock but also to the crushing machine used.
Aggregates are classified according to shape into the following types
Rounded aggregates
Irregular or partly rounded aggregates
Angular aggregates
Flaky aggregates
Elongated aggregates
Flaky and elongated aggregates
Rounded Aggregate The rounded aggregates are completely shaped by attrition and available in the form of seashore gravel. Rounded aggregates result the minimum percentage of voids (32 – 33%) hence gives more workability. They require lesser amount of water-cement ratio. They are not considered for high strength concrete because of poor interlocking behavior and weak bond strength.
Irregular Aggregates
Department of Civil Engineering
Concrete Technology The irregular or partly rounded aggregates are partly shaped by attrition and these are available in the form of pit sands and gravel. Irregular aggregates may result 35- 37% of voids. These will give lesser workability when compared to rounded aggregates. The bond strength is slightly higher than rounded aggregates but not as required for high strength concrete.
Angular Aggregates The angular aggregates consist well defined edges formed at the intersection of roughly planar surfaces and these are obtained by crushing the rocks. Angular aggregates result maximum percentage of voids (38-45%) hence gives less workability. They give 10-20% more compressive strength due to development of stronger aggregate-mortar bond. So, these are useful in high strength concrete manufacturing.
Flaky Aggregates When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate. Or in the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate.
Elongated Aggregates When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension.
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Concrete Technology
Flaky and Elongated Aggregates When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. The above 3 types of aggregates are not suitable for concrete mixing. These are generally obtained from the poorly crushed rocks.
Classification of Aggregates Based on Size Aggregates are available in nature in different sizes. The size of aggregate used may be related to the mix proportions, type of work etc. the size distribution of aggregates is called grading of aggregates.
Following are the classification of aggregates based on size:
Aggregates are classified into 2 types according to size
Fine aggregate
Coarse aggregate
Fine Aggregate When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it called as fine aggregate. Natural sand is generally used as fine aggregate, silt and clay are also come under this category. The soft deposit consisting of sand, silt and clay is termed as loam. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.
Fine aggregate Size variation
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Concrete Technology
Coarse Sand 2.0mm – 0.5mm
Medium sand 0.5mm – 0.25mm
Fine sand 0.25mm – 0.06mm
Silt 0.06mm – 0.002mm
Clay <0.002
Coarse Aggregate When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete. the size range of various coarse aggregates given below.
Coarse aggregate Size
Fine gravel 4mm – 8mm
Medium gravel 8mm – 16mm
Coarse gravel 16mm – 64mm
Cobbles 64mm – 256mm
Boulders >256mm
Department of Civil Engineering
Concrete Technology Mechanical properties of aggregates
Several mechanical properties of aggregate are of interest for the manufacture of concrete, specially high strength concrete subjected to high wear. Some of them are discussed here in brief: 1. Toughness 2. Hardness 3. Specific Gravity 4. Porosity of Aggregate 5. Bulking of Sand.
1. Toughness: It is defined as the resistance of aggregate to failure by impact. The impact value of bulk aggregate can be determined as per I.S. 2386, 1963.
The test procedure is as follows: The aggregate shall be taken as in the case of crushing strength value test i.e., the aggregate should pass through 12.5 mm I.S. sieve and retained on 10 mm I.S. sieve. It should be oven dried at 100°C to 110°C for four hours and then air cooled before test.
Now the prepared aggregate is filled upto 1/3rd height of the cylindrical cup of the equipment. The diameter and depth of the cup are 102 mm and 50 mm respectively. After filling the cup upto 1/3rd of its height, the aggregate is tamped with 25 strokes of the rounded end of the tamping rod.
After this operation the cup shall be further filled upto 2/3rd of its height and a further tamping of 25 strokes given. The cup finally shall be filled to over flowing and tamped with 25 strokes and surplus aggregate removed and the weight of aggregate noted. The value of weight will be useful to repeat the experiment.
Now the hammer of the equipment weighting 14.0 kg or 13.5 kg is raised till its lower face is 380 mm above the upper surface of the aggregate and., allowed to fall freely on the aggregate and the process is repeated for 15 times.
The crushed aggregate is now removed from the cup and sieved through 2.36 mm I.S. sieve. The fraction passing through the sieve is weighed accurately.
Let the weight of oven dry sample in the cup = W kg.
Weight of aggregate passing 2.36 mm sieve = W1 kg.
Then impact value = [(W1/W) x 100] This value should not be more 30% for aggregate to be used in concrete for wearing surfaces as road and 45% for other type of concrete.
2. Hardness: It is defined as the resistance to wear by abrasion, and the aggregate abrasion value is defined as the percentage loss in weight on abrasion.
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Concrete Technology For testing hardness of aggregate following three methods can be used: (a) Deval Attrition test.
(b) Dorry abrasion test.
(c) Los Angeles test.
(a) Deval Attrition Test: This test has been covered by IS 2386 Part (IV)-1963. In this test particles of known weight are subjected to wear in an iron cylinder rotated 10,000 (ten thousand) times at the rate of 30 to 33 revolutions per minute. After the specified revolution of the cylinder the material is taken out and sieved on 1.7 mm sieve and the percentage of material finer than 1.7mm is determined. This percentage is taken as the attrition value of the aggregate. The attrition value of about 7 to 8 usually is considered as permissible.
(b) Dorry Abrasion Test: This test has not been covered by Indian standard specifications. In this test a cylindrical specimen having its diameter and height of 25 cm is subjected to abrasion against a rotating metal disk sprinkled with quartz sand. The loss in weight of the cylinder after 1000 (one thousand) revolutions is determined.
Then the hardness of rock sample is expressed by an empirical relation as follows: Hardness or sample = 20 – Loss in weight in grams/3
For good rock this value should not be less the 17. The rock having this value of 14 is considered poor.
(c) Los-Angeles Test: This test has been covered by IS 2386 (Part-IV) 1963. In this test aggregate of the specified grading is placed in a cylindrical drum of inside length and diameter of 500 mm and 700 mm respectively. This cylinder is mounted horizontally on stub shafts. For abrasive charge, steel balls or cast iron balls of approximately 48 mm diameter and each weighting 390 grams to 445 gram are used. The numbers of balls used vary from 6 to 12 depending upon the grading of the aggregate. For 10 mm size aggregate 6 balls are used and for aggregates bigger than 20 mm size usually 12 balls are used.
Procedure: For the conduct of test, the sample and the abrasive charge are placed in the Los-Angeles testing machine and it is rotated at a speed of 20 to 33 revolutions per minute. For aggregates upto 40 mm size the machine is rotated for 500 revolutions and for bigger size aggregate 1000 revolutions. The charge is taken out from the machine and sieved on 1.7 mm sieve.
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Concrete Technology Let the weight of oven dry sample put in the drum = W Kg.
Weight of aggregate passing through 1.7 sieve = W1 Kg.
Then abrasion value = [(W1/W) x 100] The abrasion value should not be more than 30% for wearing surfaces and not more than 50% for concrete used for other than wearing surface. The results of Los Angeles test show good correlation not only the actual wear of aggregate when used in concrete, but also with the compression and flexural strength of concrete made with the given aggregate.
Table 4.8 gives an idea of toughness, hardness, crushing strength etc. of different rocks.
3. Specific Gravity: The specific gravity of a substance is the ratio of the weight of unit volume of the substance to the unit volume of water at the stated temp. In concrete making, aggregates generally contain pores both permeable and impermeable hence the term specific gravity has to be defined carefully. Actually there are several types of specific gravity. In concrete technology specific gravity is used for the calculation of quantities of ingredients. Usually the specific gravity of most aggregates varies between 2.6 and 2.8.
Specific gravity of certain materials as per concrete hand book CA-1 Bombay may be assumed as shown in Table 4.9.
Method of Determination of Specific Gravity of Aggregate:
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Concrete Technology IS-2386-Part-III-1963 describes various procedures to find out the specific gravity of aggregates of different sizes. Here the method applicable to aggregates larger than 10 mm in size has been described as follows ―
A sample of aggregate not less than 2 kg in weight is taken and washed thoroughly to remove dust, and silt particles etc. The washed sample is placed in a wire basket and immersed in distilled water at a temperature of 27 ± 5°C.
Immediately after immersion, the entrapped air is removed from the sample by lifting the basket containing sample 25 mm above the bottom of the jar or tank and allow it drop 25 times at the rate of 1 mm per sec. During this operation, care should be taken that basket and aggregate remain fully immersed in water. After this, the sample is kept in water for about 24 ± ½ hour.
After this period the basket and aggregate is given a jerk to remove the air etc. and weighed in water at the temperature of 27 ± 5°C. Let the weight of basket and aggregate be A1. The basket and sample of aggregate is removed from the water and allowed to drain for a few minutes. Then the aggregate is taken out from the basket and placed on a dry cloth and dried further. The empty basket is again immersed in water and weighed in water after giving 25 jolts. Let this weight be A2. The aggregate is surface dried in shade for not more than 10 minutes and the aggregate is weighed in air. Let this weight be B. Now the aggregate is oven dried for 24 ± ½ hour at a temperature of 100 to 110°C. It is then cooled in air tight container and weighed. Let this weight be C.
Thus,
Weight of sample in water = (A1 – A2) = A Weight of saturated surface dry in air sample = B
Weight of oven dry sample = C
(a) Then specific gravity = [C/(B – A)]
(b) Apparent specific gravity = [C/(C – A)]
(c) Water absorption = 100 (B – C)
(d) Bulk density = Net weight of the aggregate in kg./capacity or the container in litres
Absolute Specific Gravity: It can be defined as the ratio of the weight of the solid, referred to vacuum, to the weight of an equal volume of gas free distilled water both taken at the standard or a stated temperature, usually it is not required in concrete technology. Actually the absolute specific gravity and particle density
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Concrete Technology refer to the volume of solid material excluding all pores, while apparent specific gravity and apparent particle density refer to the value of solid material including impermeable pores, but not the capillary pores. In concrete technology apparent specific gravity is required.
Apparent Specific Gravity: It can be defined as the ratio of the weight of the aggregate dried in an oven at 100°C to 110°C for 24 hours to the weight of water occupying a volume equal to that of the solid including the impermeable pores. This can be determined by using pycno-meter for solids less than 10 mm in size i.e., sand.
Bulk Specific Gravity: It can be defined as the ratio of the weight in air of a given volume of material (including both permeable and impermeable voids) at the standard temperature to the weight in air of an equal volume of distilled water at the same standard temperature (20°C). The specific gravity of a material multiplied by the unit weight of water gives the weight of 1 cubic metre of that substance. Some times this weight is known as solid unit weight. The weight of a given quantity of particles divided by the solid unit weight gives the solid volume of the particles.
Solid vol. in m3 = 3 wt. of substance in kg/specific gravity x 1000 Bulk Density: The weight of aggregate that would fill a container of unit volume is known as bulk density of aggregate. Its value for different materials as per concrete hand book CIA Bombay is shown in Table 4.10.
Voids: With respect to a mass of aggregate, the term voids refers to the space between the aggregate particles. Numerically this voids space is the difference between the gross volume of aggregate mass and the space occupied by the particles alone. The knowledge of voids of coarse and fine aggregate is useful in the mix design of concrete.
Percentage voids = [(Gs – g)/Gs] x 100 where Gs = specific gravity of aggregate and g is bulk density in kg/litre. Unit Weight: The weight of a unit volume of aggregate is called as unit weight. For a given specific gravity, greater the unit weight, the smaller the percentage of voids and better the gradation of the particles, which affects the strength of concrete to a great extent.
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Concrete Technology 4. Porosity and Water Absorption by Aggregate: All aggregates, particles have pores with in their body. The characteristics of these pores are very important in the study of the properties of aggregate. The porosity, permeability, and absorption of aggregates influence the resistance of concrete to freezing and thawing, bond strength between aggregate and cement paste, resistance to abrasion of concrete etc.
The size of pores in the aggregate varies over a wide range, some being very large, which could be seen even with naked eye. The smallest pore of aggregate is generally larger than the gel pores in the cement paste, pores smaller than 4 microns are of special interest as they are believed to affect the durability of aggregates subjected to alternate freezing and thawing. Some of the pores are wholly within the body of the aggregate particles and some of them are open upto the surface of the particle.
The cement paste due to its viscosity cannot penetrate to a great depth into the pores except the largest of the aggregate pores. Therefore, for the purpose of calculating the aggregate content in concrete, the gross volume of the aggregate particles is considered solid. However water can enter these pores, the amount and rate of penetration depends upon the size, continuity and total volume of pores.
When all the pores in the aggregate are full with water, then the aggregate is said to be saturated and surface dry. If this aggregate is allowed to stand in the laboratory, some of the moisture will evaporate and the aggregate will be known as air dry aggregate. If aggregate is dried in oven and no moisture is left in it, then it is known as bone dry aggregate. Thus the ratio of the increase in weight to the dry weight of the sample, expressed as a percentage is known as absorption.
The knowledge of absorption of aggregate is important in adjusting water-cement ratio of the concrete. If water available in the aggregate is such that it contributes some water to the dilution of cement paste, in that case the water-cement ratio will be more than the required and the strength will go down.
On the other hand, if the aggregate is so dry that it will absorb some of the mixing water, in that case the mix will have lower water-cement ratio and the mix may become unworkable. Hence, while deciding the water-cement ratio, it is assumed that the aggregate is in saturated but surface dry condition, i.e. neither it will add water to cement paste, nor it will absorb water from the mix.
It has been observed that absorption of water by dry aggregate slows down due to the coating of particles with cement paste. The water absorption by aggregate should be determined for 10 to 30 minutes instead of total water absorption. The value of absorption of water may be taken as follows as recommended by concrete hand book CAI Bombay in Table 4.11.
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Concrete Technology
Surface Water: While using aggregate in the concrete, water on the surface of the aggregate should be taken into account, as it will contribute to the water in the mix and will affect the water-cement ratio of the mix, causing lower strength of the concrete. It is difficult to measure surface water of the aggregate. Therefore its value may be assumed according to I.S. 456, 1964 given in Table 4.12.
Bulking of Sand: The moisture present in fine aggregate causes increase in its volume, known as bulking of sand. The moisture in the fine aggregate develops a film of moisture around the particles of sand and due to surface tension pushes apart the sand particles, occupying greater volume. The bulking of the sand affects the mix proportion, if mix is designed by volume batching. Bulking results in smaller weight of sand occupying the fixed volume of the measuring box, and the mix becomes deficient in sand and the resulting concrete becomes honeycombed and its yield is also reduced.
The extent of bulking depends upon the percentage of moisture present in sand and its fineness. The increase in volume relative to that occupied by a saturated and surface dry sand increases with an increase in the moisture content of the sand upto a value of 5 to 8%, causing bulking ranging from 20 to 40% as shown in Table 4.13. Fig. 4.8 and Table 4.13 shows bulking of sand with various moisture contents as suggested by concrete hand book CAI, Bombay.
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Concrete Technology
As the moisture content increases, the film of water formed around the sand particles merge and the water moves into the voids between the particles so that the total volume of sand decreases, till the sand is fully saturated. The volume of fully saturated sand is same as that of the dry sand for the same method of filling the container.
Determination of Bulking of Sand: Since the volume of saturated sand is same as that of dry sand, the most convenient way of determining bulking of sand is by measuring the decrease in volume of the given sand on saturation. For the measurement of bulking of sand, usually a container of known volume, a 30 cm long steel rule, and a 6 mm iron rod is required.
Procedure: Put sufficient quantity of sand loosely into the container, till it is about two-thirds full. Level off the top of the sand with steel rule, and push this rule at the middle of the surface to the bottom of the container and measure its height. Let the height be h cm.
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Concrete Technology Now empty this sand into another container. While emptying, care should be taken that no sand particles are lost. Take about 1/3rd to half-full the first container with water and add about half the sand to it and rod it with 6 mm diameter steel rod. The sand should be rodded till the air bubbles cease to come out. At this stage the volume of sand is minimum. At this stage add the remaining sand and rod it also till air bubbles cease to come out. Smooth and level the top surface of the saturated sand and measure its height by pushing the steel rule at the middle of the surface to the bottom of the container. Let this height be h1 cm.
Then % bulking = [(h1/h1) x 100] Effect of Bulking of Sand: For volume batching, bulking has to be allowed for by increasing the total volume of sand used, otherwise the mix will be deficient in sand and segregation of the mix may take place. Also the resulting concrete will be honeycombed and its yield will be reduced, raising its cost of production. The volume to be increased can be calculated either by knowing this percentage of bulking as shown above or by bulking factor.
If,
Vm = vol. of moist sand
Vs = vol. of saturated sand then bulking = [(Vm – Vs)/Vs] and bulking factor = 1 + [(Vm – Vs)/Vs] = Vm/Vs
Hence to know the total volume of sand to be used can be calculated by multiplying the vol. Vs by the bulking factor. The value of bulking factor can be determined by the curves of Fig. 4.9. Fig. 4.9 gives bulking factor against moisture content upto 20% for three types of sands.
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Concrete Technology DELETERIOUS SUBSTANCES IN AGGREGATES:
Deleterious materials in aggregate are those substances which detrimentally effect the fresh and hardened properties of concrete for instance strength, workability, and long-term performance of the concrete in which such are used. Deleterious materials and highly undesirable constituents.
Organic impurities, clay, silt and crushed dust, salts, unsound particles, and alkali aggregate reactions. Adverse effects of deleterious materials on concrete includes the increase of water demand in concrete, impair bond strength between cement and aggregate, reduce durability, result in concrete popouts, and impair wear resistance.
There are tests such as colorimetric test recommended by ASTM C 40-92 which are used to determine aggregate organic content. The colorimetric test does not show the adverse effect of deleterious materials in aggregate. This is because high aggregate deleterious substance content does not infer that the aggregate is not fit for utilization that is why strength test based on ASTM C 87-90 is recommended for mortars with questionable sand.
1. Organic Impurities
Organic impurities interfere with the hydration reaction. Frequently, it is found in sand and consists of products of decay of vegetable matter. Organic matter may be removed from sand by washing. Colorimetric test recommended by ASTM C 40-92 can be used to determine aggregate organic content. The colorimetric test does not show the adverse effect of the organic impurity since high organic content does not necessarily mean that the aggregate is not fit for use in concrete. For this reason, strength test on mortar with questionable sand as per ASTM C 87-90 is recommended. This strength has to be compared with the strength of mortar with washed sand
2. Clay
Clay may coat the surface of aggregates which impair bond strength between aggregate and cement paste. Consequently, it adversely affecting the strength and durability of concrete it is necessary to control the amount of clay in aggregate Since no test is available to determine separately the clay content, the limits of fine materials are prescribed in terms of the percentage of material passing sieve No. 200.
3. Silt and crusher dust
Silt and dust, owing to their fineness, increase the surface area and therefore increase the amount of water necessary to wet all the particles in the mix. Impair wear resistance Reduce durability
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Concrete Technology They may result popouts It is necessary to control the amount of silt and fine dust in aggregate. Since no test is available to determine separately the silt and dust, the limits of fine materials are prescribed in terms of the percentage of material passing sieve No. 200
4. Salts
Salts are present in certain types of aggregates such as Sand from seashore, sand and Coarse aggregate dredged from the sea or a river estuary, and desert sand. Salts coming through aggregates cause reinforcement corrosion and also absorb moisture from the air and cause efflorescence. The BS 882:1992 limits on the chloride ion content of aggregate by mass, expressed as a percentage of the mass of total aggregate.
5. Unsound Particles
Two major classes of unsound particles are materials fail to maintain their integrity, and substances lead to disruptive expansion on freezing or even on exposure to water. Shale, particles with low density, clay lumps, wood, coal, mica, gypsum, and iron pyrites are examples of unsound particles. Unsound particles if present in large quantities (over 2 to 5% of the mass of the aggregate) may adversely affect the strength of concrete. These materials should not be allowed in concrete which is exposed to abrasion. Mica is very effective in reducing strength (15% reduction in 28-d f’c with 5% mica). Gypsum and iron pyrites are mainly responsible for expansion of concrete
6. Alkali- Aggregate Reactions
Reaction between alkali from cement and silica or carbonate from aggregate is called “alkali- aggregate reaction”. The most common reaction is that between the active silica constituents of the aggregate and that alkalis in cement, called as “alkali-silica reaction” Another type of the alkali-aggregate reaction is that between dolomitic limestone aggregates, containing carbonate, and alkalis in cement, called as “alkali-carbonate reaction”. Both types of the reactions cause deterioration of concrete, mainly cracking. The reactive forms of silica opal (amorphous, i.e. shapeless), Chalcedony (cryptocrystalline fibrous), and tridymite (crystalline). The gel formation on the surface of aggregate particles destroys the bond between the aggregate and cement paste. The swelling nature of the gel exerts internal pressure and eventually lead to expansion, cracking and disruption of the hydrated cement paste. In case of alkali-carbonate reaction also, gel is formed, which upon swelling cause expansion of concrete. Gel is formed around the active aggregate particles, causing cracking within rims and leads to a network of cracks and loss of bond between the aggregate and the cement paste.
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Concrete Technology ALKALI AGGREGATE REACTION Alkali Aggregate Reaction in Concrete – Types, Causes, and Effects Alkali aggregate reactions (AAR) occur when aggregates in concrete react with the alkali hydroxides in concrete producing a hygroscopic gel which, in the presence of moisture, absorbs water and causes expansion and cracking over a period of many years. This alkali-aggregate reaction has two forms, namely: Alkali-silica reaction (ASR) and Alkali-carbonate reaction (ACR). The former is of higher concern since aggregates containing various forms of silica materials are very common whereas the latter occurs rarely because of the unsuitability of carbonates for use in concrete.
Nonetheless, concrete deterioration caused by each type of alkali-aggregate reaction is similar. It should be known that no structure has ever collapsed due to alkali-aggregate reactions, but there are cases in which structural concrete members demolished due to the effect of alkali-aggregate reactions. Most of the structures severely cracked by AAR are exposed to the weather or are in contact with damp soil. This is because- for a significant amount of expansion to occur, sufficient presence of moisture is essential. Apart from moisture, high content of alkali in the concrete is also essential.
Types of Alkali Aggregate Reaction
Alkali-silica reaction (ASR)
Random map cracking and closed joints and attendant spalling concrete are indicators of alkali-silica reactions.
Petrographic examination can identify alkali-silica reactions.
It occurs broadly because aggregates containing reactive silica materials are more common.
Alkali-silica reaction generates enough expansive pressure to damage concrete.
Cracking initiates in areas with a frequent supply of moisture, such as close to the waterline in piers, near the ground behind retaining walls, or in piers or columns subject to wicking action.
It can be controlled using proper portions of supplementary cementitious materials like silica fume, fly ash, and ground granulated blast-furnace slag.
Lithium compounds can be used to decrease alkali-silica reactions.
Alkali-carbonate reaction (ACR)
It is observed with certain dolomitic rocks.
It may cause considerable expansion.
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Concrete Technology Compare to alkali-silica reactions, ACR is fairly rare because aggregates susceptible to this phenomenon are less common.
The use of supplementary cementing materials does not prevent deleterious expansion due to ACR.
So, it is recommended that ACR susceptible aggregates not be used in concrete.
Conditions for AAR Occurrence
1. Sufficient moisture supply,
2. High content of alkali in concrete It is shown that when the total alkali content, in terms of equivalent sodium oxide, is less than 3 kg/m3, damage expansion due to AAR is unlikely to happen, provided that known highly alkali- reactive minerals, such as opal and glass, are not present in the concrete.
Sources of Alkalis in Concrete
1. Cement All ingredients of concrete may contribute to the total alkali content of the concrete, the major source of alkali is from cement.
2. Aggregate Aggregate containing feldspars, some micas, glassy rock and glass may release alkali in concrete. Sea dredged sand, if not properly washed, may contain sodium chloride which can contribute significant alkali to concrete.
3. Admixtures Admixture in the context of AAR in concrete means chemical agents added to concrete at the mixing stage. These include accelerators, water reducers (plasticizers), retarders, superplasticizers, air- entraining, etc. Some of the chemicals contain sodium and potassium compounds which may contribute to the alkali content of concrete.
4. Water Water may contain a certain amount of alkali.
5. Alkalis from Outside Concrete In the areas of cold weather, de-icing salt containing sodium compounds which may increase the alkali content on the surface layer of concrete. Soils containing alkali may also increase alkali content on the surface of concrete.
Effects of Alkali-Aggregate Reaction
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Concrete Technology 1. Loss of strength, stiffness, impermeability
2. Affects concrete durability and appearance
3. Premature failure of concrete structures
4. Consequently, life of concrete structure is declined
5. Maintenance cost is increased
Tests for Aggregate Reactivity
1. Petrographic Examination (ASTM C 295, BS 812: Part 104)
2. Chemical Test (ASTM C289)
3. Accelerated Mortar Bar Test (ASTM C 1260, CSA A23.2 25A, DD 249: 1999)
4. Concrete Prism Test (ASTM C1293, CSA A23.2 14A, BS 812: Part 123)
5. Accelerated Concrete Prism Test
Preventive Measures against AAR
1. Use low alkali cement to limit alkali content in concrete
2. Use of Cementitious Replacement Materials such as PFA and GGBS in concrete to decrease alkali content in concrete
3. Reduce the access of moisture and maintain the concrete in a sufficiently dry state
4. Avoid utilization of reactive aggregate otherwise necessary precautions shall be employed to prevent influences of alkali-aggregate reactions.
5. Modify the properties of any gel such that it becomes non-expansive, for instance, using lithium salts.
GRADING OF AGGREGATES
On dividing this sum by 100, The Fineness Modulus of that aggregate is determined. This helps in deciding about the quantity of aggregates of known fineness moduli to be mixed for obtaining a concrete of desired density. The basis for mixing coarse and fine aggregates of specific fineness modulus is the presence of voids or open spaces when the aggregates are packed together.
In pure coarse aggregates packing may leave 30-40 percent voids, which can be removed only by filling with finer particles. Similarly, in fine aggregates also, voids are left that have to be filled with still finer particles (of cement). This is essential to obtain concrete of compact and void-free character.
When some pieces of aggregates of equal size are packed together, voids or open spaces are always left. The percentage of voids may be as high as 45 percent of the total volume of aggregates.
Department of Civil Engineering
Concrete Technology It has been observed that this result (presence of voids) is independent of the size of aggregates used in packing, whether coarse, medium or fine.
Either all of them should be coarse, or all of them should be medium or fine grade. This implies that even if sand alone is packed, voids to the tune of 40-45 percent are again left.
This principle of formation of voids is the governing principle for the preparation of concrete under this method.
When the coarse aggregates are packed to make the concrete, the voids formed within the mass must be filled by some finer material. Sand is used for this purpose.
But, there will be voids left between the sand grains too. These are filled with the cement particles.
In this way, the resulting concrete mass is a void-less or dense mass. The binding property of cement is made use of to give this dense mass a cohesive stone-like character.
Concrete is, an artificial stone, in a broad sense.
Grading of aggregates is aimed at determining the mean size of the particle in a given batch of aggregates.
This is commonly found by the Method of Fineness Modulus. The method can be used to determine fineness modulus of coarse aggregates, fine aggregates, and all-in aggregates, i.e., mixed aggregates.
In this method, a convenient weight of the sample is taken and sieved through a set of sieves one after another. The number of sieves is five for the coarse aggregates and ten for all-in-aggregates.
It is only six in the case of fine aggregates.
Sieve Size for Grading of Aggregates.
Coarse Aggregates: 80 mm, 40 mm, 20 mm, 10 mm, IS Nos. 480 Fine Aggregates: IS No. 480, 240, 120, 60, 30 and 15. All in aggregates: 80 mm, 40 mm, 20 mm, 10 mm, Nos: 480, 240, 120, 60, 30 and 15.
It will be noted that each successive sieve has the diameter of mesh reduced to 50 percent.
Calculations involve dividing the cumulative percentage of weights retained on these set of sieves by 100.
The resulting figure gives fineness modulus of the respective aggregate. This is illustrated in the following example.
A weight of 10 kg of Coarse aggregate and 5 kg of fine aggregate has been taken to determine fineness modulus in each case separately.
Let us assume that the weight of aggregate retained in each case and the calculated cumulative percentage of retained weights are as follows:
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Concrete Technology IS Sieve No. Weight Retained Total Weight (kg) Retained Weight (gm)
80 mm 0 0 0
40 mm 0 0 0
20 mm 2 2 20
10 mm 4 6 60
480 3.5 9.5 95
240 0.5 10 100
120 100
60 100
30 100
15 100 Showing 1 to 10 of 11 entries PreviousNext
Cumulative Percentage of weight retained on all the ten sieves – 675 Fineness modulus of coarse aggregate = 675/100 = 6.75 kg.
IS Sieve No. Weight retained Total Weight (kg) Retained (gm)
480 0.2 0.2 4
240 0.8 1.0 20
120 0.5 1.5 30
60 1.2 2.7 54
30 1.5 4.2 84
15 0.8 5.0 100
Total 5 kg 2.92 kg Cumulative percentage of weight retained on all the six sieves = 292 Fineness Modulus of Fine Aggregates = 292/100 =2.92 kg.
Now, when it is desired to obtain a combined aggregate of a definite (required) fineness modulus, F, the amount of fine aggregate (x) to be mixed with 1 volume of the coarse aggregate of a given fineness modulus can be easily determined from the relationship: x=Fc – F / F – Ff
Where “x” is the amount of fine aggregate (by volume)for 1 volume of coarse aggregate.
Fc = Fineness modulus of coarse aggregate. F = Fineness modulus of desired mixed aggregate.
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Ff = Fineness modulus of fine aggregate. The above relationship is illustrated in the following example.
F = 5.5
Fc = 6.75 Ff = 2.92 To Determine: Volume of fine aggregate (x) in percentage terms of volume of coarse aggregate. x=Fc – F / F – Ff => 6.75 – 5.3 / 5.5-2.92 x 100
=> 48.5 % approximately.
Limits of Fineness.
Repeated trials with mix designs using different aggregates have shown that following limits of fineness moduli hold good for obtaining concrete mixes of good workability.
Cement Consumption is also reasonable when limits are followed.
Aggregate Maximum Recommended Moduli Recommended Moduli Type Size (mm) Maximum. Manimum.
Fine 2.00 3.5
Coarse 20 6 6.90
40 6.90 7.50
80 7.50 8.00
160 8.00 8.50
Mixed 20 4.70 5.10
25 5.10 5.5
32 5.2 5.7
40 5.4 5.9
80 5.8 6.3
The upper limit of fineness modulus is, therefore, always below 8.5 for coarse and 7.0 for mixed aggregates.
REFERENCE https://nptel.ac.in/courses/105102012/ -Link for aggregates and their properties and tests- 3 different videos will be available within the site https://www.youtube.com/watch?v=LFwn8OExgX4- grading of aggregates
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Concrete Technology
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Concrete Technology Workability of Concrete Workability of concrete is the property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished’ as defined by ACI Standard 116R-90 (ACI 1990b).
ASTM defines it as “that property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity”.
The workability of concrete depends on many factors which are explained in factors affecting workability of concrete. Water cement ratio has much effect in the workability. Workability is directly proportional to water cement ratio. An increase in water-cement ratio increases the workability of concrete.
Types of Workability of Concrete Workability of concrete can be divided into following three types:
1. Unworkable Concrete
2. Medium Workable
3. Highly Workable Concrete
1. Unworkable Concrete – Harsh Concrete An unworkable concrete can also be called as harsh concrete. It is a concrete with very little amount of water. The hand mixing of such concrete is not easy.
Such type of concrete has high segregation of aggregates as cement paste is not lubricated properly to stick to the aggregates. It is very difficult to maintain the homogeneity of concrete mix and compaction of concrete requires much effort. Water cement ratio of such concrete is below 0.4.
2. Medium Workable Concrete This type of concrete workability is used in most of the construction works. This concrete is relatively easy to mix, transport, place and compact without much segregation and loss of homogeneity.
This type of concrete workability is generally used in all concrete construction with light reinforcement (spacing of reinforcement is which allows the concrete to be compacted effectively). Water cement ratio for medium workable concrete is 0.4 to 0.55.
3. Highly Workable Concrete A highly workable concrete is very easy to mix, transport, place and compact in structures. Such concrete is used where effective compaction of concrete is not possible or in mass concrete. Such concrete flow easily and settle down without much effort. But there is high chances of segregation and loss of homogeneity in this case.
Department of Civil Engineering
Concrete Technology The coarse aggregates tend to settle at the bottom and the concrete paste comes up. Such concrete is used in case of heavy reinforcement is used where vibration of concrete is not possible. Example of highly workable concrete is self-compacting concrete. Water cement ratio of such concrete is more than 0.55.
Workability requirement of concrete varies with each type of construction and compaction method used. For example, concrete workability required for a slab construction can be same as a mass concrete footing construction.
Workability requirement when vibrators are used for construction are different from when vibrators are not used. Similarly, concrete workability used in thick section is not workable when used in thin sections.
Factors Affecting Workability of Concrete
The workability requirements for a concrete construction depends on:
Water cement ratio
Type of construction work
Method of mixing concrete
Thickness of concrete section
Extent of reinforcement
Method of compaction
Distance of transporting
Method of placement
Environmental condition
Workability Vs. Strength of Concrete The following figure explains the relation between workability and compressive strength of concrete:
Fig: Workability Vs. Strength of Concrete
Department of Civil Engineering
Concrete Technology As you can see from figure, the strength of concrete decreases with increase in water cement ratio. The increase in water cement ratio indicates increase in workability of concrete. Thus, the strength of concrete inversely proportional to the workability of concrete.
The reason for this relation is that water from the concrete dries up and leaves voids when setting of concrete occurs. The more the water is, the more will be the number of voids. Thus, increase in number of voids decreases the compressive strength of concrete. Thus it is important to balance the strength and workability requirement for concrete work.
The workability of concrete can be enhanced by use of rounded aggregates and by the use of workability enhancing admixtures. With the use of admixture such as air-entraining admixtures, the workability in increased without increase in water-cement ratio. This helps in attaining required strength and workability for concrete work.
MEASUREMENT OF WORKABILITY
DIFFERENT TEST METHODS FOR WORKABILITY MEASUREMENT Depending upon the water cement ratio in the concrete mix, the workability may be determined by the following three methods.
1. Slump Test 2. Compaction Factor Test 3. Vee-bee Consistometer Test
1. SLUMP TEST SUITABILITY This method is suitable only for the concrete of high workability. This test is carried out with a mould called slump cone whose top diameter is 10 cm, bottom diameter is 20 cm and height is 30 cm.
PROCEDURE The test is performed in the following steps:
1. Place the slump mould on a smooth flat and non-absorbent surface.
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Concrete Technology 2. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water in it. 3. Place the mixed concrete in the mould to about one-fourth of its height. 4. Compact the concrete 25 times with the help of a tamping rod uniformly all over the area. 5. Place the mixed concrete in the mould to about half of its height and compact it again. 6. Similarly, place the concrete upto its three-fourth height and then up to its top. Compact each layer 25 times with the help of tamping rod uniformly. For the second and subsequent layers, the tamping rod should penetrate into underlying layer. 7. Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top. 8. Remove the mould immediately, ensuring its movement in vertical direction. 9. When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete.
RECOMMENDED SLUMP VALUES FOR VARIOUS CONCRETE WORKS
Recommend slump in mm Type of Construction Minimum Maximum
Pavements 25 50
Mass concrete structure 25 50
Unreinforced footings 25 75
Caissons and bridge decks 25 75
Reinforced foundation, footings and 50 100 walls
Reinforced slabs and beams 30 125
Columns 75 125
LIMITATIONS OF SLUMP TEST Following are the limitations
. Not suitable for concrete containing aggregates larger than 40 mm. . Not suitable for concrete of dry mix. . Not suitable for very wet concrete. . Not reliable because slump may be of any shape.
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Concrete Technology 2. COMPACTION FACTOR TEST According to this test, the workability may be defined as the amount of applied work required to compact the concrete to its maximum density. SUITABILITY This method is adopted for determining the workability of concrete mix in laboratories. It gives fairly good results for concrete of low workability. PROCEDURE The apparatus required for performing the compaction factor test is shown below.
1. The test is performed in the following steps: 2. Clean and dry the internal surface of the mould. 3. With the help of hand scoop, place the concrete in upper hopper A. 4. Open the trap door of hopper in order to facilitate the falling of the concrete into lower hopper B. the concrete sticking to the sides of the hopper A, should be pushed downward with the help of a steel rod. 5. Open the trap door of the hopper B and allow the concrete to fall into cylinder C. 6. Remove the surplus concrete from the top of the cylinder with the help of a trowel. Wipe and clean the outside surface of the cylinder. 7. Weigh the cylinder with partially compacted concrete nearest to 10 g. 8. Fill in the cylinder with fresh concrete in layers not exceeding 5 cm in thickness and compact each layer till 100 percent compaction is achievd. 9. Wipe off and clean the outside surface of the cylinder and weigh the cylinder with fully compacted concrete nearest to 10 g. 10. Calculate the value of compaction factor using the following formula.
Compaction factor = weight of partially compacted concrete/weight of fully compacted concrete RECOMMENDED VALUES OF WORKABILITY FOR VARIOUS PLACING CONDITIONS
Conditions Degree Values of Workability
Very low 20 – 10 seconds Vee-Bee time Concreting of shallow
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Concrete Technology sections with vibrations or 0.75 to 0.80 compacting factor
Concreting of lightly 10 – 5 seconds Vee-Bee time or reinforced sections with Low 0.80 to 0.85 compacting factor vibrations 5-2 seconds Vee-Bee time or Concreting of lightly 0.85 to 0.92 compacting factor reinforced sections without or vibrations or heavily Medium reinforced sections with 25 – 75 mm slumps for 20 mm vibrations aggregates
Concreting of heavily Above 0.92 compacting factor reinforced sections without High or 75 – 125 mm slump for 20 vibrations mm aggregates. ADVANTAGES OF COMPACTION FACTOR TEST Following are the advantages:
. Suitable for testing workability in laboratories . Suitable for concrete of low workability . Suitable to detect the variation in workability over a wide range . Its results are more precise and sensitive.
3. VEE-BEE CONSISTOMETER TEST The apparatus used in this method of test is shown below.
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Concrete Technology SUITABILITY This method is suitable for dry concrete having very low workability PROCEDURE The test is performed as given described below
1. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water. 2. Pour the concrete into the slump cone with the help of the funnel fitted to the stand. 3. Remove the slump mould and rotate the stand so that transparent disc touches the top of the concrete. 4. Start the vibrator on which cylindrical container is placed. 5. Due to vibrating action, the concrete starts remoulding and occupying the cylindrical container. Continue vibrating the cylinder till concrete surface becomes horizontal. 6. The time required for complete remoulding in seconds is the required measure of the workability and it is expressed as number of Vee-bee seconds.
COMPARISON OF WORKABILITY MEASUREMENTS BY VARIOUS METHODS
Workability Vee-bee Time Compacting Slump in mm Description in Seconds Factor
Extremely dry – 32 – 18
Very stiff – 18 – 10 0.70
Stiff 0 – 25 10 – 5 0.75
Stiff plastic 25 – 50 5 – 3 0.85
Plastic 75 – 100 3 – 0 0.90
Flowing 150 – 175 – 0.95
Kelly Ball Test
Kelly ball test apparatus consists of a cylinder with one end having a hemispherical shape of 15cm weighing 13.6kg, and the other end is attached to a graduated scale and handle. The whole arrangement is secured on a fixed stand. The results have been specified by measuring the penetration made by the hemisphere when freely placed on fresh concrete. The impression is measured by a graduated scale immediately.
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Concrete Technology
Procedure for Kelly ball test:
1. Freshly mixed concrete (test sample) is poured into a container up to a depth of 20cm. Once the container is filled with the concrete, the top surface is leveled and struck off. 2. The Kelly ball setup is kept on concrete as shown below by holding the handle of hemisphere such that the frame touches the surface of the concrete. 3. Ensure that the setup is kept at minimum 23cm away from the container ends. (Place at the middle portion of the container) 4. Now release the handle and allow the ball to penetrate through the concrete. Once the ball is released, the depth of penetration is immediately shown in the graduate scale to the nearest 6mm. 5. Note down the depth of penetration from the attached graduated scale. 6. Repeat the same experiment for three times at different portions in the container and average the value.
Formula for Kelly ball test:- The results of the Kelly ball test is correlated with the Slump test
Average Value of the reading = Slump value
Advantages of Kelly ball test: 1. The test results are more accurate when compared with the slump test. 2. This test is a simple and instant which can perform on site. 3. It doesn’t require lengthy calculation to find the workability of concrete. Disadvantages of Kelly ball test: 1. Kelly ball test is not suitable for larger size aggregates. 2. The surface of the concrete should be leveled to test the concrete. 3. This test is not recognized and used by Indian standards. 4. Large aggregate can influence the results.
Department of Civil Engineering
Concrete Technology Flow table test of concrete: As the name suggests, in this test the workability of concrete is determined by examining the flowing property of concrete.
Flow table test of concrete also determines the Quality of Concrete concerning its consistency, cohesiveness and the proneness to segregation.
As there are two methods to find the flow value of concrete which one is outdated. Here we are explaining the new method of flow table test. This new flow table test is covered with BS 1881 part 105 of 1984 and DIN 1048 part I. Apparatus of Flow Table Test: Flow table made of metal having thickness 1.5mm and dimensions 750mmx 750mm, tamping rod made of hardwood, Scoop, Centimeter Scale, Metal Cone or mould (Lower Dia = 20cm, upper Dia = 13 cm, Height of Cone = 20cm). The middle portion of flow table is marked with a concentric circle of dia 200mm to place a metal cone on it. A lift handle
The more details about Flow table is depicted in the below image go through it if required.
Procedure of flow table test: 1. Prepare concrete as per mix design and place the flow table on a horizontal surface. 2. Clean the dust or other gritty material on Flow table and Sprinkle a hand of water on it. 3. Now place the metal cone at the middle portion of the flow table and stand on it. 4. Pour the freshly mixed concrete in the mould comprising two layers; each layer should be tamped with tamping rod for 25times. After tamping the last layer, the overflowed concrete on the cone is struck off using a trowel. 5. Slowly, lift the mould vertically up & let concrete stand on its own without any support. 6. The flow table is raised at the height of 12.5mm and dropped. The same is repeated for 15times in 15secs. 7. Measure the spread of concrete in Diameter using centimetre scale horizontally and vertically. The arithmetic mean of the two diameters shall be the measurement of flow in millimetres.
Setting Time of Concrete:
First of all, understand the fact that setting time of concrete and setting time of cement are two different parameters. For finding out setting time of cement we use standard Vicat apparatus and
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Concrete Technology this test is performed in a laboratory in control of temperature. Both the final and initial setting of cement indicates the quality of cement.
Setting time of concrete completely differs from setting time of cement. Setting time of cement does not match or compare cement setting time with which the concrete is made. The concrete setting time mostly depends upon the w/c ratio, temperature conditions, type of cement, use of mineral admixture, use of plasticizer, in particular, retarding plasticizer. The significance of setting parameter of concrete is more important for site engineers than setting time of cement. For keeping the concrete we use retarding plasticizers, which increases setting time and the duration up to which concrete remains in the plastic condition is of special interest.
Setting is defined as the onset of rigidity in fresh concrete. Hardening is the development of useable and measurable strength; setting precedes hardening. Both are gradual changes controlled by hydration. Fresh concrete will lose measurable slump before initial set and measurable strength will be achieved after final set.
Setting is controlled by the hydration of C3S. The period of good workability is during the dormant period, (stage 2). Initial set corresponds to the beginning of stage 3, a period of rapid hydration. Final set is the midpoint of this acceleration phase. A rapid increase in temperature is associated with stage 3 hydration, with a maximum rate at final set.
If large amounts of ettringite rapidly form from C3A hydration, the setting times will be reduced. Cements with high percentages of C3A, such as expansive or set-regulated cements, are entirely controlled by ettringite formation.
Effect of time and temperature on workability
When fresh concrete is laid at the site then proper curing of concrete is required, because structures are exposed to the environment and in these conditions if there is no such an arrangement against the environment, then there are many factors that affect the workability of concrete and temperature is One of them. Temperature, almost in every aspect has negative effects on the properties of concrete and same is the case with the workability of fresh concrete.
When temperature increases, then in the same proportion workability of fresh concrete decreases. The reason that stands behind is “ when temperature increases then evaporation rate also increases due to that hydration rate decreases and hence, concrete will gain strength earlier “. Due to fast hydration of concrete, a hardening comes in concrete and that decreases the workability of fresh concrete. Therefore, In return manipulation of concrete become very difficult.
EFFECT ON FLOWABILITY OF CONCRETE When temperature increases then fluid viscosity increases too and that phenomenon affects the flow ability of fresh concrete. Flow ability of concrete starts reducing and hence, as a result concrete workability decrease. And when workability of concrete decreases, then due to the less flow ability of a fluid, voids within the mass of concrete develops more.
This is because deeper air voids in concrete only fill, if freshly mixed fluid has the ability to move deeper inside the small opening in the concrete. As in the present case due to higher temperature, viscosity of fluid increases and that viscous of fluid resists the movement of fluid.
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Concrete Technology Now In case when empty voids left in the concrete, then number of weak points rise in concrete and that became the reason of a reduction in the strength of concrete.
CONCLUSION
It indicates that the temperature has a negative effect on the workability of concrete as well as strength up to some extent. Temperature decreases the setting time by increasing hydration rate and that increase the early age strength of the concrete.
This is an advantage that less time will be required before removing of form works on site, but this decrease the use of proper placement of concrete in the initial stages. And if concrete is not properly laid, then strength distribution will not remain the same throughout the cross-section.
With the passage of time after mixing ingredients of concrete with water, workability of concrete starts shrinking. This happens because of fluidity loss from the concrete. Fluidity is the amount of available water in concrete that is being utilized in hydration of cement compounds for the sake of bonding.
When hydration of cement compounds C3S and C3A occurs than water within a concrete gets absorbed by these compounds and now the least amount of water will remain for workable concrete. If the temperature at the site varies, then some amount of water also lost due to evaporation. EFFECT ON SLUMP As the time further proceeds, loss in slump value of concrete becomes effective. Slump indicates how much concrete is workable? And hence slump value is almost directly related to the time passes. When time further proceeds slump loss will show almost linear behavior.
Slump loss increases likewise with the increase of temperature and it also start reducing if increasing any ingredient in concrete more than the required amount. Generally with the increase in cement contents then, then there is a decrease in the required amount of water and hence again it effects workability of freshly laid concrete.
SEGREGATION AND BLEEDING OF CONCRETE
Segregation of concrete is the separation of constituent materials (cement paste and aggregates) of concrete from each other during handling and placement. segregation may be divided into the following three types :-
Cement paste separated from the concrete during its plastic stage before hardening Separation of Coarse aggregate from the concrete mixture Water Separate from the concrete mix.
Segregation of concrete affects the structural strength and durability of structures . A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. Cause of Segregation in Concrete The primary cause of Segregation in concrete is the differences in specific gravity of the concrete material (fine, coarse aggregate and cement). The specific gravity of aggregate is in between 2.6-2.7g/cc and for cement its lies between 3.1-3.6 g/cc, due to these differences, the aggregate separates from the matrix and causes segregation in concrete. So much vibration of concrete with needle vibrators makes heavier particles settle at bottom and lighter cement sand paste comes on top. Dropping concrete from more than 1.5meter height can cause segregation.
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Concrete Technology Use of high water-cement ratio in concrete . Transporting concrete mixtures for long distances.
How to Prevent Concrete Segregation
Segregation can be control by adding , admixtures, air entraining agents and supplemantry cementious materials in the mix. Dropping concrete from more than 1.5 meter height can cause segregation so use chute or Boom Placer for Pouring . The angle of inclination may be kept between 1:3 and 1:2 so that concrete from top of chutes travels smoothly to bottom, use of small quantity of free water from top at intervals helps in lubricating the path of flow of concrete to bottom smoothly. Segregation can be minimized by maintaining proper proportioning the mix. Segregation can be controlled by using proper diameter needle for vibration Bleeding in concrete Bleeding in concrete may be considered as the physical migration of water towards the top surface . Bleeding is a form of segregation in which water present in the concrete mix is pushed upwards due to the settlement of cement and aggregate. The specific gravity of water is low, due to this water tends to move upwards. Bleeding ordinarily occurs in the wet mix of concrete. Note :- Excessive bleeding breaks the bond between the reinforcement and concrete. Forming of water at the top surface of concrete results in delaying the surface finishing.
WAYS TO REDUCE THE BLEEDING OF CONCRETE Proper proportioning of concrete . A complete and uniform mixing of concrete If we can increase the traveling length of water to be bleeded, the bleeding can be reduced considerably. For this purpose we can use finely divided pozzolanic materials. An introduction of air-entrainment by using air entraining agent can reduce bleeding. The use of finer cement. Application of cement of alkali-content. By using of a rich mix rather than lean mix. Controlled vibration can reduce bleeding. When bleeding is appeared in the fresh and plastic concrete, revibration of concrete in controlled way can overcome detrimental impact of bleeding.
CONCRETE MANUFACTURING PROCESS
The main Concrete Manufacturing Process is as follows
Batching Mixing Transporting Placing Compacting Curing
Department of Civil Engineering
Concrete Technology 1. Batching It is the main thing in the Concrete Manufacturing Process. The measurement of materials like aggregates, cement, water necessary for preparing different grades of concrete is Batching. It is by two processes. One is volume and other is weight batching. The volume batching is by mixing materials with its volume. And weight batching is by the self-weight ratio of materials. It has ratios according to standard codes. Some of the different grades of concrete are M10, M20, M25, M30.
2. Mixing
Mixing is to produce uniform, high-quality concrete. The mixing equipment is capable of effective concrete material. Separate paste mix shows the mixing of cement and water into a paste before combines with aggregates. This increase the compressive strength of concrete. This paste mix in high-speed, shear-type mixer at a water-cement ratio of 0.30 to 0.45 by mass. The premix paste blends with aggregates. The remaining batch water and final mix complete in a rotating concrete mixing equipment.
3. Transporting
Transporting concrete require great care. After mixing, the concrete transports to site. The mixing carries near the construction site. A bucket, ropeway, belt conveyor use to transport concrete. Readymix conveyor trucks use mostly in the modern construction times. The concrete transporting by conveyor truck has time limits. The concrete transporting by trucks reach the construction plant early.
4. Placing
Concrete to place in a good manner. It places without segregation to reach maximum efficiency. The concrete not to pour over a height of 1.5m. As the height of pouring concrete increases, it leads to separation of aggregates and cement paste. It causes segregation of concrete and causes honeycomb. Many concrete placing equipment use in modern construction times. some of the equipment are concrete pumps and boom lifts. Concrete pumps are normal pumps with joints to connect and remove according to needs.
Boom lifts are concrete pumps with movable arms. With the help of movable arm, a concrete place easily at heights and corners which is difficult with the normal pump.
5. Curing
Curing is to provide best strength and hardness to concrete. It is the next step after placing of concrete. Curing helps concrete to gain strength early. The concrete requires a moist and wet environment to gain full strength. Proper curing of concrete leads to increase the strength of concrete. And prevent cracking at surface dries. Avoid freezing and overheating of concrete to gain its full strength.
REFERENCE
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Concrete Technology https://civilread.com/flow-table-test-concrete/ https://civilread.com/kellyballtest/ https://www.youtube.com/watch?v=yzpWGrh9j6Y https://www.youtube.com/watch?v=0d2dUXbdVzs https://www.youtube.com/watch?v=B8va6peUJqE https://www.youtube.com/watch?v=Cg8c0649DIU https://civilblog.org/2015/04/23/how-hot-cold-weather-can-affect-concrete-is-7861/ https://www.youtube.com/watch?v=LUkZFD6auKc https://nptel.ac.in/courses/105102012/ https://www.youtube.com/watch?v=yK4GcVDYLH0 https://www.youtube.com/watch?v=_Rh1ZHGuCdQ https://www.youtube.com/watch?v=-O1aIdJdIyU
Department of Civil Engineering
Concrete Technology WATER- CEMENT RATIO AND ABRAM’S LAW
Duff Abrams published data that showed that for a given set of concreting materials, the strength of the concrete depends solely on the relative quantity of water compared with the cement. In other words, the strength is a function of the water to cement ratio (w/c) where w represents the mass of water and c represents the mass of cement. This became known as Abrams law and it remains valid today as it was in 1918. However, more often, w/cm is used and cm represents the mass of cementing materials, which includes the portland cement plus any supplementary cementing materials such as fly ash, slag cement, or silica fume. Unnecessarily high water content dilutes the cement paste (the glue of concrete) and increases the volume of the concrete produced. Some advantages of reducing water content include:
Increased compressive and flexural strength. Lower permeability and increased water tightness. Increased durability and resistance to weathering. Better bond between concrete and reinforcement. Reduced drying shrinkage and cracking. Less volume change from wetting and drying.
The less water used, the better the quality of the concrete provided the mixture can still be consolidated properly. Smaller amounts of mixing water result in stiffer mixtures; with vibration, stiffer mixtures can be easily placed. Thus, consolidation by vibration permits improvement in the quality of concrete. Reducing the water content of concrete, and thereby reducing the w/cm, leads to increased strength and stiffness, and reduced creep. The drying shrinkage and associated risk of cracking will also be reduced. The concrete will have a lower permeability or increased water tightness that will render it more resistant to weathering and the action of aggressive chemicals. The lower water to cementitious materials ratio also improves the bond between the concrete and embedded steel reinforcement.
GEL SPACE RATIO
Knowledge of porosity is very useful since porosity has a strong influence on strength and durability. In hardened cement paste, there are several types of porosity, trapped or entrained air (0.1 to several mm in size), capillary pores (0.01 to a few microns) existing in the space between hydration products, and gel pores (several nanometres or below) within the layered structure of the C-S-H. The capillary pores have a large effect on the strength and permeability of the hardened paste itself. Of course, the presence of air bubbles can also affect strength.
From experiments, the porosity within the gel for all normally hydrated cements is the same, with a value of 0.26. The total volume of hydration products (cement gel) is given byVg = 0.68α cm3/g of original cement Where, α represents the degree of hydration. The capillary porosity can be calculated by,
Pc =(w/c) −0.36αcm3/g of original cement
Where, w is the original weight of water and c is the weight of cement and w/c is the water-cement ratio. It can be seen that with increase of w/c, the capillary pores increase.
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Concrete Technology
Department of Civil Engineering
Concrete Technology The gel / space ratio (X) is defined as
vol of gel (including gel pores) X vol of gel + vol of capillary pores
0.68α 2α+ w/c
The minimum w/c ratio for complete hydration is usually assumed to be 0.36 to 0.42. It should be indicated that complete hydration is not essential to attain a high ultimate strength. For pastes of low w/c ratio, residual unhydrated cement will remain.
To satisfy workability requirements, the water added in the mix is usually more than that needed for the chemical reaction. Part of the water is used up in the chemical reaction. The remaining is either held by the C -S-H gel or stored in the capillary pore. Most capillary water is free water (far away from the pore surface). On drying, they will be removed, but the loss of free water has little effect on concrete. Loss of adsorbed water on surfaces and those in the gel will, however, lead to shrinkage. Movement of adsorbed and gel water under load is a cause of creeping in concrete.
NATURE OF STRENGTH OF HARDENED CONCRETE
Strength is defined as the ability of a material to resist stress without failure. The failure of concrete is due to cracking. Under direct tension, concrete failure is due to the propagation of a single major crack. In compression, failure involves the propagation of a large number of cracks, leading to a mode of disintegration commonly referred to as ‘crushing’. The strength is the property generally specified in construction design and quality control, for the following reasons: It is relatively easy to measure, and Other properties are related to the strength and can be deduced from strength data.
The 28-day compressive strength of concrete determined by a standard uniaxial compression test is accepted universally as a general index of concrete strength.
MATURITY CONCEPT
Concrete maturity is an index value that represents the progression of concrete curing. It is based on an equation that takes into account concrete temperature, time, and strength gain. Concrete maturity is an accurate way to determine real-time strength values of curing concrete.
The maturity method is a convenient approach to predict the early age strength gain of concrete, using the principle that the concrete strength is directly related to the hydration temperature history of cementitious paste. The maturity concept for estimating the strength gain of concrete is described in ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method. This method can potentially address many immediate challenges facing the concrete industry such as predicting appropriate time for formwork stripping and post-tensioning, especially at low temperatures while the strength development of concrete is hindered; and optimizing concrete mix formula and concrete curing
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Concrete Technology conditions (e.g. concrete heating at low temperatures or surface protection in hot-dry weathers). Lack of an accurate estimation of strength at early ages of construction is twofold: contractors either wait too
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Concrete Technology long for next action (e.g. stripping formwork) which is costly due to delays in completing the project, or they act prematurely which could cause the concrete structure to crack - that would lead to future durability and performance issues - or even structural collapse.
Principle of the Maturity Method
Maturity method is a relatively simple approach for estimating the in-place compressive strength of concrete, specifically at early ages less than 14 days. Once the maturity curve is developed in the laboratory for a specific project, it can be used for on-site estimation of compressive strength of concrete in real-time. The maturity value is governed by the fundamental assumption that a given concrete mix design poured during course of a specific project has the same compressive strength when it has the same “maturity index”. It means that a given concrete mix formula or mix, for example, may reach the same compressive strength after 7 days of curing at 10 °C when it is cured at 25 °C for 3 days. The maturity method based on the ASTM C1074 is the most commonly used method to estimate the in-situ strength of concrete. ASTM C1074 provides two maturity functions: 1) Nurse-Saul function; and 2) Arrhenius function. Based on the Nurse-Saul method, there is a linear relationship between the maturity and the temperature in real time. The underlying assumption is that the strength development in concrete is a linear function of hydration temperature. Equation 1 shows the relationship between maturity and hydration temperature history.
M(t)=∑[(Ta-T0)x∆t], Eq.1
Where: M(t) is the maturity index at age t;
Ta is the average temperature during time interval ∆t; T0 is the datum temperature.
ASTM C1074 provides a standard procedure to find the datum temperature for a specific mix design. However, most of previous studies suggest a practical estimation of the datum temperature that is between 0 °C and -10 °C. Indeed, this is the temperature at which the hydration of cementitious paste stops; hence the strength development of concrete ceases. Unpublished results by Giatec Scientific Inc. show that this temperature resides within an interval between 0 °C and -5 °C dependent on the concrete mix formula.
The second approach is the Arrhenius function that assumes there is an exponential relationship between the compressive strength and hydration temperature. The maturity index is defined in form of an equivalent age at a reference temperature. It means the actual age should be normalized to the reference temperature in order to estimate the compressive strength. This function needs a value of activation energy that can be determined as the procedure detailed in ASTM C1074.
Despite the fact that the Arrhenius function is scientifically more accurate, the Nurse-Saul function is more commonly used by concrete industry because of the following reasons:
The accuracy of the Nurse-Saul function is adequate for most field applications; The Nurse-Saul function is relatively simpler compared with the Arrhenius function.
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Concrete Technology
Department of Civil Engineering
Concrete Technology COMPRESSIVE STRENGTH AND CORRESPONDING TESTS
(a) Failure mechanism
a b c d
a. At about 25-30% of the ultimate strength, random cracking (usually in transition zone around large aggregates) are observed b. At about 50% of ultimate strength, cracks grow stably from transition zone into paste. Also, microcracks start to develop in the paste. c. At about 75% of the ultimate strength, paste cracks and bond cracks start to join together, forming major cracks. The major cracks keep growing while smaller cracks tend to close. d. At the ultimate load, failure occurs when the major cracks link up along the vertical direction and split the specimen
The development of the vertical cracks results in expansion of concrete in the lateral directions. If concrete is confined (i.e., it is not allow to expand freely in the lateral directions), growth of the vertical cracks will be resisted. The strength is hence increased, together with an increase in failure strain. In the design of concrete columns, steel stirrups are placed around the vertical reinforcing steel. They serve to prevent the lateral displacement of the interior concrete and hence increase the concrete strength. In composite construction (steel + reinforced concrete), steel tubes are often used to encase reinforced concrete columns. The tube is very effective in providing the confinement. The above figure illustrates the case when the concrete member is under ideal uniaxial loading. In reality, when we are running a compressive test, friction exists at the top and bottom surfaces of a concrete specimen, to prevent the lateral movement of the specimen. As a result, confining stresses are generated around the two ends of the specimen. If the specimen has a low aspect ratio (in terms of height vs width), such as a cube (aspect ratio = 1.0), the confining stresses will increase the apparent strength of the material. For a cylinder with aspect ratio beyond 2.0, the confining effect is not too significant at the middle of the specimen (where failure occurs). The strength obtained from a cylinder is hence closer to the actual uniaxial strength of concrete. Note that in a cylinder test, the cracks propagate vertically in the middle of the specimen. When they get close to the ends, due to the confining stresses, they propagate in an inclined direction, leading to the formation of two cones at the ends.
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Concrete Technology
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Concrete Technology
(b) Specimen for compressive strength determination The cube specimen is popular in U.K. and Europe while the cylinder specimen is commonly used in the U.S.
i. Cube specimen BS 1881: Part 108: 1983. Filling in 3 layers with 50 mm for each layer (2 layers for 100 mm cube). Strokes 35 times for 150 mm cube and 25 times for 100 mm cube. Curing at 20±5 C and 90% relative humility.
ii. Cylinder specimen ASTM C470-81. Standard cylinder size is 150 x 300 mm. Curing condition is temperature of 23±1.7C and moist condition. Grinding or capping is needed toprovide level and smooth compression surface. (c) Factors influencing experiment results
(i) End condition. Due to influence of platen restraint, cube's apparent strength is about 1.15 times of cylinders. In assessing report on concrete strength, it is IMPORTANT to know which type of specimen has been employed. (ii) Loading rate. The faster the load rate, the higher the ultimate load obtained. The standard load rate is 0.15 -0.34 MPa / s for ASTM and 0.2-0.4 MPa/s for BS. (iii) Size effect: The probability of having larger defects (such as voids and cracks) increases with size. Thus smaller size specimen will give higher apparent strength. Standard specimen size is mentioned above. Test results for small size specimen needs to be modified.
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Concrete Technology
Department of Civil Engineering
Concrete Technology
TENSILE STRENGTH AND CORRESPONDING TESTS
(d) Failure mechanism
a. b. c.
a. Random crack development (microcracks usually form at transition zone) b. Localization of microcracks c. Major crack propagation
It is important to notice that cracks form and propagate a lot easier in tension than in compression. The tensile strength is hence much lower than the compressive strength. An empirical relation between f and f is given by:
f = 0.615 (f ) (for 21 MPa < f < 83 MPa)
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Concrete Technology
Department of Civil Engineering
Concrete Technology Substituting numerical values for f , f is found to be around 7 to 13% of the compressivestrength, with a lower f /f ratio for higher concrete strength. In the above formula, f isobtained from the direct compression of cylinders while f is measured with the splitting
tensile test, to be described below.
(e) Direct tension test methods Direct tension tests of concrete are seldom carried out because it is very difficult to control. Also, perfect alignment is difficult to ensure and the specimen holding devices introduce secondary stress that cannot be ignored. In practice, it is common to carry out the splitting tensile test or flexural test.
(f) Indirect tension test (split cylinder test or Brazilian test) BS 1881: Part 117:1983. Specimen 150 x 300 mm cylinder. Loading rate 0.02 to 0.04 MPa/s ASTM C496-71: Specimen 150 x 300 mm cylinder. Loading rate 0.011 to 0.023 MPa/s The splitting test is carried out by applying compression loads along two axial lines that are diametrically opposite. This test is based on the following observation from elastic analysis. Under vertical loading acting on the two ends of the vertical diametrical line, uniform tension is
introduced along the central part of the specimen.
The splitting tensile strength can be obtained using the following formula:
According to the comparison of test results on the same concrete, f is about 10-15% higher than direct tensile strength, f .
Department of Civil Engineering
Concrete Technology FLEXURAL STRENGTH AND CORRESPONDING TESTS
BS 1881: Part 118: 1983. Flexural test. 150 x 150 x 750 mm or 100 x 100 x 500 (Max. size of aggregate is less than 25 mm). The arrangement for modulus of rupture is shown in the above figure. From Mechanics of Materials, we know that the maximum tension stress should occur at the bottom of the constant moment region. The modulus of rapture can be calculated as:
This formula is for the case of fracture taking place within the middle one third of the beam. If fracture occurs outside of the middle one-third (constant moment zone), the modulus of rupture can be computed from the moment at the crack location according to ASTM standard, with the following formula.
However, according to British Standards, once fracture occurs outside of the constant moment zone, the test result should be discarded.
Although the modulus of rupture is a kind of tensile strength, it is much higher than the results obtained from a direct tension test. This is because concrete can still carry stress after a crack is formed. The maximum load in a bending test does not correspond to the start of cracking, but correspond to a situation when the crack has propagated. The stress distribution along the vertical section through the crack is no longer varying in a linear manner. The above equations are therefore not exact.
NON DESTRUCTIVE TESTING METHODS
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Concrete Technology Non-destructive tests of concrete is a method to obtain the compressive strength and other properties of concrete from the existing structures. This test provides immediate results and actual strength and properties of concrete structure. The standard method of evaluating the quality of concrete in buildings or structures is to test specimens cast simultaneously for compressive, flexural and tensile strengths. The main disadvantages are that results are not obtained immediately; that concrete in specimens may differ from that in the actual structure as a result of different curing and compaction conditions; and that strength properties of a concrete specimen depend on its size and shape. Although there can be no direct measurement of the strength properties of structural concrete for the simple reason that strength determination involves destructive stresses, several non- destructive methods of assessment have been developed. These depend on the fact that certain physical properties of concrete can be related to strength and can be measured by non-destructive methods. Such properties include hardness, resistance to penetration by projectiles, rebound capacity and ability to transmit ultrasonic pulses and X- and Y-rays. These non-destructive methods may be categorized as penetration tests, rebound tests, pull-out techniques, dynamic tests, radioactive tests, maturity concept. It is the purpose of this Digest to describe these methods briefly, outlining their advantages and disadvantages. Methods of Non-Destructive Testing of Concrete Following are different methods of NDT on concrete:
1. Penetration method 2. Rebound hammer method 3. Pull out test method 4. Ultrasonic pulse velocity method 5. Radioactive methods
Penetration Resistance Test on Hardened Concrete – Purpose and Application
Penetration resistance test is conducted on concrete structures using Windsor Probe test machine. In this test method, a steel probe is fired on the concrete surface by a sudden explosion. The penetration is inversely proportional to the strength of concrete. The result of the test is influenced by aggregate strength and nature of formed surfaces of concrete.
The purpose of the penetration resistance test is used to determine the uniformity of concrete, specify the poor quality or deteriorated concrete zones, and evaluate the in-place strength of concrete. It is sometimes necessary to estimate the strength of concrete on-site for early form removal or to investigate the strength of concrete in place because of low cylinder test results.
Due to the nature of equipment, it cannot and should not be expected to yield absolute values of strength. The penetration resistance test on hardened concrete can be carried out based on the procedures and specifications of Standard Test Method for Penetration Resistance of Hardened Concrete (ASTM C 803/ 803M- 97) or British Standard (BS 1881 Part 207).
Purpose of Penetration Resistance Test
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Concrete Technology 1. Determine the uniformity of concrete 2. Specify exact locations of poor quality or deteriorated concrete zones 3. Assess in-place strength of concrete
Application of Penetration Resistance Test Penetration resistance test is conducted to estimate the strength of concrete on-site for early form removal or to investigate the strength of concrete in place because of low cylinder test results.
Apparatus 1. Probe Probe consists of driver unit used to drive the probe into the concrete and probe manufactured from alloy-steel rod plated for corrosion protection, with a blunt conical end that can be inserted into the driver unit and driven into the concrete surface.
Probes of 79.4-mm overall length and 7.9-mm diameter, with the penetrating end diameter reduced to 6.4 mm for approximately 14.3 mm in length, is suitable for testing concrete with a unit weight of 2000 kg/m3 or greater. Pins can be used instead of probe when the penetration resistance test is carried out using this tool.
Fig. 1: Probe Device Used for Penetration Resistance Testing
2. Measurement Equipment Measurement equipment such as a Vernier caliper or depth gauge to measure the exposed length of a probe to the nearest 0.5 mm. The measuring equipment shall include a reference base plate which is supported on the concrete surface at three equally spaced points at least 50 mm from the probe to be measured.
3. Positioning Device A single device or a triangular device with holes at the three corners can be used for positioning and guiding the probe and driver unit.
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Fig. 2: Positions of Testing Stations are Specified by Tringular Device through Which a Probe is Fired
Sampling
Concrete to be tested should have gain enough strength so that the probe would not penetrate more than one half the thickness of the concrete member Maximum spacing between probes is 175 mm Minimum spacing between probes is 100 mm Minimum spacing between concrete and the edge of a concrete surface is 100mm. A minimum of three firmly embedded test probes in a given test area constitute one test.
Testing Procedure
1. Place the positioning device on the surface of the concrete at the location to be tested. 2. Mount a probe in the driver unit 3. Position the driver in the positioning device 4. Fire the probe into the concrete. 5. Remove the positioning device and tap the probe on the exposed end with a small hammer to ensure that it has not rebounded and to confirm that it is firmly embedded. 6. Place the measuring base plate over the probe and position it so that it bears firmly on the surface of the concrete without rocking or other movement.
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Concrete Technology Fig. 3: Penetration Resistance Testing
Testing Considerations
1. If the probe is sloped with respect to the surface of the concrete, take four measurements equally spaced around and parallel to the probe and average them to get the measurement. 2. If the probe is not firmly embedded then the test is not valid and hence it should be repeated. 3. Similarly, the test should be repeated if the range of depth of penetration for three tests is more than 8.4mm in concrete made with 25mm maximum aggregate size, and 11.7 in concrete made with 50mm maximum aggregate size. 4. When tests are to be made on concrete having a density of approximately 2000 kg/m3 or less, and on all concrete with strengths less than 17 MPa, decrease the amount of energy delivered to the probe by the driver or use a larger-diameter probe, or both.
Fig. 4: Penetration Test Resistance on Hardened Concrete
Results The penetration resistance of concrete is computed by measuring exposed length of probes driven into concrete. In order to estimate concrete strength, it is necessary to establish a relationship between penetration resistance and concrete strength.
Such a relationship must be established for a given test apparatus, using similar concrete materials and mixture proportions as in the structure. Procedures and statistical methods provided in ACI 228.1R can be used for developing and using the strength relationship.
Factors Influence the Penetration Resistance Test Result Nature of the formed surfaces for instance wooden forms versus steel forms. That is why correlation testing should be performed on specimens with formed surfaces similar to those to be used during construction. Probe penetration resistance is affected by concrete strength as well as the nature of the coarse aggregate.
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Fig. 5: Penetration Test Result for Soft and Hard Surfaces
2. Rebound Hammer Test on Concrete – Principle, Procedure, Advantages & Disadvantages
Rebound Hammer test is a Non-destructive testing method of concrete which provide a convenient and rapid indication of the compressive strength of the concrete. The rebound hammer is also called as Schmidt hammer that consist of a spring controlled mass that slides on a plunger within a tubular housing.
The operation of rebound hammer is shown in the fig.1. When the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass with a constant energy is made to hit concrete surface to rebound back. The extent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A concrete with low strength and low stiffness will absorb more energy to yield in a lower rebound value.
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Concrete Technology Operation of the rebound hammer Objective of Rebound Hammer Test As per the Indian code IS: 13311(2)-1992, the rebound hammer test have the following objectives:
1. To determine the compressive strength of the concrete by relating the rebound index and the compressive strength 2. To assess the uniformity of the concrete 3. To assess the quality of the concrete based on the standard specifications 4. To relate one concrete element with other in terms of quality Rebound hammer test method can be used to differentiate the acceptable and questionable parts of the structure or to compare two different structures based on strength.
Principle of Rebound Hammer Test
Rebound hammer test method is based on the principle that the rebound of an elastic mass depends on the hardness of the concrete surface against which the mass strikes. The operation of the rebound hammer is shown in figure-1. When the plunger of rebound hammer is pressed against the concrete surface, the spring controlled mass in the hammer rebounds. The amount of rebound of the mass depends on the hardness of concrete surface.
Thus, the hardness of concrete and rebound hammer reading can be correlated with compressive strength of concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.
Procedure for Rebound Hammer Test
Procedure for rebound hammer test on concrete structure starts with calibration of the rebound hammer. For this, the rebound hammer is tested against the test anvil made of steel having Brinell hardness number of about 5000 N/mm2.
After the rebound hammer is tested for accuracy on the test anvil, the rebound hammer is held at right angles to the surface of the concrete structure for taking the readings. The test thus can be conducted horizontally on vertical surface and vertically upwards or downwards on horizontal surfaces as shown in figure below
If the rebound hammer is held at intermediate angle, the rebound number will be different for the same concrete.
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Rebound Hammer Positions for Testing Concrete Structure The impact energy required for the rebound hammer is different for different applications. Approximate Impact energy levels are mentioned in the table-1 below for different applications.
Table-1: Impact Energy for Rebound Hammers for Different Applications As per IS: 13311(2)-1992
Approximate Impact Energy for Applications Sl.No Rebound Hammer in Nm
1 For Normal Weight Concrete 2.25
For light weight concrete / For small 2 0.75 and impact resistive concrete parts
For mass concrete testing Eg: In 3 roads, hydraulic structures and 30.00 pavements
Points to Remember in Rebound Hammer Test
1. The concrete surface should be smooth, clean and dry.
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Concrete Technology 2. Ant loose particles should be rubbed off from the concrete surface with a grinding wheel or stone, before hammer testing. 3. Rebound hammer test should not be conducted on rough surfaces as a result of incomplete compaction, loss of grout, spalled or tooled concrete surface. 4. The point of impact of rebound hammer on concrete surface should be at least 20mm away from edge or shape discontinuity. 5. Six readings of rebound number is taken at each point of testing and an average of value of the readings is taken as rebound index for the corresponding point of observation on concrete surface. Correlation between compressive strength of concrete and rebound number
The most suitable method of obtaining the correlation between compressive strength of concrete and rebound number is to test the concrete cubes using compression testing machine as well as using rebound hammer simultaneously. First the rebound number of concrete cube is taken and then the compressive strength is tested on compression testing machine. The fixed load required is of the order of 7 N/ mm2 when the impact energy of the hammer is about 2.2 Nm.
The load should be increased for calibrating rebound hammers of greater impact energy and decreased for calibrating rebound hammers of lesser impact energy. The test specimens should be as large a mass as possible in order to minimize the size effect on the test result of a full scale structure. 150mm cube specimens are preferred for calibrating rebound hammers of lower impact energy (2.2Nm), whereas for rebound hammers of higher impact energy, for example 30 Nm, the test cubes should not be smaller than 300mm.
The concrete cube specimens should be kept at room temperature for about 24 hours after taking it out from the curing pond, before testing it with the rebound hammer. To obtain a correlation between rebound numbers and strength of wet cured and wet tested cubes, it is necessary to establish a correlation between the strength of wet tested cubes and the strength of dry tested cubes on which rebound readings are taken.
A direct correlation between rebound numbers on wet cubes and the strength of wet cubes is not recommended. Only the vertical faces of the cubes as cast should be tested. At least nine readings should be taken on each of the two vertical faces accessible in the compression testing machine when using the rebound hammers. The points of impact on the specimen must not be nearer an edge than 20mm and should be not less than 20mm from each other. The same points must not be impacted more than once.
Interpretation of Rebound Hammer Test Results
After obtaining the correlation between compressive strength and rebound number, the strength of structure can be assessed. In general, the rebound number increases as the strength increases and is also affected by a number of parameters i.e. type of cement, type of aggregate, surface condition and moisture content of the concrete, curing and age of concrete, carbonation of concrete surface etc.
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Relationship Between Cube Strength and the Rebound Number Moreover the rebound index is indicative of compressive strength of concrete up to a limited depth from the surface. The internal cracks, flaws etc. or heterogeneity across the cross section will not be indicated by rebound numbers.
Table- below shows the quality of concrete for respective average rebound number.
Table. Quality of Concrete for different values of rebound number
As such the estimation of strength of concrete by rebound hammer method cannot be held to be very accurate and probable accuracy of prediction of concrete strength in a structure is ± 25 percent. If the relationship between rebound index and compressive strength can be found by tests on core samples obtained from the structure or standard specimens made with the same concrete materials and mix proportion, then the accuracy of results and confidence thereon gets greatly increased.
Advantages and Disadvantages of Rebound Hammer Test
The advantages of Rebound hammer tests are:
1. Apparatus is easy to use 2. Determines uniformity properties of the surface 3. The equipment used is inexpensive 4. Used for the rehabilitation of old monuments
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Concrete Technology The disadvantages of Rebound Hammer Test
1. The results obtained is based on a local point 2. The test results are not directly related to the strength and the deformation property of the surface 3. The probe and spring arrangement will require regular cleaning and maintenance 4. Flaws cannot be detected with accuracy Factors Influencing Rebound Hammer Test
Below mentioned are the important factors that influence rebound hammer test:
1. Type of Aggregate 2. Type of Cement 3. Surface and moisture condition of the concrete 4. Curing and Age of concrete 5. Carbonation of concrete surface Type of Aggregate
The correlation between compressive strength of concrete and the rebound number will vary with the use of different aggregates. Normal correlations in the results are obtained by the use of normal aggregates like gravels and crushed aggregates. The use of lightweight aggregates in concrete will require special calibration to undergo the test.
Type of Cement
The concrete made of high alumina cement ought to have higher compressive strength compared to Ordinary portland cement. The use of supersulphated cement in concrete decrease the compressive strength by 50% compared to that of OPC.
Type of Surface and Moisture Condition
The rebound hammer test work best for close texture concrete compared with open texture concrete. Concrete with high honeycombs and no-fines concrete is not suitable to be tested by rebound hammer. The strength is overestimated by the test when testing floated or trowelled surfaces when compared with moulded surfaces.
Wet concrete surface if tested will give a lower strength value. This underestimation of strength can go lower to 20% that of dry concrete.
Type of curing and age of concrete
As time passes, the relation between the strength and hardness of concrete will change. Curing conditions of concrete and their moisture exposure conditions also affects this relationship. For concrete with an age between 3days to 90 days is exempted from the effect of age. For greater aged concrete special calibrated curves is necessary.
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Concrete Technology Carbonation on Concrete Surface
A higher strength is estimated by the rebound hammer on a concrete that is subjected to carbonation. It is estimated to be 50% higher. So the test have to be conducted by removing the carbonated layer and testing by rebound hammer over non-carbonated layer of concrete.
3. Pullout Tests On Hardened Concrete
The fundamental principle behind pull out testing is that the test equipment designed to a specific geometry will produce results (pull-out forces) that closely correlate to the compressive strength of concrete. This correlation is achieved by measuring the force required to pull a steel disc or ring, embedded in fresh concrete, against a circular counter pressure placed on the concrete surface concentric with the disc/ring.
Types of Pull Out Tests:
Depending upon the placement of disc/ring in he fresh concrete, pull out test can be divided into 2 types,
1. LOK test 2. CAPO test (Cut and Pull out Test)
LOK Test: The LOK-TEST system is used to obtain a reliable estimate of the in-place strength of concrete in newly cast structures in accordance with the pullout test method described in ASTM C900, BS 1881:207, or EN 12504-3.
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LOK Test A steel disc, 25 mm in diameter at a depth of 25 mm, is pulled centrally against a 55 mm diameter counter pressure ring bearing on the surface. The force F required to pullout the insert is measured. The concrete in the strut between the disc and the counter pressure ring is subjected to a compressive load. Therefore the pullout force F is related directly to the compressive strength.
LOK Test Process. H indicated the highest pullout force. CAPO test (Cut and Pull out Test) The CAPO-TEST permits performing pullout tests on existing structures without the need of preinstalled inserts. CAPO-TEST provides a pullout test system similar to the LOK-TEST system for accurate on-site estimates of compressive strength. Procedures for performing post-installed pullout tests, such as CAPO-TEST, are included in ASTM C900 and EN 12504-3.
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Concrete Technology
Cut and Pull out Test When selecting the location for a CAPO-TEST, ensure that reinforcing bars are not within the failure region. The surface at the test location is ground using a planing tool and a 18.4 mm hole is made perpendicular to the surface using a diamond-studded core bit. A recess (slot) is routed in the hole to a diameter of 25 mm and at a depth of 25 mm.
A split ring is expanded in the recess and pulled out using a pull machine reacting against a 55 mm diameter counter pressure ring. As in the LOKTEST, the concrete in the strut between the expanded ring and the counter pressure ring is in compression. Hence, the ultimate pullout force F is related directly to compressive strength.
CAPO Test on Concrete Slab
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Concrete Technology Relationship between the pullout force and compressive strength:
The relationship between the pullout force Fu in kN and compressive strength Fc in MPa is given below,
Typical Pull out Force Calibration Chart By measuring the pull-out force of a cast-in disc or expanded ring, the compressive strength of in- situ concrete can be determined from the relationship in fig.4 to a great degree of confidence.
Pull off force compressive strength relationship The pullout test produces a well defined in the concrete and measure a static strength property of concrete. The equipment is simple to assemble and operate.
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Concrete Technology The compressive strength can be considered as proportional to the ultimate pullout force. The reliability of the test is reported as good. It is superior to rebound hammer and Windsor probe test because of greater depth of concrete volume tested. However this test is not recommended for aggregates beyond size of 38mm.
The major limitation of this test is that it requires special care at the time of placement of inserts to minimize air void below the disc besides a pre-planned usage.
Uses:
1. Determine in-situ compressive strength of the concrete 2. Ascertain the strength of concrete for carrying out post tensioning operations. 3. Determine the time of removal of forms and shores based on actual in-situ strength of the structure. 4. Terminate curing based on in-situ strength of the structure. 5. It can be also used for testing repaired concrete sections.
Post Test Process: After the concrete has fractured by this test, the holes left in the surface are first cleaned of the dust by a blower. It is then primed with epoxy glue and the hole is filled with a polymer-modified mortar immediately thereafter and the surface is smoothened.
4. Ultrasonic Testing of Concrete for Compressive Strength
Ultrasonic testing of concrete or ultrasonic pulse velocity test on concrete is a non-destructive test to assess the homogeneity and integrity of concrete.
With this ultrasonic test on concrete, following can be assessed:
1. Qualitative assessment of strength of concrete, its gradation in different locations of structural members and plotting the same. 2. Any discontinuity in cross section like cracks, cover concrete delamination etc. 3. Depth of surface cracks.
Ultrasonic Testing of Concrete Ultrasonic pulse velocity test consists of measuring travel time, T of ultrasonic pulse of 50 to 54 kHz, produced by an electro-acoustical transducer, held in contact with one surface of the concrete member under test and receiving the same by a similar transducer in contact with the surface at the other end.
With the path length L, (i.e. the distance between the two probes) and time of travel T, the pulse velocity (V=L/T) is calculated.
Higher the elastic modulus, density and integrity of the concrete, higher is the pulse velocity. The ultrasonic pulse velocity depends on the density and elastic properties of the material being tested.
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Concrete Technology
Ultrasonic Pulse Velocity Testing Instrument Though pulse velocity is related with crushing strength of concrete, yet no statistical correlation can be applied.
The pulse velocity in concrete may be influenced by:
a. Path length b. Lateral dimension of the specimen tested c. Presence of reinforcement steel d. Moisture content of the concrete
The influence of path length will be negligible provided it is not less than 100mm when 20mm size aggregate is used or less than 150mm for 40mm size aggregate. Pulse velocity will not be influenced by the shape of the specimen, provided its least lateral dimension (i.e. its dimension measured at right angles to the pulse path) is not less than the wavelength of the pulse vibrations. For pulse of 50Hz frequency, this corresponds to a least lateral dimension of about 80mm. the velocity of pulses in steel bar is generally higher than they are in concrete. For this reason pulse velocity measurements made in the vicinity of reinforcing steel may be high and not representative of the concrete. The influence of the reinforcement is generally small if the bars runs in a direction at right angles to the pulse path and the quantity of steel is small in relation to the path length. The moisture content of the concrete can have a small but significant influence on the pulse velocity. In general, the velocity is increased with increased moisture content, the influence being more marked for lower quality concrete.
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Fig.2: Method of propagating and receiving pulses
Measurement of pulse velocities at points on a regular grid on the surface of a concrete structure provides a reliable method of assessing the homogeneity of the concrete. The size of the grid chosen will depend on the size of the structure and the amount of variability encountered.
Table: 1 – Concrete Quality based on Ultrasonic Pulse Velocity Test
PULSE VELOCITY CONCRETE QUALITY
>4.0 km/s Very good to excellent
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Concrete Technology 3.5 – 4.0 km/s Good to very good, slight porosity may exist
3.0 – 3.5 km/s Satisfactory but loss of integrity is suspected
<3.0 km/s Poor and los of integrity exist.
Table 1 shows the guidelines for qualitative assessment of concrete based on UPV test results.
To make a more realistic assessment of the condition of surface of a structural member, the pulse velocity can be combined with rebound number.
Table 2 shows the guidelines for identification of corrosion prone locations by combining the results of pulse velocity and rebound number.
Table:2 – Identification of Corrosion Prone Location based on Pulse Velocity and Hammer Readings
Sl. Test Results Interpretations No.
High UPV values, high 1 Not corrosion prone rebound number
Medium range UPV values, Surface delamination, low quality of surface 2 low rebound numbers concrete, corrosion prone
Low UPV, high rebound Not corrosion prone, however to be confirmed 3 numbers by chemical tests, carbonation, pH
Low UPV, low rebound Corrosion prone, requires chemical and 4 numbers electrochemical tests.
Detection of Defects with Ultrasonic Test on Concrete When ultrasonic pulse travelling through concrete meets a concrete-air interface, there is a negligible transmission of energy across this interface so that any air filled crack or void lying directly between the transducers will obstruct the direct beam of ultrasonic when the void has a projected area larger than the area of transducer faces.
The first pulse to arrive at the receiving transducer will have been directed around the periphery of the defect and the time will be longer than in similar concrete with no defect.
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Concrete Technology Estimating the depth of cracks An estimate of the depth of a crack visible at the surface can be obtained by the transit times across the crack for two different arrangements of the transducers placed on the surface.
One suitable arrangement is one in which the transmitting and receiving transducers are placed on opposite sides of the crack and distant from it. Two values of X are chosen, one being twice that of the other, and the transmit times corresponding to these are measured.
An equation may be derived by assuming that the plane of the crack is perpendicular to the concrete surface and that the concrete in the vicinity of the crack is of reasonably uniform quality. It is important that the distance X be measured accurately and that very good coupling is developed between the transducers and the concrete surface.
The method is valid provided the crack is not filled with water.
This ultrasonic test is done as per IS: 13311 (Part 1) – 1992. Procedure for Ultrasonic Pulse Velocity i) Preparing for use: Before switching on the ‘V’ meter, the transducers should be connected to the sockets marked “TRAN” and ” REC”. The ‘V’ meter may be operated with either:
The internal battery, An external battery or The A.C line. ii) Set reference: A reference bar is provided to check the instrument zero. The pulse time for the bar is engraved on it. Apply a smear of grease to the transducer faces before placing it on the opposite ends of the bar. Adjust the ‘SET REF’ control until the reference bar transit time is obtained on the instrument read-out. iii) Range selection: For maximum accuracy, it is recommended that the 0.1 microsecond range be selected for path length upto 400mm. iv) Pulse velocity: Having determined the most suitable test points on the material to be tested, make careful measurement of the path length ‘L’. Apply couplant to the surfaces of the transducers and press it hard onto the surface of the material. Do not move the transducers while a reading is being taken, as this can generate noise signals and errors in measurements. Continue holding the transducers onto the surface of the material until a consistent reading appears on the display, which is the time in microsecond for the ultrasonic pulse to travel the distance ‘L’.
The mean value of the display readings should be taken when the units digit hunts between two values.
Pulse velocity=(Path length/Travel time) v) Separation of transducer leads: It is advisable to prevent the two transducer leads from coming into close contact with each other when the transit time measurements are being taken.
If this is not done, the receiver lead might pick-up unwanted signals from the transmitter lead and this would result in an incorrect display of the transit time.
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Concrete Technology 5. Radioactive Methods of NDT Radioactive methods of testing concrete can be used to detect the location of reinforcement, measure density and perhaps establish whether honeycombing has occurred in structural concrete units. Gamma radiography is increasingly accepted in England and Europe.
The equipment is quite simple and running costs are small, although the initial price can be high. Concrete up to 18 in. (45 cm) thick can be examined without difficulty.
Purpose of Non-Destructive Tests on Concrete A variety of Non Destructive Testing (NDT) methods have been developed or are under development for investigating and evaluating concrete structures.
These methods are aimed at estimation of strength and other properties; monitoring and assessing corrosion; measuring crack size and cover; assessing grout quality; detecting defects and identifying relatively more vulnerable areas in concrete structures.
Many of NDT methods used for concrete testing have their origin to the testing of more homogeneous, metallic system. These methods have a sound scientific basis, but heterogeneity of concrete makes interpretation of results somewhat difficult.
There could be many parameters such as materials, mix, workmanship and environment, which influence the results of measurements.
Moreover, these tests measure some other property of concrete (e.g. hardness) and the results are interpreted to assess a different property of concrete e.g. strength, which is of primary interest.
Thus, interpretation of results is very important and difficult job where generalization is not possible. As such, operators can carry out tests but interpretation of results must be left to experts having experience and knowledge of application of such non-destructive tests.
Purposes of Non-destructive Tests
1. Estimating the in-situ compressive strength 2. Estimating the uniformity and homogeneity 3. Estimating the quality in relation to standard requirement 4. Identifying areas of lower integrity in comparison to other parts 5. Detection of presence of cracks, voids and other imperfections 6. Monitoring changes in the structure of the concrete which may occur with time 7. Identification of reinforcement profile and measurement of cover, bar diameter, etc. 8. Condition of prestressing/reinforcement steel with respect to corrosion 9. Chloride, sulphate, alkali contents or degree of carbonation 10. Measurement of Elastic Modulus 11. Condition of grouting in prestressing cable ducts
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Concrete Technology REFERENCES https://www.youtube.com/watch?v=iNmDK0-3Z5Q https://www.giatecscientific.com/education/concrete-maturity-in-situ-compressive-strength/ https://www.youtube.com/watch?v=Td_0VJPlhdA https://www.youtube.com/watch?v=e8bH26-3PCw https://www.youtube.com/watch?v=mDaWekA5Zgs https://www.youtube.com/watch?v=wI5hFajlrrM https://www.youtube.com/watch?v=JGQnbwxPiFA
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Concrete Technology MODULUS OF ELASTICITY
Modulus of elasticity (also known as elastic modulus, the coefficient of elasticity) of a material is a number which is defined by the ratio of the applied stress to the corresponding strain within the elastic limit. Physically it indicates a material’s resistance to being deformed when a stress is applied to it. Modulus of elasticity also indicates the stiffness of a material. Value of elastic modulus is higher for the stiffer materials. Modulus of Elasticity,E=fs Here, f= applied stress on a body s= strain to correspond to the applied stress
Determination of Modulus of Elasticity Concrete.
Units of Elastic Modulus Units of elastic modulus are followings:
In SI unit MPa or N/mm2 or KN/m2. In FPS unit psi or ksi or psf or ksf.
Modulus of Elasticity of Concrete
Modulus of Elasticity of Concrete can be defined as the slope of the line drawn from stress of zero to a compressive stress of 0.45f’c. As concrete is a heterogeneous material. The strength of concrete is dependent on the relative proportion and modulus of elasticity of the aggregate.
To know the accurate value of elastic modulus of a concrete batch, laboratory test can be done. Also, there are some empirical formulas provided by different code to obtain the elastic modulus of Concrete. These formulas are based on the relationship between modulus of elasticity and concrete compressive strength. One can easily obtain an approximate value of modulus of elasticity of concrete using 28 days concrete strength (f’c) with these formulas.
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Concrete Technology Elastic Modulus of Concrete from ACI Code
Different codes have prescribed some empirical relations to determine the Modulus of Elasticity of Concrete. Few of them are given below.
According to ACI 318-08 section 8.5,
Modulus of elasticity for concrete, Ec=w1.50c×0.043√f′cMPa
Ec=wc1.50×0.043fc′MPa 3 This formula is valid for values of wc between 1440 and 2560 kg/m . For normal-weight concrete, Ec=4700√f′cMPa (inFPSunitEc=57000√f′cpsi)
Ec=4700fc′MPa (inFPSunitEc=57000fc′psi)
Elastic Modulus of Concrete from BNBC According to BNBC 2006 section 5.13.2.1,
For stone aggregate concrete,
Ec=44 w1.50c√f′cN/mm2
Ec=44 wc1.50fc′N/mm2
3 2 When wc between 15 and 25 kN/m and √f’c in N/mm .
For normal density concrete , Ec=4700√f′c N/mm2
Ec=4700fc′N/mm2,
For brick aggregate concrete, Ec=3750√f′cN/mm2
DYNAMIC MODULUS:
The value of modulus of elasticity Ec determined by actual loading of concrete is known as static modulus of elasticity. This method of testing is known as destructive method as the specimen is stressed or loaded till its failure. The static modulus of elasticity does not represent the true elastic behaviour of concrete due to the phenomenon of creep. At higher stresses the modulus of elasticity is affected more seriously. Thus a non-destructive method of testing known as dynamic method is adopted for determining the modulus of elasticity. In this case no stress is applied on the specimen. The modulus of elasticity is determined by subjecting the specimen to longitudinal vibration at their natural frequency that is why this is known as dynamic modulus.
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Concrete Technology In this method either the resonant frequency through a specimen of concrete or pulse velocity travelling through the concrete is measured. From the known values of length of specimen, density of concrete and resonant frequency the value of dynamic modulus in S.I. units is determined from the relation-
Ed = K.n2L2 ρ where, Ed = dynamic modulus of elasticity K = a constant n = resonant frequency L = length of specimen ρ = density of concrete If length of specimen is measured in mm and density ρ in kg/m3 then- Ed = 4n2L2 ρ x 10–15 GPa The value of dynamic modulus of elasticity can also be determined from the relation- Ed = ρv2 [(1 + µ)(1 – 2µ)/(1 – µ)] where, v = pulse velocity in mm/s ρ = density of concrete kg/m3 µ = poisson’s ratio. The value of dynamic modulus of elasticity computed from ultrasonic pulse velocity method is somewhat higher than static modulus of elasticity as the creep remains unaffected in dynamic modulus. Creep also does not significantly affect the initial tangent modulus. Thus the value of initial tangent modulus and dynamic modulus is approximately the same, but the value of dynamic modulus is appreciably higher than secant modulus. The relation between static and dynamic modulic is given by the following relation in G.N/m2. Ec = 1.25 Ed – 19 …(i)
This relation is not applicable to very rich concrete with cement content more than 500 kg/m3 and light weight concrete. For light weight concrete relation is- Ec = 1.04 Ed – 4.1 …(ii)
CREEP
Creep is defined as the time-dependent deformation under a constant load. Water movement under stress is a major mechanism leading to creeping of concrete. As a result, factors affecting shrinkage also affect creep in a similar way. Besides moisture movement, there is evidence that creep may also be due to time-dependent formation and propagation of microcracks, as well as microstructural adjustment under high stresses (where stress concentration exists). These mechanisms, together with water loss from the gel interlayer, lead to irreversible creep. Creeping develops rapidly at the beginning and gradually decreases with time. Approximately 75% of ultimate creep occurs during the first year. The ultimate creep strain (after 20 years) can be 3-6 times the elastic strain.
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Concrete Technology Creep can influence reinforced concrete in the following aspects.
Due to the delayed effects of creep, the long-term deflection of a beam can be 2-3 times larger than the initial deflection.
Creeping results in the reduction of stress in pre-stressed concrete which can lead to increased cracking and deflection under service load.
In a R.C column supporting a constant load, creep can cause the initial stress in the steel to double or triple with time because steel is non-creeping and thus take over the force reduced in concrete due to creep.
Creep is significantly influenced by the stress level. For concrete stress less than 50% of its strength, creep is linear with stress. In this case, the burger’s body, which is a combination of Maxwell and Kelvin models, is a reasonable representation of creep behaviour. For stress more than 50% of the strength, the creep is a nonlinear function of stress, and linear viscoelastic models are no longer applicable. For stress level higher than 75-80% of strength, creep rupture can occur. It is therefore very important to keep in mind that in the design of concrete column, 0.8 f is taken to bethe strength limit.
Factors affecting Creep of concrete
w/c ratio: The higher the w/c ratio, the higher is the creep.
Aggregate stiffness (elastic modulus): The stiffer the aggregate, the smaller the creep.
Aggregate fraction: higher aggregate fraction leads to reduced creep.
Effects of Creep on Concrete and Reinforced Concrete
In reinforced concrete beams, creep increases the deflection with time and may be a critical consideration in design. In eccentrically loaded columns, creep increases the deflection and can load to buckling. In case of statically indeterminate structures and column and beam junctions creep may relieve the stress concentration induced by shrinkage, temperatures changes or movement of support. Creep property of concrete will be useful in all concrete structures to reduce the internal stresses due to non-uniform load or restrained shrinkage. In mass concrete structures such as dams, on account of differential temperature conditions at the interior and surface, creep is harmful and by itself may be a cause of cracking in the interior of dams. Therefore, all precautions and steps must be taken to see that increase in temperature does not take place in the interior of mass concrete structure. Loss of prestress due to creep of concrete in prestressed concrete structure. Because of rapid construction techniques, concrete members will experience loads that can be as large as the design loads at very early age; these can cause deflections due to cracking and early age low elastic modulus. So, creep has a significant effect on both the structural integrity and the economic impact that it will produce if predicted wrong.
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Concrete Technology SHRINKAGE
After concrete has been cured and begins to dry, the excessive water that has not reacted with the cement will begin to migrate from the interior of the concrete mass to the surface. As the moisture evaporates, the concrete volume shrinks. The loss of moisture from the concrete varies with distance from the surface. The shortening per unit length associated with the reduction in volume due to moisture loss is termed the shrinkage. Shrinkage is sensitive to the relative humidity. For higher relative humidity, there is less evaporation and hence reduced shrinkage. When concrete is exposed to 100% relative humidity or submerged in water, it will actually swell slightly.
Shrinkage can create stress inside concrete. Because concrete adjacent to the surface of a member dries more rapidly than the interior, shrinkage strains are initially larger near the surface than in the interior. As a result of the differential shrinkage, a set of internal self-balancing forces, i.e. compression in the interior and tension on the outside, is set up.
In additional to the self-balancing stresses set up by differential shrinkage, the overall shrinkage creates stresses if members are restrained in the direction along which shrinkage occurs. If the tensile stress induced by restrained shrinkage exceeds the tensile strength of concrete, cracking will take place in the restrained structural element. If shrinkage cracks are not properly controlled, they permit the passage of water, expose steel reinforcements to the atmosphere, reduce shear strength of the member and are bad for appearance of the structure. Shrinkage cracking is often controlled with the incorporation of sufficient reinforcing steel, or the provision of joints to allow movement. After drying shrinkage occurs, if the concrete member is allowed to absorb water, only part of the shrinkage is reversible. This is because water is lost from the capillary pores, the gel pores (i.e., the pore within the C-S-H), as well as the space between the C-S-H layers. The water lost from the capillary and gel pores can be easily replenished. However, once water is lost from the interlayer space, the bond between the layers becomes stronger as they get closer to one another. On wetting, it is more difficult for water to re-enter. As a result, part of the shrinkage is irreversible.
The magnitude of the ultimate shrinkage is primarily a function of initial water content of the concrete and the relative humidity of the surrounding environment. For the same w/c ratio, with increasing aggregate content, shrinkage is reduced. For concrete with fixed aggregate/cement ratio, as the w/c ratio increases, the cement becomes more porous and can hold more water. Its ultimate shrinkage is hence also higher. If a stiffer aggregate is used, shrinkage is reduced. The shrinkage strain, ε , is time dependent. Approximately 90% of the ultimate shrinkage occurs during the first year.Both the rate at which shrinkage occurs and the magnitude of the total shrinkage increase as the ratio of surface to volume increases. This is because the larger the surface area, the more rapidly moisture can evaporate.
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Concrete Technology Types of shrinkage Based on a number of local investigations in Hong Kong, the value of shrinkage strain (after one year) for plain concrete members appears to lie between 0.0004 and 0.0007 (although value as high as 0.0009 has been reported). For reinforced concrete members, the shrinkage strain values are reduced, as reinforcement is helpful in reducing shrinkage. Shrinkage of concrete can be classified into the following categories: 1. Plastic Shrinkage 2. Drying Shrinkage 3. Autogeneous Shrinkage 4. Carbonation Shrinkage.
1. Plastic Shrinkage: Plastic shrinkage takes place, soon after the concrete is placed in the form work, while the concrete is still in plastic stage. The cement paste at this stage un-goes a volumetric contraction. The magnitude of this volumetric contraction is of the order of 1% of the absolute volume of the dry cement. The plastic shrinkage is caused by the loss of water by evaporation from the surface of concrete or by suction by dry concrete below. The contraction in volume induces tensile stress in the surface layers due to the restraint caused by non-shrinking inner concrete. As the concrete is very weak in plastic state, plastic cracking takes place at the surface.
Plastic shrinkage is directly proportional to the loss of water i.e., plastic shrinkage is greater, the greater the rate of evaporation of water, which in turn depends upon the air temperature, the concrete temperature, relative humidity of the air and wind velocity etc. The rate of evaporation should not be greater than 0.5 kg/m2 per hour of the exposed concrete surface to avoid plastic cracking of the surface. A complete prevention of evaporation from the concrete surface imme- diately after casting reduces plastic shrinkage. ADVERTISEMENTS:
As stated above, that the loss of water from the cement paste is responsible for the plastic shrinkage. The plastic shrinkage has been found greater, for the larger cement content in the mix and lower for larger aggregate content. The effect of cement aggregate ration is shown in Fig 16.1.
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Concrete Technology 2. Drying Shrinkage: Withdrawal of water from hardened concrete stored in unsaturated air develops drying shrinkage. A part of this movement is irreversible. This part should be distinguished from the reversible part or moisture movement. In case of reversible moisture part, if the concrete allowed to dry in air of a given relative humidity is placed in water or at a higher humidity later on, the cement paste will absorb water and win swell. This phenomenon has been shown in Fig. 16.2 (a).
The reversible moisture movement represents 40 to 70% of the drying shrinkage but it depends on the age before the start of first drying. If concrete is fully hydrated before exposing to drying the reversible moisture movement will form the greater proportion of the drying shrinkage. On the other hand if drying is accompanied by extensive carbonation, the cement will not allow movement of moisture content and irreversible shrinkage will be larger.
However not all the initial drying shrinkage is recovered even after a prolonged storage in water. For usual range of concrete, the pattern of moisture movement under alternating wetting and drying is a common occurrence in the practice. It has been shown in Fig.16.2 (b). The magnitude of this cyclic moisture movement depends upon the duration of the wetting and drying periods, but drying is very much slower than wetting. Thus the influence of long dry weather can be reversed by a short period of rain.
The movement of moisture also depends upon the following factors:
1. Range of relative humidity. 2. Composition of the concrete. 3. Degree of hydration at the time of initial drying. Concrete made with light weight aggregate has a higher moisture movement than normal weight aggregate concrete.The irreversible part of shrinkage is associated with the formation of additional chemical and physical bond in the cement gel when adsorbed water has been removed. When concrete dries, first all the free water present in capillaries or pores evaporates and gets lost i.e., water present in concrete pores or capillaries which is not physically bound is lost. Thus under drying conditions, the gel water is lost progressively over a long time, as long as concrete is kept in drying conditions. This loss of free water does not cause any significant volumetric contraction of the
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Concrete Technology cement paste. Due to the loss of free water, an internal relative humidity gradient is induced within the cement paste structure. As the time passes water molecules are transferred from the large surface area of calcium silicate hydrate into empty pores or capillaries and then out of the concrete. This phenomenon causes contraction in cement paste. This reduction or contraction in cement paste is not equal to the volume of water removed due to the internal restraint caused to contraction by the calcium silicate hydrate structure and also due to the negligible effect of free water loss on the change of volumetric contraction. The loss of free water from the hardened concrete does not cause any appreciable change in volume. It is the loss of water held in gel pores that causes change in the volume of concrete. Fig. 16.3 shows the relationship between loss of moisture and shrinkage. Under drying conditions the gel water is lost progressively over a long time as long as the concrete is kept in drying conditions.
Theoretically it is estimated that the total linear change due to long time drying shrinkage is of the order of 10,000 x 10–6, but actually this change has been observed upto 4,000 x 10–6. Fig. 16.4 shows a typical apparatus for the measurement of shrinkage.
Further it has been observed that cement paste shrinks more than mortar and mortar shrinks more than concrete. Concrete made with smaller sized aggregate shrinks more than the concrete made
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Concrete Technology with bigger size aggregate. The magnitude of drying shrinkage is also a function of the fineness of gel, the finer the gel, greater the shrinkage. The high pressure steam cured concrete with low specific surface of gel, shrinks much less than that of normally cured concrete.
Magnitude of Shrinkage: If no other reliable data is available, the magnitude of shrinkage can be estimated by the formula suggested by Schorer.
εs = 0.00125(0.90 – h) …(16.1) where.
εs = shrinkage strain h = relative humidity expressed as a fraction. If the average humidity is 50%, then h = 0.50. If average humidity is 100%, then h = 1.0.
In these conditions, shrinkage is given as:
(i) If h = 0.5, then εs = 0.00125 (0.90 – 0.5) = 0.00125 x 0.4 = 0.0005 (ii) Uh = 1.0, then εs = 0.00125 (0.9 – 1.0) = – 0.00125. -ve sign indicates swelling.
The rate of shrinkage decreases with time. It has been observed that 14 to 34% of 20 years shrinkage takes place in two weeks’ time. Whereas 40 to 70% in 3 months and 66 to 80% in one year.
3. Autogeneous Shrinkage: If no movement of water to or from the set paste of concrete is allowed, then the shrinkage developed is known as autogeneous shrinkage. This shrinkage is caused by the loss of water consumed or used up in the hydration of cement. Autogeneous shrinkage is not distinguished from shrinkage of hardened concrete due to the loss of water to the outside except for massive structures as interior of concrete dams. The magnitude of this shrinkage is very small of the order of 50 x 10- 6 to 100 x 10-6. Hence is not of much significance.
4. Carbonation Shrinkage: In addition to drying shrinkage, concrete also undergoes carbonation shrinkage. Many experimental data include both types of shrinkage, but their mechanism is different.
Carbonation is the reaction of carbon dioxide CO2 present in the atmosphere, with the hydrated
cement minerals in the presence of moisture. The action of CO2 takes place even in small
concentrations such as present in rural air, where the content of CO2 is about 0.03% by volume. In
an un ventilated laboratory the CO2 content is about 0.1%, where as in the atmosphere of big cities, the carbon dioxide content exists about 0.3% and in exceptional cases it may go up to 1.0%. The rate
of carbonation increases with the increase in the concentration of CO2 especially at high water/cement ratio.
In the presence of moisture, CO2 forms carbonic acid, which reacts with Ca(OH)2 to form calcium
carbonate (CaCO3). Other cement compounds are also decomposed, producing hydrated silica,
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Concrete Technology alumina and ferric oxide The complete decomposition of calcium compounds in hydrated cement is
chemically possible even at low pressure of CO2 in normal atmosphere, but carbonation penetrates
beyond the exposed surface of concrete extremely slowly. The simultaneous reaction of CO2 with hydrated cement minerals in concrete induces contraction of concrete, which is known as carbonation shrinkage.
Factors Affecting the Rate of Carbonation: The actual rate of carbonation depends on the permeability of concrete, its moisture content and on
the content of CO2, relative humidity of the ambient medium and size of the specimen , grade of concrete, depth of core, whether the concrete protected and time. As the permeability of concrete is governed by the water/cement ratio, the inadequately cured concrete will be more prone to carbonation i.e., the depth of carbonation of inadequately cured concrete will be more. The effect of following factors has been found more pronounced. i. Effect of Relative Humidity: The highest rate of carbonation occurs at a relative humidity of 50 to 70%. ii. Effect of Time: The rate of carbonation depth is approximately proportional to the square root of time. It doubles between 1 and 4 years and then again doubles between 4 and 10 years. This process repeats upto about 5-years. However periodic wetting of concrete by rain slows down significantly the progress of carbonation. iii. Effect of w/c Ratio: The depth of carbonation is directly proportional to the increase of water/cement ratio i.e., the depth of carbonation increases with the increase in water/cement ratio. It has been observed that depth of carbonation at w/c ratio 0.4 is half that of at w/c ratio 0.6 and at w/c 0.8, it is 50% higher than that at w/c ratio 0.6. iv. Effect of Cement Content: It has been observed that higher the cement content lesser the depth of carbonation. The depth of carbonation is found about 50% with cement content of 500 kg/m3 of that having cemented content of 310 kg/nr. At cement content of 180 kg/m3 it has been found twice that of a cement content of 310 kg/m3. The extent of carbonation can be determined easily by treating freshly broken surface with
phenolphthalein. The portion containing free Ca(OH)2 turn pink by soaking it in Phenolphthalein, whereas carbonated portion remains unaffected. Effects of Carbonation on Concrete: Following effects of carbonation are observed: Carbonation of concrete is accompanied by: (a) Increase in the weight of the concrete.
(b) Shrinkage in concrete known as carbonation shrinkage. Carbonation shrinkage most probably is
caused due to the dissolving of crystals of Ca(OH)2, while under a compressive stress imposed by the
drying shrinkage and depositing CaCO3 in the voids of cement paste. Thus the compressibility of
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Concrete Technology cement paste is increased and the carbonation of the hydrates present in the gel does not contribute to the shrinkage. (c) Carbonation results in increased strength.
(d) Carbonation results in reduced permeability. These changes possibly are due to the water released by carbonation, which promotes the process of hydration. The calcium carbonate produced reduces the voids with in the cement paste. This applies to concrete made with Portland cement only. In case of super sulphated cement the strength of concrete decreases with carbonation. However this decrease in strength is not structurally significant.
(e) The most important effect of carbonation is the neutralization of alkaline nature of hydrated
cement paste. The pH value reduces from 12 to 8. This reduction in pH value provides protection to steel from corrosion. However, if full depth of reinforcement cover is carbonated and oxygen and moisture can penetrate into the concrete then corrosion and cracking of concrete may take place. This is a very significant development. Fig. 16.5 shows the drying shrinkage of mortar specimen
dried in CO2 free air at different relative humidities and also the shrinkage after subsequent carbonation.
It has been observed that carbonation increases shrinkage at intermediate humidities, but not at extreme limits of 100 and 25%. In the latter case there is no sufficient water present in the pores
with in the cement paste for CO2 to form carbonic acid. On the other hand at 100% humidity when the pores are full of water, the diffusion of carbon dioxide into paste will be very slow. Thus carbonation is greater in concrete protected from direct rain, but exposed to moist air, then concrete periodically exposed fully to rain. The sequence of drying and carbonation, greatly affects the total magnitude of shrinkage. Simultaneous drying and carbonation produces lower total shrinkage than when drying is followed by carbonation. Carbonation shrinkage of high pressure steam cured concrete is very small.
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Concrete Technology
Concrete subjected to alternate wetting and drying in air containing CO2, carbonation shrinkage becomes progressively more apparent.
The total shrinkage at any stage is greater than drying the concrete in CO2 free air. Thus carbonation increases the magnitude of irreversible shrinkage and crazing of exposed concrete mass occurs. However carbonation of concrete before exposure to alternate drying and wetting reduces moisture by about 50%.
REFERENCES https://www.youtube.com/watch?v=EwXrGZzxoDA https://www.youtube.com/watch?v=SaNoLHeS_yM https://www.youtube.com/watch?v=6wmzW4J6L3w
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Concrete Technology a) Theoretical thickness: The theoretical thickness is defined as the ratio of section area to the semi- perimeter in contact with the atmosphere. Higher the theoretical thickness, smaller the creep and shrinkage. b) Temperature: with increasing temperature, both the rate of creep and the ultimate creep increase. This is due to the increase in diffusion rate with temperature, as well as the removal of more water at a higher temperature. c) Humidity: with higher humidity in the air, there is less exchange of moisture between theconcrete and the surrounding environment. The amount of creep is hence reduced. d) Age of concrete at loading: The creep strain at a given time after the application of loading is lower if loading is applied to concrete at a higher age. For example, if the same loading is applied to 14 day and 56 day concrete (of the same mix), and creep strain is measured at 28 and 70 days respectively (i.e., 14 days after load application), the 56 day concrete is found to creep less. This is because the hydration reaction has progressed to a greater extent in the 56 day concrete. With less capillary pores to hold water, creep is reduced.
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Concrete Technology FACTORS IN THE CHOICE OF MIX DESIGN
Concrete mix design is defined as the appropriate selection and proportioning of constituents to produce a concrete with pre-defined characteristics in the fresh and hardened states.
Moreover, concrete mixes are designed in order to achieve a defined workability, strength and durability . Finally, this article presents factors affecting the choice of concrete mix design.
Basis for selection and proportioning of materials
The structural requirements of the concrete environmental conditions The job site conditions, especially the methods of concrete production, transport, placement, compaction and finishing The characteristics of the available raw materials The various factors affecting the choice of concrete mix design are: 1. Compressive strength of concrete
Concrete compressive strength considered as the most important concrete property. It influences many other describable properties of the hardened concrete. The mean compressive strength (fcm) required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. ISO 456-200, British Standard, and Eurocode utilize the term mean compressive strength which is slightly greater than characteristic compressive strength. However, ACI Code do not use such term. Other factors which influences the concrete compressive strength at given time and cured at a specified temperature is compaction degree. Finally, it is demonstrated that, concrete compressive strength of fully compacted concrete is inversely proportional to the water-cement ratio.
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Concrete Technology Fig.1:Means compressive strength vs characteristic compressive strength
Fig.2: compressive strength of concrete 2. Workability of concrete
Concrete workability for satisfactory placement and compaction depends on the size and shape of the section to be concreted, the amount and spacing of reinforcement, and concrete transportation; placement; and compaction technique. Additionally, use high workability concrete for the narrow and complicated section with numerous corners or inaccessible parts. This will ensure the achievement of full compaction with a reasonable amount of effort. Frequently, slump test values used to evaluate concrete workability. Lastly, ACI 211.1 provides slump test values for various reinforced concrete sections which ranges from 25 mm to 175 mm.
Fig.3: Workability of concrete
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Concrete Technology
Fig.4: Self compacting concrete, high workability concrete 3. Durability of concrete
The ability of concrete to withstand harmful environment conditions termed as concrete durability. High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the utilized water-cement ratio. Concrete durability decreases with the increase of w/c ratio.
Fig.5: Durability of concrete 4. Maximum nominal size of aggregate
Reinforcement spacing controls maximum aggregate size. Aggregate size is inversely proportional to cement requirement for water-cement ratio. This is because workability is directly proportional to size of aggregate
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Concrete Technology However, the compressive strength tends to increase with the decrease in size of aggregate. smaller aggregate size offers greater surface area for bonding with mortar mix that give higher strength. IS 456:2000 and IS 1343:1980 recommends that the nominal size of the aggregate should be as large as possible. Finally, in accordance with ACI code, maximum aggregate size shall not exceed minimum reinforcement spacing, bar diameter, or 25mm.
Fig.6: Maximum aggregate size
5. Grading and type of aggregate
Aggregate grading influences the mix proportions for a specified workability and water- cement ratio. The relative proportions between coarse and fine aggregate in concrete mix influence concrete strength. Well graded fine and coarse aggregate produce a dense concrete because of the achievement of ultimate packing density. If available aggregate, which obtained from natural source, does not confirm to the specified grading, the proportioning of two or more aggregate become essential. Additionally, for specific workability and water to cement ratio, type of aggregate affects aggregate to cement ratio. Lastly, An important feature of a satisfactory aggregate is the uniformity of the grading that achieved by mixing different size fractions.
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Concrete Technology Fig.7: aggregate grading types 6. Quality Control at site
The degree of control could be evaluated by the variations in test results. The variation in strength results from the variations in the properties of the mix ingredients, in addition to lack of control of accuracy in batching, mixing, placing, curing and testing. Finally, the lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control.
ACCEPTANCE CRITERIA FOR DESIGN MIX CONCRETE
I. The concrete shall be deemed to comply with the strength requirements if: a) Every sample has test strength not less than the characteristic value; or
b) The strength of one or more samples though less than the characteristic value, is in each case not less than the greater of:
The characteristic strength minus 1.35 times the standard deviation; and
2) 0.80 times the characteristic strength; and the average strength of all the samples is not
less than the characteristic strength plus times the standard deviation.
II. The concrete shall be deemed not to comply with the strength requirements if: a) The strength of any sample is less than the greater of: The characteristic strength mix is 1.35 times the standard deviation; and 0.80 times the characteristic strength ; or b) The average strength of all samples is less than the characteristic strength
plus times the standard deviation.
III Concrete which does not meet the strength requirements as specified in I, but has a strength greater than that required by II may, at the discretion of the designer, be accepted as being structurally adequate without further testing.
IV. Concrete of each grade shall be assessed separately.
V. Concrete shall be assessed daily for compliance.
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Concrete Technology VI. Concrete is liable to be rejected if it is porous or honey-combed; its placing has been interrupted without providing a proper construction joint; the reinforcement has been displaced beyond the tolerances specified; or construction tolerances have not been met. However, the hardened concrete may be accepted after carrying out suitable remedial measures to the satisfaction. VIII. Where the value of the average strength of the tests (preferably 30 tests or 15 tests) is less than
shall be rejected.
BIS METHOD OF MIX DESIGN
The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass.
Factors to be considered for mix design
The grade designation giving the characteristic strength requirement of concrete.
The type of cement influences the rate of development of compressive strength of concrete.
Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000.
The cement content is to be limited from shrinkage, cracking and creep.
The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction.
Procedure for Concrete Mix Design – IS456:2000
1) Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control.
2) ft = fck + 1.65 S
3) Where, S is the standard deviation obtained from the Table of approximate contents given after the design mix.
4) Obtain the water cement ratio for the desired mean target using the empirical relationship between compressive strength and water cement ratio so chosen is checked against the limiting water cement ratio. The water cement ratio so chosen is checked against the limiting water cement ratio for the requirements of durability given in table and adopts the lower of
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Concrete Technology the two values.
5) Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.
6) Select the water content, for the required workability and maximum size of aggregates (for aggregates in saturated surface dry condition) from table.
7) Determine the percentage of fine aggregate in total aggregate by absolute volume from table for the concrete using crushed coarse aggregate. 8) Adjust the values of water content and percentage of sand as provided in the table for any difference in workability, water cement ratio, grading of fine aggregate and for rounded aggregate the values are given in table.
9) Calculate the cement content form the water-cement ratio and the final water content as arrived after adjustment. Check the cement against the minimum cement content from the requirements of the durability, and greater of the two values is adopted.
10) From the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in steps 6 and 7 above, calculate the content of coarse and fine aggregates per unit volume of concrete from the following relations:
11) Where, V = absolute volume of concrete = gross volume (1m3) minus the volume of entrapped air
12) Sc = specific gravity of cement
13) W = Mass of water per cubic metre of concrete, kg C = mass of cement per cubic metre of concrete, kg 14) p = ratio of fine aggregate to total aggregate by absolute volume
15) fa, Ca = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and
16) Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively
17) Determine the concrete mix proportions for the first trial mix.
18) Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength.
19) Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.
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Concrete Technology CONCRETE MIX DESIGN EXAMPLE – M50 GRADE CONCRETE
Grade Designation = M-50 Type of cement = O.P.C-43 grade Brand of cement = Vikram ( Grasim ) Admixture = Sika [Sikament 170 ( H ) ] Fine Aggregate = Zone-II
Sp. Gravity Cement = 3.15 Fine Aggregate = 2.61 Coarse Aggregate (20mm) = 2.65 Coarse Aggregate (10mm) = 2.66
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Concrete Technology Minimum Cement (As per contract) =400 kg / m3 Maximum water cement ratio (As per contract) = 0.45
Mix Calculation: –
1. Target Mean Strength = 50 + (5 X 1.65 ) = 58.25 Mpa
2. Selection of water cement ratio:-
Assume water cement ratio = 0.35
3. Calculation of water: –
Approximate water content for 20mm max. Size of aggregate = 180 kg /m3 (As per Table No. 5, IS : 10262 ). As plasticizer is proposed we can reduce water content by 20%.
Now water content = 180 X 0.8 = 144 kg /m3
4. Calculation of cement content:-
Water cement ratio = 0.35 Water content per cum of concrete = 144 kg Cement content = 144/0.35 = 411.4 kg / m3 Say cement content = 412 kg / m3 (As per contract Minimum cement content 400 kg / m3) Hence O.K.
5. Calculation for C.A. & F.A.: –
Volume of concrete = 1 m3 Volume of cement = 412 / (3.15 X 1000) = 0.1308 m3 Volume of water = 144 / (1 X 1000) = 0.1440 m3 Volume of Admixture = 4.994 / (1.145 X 1000) = 0.0043 m3 Total weight of other materials except coarse aggregate = 0.1308 + 0.1440 +0.0043 = 0.2791 m3
Volume of coarse and fine aggregate = 1 – 0.2791 = 0.7209 m3 Volume of F.A. = 0.7209 X 0.33 = 0.2379 m3 (Assuming 33% by volume of total aggregate) Volume of C.A. = 0.7209 – 0.2379 = 0.4830 m3 Therefore weight of F.A. = 0.2379 X 2.61 X 1000 = 620.919 kg/ m3 Say weight of F.A. = 621 kg/ m3 Therefore weight of C.A. = 0.4830 X 2.655 X 1000 = 1282.365 kg/ m3 Say weight of C.A. = 1284 kg/ m3 Considering, 20 mm: 10mm = 0.55: 0.45 20mm = 706 kg. 10mm = 578 kg. Hence Mix details per m3 Increasing cement, water, admixture by 2.5% for this trial
Cement = 412 X 1.025 = 422 kg Water = 144 X 1.025 = 147.6 kg Fine aggregate = 621 kg Coarse aggregate 20 mm = 706 kg Coarse aggregate 10 mm = 578 kg
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Concrete Technology Admixture = 1.2 % by weight of cement = 5.064 kg.
Water: cement: F.A.: C.A. = 0.35: 1: 1.472: 3.043
SELF COMPACTING CONCRETE
Self compacting concrete (SCC) is an innovative concrete that does not require vibration for placing. It is able to flow under its own weight, completely filling formwork and achieving full compaction, even in the presence of congested reinforcement. The hardened concrete is dense, homogeneous and has the same engineering properties and durability as traditional vibrated concrete. Self compacting concrete (SCC) has been described as "the most revolutionary development in concrete construction for several decades". Originally developed to offset a growing shortage of skilled labour, it has proved economically beneficial because of a number of factors, including: • Faster construction • Reduction in site manpower • Better surface finishes • Easier placing • Improved durability • Greater freedom in design • Thinner concrete sections • Reduced noise levels, absence of vibration • Safer working environment. Originally developed in Japan, SCC technology was made possible by the much earlier development of super plasticizer for concrete. SCC has now been taken up with enthusiasm across Europe for both site and pre-cast concrete work. Concrete is now no longer a material consisting of cement, aggregates, water and admixture but it is an engineering material with several new constituents. Concrete alone may not pose any problem. But due to the presence of congested reinforcement compaction becomes difficult. Moreover it is becoming labour intensive. Hence there is need for concrete which gets compacted under gravity. Self-compacting concrete is a type of concrete that gets compacted under its self- weight. The concept of self-compacting concrete resulted from research into in-situ concrete piling and the filling of other inaccessible areas where compaction is essential but difficult. Poor quality of vibration of concrete, in congested locations, has often been a shortcoming of traditional concrete. In such situations SCC, which flows under its own weight and does
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Concrete Technology not require any external vibrations during compaction, has revolutionized concrete placement. SCC does not show segregation and bleeding. For SCC it is generally necessary to make use of super plasticizer in order to obtain high mobility. Possible segregation is eliminated by adding viscosity modifying admixtures or a large volume of powdered material. The powdered materials are fly ash, silica fume, lime stone powder, glass filler. Advantages of SCC At the start of the project the production of SCC is little more expensive than conventional concrete and it is difficult to maintain the desired consistency over a long period of time. However, SCC offers many advantages for the precast, pre-stressed concrete industry and for cast-in-place construction:
Easier and rapid placement in members with dense reinforcement and complicated form work results in faster construction and reduction in cost of production. Good bond between concrete and reinforcement is obtained, even in congested reinforcement. Reduction on site man power for all operations. Relatively low water-to-cement ratio results in rapid strength development, improved quality, strength and durability. Produces good surface finish particularly for slabs. Cost efficient and rational solution in thin overlays on pre-fabricated elements-thinner concrete sections can be cast easily. Reduced noise levels in the plants and at construction.
Limitations of SCC Apparent lack of reliable test standard that can qualify the physical properties of SCC. Higher material cost not only for admixtures but also for increased quality control testing needed for concrete and aggregate. Mixing and finishing times will likely be longer. Because of SCC high fluidity, grout leakage could be the problem in forms that do not completely seal. Since SCC is inherently self-leveling, filling form that is not level could conceivably cause problems.
Requirements for self compacting concrete Application area SCC may be used in pre-cast applications or for concrete placed on site. It can be manufactured in a site batching plant or in a ready mix concrete plant and delivered to site by truck. It can then be placed either by pumping or pouring into horizontal or vertical structures. In designing the mix, the size and the form of the structure, the dimension and density of reinforcement and cover should be taken in to consideration. These aspects will influence all the specific requirements for the SCC. Due to the flowing characteristics of SCC, it may be difficult to cast, unless contained in a form. SCC has made it possible to cast concrete structures of a
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Concrete Technology quality which is not possible with the existing concrete technology.
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Concrete Technology Requirements SCC can be designed to fulfill the requirements of EN 206 (referenced standards for the European guidelines for self compacting concrete) regarding density, strength development, final strength and durability. Due to the high content of powder, SCC may show more plastic shrinkage or creep than ordinary concrete mixes. These aspects should therefore be considered during designing and specifying SCC. Current knowledge of these aspects is limited and this is an area requiring further research. Special care should also be taken to begin curing the concrete as early as possible. The workability of SCC is higher than the highest class of consistence described within EN 206 and can be characterized by the following properties: Filling ability Passing ability Segregation resistance Filling ability: The ability of fresh concrete to flow into and fill all spaces within the formwork, under its own weight. Passing ability: The ability of fresh concrete to flow through tight openings such as spaces between steel reinforcing bars without segregation or blocking. Resistance to segregation: The ability of concrete to remain homogeneous in composition while in its fresh state.
TEST METHODS
Slump flow test (1) and T 50cm test (2)
Introduction The slump flow is used to assess the horizontal free flow of SCC in the absence of obstructions. It was first developed in Japan (1) for use in assessment of underwater concrete. The test method is based on the test method for determining the slump. The diameter of the concrete circle is a measure for the filling ability of the concrete.
Assessment of test
This is a simple, rapid test procedure, though two people are needed if the T50 time is to be measured. It can be used on site, though the size of the base plate is somewhat unwieldy and level ground is essential. It is the most commonly used test, and gives a good assessment of filling ability. It gives no indication of the ability of the concrete to pass between reinforcement without blocking, but may give some indication of resistance to segregation. It can be argued that the completely free flow, unrestrained by any boundaries, is not representative of what happens in practice in concrete construction, but the test can be profitably be used to assess the consistency of supply of ready- mixed concrete to a site from load to load.
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Concrete Technology Figure D.1.1
Slump cone
Flow table
units: mm
Equipment
The apparatus is shown in figure D.1.1. mould in the shape of a truncated cone with the internal dimensions 200 mm diameter at the base, 100 mm diameter at the top and a height of 300 mm, conforming to EN 12350-2 base plate of a stiff non absorbing material, at least 700mm square, marked with a circle marking the central location for the slump cone, and a further concentric circle of 500mm diameter trowel scoop ruler stopwatch (optional)
Procedure About 6 litre of concrete is needed to perform the test, sampled normally. Moisten the base plate and inside of slump cone, Place baseplate on level stable ground and the slumpcone centrally on the base plate and hold down firmly. Fill the cone with the scoop. Do not tamp, simply strike off the concrete level with the top of the cone with the trowel. Remove any surplus concrete from around the base of the cone. Raise the cone vertically and allow the concrete to flow out freely. Simultaneously, start the stopwatch and record the time taken for the concrete to reach the 500mm spread circle. (This is the T50 time). Measure the final diameter of the concrete in two perpendicular directions. Calculate the average of the two measured diameters. (This is the slumpflow in mm). Note any border of mortar or cement paste without coarse aggregate at the edge of the pool of concrete.
Interpretation of result The higher the slump flow (SF) value, the greater its ability to fill formwork under its own weight. A value of at least 650mm is required for SCC. There is no generally accepted advice on what are reasonable tolerances about a specified value, though ± 50mm, as with the related flowtable test, might be appropriate.
The T50 time is a secondary indication of flow. A lower time indicates greater flowability. The Brite EuRam research suggested that a time of 3-7 seconds is acceptable for
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Concrete Technology civil engineering applications, and 2-5 seconds for housing applications. In case of severe segregation most coarse aggregate will remain in the centre of the pool of concrete and mortar and cement paste at the concrete periphery. In case of minor segregation a border of mortar without coarse aggregate can occur at the edge of the pool of concrete. If none of these phenomena appear it is no assurance that segregation will not occur since this is a time related aspect that can occur after a longer period.
J Ring test (3)
Introduction The principle of the JRing test may be Japanese, but no references are known. The JRing test itself has been developed at the University of Paisley. The test is used to determine the passing ability of the concrete. The equipment consists of a rectangular section (30mm x 25mm) open steel ring, drilled vertically with holes to accept threaded sections of reinforcement bar. These sections of bar can be of different diameters and spaced at different intervals: in accordance with normal reinforcement considerations, 3x the maximum aggregate size might be appropriate. The diameter of the ring of vertical bars is 300mm, and the height 100 mm.
The JRing can be used in conjunction with the Slumpflow, the Orimet test, or eventually even the V- funnel. These combinations test the flowing ability and (the contribution of the JRing) the passing ability of the concrete. The Orimet time and/or slumpflow spread are measured as usual to assess flow characteristics. The JRing bars can principally be set at any spacing to impose a more or less severe test of the passing ability of the concrete. After the test, the difference in height between the concrete inside and that just outside the JRing is measured. This is an indication of passing ability, or the degree to which the passage of concrete through the bars is restricted.
Assessment of test These combinations of tests are considered to have great potential, though there is no general view on exactly how results should be interpreted. There are a number of options – for instance it may be instructive to compare the slump-flow/JRing spread with the unrestricted slump-flow: to what extent is it reduced?
Like the slump-flow test, these combinations have the disadvantage of being unconfined, and therefore do not reflect the way concrete is placed and moves in practice. The Orimet option has the advantage of being a dynamic test, also reflecting placement in practice, though it suffers from requiring two operators.
Figure D.3.1: the J Ring used in conjunction with the Slump flow
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Concrete Technology
Equipment
mould, WITHOUT foot pieces, in the shape of a truncated cone with the internal dimensions 200 mm diameter at the base, 100 mm diameter at the top and a height of 300 mm. base plate of a stiff non absorbing material, at least 700mm square, marked with a circle showing the central location for the slump cone, and a further concentric circle of 500mm diameter trowel scoop ruler JRing a rectangular section (30mm x 25mm) open steel ring, drilled vertically withholes. In the holes can be screwed threaded sections of reinforcement bar (length 100mm, diameter 10mm, spacing 48 +/- 2mm) Procedure About 6 litre of concrete is needed to perform the test, sampled normally. Moisten the base plate and inside of slump cone, Place base-plate on level stable ground. Place the JRing centrally on the base-plate and the and the slump-cone centrally inside it and hold down firmly. Fill the cone with the scoop. Do not tamp, simply strike off the concrete level with the top of the cone with the trowel. Remove any surplus concrete from around the base of the cone. Raise the cone vertically and allow the concrete to flow out freely. Measure the final diameter of the concrete in two perpendicular directions.Calculate the average of the two measured diameters. (in mm).Measure the difference in height between the concrete just inside the bars and that just outside the bars. Calculate the average of the difference in height at four locations (in mm). Note any border of mortar or cement paste without coarse aggregate at the edge of the pool of concrete.
V funnel test (4) and V funnel test at T 5minutes (5)
Introduction The test was developed in Japan and used by Ozawa et al (5). The equipment consists of a V- shaped funnel, shown in Fig.D.4.1. An alternative type of V-funnel, the O funnel, with a circular section is also used in Japan. The described V-funnel test is used to determine the filling ability (flowability) of the
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Concrete Technology concrete with a maximum aggregate size of 20mm. The funnel is filled with about 12 litre of concrete and the time taken for it to flow through the apparatus measured. After this the funnel can be refilled concrete and left for 5 minutes to settle. If the concrete shows segregation then the flow time will increase significantly.
Assessment of test Though the test is designed to measure flowability, the result is affected by concrete properties other than flow. The inverted cone shape will cause any liability of the concrete to block to be reflected in the result – if, for example there is too much coarse aggregate. High flow time can also be associated with low deformability due to a high paste viscosity, and with high inter-particle friction. While the apparatus is simple, the effect of the angle of the funnel and the wall effect on the flow of concrete is not clear.
Figure D.4.1: V-funnel test equipment (rectangular section)
Equipment
V-funnel bucket ( ±12 litre ) trowel scoop stopwatch
Procedure flow time About 12 litre of concrete is needed to perform the test, sampled normally. Set the V-funnel on firm ground. Moisten the inside surfaces of the funnel. Keep the trap door open to allow any surplus water to drain. Close the trap door and place a bucketunderneath. Fill the apparatus completely with concrete without compacting or tamping, simply strike off the concrete level with the top with the trowel. Open within 10 sec after filling the trap door and allow the concrete to flow out under gravity. Start the stopwatch when the trap door is opened, and record the time for the discharge to
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Concrete Technology complete (the flow time). This is taken to be when light is seen from above through the funnel. The whole test has to be performed within 5 minutes.
Procedure flow time at T 5 minutes Do NOT clean or moisten the inside surfaces of the funnel again. Close the trap door and refill the V-funnel immediately after measuring the flow time. Place a bucket underneath. Fill the apparatus completely with concrete without compacting or tapping, simply strike off the concrete level with the top with the trowel. Open the trap door 5 minutes after the second fill of the funnel and allow the concrete to flow out under gravity. Simultaneously start the stopwatch when the trap door is opened, and record the time for the discharge to complete (the flow time at T 5 minutes). This is taken to be when light is seen from above through the funnel.
Interpretation of result This test measures the ease of flow of the concrete; shorter flow times indicate greater flowability. For SCC a flow time of 10 seconds is considered appropriate. The inverted cone shape restricts flow, and prolonged flow times may give some indication of the susceptibility of the mix to blocking. After 5 minutes of settling, segregation of concrete will show a less continuous flow with an increase in flow time.
L box test method (6)
Introduction This test, based on a Japanese design for underwater concrete, has been described by Petersson (2). The test assesses the flow of the concrete, and also the extent to which it is subject to blocking by reinforcement. The apparatus is shown in figure D.6.1. The apparatus consists of a rectangular-section box in the shape of an ‘L’, with a vertical and horizontal section, separated by a moveable gate, in front of which vertical lengths of reinforcement bar are fitted. The vertical section is filled with concrete, then the gate lifted to let the concrete flow into the horizontal section. When the flow has stopped, the height of the concrete at the end of the horizontal section is expressed as a proportion of that remaining in the vertical section (H2/H1in the diagram). It indicates the slope of the concrete when at rest. This is an indication passing ability, or the degree to which the passage of concrete through the bars is restricted. The horizontal section of the box can be marked at 200mm and 400mm from the gate and the times taken to reach these points measured. These are known as the T20 and T40 times and are an indication for the filling ability. The sections of bar can be of different diameters and spaced at different intervals: in accordance with normal reinforcement considerations, 3x the maximum aggregate size might be appropriate. The bars can principally be set at any spacing to impose a more or less severe test of the passing ability of the concrete.
Assessment of test This is a widely used test, suitable for laboratory, and perhaps site use. It assesses filling and passing ability of SCC, and serious lack of stability (segregation) can be detected visually.
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Concrete Technology Segregation may also be detected by subsequently sawing and inspecting sections of the concrete in the horizontal section. Unfortunately there is no agreement on materials, dimensions, or reinforcing bar arrangement, so it is difficult to compare test results. There is no evidence of what effect the wall of the apparatus and the consequent ‘wall effect’ might have on the concrete flow, but this arrangement does, to some extent, replicate what happens to concrete on site when it is confined within formwork. Two operators are required if times are measured, and a degree of operator error is inevitable.
Equipment L box of a stiff non absorbing material see figure D.6.1. trowel scoop stopwatch
Figure D.6.1: L-box
Procedure
About 14 litre of concrete is needed to perform the test, sampled normally. Set the apparatus level on firm ground, ensure that the sliding gate can open freely and then close it. Moisten the inside surfaces of the apparatus, remove any surplus water Fill the vertical section of the apparatus with the concrete sample. Leave it to stand for 1 minute. Lift the sliding gate and allow the concrete to flow out into the horizontal section. Simultaneously, start the stopwatch and record the times taken for the concrete to reach the 200 and 400 mm marks. When the concrete stops flowing, the distances “H1” and “H2” are measured. Calculate H2/H1, the blocking ratio. The whole test has to be performed within 5 minutes.
Interpretation of result
If the concrete flows as freely as water, at rest it will be horizontal, so H2/H1 = 1. Therefore the nearer this test value, the ‘blocking ratio’, is to unity, the better the flow of the concrete. The EU
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Concrete Technology research team suggested a minimum acceptable value of 0.8. T20 and T40 times can give some indication of ease of flow, but no suitable values have been generally agreed. Obvious blocking of coarse aggregate behind the reinforcing bars can be detected visually.
U box test method (7)
Introduction The test was developed by the Technology Research Centre of the Taisei Corporation in Japan (4) Sometimes the apparatus is called a “box-shaped” test. The test is used to measure the filling ability of self-compacting concrete. The apparatus consists of a vessel that is divided by a middle wall into two compartments, shown by R1 and R2 in Fig.D.7.1 An opening with a sliding gate is fitted between the two sections. Reinforcing bars with nominal diameters of 13 mm are installed at the gate with centre-to-centre spacings of 50 mm. This creates a clear spacing of 35 mm between the bars. The left hand section is filled with about 20 litre of concrete then the gate lifted and concrete flows upwards into the other section. The height of the concrete in both sections is measured.
Note: An alternative design of box to this, but built on the same principle is recommended by the Japan Society of Civil Engineers. Assessment of test This is a simple test to conduct, but the equipment may be difficult to construct. It provides a good direct assessment of filling ability – this is literally what the concrete has to do – modified by an unmeasured requirement for passing ability. The 35mm gap between the sections of reinforcement may be considered too close. The question remains open of what filling height less than 30 cm. is still acceptable.
Figure D.7.1
Department of Civil Engineering
Concrete Technology Equipment
U box of a stiff non absorbing material see figure D.7.1. trowel scoop stopwatch
Procedure About 20 litre of concrete is needed to perform the test, sampled normally. Set the apparatus level on firm ground, ensure that the sliding gate can open freely and then close it. Moisten the inside surfaces of the apparatus, remove any surplus water Fill the one compartment of the apparatus with the concrete sample. Leave it to stand for 1 minute. Lift the sliding gate and allow the concrete to flow out into the other compartment. After the concrete has come to rest, measure the height of the concrete in the compartment
that has been filled, in two places and calculate the mean (H1). Measure also the height in the other compartment (H2) Calculate H1 - H2, the filling height.
The whole test has to be performed within 5 minutes.
Interpretation of result
If the concrete flows as freely as water, at rest it will be horizontal, so H1 - H2 = 0. Therefore the nearer this test value, the ‘filling height’, is to zero, the better the flow and passing ability of the concrete.
Fill box test method (8)
Introduction This test is also known as the ‘Kajima test ‘.The test is used to measure the filling ability of self- compacting concrete with a maximum aggregate size of 20mm. The apparatus consists of a container (transparent ) with a flat and smooth surface. In the container are 35 obstacles made of PVC with a diameter of 20mm and a distance centre to centre of 50mm: see Figure D.8.1. At the top side is put a filling pipe (diameter 100mm height 500mm) with a funnel (height 100mm). The container is filled with concrete through this filling pipe and the difference in height between two sides of the container is a measure for the filling ability.
Assessment oftest This is a test that is difficult to perform on site due to the complex structure of the apparatus and large weight of the concrete. It gives a good impression of the self-compacting characteristics of the concrete. Even a concrete mix with a high filling ability will perform poorly if the passing ability and segregation resistance are poor.
Figure D.8.1
Front view Side view
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Concrete Technology
Equipment Fill box of a stiff, transparent, non absorbing material scoop ca 1,5 to 2 litre ruler stopwatch
Procedure About 45 litre of concrete is needed to perform the test, sampled normally. Set the apparatus level on firm ground. Moisten the inside surfaces of the apparatus, remove any surplus water Fill the apparatus with the concrete sample. Fill the container by adding each 5 seconds one scoop with 1,5 to 2litrer of fresh concrete into the funnel until the concrete has just covered the first top obstacle. Measure after the concrete has come to rest, the height at the side at which the container is filled on two places and calculate the average(h1). Do this also on the opposite side (h2). Calculate the average filling percentage: Average filling %: F= {(h1+h2)/ 2*h1} * 100% The whole test has to be performed within 8 minutes. Interpretation of result If the concrete flows as freely as water, at rest it will be horizontal, so Average filling percentage = 100%. Therefore the nearer this test value, the ‘filling height’, is to 100%, the better the self- compacting characteristics of the concrete.
Fibre reinforced concrete . Fibre reinforced concrete (FRC) may be defined as a composite materials made with Portland cement, aggregate, and incorporating discrete discontinuous fibres. . The role of randomly distributes discontinuous fibres is to bridge across the
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Concrete Technology cracks that develop provides some post- cracking “ductility”. . The real contribution of the fibres is to increase the toughness of the concrete under any type of loading. . The fibre reinforcement may be used in the form of three – dimensionally randomly distributed fibres throughout the structural member when the added advantages of the fibre to shear resistance and crack control can be further utilised.
REFERENCES https://www.youtube.com/watch?v=DGhQYSlzTUw https://www.youtube.com/watch?v=I3u6IYWINV0 https://www.youtube.com/watch?reload=9&v=dzbi2Rne4Sg https://www.youtube.com/watch?v=XruASAXB6YM https://www.youtube.com/watch?v=LmYrCUEPavI