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Home , Reinforced concrete, Structural material

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CHAPTER – 1

INTRODUCTION

1.1 HISTORY OF REINFORCED

A French gardener by name Joseph Monier first invented the in the year 1849. If not for this reinforced concrete most of the modern buildings would not have been standing today.

Reinforced concrete can be used to produce frames, , , beams etc. Reinforcement material used should have excellent bonding characteristic, high tensile strength and good thermal compatibility. Reinforcement requires that there shall be smooth transmission of load from the concrete to the interface between concrete and reinforcement material and then on to reinforcement material. Thus the concrete and the material reinforced shall have the same strain.

1.1.1 Reinforced Concrete

The steel bars are reinforced into the concrete. The bars have a rough, corrugated surface thus allowing better bonding with steel the concrete gets extra tensile strength. The compression strength, also show marked improvement characteristic of steel rebars and concrete shall match. The shall have cross sectional are equal to 1% for slabs and beams, this can be

6% in case of columns (www.wikipedia.com). The concrete has alkaline nature, this forms a passivating film around the bars thereby protecting it from . This passivating film will not form in 2

neutral or acidic condition. Carbonation of concrete takes place along with absorption resulting in failure of steel rebar.

By comparing the tension capacity of steel bars and concrete + steel reinforcements the reinforced concrete can be called as under reinforced (tensile capacity of bars in less than concrete + bar) it is over reinforced (tensile capacity of steel is greater than concrete + steel tensile strength. The over reinforced fails without giving prior warning and under reinforced fails but gives a deformation warning before it fails. Therefore it is better to consider an under reinforced concrete.

1.2 FIBER REINFORCED CONCRETE

The construction material is continuously evolving. The demand for high strength, crack, resistant and lighter concrete resulted in development of fiber reinforced concrete (2, 3, 4, 5, 6, 7). Fibers that are used are steel, nylon, asbestos, , carbon, sisal, jute, coir, polypropylene, kenaf.

1.2.1 History of FRC

The practice of adding certain fibers to construction material dates back to the ancient times. When horse hair, straws were used to strengthen the . In 1911 Porter found that fiber could be used in concrete. Early 1900 saw the use of asbestos fiber. In 1950 fiber reinforced concrete was becoming a field of interest as asbestos being a health risk was discovered. In 1963 Romualdi and Batson published their classic paper on FRC. Since then there was no looking back, glass, steel, polypropylene fiber were used in concrete.

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1.2.2 Necessity of FRC

The use of concrete as a is limited to certain extent by deficiencies like brittleness, poor tensile strength and poor resistance to impact strength, fatigue, low and low durability.

It is also very much limited to receive dynamic stresses caused due to explosions.

The brittleness is compensated in structural member by the introduction of reinforcement (or) pre-stressing steel in the tensile zone. However it does not improve the basic property of concrete. It is merely a method of using two materials for the required performance.

The main problem of low tensile strength and the requirements of high strength still remain and it is to be improved by different types of reinforcing materials. Further concrete is also deficient in ductility, resistance to fatigue and impact. The importance of rendering requisite quantities in concrete is increasing with its varied and challenging applications in pre-cast and pre-fabricated building elements. The development in the requisite characteristics of concrete will solve the testing problems of structural engineers by the addition of fibers and admixtures.

The role of fibers are essentially to arrest any advancing cracks by applying punching forces at the rack tips, thus delaying their propagation across the matrix. The ultimate cracking strain of the composite is thus increased to many times greater than that of un- reinforced matrix. Admixtures like , , granulated blast furnace slag and can be used for such purposes. 4

However addition of fibers and mineral admixtures posses certain problems regarding mixing, as fibers tends to form balls and workability tends to decrease during mixing.

1.2.3 Behaviour of Fiber in Concrete

Fibers contribute towards reducing the bleeding in fresh concrete and renders concrete more impermeable in the hardened stage. Contribution of certain percentage of fibers in concrete towards flexural strength is smaller compared to the strength given by the rebars. Most importantly fiber restricts the growth of crack under load thereby arresting ultimate cracking. Non metallic fibers like alkali resistant and synthetic fibers provide resistance against chemicals.

Reinforcing capacity of fiber is based on length of fiber, diameter of fiber, the percentage of fiber and condition of mixing, orientation of fibers and aspect ratio. Aspect ratio is ratio of length of fiber to its diameter which plays an important role in the process of reinforcement.

1.3 TYPES OF FIBER

1.3.1 Asbestos Fiber

This comes under naturally occurring mineral fiber. Asbestos fiber shows very good resistance to heat, electrical, chemical damage and fire. It has average tensile strength. Hence it became very popular is the late 19 th century. Asbestos is a combination of six naturally occurring silicates. They were originally used in building insulation electrical insulation for hot plate curing. More is required when asbestos fiber is mixed with due to high absorption. But later 5

it was discovered that asbestos was carcinogenic in nature, hence very likely to human health that it was totally banned.

1.3.2 Carbon Fiber

Carbon fiber improves the elasticity and gives good tensile strength. They are formed by oxidation of poly-acronitrile fibers. After oxidation thermal pyrolysis is carried out thereby producing carbon fibers. They exhibit high elasticity and give good tensile strength.

Rudder of aeroplanes is manufactured using this fiber.

1.3.3 Aramid Fiber

This is synthetic fiber. As name it is aromatic polyamide. Aramid fiber is another reinforcing material that could be used. They are formed by reaction of an amine group and a carboxylic acid halide group. This fiber is commercially known as technora, kevlar, nomex.

Kevlar was originally used as for manufacturing the air frame of commercial aeroplane, as they are very light weight and high strength material. In these fibers, chain molecules are all oriented along fiber axis, so high strength chemical bond results in its high strength. This was first discovered by DuPont. They were excellent substitute for asbestos.

1.3.4 Metallic Fibers

They are manufactured by heating the metal until it evaporizes, then depositing it at very high pressure on to polyster film. Metallic fiber is usually aluminized nylon yarn. Metallic fiber is actually a combination of plastic and metal. They can be drawn from steel wool too. The metallic fibers are fiber or fiber. 6

1.3.5 Polypropylene, Polyethylene, Nylon Fiber

These show high alkaline resistive and acid resistive property.

Polypropylene is a of polyolefin. Polypropylene fiber in the form of fibrillated film fibers show excellent bonding with matrix as the matrix can easily blend into this fibrils thus giving good impact resistance. The nylon and polypropylene have very high tensile strength 561.0 – 867.0N/mm 2. They could be used where high energy absorption is required because their high elongation (15-25%) absorbs more energy. The low modulus of this fiber reduces the reinforcing property. They are extensively used in pile shell, non-load bearing corrosion proof member, cladding panels floatation unit, guniting crack inhibitor. It is a very good steel reinforcement substitute in the aspect of transportation and handling purpose in case of precast components because using plastic fiber reduces the size (thinner section are formed) and increases the crack resistance thereby saving material, transportation and erection cost.

1.3.6 Glass Fiber

Glass reinforced cement consists of 4 to 4.5 % by volume of glass fiber mixed into cement or cement sand mortar. This glass reinforced cement mortar is used for fabricating concrete products having section of 3 to 12mm in thickness. Methods of manufacture vary and include spraying, casting, spinning, extruding and pressing. Each technique imparts different characteristics to the end product. Spray deposition constitutes a very appropriate and by far the most developed method of processing. In the simplest form of spray 7

processing, simultaneous sprays of cement sane mortar slurry and chopped glass fiber are deposited from a dual spray gun into, or onto a suitable mould. Mortar slurry is fed to the spray gun from a metering pump unit and it is atomized by compressed air. Glass fiber is fed to a chopper and feeder unit that is mounted on the same gun assembly. The fibers are manufactured from Glass quarry products.

The glass quarry products are melted in furnace, and then from the process of bushing, the fiber filaments will be obtained . These are best suited for application as renovating construction material for restoration of old heritage buildings and for architectural applications.

1.3.7 Natural Fiber

Natural fiber are fiber consisting of bamboo seed etc., fruit fiber (coir), stem fiber i.e. jute, kenaf, san, flax etc., and leaf fiber like henqueen, sisal, coconut. The cost efficient and energy efficient production of this fiber is a natural advantage. But they have high water absorption, low alkali resistant, are prone to insect and fungal attack and have low elastic modulus making it a deterrent for usage in concrete. Sisal fibers are extracted from agave sisalana leaves. It comprises of pectin, lignin and hemicellulose. They are strong but are prone to alkali attack. Wood fiber or cellulose fiber is the most popularity used natural fiber in concrete. The high modulus of elasticity, tensile strength, and abundance of availability are the major advantage. Wood fiber is extracted from wood by the process called pulping. Wood fiber contains cellulose, hemicellulose and lignin.

Lignin reduces the strength of fiber, hence chemical pulping process 8

called kraft or sulphate is used to remove the lignin. The very low

alkali resistant property of wood fiber can be improved by using

processes that would limit the disintegration of fiber in alkaline

environment.

1.3.8 The Properties of Common Fibers

Table 1 shows the typical properties of various Fibers

Table 1: Typical Properties of Fibers (ACI 544.1R)

Tensile Young’s Ultimate Specific Type of Fiber Strength (MPa) Modulus (Gpa) Elongation % Gravity Acrylic 210-420 2.1 25-45 1.1 Asbestos 560-980 84-140 0.6 3.2 Carbon 1800-2600 230-380 0.5 1.9 Glass 1050-3850 70 1.5-3.5 2.5 Nylon 770-840 4.2 16-20 1.1 Polyester 735-875 8.4 11-13 1.4 Polyethylene 700 0.14-0.42 10 0.9 Polypropylene 560-770 3.5 25 0.9 Rayon 420-630 7 10-25 1.5 Rock Wool 490-770 70-119 0.6 2.7 Steel 280-2800 203 0.5-3.5 7.8

1.4 FORMS OF FIBER REINFORCED CONCRETE

1.4.1 Steel Fiber Reinforced Concrete

Steel fiber reinforced concrete (SFRC) is made with hydraulic

and containing fine or fine and coarse aggregates along with

discontinuous discrete steel fibers. (ACI 544.4R). Steel fibers are

obtained by cutting drawn wires. Fiber can be indented, crimped,

shaped up in irregular order to provide better mechanical bonding. 9

The steel fiber geometry, mixing technique, content, size and shape of aggregates decide the fiber distribution efficiency. Steel fibers are short, length is discrete and has an aspect ratio in the range 20-100 with diameter ranging between 0.15 mm to 1 mm (17). When steel fibers are added to concrete mix they are uniformly distributed and randomly dispersed. This is then called steel fiber reinforced concrete.

SFRC shows a marked increase in impact, strength, toughness ductility, tensile toughness and flexural strength properties compared to plain concrete. But and shrinkage are unaffected by adding steel fiber. SFRC is very effective in controlling the progress of crack into becoming visible cracks, and also improves impact and abrasion resistance. The SFRC is used in refractory linings, blast resistance structure, tunnel linings, pavements and unit.

Balling or clumping is a problem when these fibers are used in concrete in higher percentages and for aspect ratios greater than

100 (5).

1.4.1.1 The Applications of Steel Fibers

1. Cast in place application involve slabs on grade in the form of

industrial and parameters.

2. Repairs and re-habitation of marine structures.

3. Slip formed cast in place tunnel lining.

4. Precast panels.

5. Highway construction and repair

6. Airport runways, taxiways, aprons

7. construction and repair 10

8. Structural building elements

9. Railroad ties, Machine bases and frames

1.4.2 Glass Fiber Reinforced Concrete

Russians were the first to realize the potential of glass as a construction material in the 1940’s. But since the glass has very low alkali resistance it became very difficult to mix it with concrete which is alkaline in nature. Thus a better glass which is alkali resistant was made by adding Zirconium to the slurry in 1970 by the British.

Several manufacturing processes for producing glass fiber reinforced concrete premix products have been developed, such as casting, spray premix, press molding, extrusion, and pultrusion.

Glass fiber-reinforced concrete premix is a mixture of AR glass fiber, sand, cement, water, chemical and mineral admixtures, and aggregate

(7, 62). These fibers reduce crack width and spacing between cracks.

They are very high temperature resistant as they absorb high energy thereby providing the property of ductility. Their light weight property makes them very popular for concrete mix. They have found varied use in industry today. They are used as sound reducers when used in thickness of 10 mm and surface mass of 20 kg/m 2. They are used for repair material for historical buildings and also for extension of old buildings. Any shape product can be formed with good binding strength due to their excellent design flexibility. They are used in sewer relining, earth retaining , architectural product as building facades, claddings, cable troughs and noise protection barrier.

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1.4.2.1 Advantage of GRC

The GRC is

1. Light weight

2. Improved shrinkage properties over plain concrete

3. Accurate properties of GRC, environmental friendly.

4. Used in precast manufacturing.

5. Anti corrosive, highly resistant to chemicals, High flexural

strength, Impact strength and tensile strength.

6. Excellent design flexibility. Any shape products can be formed

with good bending strength.

7. Considerable heat saving is achieved because of low thermal

conductivity.

8. Specific resistance of Glass is higher therefore can be used to

manufacture strands.

9. The thickness of glass filament varies between 10 micron to 20

micron.

The Alkali Resistant glass fibers are manufactured by Saint

Gobain, Cem-FIL (22) are used in the experimental investigations and are purchased from Ecmas Resins Pvt. Ltd. Hyderabad and designated as ARC 14 306 HD chopped strands, where the notations ARC is

Alkali Resistant Continuous filament. 14 is the filament diameter in micron, 306 is tex of strand and HD is high dispersion product code.

1.4.2.2 Comparison of GRC with other Fibers

Though polypropylene is cheaper and easily available the flexural strength of polypropylene is very less compared to GRC. Aramid and 12

carbon fibers have better strength than GRC but cost factor makes them unpopular. The property of is almost same in

Steel and GRC but then GRC can be moulded in any shape which is not the case with steel fiber. GRC are of three types, continuous strands, mesh type and small cut strands type. Glass fiber types readily available are E, R, D, C and AR. AR glass is most suitable for

GRC.

1.4.3 Hybrid Fiber Reinforced Concrete

When two different fibers are mixed in a common matrix we call it hybrid fiber reinforced concrete. The fibers are chosen such that their properties complement each other. That is if one kind of fiber is stiff and strong we can choose the second fiber as flexible and ductile.

With this combination we can achieve improved first crack , ultimate strength and improved strain and toughness after cracking.

This Hybrid fiber reinforced concrete has achieved excellent results for bridging the early micro cracks and macro cracks in the later stages by choosing the appropriate fiber. The crack bridging capability will make low modulus fiber to increase the strength of the matrix. But excessive presence of fiber is also harmful as it may lead to defect during production because fiber and particles will not achieve the required packing thus reducing the strength. So the quality of fiber shall be carefully chosen.

1.5 USE OF ADMIXTURES

Varied properties of cement can be obtained by changing the percentage of ingredient in the cement and if this becomes economical an extra ingredient is added 13

to get the disused property for cement. Thus any material that is added to the cement aggregate and water is called admixture.

Admixture could alter the setting time of cement, improve workability, provide pigmentation, air entrainment etc. They could be added before or during mixing.

1.5.1 History of Admixture

The usage of admixtures or additives dates back to Roman and

Egyptian times, where volcano ash was used to allow to set under water, horse hair was used to reduce cracks during hardening process and addition of blood resulted in frost resistance structure.

1.5.2 Types of Admixtures

The admixtures that could bring about air entrainment are gas performing agent like aluminum, zinc powder hydrogen peroxide or using surface tension reducers like vinsol, resin, animal and vegetable fats. Air entrainment improves the workability and makes the mix frost free. So it is used in places where resistance to weather is more important than strength of concrete. It may be required to delay setting time of cement specially in tropical countries, oil well constructions and soil cement stabilization. Delayed setting time is obtained reducing the rate of chemical reaction. The popular additive used is gypsum or plaster of paris. This is added during manufacture of cement. The other additives that could be used are Na 2Co 3, FeCl 3, tannic acid, gallic acid and sulphonic acid etc. Sometimes it may be required to speed up the setting time to shorten curing time and quickly bring the structure to service or to offset lower temperature 14

retardation effect. The commonly used additive for this purpose is

CaCl 2. This reduces the setting time from 3 hr. to 1 hr. But if 3 % of

CaCl 2 is added, the setting time reduces drastically there by disturbing volume stability. The other additives for this purpose are

Na 2SO 4, K 2SO 4, NaOH, and KOH. Water proof property is displayed by concrete when it has proper mix design and low water cement ratio.

The water proof property could be increased by using pore filling material or water repellant materials. Pore filling material like Al and

Zn sulphates, silicates of soda, Al 2Cl 3, CaCl 3, talc, chalk and fuellers earth (multani mitti) are some of the pore filling materials. The water repellant materials are like soda, potash soaps, vegetable oils, waxes, fats and coal tar residue. Another very important additive that has revolutionized the concept of building materials are the pozzolonas.

These materials have no cementitious properties of their own, but react with lime in presence of water to form compound which have low solubility and cementation properties. Pozzolana derives its name from

Pozzuoli, a place in Italy near the volcanic mountain of Vesuvius. The main attraction of this is it allows the cement to set under water and is also corrosion resistive. Some industrial pozzolanas are fly ash, silica fume, rice husk ash, ground granulated blast furnace slag and metakaolin. Some naturally occurring pozzolanas are clay sand shales, diatomaceous earth, pumices, volcanic tuffs etc. All these admixtures are also called mineral admixtures (57).

1.5.3 Microsilica in concrete 15

Microsilica otherwise referred to as condensed silica fume or just silica fume (1) has evolved as a major almost replacing the good old binder. This is because of its extreme finesse that can easily move into the particles of cement thereby enhancing the packing of the mix. The micro silica consists of glassy sphere of . This silicon dioxide is obtained when pure quarts and coal are reacted in an electric arc furnace emitting silicon oxide. This silicon oxide combines with oxygen present in the upper part of furnace to form silicon dioxide (SiO 2) which is condensed into spherical form of pure silicon dioxide. The particle size is so small that it does not exceed 0.5 microns which makes it so fine that it could more than compete for the finest binding material available. The specific gravity of this micro silica is 2.20. When the micro silica is mixed with cement, it enhances the properties like compressive strength, durability, resistance to chemical attack and reduces the undesirable properties like water permeability and chloride ion permeability.

The high binding property of microsilica arises because it combines with the hydroxide generated during hydration process and forms calcium silica hydrate (CSH) which enhances the strength and durability of concrete. The large surface area of micro silica along with its increased packaging property contributes to the prevention of bleeding and segregation. Addition of micro silica brings about a refinement of the concrete pore structure and reduces the amount of pores there by making the mix almost impenetrable for 16

chloride ions. This makes silica fume or micro silica an Ideal mineral admixture which can be used along with cement in concrete mix to build structures in marine environment, coastal areas, and sewerage treatment plant and shotcrete projects. Sulphate attack on concrete is another major factor for premature concrete failure. Sulphate reacts with forming gypsum. This gypsum in turn reacts with tricalcium aluminates (C 3A) in concrete to form mono sulfoaluminate and . This formation results in increase in volume thereby resulting in cracking or peeling, the addition of microsilica reduces the Ca(OH) 2 content as it forms tricalcium silicates. The reduction of Ca(OH) 2 which is the main cause of the other reaction makes sulphate ions impenetrable in concrete. Thus micro silica makes the material very safe to be used in sulphate environment like sea water and also ground water thereby protecting the foundation of structure in such places as concrete pipelines and marine structures. Water seepage is another major problem especially in concrete structures, where water presses against the concrete surfaces. The microsilica mixed in concrete reduces water permeability thereby preventing dampness and seepage thus making it useful during construction of swimming pools, tanks, sewerage treatment plant and underground structures. Concrete with microsilica exhibits high abrasion resistance property. This makes it very suitable in environs of high wear and tear like highway toll booths where braking zones and accelerating zones are present and also on pavements. The microsilica used in these experimental 17

investigations is densified microsilica 920 D supplied by M/S Elkem

India Pvt. Ltd. Mumbai. The typical bulk density is ranged between

500 Kg/m 3 – 700 Kg/m 3, conforming to standard ASTM C 1240 (20) is shown in Table 1.1.

Table 1.1 Properties of Microsilica 920 D

1.5.4 Super Plasticizer

It is desirable to have lower water cement ratio during the making of concrete because this will reduce capillary pores, this in turn reduces permeability there by giving a high durable concrete, but the problem is lower water cement ratio reduces workability. In order to solve this problem the plasticizers and super plasticizers were introduced. This water reducing admixtures when introduced into the 18

manufacture of concrete lowers the water requirement by 20 % to 40

% without reducing the workability leading to high strength concrete.

Plasticizer and super plasticizers comes under the category of chemical admixtures. Plasticizers are manufactured from a by product of paper industry called lingo sulphonates. The super plasticizers otherwise referred to as high range water reducers are organic manufactured from sulphonated naphthalene condensate.

This lignin and naphthalene sulphonate wrap their long molecular chain around the cement particles giving them a negative charge. As the like charges will repel, the concrete shows good workability with less water. Nowadays poly carboxylate ether super plasticizers are being used. They work on the principles of steric stabilization.

1.6 OBJECTIVE AND SCOPE OF THE PRESENT RESEARCH

WORK

The objective of the present work is to investigate experimentally various properties of Mixed Fiber Reinforced Concrete (MFRC) for structural applications. To develop MFRC that can overcome the draw backs like inherent brittleness, multiple cracking under crushing loads and not so well defined flexural behaviour particularly when used with random orientation. Another drawback with SFRC is the balling effect particularly when the fiber percentage is more than 1.0 with aspect ratios of more than 40 that were observed in the literature review, an approach is made by using mixed fibers of steel and glass

(MFRC) to various proportions are studied in various total fiber 19

percentages along with certain percentage of microsilica, an optimum

Mixed Fiber Reinforced Concrete is proposed.

In the present research study it is planned to make use of glass fiber for structural concrete. The study includes the introduction of glass fiber into concrete at various percentages, combining glass fiber with steel fiber to enhance certain properties like elastic properties, durability properties, flexural properties etc.

It is hoped that mixed fibers with steel and glass would be accepted for making of structural components.

1.7 METHODOLOGY

The present thesis work aims to make use of mixed fiber concept in fiber reinforced concrete and covers the following aspects

1. Study of Basic strength properties of glass fiber reinforced concrete

(GFRC) at various percentages.

2. Study of Basic strength properties of mixed fibers of glass and steel

at various percentages (MFRC).

3. Study of Elastic properties of mixed fiber reinforced concrete

(MFRC)

4. Study of Impact strength of mixed fiber reinforced concrete (MFRC).

5. Study of Permeability properties of mixed fiber reinforced concrete

(MFRC)

6. Study of Chemical resistance properties of mixed fiber reinforced

concrete (MFRC).

7. Testing of Specimen RCC Beams of MFRC.