UNIT 3 MATERIALS FOR PRESTRESSED CONCRETE

Structure 3.1 Introduction Objectives 3.2 Materials 3.3 Some Phenomena Related with Steel 3.4 Summary 3.5 Answers to SAQs

INTRODUCTION

We know that concrete in prestressed concrete members is subjected to high stresses. These high stresses may be produced due to a high value of the prestresseses or due to a combination of prestresses and other stresses (produced due to self weight and external loads). We also understand that steel tendons used in prestressed concrete members must have a high value of ultimate strength. Mild steel or even High Yield Strength bars may not be used as tendons as sufficiently high values of stresses (which are required to be introduced in tendons) can not be induced in these materials. It is not only the strength of these materials, namely concrete and steel, which affects the performance of prestressed concrete mkmbers. Other properties such as shrinkage, creep, maximum elongation - to name a few of those properties - are also important. Only if we have a clear concept of these properties and their likely effects on the performance of the materials, we shall be able to assess the likely performance of structural members, constructed by using these materials. Objectives After studying this unit, you should be able to understand the properties of concrete and steel that affect the properties and performance of prestressed concrete members, understand how these properties of concrete and steel affect the properties and performance of prestressed concrete members, and appreciate the standard guidelines, given in this regard, in the Indian Standard Code of Practice for prestressed concrete.

3.2 MATERIALS

Properties of various materials used in prestressed concrete must suit the requirements which are connected with the performance of prestressed concrete. In prestressed concrete we require a high strength - compressive as well as tensile - at an early age. It means that the rate of development of strength of concrete used in prestressed concrete construction should be high. This shall help us in improving the speed of construction of structural members, especially in pre-tensioned construction, as we can transfer the prestress to the concrete at an Prestressed Concrete early age. A greater strength, compared to that of reinforced cement concrete, is generally required in prestressed concrete as the compressive stresses induced in prestressed concrete are generally of a higher order of magnitude. Low shrinkage, minimum creep characteristics and a high value of Young's modulus of elasticity are generally deemed necessary for prestressed concrete. In the unit devoted to 'Losses of prestress' you shall learn how these properties influence the losses of prestress which take place in prestressed concrete. We can notify some of the needed requirements of prestressed concrete in terms of various properties as under.

A high value of strength - compressive, tensile and shear strengths. These properties of concrete may be associated with a high value of Young's modulus of elasticity, greater density, etc. Low early shrinkage and small creep deformations. These properties of concrete are associated with the mix of concrete and are influenced by the richness of mix and water-cement ratio. Durability of concrete. This property is influenced by the quality of concrete. The durability of concrete depends on its resistance to deterioration and the environmental conditions around concrete. The resistance of concrete to weathering, abrasion, chemical attack, frost and fire depends on its quality. The strength of concrete alone is not a reliable guide to the quality and durability of concrete. For having a durable concrete, it is important to have a low water-cement ratio of the concrete mix and an adequate quantity of cement. Alongwith this, low permeability of concrete also is desired. It can be obtained by thorough compaction of the concrete mix. Several guidelines have been provided in the code so that a durable concrete could be obtained. Following details should give us a better understanding of types of different constituents of prestressed concrete. These details have been taken from the recommended standard guidelines of various Indian codes of practice. Cement Any of the following types of cements may be used in prestressed concrete construction. (a) Ordinary Portland cement, (b) Portland slag cement, but with not more than 50% slag content, (c) Rapid hardening Portland cement, and (d) High strength ordinary Portland cement. Aggregates in Prestressed Concrete Aggregates having a maximum nominal size of 20 mrn or smaller are generally considered satisfactory. Coarse and fine aggregates should be batched separately. All aggregates should comply with the requirements of IS : 383-1970. The nominal maximum size of the aggregate should be as large as possible subject to the following : (a) It should not be greater than one-fourth the minimum thickness of the member. (b) It should be 5 rnrn less than the spacing between the cables, Materials for strands or sheathings provided in the member. Prestressed Concrete (c) It should not be more than 40 mm. Water The requirements of water used for mixing and curing should conform to the requirements given in IS : 456-1978 (Indian Standard Code of Practice for plain and reinforced concrete [third revision]). The use of sea water in the construction of prestressed concrete members is prohibited. This is because the salts in the sea water can deteriorate the quality of concrete and may lead to corrosion of reinforcement and tendons. Both the quality and quantity of water used in a concrete mix are important. For this purpose, water-cement ratio of the concrete mix as well as other parameters such as permissible limits of chlorides and sulfates present in mixing water are specified in the code. Use of Admixtures in Concrete The use of chemical admixtures may be made in prestressed concrete construction. But the admixture should not contain chlorides in any form. This guideline is once again to protect tendons from corrosion as chlorides help the corrosion of steel inside concrete. The admixtures should conform to IS : 9103-1979 (Specifications for admixtures for concrete). Concrete Mix Proportions Concrete mix proportions should be chosen in such a way that concrete of adequate workability is obtained. Workability of concrete mix should be such that it can be compacted well using the available means of compaction. The recommended values of workability of concrete can be . taken from IS : 456-1978. Concrete should surround all the tendons and other reinforcement well and should fill the formwork space completely so that the maximum density is achieved. When concrete hardens, it should have the required strength, durability and surface finish. Determination of the proportions of cement, aggregates and water to achieve the required strengths in concrete is made by designing the concrete mix. Such a concrete is called as a 'Design Mix concrete'. Use of only the design mix concrete is recommended in the case of prestressed concrete construction. A maximum cement content of 530 kg/m3 is specified in the code so that shrinkage stains of concrete may be restricted within limits. In specifying a particular grade of concrete, following information about concrete should be specified : Grade designation Type of cement to be used Maximum nominal size of aggregates Minimum and maximum cement content for the mix Maximum water-cement ratio of the mix Workability to be achieved Type of aggregates to be used Use of any admixture, if required, stating the conditions of use Prestressed Concrete The design mix shall provide the required strength and workability in the concrete. The Indian standard code procedure for the design of concrete mixes should be followed for the design of concrete mixes. In the case of prestressed concrete construction, no hand mixing of concrete is permitted by the code. With the application of vibration techniques, use of chemical admixtures, etc. we can produce concrete of a compressive strength in the range of 70-100 N/rnrn2. These concretes, called Ultra High-Strength concretes, are required to be used in prestressed concrete construction. Experimental investigations, in recent years, have shown that in high-strength concrete mixes, workability, type of aggregates (in terms of size, shape, their strength, porosity, permeability, water absorption, surface texture, etc.) and the strength requirements influence the selection of water-cement ratio. High-strength concrete mixes can be designed by using any one of the following methods : e Indian standard code method. Erntroy and Shacklock's empirical method. Mix design procedure of American Concrete Institute. British method, based on the work of Emtroy, which has replaced the Road Note No. 4 method of concrete mix design. Light-weight Aggregate Prestressed Concrete The use of light-weight aggregate concrete for prestressed concrete construction is well established. The main advantage of light-weight concrete is that it reduces the self-weight of the structural components. Because of this the amount of concrete and prestressing steel required for carrying the load is reduced. This type of concrete becomes important in long span structures where the self-weight of the member is an important factor to be considered for the determination of the design load of the structure. This eases the transportation of the members also. In the present state of the art, it is possible to produce high-strength light-weight concrete of 28 day cube compressive strength in the range of 30 to 50 N/rnm2. The light-weight aggregates generally used for prestressed concrete are foamed slag, lytag and aglite. The modulus of elasticity of light-weight concrete is about 50 to 55% of that of normal-weight concrete. The loss of prestresses in light-weight concrete due to elastic deformation is higher than the normal-weight concrete. But shrinkage and creep are comparable to those in normal-weight concrete. Moulds in Prestressed Concrete Moulds for pre-tension work should be sufficiently strong and rigid so that they can withstand the effects of placing and compacting the concrete without any distortion. Some moulds are required to withstand the tendon forces before the transfer of prestress. They should be sufficiently strong in this respect. Grout for Prestressed Concrete We know that in grouted post-tensioned members, ducts are filled with grout. The grouting material should be fluid and it should give low sedimentation or bleeding in the plastic stage. In the hardened state, it Materials for should be dense and durable and should be subject to low shrinkage. A Prestressed Concrete grouting technique which ensures the easy filling of ducts with the grout material should be adopted. Grout should be made with any type of recommended cement and using water of a required quality. If very large size ducts are to be grouted, fine sand passing 150 micron sieve size can be used in the grout. Admixtures may also be used in the preparation of grout. The code specifies that water cement ratio for the neat cement grouts should be approximately 0.50 by mass. It should never exceed 0.55 by mass of cement. The compressive strength of the 100 mm cubes of the grout material should not be less than 17 N/mrn2 at the age of 7 days. It is specified that the cubes should be cured in a moist atmosphere for the first 24 hours and subsequently in water.

I Properties of Concrete " 1 i i Following are some standard guidelines prescribed in the Indian standard code IS : 1343-1980 with respect to quality of materials used in prestressed concrete. The minimum 28 day compressive strength of concrete cubes of 15 cm size, prescribed in the Indian standard code IS : 1343-1980, is 40 N/mm2 for pre-tensioned concrete and 30 N/dfor post-tensioned concrete. The characteristic strength of concrete is defined as the strength of the concrete below which not more than 5 percent results are expected to fall. The code specifies standard guidelines regarding sampling and strength test of concrete to be use in prestressed concrete. .These requirements are given in the table below. Optional Test Requirement of Concrete

Grade of Assumed Compressive Strength Modulus of Rupture by Concrete Standard of 15 cm cubes Beam Test (Minimum) Deviation in (Minimum at 7 Days) (N/mm2) (N/mm2) At 72 * 2 At 7 Days Hours (N/mm2) (N/mm2) M 30 6.0 20.0 2.1 3 .O M 35 6.3 23.5 2.3 3.2 M 40 6.6 27.0 2.5 3.4 M 45 7.0 30.0 2.7 3.6 - M 50 7.4 33.5 2.9 3.8 M 55 7.7 37.0 3.1 4.0 M 60 7 8 40.0 3.3 4.2 Strength of Concrete at the Transfer Stage Concrete strength in the member at the transfer stage should conform to the design requirements. The frequency of the sampling and the number of cubes to be cast for this purpose has to be decided by the engineer-in-charge. Sampling of concrete shodd preferably be at the point of placing and the cubes should be stored as far as possible under the same conditions as the concrete in the members. Prestressed Concrete Increase in Strength with Age If a member is not supposed to receive its full design stress within a period of 28 days after the casting of the member (It may happen in the case of foundations or ground floor columns) the characteristic compressive strength of the concrete may be increased by multiplying by the factors given below :

Minimum Age of Member Age Factor When Full Design Stress is Expected (Months) 1 1.o 3 1.10 6 1.15 12 1.20 In this respect, it is to see that when members are subjected to lower direct load during construction, they should be checked for stresses resulting from combinations of direct load and bending during construction. It is also recommended by the code that the design strength shall be based on increased value of compressive strength in such cases. Tensile strength of concrete Tensile strength of concrete is defined by the code to be equal to 0.7 times of square root of the characteristic strength of concrete. This expression is to be used when the designer wishes to use an estimate of the flexural strength of concrete from the conlpressive strength. Otherwise, the flexural strength may be obtained from IS : 5 16- 1959, which specifies tests for concrete for strength.

f,, = 0.7 (f,,

where, fcr = Tensile strength of concrete in N/mm2, and fc, = characteristic strength of concrete in 1V/mm2. Modulus of Elasticity of Concrete The Young's modulus of elasticity of concrete is primarily influenced by the elastic properties of the aggregate and to a lesser extent by the conditions of curing and age of concrete. The mix proportions of concrete and type of cement also affect it. It is normally related to the compressive strength of concrete. If any test data for the determination of the modulus of elasticity of concrete is not available, it may be assumed in (N/mm2) as follows : 1 E, = 5700 (f, )Z This modulus of elasticity of concrete is called as short term modulus of elasticity as long term effects are not considered in this expression. Shrinkage of Concrete Shrinkage of concrete is primarily due to gradual loss of moisture which results in change in volume. The shrinkage of concrete depends on the mix constituents of concrete, size of the member, type of member and the environimental conditions. It is most influenced by the total amount of water present in the concrete at the time of mixing and by the cement Materials for content to a lesser extent. For pre and post-tensioned concrete, the code Prestressed Concrete gives the following values of shrinkage strains. For pre-tensioning, shrinkage strain = 0.0003 For post-tensioning, shrinkage strain = 0.0002/Log,, (t + 2) Where, 't' is the age of concrete at transfer (in days). The rate and amount of shrinkage of the structural members under ambient conditions will depend very much upon the ratio of surface area to volume of the member. Aggregates of rock types having high moduli of elasticity and low values of deferred strains are more effective in restraining the

I contraction of the cement paste and their use reduces the shrinkage of concrete. Light weight aggregates with low modulus of elasticity may lead L b to higher shrinkage than normal concrete. The vaiue of shrinkage strain for post-tensioned concrete may be increased by 50% in dry atmospheric b conditions. But in no case it can exceed 0.0003. For the calculation of deformation of concrete at some stage before the maximum shrinkage strain is reached, it may be assumed that half of the shrinkage takes place during the first month and about 75% of it takes place in the first six months after commencement of drying of concrete. Creep of Concrete Creep of concrete is another important characteristic. It represents progressive strains in concrete which occur at constant value of stresses. It depends on the level of stress in the concrete, age at loading and the duration of loading in addition to those factors which affect the shrinkage of concrete. As long as the stress in concrete does not exceed one-third of characteristic strength of concrete, creep may be assumed to be proportional to stress. The ultimate creep strain may be estimated from the following values of creep coefficient (which is defined as the ratio of ultimate creep strain and elastic strain at the age of loading). The ultimate creep strain does not include the elastic strain. Age at Loading Creep Coefficient 7 days 28 days 1 year For the calculation of deformation at some stage before the total creep is present, it may be assumed that about 50% of total creep takes place in the first month after loading and about 75% of total creep takes place in the first six months after loading. Thermal Expansion of Concrete For determining the thermal expansion of concrete, IS : 456-1978 may be referred to. It depends on the nature of cement, the aggregate, the cement content, the relative humidity and the size of sections, in addition to other factors. Dumbility Requirements for Concrete Even though the alkaline environment of concrete protects steel, carbonation of hydrated cement may reduce the effective protection. The codes provide guidelines regarding the minimum cover to be provided to the reinforcement and the density of concrete for this purpose. Prestressed Concrete The Indian code provides for a minimum clear cover of 20 mrn for protected pre-tensioned concrete members while it is 30 mm or the size of the cable (whichever is larger) in the case of post-tensioned members. If the prestressed members are exposed to an aggressive environment, these cover requirements are increased further by 10 mm. In the Indian standard code, the minimum amount of cement and the maximum value of water- cement ratio of the concrete mix is recommended for this purpose. Prestressing Steel For prestressed concrete members, the high-tensile steel, used generally, consists of wires, bars or strands. The high tensile strength of steel is generally achieved by marginally increasing the carbon content in steel in comparison to mild steel. High-tensile steel usually contains 0.6 to 0.85% carbon, 0.7 to 1% manganese, 0.05% of sulphur and phosphorus. The high carbon steel ingots are hot rolled into rods and cold drawn through a process of dies to reduce the diameter and increase the tensile strength. The durability of wires gets improved due to the cold-drawing operation. The cold-drawn wires are then tempered to improve their properties. Tempering or ageing or stress relieving by heat treatment of wires at 150-420°C improves the tensile strength. These cold-drawn wires are generally available in nominal sizes of 2.5, 3,4,5,7 and 8 mm diameter. The prestressing steel, as per the code, should be any one of the following types : Plain hard-drawn steel wire conforming to IS : 785(Part 1)- 1966 and IS : 1785(Part 2)-1967, , Cold-drawn indented wire, High tensile steel bar conforming to IS : 2090- 1962, and Uncoated stress relieved strand conforming to IS : 6006- 1970. Hard-drawn steel wires which are indented or crimped are preferred for pre-tensioned members as their bond characteristics are superior to the plain wires. Strands normally comprise two, three or seven wires of 2 to 5 mm size. The helical form of twisted wires in the strand improves the bond strength. Two and three-ply strands are made up of 2 mm and 3 mm diameter wires while a seven-ply strand is made by twisting 2 to 5 mm wires. High-tensile steel bars commonly used in prestressing are manufactured in nominal sizes of 10, 12, 16, 20,22, 25, 28 and 32 mm diameter. The ultimate tensile strength of bars does not vary appreciably with the diameter. This is because the high strength of the bars is due to alloying rather than due to cold-worlung as in the case of wires. The minimum characteristic tensile strength of high-tensile strength bars as per code i.s 980 N/mm2. Their proof stress should not be less than 80% of the mini:mum specified tensile strength. Elongation at rupture should be 10% for the specified gauge length. It is specified that all prestressing steel should be free from splits, harnnful stratches, surface flaws, rough, jagged and imperfect edges and other defects likely to impair its use in prestressed concrete. Slight rust on the surface of reinforcement may be permitted provided there is no visibl~e surface pitting. If any coupling is made or any other similar fixture is used in conjunction Materials for with wires or bars, it should have an ultimate strength not less than the Prestressed Concrete individual strength of the wires or bars which are being joined. It is specified in the code that the modulus of elasticity of steel tendons should be determined by tests on samples of steel to be actually used in construction. Otherwise the value may be obtained from the manufacturer of the steel. If it also is not possible, then the following values may be used.

Type of Steel Young's Modulus of Elasticity (E) in kN/mmZ Plain cold-drawn wires 210 High tensile steel bars rolled or 200 heat treated Strands 195 The ultimate tensile strength of a plain hard-drawn steel wire varies with its diameter. The tensile strength decreases with increase in the diameter of the wires. Tensile strengths and elongation characteristics of cold-drawn stress relieved wires as per IS : 1785 (Part1)-1983 is as given in the following table.

Permissible Stresses in Steel As per the Indian code, the permissible stresses in the tendons at the time of initial prestressing should not exceed 80% of the characteristic tensile strength of the tendons. The final prestresses after allowing for all losses of prestress should not be less than 45% of the characteristic tensile strength of tendons. Untensioned Reinforcement In prestressed concrete construction, use of untensioned reinforcement also has to be made alongwith tendons. This is necessary as tensile stresses produced due to prestress act in lateral directions. These stresses are taken up by untensioned reinforcement which is provided in addition to tendons in lateral as well as longitudinal directions. The untensioned reinforcement is also needed to take on stresses produced during the transport of the prestressed concrete members. Reinforcement used as untensioned steel should be one of the following types : (a) Mild steel and medium tensile steel bars, (b) Hot-rolled deformed bars, (c) Cold-twisted bars, and (d) Hard-drawn steel wire fabric. Prestressed Concrete 3.3 SOME PHENOMENA RELATED WITH STEEL

Relaxation of Stress in Steel If we keep a high-tensile steel wire stretched and maintained at a constant strain, the value of the force in the wire does not remain constant but decreases with time. This phenomenon is known as relaxation of stress in steel. Due to relaxation of stresses in tendons, loss of prestress takes place in prestressed concrete members. The values of loss of prestress due to relaxation of steel are specified in the code. Stress Corrosion Stress corrosion takes place in prestressed concrete tendons due to the combined actions of corrosion and static tensile stresses. This type of attack in alloys is due to the internal metallurgical structure of steel and may lead to brittle fractures. Other possible types of corrosion in steel tendons are pitting corrosion and chloride corrosion. Hydrogen Embri ttlement As a result of the action of acids on high-tensile steels, liberation of hydrogen takes place. It penetrates into the steel surface and makes it brittle and fracture prone. Even small amounts of hydrogen can cause significant damage to the tensile strength of high-tensile steel wires. Use of high-alumina cement and blast furnace slag cement can cause hydrogen embrittlement in wires. In order to prevent it taking place, it is essential to properly protect steel from the action of acids. Protective coverings should be provided over the tendons for this puruose.

(a) Why a larger value of characteristic strength is specified for pre-tensioned members than in post-tensioned members? (b) Write names of some methods for the design of concrete mixes for prestressed concrete.

3.4 SUMMARY

Prestressed concrete is a material different from other conventional construction materials as it is subjected to prestresses along with other stresses produced due to various internal and external factors. Because of this fact, a prestressed concrete member generally behaves in a different manner. Due to the presence of a high degree of stresses in prestressed concrete members, a better material quality control is generally needed. For this purpose, Indian standard guidelines with regard to the materials, used in prestressed concrete construction, are available. We have also to appreciate that there are some properties and phenomena intrinsic to steel and concrete which may change the material behaviour with time. All of the above aspects have been discussed in this unit to give the required Materials for basic knowledge to the student. Understanding of some of these phenomena - Prestressed Concrete such as creep and shrinkage of concrete and relaxation of steel - shall go a long way in understanding various losses of prestresses in such types of members.

3.5 ANSWERS TO SAQs

Please refer the preceding text for all the Answers to SAQs.

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