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Unit 3 Materials for Prestressed Concrete

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Unit 3 Materials for Prestressed Concrete 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.
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