Cement
The reinforced and/or pre-stressed concrete precast element could be used. The behaviour of reinforced and pre-stressed concrete under service loads and at the ultimate limit is heavily influenced by the material composition and manufacturing process. The versatility of this structural and architectural product is enhanced by factory production. It is critical that specifiers and users understand the nature of the materials used in precast concrete, which this chapter helps to accomplish.
For satisfactory performance in a structure, cement should meet the requirements of 456: 2000. The cement standard divides it into two broad categories: general purpose and multipurpose.
– Multipurpose: In precast concrete construction, ordinary Portland Cements [OPC] 43 grade [IS:8112] and 53 grade [IS:12269] are commonly used. High alumina cement should not be used in precast concrete because it reverts and loses a significant amount of strength in warm and humid conditions. When mixed with Portland cement, it may cause a ‘flash set.’ Precast manufacturers also offer advice on the implications of different mix designs, including the cement type.
SCMs (Supplemental Cementitious Materials)
Fly ash, ground granulated blast-furnace slag, Metakaolin, and silica fume are examples of supplementary cementitious materials [SCMs]. They must meet the requirements of the relevant parts of IS:12089 for GGBS and IS:15388 for silica fumes. Reduced cost, improved workability, lower heat of hydration, improved durability, improved chemical resistance, and increased long-term strength are all advantages of using SCMs. They should not be used in large areas where adjacent elements can be seen at the same time and a uniform colour is required. Steam curing improves the hydration of SCMs. Adequate guidance on best practises for using silica fume should be provided.
Aggregates
IS:383 requirements should be met by general aggregates. It includes clauses that address a variety of properties that must be known for mix design, such as particle density, water absorption, and particle size distribution material finer than 75/um, project specifications, and so on. Special Aggregates may be required to provide desired architectural features such as colour and texture for surfaced exposed aggregates. When using special aggregates, it may be advantageous to stockpile them at the start of the project to minimise colour and other variations in supply during the project. The most uniform exposed aggregates surface will be provided by gap-graded aggregates.
Water
Water should be free of impurities that would reduce the strength and durability of the concrete. When recycled water is used, the proportion of soluble salts and alkalis in the concrete may increase. Testing should be performed to ensure that the limits are not exceeded, and where potentially alkali-reactive aggregates are used, the guidelines’ limits should be followed. Chemical Admixtures When compared to the manufacturer’s declared values, chemical admixtures should meet the requirements of IS: 9103- 1999. Calcium chloride should not be used as a direct additive in reinforced or prestressed concrete because it can cause reinforcement corrosion, posing a severe durability risk.
Pigments
Pigments are extremely fine particles. They do not dissolve or stain the concrete, but are distributed as fine solids throughout the matrix and bound in the same way as the other aggregates.
- To avoid interfering with the chemical reaction between cement and water, pigments should be chemically inert.
- Alkali resistance to prevent water leaching; pigments can be natural or synthetic. Because of its controlled chemical composition and extremely fine particle size, the latter type provides a superior product. Nature oxides have a low tinting strength by definition.
Augmentation
Steeling reinforcing bars, prestressing tendons, steel fibres, and non-metallic reinforcement are the most commonly used reinforcement materials. After fabrication, reinforcement should be clean and free of harmful matter that could impair the bond with the concrete, such as loose mill scale, loose rust oil, grease, and retarders.
Reinforcing Bars
IS: 1786-2008 covers high strength deformed bars for concrete reinforcement, IS 432 covers mild steel and medium tensile steel bars, and IS 432 covers hard-drawn wires. Bars are classified according to their shape, ductility class, strength grade, and size. Welded wire fabric reinforcement is widely used in developed countries, and its popularity is growing in India.
Tendon Strengthening
Steel tendons for prestressed concrete can take the form of wires, strands, or bars. For pre-tensioned members, wires and strands are commonly used. The most common system for post-tensioned members is a 7-wire strand system. Prestressing tendons should be clean and free of harmful matter such as loose mill scales, loose rust, mud, oil, grease, or any other coating that may weaken the bond between the concrete and the steel.