Materials for Prestressed Concrete
High strength concrete mix:
- Pre-stressed concrete requires concrete which has a high compressive strength, with comparatively higher tensile strength.
- Low shrinkage, minimum creep characteristics and a high value of Young’s modulus are generally deemed necessary for concrete used for concrete used for pre-stressed members.
- A minimum cement content of 300 to 360 kg/m3 is prescribed mainly to cater to the durability requirements. In high-strength concrete mixes, the water content should be as low as possible with due regard to adequate workability.
- To safeguard against excessive shrinkage, the code prescribes that the cement content in the mix should preferably not exceed 530kg/m3.
- Aggregate of rock types having high moduli of elasticity and low values of differed strain are more effective in restraining the contraction of the cement paste and their use reduces the shrinkage of concrete. The commonly used aggregates, in increasing order of effectiveness in restraining shrinkage, are sand-stone, basalt, granite, quartz and limestone.
- The values of total residual shrinkage strain recommended in the I.S. code for the purpose of design are 3.0X10-4 for pre-tensioned members and(2.0X10-4)/log(t+2) for post-tensioned members, where t is the age in days of the concrete at transfer.
LIGHT-WEIGHT AGGREGATE PRE-STRESSED CONCRETE:
- The main advantage of light weight concrete is that it reduces the self –weight of the structure, thus minimizing the amount of concrete and steel required for carrying the load.
- The light weight criterion becomes important especially in long span structures where dead load forms the major portion of the total design load on the structure, or when the self-weight of the member is a factor to be considered in the transportation and erection, as in pre-cast concrete construction.
- The light-weight aggregates, generally used for pre-stressed concrete are foamed slag, lytag and aglite.
- The modulus of elasticity of light-weight concrete is about 50 to 55 percent of the modulus of elasticity of normal- weight concrete and hence the loss of pre-stress due to elastic deformation is higher and deflections of flexural members are comparatively higher due to the lower values of modulus of elasticity.
- The unit weight of light-weight concrete varies considerably between 1450 and 1750 kg/m3.
- The shrinkage and creep of light-weight concrete is comparable, with marginal variations, to that of sand and gravel concrete.
- For pre-stressed concrete members, the high tensile steel used generally consists of wires, bars, or strands.
- The higher tensile strength is generally achieved by marginally increasing the carbon content in steel in comparison with mild steel.
- High tensile steel usually contains 0.60 to 0.85 percent carbons, 0.70 to 1.00 percent manganese, and 0.05 percent of sulphur and phosphorus with traces of silicon.
- The high-carbon steel ingots are hot-rolled into rods and cold-drawn through a series of dies to reduce the diameter and increase the tensile strength.
- The process of cold-drawing through dies decreases the durability of the wires. The cold-drawn wires are subsequently tempered to improve their properties. Tempering or ageing or stress relieving by heat treatment of the wires at 150-420oC enhances the tensile strength. The cold drawn stress relieved wires are generally available in nominal sizes of 2.5,3,4,5,7 and 8mm diameter and they should confirm to the Indian standard code IS: 1785-1983.
- The hard drawn steel wires which are indented or crimped are preferred for pre-tensioned elements because of their superior bond characteristics. The small diameter wires of 2 to 5 mm are mostly used in the form of strands comprising two, three or seven wires.
- The high-tensile steel bars commonly employed in pre-stressing are manufactured in nominal sizes of 10,12,16,20,22,25,28 and 32mm diameter and are covered in IS:2090-1983.
- The ultimate tensile strength of a plain-drawn steel wire varies with its diameter. The tensile strength decreases with increases in the diameter of the wires. And referred in the relevant Indian standard codes.