Correct Answer - Option 4 : High modulus of elasticity and creep values are lower
Concept-
For pre-stressed structural elements high strength concrete and steel are used,the reasons of using high grade steel and concrete is explained below.
Steel-
In general magnitude of pre-stressing force is very high which can be achieved by either increasing area of steel or by increasing stress of steel.
Increase in area of steel beyond a certain limit creates problem in concreting and it also affects the behavior of the member so it is preferable to use high tensile steel.
Generally 150 to 200N / mm2 loss of pre-stress is invariably present in pre-stressed concrete. If low grade steel is used then stress remaining after all losses is very less so it is preferable to use high tensile steel of ultimate stress lies between 1000 to 2000 N / mm2.
Concrete-
For high bond stress between wire and concrete in pre-tensioning member.
To resist high anchorage stress in anchorage zone in post tensioning member.
To reduce loss of concrete due to elastic shortening of concrete and creep of concrete, as these losses are inversely proportional to the modulus of elasticity of concrete, so with increase in grade of concrete the modulus of elasticity will also increase.
