Hardened Concrete Properties Questions and Answers

Answer: B

About 95 of the 100 cubes will exceed 20 MPa.

IS 456 defines the characteristic strength as the strength below which not more than 5 percent of test results are expected to fall. So for M20 concrete, 5 cubes in 100 may fall below 20 MPa and about 95 should exceed it.

50 cubes would correspond to the mean strength, and 100 would mean no failures at all, which the statistical definition does not promise. 80 implies a 20 percent failure rate.

Answer: D

The modulus of elasticity of concrete increases with Increase in age.

The modulus of elasticity rises with the strength of concrete (IS 456 relates it to the square root of the characteristic strength), and strength grows with age as hydration continues.

The other three all lower strength and so lower the modulus: a higher water-cement ratio makes weaker, more porous concrete, a shorter curing period stops hydration early, and less vibration leaves air voids.

Answer: B

IS 456: 2000 gives \(E_c = 5000\sqrt{f_{ck}}\).

Clause 6.2.3.1 of IS 456: 2000 gives the short-term static modulus of elasticity of concrete as \(E_c = 5000\sqrt{f_{ck}}\), with \(E_c\) and \(f_{ck}\) in N/mm². For M20 concrete this gives about 22,360 N/mm².

The older IS 456: 1978 used \(5700\sqrt{f_{ck}}\), which is why nearby numbers appear among the options, but 500, 700 and 5500 are not the code coefficients.