Concrete Technology And Design Of Concrete Structures MCQ Questions & Answers | Civil Engineering

The Indian Standard IS 456:2000 specifies minimum reinforcement in slabs. When high-strength deformed (HYSD) bars are used, the minimum steel in either direction shall not be less than 0.12% of the gross cross-sectional area (bD).

For comparison, when plain/mild steel is used the minimum is 0.15% — the reduction to 0.12% is permitted because HYSD bars have higher yield strength and provide equivalent ductility with slightly less area of steel.

This requirement (clause on minimum reinforcement for slabs) ensures adequate cracking control and ductile behaviour of slabs while taking advantage of HYSD bar properties.

Concept: When all footings are proportioned on the basis of total load (dead + full live load), the design aims to keep the same allowable soil pressure under every footing at the notional maximum loading condition.

Reality of loading: In service, the live load is not fully present everywhere at the same time, and interior columns generally have larger tributary areas than exterior columns. Hence, interiors attract higher actual loads more often.

Resulting behavior: Because interior footings experience higher sustained/actual pressure relative to exterior footings, they undergo greater compression of the supporting soil.

Conclusion: Differential settlement occurs with the interior columns settling more than the exterior columns.

The modular ratio is defined as: m = Es / Ec Where: Es = Modulus of elasticity of steel (approximately constant) Ec = Modulus of elasticity of concrete

What is Creep? Creep is the long-term deformation of concrete under a sustained load. Over time, concrete subjected to constant stress will slowly deform. This is a time-dependent increase in strain, even when the stress remains constant.

How Creep Affects Modulus of Elasticity: Due to creep, the concrete shows more strain for the same level of stress. This means its effective modulus of elasticity reduces over time. In other words, Ec becomes smaller than its short-term value.

Impact on Modular Ratio: Since modular ratio = Es / Ec, and Es remains constant, a reduction in Ec (due to creep) results in an increase in the modular ratio.

Why Some Get Confused: In some design approaches (e.g. working stress method), to indirectly account for creep and long-term deflection, a lower modular ratio is used empirically. This is a correction based on behavior, not a direct mathematical consequence of the creep effect on elasticity.

But theoretically and fundamentally, the effect of creep is to increase the modular ratio.

Therefore, the correct answer is: Option B — To increase it.