IS 456 (1978) – Minimum Clear Cover Requirements According to IS: 456–1978, what are the minimum clear covers (in mm) for main reinforcement in the following members, respectively: slab, beam, column, and footing?

Introduction / Context:
Clear cover is the distance from the surface of the concrete to the nearest surface of reinforcing steel. Adequate cover protects steel from corrosion and fire and ensures proper bond. IS: 456 specifies minimum covers for different members, reflecting exposure and structural role. This question checks recall of the typical minimums used historically in IS: 456–1978 for common members.

Given Data / Assumptions:

• Code reference: IS: 456–1978 (earlier edition conventions).
• Members in order: slab, beam, column, footing.
• Exposure assumed mild/normal unless otherwise specified; footing in contact with soil typically needs higher cover.

Concept / Approach:

Historically adopted minimum covers were approximately: slabs ≈ 15 mm (or bar diameter, whichever greater); beams ≈ 25 mm; columns ≈ 40 mm; footings ≈ 75 mm where in contact with soil. These values serve as baseline detailing targets before applying any additional cover for severe exposure or fire rating requirements.

Step-by-Step Solution:

Verification / Alternative check:

Later editions (e.g., IS: 456–2000) refine cover by exposure class, but the baseline numbers above remain consistent with many detailing guides and legacy drawings for typical building work.

Why Other Options Are Wrong:

• 20, 25, 30, 75: Column cover 30 mm is inadequate; standard practice is closer to 40 mm.
• 5, 15, 25, 50: Slab and footing covers are too small; these values would not ensure durability.
• None of the above: Incorrect because option listing 15, 25, 40, 75 is available.

Common Pitfalls:

Ignoring the rule “not less than bar diameter”; neglecting extra cover required for aggressive environments or for fire resistance; mixing clear cover with nominal cover terminology in later code editions.

Final Answer:

15, 25, 40, 75