Cement Concrete Flooring – Layer composition (identify the incorrect layer specification) For a typical multilayer cement concrete floor over ground, which of the following layer specifications is incorrect?

Introduction / Context:
Ground-supported concrete floors are built in layers to control dampness, provide uniform support, and deliver a durable wearing surface. Knowing typical thicknesses and mixes helps spot incorrect specifications that would be uneconomical or technically unsuitable.

Given Data / Assumptions:

• Floor constructed over compacted subgrade.
• Sand blinding to even out the surface and break capillarity.
• Lean base concrete (often lime concrete or weak cement concrete).
• Wearing course (topping) of denser concrete.

Concept / Approach:
Common practice is: compacted subgrade → 7.5–10 cm sand → about 7.5–10 cm lean base (e.g., 1:4:8) → wearing course 2.5–4 cm thick (e.g., 1:2:4 or granolithic). A 10 cm thick 1:2:4 topping is excessively thick and atypical for a wearing course, making that statement incorrect.

Step-by-Step Solution:
Check base: consolidated ground (correct).Sand layer: ~10 cm clean sand (correct and common).Lean base: 1:4:8 around 7.5–10 cm (acceptable).Wearing course: usually 2.5–4 cm, not 10 cm ⇒ the given top-layer spec is incorrect.

Verification / Alternative check:
Flooring handbooks and schedules of rates list thin toppings for economy and performance; thicker structural concrete is reserved for slabs, not merely as wear layer.

Why Other Options Are Wrong:

• Options a–c reflect commonly accepted construction sequences.
• Option e is wrong because one option (d) is indeed incorrect.

Common Pitfalls:
Confusing structural slab thickness with wearing course thickness, or omitting the sand layer that assists in leveling and damp-proofing.

Final Answer:
Provide a 10 cm thick 1:2:4 cement concrete wearing course as the top layer