Pavement Design MCQ with answer and solution

Concept

Frictional temperature stresses and contraction cracking in rigid pavements

Formula

σ_f = (W × L × f) / (2 × 10⁴)

Step-by-step solution

1. Uniform cooling causes the entire concrete pavement slab to undergo contraction. 2. Subgrade friction opposes this movement from both ends towards the interior. 3. Frictional resistance builds up towards the center: zero at edges, maximum at the slab midpoint. 4. Consequently, maximum frictional tensile stress occurs at the center of the slab. 5. Therefore, cracking due to uniform cooling is most likely to initiate at the center. Answer: D

Explanation

In rigid concrete pavements, uniform cooling (such as seasonal temperature drops) causes the slab to contract uniformly along its length. This contraction is resisted by subgrade friction acting against the bottom face of the slab. Frictional restraint accumulates symmetrically from the free outer edges inward, meaning tensile stress is zero at the slab edges and reaches its maximum at the mid-length (center) of the slab (σf = W·L·f / 2). Once this tensile stress exceeds concrete tensile capacity, transverse cracking develops at the center.

Why the other options are wrong

A) Incorrect: Frictional stress is zero at the free edge, so cracking will not initiate at the edge. B) Incorrect: Corner stresses are minimal during uniform contraction. C) Incorrect: Edges are free to contract without frictional restraint accumulation.

Shortcut

Uniform cooling -> Contraction resisted by friction -> Maximum tensile stress at CENTER.

Common mistake

Confusing uniform cooling (frictional stress, maximum at center) with warping temperature stresses (which vary across slab depth and can cause corner or edge curling).