Extreme Weather Concreting MCQ with answer and solution

Concept

Prevention of plastic shrinkage cracks

Formula

E = 5 × ([T_c + 18]^2.5 - r × [T_a + 18]^2.5) × (v + 4) × 10^-6

Step-by-step solution

1. Plastic shrinkage occurs during the first few hours after casting while concrete is in a plastic state. 2. High ambient temperature, low relative humidity, and high wind velocity cause rapid surface moisture evaporation. 3. When surface evaporation rate exceeds bleeding rate (> 1.0 kg/m2/h), capillary tensile stresses tear the weak fresh paste. 4. Erecting windbreaks, using sunshades, and applying immediate moist curing reduce evaporation below critical thresholds. Answer: C

Explanation

Plastic shrinkage cracking occurs when the rate of evaporation from fresh concrete surfaces exceeds the bleeding rate (typically greater than 1.0 kg/m2/h), causing tensile stresses in weak unhydrated paste. Erecting windbreaks, using sunshades, and initiating immediate moist curing or applying aliphatic evaporation retarders drastically lowers surface evaporation and eliminates tensile tearing.

Why the other options are wrong

A) Increasing cement content raises hydration temperatures and drying shrinkage, worsening cracking. B) Using warm mixing water accelerates setting and increases evaporation rate, aggravating cracking. D) Delaying finishing prolongs exposure of unprotected fresh paste to drying ambient winds.

Shortcut

Plastic shrinkage cracking = High evaporation rate > bleeding rate; Prevented by windbreaks, sunshades, and early curing.

Common mistake

Increasing cement content, which increases heat of hydration and aggravates plastic shrinkage cracking.