During a hot summer pour, a concrete mix starts setting too quickly before finishing. Which of the following is the most effective corrective action?
- AIncrease water content in the mix
- BUse a retarding admixtureCorrect
- CAdd high-range water reducer
- DSwitch to high early-strength cement
Solution
Concept
Retarder action in hot weather concreting
Formula
NONE
Step-by-step solution
1. Elevated temperatures in summer concreting accelerate hydration kinetics and speed up the crystallization of hydration products. 2. This causes rapid slump loss and premature initial setting before placement and finishing can be completed. 3. Retarding admixtures delay the hydration of C3A and C3S, prolonging the plastic state of concrete. 4. Thus, using a retarding admixture is the most effective corrective action. Answer: B
Explanation
In hot weather concreting, elevated ambient temperatures drastically accelerate the rate of cement hydration, causing rapid slump loss and premature initial stiffening. Retarding admixtures delay the chemical hydration of tricalcium aluminate and silicates, extending the setting time to permit proper transport, placement, and surface finishing.
Why the other options are wrong
A) Adding water increases w/c ratio, drastically weakening compressive strength and durability. C) Standard water reducers increase workability temporarily without delaying setting time unless formulated as retarders. D) High early-strength cement accelerates hydration further, exacerbating flash setting.
Shortcut
Premature setting in hot weather -> Add retarding admixture.
Common mistake
Adding extra water to restore slump, which severely degrades 28-day compressive strength and increases drying shrinkage.
Correct answer: B) Use a retarding admixture