CSLB General Building (B)Site Work, Foundations and ConcreteMedium
A crew is placing a concrete slab on a day when the ambient temperature is 98°F with low humidity and moderate wind. Per ACI 305 hot weather concreting guidance, which practice is most important to prevent plastic shrinkage cracking?
- APouring the concrete at the hottest part of the day for faster set time
- BDelaying curing until the concrete reaches final set
- CIncreasing the water-cement ratio to speed up placement
- DUsing fog spraying and wind breaks to reduce surface moisture evaporation before finishing
Show answer & explanationAnswer & explanation
Correct answer: D. Using fog spraying and wind breaks to reduce surface moisture evaporation before finishing
In hot, dry, windy conditions, high evaporation rates can exceed the rate of bleed water rising to the surface, causing plastic shrinkage cracks. ACI 305 recommends fog spraying, sunshades, and windbreaks to reduce evaporation and protect the surface until finishing and curing begin.
Why the other options are wrong
- A. Pouring during the hottest part of the day worsens evaporation and workability issues.
- B. Delaying curing increases moisture loss and cracking risk rather than preventing it.
- C. Increasing water content weakens the concrete and increases shrinkage risk.
Hot Weather Concreting (ACI 305)
Guidance for placing and curing concrete in hot, dry, or windy conditions to prevent rapid moisture loss and plastic shrinkage cracking.
- High evaporation rate causes plastic shrinkage cracks
- Fog spraying and windbreaks reduce evaporation
- Avoid adding water to the mix to combat heat
Memory trick: Fight heat with fog, not more water.