Your crew is pouring a concrete foundation wall in hot Florida weather (95°F). What is the most critical concern for concrete placement and curing?
Correct Answer
B) Rapid moisture loss leading to plastic shrinkage cracking
In hot weather, rapid evaporation of surface moisture can cause plastic shrinkage cracks before the concrete has gained sufficient strength. Proper curing methods, wind protection, and surface treatments are essential to prevent this.
Why This Is the Correct Answer
In hot weather (above 90°F), the surface of freshly placed concrete loses moisture through evaporation faster than bleed water can replace it. This rapid moisture loss causes plastic shrinkage cracking before the concrete has set or gained sufficient strength. Contractors must use windbreaks, evaporation retarders, fogging, and curing blankets to maintain surface moisture. This is the primary concrete placement risk in Florida's heat and is consistently tested on contractor exams.
Why the Other Options Are Wrong
Option A: Forms expanding and causing blowouts
While thermal expansion can affect concrete formwork, form blowouts are caused by excessive concrete pressure and pour rate — not primarily by heat. Properly designed forms account for hydrostatic pressure. This concern exists regardless of temperature and is not the most critical hot-weather specific risk.
Option C: Rebar becoming too hot to handle safely
Hot rebar is a worker comfort and handling concern, but it does not affect concrete quality or structural integrity once encased. Workers can handle hot rebar with gloves or by scheduling pours in cooler morning hours. It is not a critical concrete placement concern.
Option D: Concrete setting too slowly
Hot weather actually accelerates concrete hydration, causing it to set faster — not slower. Accelerated setting is a concern because it reduces workability time, but the primary structural risk in hot weather remains plastic shrinkage cracking from rapid surface evaporation.
Memory Technique
Hot + Fast + Dry = Cracks. In Florida heat, concrete loses moisture fast before it can cure, causing plastic shrinkage. Keep it wet, keep it covered.
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