When casting in hot weather, which timing is recommended?

• A. Cast in the middle of the day
• B. Cast indoors
• C. Cast after dusk
• D. Cast early in the day or late at night when it’s cooler

Answer: (D)

When casting in hot weather, controlling the temperature and moisture environment during placement and early curing is crucial. Heat from hydration plus intense outdoor heat can cause concrete to gain strength quickly at the surface while interior portions lag, leading to high thermal and moisture gradients that promote cracking and surface drying issues. Placing during the cooler parts of the day—early morning or late at night—helps keep the concrete temperature more moderate, preserves workability, and slows the early curing rate enough to allow even filling, finishing, and initial curing. This timing reduces stress from rapid temperature rise and minimizes plastic shrinkage and surface cracking, resulting in a better finish and more reliable strength development. Casting in the middle of the day subjects the concrete to peak temperatures and faster evaporation, increasing cracking risk, while casting after dusk can still leave you with a shortened working window depending on the conditions.