Retarding admixtures used in hot-weather concrete reduce the possibility of surface crusting. True or False?

• A. True
• B. False
• C. Not sure
• D. It depends

Answer: (B)

Crusting in hot weather happens when the surface dries and stiffens faster than the interior can set, forming a hard skin that can affect finish and durability. Retarding admixtures slow the chemical reaction and the heat of hydration, which helps with workability and reduces the risk of thermal cracking, but they don’t stop water from evaporating at the surface. Because crusting is driven primarily by moisture loss at the surface rather than just how quickly the mix begins to set, a retarder doesn’t inherently reduce the chance of crust formation. In fact, if curing and moisture control aren’t managed, delaying set can keep the surface in a plastic state longer while evaporation continues, potentially keeping a crust from forming the way you’d want it to. The practical way to curb crusting is proper curing and moisture retention (fogging, misting, covering, curing compounds), along with finishing procedures that minimize surface drying.