How to judge whether the concrete is frozen when the cast-in-place roof encounters a low temperature of MINUS 4 degrees the next day?

Yes, the freezing critical strength of concrete is the pre-curing strength of concrete whose compressive strength loss is less than 5% after freezing. The hardening and strength of concrete is due to the hydration of cement. Under certain humidity conditions, the hydration rate mainly depends on the temperature.

When the temperature drops to 0℃, the hydration basically stops. When the temperature drops to -2 ~-4℃, the water in the concrete begins to freeze, and the volume of the frozen water expands by about 8 ~ 9%, which produces ice crystal stress in low-strength concrete, causing micro-cracks in the cement stone structure and weakening the bonding force between cement, aggregate and steel bars, thus reducing the strength of concrete.