Pouring concrete in cold and hot weather

Concrete and screed shouldn’t be placed below about 5°C and falling, and a fresh pour must not freeze in roughly the first 24 hours. Water expands as it freezes, so ice in a green mix disrupts the structure and you lose strength permanently. A frosted slab usually means rip-out and re-pour.

Masonry mortar and cement render are more sensitive again: don’t lay them much below 3°C and falling, and protect new work from frost overnight (BS 8000-3 / NHBC guidance). If the job can’t wait, hold the pour above freezing with insulated blankets or a heated enclosure until it has set.

Raw heat matters: above about 30°C concrete goes off fast and you lose workability, and above 35°C it’s usually a no-go. But the bigger summer problem is the evaporation rate: hot, dry and windy together pull water out of the surface faster than bleed water can rise, and the surface cracks as it shrinks (plastic-shrinkage cracking).

The recognised measure is the ACI 305 evaporation rate. At or above roughly 1.0 kg/m²/h the risk is high and precautions are essential, and 0.5 to 1.0 calls for care. Windbreaks, an evaporation retarder, a dampened subbase, fog-spraying and covering the pour quickly all help, and pour early or late, not in the heat of the day.

Light rain once the surface has taken its initial set is usually fine. But rain on a fresh, unworked pour washes out the cement, raises the water/cement ratio and leaves a weak, dusty, pock-marked finish. Have sheeting ready, finish and cover before the rain arrives, and treat a real downpour during placing as a no-go. Interior screed shouldn’t get wet at all.

These are rules of thumb, and your site has its own microclimate, mix and schedule. CanIPour leans cautious on purpose, because a wrong “go” is the expensive mistake. Treat it as the night-before sense check before you order the load and book the gang, and always read your own site on the day.