Concreting in hot weather

Above roughly 30°C concrete goes off quickly, so you lose workability and risk cold joints between loads. Above about 35°Cit is usually a no-go on heat alone. But a 28°C day with dry air and a stiff breeze can be more dangerous than a still 32°C one, which is why temperature on its own doesn't tell the whole story.

As concrete sits, bleed water rises to the surface. If the air pulls that water away faster than it can rise, the surface shrinks and cracks before it has set. This is plastic-shrinkage cracking, and it is driven by heat, low humidity and wind together.

The recognised measure is the ACI 305 evaporation rate. At or above about 1.0 kg/m²/h the risk is high and precautions are essential; 0.5 to 1.0 calls for care. CanIPour computes this rate from the forecast temperature, humidity and wind, so you see the real risk rather than guessing from the thermometer.

Pour early or late, not in the heat of the day. Set up windbreaks, dampen the subbase, and have an evaporation retarder ready. Get a curing membrane or cover on as soon as the surface allows, and fog-spray to keep it damp. Smaller, well-planned loads beat one big pour you can't finish in time.