Hot-weather concreting is in every specification and almost every specification understates it. The rules assume “hot” means 32 °C. In Riyadh in July, ambient hits 46 °C and the aggregate stockpile is hotter than that.
What actually goes wrong
Three things, in this order:
Slump loss. The mix stiffens in the truck. By the time it reaches the pump it is no longer the mix you designed, and the temptation to add water at the truck is the single most destructive habit on a hot site.
Plastic shrinkage cracking. Surface water evaporates faster than bleed water rises. You get a map of fine cracks across the top surface within hours — before the concrete has any tensile strength to resist them.
Thermal differential. In a thick pour, the core hydrates and heats while the surface cools to ambient overnight. Past roughly 20 °C of differential you are building cracks into the element deliberately.
The 900 m³ raft
Last July we had a raft foundation to pour: 900 m³, 1.8 m thick, in a single continuous operation because a construction joint through the middle was not acceptable to the structural engineer.
What we did:
- Chilled the mix with ice, replacing part of the batch water. Placement temperature came in at 29 °C against a 32 °C limit.
- Started at 20:00 and finished at 06:30. The entire pour happened in the coolest ten hours available.
- Retarder dosed up, deliberately extending set time so the pour stayed monolithic — the last truck had to knit into concrete placed nine hours earlier.
- Thermocouples at three depths, read hourly, with insulating blankets on standby to slow surface cooling rather than speed it.
- Two pumps, one spare on site, primed. A pump failure at hour six is a cold joint through the middle of your raft.
Peak core temperature reached 68 °C. Maximum differential was 17 °C, inside the 20 °C limit. No cracking on the exposed face.
What we changed permanently
Aggregate shading. Stockpiles are now shaded and sprinkled at every batching plant we run. It is a cheap intervention that removes several degrees before the mix is even designed.
No water at the truck. Ever. If the slump is wrong the load goes back. We have rejected loads and we will again. This is a cultural rule more than a technical one — it survives only if the site team knows nobody will be blamed for sending concrete away.
Night pours are the default in summer, not the exception requiring approval. Fighting for a daytime pour permit in July was wasting everyone’s time.