A driveway going in on a ninety degree afternoon
Standing in a yard in July watching a truck back up, the worry is usually that the heat is going to hurt the slab somehow. It is not the enemy that cold is. Concrete cures the same chemical way whether it is hot or cold out, and heat does not damage the finished strength the way a hard freeze on green concrete can.
What heat does is speed everything up and dry things out faster, and both of those need to be managed rather than ignored.
The real risk is water leaving the surface too fast
Fresh concrete needs the water in its mix to stay put long enough to do its job, both for the chemical reaction that builds strength and for the finishing work that happens in the first hours. Sun, heat, and wind together pull that water off the surface faster than usual, and a surface that dries out too quickly while the concrete underneath is still soft is prone to a network of fine cracks called crazing, or to shrinkage cracking that runs deeper than the joints intended.
Bleed water, the moisture that naturally rises to the surface as concrete sets, also behaves differently in the heat. It can evaporate as fast as it rises, which fools a crew into thinking the surface is ready to finish before it actually is underneath.
Why we start before the sun is up
Summer pours here get an early start, sometimes before daylight, so the crew has the coolest hours to place and finish the concrete before the afternoon heat is working against them. Loads get spaced so material is not sitting and heating up before it goes into the forms.
A retarder is a common admixture in a hot weather mix, used to hold the set back closer to what it would be on a mild day rather than letting the heat rush it. That gives the crew the working time they need to finish the surface properly instead of racing a slab that wants to set before the float work is done.
- Early start, ahead of the afternoon heat
- Truck spacing planned so concrete is not sitting and heating up
- A retarder used where the mix needs the extra working time
- Curing steps started right after finishing, not left for later
What happens after the finish goes on
Once the surface is finished, keeping it from drying out too fast becomes the job. That might mean a curing compound applied right behind the finishing crew, or keeping the surface damp in the first day, depending on the pour and the forecast. That step matters more in July than it does in a mild October week, because the heat is working against the concrete the whole time.