Every summer we get asked the same question. It is going to be 105 on Thursday, can you still pour. The answer is yes, and we pour in that heat regularly, but only because the whole day gets built around the heat instead of pretending it is not happening. A crew that pours a July slab in Hemet the same way they would pour one in March is going to hand somebody a slab with problems in it.
Here is what the heat actually does and what changes because of it.
The concrete sets faster than you can work it
Cement hydration is a chemical reaction and heat speeds it up. Hot ambient air, hot sun on the slab, hot aggregate that has been sitting in a stockpile all week, hot mix water, all of it means the concrete starts stiffening sooner and stiffens quicker once it starts.
That is a scheduling problem, not a quality problem, right up until it becomes a quality problem. Finishing concrete is a sequence. Screed, bull float, wait, edge and joint, wait, float, wait, trowel or broom. Each of those steps has a window where the concrete is ready for it. In heat those windows come sooner and they close faster, and if the crew is short handed or the truck is late, the slab moves past a step before anybody got to it. You cannot go back. A surface that stiffened before it got floated stays that way.
So the answer is more people on the pour, smaller sections at a time, and everything staged and ready before the first truck shows up.
The surface skins over while the body is still soft
This is the one that causes real damage and it is worth understanding.
The top of the slab is exposed to sun and wind. The body of the slab, an inch down, is not. So the surface dries and stiffens while the concrete underneath is still plastic and still settling and still bleeding water upward. Now you have a crust over a soft interior.
Two things go wrong from there. First, if somebody trowels that crust closed early, the water still coming up from below gets trapped under a sealed surface, and later that layer delaminates. It sounds hollow when you tap it and eventually it flakes off in sheets. Second, the crust itself is shrinking hard and fast while the slab below it is not, and the surface tears. That is plastic shrinkage cracking, and it looks like short random cracks in the top surface, often at an angle, in the first hours. Those are not structural cracks and they are not from the base. That is the surface drying too fast, and once they are in, they are in.
Low humidity and wind pull water out faster than the cement can use it
People blame temperature for everything, and temperature is only part of it. What actually drives water out of fresh concrete is evaporation rate, and that is temperature plus humidity plus wind speed plus how hot the slab surface itself is.
We are in a dry inland valley. Summer afternoon humidity here gets very low, and an afternoon breeze across an open slab is the single most aggressive thing you can do to it. A breezy 95 degree day with single digit humidity can be harder on a pour than a still, humid 105.
The reason that matters is that the cement needs that water. Hydration is not drying. Concrete does not get hard by drying out, it gets hard by the cement reacting with water, and that reaction runs for a long time after the slab looks finished. If the water leaves early, the reaction stops early and the concrete never reaches the strength it was mixed for. The top half inch is where that shows up worst, which is exactly the part that has to resist tires, scraping and weather for the next thirty years. Concrete that dried instead of cured has a soft, dusty, easily damaged surface, and you cannot fix that afterward.
What we actually change on a hot pour
We start early. First truck at first light, so the flatwork is placed and finished before the worst of the afternoon. On a big pour that means the crew is on site well before sunrise. Sometimes it means splitting a job across two mornings rather than trying to get it all in one long hot day.
We cool what we can. Wetting down the subgrade and the forms the night before or early that morning, so bone dry ground and hot forms are not pulling water out of the mix from underneath. Keeping tools, wheelbarrows and the pump line out of direct sun. Talking to the plant about mix temperature.
We block the evaporation. Wind breaks where we can set them, shade over the work when it makes sense, and an evaporation retarder or fog misting on the surface between finishing steps so the crust does not form ahead of the schedule. This is the difference between a slab with plastic shrinkage cracks and one without.
We adjust the mix with the plant, not on the truck. The tempting move in heat is adding water at the site to keep it workable. That is the worst thing you can do, because extra water means lower strength, more shrinkage, and more cracking, and it is how slabs end up soft on top. If the mix needs to stay workable longer, that is handled with admixtures at the plant, which is a phone call ahead of the pour rather than a decision made with a hose.
We treat curing as work, not as waiting. Curing is an active step. Keeping the slab wet or sealed so hydration can keep running, starting as soon as the finish will take it and continuing for days. Wet burlap, plastic, a curing compound, depending on the finish and what the slab is. In our summer, a slab left to its own devices after the crew drives away is losing strength it was paid for. This is the step people skip because the slab already looks done.
We cut joints on the slab’s schedule. Heat shortens the window for saw cutting, sometimes to a few hours, so somebody is coming back that evening. Cut late and the slab has already picked its own crack.
Sometimes the right answer is moving the date
There are days we will tell you not to pour. A forecast with a hot afternoon and a strong wind, on a large open slab with a finish that needs time, is a day where the risk is not worth it. Pushing to Monday morning costs a few days. A slab with a delaminated surface or a field of surface cracks costs a tear out.
We would rather have that conversation before the trucks are ordered. Nobody enjoys being told their concrete is waiting on the weather, and we say it anyway, because the slab is going to be there a lot longer than the inconvenience.
If your slab went in during a heat wave
Things worth looking for in the first weeks:
- A surface that dusts off under your hand or scuffs easily. That is a curing problem in the top layer.
- A hollow sound when you tap the surface, which points to delamination.
- Short, random, shallow cracks in the surface that appeared within a day or two. Plastic shrinkage, from drying too fast.
- Whether anybody kept the slab wet or covered after the finish, because that single answer explains most summer surface problems.
If you are planning work for this summer, call (951) 533-8821 and we will talk about the schedule before anything gets ordered.