It's 92°F, the wind picked up to 12 mph, humidity's at 25%, and the trucks are an hour out. Here's what an experienced foreman knows that a thermometer doesn't tell you.
Most contractors look at the air temp and call it a hot-weather pour. That's only part of the story. The thing that actually wrecks slabs in summer is evaporation rate — how fast moisture leaves the surface. And evaporation rate depends on four things, not one.
Run those four through the ACI 305R formula and you get a number in lb/ft²/hr. Anything over 0.15 means you're losing moisture faster than bleed water can replace it — and that's when plastic shrinkage cracking starts.
It's the most useful number in hot weather concreting. ACI 305R says: when evaporation rate exceeds 0.15 lb/ft²/hr, take precautions. Above 0.20, most experienced contractors will reschedule unless they have no choice.
The hidden problem with Type 1L blended cement — which most of the country has been pouring since 2022 — is that it has less bleed water than the old Type 1. So the 0.15 threshold is even more critical now than it was when your mentor learned the trade.
When the forecast says you're going to be over 0.15 but under 0.20, you don't have to walk away. You have to take precautions. Here are the four that actually work in the field:
1. Time of day. The single biggest lever. Pour at 5am instead of noon and you might cut your evaporation rate in half. Cooler air, lower wind, higher humidity. If you're looking at a 95°F afternoon, the morning pour isn't a nice-to-have — it's the whole strategy.
2. Cool the concrete. Ask the supplier for chilled water or ice in the mix. A 10°F drop in concrete temp can knock your evaporation rate down 20%. Keep aggregate piles shaded and wetted before the truck loads.
3. Block the wind, raise the humidity. Set up windbreaks (plywood walls, tarps, anything that breaks the airflow over the slab). Fog the subgrade and forms before placement. Some crews fog above the slab during finishing — works great if the wind isn't too strong.
4. Evaporation retarder + immediate curing. Spray an evaporation retarder during finishing. Apply curing compound the second the final pass is done — don't wait, don't take a break, don't let the surface dry for 10 minutes. In the worst conditions, do both.
Don't add water at the truck to compensate for stiffening — you'll weaken the mix. Don't spray water on the slab during finishing — you'll trap weak paste at the surface. Don't skip the morning safety meeting because "everyone knows the drill" — hot weather pours are exactly when the new guy needs to hear what's different.
A foreman's 5am hot-weather routine, in plain English:
The 0.15 rule, automated. PourDay runs the ACI 305R evaporation rate formula live for your job site and gives you a GO, CAUTION, or NO GO every morning. Free on iOS.
If you want the formal version: ACI 305R defines hot weather concreting as conditions that cause excessive evaporation, accelerated set, or both. It triggers at concrete temperatures above 95°F or any combination of weather variables that pushes evaporation rate over 0.15 lb/ft²/hr. The standard recommends specific precautions when those conditions are met. The standard also assumes someone is actually measuring evaporation rate — which is the part most jobs skip.
Three weather conditions where the right call is to push the pour to another day.
Read articleThe three real causes of premature cracking — and the one you control on pour day.
Read articleReal-time pour conditions. 16-day forecast. Pour logging. Free for contractors.