There's a kind of pressure on every job to keep the schedule moving. The trucks are rolling, the crew's on site, the GC's breathing down your neck. Sometimes the right call is still "not today." Here's when.
Concrete needs heat to hydrate. Below 50°F it slows down. Below 40°F it basically stops. If you place at 45°F and the temperature drops to 35°F overnight, you've got a slab that'll be at maybe 15% strength when it should be at 25%. If it freezes before reaching 500 psi (which takes a day or two in cold conditions), water in the mix expands and you get permanent damage — surface flaking, low strength, the whole package.
What to actually check: Look at the next 48 hours, not just tomorrow morning. If it's 50°F at 7am but dropping to 30°F by midnight and you've got no cure blankets staged, push the pour. If you can heat the mix, insulate the slab, and keep the surface above 50°F for 3 days minimum, you can pour into colder weather — but only if you've got the gear and the plan.
The quick rule: Don't pour if you can't keep it above 40°F for the first 3 days. Don't pour if it's going to freeze in the first 24 hours. Period.
This is the day that catches good crews off guard. It might not feel that hot — 85°F isn't unusual. But pair it with 20% humidity and a 15 mph wind, and the surface is losing moisture faster than the slab can supply it. That's plastic shrinkage cracking territory.
ACI 305R sets the action threshold at 0.15 lb/ft²/hr. Above that, you start taking precautions. Above 0.20, most experienced crews will reschedule unless there's no choice. The math involves four variables — air temp, concrete temp, humidity, and wind speed at 18 inches above the slab — and there's a chart in the ACI document for it. Or you let an app do it for you live.
What to actually check: Run the evaporation rate at 5am. If it's under 0.15, you're probably fine. 0.15 to 0.20, take precautions: morning pour, fogging, retarders staged, curing compound on the truck. Over 0.20 — push it.
A light drizzle after final set won't hurt anything — and might actually help curing. But rain hitting fresh concrete before it sets is a problem. It washes cement paste off the surface, increases the water-to-cement ratio in the top layer, knocks out the air entrainment, and leaves a finish that'll dust and crumble.
What to actually check: Look at the radar and the hourly forecast. If there's a 60%+ chance of more than 0.1" of rain in the next 4-8 hours, you're rolling the dice. If you can place and finish, then tent or cover the slab before the rain hits, you can sometimes save the pour. If the timing's tight or the rain's heavy, push it.
Rescheduling costs you a short load fee, a half-day of crew, and an awkward call to the GC. A bad pour costs you saw-cutting and patching at best, full demo and replacement at worst — plus the relationship with the customer who watches you grind out a slab they paid for two months ago.
The math almost always favors waiting. The hard part is having the data to make the call confidently at 5am, before the trucks leave the yard. That's exactly what PourDay does.
Don't push a pour that wants to be pushed. PourDay flags GO, CAUTION, and NO GO conditions for your job site — 16 days out and live. Free on iOS.
Twenty years in, you know the feeling. Wind's shifty. Sky looks wrong. Crew's slow. Mix design's borderline. The numbers say go but something's off. Trust it. The data tells you when conditions are bad. The gut tells you when something else is bad. Both are valid. Neither one alone is enough.
The four levers you can pull when conditions push your evaporation rate over the line.
Read articleThe temperature where curing stops, and what to do when the forecast turns on you.
Read articleThe three real causes of premature cracking and which ones you can prevent on pour day.
Read articleReal-time pour conditions. 16-day forecast. Pour logging. Free for contractors.