What we check
- Work window: 4 hours, starting between 6 AM and 12 PM
- Temperature while working: 40–100°F required, ideally 50–90°F
- After you finish: stays at or above 33°F for 48 hours (required)
- After you finish: stays at or above 40°F for 72 hours (recommended)
- Wind: at most 30 mph
- Rain after: no likely rain for 8 hours (ideally 24 hours)
- Evaporation rate: under 0.2 lb/ft²/hr (ACI plastic-shrinkage cracking threshold)
These limits reflect typical manufacturer and industry guidance. Your product label always takes priority.
The ideal weather to pour concrete
The easiest pours happen on days between about 50°F and 80°F, with light wind, no rain during placing and finishing, and no freeze in the days after. Overcast is a bonus: it slows evaporation and gives you more time to finish.
How cold is too cold to pour concrete?
The American Concrete Institute (ACI 306) treats it as cold weather when the air temperature has fallen to, or is expected to fall below, 40°F during the protection period. Cement hydration slows sharply in the cold. If the water in fresh concrete freezes before the concrete reaches roughly 500 psi, which normally takes a couple of days, it can permanently lose strength and durability.
- This tool marks a day No if temperatures are forecast to hit freezing within 48 hours of your pour.
- It marks Maybe if lows dip below 40°F within 72 hours. Plan on insulated curing blankets, a heated or accelerated mix from your supplier, and never pouring on frozen ground.
Hot, dry, windy days: the evaporation rate
Concrete can crack on a mild day too. When surface water evaporates faster than bleed water rises, the surface shrinks and plastic shrinkage cracks appear within hours. ACI 305 guidance says to take precautions when the evaporation rate approaches 0.2 lb/ft²/hour (1.0 kg/m²/hour).
This forecast estimates that rate every hour from air temperature, humidity, and wind (the Uno equation, assuming concrete temperature equals air temperature). When it's high: pour early in the morning, set up windbreaks and sunshades, fog the air above the slab, use an evaporation retarder, and start curing immediately after finishing.
Can it rain on new concrete?
Rain during placement and finishing is the real problem. Extra water on the surface weakens the top layer and leads to dusting and scaling. Light rain several hours after finishing usually does little harm once the concrete has set, and slow, moist curing is actually good for it. Keep plastic sheeting on site whenever the chance of rain is above 30%.
After the pour, keep the slab moist (curing compound, wet burlap, or plastic) for about 7 days for best strength.
Frequently asked questions
Is 40 degrees too cold to pour concrete?
At 40°F you are in cold-weather concreting territory. It can be done if temperatures stay above freezing and you protect the concrete with insulated blankets and possibly an accelerated or heated mix. Avoid pouring if a freeze is forecast in the next 48 hours without protection.
How hot is too hot to pour concrete?
Above about 90°F, especially with low humidity and wind, concrete sets fast and is prone to cracking. Pour early in the morning, dampen the subgrade, use shade and windbreaks, and consider a set-retarding admixture.
How long does concrete need before it can freeze?
Concrete should be protected from freezing until it reaches about 500 psi, which typically takes around 48 hours for a normal mix kept above 50°F. It takes longer in colder conditions.
Can I pour concrete if rain is forecast?
Yes if the rain is expected well after you finish. Rain during placement and finishing damages the surface. Keep plastic sheeting ready, and cover the slab if rain arrives within the first few hours.