Temperature affects how fast concrete sets and how strong it ends up. These are the basics from the National Ready Mixed Concrete Association (NRMCA). For anything structural, or a big pour in extreme weather, talk to your supplier: they can adjust the mix.
Cold weather
What counts as cold: NRMCA defines cold weather as more than 3 days in a row when the average daily temperature is below 40°F (5°C) and the air doesn’t rise above 50°F (10°C) for more than half of any 24 hours.
What goes wrong:
- Fresh concrete freezes below about 25°F (−4°C). Frozen concrete can lose more than half its potential strength and won’t be durable.
- Cold concrete sets slowly. As a rule of thumb, a 20°F (10°C) drop in concrete temperature roughly doubles the setting time.
Basic precautions:
- Protect new concrete from freezing until it reaches 500 psi, about two days after placing.
- Use insulating blankets, especially on thin sections like slabs.
- Don’t water-cure when freezing is likely; use curing compounds or plastic sheeting instead.
- Ask your supplier about heated concrete and accelerating admixtures. Accelerators speed setting but don’t prevent freezing.
- Remove blankets gradually to avoid thermal shock.
Hot weather
What counts as hot: following ACI 305R, NRMCA describes hot weather as any mix of high air temperature, high concrete temperature, low humidity, high wind and strong sun that speeds up moisture loss and setting.
What goes wrong:
- Hot concrete needs more water, which lowers strength and durability.
- It sets faster, so you have less time to place and finish.
- Fast evaporation causes plastic shrinkage cracks before the concrete sets.
- A hot day followed by a cool night can cause thermal cracking.
Basic precautions:
- Avoid the hottest part of the day.
- Schedule deliveries so trucks can unload immediately, and have enough help ready.
- Don’t sprinkle water on the surface to make finishing easier.
- In wind and low humidity, use windbreaks, sunshades, fogging or evaporation retarders.
- Ask your supplier about set-retarding admixtures.
Either way
Plan the pour, have your crew and tools ready before the concrete arrives, and cure it properly afterward. Use the concrete calculator to order the right amount so the pour isn’t interrupted by a second trip or truck.