Pouring Concrete in Hot or Cold Weather: What Changes

Concrete hardens through a chemical reaction with water, not by drying out. Heat, sun and wind remove that water too fast; cold slows the reaction and can freeze the water before the concrete is strong enough. Both can be managed on a small DIY pour if you plan for them.

Hot, dry and windy weather

The main risk is water evaporating from the surface faster than it rises from below. The surface then shrinks and cracks while the concrete is still plastic (plastic shrinkage cracking). Heat also shortens the time you have to place and finish.

• Pour early in the morning, before the hottest part of the day.
• Dampen the ground and forms before the pour so they don’t pull water from the mix, but leave no standing water.
• Have enough help to finish quickly, and don’t add extra water to make the mix easier to work: it weakens the concrete.
• Shade the work and block wind if you can. Wind speed matters as much as temperature for evaporation.
• Start curing as soon as finishing is done: a curing compound, wet burlap kept damp, or plastic sheeting, for at least the first several days.

The American Concrete Institute’s hot-weather guidance uses air temperature, concrete temperature, humidity and wind to estimate the evaporation rate. Our concrete forecast uses the same kind of estimate hour by hour.

Cold weather

Cold slows the chemical reaction, so concrete gains strength slowly. The serious risk is freezing: if the water in fresh concrete freezes before the concrete has gained early strength, the surface can scale and the slab can be permanently weakened.

• Plan pours when the forecast stays above freezing for at least the first few days, and ideally above about 40 to 50 °F.
• Never pour on frozen ground, snow or ice. Thaw and dry the subgrade first.
• Cover the slab with insulated curing blankets or straw under plastic as soon as it is finished, and leave them on through cold nights.
• Ask the supplier about a mix for cold weather. Bagged mixes list their minimum temperatures on the bag.
• Expect finishing to take longer as the concrete sets more slowly.

Rain

Rain on fresh concrete washes out the surface paste and leaves a weak, dusty or pitted top. Keep plastic sheeting ready to cover the slab without touching the surface (support it on boards or stakes). Once the concrete has set hard, light rain is not a problem and can even help curing.

How long before you can use it

Times vary with the mix and temperature, but typical guidance for a residential slab is foot traffic after a day or two, light use after about a week, and vehicles after about a week to a month depending on the mix and thickness. Concrete gains strength more slowly in cool weather, so add time when it has been cold. Check the bag or ask your supplier for the mix you used.

For sealers and coatings on concrete, see the driveway sealing forecast and the garage floor epoxy forecast.

A DIY Day is an independent site. Figures above are typical label ranges; always follow the instructions on the product you use. Spotted a mistake? Tell us.