Hot and Cold Weather Concrete Placement

The same mix behaves like a different material in July and in January. In the heat it stiffens fast, wants more water and can crack before you finish it. In the cold it sits for hours, and if it freezes early it may never reach its strength. Neither problem can be fixed afterward, so the work happens before the truck arrives.

You already know the basics of protecting a slab. This guide is about planning: reading the forecast, choosing what to change in the mix and on site, and deciding when protection can come off. Numbers here come from American Concrete Institute (ACI) guides and National Ready Mixed Concrete Association (NRMCA) practice. Your project specification controls, and mix changes go through the foreman and the producer.

What You Will Be Able to Do

• Tell from the forecast whether a pour needs a weather plan.
• Estimate how temperature changes your set time and your crew's day.
• Control evaporation on a hot, windy pour.
• Set placing temperatures by section thickness and protect concrete until it is safe.
• Remove protection without shocking the concrete.

Temperature Runs the Clock

A rule of thumb is that a drop of 20°F in concrete temperature about doubles the setting time. Heat works the other way, and everything speeds up.

Worked example. A slab that reaches finishing stage in about 3 hours at 70°F will take about 6 hours at 50°F. A crew that planned to be off the slab by 2 p.m. is now finishing at dusk, in the cold, with tired people. Plan the lights, the extra hands and the start time before the pour.

Below about 40°F, strength gain slows to a crawl. Fresh concrete itself freezes at about 25°F.

Hot Weather

ACI does not define hot weather as one temperature. It is any mix of high air temperature, high concrete temperature, low humidity, high wind and strong sun that hurts the concrete. A dry 85°F day with wind can be worse than a humid 95°F day.

Evaporation

Plastic shrinkage cracks are parallel, shallow cracks, often 1 to 3 feet apart, that show up in the first hours. They form when the surface loses water faster than bleed water can replace it.

ACI 305R has a chart that estimates evaporation from air temperature, concrete temperature, humidity and wind. A common trigger for extra care is about 0.2 pounds per square foot per hour. Treat that as a warning line, not a safe line. The chart does not count direct sun. Mixes that bleed little, such as those with lots of fine material, entrained air or silica fume, can crack at much lower rates. Wind over 5 mph is a sign to take evaporation seriously, and a vapor retarder under the slab adds risk too.

The Hot Weather Plan

Before the pour:

1. Set the start time. Late afternoon or evening helps with sun and wind. In very hot areas, early morning gives better control of concrete temperature.
2. Put up windbreaks and sunshades where you can.
3. Dampen the subgrade, forms and steel. Leave no standing water.
4. Space the trucks so each can unload right away. Trucks waiting in line are losing slump.
5. Bring more people, not fewer. Finishing goes faster and there is less time to fix delays.

In the mix. Ask the producer about retarders, water reducers, less heat-producing cement, and fly ash or slag. Chilled water can drop concrete temperature by up to about 10°F, and ice by up to about 20°F. Many specifications cap concrete at 95°F at discharge.

During the pour:

• Fog upwind, into the air above the slab. You are raising humidity over the surface, not wetting it.
• Use an evaporation retarder behind the screed and between finishing passes if needed. It is not a curing compound.
• Cover the slab with wet burlap or plastic if finishing is delayed.
• Never sprinkle water on the surface to help finishing.

After the pour. Start curing the moment finishing ends. Keep curing for the time your specification sets, with 3 to 7 days as a common range. White pigmented compound and reflective sheets keep the slab cooler, and they let you see that coverage is complete. Watch for hot days followed by cool nights, because that sudden swing can cause thermal cracks.

If plastic shrinkage cracks open during final finishing, a finisher may be able to close them by working them back together. Treat that as a warning, and tighten up evaporation control right away.

Warning: Heat stroke hits fast on an exposed slab, and hot concrete days are long. Drink water often, take breaks in the shade, and know the signs. Someone who is confused or stops making sense needs emergency care right now.

Cold Weather

ACI 306R treats cold weather as any time the air has fallen to, or is expected to fall below, 40°F during the protection period. The protection period is the time after the pour when the concrete cannot be left on its own. So a mild day with a hard frost coming on the third night is still a cold weather pour.

Two strength numbers matter:

• 500 psi. Concrete must not freeze before reaching this. It usually takes about two days with proper protection. Fresh concrete that freezes can lose more than half of its potential strength and will not be durable.
• 3,500 psi. Saturated concrete that will go through repeated freezing and thawing needs this strength before those cycles begin. New concrete is full of water, which is why a driveway poured late in the fall can scale.

Placing Temperatures

The minimum depends on the thinnest dimension of the section, because thin work loses heat fast.

That minimum applies as placed and all through the protection period. Do not place concrete more than about 20°F above the minimum. Hotter concrete needs more water, loses slump faster and is more likely to crack.

Worked example. You are pouring a 6-inch slab and a 16-inch wall with frost forecast.

• The slab's least dimension is under 12 inches, so hold it at 55°F or more, and place it no warmer than about 75°F.
• The wall is in the 12 to 36-inch row, so hold 50°F or more, and place it no warmer than about 70°F.
• The slab is the harder job. It has more surface and less mass to hold heat.

The Cold Weather Plan

Before the pour:

1. Remove snow and ice from forms, steel and the base.
2. Thaw the ground. Never place on frozen subgrade. It settles when it thaws, and it chills the concrete from below. Avoid placing on a surface under 40°F, which also stiffens the top before the bottom and leads to delamination.
3. Warm metal in contact with the concrete to above freezing.
4. Have the blankets, tarps and heaters on site before the first truck comes.

In the mix. Ask for heated water and aggregate, Type III cement, more cement, or less fly ash and slag. Use the lowest slump that still places well. Extra water slows set and stretches out bleeding.

Accelerators speed up the set, but they do not stop concrete from freezing, and they never replace protection. Calcium chloride is limited to 2 percent by weight of cement and is left out of reinforced work where steel corrosion is a concern. Use a non-chloride accelerator there.

Protection. Insulated blankets, tarps, and straw covered with plastic are all common. Corners and edges lose heat first, so lap them well. Concrete makes its own heat as it cures, and blankets keep that heat in. Metal forms give little protection, while wood forms hold heat better.

Warning: Fuel heaters give off carbon monoxide. Vent them to the outside of any enclosure, and never run an unvented heater in a tent with people inside. Exhaust can also react with the fresh surface, called carbonation, and leave a slab that dusts for the rest of its life.

Curing. Do not water cure when freezing is close. Use curing compound, plastic or curing paper to hold in the moisture already there.

Finishing. Cold concrete bleeds for a long time. Warm, humid air can also condense on a cold slab in a closed building. Do not trowel that water into the surface. It causes the same dusting as troweling in bleed water.

When Can Protection Come Off?

Do not go by the calendar. Forms in the cold can need anywhere from 1 to 7 days. Use field-cured cylinders, or the maturity method (ASTM C1074), which estimates strength from the concrete's actual temperature history using sensors in the pour.

Then take protection off slowly. Temperature shock cracks concrete.

• Do not let concrete cool faster than about 5°F an hour in the first 24 hours.
• Surface and core more than about 35°F apart can cause thermal cracks.
• Remove layers of insulation in stages, and let the concrete cool under them.
• Stay within the maximum drop in the table above.

Worked example. A 10-inch wall is at 62°F under blankets and the air is 18°F. That is a 44°F difference, so pulling the blankets at once would drop the surface fast, at far more than 5°F an hour. Take off one layer, or wait for a warmer part of the day, and let the concrete come down over a full day.

Cylinders in Bad Weather

Test specimens need protection too. In either season they must stay at 60 to 80°F for their first 24 to 48 hours, with a thermometer record. Hot jobs may need ice or refrigeration in the curing box, and cold jobs may need a heater. A frozen cylinder or one cooked in the sun will report a low strength for a slab that may be fine.

Weather Troubleshooting

Check Your Understanding

1. It is 62°F at noon with a clear sky, and the forecast calls for 34°F tonight and 31°F tomorrow night. Is this a cold weather pour?

Yes. The air is expected to fall below 40°F during the protection period. Have blankets ready, even though it is mild when you pour.

2. A dry wind is blowing at 12 mph over a slab on a vapor retarder. Bleed water has dried early and the surface looks ready. What is your concern?

Evaporation is beating bleeding, so plastic shrinkage cracks are likely. The wind and the retarder both add risk. Use windbreaks, fog or evaporation retarder, and keep curing supplies close.

3. Your foreman says the mix has an accelerator, so blankets can stay on the truck tonight. It will hit 22°F. What do you say?

An accelerator speeds the set, but it does not stop concrete from freezing. Protect the slab until it reaches at least 500 psi.

4. What minimum temperature applies to a 40-inch-thick mat footing?

45°F. It falls in the 36 to 72-inch row.

5. A wet burlap cure is planned, but overnight lows have dropped to 28°F. What should change?

Do not water cure with freezing coming. Switch to curing compound, plastic or paper, plus insulation.

Key Takeaways

• A 20°F drop roughly doubles setting time. Plan your crew around it.
• Hot weather is a mix of heat, wind, dry air and sun. Watch evaporation, not only the thermometer.
• Fog the air, do not wet the slab, and start curing the moment finishing ends.
• Cold weather starts at 40°F during the protection period, even if the pour day is mild.
• Protect concrete from freezing until 500 psi. Accelerators do not replace blankets.
• Vent heaters, and bring temperature down slowly when protection comes off.