Curing Concrete in Hot, Windy Weather

Hot weather trouble on flatwork is mostly a water problem. Heat, wind and dry air pull water off the surface faster than the concrete can bleed it back up, and the top dries and cracks before it sets. The fix is to measure that drying, slow it while you finish, and seal the surface the moment you are done.

What counts as hot weather for concrete?

There is no single temperature. NRMCA’s hot weather concreting guide quotes the ACI 305R definition: any mix of high air temperature, high concrete temperature, low relative humidity, high wind and sun that speeds up moisture loss and hydration enough to hurt the concrete.

So a mild day with a dry wind can strip water off a slab faster than a hot, still, humid one. NRMCA notes these conditions “can occur at any time, especially in arid or tropical climates,” and that high humidity tends to offset high temperature.

There is a firm number for the concrete itself. ACI’s answer on hot weather temperature limits says ACI 301-20 and ACI 305.1-14 cap concrete at 95°F (35°C) at discharge. A hotter mix needs “successful past field experience or preconstruction testing” and the engineer’s approval before placement.

How do you find the evaporation rate?

Use the evaporation chart in ACI 305, Figure 2.1.5. Euclid prints it on the back of its EUCOBAR data sheet with four steps: enter with air temperature, move up to relative humidity, move right to concrete temperature, move down to wind speed, then move left and read the rate.

You need four readings on site: air temperature, relative humidity, concrete temperature from the first truck, and wind speed at slab height. A handheld weather meter reads the air side and a probe thermometer reads the concrete. Take readings again at midday, because the rate changes fast as the wind picks up.

The 1.0 kg/m²/h line is old guidance. FHWA’s Appendix A summary quotes ACI 308-92: “A value of evaporation of 1.0 kg/m2/h or more requires measures to prevent excessive moisture loss,” and over 0.5 “may also require some measures.” Euclid’s hot weather article gives the same pair in US units.

Why the chart is only a starting point

The chart estimates evaporation, not how much water your mix can bleed back. FHWA’s pavement curing research found paving mixes may bleed less than 0.5 kg/m²/h even at their peak, and that rates as low as 0.3 kg/m²/h “may cause critical losses from some concrete if not protected.” The same study found the chart overestimates rates above 0.5.

Low-slump, sticky mixes are the ones to watch. NRMCA’s plastic shrinkage guide says mixes with high cement content, high fines, low water, entrained air, high temperature or silica fume bleed less and can crack “even when evaporation rates are low.”

Why do plastic shrinkage cracks form?

They form when the surface loses water faster than bleed water rises to replace it. NRMCA calls that “the critical condition.” As water drops below the surface, tiny menisci between particles pull the top layer into tension, and it cracks as soon as it starts to stiffen.

The cracks show up while the concrete is still plastic or just after. NRMCA describes them as “usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow,” and generally not running to the slab edge. Wind over 5 mph, low humidity and high air or concrete temperature all raise the risk.

Anything that delays set also raises it. NRMCA lists retarders, some water reducers, fly ash and slag, cool subgrades and high water content. That matters in summer, because the set retarder that buys you finishing time also leaves the surface exposed longer.

How do you stop concrete cracking in hot weather?

Slow the drying from the moment the concrete hits the ground until the curing goes on. NRMCA’s CIP 5 and CIP 12 boil down to this list, most of which has to be set up before the first truck.

1. Dampen the subgrade, forms and steel, but do not leave standing water.
2. Put up windbreaks and sunshades on the windward side.
3. Schedule an early start. NRMCA says to avoid the hottest part of the day.
4. Ask the supplier about cooler concrete. NRMCA says chilled water can drop it up to 10°F (6°C) and ice up to 20°F (12°C).
5. Have enough finishers that nothing waits. If delays happen, cover the slab with wet burlap or plastic between passes.
6. Fog upwind of the slab, into the air above it, until curing materials go on.
7. Spray an evaporation retarder behind the screed and after each float pass as needed.
8. Never sprinkle water on the surface to help finishing. NRMCA says not to.
9. Consider synthetic fibers (ASTM C1116) in the mix for slabs that crack every summer.

Thin residential slabs on a vapor retarder need extra care. NRMCA notes that placing directly on a vapor retarder can increase the risk of plastic shrinkage cracking.

Evaporation retarder or curing compound: what is the difference?

An evaporation retarder is a finishing aid; a curing compound is the cure. The retarder goes on between screeding and final finishing to keep the surface from drying out. The compound goes on after final finishing and stays on to hold water in for days.

The EUCOBAR data sheet, a typical product, says to dilute it 9 to 1 with water and spray it “as soon as possible after floating,” with “additional applications” in extreme drying. Its precautions say plainly: “Do not use as a curing compound.” NRMCA’s curing guide agrees that evaporation retardants “should not be used for final curing.”

When do you apply curing compound in hot weather?

As soon as final finishing is done and the surface water sheen is gone. NRMCA’s hot weather rule is “start curing as soon as finishing is completed” and keep it going for at least 3 days. In hot wind the sheen can vanish in minutes, so have the sprayer mixed and on the slab before the last pass.

For the full timing rules, see when to apply curing compound.

Do not spray into standing bleed water to beat the heat. FHWA says applying compound too early “causes a delamination of a thin layer of surface mortar.” Its suggestion for strong drying is to wait out the main bleeding, apply half the compound, then apply the rest later in a second coat.

Cure longer than you think. NRMCA’s 3 days is a hot weather minimum; specs written to ACI 308.1 curing requirements call for 7. Wet burlap under plastic or a wet cure works too, and NRMCA notes water curing also keeps the slab cooler. Compare the options in wet curing vs curing compound.

Should you use white pigmented curing compound?

On exterior paving in full sun, yes. ACI 305R, as summarized by FHWA, says “white pigmented curing compound is recommended to reflect light and reduce solar heating.” NRMCA adds that the white color helps you spray it evenly, since thin spots and misses show.

ACI 305R also asks more of compounds in hot weather: a water loss limit of 0.39 kg/m², tighter than the 0.55 kg/m² in the C309 limit. For mixes with a water to cement ratio below 0.40, it says to water cure instead of using a membrane.

White compound is not for every slab. The W.R. Meadows 1600-WHITE data sheet says it reflects “the sun’s rays to help keep the concrete surface cooler,” but it is meant for highways, airports and street and curb paving.

The same sheet says not to use it on residential work, patios or sidewalks, or on surfaces that will later be painted, tiled or sealed. Interior slabs that get flooring need a different plan; see curing compound and flooring.

Does concrete cure faster in hot weather?

It sets and gains early strength faster, and pays for it later. NRMCA says concrete “cured at high temperatures at an early age will not be as strong at later ages” as the same mix cured around 70°F (20°C). Heat also raises water demand, so a mix that gets extra water on site loses strength and durability twice.

Faster set means a shorter finishing window, not a shorter curing period. Count curing days from the end of finishing, the same as any other season.

If your slab already has plastic shrinkage cracks

If you catch them during finishing, the finisher can often close them by refinishing, NRMCA says. Then fix the cause right away: fog, windbreak or evaporation retarder for the rest of the pour, and cure the moment you finish.

If the slab has hardened, check what you have. Plastic shrinkage cracks are short, roughly parallel, shallow and stop short of the edges. Cracks that follow the rebar or a change in slab depth are settlement cracks, a different problem. Fine map-pattern cracks that only show when wet are crazing, covered in dusting, crazing and curling.

NRMCA says plastic shrinkage cracks “rarely impair the strength” of floors and pavements, but they can let in water and chemicals and become weak points for later cracking. Keep curing the slab for the full period even after the cracks appear. Ask your concrete supplier or a repair contractor about sealing them before the first freeze or salt exposure.