Concreting in hot weather

Unlike most materials, concrete is often cast and cured on site rather than in the (more) controlled conditions of a factory. That means that temperature and humidity—as well as exposure to sun, wind, and precipitation—can affect cast-in-place concrete. Let’s look at the materials science of concreting in hot weather.

Portland cement clinker comprises four components:

  • Tricalcium silicate, or C3S in cement chemistry notation
  • Dicalcium silicate, or C2S
  • Tricalcium aluminate, or C3A
  • Tetracalcium aluminoferrite, or C4AF

The calcium silicates, C3S and C2S, form calcium silicate hydrate, which is responsible for the strength of concrete, and calcium hydroxide, or CH. The C3A is responsible for its setting. It’s not clear what C4AF does, if anything. Its slow hydration has little effect on the properties.


If portland cement consisted only of ground clinker, it would set rapidly due to the hydration of C3A. To prevent this “flash set” and maintain a reasonable working time, cement manufacturers intergrind gypsum into it. The sulfates in the gypsum control the setting.

Laboratory temperatures are maintained at about 73˚F, and the dosage of gypsum, about 5% by mass of cement, is adjusted to provide an acceptable setting time at that temperature. So what happens at higher temperatures?

Both solubility and rate of dissolution of most materials increase with temperature. This is true of C3A. However, gypsum is less soluble as the temperature increases. That is, the sulfates become less readily available just when they’re more necessary to control setting. When the temperature is high enough, there won’t be enough sulfates in solution to control flash set. Concreting in hot weather makes the concrete more susceptible to flash set.

Modern concretes almost never contain just portland cement. Supplementary cementitious materials such as fly ash, slag cement, and silica fume comprise a growing share of concrete volume. Judicious use of these materials can enhance the performance of fresh- and hardened concrete as well as reduce its carbon footprint. However, they can also produce unpleasant surprises. Class C fly ash often contains appreciable amounts of calcium and aluminates, which will affect the setting characteristics. If that fly ash is a component of a blended cement, the cement manufacturer can adjust the gypsum content accordingly to control the set. Adding the fly ash at the batch plant doesn’t allow opportunity for this.

Because contractors often want the concrete to gain strength quickly, cement producers grind cement finely to make it more reactive. Naturally the finer the cement, the more readily the C3A dissolves and the more sulfate is necessary to control flash set. Fine cement is likelier to cause problems when concreting in hot weather.

So if you know you’re going to be placing concrete in hot weather, qualify the mixture under those conditions. That is, turn up the heat in the lab and/or store your ingredients and mixing pan in a hot room overnight. See how it behaves and how much admixture you need to use to get the performance you want.

Air entrainment

Exterior concrete that will undergo cycles of freezing and thawing—particularly in the presence of deicing salts—needs a good air-void system to protect it. The severity of exposure depends on how saturated the concrete is when it freezes and whether deicing salts are present. Water expands on freezing, and the air voids give it a place where it can expand without inducing excessive tensile stresses in the concrete.

When concreting in hot weather, the air content is harder to maintain. Effectively, the air voids boil out of the fresh concrete. You may find that a higher dosage of air-entraining admixture is necessary to maintain the same air content.


Hot weather may promote drying of the concrete surface, leading to plastic shrinkage and then drying shrinkage. High temperatures, low humidity, and even a light breeze can cause rapid drying, which leads the surface to contract relative to the rest of the concrete. Early-age concrete has little or no tensile strength, so it’s not able to resist the stresses due to restrained shrinkage.

Of course the concrete doesn’t know what’s causing the stress; it just knows when it’s too much. Thermal gradients can also add to the stress. For example, rain on fresh concrete can rapidly cool the surface, causing it to contract relative to the concrete underneath. Similarly, rapid diurnal temperature changes—which occur most commonly in the spring and fall—can cause the surface temperature to drop.

Once the total tensile stresses exceed the tensile capacity of the concrete, it will crack. And once cracking starts, any additional tensile stresses will widen the cracks.

To minimize cracking, consider whether you need a larger crew to place and finish the concrete more quickly. If it’s not under a roof, use windbreaks and misting if necessary. And get the curing compound on it right away. Watch the weather forecast so you don’t place the concrete just before it rains. And consider whether fiber reinforcement would be worthwhile. Polypropylene fibers won’t take the place of temperature steel or structural reinforcement, but they can limit plastic shrinkage cracks.

Another possibility is to place the concrete beginning in late afternoon or evening so it doesn’t reach final set before the external temperature begins to fall. We can help you time your placement to take advantage of diurnal temperature changes using a thermal control plan.

Tortoiseshell cat with lion cut on Tibetan chest
Nisse gets a lion cut every spring to help her cope with hot weather. Photo: Rachel Detwiler.

Cement hydration and microstructure

The microstructure of hydrating cement paste develops via two processes, dissolution and diffusion. The rates of both increase with temperature, but the rate of dissolution increases more. That is, the component minerals C3S, C2S, and C3A all dissolve rapidly and begin to diffuse, but their hydration products precipitate quickly without moving too far from the original cement grain.

We’ve discussed previously how the microstructure forms under these conditions. Essentially the hydration products precipitate near the original cement grains, forming hard, impenetrable shells around them. Between these shells are large, continuous pores. That is, the concrete sets and gains strength quickly, but its later-age strength is lower than it would be under ideal curing conditions. More important, though, is that it’s more permeable and less durable.

If concreting in hot weather is unavoidable, you can adjust the mixture proportions to compensate. If your primary concern is strength, you may need to reduce the water-cementitious materials ratio. Supplementary cementitious materials such as fly ash and slag cement will help you obtain a more favorable microstructure if your main concern is permeability or durability. To get both strength and durability, do both. Because they react more slowly than cement and give off less heat of hydration, they’ll also give you more working time and moderate the internal temperature.

Working in hot weather

Hot weather is hard on workers, too, so make sure they have plenty of water to drink and a place to cool off. And allow for more frequent breaks.