With heatwaves hitting several states, I thought it would be a good time to revisit hot weather concrete repair and installation. Repairing concrete in hot weather can be difficult. Listed below are a few tips for working with concrete in the heat as well as some alternatives for repairing concrete.

When concrete sets up or hardens, it is an exothermic reaction, meaning it generates heat. That reaction goes faster when the concrete is hot. The primary concern with concrete’s strength and set time is not the air temperature but the concrete temperature. As cement hardens, it sucks up water and grows crystals around the aggregate particles. When it’s hot, that reaction is fast and the crystals grow quickly without having time to become strong. Early strength will be higher but as it ages (around 28 days) the strength decreases. If the concrete is about 18° hotter than normal (for example, 88° instead of 70°), the ultimate compressive strength will be about 10% lower.

In hot weather, as the cement sets up, more mixing water is needed. This can weaken a concrete repair (as much as another 10% lower strengths) and cause color differences. Another potential problem in hot weather is surface drying—if the concrete is warm and the sun is shining and there is a hot dry wind across the concrete, more drying and surface shrinkage are likely. Also, maintaining air content can be a problem in warm concrete. Mixing is more likely to drive air out of the concrete making the level difficult to control.

Using polyurethane concrete repair materials are a solid alternative to repairing with concrete as they are much less susceptible to temperature fluctuations. Resins, epoxies, patches and caulking can all be affected by temperatures so it is important to identify their workable range. Some polyurea materials will have almost no working time in high temperatures.

Concrete Welder for cracks and small concrete repairs is virtually unaffected by high temperatures because of the quick and simple application. Concrete Welder comes in a cartridge that can be squirted into repairs and leveled off quickly and easily. FlexSet can be used for larger concrete repairs but in extreme temperatures be aware of shortened working time. If the materials can be kept inside a cooler area until it is time to apply them, the heat will make little difference in the cure time.

If using concrete, the objective is to keep that concrete that is as cool as possible because any concrete warmer than about 80° could be a problem. Since there’s more aggregate than anything else in concrete, aggregate temperature has the greatest effect on concrete temperature. Providing shade and using cool water is another way to get cool concrete. Retarding admixtures are another tool to control concrete in warm weather. If too much retarder is added to concrete used for a slab it can lead to crusting, where the surface sets but the concrete below is still soft. This can lead to delamination of the surface.

Another way to slow set times is by using fly ash for part of the cementitious material, although this will change the color. Fly ash will not make much difference if your concrete is more than 75°. Keep the sun off the concrete surface if at all possible. In very hot weather, place the concrete early in the morning or later in the evening and remember that friction within the concrete during mixing can generate enough heat to raise the concrete temperature 5° in 30 minutes. Sometimes in very hot weather it is safest to work with concrete in the dark.

Polyurethane repairs can be simpler and faster when the weather is hot but if your repair is very large it might be best to repair with concrete at night when the temperatures are lower.