Category: hot weather

Heat Can Cause Concrete Problems

Hot Weather = Weak Concrete

Pouring concrete is as much an art as it is a science. Everything has to be right, or you run the risk of your concrete being weak, developing cracks, or showing other signs of damage. We’ve discussed the effects of cold weather on concrete before, so now we’ll address the impact of the weather being too hot.

The Problem is Crystal Clear

Concrete is hard because it grows crystals around the aggregate particles. This happens by the cement drawing the water out of the surrounding material. This reaction, called hydration, generates heat, so the reaction accelerates when the concrete is hot. If the concrete is hot, the rapid reaction will not allow the crystals time to grow strong. According to, the early strength will be higher, however, the 28-day strength is less. If the concrete is formed at 90 degrees, instead of 70, the concrete’s compressive strength will be approximately 10% lower. If the weather is too hot, more mixing water will need to be added. This can contribute to even weaker concrete and changes in color between slabs.

Hot Weather Can Cause Shrinkage

hot weather, concrete project

Start at air temperature, draw a vertical line up to relative humidity, then a horizontal line to the right to concrete temperature, then go vertically down to wind velocity, and horizontally to the left to determine the evaporation rate. If the evaporation rate is greater than 0.1, plastic shrinkage cracking is likely.

As stated earlier, there are a lot of variables that can come into play regarding a successful concrete pour. Warm concrete and a hot, dry wind can cause shrinkage. G.E. Munro wrote an article about concrete shrinkage and weather for the Lafarge Cement newsletter in 1987. He gives the example of a mug of beer. If you have a cold glass of beer on a hot day, what happens on the outside of the mug? The humidity in the air will condense on the glass. He states that the same thing can occur with concrete. If the concrete is 18 degrees or cooler than the air, the humidity will condense on the concrete, preventing it from drying out as quickly. On the other hand, if the concrete is warmer than the air, you may have a problem with the concrete drying out and shrinking.

The image from ACI 305 can help you determine if evaporation will be a problem with concrete shrinkage. You will have to know the air and concrete temperature, humidity, and wind speed to calculate the rate of evaporation. The chance of shrinkage and cracking is possible if the sum is >.01 lbs per square foot per hour.

Keep An Eye On The Weather

Any concrete project can be tricky, but the problems grow exponentially with the size of the project. You have to keep an eye on the weather for the duration of the project. Temperature, precipitation, and other variables need to be taken into account. It is important that you hire someone that has experience with these kinds of situations and knows what needs to be done for the successful completion of a project. Matthews Structural Solutions has structural engineers and expert contractors to give you the peace of mind to know that your project will turn out right.

Contact Matthews Structural Solutions today and learn more about how we can make your concrete project a success, no matter the size. We look forward to hearing from you.