As high temperatures greatly influence the concrete curing process, there are many challenges when concreting in hot weather conditions.
The high temperatures found during hot weather concreting accelerate the cement hydration process, which may seem like an advantage as the concrete gains strength quickly. However, if the temperatures are too high and the reaction goes too fast the final 28-day strength will be lower than expected. This can be a very frustrating situation as you may think that everything is going according to plan until you get the low compression test result from the 28-day tests.
The graph above shows the strength development of the same concrete mix. One was cured at a temperature of 20 °C while the other was cured at 40 °C. You can see that the 40 °C, indicated with a red line, started gaining strength quickly but around the 14 days, the strength development almost stopped. On the other hand, the 20 °C one gained strength slower but ended up with higher-end final strength. This phenomenon is called the crossover effect.
Sometimes to counteract the loss of moisture and improve the workability of the concrete during hot weather, additional water is added to the concrete mix. This might help to balance out the increased water demand, but if overdone, you create a high water/cement ratio. This can result in lower final strength and a porous surface which will affect the overall durability of the concrete.
During hot weather, the risk of cracking increases. The causes of these cracks are:
The hot temperatures and high humidity in many cases increase the water evaporation. If too much of the water evaporates, the concrete volume shrinks, creating small cracks on the concrete’s surface. These cracks may look small, but in the long run, they can become channels for water and chemicals to enter the concrete. This compromises the concrete durability and creates concerns for the contractor.
During the cement hydration process, the reaction emits heat along the structure. However, the temperature is in most scenarios not the same in different parts of the structure. This is because, the heat dissipates more easily from the concrete’s surface, due to its location and the cooling effect of the wind, than in the concrete’s core. You can see this in the illustration below:
In the illustration, you can see that the temperature is higher at the core and then it decreases in the sides. This means that while the surface is contracting and hardening, due to the lower temperatures, the core is still warm and expanded. If the temperatures are too different, the internal tensions inside the concrete will increase and cause cracks. These cracks vary in size and severity, some will be simply aesthetic while others may present a structural problem, which can end as a costly affair.