A thorough description of why concrete puts off heat when it is curing, how much heat it puts off, and how you can get the best concrete possible.
Concrete is an excellent conductor of heat, but it also produces heat which raises the concrete temperature while curing. Why does curing concrete do this? The simple answer is that water, aggregates (stone, sand, and gravel), and portland cement start a chain of chemical reactions called hydration which raises the concrete temperature.
What are the types of concrete?
The type of concrete has a massive impact on the heat it produces and how long it takes to cure. Here is a handful of the most popular types of concrete and how the style affects the concrete temperature.
Type 1 cement
This will be the most common style of concrete. It has no additives to change the curing process, which means that it will only be able to be used on smaller projects where heat dissipation is not a concern. It will be the most common type that you will find in the construction industry. It is used as interior and exterior concrete.
Type 2 cement
This concrete mix has been reformulated with additives to help the curing process create less heat. It will be the more popular option for large structures that will already struggle while dealing with the high heat produced because of the reactions in the hydration process.
Type 4 cement
This concrete mixture produces the least amount of heat generated. The concrete you will find is used for massive projects like dams, bridges, and huge foundations.
These projects must be meticulous about how much temperature rise they allow to build within the concrete, or it could cause thermal cracking and would ruin the project. They will routinely use large machines such as coolers and chillers to help cool the concrete as they speed up the process while attempting to keep the concrete from overheating.
Does concrete give off heat while it is curing?
As concrete dries, it goes through an exothermic reaction. The chemical reaction begins when the water encounters the fresh concrete, starting the cement hydration process. The common misconception is that concrete is drying out, so it hardens. This is not entirely right. It does dry out, but it happens when a chemical reaction occurs, an exothermic process that continues utilizing the water molecules until they have used the moisture and develop what is left into a rigid material. It is not due to standard water evaporation like most people believe.
The chemical reaction generates heat to expel the energy involved in the chemical reaction, which causes a high concrete temperature. When hydration produces heat, it is not a problem if you deal with small construction projects such as small concrete structures, walkways, or a concrete slab where the high concrete temperature can dissipate into the air and surrounding ground.
You must be careful with the concrete pour when you get into massive civil engineering projects such as bridges and dams. Specific preventive measures must be taken against hot weather, air temperature, and internally generated heat, or you will cause heat stress and be left with an unusable concrete structure.
How hot does concrete get when setting?
The Portland Cement Association states that it will have a 10-15 degrees temperature rise for every one hundred pounds of concrete. This means that it is essential to control the heat produced, not only the heat generation of the concrete specimens but also the external temperature.
While the concrete materials are setting, you will want to ensure that it does not exceed the ideal concrete curing temperature, or the cement particles will harden faster. Although this may sound good if you are pouring concrete and dries too quickly, it can experience thermal shock, affecting the curing concrete. You will not have a solid final product; it will become brittle and have a low final compressive strength, so you have to redo your concrete pour.
Concrete cured in lower ambient temperatures, as in 50 degrees or cooler, will be moist longer. Unlike hot weather concreting, cold weather concreting may cause the concrete to freeze. If it freezes, the concrete stays cold, will not reach a proper concrete curing temperature, and will not gain strength as it should. This will also create an inferior substantial element that may crack or fail.
In ideal situations, concrete cures should always be poured in warm weather such as 50-90 degrees ambient temperature. This may mean that whoever oversees the concrete curing may have to insulate or wrap the concrete in cold weather. Or they may have to attempt to cool it off while hot weather is concreting. Water can be an effective way to lower the concrete curing temperature if done correctly.
How long does the concrete produce heat?
It will take the concrete approximately 28 days to complete the curing process. Although this will not be the end of the heat in its life, concrete is a thermal material and will absorb any heat.
Concrete is a dull, unreflective material that absorbs the heat from direct sunlight when it comes into contact. Light is a form of energy that cannot reflect away from the concrete, so it is absorbed and causes the concrete to reach higher temperatures. It also absorbs the heat from the ground and the air around it, which raises the temperature of the concrete.
Why is the temperature so crucial while curing?
The temperature that the concrete cures at is essential because it will play a massive role in the final strength of the concrete. If the concrete is exposed to high ambient temperatures while curing, the hot weather will raise the concrete temperature, and it will dry too quickly, which will cause it to be a weaker product.
The same concept applies when the concrete temperature is exposed to cold weather. It will cause the concrete to dry too slowly and cause it to be weaker and unusable.
Why could a hot day cause a disaster?
The reactions that happen during curing rely on water molecules to be able to complete the process. If the ambient temperature outside is too hot and the concrete temperature gets too high, then the water in the concrete will begin to evaporate in the hot weather. If the water starts evaporating, there will not be enough water molecules in the mix, resulting in a weaker final product.
The inside concrete temperature should not exceed 36 degrees Fahrenheit compared to the exterior of the concrete. If this happens, the concrete may begin to form thermal cracks and can be rendered much weaker.
Professionals who mix their own concrete on-site may adjust the ingredients to help offset the hot weather. Having more of one component than the other can drastically change how the concrete temperature is affected by hydration.
Professionals will most often choose to pour the concrete in the cooler part of the day to avoid the majority of the hot weather. They will continue to spray water on it as it dries to help it stay moist and control the concrete’s temperature. Or they can put plastic over the surface to raise the relative humidity and help contain the evaporated water before it escapes.
If the concrete job is a much larger project, such as a dam, bridge, or skyscraper foundation, the concrete professionals will have to be more careful while placing concrete. With projects of this size, they will most likely face hot weather and cold weather concreting. They may have to bring in commercial chillers to chill the concrete to prevent it from getting too hot. They may have to spend a majority of their time placing concrete in small segments at a time to help keep the internal temperature down.
Why can a cold day cause disaster?
The curing process of concrete produces heat, but it also needs to be in a semi-warm environment to work correctly. Suppose it is below the lowest manageable temperature of 50 degrees Fahrenheit outside. In that case, the water used for the chemical curing reaction will begin to freeze, and you will not be able to get your concrete to dry.
Getting concrete poured in the cold is as bad as pouring concrete in excessive heat. Like there are ways to cool off the hot concrete so that it can cure, there are also ways to keep your cold concrete warm to cure correctly.
The most common and one of the best options is to use a thermal drying blanket. It will be spread over the concrete and insulate it from the cold while maintaining an appropriate relative humidity and keeping the heat from the hydration process in the concrete. Some companies also offer these thermal blankets a heated blanket, which will get warmer, causing the concrete to hold its temperature more effectively.
Concrete will go through a series of reactions as it dries. While going through the responses will put off a large amount of heat, which needs to can be kept under control with external methods. You now know the different ways to keep the concrete cool and the optimal temperatures to help you have a successful curing process.
We discussed how the cold can be just as detrimental as the heat and that if attempting to pour concrete in the cold weather, you have options to assist you in keeping the concrete warm. You will have to do if you hope to have a useable piece of concrete.
Concrete does give off heat. Whether you are a contractor pouring a foundation for a residential home or pouring the concrete for the next hoover dam, you must be aware of your concrete temperatures, keep it under control, and hope you have a useable piece when you are done.