Thermal cracking in concrete occurs because of extreme temperature differences within a concrete structure and its surrounding surfaces and environment. When cooler segments of concrete contract more than warmer portions of concrete it creates tensile stresses that can outweigh a concrete’s rated tensile strength. This results in cracks—or more technically, thermal cracks. There are several reasons thermal cracking occurs and some things you can do to help prevent it from happening.
We have replaced several thousand yards of concrete due to the following reasons for thermal cracking.
1. Thermal Cracking and Mass Concrete
As a commercial concrete contractor, we deal with mass concrete on a regular basis. Concrete spaces are only growing larger, and yet mass concrete is at an increased risk of thermal cracking.
The first issues arise when Portland cement is used for mass concrete. Portland cement is not a good mix for mass concrete because it creates too much heat during the hydrating process. In smaller slabs, the heat can cool quickly and evenly across surfaces. The same results are not true in mass concrete. Thick foundations, long slabs, and columns trap heat faster than it can dissipate. This results in the exact phenomenon that creates thermal cracks in the first place—temperature fluctuations in slabs of interconnected concrete.
What is Mass Concrete?
In general, mass concrete is defined as any type of concrete feature that measures four-feet to six-feet deep. Another defining feature is the type of concrete mix applied. Mixes that are heavy on Portland cement generate and store more heat. These mixes are better suited to smaller concrete segments as opposed to mass concrete.
It’s important to define mass concrete during the beginning stages of project development. “Some project specifications increase the risk of thermal failure by changing the definition of mass concrete (to increase the thickness) to avoid the added expense of designing low heat-of-hydration mixes,” reads forconstructionpros.com.
Concrete mixes with reduced temperature rise are better suited to mass concrete projects. Using these types of mixes is the best way to prevent thermal cracking in mass concrete. Project specifications should include a maximum temperature for placement, as well as a maximum difference in the interior and surface temperature. In general, temperatures should not exceed 160 degrees F and should have no more than a temperature difference of 35 degrees F. Thermal cracks can be serious in mass concrete and undermine the entire structure.
It is possible for a contractor to do everything right and still encounter thermal cracking.
2. Too Much Heat
Another reason for thermal cracks is if the interior or surface of concrete becomes hotter than the air or ground temperatures by 35 degrees F or more. Yet, even concrete surfaces that are just 10 degrees F warmer than the daily high can result in issues. The chance of thermal cracking increases as the concrete temperature increases in relation to ambient temperatures.
3. Adding Hot Water to the Mix
When applying concrete in cold water it is normal for contractors to add hot water to the ready-mix in order to accelerate the initial set process. The goal is for concrete temperatures to remain between 65 and 75 degrees F at the time placement begins. Yet, if air temperatures are 10 degrees below this (so 55 to 65 degrees F), the risk for thermal cracking increases.
4. Too High Portland Cement Content
As mentioned above, Portland cement produces more heat in concrete and therefore increases the risk of thermal cracking in concrete. Try replacing Portland cement with slag cement or class F fly ash to reduce heat of hydration.