How To Reduce Thermal Cracking

What is Thermal Cracking?

Thermal cracking occurs when the temperatures of different portions within a concrete slab exceed a certain limit. These differentials exist because while the internal temperature of the concrete slab rises and drops slowly, due to concrete’s exothermic reaction, the peripheral temperature of the slab cools rapidly due to the ambient temperature.

The difference between internal and peripheral temperatures cause restraint in the concrete member, which can then result in tensile stress that exceeds the in-situ tensile strength, causing Thermal Cracks.

Internal temperature vs. Peripheral temperature of a concrete member.

Although Thermal Cracking tends to occur at an early age, there is a similar temperature-related cracking phenomenon that can occur at a later age as well. Concrete bodies will expand and contract when exposed to hot and cold ambient temperatures, respectively. When this bulk volume change, resulting from temperature variations is restrained, this later age cracking can occur.

How Do You Minimise Thermal Cracking?

Thermal Cracking can be dangerous to your structure, so it is important to recognise when it might occur, and proactively prevent and/or minimise its chance of happening.

Typically, design specifications will include a maximum temperature, and a maximum temperature differential. The former is used to identify the timeframe in which preventive measures can be taken, whereas the latter outlines when cracking might occur. The permitted temperature differential range varies, but is usually limited to 20℃.

Connecting a sensor to a rebar on the peripheral of the pour.

In order to proactively prevent Thermal Cracking, you must be aware when the temperature differentials are in danger of exceeding the specified limit. This is done by using temperature sensors to actively measure the temperatures of the different areas within your concrete slab. Ideally, multiple sensors will be placed in various areas of your slab (centre, peripheral, etc.) in order to accurately measure the differentials.

Once aware of the temperature, there are different actions which can be taken in order to control the temperature of the slab. Misting the slabs, providing shade, and placing the concrete at different times of the day are all methods of reducing heat exposure from sun radiation. In addition, auxiliary components, like frost/curing blankets, may also be used to help control the in-situ temperature.

Workers place a curing blanket to control concrete temperature.

Optimising your concrete mix can also help reduce Thermal Cracking. Adjusting the amount of portland cement, or water, for instance, can actively reduce the heat of hydration.

The Importance of Temperature

Measuring temperature is not only useful for the prevention of Thermal Cracking, but can be used for a myriad of other useful processes. Compressive strength, for instance can also be calculated based on your in-situ temperature (read our post on why cubes shouldn’t be used to measure concrete strength). If you are already measuring temperature, for any reason, be sure to see how you can leverage that to save time on other on-site processes.