When it comes to construction, cracks can be a common occurrence. Thermal cracking is a common issue in concrete construction that can compromise the integrity and durability of the structure. In this blog, we’ll explore the causes, prevention and remedies of thermal cracking in concrete, and what they mean for the longevity of your building.

What is the Thermal Cracking of concrete?

Thermal cracking of concrete refers to the phenomenon of cracking in concrete structures due to temperature changes. When concrete is exposed to temperature variations, it expands or contracts, which can cause it to crack. This is because concrete is a rigid material that cannot easily deform to accommodate the stresses caused by thermal expansion or contraction. Thermal cracking is a common problem in concrete structures, and it can lead to serious structural issues if not addressed in a timely manner. Proper design, construction techniques, and maintenance can help prevent the thermal cracking of concrete.

What are the causes for Thermal Cracking?

Hydration of cementitious materials generates heat for several days after placement in all concrete members. This heat dissipates quickly in thin sections and causes no problems.
In thicker sections, the internal temperature rises and drops slowly, while the surface cools rapidly to ambient temperature.
Surface contraction due to cooling is restrained by the hotter interior concrete that doesn’t contract as rapidly as the surface, creating tensile stresses that can result in cracking.
Thermal cracking typically occurs at early ages, although exposure to extreme temperature changes can cause cracking in older concrete.
Mass concrete members, defined by a minimum dimension of 4 feet (1.3 m), are particularly susceptible to thermal cracking.
However, temperature cracking can occur in non-mass concrete structures, such as the upper surface of pavements and slabs.
Concrete has a thermal coefficient of expansion ranging from 5.5 to 14.5 millionths/°C, contributing to the risk of thermal cracking in concrete structures.

Identifying Thermal Cracking in Concrete: Signs to Watch Out For

Random pattern cracking on the surface of mass concrete members is a common sign of thermal cracking caused by excessive temperature differentials.
Checkerboard or patchwork cracking may appear within a few days after formwork stripping.
Temperature-related cracks in pavements and slabs can resemble drying shrinkage cracks and typically occur perpendicular to the longest axis of the concrete.
These cracks may become visible any time after concrete placement, but usually appear within the first year or summer-winter cycle.

Exploring Predictive Methods for Thermal Cracking in Concrete

PCA Method:

Calculates 10°C temperature rise for every 100kg of cement, assuming a minimum dimension of 1.5 meters.
Does not calculate time to maximum temperature or temperature differentials, and does not account for differences in cement composition.
Adjustments for supplementary cementitious materials (SCMs) such as fly ash or slag are crude at best.

ACT 207.2R Graphical Method:

Uses charts and equations based on empirical data, but makes assumptions for boundary conditions and cement fineness based on test methods that are rarely used anymore.
Generally underestimates maximum temperature and poorly predicts time to maximum temperature.

Shmidt’s Method:

Developed in the 1920s as a simplified finite difference method, but offers little guidance for boundary conditions and can be difficult to model.
Adjustments for SCMs are crude and the method should be performed by an experienced engineer.

Preventing Thermal Cracks in Concrete: Tips and Techniques

The key to preventing thermal or temperature-related cracking is to recognize when it might occur and take steps to minimize it.
Reduce the heat of hydration by optimizing cementitious materials using supplementary cementitious materials like fly ash or slag or using a Portland cement that generates lower heat of hydration.
Avoid specifying an excessively low water-cement ratio (w/cm), as retarding chemical admixtures may only delay peak concrete temperatures.
Consider starting with a cooler initial concrete temperature to reduce the peak temperature in the structure, but be mindful of practical feasibility and project costs.
For pavements and slabs, reduce heat gain from solar radiation by using misting or shading techniques during placement.
Placing concrete in the early morning may result in a more critical situation if peak hydration temperature coincides with peak ambient temperature.
Be cautious with wind breaks, as they may inhibit evaporative cooling and increase heat gain. Use curing blankets to reduce heat loss from slabs and pavements during cold weather conditions.
If cracks appear during final finishing, the finisher may be able to close them by refinishing. However, to avoid further cracking, take necessary precautions as discussed above.