What is Thermal cracking of concrete?
Thermal cracking is most often formed within the first two or three weeks after casting and caused by restraint to early thermal movements but can also be created at any time in the life of a concrete structure by extreme thermal gradients.
Excessive temperature differences within a concrete structure or its surroundings cause the cooler portion to contract more than the warmer section, restraining contraction and creating tensile stresses and surface cracking.
Cracks form when the tensile stress within the concrete exceeds its tensile strength, most often due to temperature differences between the core and the surface of the concrete but also along the length of a beams or depth of columns.
This differential can be caused by the heat generated during cement hydration or rapid changes in ambient temperature especially directly after demoulding.
What causes Thermal cracking in concrete structures?
Most often thermal cracking occurs in large volume pours with significant quantities of cement providing high heat of hydration and high core temperatures in the first few days after casting. As the surface of the concrete does not generate as much heat, and radiates much of the heat it does produce into the atmosphere, it will be significantly cooler than the core, especially in cold ambient conditions.
Casting concrete in locations with extremely high ambient temperatures can accelerate the hydration process, produce heat more quickly and exhibit higher peak teak temperatures; as concrete has low thermal conductivity this can increase the thermal gradient between the core and the surface and create thermal cracking.
Even when using suitable concrete mixes with limited heat of hydration rapid changes in surface temperature such as when demoulding concrete units in freezing temperatures can shock the concrete into cracking due to the rapid contraction of the surface and large tensile stresses it creates.
Casting concrete members with uneven geometry, such as large step or hammerhead units, can allow the generation of thermal gradients between the more massive section and the thin edge section (Figure 2).
Similarly, when different sections of the same unit endure different environmental conditions such as in partially buried columns, the insulation provided by the soil contact will create a thermal gradient compared to the top of the column which can radiate its heat more freely into the surrounding air.
What are the signs of Thermal cracking in concrete structures?
Volumetric expansion.
How can I identify Thermal cracking in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
Identification of cracks distribution | - |
How can I prevent Thermal cracking in concrete structures?
Preventing thermal cracking in concrete structures begins with thoughtful design and extends through material selection, temperature control, and construction practices.
The design phase is crucial, with considerations for concrete movement due to thermal expansion and autogenous shrinkage. This involves strategic planning around concrete cover, dimensions, and geometry to effectively manage temperature gradients, ensuring that the stress/strength ratio does not exceed critical limits. Breaking large members into smaller connected pours can help reduce hydration heat and eliminate thermal cracking.
Opting for aggregates with a low coefficient of thermal expansion, cement types that generate less heat during hydration, and the use of supplementary cementitious materials and admixtures can improve concrete's performance and reduce early-age cracking. These materials influence the heat of hydration and tensile strength of the concrete, making the mix less prone to thermal stresses.
Controlling the temperature of fresh concrete is vital for preventing thermal cracking. Techniques such as pre-cooling the concrete mix with ice or liquid nitrogen, using thermally insulating formwork, and ensuring the concrete temperature at placement is optimized can significantly reduce the risk of thermal cracking. Proper curing practices, including insulating the concrete surface to control the cooling rate and employing electronic temperature sensors for real-time monitoring, allow for immediate adjustments to prevent cracking.
By integrating these strategies from the initial design through the construction and curing stages, the risk of thermal cracking can be significantly reduced, ensuring the structural integrity and longevity of concrete structures. Adapting to environmental conditions and following best practices in construction procedures further supports the effective management of thermal cracking
How can I repair the damage from Thermal cracking on concrete structures?
If cracks are detected, it is important check that they:
do not progress with time.
do not significantly increase the corrosion risk.
do not penetrate through the structure element.
In some cases crack monitoring may be necessary.
In aggressive exposure conditions Concrete Coating could be applied to mitigate further deterioration processes, namely, corrosion.