Thermal cracking in concrete (a detailed study)

Thermal cracks in concrete must be avoided, especially when the thickness of the concrete increases. Let's see what types of thermal cracks exist.

A common problem that designers and construction teams face is cracking due to thermal influences. Heat build-up in concrete is inevitable, but it can be controlled or minimized to some extent. Furthermore, the problem can also be resolved by providing calculated reinforcement based on design assumptions. Cracks in concrete can be treated in the following categories.

• Freeze/thaw cycles
• Outdoor seasonal temperature fluctuations
• Early thermal contractions

Cracks caused by freeze-thaw cycles

The water molecules in concrete freeze, thus increasing their volume compared to the volume contained in the concrete.

The damage caused by this process is known as freeze-thaw damage. If the water content in concrete is more than 91%, it is called saturated concrete.

When saturated concrete freezes, its volume increases by 9%. As there is no space to accommodate the increase in volume, the concrete is subject to additional stress. As a result, the following can be observed.

• Cement and aggregates can come loose
• Concrete can crack (random cracks)
• Surface sizing
• D cracking (durability cracking)

The effects of freeze/thaw effects are due to the following reasons and can be minimized as suggested here.

• Insufficient air pockets. If there is enough air to compensate for the increase in volume, the three problems listed above may not initially occur.
• Using non-durable materials leads to D cracking. Therefore, it is recommended to avoid this.
• A high water-cement ratio also directly contributes to this problem. Therefore, the water content could be limited by the use of water-reducing additives.

Cracks in concrete due to seasonal external temperature fluctuations

Buildings are exposed to direct sunlight and subject to significant temperature fluctuations compared to covered buildings. They absorb heat and release it again at night. The increase in temperature depends on environmental conditions.

When concrete heats up it expands and when it cools it contracts. When heated, tensile and compressive stresses arise, and the same process can also be observed during cooling. As a result, thermal cracks develop in concrete over time as stresses normally develop.

Cracks in concrete due to seasonal external temperature fluctuations can be minimized if they are taken into account at the design stage.

Depending on the tension created by the applied loads, the reinforcement of a section is calculated. Due to temperature fluctuations in the section, additional stresses arise. This must be taken into account when sizing the reinforcement.

To deal with the thermal effect, different load combinations can be taken into account in the design.

The design crack width can be limited for the following load combinations.

A structure is subject to bending moments (M), axial forces (T – tensile forces; causes cracks) due to loading, in addition to temperature fluctuations, such as the increase in temperature in relation to the ambient temperature during the hydration process (T ₁ ) and temperature fluctuations due to seasonal effects (T ₂ ).

1. Crack width (M + T + T ₂ )
2. Crack width (T ₁ only)
3. Crack width (T ₁ +T ₂ )

If the design is carried out taking into account the above cases, cracking can be minimized.

Early thermal contractions

Early thermal cracking occurs due to tensile stresses in concrete exceeding its capacity, and tensile stresses occur due to thermal contractions or due to temperature fluctuations within a section. Thermal cracks in concrete can take two to three weeks to develop.

The reaction between cement and water produces heat. The amount of heat and the rise in temperature depend on many factors.

• Type of cement
• Quantity of cement
• Placement temperature (concrete temperature at the time of concreting)
• Environmental conditions
• Component geometry including concrete thickness
• Type of formwork

Concrete reaches its maximum temperature on the first day and then begins to gradually cool down. The following figure shows temperature fluctuations in model tests when there are concerns about the type of formwork to be used, the maximum temperature in the core, the temperature gradient, the temperature difference between the core and the formwork, etc.

As explained above, there are many reasons for the development of additional stresses in concrete. Therefore, it is recommended to check the temperature fluctuations of concrete through model tests when the element sizes are larger (1000mm, 1500mm, etc.) and the quality of the concrete is also higher. The cement content increases as the quality of concrete increases, so the heat of hydration is higher compared to low-quality concrete.

The following factors can be considered as the main cause of premature thermal cracking.

• Temperature increase
• Coefficient of thermal expansion of concrete
• Concrete's ability to absorb tensile stresses
• Internal and external stains

Early thermal cracking can be prevented to some extent by structural reinforcements. Reinforcement calculations must be carried out based on relevant constraints and temperature fluctuations during the hydration process. These calculations take into account the peak temperature during the hydration process and the ambient temperature. Furthermore, after carrying out model testing, the following restrictions and measures will be taken. During the test it is also checked whether the specified values ​​are respected.

• Concrete, when placed has a temperature of no more than 30 ⁰ C. ( May depend on the project )
• The maximum increase in concrete hydration temperature should not exceed 70. ⁰ C. ( Normally this value is used. However, it may vary from project to project and the value may be increased depending on the recommendation of different references. A maximum peak temperature limit of up to 80 °C could be considered. ^Ó C with a content of fly ash and silica fume greater than 30% to minimize the risk of delayed ettringite formation. A deviation from the general value (70 ^Ó C) will always create uncertainty.
• No temperature gradient in concrete may exceed 20. ⁰ C , at a distance of 1 m and the maximum temperature difference between two parts of the same casting shall not exceed 25 ⁰ C. ( Both limit values ​​must correspond to the respective design specifications. These values ​​may change slightly according to the specifications. )
• To meet the above requirements, Portland cement with fly ash or low-heat Portland cement or mixed hydraulic cement that meets the requirements of the relevant cement standard may be used, in addition to other temperature control measures during concreting (e.g. use of ice or chilled water for concrete mixing, special curing/insulation methods during concrete hydration).
• The contractor prepares a 2mx2mx2m sample concrete block ( Size may vary and depends on the actual sizes of the item. ) using the concrete mix and aims to demonstrate the fluctuations in temperature rise during the hydration process to verify the suitability of the mixture composition and other temperature control measures for use in the shielding structure.
• The Contractor shall submit to the Engineer at least two months before the commencement of specific work on the shielding structures a description of the procedure detailing all measures taken to achieve the above temperature control measures.

Furthermore, special attention should be paid to the formation of delayed ettringites, as they can significantly affect structural strength. It is necessary to carry out the necessary checks and select the type of formwork that meets the above requirements.