Thermal Cracking in Concrete: Causes, Prevention and Repair Methods

Thermal cracking in concrete occurs due to temperature differentials within a structure, leading to tensile stresses that exceed the material’s tensile strength. This issue is particularly common in mass concrete, where the heat generated during the hydration of cement causes the interior to expand while the surface cools and contracts, creating stress. Thermal cracks can compromise structural integrity, allowing moisture and aggressive agents to penetrate, which accelerates deterioration.

Where it occurs?

Thermal cracking in concrete typically occurs in areas where significant temperature differentials develop, such as:

1. Mass Concrete Structures: Foundations, dams, bridge piers, or large columns, where the heat generated during hydration cannot dissipate uniformly.
2. Concrete Pavements: Exposed to rapid temperature changes between day and night or during seasonal shifts.
3. Walls and Slabs: Particularly thin sections that cool faster than their thicker counterparts, leading to uneven contraction.
4. Exposed Surfaces: Structures exposed to harsh environmental conditions, like cold winds or direct sunlight, causing surface cooling or heating at different rates than the interior.
5. Early-Age Concrete: Freshly poured concrete is especially vulnerable as it undergoes significant temperature changes during hydration.

Causes of Thermal Cracking in Concrete:

1. Heat of Hydration: Excessive heat generated during cement hydration.
2. Rapid Cooling: Sudden temperature drops causing uneven contraction.
3. Rapid Heating: Surface expansion due to sunlight or high temperatures.
4. Inadequate Curing: Uneven drying leading to thermal stress.
5. High Cement Content: Increased internal heat generation in the mix.
6. Improper Aggregate Selection: Aggregates with high thermal expansion properties.
7. Structural Restraints: Restricted movement causing stress accumulation.
8. Large Temperature Gradients: Core and surface cooling at different rates.
9. Improper Construction Practices: Ignoring environmental conditions during placement.
10. Poor Mix Design: Lack of supplementary materials to control heat.

Types of Thermal Cracking in Concrete:

1. Plastic Shrinkage Cracks: Plastic shrinkage cracks occur during the early stages of concrete setting, typically within a few hours after placement. These cracks form when the concrete surface dries out more quickly than the underlying layers, creating internal stresses. This can happen due to rapid moisture loss, high ambient temperatures, or strong winds, which cause the top layer of the concrete to shrink faster than the rest. These cracks are typically shallow and irregular, and while they do not always compromise the structural integrity, they can affect the surface finish and durability of the concrete.
2. Temperature Differential Cracks: Temperature differential cracks occur when there is a significant temperature gradient between the surface and the interior of a mass concrete structure, such as a large foundation or dam. The core of the concrete retains heat from hydration, while the surface cools more quickly, creating internal stress. This differential expansion and contraction cause the concrete to crack, especially if the temperature difference is large enough to exceed the material’s tensile strength. Such cracks are commonly observed in large concrete pours, where the size of the structure amplifies the temperature differences between the core and surface.
3. Restraint Cracking: Restraint cracking occurs when there are internal or external restrictions that prevent the concrete from expanding or contracting freely with temperature changes. Reinforcement bars, adjacent structural elements, or the surrounding soil can restrict the natural thermal movement of the concrete. When the concrete expands due to heat or contracts during cooling, these restraints create tensile forces that the concrete cannot resist, leading to cracks. This type of cracking is particularly common in reinforced concrete structures, where the steel reinforcement may limit the movement of the concrete.
4. Surface Cracks from Rapid Cooling: Surface cracks from rapid cooling are common when concrete that has been exposed to high temperatures (such as during curing or after initial setting) is suddenly exposed to a cold environment. This can occur when freshly placed concrete is subjected to sudden rain, frost, or rapid temperature drops in colder climates. The surface layer contracts much faster than the interior, leading to the development of cracks. These cracks are typically shallow but can compromise the aesthetic quality and durability of the concrete surface.
5. Expansion Cracks: Expansion cracks develop when concrete is exposed to prolonged high temperatures or intense sunlight, causing it to expand. This type of cracking is particularly problematic in regions where concrete is exposed to extreme heat or direct sunlight. When the concrete expands, the tensile stress may exceed the material’s strength, leading to cracks, particularly in concrete elements that are restrained or confined by other structural elements. These cracks often appear near the surface and can create significant damage over time if not addressed.
6. Cyclic Thermal Cracks: Cyclic thermal cracks are caused by repeated cycles of heating and cooling, a phenomenon common in outdoor or exposed structures like pavements, bridges, and building facades. As the concrete heats up during the day and cools down at night, the expansion and contraction stresses may build up over time. This cyclical pattern of thermal loading can eventually lead to cracking as the concrete is unable to fully accommodate the repeated changes in temperature. Over time, these cracks can grow, leading to durability concerns.
7. Edge or Joint Cracking: Edge or joint cracking typically occurs near the edges or joints of concrete structures, where temperature gradients and movement restrictions are more pronounced. These cracks are often caused by uneven temperature distribution, such as when one side of a concrete slab is exposed to the sun while the other side is shaded. Additionally, the presence of joints or weak points in the concrete can exacerbate the effect of thermal stress, as the concrete may not have the same capacity to expand or contract across these areas. These cracks can compromise the structural integrity and may lead to water infiltration if not properly sealed.

How  to prevent thermal cracking in concrete? 

1. Use low-heat cement or blended cements to reduce heat during hydration.
2. Ensure adequate curing methods to retain moisture and control temperature variations.
3. Apply insulating blankets or forms to minimize temperature differences between the concrete core and surface.
4. Gradually cool large concrete pours to avoid sudden temperature changes.
5. Avoid pouring during peak heat and use chilled water or ice in extreme conditions.
6. Use aggregates with low thermal expansion properties to reduce internal stress.
7. Incorporate expansion joints to allow for thermal expansion and contraction.
8. Prevent rapid drying by using water-retention techniques and maintaining moisture in the concrete.
9. Reduce the thickness of large concrete pours to limit temperature gradients.
10. Properly place reinforcement to control cracking and resist tensile stresses.

Different repair method for thermal cracking in concrete:

1. Surface Sealing- Surface sealing involves applying sealants or coatings over the cracks to prevent the entry of moisture, chemicals, or other harmful substances. This method helps to protect the concrete from further deterioration, especially in environments subject to freeze-thaw cycles or exposure to aggressive chemicals.
2. Epoxy Injection- Epoxy injection is a widely used method for repairing thermal cracks. It involves injecting a two-part epoxy resin into the cracks under pressure to bond the crack faces together. This process restores the load-carrying capacity of the concrete by re-establishing the continuity of the material.
3. Routing and Sealing- Routing and sealing is a two-step process where the crack is widened slightly using tools to create a reservoir for a sealant or filler. This technique allows the sealant to bond more effectively and ensures that the repair material adheres better to the crack.
4. Grouting- Grouting is used to fill larger cracks or voids in concrete that cannot be repaired with simple sealants. A non-shrink grout, often made from cement-based or epoxy-based materials, is injected into the crack. 
5. Concrete Patching- Concrete patching is used when the crack has caused significant surface damage or when the concrete around the crack is deteriorating. The loose or damaged concrete is removed, and the area is cleaned before applying a patching compound. This compound is typically a mix of cement, sand, and additives designed to bond strongly to the existing concrete and match its properties. 
6. External Reinforcement- In cases where the structural strength of the concrete has been compromised due to thermal cracking, external reinforcement methods can be used. This can involve the use of carbon fiber reinforced polymers (CFRP), steel plates, or mesh, which are bonded to the surface of the concrete. 
7. Carbonation- Carbonation is a controlled process where carbon dioxide is introduced to react with calcium hydroxide in the concrete, forming calcium carbonate. This can sometimes help reduce the width of the crack, especially if the concrete has been exposed to CO₂. 
8. Surface Repair with Fiber-Reinforced Polymers (FRP)- Fiber-reinforced polymers (FRP) are advanced composite materials used to reinforce and repair concrete. In thermal cracking cases, FRP sheets or wraps are applied to the concrete surface to provide additional strength and prevent further cracking. 
9. Application of Concrete Bonding Agents- Concrete bonding agents are used to improve the adhesion between the original concrete and new repair material. When cracks are repaired with patching compounds or resurfacing materials, bonding agents help ensure that the new material adheres properly to the old concrete, enhancing the durability of the repair. 

Conclusion

Thermal cracking in concrete can be prevented through proper curing, material selection, and temperature control. Addressing these factors ensures the durability and integrity of concrete structures over time.