Thermal cracks typically appear in the early days after concrete placement, especially in mass concrete structures or under conditions with significant temperature fluctuations. This phenomenon occurs when the internal temperature of the concrete rises sharply due to the heat of cement hydration, while the surface, exposed to cooler air or wind, cools down more rapidly. This temperature gradient creates tensile stresses at the surface, which, if exceeding the tensile strength of the concrete, result in cracking. Thermal cracks not only affect the aesthetic appearance of the structure but may also, over time, reduce its durability and overall performance.
Under such conditions, the use of specific concrete admixtures can serve as an effective engineering solution to prevent or reduce the severity of these cracks. Admixtures such as supplementary cementitious materials (SCMs), air-entraining agents, shrinkage reducers, set retarders, and reinforcing fibers each contribute in their own way to controlling concrete temperature, reducing internal stresses, and improving tensile strength. Through the proper combination of these admixtures and the application of complementary construction techniques, it is possible to produce concrete with high resistance to temperature variations—ensuring reliable performance in both hot and cold climates.
Thermal cracks often appear in the early days after concrete placement, especially in mass concrete structures such as foundations, thick walls, or dams. These cracks occur when:
According to ACI technical guidance and CIP 42, thermal cracking can occur not only in mass concrete but also in slabs, pavements, and surfaces exposed to significant temperature fluctuations.
Supplementary Cementitious Materials (SCMs): An Effective Solution for Heat Control in Mass Concrete
Supplementary cementitious materials such as fly ash and ground granulated blast furnace slag (GGBFS) are widely used admixtures in the concrete industry, playing a crucial role in reducing the risk of thermal cracking. These pozzolanic compounds, when used to partially replace Portland cement, lower the amount of heat generated during hydration — a critical factor in mass concrete, especially under high temperature conditions.
By reducing internal temperatures and promoting a more uniform heat distribution within the concrete matrix, these admixtures help prevent damaging thermal stresses and significantly decrease the likelihood of cracking. Moreover, SCMs contribute to a denser microstructure through slower but more stable pozzolanic reactions, enhancing concrete durability against permeability, sulfate attack, and thermal cycling.
🔹 Key Applications:
The use of SCMs in projects such as dam construction, deep foundations, thick walls, industrial footings, and concrete placement in hot and dry climates is considered one of the most effective engineering strategies for controlling concrete temperature and preventing both early-age and long-term cracking.
Set Retarding Admixtures: Thermal Balance in Challenging Concrete Placements
Shrinkage-reducing admixtures are among the most effective tools for controlling cracks caused by plastic and early drying shrinkage, playing a key role in enhancing the durability and uniformity of concrete. These admixtures reduce capillary tension in the pore water, preventing rapid moisture loss during the early stages of setting — a common cause of surface cracks and initial tensile stress-related cracking.
These compounds perform especially well in dry, hot, or windy climates where ambient humidity is low and the risk of surface moisture loss is high. By better regulating internal moisture and reducing the rate of shrinkage, they contribute to the dimensional stability of concrete at early ages and provide ideal conditions for uniform, crack-free setting.
🔹 Key Applications:
Ideal for industrial slabs, flooring, water tanks, tunnels, and large concrete surfaces exposed to direct sunlight, dry winds, or rapid temperature changes — where precise surface crack control is essential.
Set Retarding Admixtures (Retarders): Thermal Balance in Challenging Concrete Placements
Set retarding admixtures, commonly known as retarders, are essential chemical components used to manage the cement hydration process. By extending the initial setting time of concrete, they provide more flexibility for construction activities while also helping to moderate internal temperature fluctuations within the concrete. This is particularly important in high ambient temperatures or when placing large volumes of concrete.
By slowing down the hydration rate, retarders prevent the simultaneous peaking of internal concrete temperature and ambient heat — a condition that could otherwise lead to increased thermal stresses and the formation of surface and internal cracks. Retarders are especially beneficial in projects that require extended working time, such as structures with complex reinforcement or concrete placement in hot weather, as they prevent early setting and loss of workability.
🔹 Key Applications:
Suitable for hot-weather concreting, mass concrete pours, multi-phase placements, bridges, large slabs, and projects with potential construction delays that require precise setting time control and effective temperature management.
Air-Entraining Agents: Enhancing Concrete Flexibility Against Thermal Stress and Freezing
Air-entraining agents are essential admixtures in the design of concrete resistant to harsh weather conditions, particularly in cold regions. These materials introduce very fine and uniformly distributed air bubbles into the concrete matrix, creating small voids that act as cushions during water expansion caused by freezing or sudden temperature rises.
This process reduces internal pressure and helps prevent cracking due to freeze-thaw cycles. Additionally, the micro air bubbles enhance the concrete’s resistance to water penetration and other aggressive environmental factors, significantly improving the long-term durability of the structure.
🔹 Key Applications:
Ideal for concrete pavements, sidewalks, exterior slabs, bridges, water-contact structures, and projects in cold or mountainous regions exposed to severe temperature fluctuations or frequent freezing conditions.
Fiber Reinforced Concrete (FRC): Internal Reinforcement for Combating Thermal Cracks
Fiber Reinforced Concrete (FRC) is one of the most advanced modern technologies for enhancing concrete’s resistance to both thermal and mechanical cracking. By incorporating synthetic or natural fibers—such as polypropylene, steel, glass, or carbon—into the concrete mix, a more flexible and resilient structure is achieved. This type of concrete excels at controlling and containing micro-cracks caused by thermal stresses, shrinkage, or dynamic loading.
The fibers act as a stress-distributing network within the concrete, helping to spread stress evenly throughout the section. As a result, stress concentration in a single point is reduced, preventing the formation and propagation of both surface and internal cracks. This property is especially valuable in environments subject to frequent heating and cooling cycles or repeated loading.
🔹 Key Applications:
Ideal for industrial floors, on-grade slabs, concrete walls, road pavements, tunnels, shotcrete, and projects involving high thermal or dynamic loads—where structural stability and crack reduction are top execution priorities.
In addition to the use of admixtures, the following technical measures are essential for controlling thermal cracking:
✅ Using concrete with a low initial temperature
✅ Implementing a Thermal Control Plan
✅ Insulating formwork and controlling the rate of cooling
✅ Cooling mass concrete using embedded cooling pipes
✅ Monitoring with digital temperature sensors
✅ Delaying the removal of insulating blankets or protective formwork
In mass concrete, the temperature difference between the core and the surface should not exceed 20°C.
Preventive Strategies for Avoiding Thermal Cracking in Concrete (Non-Admixture-Based)
🌞 In Hot Weather:
❄️ In Cold Weather:
Thermal cracking is a silent yet significant risk in concrete projects. However, with proper mix design, smart use of admixtures, and precise temperature control measures, this issue can be effectively prevented.
Whether in hot and dry climates or in cold and snowy winters, concrete admixtures are key tools for engineers to ensure the durability, safety, and aesthetics of structures.
