Keywords: Pumping concrete; Temperature crack; Cause analysis; Control measures

1. Introduction

With the advancement of building technology, pumping concrete has become widely used in modern construction. This technique not only improves the workability of concrete but also allows for the construction of thin-walled structures with minimal vibration. Additionally, it enhances the impermeability and durability of the final structure. However, the use of pumping concrete often involves a high proportion of cementitious materials and specific aggregate gradations, which can lead to significant heat generation during hydration. This heat accumulation can cause temperature differences between the internal and external parts of the concrete, leading to temperature-induced cracks. These cracks can compromise the structural integrity and long-term performance of the building. Therefore, understanding the mechanisms behind temperature cracks and implementing effective preventive measures is essential.

2. Mechanism and Characteristics of Temperature Cracks

During the hardening process, cement hydration generates a large amount of heat. In thick sections of concrete, this heat tends to accumulate, causing a sharp rise in internal temperature. Meanwhile, the surface cools more rapidly, creating a significant temperature gradient. This difference leads to uneven expansion and contraction, resulting in tensile stress on the surface. When this stress exceeds the tensile strength of the concrete, cracks form. These cracks typically appear after the initial setting of the concrete. In some cases, sudden temperature drops due to cold weather can cause rapid cooling of the surface, while the interior remains warm. This contrast can create additional tensile stress, especially near the surface, leading to shallow cracking.

Temperature cracks usually have no fixed pattern. Large-scale cracks often appear in a random, crisscross manner. In beams and slabs, cracks tend to run parallel to the shorter sides. Deep and through-cracks are generally aligned along the longer edges of the section, with more frequent occurrences in the middle. The width of the cracks varies depending on temperature fluctuations—wider in winter and narrower in summer. Cracks caused by thermal expansion are usually thinner in the middle and wider at the ends, whereas cold shrinkage cracks show less variation in thickness. Such cracks can lead to steel corrosion, carbonation of concrete, reduced freeze-thaw resistance, and lower durability.

3. Influencing Factors and Prevention Measures

The internal temperature of concrete is influenced by its thickness and the type and quantity of cement used. Thicker sections and higher cement content result in greater heat generation, increasing the risk of temperature cracks. For mass concrete, larger structural dimensions lead to higher temperature stresses, making cracking more likely. Thus, controlling the temperature difference between the core and surface of the concrete is the most critical measure in preventing temperature cracks in such cases.

3.1 Selection of Raw Materials and Mix Design

(1) It is advisable to use low- or medium-heat cement and reduce the cement content. High cement content increases hydration heat, leading to greater temperature differentials and potential cracking. Using fly ash or slag cement instead of ordinary Portland cement can help lower the heat of hydration. Additionally, incorporating superplasticizers and optimizing aggregate grading can further reduce the need for cement and minimize cracking risks.

(2) Adding admixtures such as fly ash or water-reducing agents can improve the workability of the mix, enhance cohesion, and reduce the heat generated during hydration. These additives also help delay the peak temperature, allowing for better heat dissipation and reducing the likelihood of cracks forming.

3.2 Improving Construction Processes

(1) Optimizing the mixing process, such as using a two-step mixing method or wrapping aggregates in mortar, can improve the bonding between cement and aggregates, resulting in stronger concrete and reduced hydration heat. Implementing new cooling techniques can also help lower the pouring temperature, especially in hot weather conditions.

(2) Careful control of the pouring process is essential. Proper layering and sub-block pouring allow for better heat dissipation and reduce constraints. Vibration before the final set helps eliminate voids and improve bond strength, minimizing internal cracks. During hot seasons, covering the concrete with wet burlap or using cooling methods can help regulate the temperature.

(3) Effective curing is crucial for maintaining the required temperature and moisture levels. Insulating the surface helps slow down the cooling rate, reducing the risk of surface cracks. In cold weather, additional insulation should be applied to protect against sudden temperature drops.

4. Treatment of Temperature Cracks

Various methods are available to repair temperature cracks, depending on their severity and location. Common approaches include surface sealing, grouting, structural reinforcement, and concrete replacement.

4.1 Surface Repair Method

This method is suitable for minor surface cracks that do not affect the structural integrity. It involves applying sealants, epoxy resins, or protective coatings to the affected area. Sometimes, reinforcing materials like fiberglass cloth are used to prevent further cracking and enhance the surface’s durability.

4.2 Caulking Method

Caulking involves filling cracks with flexible or rigid waterproof materials, such as PVC, rubber, or polymer cement mortar. This method is effective for sealing cracks and preventing water infiltration, which can accelerate deterioration.

4.3 Structural Reinforcement Method

If cracks significantly impact the structure’s performance, reinforcement techniques such as adding steel plates, increasing cross-sectional area, or using prestressed reinforcement may be necessary. These methods help restore the structural capacity and improve long-term stability.

4.4 Concrete Replacement Method

In cases of severe damage, removing the affected concrete and replacing it with new material is the most effective solution. This approach ensures the structural integrity of the building and restores its functional properties.

5. Conclusion

Temperature cracks are a common challenge in concrete construction, particularly in pumped concrete applications. While they are inevitable to some extent, their presence can weaken the structure, reduce impermeability, and shorten the lifespan of the building. Therefore, it is crucial to recognize the risks associated with these cracks and implement proper prevention and repair strategies. By adopting advanced materials, optimized construction techniques, and timely maintenance, the quality and safety of concrete structures can be significantly improved, meeting the demands of modern construction standards.

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