Reasons for Cracking of Precast Concrete Houses
The reasons for cracking of precast concrete houses mainly include temperature stress, material shrinkage, foundation settlement, production process defects, inadequate curing, improper concrete mix proportions, issues with construction materials, and unreasonable reinforcement layout.
Temperature Stress: The coefficient of thermal expansion of concrete is 1×10⁻⁵/℃. When the ambient temperature difference exceeds 20℃, the wall may develop fine cracks of 0.1-0.2mm due to thermal expansion and contraction. For example, in northern winters, when the day-night temperature difference reaches 15℃, the walls on the sunny and shady sides are prone to cracking due to uneven temperature distribution.
Material Shrinkage: During the hardening process of concrete, water evaporation causes shrinkage deformation, with a shrinkage rate of approximately 0.02%-0.04%. If the cement content is too high (>400kg/m³) or curing is insufficient, shrinkage cracks may increase to 0.25mm. Additionally, when the mud or powder content of sand and gravel is excessive, the materials do not bond tightly with the cement, also leading to shrinkage cracks.
Foundation Settlement: When the differential settlement of the foundation exceeds 30mm, the wall is subjected to shear stress, which may result in diagonal cracks with a width of more than 0.3mm.
Production Process Defects: If the mold is not vibrated densely (bubble rate >5%) or the temperature difference during steam curing is too large (>20℃/h), surface cracks of less than 0.1mm may occur during the prefabrication stage.
Inadequate Curing: If water curing is not carried out according to standards, such as failing to keep the concrete surface moist within the specified time or the curing period is insufficient (ordinary cement for no less than 7 days, composite cement for no less than 14 days), the concrete surface may dry and crack.
Improper Concrete Mix Proportions: When the cement content is excessive, the concrete surface has a thick layer of laitance, resulting in insufficient surface strength and a stress difference with the interior, which easily leads to cracking.
Issues with Construction Materials: When the mud or powder content of sand and gravel is excessive, the materials do not bond tightly with the cement, leading to shrinkage cracks.
Unreasonable Reinforcement Layout: Insufficient reinforcement ratio, especially when the reinforcement ratio of small-diameter bars is too low, reduces the ultimate tensile strain capacity of the concrete, resulting in local cracking.
