Causes of Cracks in Prefabricated House Roofs
The causes of cracks in prefabricated house roofs mainly include aspects such as design, construction, inherent characteristics of concrete, and other factors.
Design Aspect
Insufficient design of floor slab thickness makes it difficult to withstand corresponding loads, easily leading to cracks. A lack of reinforcing steel bars at the side spans or corners, such as at the external corners of cantilevered slabs, where seven radial bars should be installed at each external corner to cope with stress concentration. Without these, the external corners are prone to cracking. When the floor slab has single-layer reinforcement, radial bars are even more essential. At the same time, if temperature stress is not fully considered during design, and there is a lack of thermal insulation layers and temperature joints, the roof will generate significant stress during temperature changes, resulting in cracks. Additionally, improperly shaped pre-embedded holes can cause stress concentration, leading to cracks.
Construction Aspect
Excessive concentration of construction loads or early access by personnel can exert excessive pressure on the roof structure that has not fully set, causing cracks to appear. Insufficient stiffness or stability of the formwork support system may lead to deformation during construction, affecting the quality of the roof structure and resulting in cracks. Early formwork removal, when the concrete strength has not reached the required level, makes it unable to bear its own weight and external loads, leading to cracking. Inadequate curing, such as insufficient curing time or improper methods, can cause rapid water evaporation and concrete setting in hot weather, preventing air bubbles inside from being squeezed out. Over time, the bursting of these air bubbles can create撑开 (expanding) cracks in the concrete. Improper control of the steel bar protective layer, unreasonable layout of pre-embedded pipelines, uneven concrete vibration, or improper troweling techniques can also affect the integrity and strength of the roof structure, leading to cracks. Furthermore, suboptimal steel bar placement, such as the haphazard addition of steel bars, can alter the overall tensile force, causing inconsistent shrinkage and setting. Even the addition of light fixture boxes in the middle of the house can change the tensile force, resulting in cracks.
Inherent Characteristics of Concrete
Plastic shrinkage during the initial pouring stage, drying shrinkage during the hardening process, and temperature shrinkage caused by cement hydration are inherent physical and chemical changes in concrete materials that inevitably lead to a certain degree of cracking on the roof.
Other Factors
In areas with large temperature differences, if the roof lacks expansion joints, it will undergo significant deformation due to thermal expansion and contraction, resulting in cracks. Shoddy construction practices, such as uneven pouring of cement, overly spaced steel bars, and insufficient cement curing time, can also seriously affect the quality of the roof structure and cause cracks.
