Cracks in Prefabricated Floor Slabs of Houses
Cracks in the prefabricated floor slabs of houses are mainly caused by factors such as structural design, raw materials, construction, materials, and others, as detailed below:
Structural Design Issues
Design units conduct floor slab structural design under critical conditions without fully considering multiple factors in the construction environment. For example, they fail to accurately calculate the stress on the floor slab in different environments, resulting in cracks due to the floor slab bearing loads beyond the design expectations during actual use.
Raw Material Issues
Improper selection of concrete raw materials is a significant cause. High hydration heat of cement can raise the internal temperature of the concrete, generating thermal stress and leading to cracking. Improper use of admixtures may affect the setting time and strength development of the concrete. Unreasonable mix design, such as an excessively high water-cement ratio, increases the shrinkage of the concrete during hardening. Poor aggregate grading affects the compactness and strength of the concrete, thereby causing cracks.
Improper Construction
- Unauthorized alteration of the water-cement ratio: During pumped concrete construction, adding water without authorization for convenience leads to increased shrinkage of the concrete during hardening, resulting in irregular cracks in a mesh pattern or of varying lengths on the structure.
- Support system issues: The support system does not conform to the plan, and the horizontal and vertical tie rods of the formwork support system are unreasonably arranged, resulting in insufficient support stiffness. When the concrete strength has not reached a certain value, the floor load causes increased deformation of the formwork support, leading to excessive twisting of the floor slab and causing cracks.
- Deviation in steel bar positions: Negative moment bars at supports are not effectively positioned, and no access walkway is provided during concrete pouring, causing the steel bars to be trampled and sink. This reduces the actual effective height of the floor slab, decreases its ability to resist external loads, and makes it prone to cracks.
- Improper handling of pipeline areas: There are many crossings of pipelines in the cast-in-place slab, and no anti-cracking measures are taken, which excessively weakens the concrete cross-section and makes the floor slab prone to cracking along the direction of the embedded pipelines.
- Inadequate curing: After concrete pouring and tamping, the surface is not covered, and watering for curing is not timely. The rapid evaporation of free water on the surface, lack of water for cement hydration, and sudden volume shrinkage occur. Due to the low early strength, the concrete cannot resist the stress and cracks. Improper curing in high summer temperatures can also easily lead to temperature difference cracks.
- Early loading and premature removal of the support system: Applying loads or removing the support system before the concrete reaches the specified strength can deform the floor slab and increase the risk of cracks.
Material Issues
Excessively large spacing between steel bars or the use of single-layer bidirectional steel bars without cushion blocks and horse stools for support below the steel bars; poor quality of the materials themselves, such as low-quality materials provided by small concrete mixing stations, can all lead to floor slab cracking.
Other Factors
The lack of reinforcing steel bars at the corners of the floor slab or early loading can cause diagonal cracks at the corners. Cracking along the direction of the embedded water and electricity pipelines usually occurs when the pipeline diameter exceeds one-third of the slab thickness and no reinforcing mesh is provided. Irregular crazing is mostly caused by inadequate curing or abnormal concrete setting.
