Causes of Cracking in Prefabricated Floor Slabs of Houses
The main causes of cracking in prefabricated floor slabs of houses are as follows:
Structural Design Issues: Design units conduct floor slab structural design under critical conditions without fully considering multiple factors in the construction environment, such as temperature changes, humidity differences, and foundation settlement. This leads to uneven stress distribution in the floor slab when it bears loads, thereby causing cracking.
Raw Material Problems: Improper selection of concrete raw materials, such as cement with high hydration heat, can cause the internal temperature of the concrete to rise, generating thermal stress and subsequently leading to cracking. Improper use of admixtures or unreasonable mix design can also affect the strength and durability of the concrete, increasing the risk of cracking.
Unauthorized Alteration of Water-Cement Ratio: During pumped concrete construction, to facilitate construction, water is added without authorization to increase the fluidity of the concrete, altering the water-cement ratio. This results in increased shrinkage during concrete hardening, leading to the formation of irregular cracks in a mesh pattern or of varying lengths in the structure.
Inconsistency Between Support System and Plan: The horizontal and vertical tie rods in the formwork support system are improperly arranged, resulting in insufficient support stiffness. When the concrete strength has not reached a certain level, due to the influence of floor loads, the deformation of the formwork support increases, causing excessive twisting of the floor slab and thereby triggering cracks.
Significant Deviation in Rebar Position: Effective positioning measures are not adopted for negative moment rebars at supports. During concrete pouring, without a passage for workers, rebars are often stepped on and sink by construction personnel, reducing the actual effective height of the floor slab. This prevents it from effectively resisting negative moments, lowering the ability of the structural slab to withstand external loads and making it prone to cracking.
Lack of Anti-Cracking Mesh at Pipeline Locations: There are many instances of cross-shaped intersections of conduits in cast-in-place slabs, and without anti-cracking measures, these locations excessively weaken the concrete cross-section, causing the floor slab to be prone to cracks along the direction of the embedded conduits.
Inadequate Curing: After concrete pouring and tamping, surface covering is not carried out, and watering for curing is not timely. Combined with exposure to wind and sun, the free water on the concrete surface evaporates too quickly, and the cement hydration process lacks the necessary moisture, resulting in rapid volume shrinkage. The tensile stress generated by this shrinkage cannot be resisted by the concrete at its early low strength, leading to cracking.
Premature Loading and Early Removal of Support System: Applying loads to the concrete before it reaches the specified strength or removing the props before the concrete meets the formwork removal conditions can both lead to deformation of the concrete floor slab, thereby triggering cracking.
