Cracking of Prefabricated Floor Slabs in New Houses

Cracking of prefabricated floor slabs in new houses can be caused by various factors, including structural design, raw materials, construction, materials, and environmental conditions. Details 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. For example, insufficient estimation of the loads borne by the floor slab and the usage environment can lead to cracking during normal use due to stress concentration or insufficient load-bearing capacity.

Raw Material Issues

Improper selection of concrete raw materials is a common cause. High hydration heat of cement can increase the internal temperature of concrete, generating thermal stress and leading to cracks. Improper use of admixtures can affect the setting time and strength development of concrete. Unreasonable mix design, such as an excessively high water-cement ratio, can increase the shrinkage of concrete during hardening. Poor aggregate gradation or high silt content can affect the compactness and strength of concrete, leading to floor slab cracking.

Improper Construction

• Unauthorized alteration of the water-cement ratio: During pumped concrete construction, water is added without authorization to improve fluidity, increasing the shrinkage of concrete during hardening and resulting in mesh or irregular cracks.
• Inconsistency between the support system and the plan: Unreasonable horizontal and vertical tie bar settings in the formwork support system and insufficient support stiffness can increase deformation of the formwork support under floor loads, causing excessive twisting and cracking of the floor slab.
• Significant deviation in steel bar positions: Negative moment bars at supports are not effectively positioned, and no access walkway is provided during concrete pouring, causing 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 cracking.
• Failure to install anti-cracking mesh at pipeline locations: In cast-in-place slabs, multiple crossings of pipelines without anti-cracking measures can excessively weaken the concrete section, leading to floor cracks along the direction of embedded pipelines.
• Inadequate curing: After concrete pouring and tamping, the surface is not covered, and curing with water is not timely. Rapid evaporation of free water on the surface, lack of water for cement hydration, and rapid volume shrinkage can cause cracking due to low early strength. Improper curing in high summer temperatures can more easily lead to temperature cracks.
• Early loading: Applying loads before the concrete reaches the specified strength can cause deformation and cracking of the floor slab.
• Early removal of the support system: Due to fast construction progress and insufficient support systems, upright posts are removed too early before the concrete reaches the formwork removal conditions, increasing the risk of cracking due to increased deflection of the floor slab.

Material Issues

Excessive spacing between steel bars or the use of single-layer bidirectional steel bars without pads and horse-shaped supports underneath; problems with the materials themselves, such as poor-quality concrete supplied by small ready-mixed concrete plants, can affect the structural performance of the floor slab and lead to cracking.

Environmental Factors

Temperature changes can cause concrete to shrink or expand, potentially leading to temperature cracks.