Cracks in Prefabricated Floor Slabs of Houses

Cracks in the prefabricated floor slabs of houses need to be evaluated based on their type to determine if they pose a safety risk. Normal cracks do not present structural risks, while abnormal cracks require prompt attention.

1. Types and Characteristics of Cracks

1. Normal Cracks

   • Shrinkage Cracks: These occur when the cement in the prefabricated slab undergoes hydration, causing volume shrinkage. When the shrinkage stress exceeds the tensile strength of the concrete, cracks form. These cracks are fine and uniform, typically ranging from 0.05 to 0.2 millimeters in width, and are irregularly distributed. They usually do not affect structural safety but may cause water leakage.
   • Temperature Cracks: These result from environmental temperature changes causing the prefabricated slab to expand and contract. Under structural constraints, cracks form, commonly seen on the top floors of houses. They often appear as regular diagonal or horizontal lines and are more pronounced in winter due to greater temperature differences. The width of these cracks is generally less than 0.3 millimeters.
   • Settlement Cracks: These are caused by slight uneven settlement of the house's foundation. The cracks are relatively wide (possibly exceeding 0.3 millimeters) and may extend from the bottom to the top of the slab, with their direction related to the settlement. If the settlement stabilizes, the cracks will not continue to develop.

2. Abnormal Cracks

   • Stress Cracks: These occur due to insufficient reinforcement, misaligned steel bars, inadequate concrete strength, or excessive loads. Cracks appear along the edges of beams (walls) at the upper supports of the floor slab and as diagonal cracks at the lower part, possibly penetrating the entire slab. When wide, these cracks pose a safety risk.
   • Steel Corrosion Cracks: When steel bars rust, their volume expands, causing the concrete to crack. These cracks run along the direction of the steel bars, appearing straight and horizontal or vertical. Rust stains are visible on the slab's surface, and the cracks gradually widen over time until the steel bars rust through, reducing the slab's load-bearing capacity.

2. Crack Evaluation and Treatment

1. Normal Crack Evaluation: If the crack characteristics match those of shrinkage, temperature, or settlement cracks, and the width does not exceed 0.3 millimeters, the cracks are not continuously developing, and there is no accompanying house tilt or significant deflection, they can be considered normal. Cracks that are unidirectional and parallel to one side wall, with no slab deformation, generally do not pose a safety risk but require waterproofing.
2. Abnormal Crack Evaluation: If the crack width exceeds 0.3 millimeters, continues to expand, or is accompanied by house tilt, significant deflection (such as slab sagging), or if indoor cracks penetrate and are wider than 0.7 millimeters, immediate structural inspection and evaluation are necessary, which may involve reinforcement.
3. Treatment Methods:
   • Normal Cracks: During renovation, use Dacron cloth or mesh fabric to fill the cracks, and apply an outer layer of carbon fiber cloth to enhance crack resistance, while ensuring proper waterproofing.
   • Abnormal Cracks: Commission a professional agency for inspection and evaluation, and based on the results, adopt measures such as pressure grouting, steel plate bonding, or carbon fiber reinforcement to ensure structural safety.