Prefabricated Slab House with Cracks

Cracks in prefabricated slab houses require safety assessment and corresponding treatment measures based on specific causes. Common causes and treatment methods are as follows:

Crack Cause Analysis

1. Unsynchronized deformation of two prefabricated slabs
   Cracks are usually unidirectional and parallel to one side of the wall. If there is no significant deflection (bending deformation) in the floor slab, there is generally no safety hazard, but leakage may occur due to the gaps. Such cracks are often caused by loose splicing of prefabricated slabs during construction or differential settlement in the later stage.

2. Stress issues in concrete floor slabs
   Common in two-way slabs (supported on all four sides), if there are issues such as insufficient reinforcement, reinforcement displacement, insufficient concrete strength, excessive loads, or premature loading, the floor slab may crack due to uneven stress distribution:

• Cracks at supports: Cracks appear along the edges of beams (walls) on the upper part of the floor slab, often due to insufficient configuration of negative reinforcement at supports or insufficient shear resistance caused by unqualified concrete strength.
• Diagonal cracks: Diagonal cracks appear on the lower part of the floor slab and may penetrate through the slab, often caused by excessive positive bending moment in the mid-span or insufficient tensile strength of the concrete.
  If the crack width is small and there is no significant deflection in the floor slab, there is usually no safety problem; if the crack width is large (e.g., exceeding 0.3mm) or the floor slab shows obvious deformation, there is a structural hazard, and professional testing and assessment are required.

3. Concrete shrinkage
   During the hardening process of the floor slab concrete, shrinkage occurs due to water evaporation, and internal forces are released at weak points, causing cracking. Cracks are often vertically penetrating and commonly found at the corners of the floor slab or in areas with weak constraints (e.g., at the junctions with walls). When the crack width is not large (e.g., less than 0.2mm), it does not affect structural safety but may reduce durability (e.g., increase the risk of steel bar corrosion).

4. Steel bar corrosion
   The steel bars inside the floor slab rust due to damage to the protective layer or chloride ion erosion. The volume expansion of the rust causes concrete cracking. Cracks are distributed along the direction of the steel bars (horizontal and vertical), and rust marks are visible on the surface of the slab. Such cracks will expand over time, eventually leading to rusting and breakage of the steel bars and a decrease in the load-bearing capacity of the floor slab, requiring timely repair.

Treatment Methods

1. Routine maintenance: During redecoration, the cracks can be filled with Dacron cloth or mesh cloth and reinforced with carbon fiber cloth (which can be purchased online). This method is suitable for cases where the crack width is small and there is no structural hazard.
2. Professional testing and reinforcement: If the floor slab shows visible deflection or deformation, or if the indoor cracks penetrate and the width is greater than 0.7mm, a professional organization should be commissioned for testing and reinforcement.

• Non-developing cracks: Fill and seal with crack injection glue.
• Continuously expanding cracks: Measures such as carbon fiber reinforcement, steel plate bonding reinforcement, or adding supports should be combined to improve structural safety.

Tip: Before treatment, it is necessary to first determine the cause of the cracks to avoid blindly repairing and covering up potential risks. If you are unable to determine the nature of the cracks, it is advisable to contact a structural engineer for on-site assessment first.