Cracks in the Ceiling of Prefabricated Slab Houses

Cracks in the ceiling of prefabricated slab houses should be evaluated for danger based on their type, width, and development trend, and targeted treatment measures should be taken.

I. Types and Causes of Cracks

1. Normal Cracks

   • Shrinkage Cracks: During the production of prefabricated slabs, the hydration reaction of cement causes volume shrinkage, resulting in fine, uniform cracks with widths ranging from 0.05 to 0.2 millimeters. These cracks generally pose no structural risk.
   • Temperature Cracks: Changes in ambient temperature cause thermal expansion and contraction of the prefabricated slabs, leading to regular diagonal or horizontal cracks under structural constraints. These cracks are commonly found on the top floors of houses and are considered a normal phenomenon.
   • Settlement Cracks: Slight uneven settlement of the house foundation can cause cracking in the prefabricated slabs. These cracks are relatively wide and may extend from the bottom to the top of the slab, with their direction related to the settlement direction. The severity of these cracks should be assessed in conjunction with settlement observations.

2. Criteria for Dangerous Cracks

   • Excessive Width: According to the Technical Guidelines for Structural Safety Inspection of Self-built Houses, cracks with widths exceeding 1 millimeter should be regarded with caution as they may affect structural safety.
   • Stress-Related Cracks: Horizontal cracks that appear when the roof sags, even if only 0.8 millimeters wide, should be taken seriously as they may indicate abnormal stress on the prefabricated slab.
   • Critical Location: Cracks that extend from the roof to two-thirds of the wall height or are located on load-bearing beams significantly increase the risk factor.
   • Dynamic Development: Cracks that continue to lengthen and widen, even if they do not reach 1 millimeter, should be inspected by a professional organization to rule out the risk of structural instability.

II. Treatment Methods

1. Micro-cracks (as thin as a hair): These can be directly sealed with specialized adhesive to prevent moisture and dust from entering and to avoid further expansion of the cracks.
2. Medium-width cracks: These require the injection of structural adhesive. After injection, the adhesive should be allowed to cure completely for 72 hours to ensure adequate bonding strength.
3. Construction Precautions: When installing prefabricated slabs, a 10-15 millimeter thick layer of mortar should be laid at the slab joints to enhance the connection between slabs and reduce the risk of cracks caused by construction defects.

III. Key Recommendations

If cracks meet the criteria for danger (such as excessive width, stress-related, critical location, or dynamic development), the use of the affected area should be immediately suspended, and a qualified structural inspection agency should be commissioned to conduct an assessment. Based on the results, measures such as reinforcement or replacement of the prefabricated slabs should be taken to ensure居住 (living) safety.