Cracks in Prefabricated Concrete Houses

The occurrence of cracks in prefabricated concrete houses is closely related to construction techniques, material properties, environmental conditions, and component design, necessitating targeted measures based on the type and width of the cracks.

Main Causes of Crack Formation

1. Construction Technique Issues

   • Excessive mixing or transportation time leads to concrete moisture evaporation, reduced slump, and irregular shrinkage cracks during hardening.
   • Inadequate or prolonged vibration causes internal defects in the concrete (such as honeycombing and voids), resulting in plastic shrinkage cracks; unauthorized increases in the water-cement ratio can damage concrete properties, leading to increased shrinkage during hardening and the formation of mesh cracks.
   • Inadequate vibration in the mold (air bubble content > 5%) or excessive temperature differentials during steam curing (> 20°C/h) may cause cracks up to 0.1mm on the surface of prefabricated components.

2. Material and Environmental Factors

   • The coefficient of thermal expansion for concrete is 1×10⁻⁵/°C. When the ambient temperature difference exceeds 20°C, walls may develop fine cracks ranging from 0.1 to 0.2mm due to thermal expansion and contraction.
   • Shrinkage deformation (shrinkage rate approximately 0.02%-0.04%) occurs during the hardening process due to moisture evaporation. Excessive cement usage or inadequate curing may increase cracks to 0.25mm.
   • When differential settlement of the foundation exceeds 30mm, walls may develop diagonal cracks (width > 0.3mm) due to shear stress.

3. Design Defects in Prefabricated Components

   • Improper handling of prestressed tendons (such as failure of transverse reinforcement at the slab end to secure the prestressed tendons) may lead to spalling of the concrete cover.
   • Improper support positions during transportation and stacking, uneven ground, or irregular layering of timber may cause cracks.
   • Design defects in components (such as cutting off longitudinal reinforcement in beams at mid-span or insufficient anchorage length) may result in diagonal or vertical cracks.

Crack Classification and Safety Standards

According to the "Code for Acceptance of Construction Quality of Concrete Structures" (GB50204-2015), crack control in prefabricated components is classified into three levels:

• Indoor Dry Environment: Maximum allowable crack width of 0.30mm (non-load-bearing walls, secondary beams).
• Humid Environment: Allowable crack width of 0.20mm (kitchen and bathroom walls, exposed beams and columns).
• Outdoor or Corrosive Environment: Allowable crack width of 0.15mm (roofs, exterior basement walls). In ordinary residential environments, cracks with a width ≤ 0.25mm are considered "harmless cracks" and do not require structural reinforcement; if the crack width > 0.3mm or is continuous, it is necessary to determine whether it is caused by foundation settlement or excessive structural loads and have it repaired by a professional team.

Prevention and Repair Measures

1. Construction Phase: Strictly control mixing and transportation time, vibration techniques, and concrete mix proportions; ensure adequate vibration in the mold and maintain a steam curing temperature differential ≤ 20°C/h.
2. Material Selection: Use cement within its shelf life, control cement usage (≤ 400kg/m³), and enhance curing to reduce shrinkage.
3. Component Handling: Optimize prestressed tendon configuration, add spiral stirrups or steel mesh; standardize support positions and stacking methods during transportation.
4. Crack Repair: Non-structural cracks (such as craze cracks and horizontal cracks) can be repaired with fillers; structural cracks (such as diagonal and vertical cracks) require intervention from a professional team.