Cracks in Prefabricated Beams of Houses

Cracks in the prefabricated beams of houses require targeted treatment based on the type of crack, along with cause investigation and preventive measures to ensure structural safety.

Analysis of Crack Causes

1. Improper Construction Operations

   • Premature Handling: Moving the concrete before it has fully hardened leads to uneven structural stress. Post-tensioned prestressed beams lifted before the grout or end caps have dried, or pre-tensioned beams with the steel strands released before the concrete has hardened, can both result in prestress loss and cracking.
   • Curing Defects: Failure to extend the curing time in low-temperature or dry environments, or an improper ratio of cement to sand and gravel, can lead to insufficient concrete strength and reduced crack resistance.
   • Site Constraints: Stacking unhardened beams prematurely to free up space increases the risk of cracking.

2. Design and Material Issues

   • Design Flaws: Such as abrupt changes in the concrete structure's cross-section leading to stress concentration, insufficient steel reinforcement ratio, or an excessively high concrete grade with improper construction mix proportions.
   • Material Quality: Substandard water quality, an imbalanced sand-to-gravel ratio, or rapid surface drying of the concrete in high-temperature or windy conditions can trigger plastic shrinkage cracks.

3. Accumulated Structural Damage

   • Combined factors such as uneven foundation settlement, defects in the beam's steel reinforcement ratio, or unreasonable structural design can lead to crack propagation.

Remedial Measures for Cracks

1. Large Cracks (Deep and Wide)

   • Chiseling Treatment: Chisel away the concrete along the crack until the crack disappears, and clean away the debris and dust.
   • Rapid Repair Mortar: Mix ordinary cement (400kg), fine sand (600kg), special adhesive powder (15-20kg), and water (155-165kg) according to the specified ratio, and quickly fill the crack to ensure compactness.

2. Small Cracks (Superficial)

   • Special Adhesive Treatment: Use ready-made adhesive (such as HQG-4# Huaqian Adhesive) or self-prepared adhesive (100kg HQG-4# adhesive + 10kg ordinary cement), stir evenly, and apply 2-3 coats to ensure penetration into the crack.

Precautions

• Thoroughly clean the crack surface of oil, dust, and other impurities before repair.
• Strictly follow the specified material ratios to avoid errors that could affect the repair effectiveness.
• For complex cracks or difficult repairs, it is advisable to contact a professional construction team.

Preventive Measures

1. Strictly Adhere to Hardening Time: Move the concrete only after it has fully hardened (usually 7-28 days, depending on the environment), and use a rebound hammer to test the strength before operation.
2. Optimize Curing Conditions: In low-temperature or dry weather, take measures such as covering for insulation and spraying water for curing; adjust the cement-to-sand-and-gravel ratio to ensure the strength meets design requirements.
3. Reasonably Plan the Site: Reserve stacking space according to the production speed to avoid premature handling; manage the prefabricated beams in zones and move them in batches according to the hardening progress.

Special Reminder: The allowable crack width for concrete beams in ordinary residential buildings is 0.3 millimeters (as per the "Code for Design of Concrete Structures" GB50010). If the crack continues to lengthen and widen, horizontal or 45° diagonal cracks appear at the bottom of the beam, or is accompanied by door and window deformation, tile bursting, or floor tilting, it may indicate foundation settlement or structural damage, and the house safety appraisal station should be contacted immediately (within 24 hours).