Cracking of Precast Concrete Houses

Cracking in precast concrete houses may be caused by temperature stress, material shrinkage, foundation settlement, or production process defects, and the need for repair should be determined based on the width and nature of the cracks.

I. Common Causes of Cracking

1. Temperature Stress: Concrete materials have thermal expansion and contraction properties. If expansion joints are not reserved during the production or installation of precast components, or if there are significant environmental temperature changes (such as large day-night temperature differences), tensile stresses can develop inside the components, leading to surface or internal cracks.
2. Material Shrinkage: During the hardening process, concrete undergoes shrinkage due to water evaporation. If curing is insufficient (e.g., low humidity or short duration), shrinkage stresses may exceed the material's tensile strength, causing cracking. Additionally, the type of cement (such as high early-strength cement) or improper aggregate grading can also exacerbate shrinkage.
3. Foundation Settlement: If the soil under the house is uneven (e.g., at the junction of soft and hard soil), the backfill is not compacted, or there are changes in the groundwater level, local settlement differences can occur, leading to cracks at the connections between precast components or in the walls. These cracks are usually inclined or through-going.
4. Production Process Defects: During the production of precast components, if the thickness of the steel reinforcement cover is insufficient, the prestressing tension is improper, or the concrete is not vibrated densely, the crack resistance of the components can be reduced, making them prone to cracking under load.

II. Criteria for Crack Assessment and Repair Principles

According to the Code for Acceptance of Constructional Quality of Concrete Structures (GB50204-2015):

• Allowable 'Harmless Cracks': Cracks with a width of ≤0.25mm are usually caused by material shrinkage or temperature changes and only affect aesthetics, requiring no structural reinforcement. Surface sealing treatments (such as applying epoxy resin or cement-based capillary crystalline coatings) can be used to prevent water ingress.
• Cracks Requiring Inspection and Repair: Cracks with a width exceeding 0.3mm or that are through-going (penetrating the wall) may be caused by foundation settlement or excessive structural loads and require inspection by a professional team. Inspection contents include crack depth, foundation settlement observation, and structural load analysis. Repair methods include pressure grouting (filling with epoxy resin or cement slurry), adding steel reinforcement, or foundation treatment (such as grouting reinforcement).

III. Preventive Measures

1. Design Stage: Reasonably set expansion and settlement joints and optimize component reinforcement (e.g., increase the number of distribution bars).
2. Construction Stage: Ensure uniform foundation compaction, control concrete curing conditions (humidity ≥90%, duration ≥14 days), and avoid premature formwork removal or loading.
3. Usage Stage: Avoid heavy loading or excavation around the house, regularly inspect crack changes, and record width data.