Cracks on the Roof of a Prefabricated Slab Building

Whether cracks on the roof of a prefabricated slab building are safe requires a comprehensive assessment. If the crack width exceeds 1 millimeter, the depth is through, it is located in a critical position, or it continues to develop, there are safety hazards, and timely treatment is required.

Causes of Cracks

1. Shrinkage Cracks: During the production of prefabricated slabs, the hydration reaction of cement causes volume shrinkage. When the shrinkage stress exceeds the tensile strength of the concrete, shrinkage cracks occur. These cracks are fine, with widths mostly ranging from 0.05 to 0.2 millimeters, and are evenly distributed.
2. Temperature Cracks: Changes in ambient temperature cause the prefabricated slabs to expand and contract. Excessive temperature stress under structural constraints leads to temperature cracks, which are commonly found on the top floor. They are mostly distributed in a regular diagonal or horizontal pattern and are more pronounced in winter. Cracks may also occur if the concrete is not watered for curing in a timely manner after pouring, and these cracks are relatively shallow.
3. Improper Formwork Removal Time: If the formwork is removed too early, when the concrete strength has not reached the specified time and the formwork removal report is unqualified, it will cause the concrete to crack.
4. Manual Disturbance: Construction is carried out before the concrete reaches a certain strength, and the construction period is shortened due to rushing the schedule, disturbing the concrete and forming cracks.
5. Settlement Cracks: Slight uneven settlement of the building foundation causes cracks in the prefabricated slabs. These cracks are relatively wide and may extend from the bottom of the slab to the top. They have a certain directionality, related to the direction of settlement. Although they occur less frequently, when they do, they often indicate structural cracks and will worsen, requiring professional assessment.

Methods for Judging the Safety of Cracks

1. Crack Width: Conventional shrinkage cracks are almost all below 3mm. Cracks in prefabricated slabs exceeding 1 millimeter require caution, and those above 1 millimeter are considered dangerous. For stress cracks such as horizontal cracks caused by roof sagging, even 0.8 millimeters should be noted.
2. Crack Depth: When the crack is small and difficult to measure, go upstairs and check the position corresponding to the crack for through cracks. If they cannot be seen, pour water on the upstairs floor. If the water leaks down quickly, it indicates a through crack, which requires attention.
3. Crack Location: If the crack extends from the roof to two-thirds of the wall height or is exactly on a load-bearing beam, the risk factor doubles.
4. Crack Development: If the crack continues to lengthen and widen, even if it has not reached 1 millimeter, a professional organization should be consulted for inspection.

Crack Repair Methods

1. For cracks as thin as a hair, they can be directly sealed with glue.
2. For medium-sized cracks, special glue should be injected, and it takes 72 hours to dry completely.
3. During construction, pay attention to slab joint treatment. When installing prefabricated slabs, a 10 - 15 millimeter thick mortar bed should be provided.