Cracking in the Roof of Old Houses with Prefabricated Slabs

The cracking in the roof of old houses with prefabricated slabs should be addressed based on the cause of the cracks. Common causes include aging at the joints, insufficient load-bearing capacity of the floor slabs, and disturbances from renovation work. The specific repair methods are as follows:

I. Repair of Cracks at Joints (for Aging and Loosening of Mortar)

Cracks at the joints of prefabricated slabs, caused by mortar cracking and debris falling, leading to looseness between the slabs, are a typical cause of roof cracking. During repair, first remove the loose mortar around the cracks and clean the area to a solid base using a high-pressure water jet or wire brush, ensuring no floating dust remains. After the base is dry, apply an interface agent at the cracks to enhance adhesion, then embed an elastic waterproof sealant (such as polyurethane-based) with a filling depth of at least 1.5 times the width of the cracks. To improve crack resistance, alkali-resistant glass fiber mesh can be pasted on the surface of the sealant, with the mesh extending 10-15 cm beyond both sides of the cracks. Smooth it out with a special adhesive mortar to form a flexible transition layer, reducing secondary cracking caused by temperature changes.

II. Strengthening of Floor Slab Load-Bearing Capacity (for Cracks Caused by Heavy Loads)

The hollow structure of prefabricated slabs and uneven distribution of steel reinforcement can easily lead to localized overloading and cracking. If the crack width exceeds 0.3 mm or is accompanied by deflection of the floor slab, structural reinforcement is required:

1. Carbon Fiber Sheet Reinforcement: Apply carbon fiber sheets along the direction of the cracks, with a width of 20-30 cm and the number of layers determined by calculations (usually 1-2 layers). Impregnate and cure with epoxy resin to enhance the flexural capacity of the floor slab.
2. Additional Supports: Install steel supports or concrete columns below the areas with concentrated cracks to transfer the load to the foundation and reduce stress concentration in the floor slab.
3. Load Reduction: Remove overweight items from the floor, avoid concentrated stacking, and redistribute the design value of live loads on the floor if necessary.

III. Protection During Renovation Work (for Cracks Caused by Renovation Disturbances)

During the renovation of old houses, construction activities such as hammering and drilling can easily cause vibration and cracking of prefabricated slabs. Before repair, assess the scope of impact of the construction:

1. Crack Monitoring: Regularly measure the width changes of the cracks using a crack width gauge. If the cracks continue to expand, suspend construction and reinforce immediately.
2. Flexible Connections: At locations where new equipment or pipelines penetrate the floor slab, use elastic sleeves or flexible sealing materials to avoid stress transfer caused by rigid connections.
3. Overall Reinforcement: For areas frequently subjected to renovation, lay steel reinforcement mesh (φ6@150mm) on the surface of the floor slab and pour fine aggregate concrete (with a thickness of at least 40 mm) as a whole to form a composite floor slab structure.

After repair, note the following: Regularly check for recurrence of cracks and avoid applying impact loads directly to the repaired areas. If the roof leaks, treat the waterproof layer simultaneously. It is recommended to use SBS modified bitumen membrane or polymer cement waterproof coating to form a composite waterproof system with the crack repair layer.