Can Prefabricated Floor Slabs Be Converted to Cast-in-Place?

Prefabricated floor slabs can be converted to cast-in-place, but this must be implemented after a comprehensive assessment based on the building structure, renovation needs, and safety considerations, and must adhere to professional standards. Here are the specific details:

I. Conventional Renovation Process

1. Demolition and Support: First, remove the existing prefabricated slabs to clear the floor space. Then, install floor formwork as a support structure for the cast-in-place concrete. The formwork must be flat on the surface and tightly joined to prevent deformation during pouring.
2. Reinforcement: Install a steel reinforcement mesh on the formwork. All steel bars must be inserted and welded into the walls or columns to form an integrated load-bearing structure. This step is crucial for ensuring the floor's load-bearing capacity; if the steel connections are not secure, it may lead to floor cracking or settlement.
3. Concrete Pouring and Curing: Use standard concrete (e.g., C30 or higher strength) for pouring, and vibrate it in layers to avoid voids. After pouring, natural curing is required for at least 28 days, keeping the surface moist during this period to prevent cracks caused by rapid drying shrinkage.

II. Mainstream Renovation Approaches

1. Complete Demolition and Reconstruction of Cast-in-Place Layer: Suitable for scenarios where the prefabricated slabs are severely damaged or a complete change in the floor structure is needed. This method has a long construction period and high cost but can thoroughly resolve issues such as joint leakage and poor sound insulation in prefabricated slabs.
2. Retention of Prefabricated Slabs + Addition of Composite Layer: Pour a layer of cast-in-place concrete (typically 5-8 cm thick) directly onto the existing prefabricated slabs to form a composite structure. This method causes less disturbance to the building but must meet the following conditions:
   • The prefabricated slabs must be structurally assessed to confirm no severe aging or damage;
   • The interface between the composite layer and the prefabricated slabs must be roughened or treated with a bonding agent to enhance adhesion;
   • Additional steel bars must be added and connected to the original structure to prevent interlayer slippage.

III. Key Considerations

1. Professional Assessment First: Before renovation, a structural engineer must be commissioned to inspect the building, assessing the load-bearing capacity of the prefabricated slabs, wall stability, and renovation feasibility. If the building is a brick-and-mortar structure or the prefabricated slabs have a large span, the renovation risk may increase significantly.
2. Construction Qualification Requirements: Cast-in-place renovation involves structural safety and must be carried out by a qualified construction team. Unlicensed construction is strictly prohibited.
3. Cost and Duration: The cost of complete demolition and reconstruction of the cast-in-place layer is approximately 800-1200 yuan per square meter (including materials, labor, and curing costs), with a duration of about 30-45 days. The cost of renovating with a composite layer is about 500-800 yuan per square meter, with a duration of about 15-20 days. However, long-term use may lead to cracking due to inconsistent deformation between the composite layer and the prefabricated slabs.

Summary: Converting prefabricated slabs to cast-in-place is technically feasible but must strictly follow the "assessment-design-construction-acceptance" process. Priority should be given to schemes that cause less disturbance to the building, and key nodes (such as steel connections and concrete curing) must comply with standards to ensure the structural safety and durability of the renovated building.