Conversion of House Prefabricated Slabs to Cast-in-Place Concrete

The conversion of house prefabricated slabs to cast-in-place concrete should be carried out with caution. The core objective is to address structural defects in prefabricated slabs, but it is essential to strictly adhere to technical specifications and assess feasibility.

I. Core Reasons for Conversion

Prefabricated slabs, due to production process and historical technical limitations, have the following issues:

1. Poor Integrity: Prefabricated slabs are installed in sections, with weak connections between slabs, making them prone to cracks due to concentrated loads or foundation settlement.
2. Insufficient Load-Bearing Capacity: Original design standards are relatively low, making it difficult to meet modern usage requirements (such as adding floors or installing equipment).
3. Weak Seismic Performance: The connection between prefabricated slabs and walls is not firm, making them prone to detachment or collapse during earthquakes.
4. Poor User Experience: They have poor sound insulation and waterproofing performance, and after long-term use, the underside of the slabs is prone to cracks and exposed reinforcement.

II. Key Steps Before Conversion

1. Structural Inspection and Assessment:
   • Inspect the concrete strength, reinforcement position, and rust condition, focusing on assessing whether the lap length of the prefabricated slab ends on the wall (beam) complies with specifications (e.g., the "Code for Design of Concrete Structures" requires a lap length of at least 1.5 times the slab thickness).
   • Check the width of cracks on the underside of the slabs (if exceeding 0.3mm, treatment is required) and the degree of rust on exposed reinforcement.
2. Load Verification and Scheme Demonstration:
   • Recalculate the dead load (such as the weight of decoration materials) and live load (such as the weight of personnel and equipment) of the floor based on the post-conversion usage (e.g., residential to commercial).
   • Verify the load-bearing capacity of the original walls, columns, and foundations, and reinforce them if necessary (such as adding structural columns or expanding the foundations).

III. Mainstream Conversion Paths

1. Complete Demolition and Reconstruction:
   • Demolish the prefabricated slabs and clear the floor → Install flat formwork → Install reinforcement (which must be inserted into the wall or column by at least 15d, where d is the diameter of the reinforcement) → Pour concrete with a strength of C30 or higher → Cure until the strength meets the standard and then remove the formwork.
   • Applicable Scenarios: The prefabricated slabs are severely damaged or a complete change in structural layout is required.
2. Adding a Composite Layer:
   • Retain the original prefabricated slabs and directly pour a 40-60mm thick cast-in-place layer on top, equipped with a reinforcement mesh and anchored to the original slabs.
   • Advantages: Minimal construction disturbance and short construction period, but it is necessary to ensure that the load-bearing capacity of the original slabs is sufficient.
3. Frame Reinforcement Conversion (e.g., a case of old houses in Shaanxi in the 1990s):
   • Add new beams and columns to form a frame structure → Demolish the prefabricated slabs → Pour a cast-in-place roof.
   • Applicable Scenarios: A significant improvement in the overall structural integrity and seismic performance is required.

IV. Conversion Risks and Precautions

• Cost and Construction Period: The conversion cost is approximately 300-500 yuan/㎡, with a construction period of 15-30 days per floor, and usage must be suspended during this time.
• Structural Disturbance: Demolishing the prefabricated slabs may damage the stability of the original walls, and temporary support measures must be taken.
• Compliance: An identification report and reinforcement scheme must be issued by a professional institution and approved by the local housing and construction department.

Conclusion: The conversion of prefabricated slabs to cast-in-place concrete should be based on structural safety, with priority given to the process of inspection and assessment → scheme demonstration → professional construction to avoid secondary risks caused by blind conversion.