Conversion of Old Precast Slabs to Cast-in-Place in Houses
Converting old precast slabs to cast-in-place in existing houses requires careful assessment of structural safety, adherence to professional procedures, and selection of appropriate solutions. The need for conversion primarily arises from issues such as poor integrity, insufficient load-bearing capacity, poor user experience, and weak seismic resistance of precast slabs. The conversion process must ensure safety through structural inspections, load verification, and reinforcement design. A common approach involves completely removing the precast slabs and then casting in place, but this requires weighing costs, construction time, and the impact on the house.
Structural inspection and assessment are essential before conversion. It is crucial to check whether the lap length of the precast slab ends on the wall (beam) meets regulatory requirements. Insufficient lap length can lead to weak connections between the floor and the wall, affecting overall stability. Additionally, inspect the slab bottom for severe cracks or exposed rebar. Cracks reduce the load-bearing capacity of the precast slabs, while exposed rebar can lead to corrosion, further weakening structural performance. Furthermore, recalculate the dead and live loads on the floor based on the post-conversion usage. For example, if the house is repurposed for commercial use or additional equipment is installed, the loads will increase significantly. Verify the load-bearing capacity of the existing walls, columns, and foundations. If they are insufficient, reinforce the walls or foundations simultaneously; otherwise, the cast-in-place floor may fail due to the inability of the lower structure to bear the additional loads.
Completely removing precast slabs and then casting in place is a common method, but it has limitations. This approach involves removing the precast slabs and clearing the floor, installing flat floor formwork as support for the cast-in-place work, and ensuring that all rebar is inserted and welded into the walls and columns to guarantee reliable connections between the floor and the main structure. Finally, pour concrete and wait for it to dry naturally before removing the formwork. However, this method is highly risky, as the removal process may lead to slab fractures or unstable supports, causing collapses. It is also costly and time-consuming, requiring expenses for removal, formwork installation, rebar welding, etc., and taking a considerable amount of time from removal to completion of the cast-in-place work. It significantly disrupts the house, potentially destroying the original structural balance. Therefore, it must be implemented cautiously based on professional structural assessments and justifications to avoid new safety hazards caused by the conversion.
