A combination floor is a floor system consisting of precast concrete elements (lattice girder slabs or hollow-core slabs) topped with an in-situ concrete layer.
How does a combination floor work?
The precast element serves as permanent formwork during construction and as tensile reinforcement in the finished floor. Once placed on the supporting walls or beams, it spans the gap without temporary propping (for shorter spans) or with minimal propping (for longer spans). The in-situ concrete topping is then poured on site. As it cures, the rough upper surface of the precast element and protruding lattice girders or reinforcement loops create a mechanical bond, forming a monolithic composite section.
This composite action means the precast element handles the tensile stresses in the bottom zone while the in-situ topping takes the compressive stresses in the top zone — exactly how a reinforced concrete slab is designed to behave.
Types of combination floors
- Lattice girder slabs — thin precast planks (50-80 mm) with welded steel lattice trusses projecting upward into the topping
- Hollow-core slabs with structural topping — prestressed hollow-core units with a cast-in-situ layer for additional capacity or continuity
- Filigree slabs — similar to lattice girder slabs, widely used in Central European construction
Applications
- Residential apartment buildings and multi-storey housing
- Commercial office floor construction
- Parking structures requiring long spans
- Industrial buildings with high floor loads
- Renovation projects where crane capacity is limited
Related terms
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