What is a structural topping?
A structural topping (also called compression layer) is a thin reinforced in-situ concrete layer cast on top of a precast floor element so it can carry compressive forces and act as a diaphragm. Together the precast unit and the topping behave as one monolithic floor that bends, distributes loads and transfers forces to the walls.
How does a structural topping work?
In a precast floor (hollow-core slab, beam-and-block or lattice girder slab) the top fibre is in compression and the bottom in tension. The precast unit carries the tensile force with pre-stressed reinforcement at the bottom. The structural topping is cast on top:
- Carrying compression — concrete is strong in compression and resists hogging moments
- Spreading point loads — the topping distributes concentrated loads across several units
- Diaphragm action — the floor acts as a stiff plate that transfers horizontal loads to walls
- Closing joints — joints between precast units are connected monolithically
A structural topping is usually 50 to 80 mm thick and contains a light mesh to control cracking.
Application
- Lattice girder floors — precast soffit with a topping
- Hollow-core slab floors — topping for diaphragm action and unit interconnection
- Beam-and-block floors — composite construction with a cast topping
- Renovation — extra topping to strengthen an existing floor
- Roofs and access decks — trafficable surface on precast units
Points of attention
- Roughen the precast surface or apply a bonding agent
- Use concrete of sufficient strength class (often C20/25 or higher)
- Place reinforcement at the correct height with spacers
- Cast the topping in a single pour and compact carefully
- Protect against drying out during curing
Related terms
- Lattice girder floor
- Beam-and-block floor
- Composite floor
- Reinforcement
- Structural concrete
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