Concrete @ your Fingertips

Chair spacers for only top reinforcement

Continuous wire chairs which just support the top reinforcement directly off the ground or soffit formwork should be placed on cementitious line spacers, each of approximately 300mm in length and transverse to the line of the continuous chair. See BS 7973-2: 20xx Figure 3(b).

For suspended slab soffits, the reason is obvious, as the bottom wire would be visible once the concrete is struck and may corrode. For ground supported slabs, the chairs should be put on cementitious spacers to prevent penetration of the damp proof membrane (dpm) under the weight of the reinforcement, the steel fixers and the concrete being poured and compacted, defeating the purpose of the dpm. Where the plastic sheeting on the ground is acting as a slip membrane only, the high point loads from the wires of the chair may act as a restraint to the concrete shrinkage and potential increased risk of cracking. The cementitious spacers provide a flat undersurface and lower bearing pressure, so less resistance to the slip membrane performing its function correctly.