Woodwool is a man-made board material about 50 to 75mm thick comprising shredded timber bound together in a cement paste. It was used generally during the 1960s to provide insulation often at roof level and as a permanent shutter to in-situ concrete.
Unfortunately, when woodwool was incorporated as a permanent shutter, unless great care was taken when placing concrete against the woodwool soffit shutters, inadequate compaction of the concrete could occur due to the compressible nature of the board. This lack of compaction resulted in voids and honeycombing on the soffit of the slab sometimes leaving no cover to the reinforcing steel. As the woodwool boards were left in place, these defects then remained undetected.
Woodwool boards were used both as a permanent shuttering to flat slabs but also as a void former between concrete ribs.
At a building in west London the following construction was found:
The ribs were 75mm wide, 220mm deep and at 390mm centres, spanning between a central spine beam and the building edge beams. The woodwool beneath the ribs was about 30-40mm thick.
The concrete around the reinforcement in the bottom of the ribs was found to be honeycombed, in some cases leaving the steel fully exposed. In this instance, the poor compaction was probably due to the inherent difficulty of compacting concrete around a reinforcing bar in a relatively narrow rib being exacerbated by the woodwool compressing and absorbing energy. However, as the building had been well maintained and was dry internally, the exposed steel was in a reasonably good condition with only surface corrosion.
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