CROSS Safety Report
Correct uses of concrete spacers
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A reporter raises concerns about how concrete strip spacers are being installed on some construction sites.
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Concrete strip spacers are increasingly being used to support the bottom reinforcement in slabs says a reporter. They come in a variety of sizes to suit the required cover and generally are 1m in length. However, if used incorrectly they can act as crack inducers. The problem, which is believed to be widespread, arises if when they are laid end to end in a continuous unbroken line and form a discontinuity in the structure (Figure 1).
Spacer strips regardless of their material should be installed in accordance with the requirements of BS7973-1:2001, ‘Spacers and chairs for steel reinforcement and their specification’. Corrosion caused by the lack of cover is hidden from view e.g. in the case of ground bearing slabs, and will lead to failure if left unchecked. The worst example seen by the reporter’s firm was on a wind turbine base subject to aggressive ground conditions.
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This seems to be a common issue as it has frequently been observed by others that spacers are placed in long lines. Whilst this is unlikely to result in immediate failure it could result in long term problems such as corrosion of reinforcement. It illustrates the need for designers to ensure that specifications reflect current practice and techniques. Where such spacers may be used the requirement for staggering should be specified are reference made to BS 7973-1:2001 Spacers and chairs for steel reinforcement.
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