CROSS Safety Report
Plastic spacers in slabs
This report is over 2 years old
A reporter has spotted plastic rails about 1.5 m long being used to support the bottom rebar mat of a slab. They are 'U' shaped with the open side being placed against the soffit.
Key Learning Outcomes
For construction professionals:
- Spacers for steel reinforcement should be in accordance with BS7973 which gives requirements for plastic spacers and requires an open area to allow for aggregate to pass through
These products should be installed in accordance with the manufacturers’ recommendations
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A reporter has spotted plastic rails about 1.5 m long being used to support the bottom rebar mat of a slab. They are 'U' shaped with the open side being placed against the soffit. They are quickly put down before the rebar is placed and avoid the need to fix on the traditional type of spacer. There are openings along the sides to allow concrete to fill the void.
Having looked after the soffit was stripped, the reporter could not see where they were, which was encouraging. However, he is uncomfortable with how they may affect the slab strength near supports, where the slab is hogging and the concrete around the rail is in compression.
He has heard that some such spacers have holes which are too small to allow aggregate to enter the void, just 'grout'. He has also heard that they may move around as the concrete is placed as they are not wired on, and presumably some may end up being loose. Are CROSS aware of these spacers being used and do others have any concerns?
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Such spacers have been in use for many years. BS7973 gives requirements for plastic spacers and requires an open area to allow for aggregate to pass through. To prevent movement, they can be wired to the rebar. As with every product, they need to be used in accordance with manufacturers’ recommendations. Care does need to be taken where very heavy rebar cages are supported, as plastic spacers may not have the same capacity as concrete spacers.