URGENT! Can 100 standard Celcon blocks be used below ground?


31 Oct 2014
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United Kingdom
I've got 700mm of concrete in the ground for single storey extension.

Thought I was okay to build up from the concrete a cavity wall using standard 100mm Celcon blocks (3.6n) 2 courses inner skin and 1 course + 3 of face bricks to outer skin with weak concrete infill to DPC – thought I'd asked Celcon Tech Dept about this but not sure now (and of course they're closed now for w/end).

A builder just said he hadn't seen them used like that before, he always uses the heavy blocks which has got be concerned.
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Your building inspector might not like it, especially if the soil is heavy in sulphates. Least said soonest mended. Try and avoid mentioning the strength if you can. My BI let me build a bungalow on 3.6n trenchblocks, but did say "I do wish you'd stopped at saying they were trenchblocks without giving me the strength"

From a loading point of view, they'll be fine right now.. It's just the sulphates that attack and weaken them over time. Hopefully you don't have that problem but if you do you can take steps to mitigate it
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They can be used below ground level. Have to differ here because I don't think that sulphates are a problem for them.
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