Building up levels

[GaryN]

Hi Tony,
I use a 6 sharp sand to 1 cement wet mix when laying slabs.
In the past, when I've had to build up levels to prevent this bed being too deep, and when I've had no sub base material on site, I've built up the level using a damp 10 sharp sand to 1 cement mix which I've trampled down by foot.
Is this accepatable and if so what's the maximum depth this 10 to 1 mix can be?
Regards
Gary

[Tony McC]

The maximum thickness of any build-up layer is determined by how well it can be compacted. If you were using a granular sub-base or fill material, then typical compaction equipment such as a vibrating plate or a roller, can consolidate to a depth of around 200mm, but 150mm is a safer limit. With cementitious material, such as your 10:1 mix or a concrete, consolidation or 'compaction to refusal' isn't as critical as the cement will hydrate and harden the material even if it isn't fully compacted. However, any cementitious material is far better, from a structural point of view, when properly compacted, so again, I'd say that you should try to use layers of not more than 150mm depth, ensuring each layer is fully consolidated before placing the next.

The final bedding layer, whether it is cement-bound or clean sand, should never be more than 50mm depth.

[GaryN]

Thanks Tony,
Am I right in thinking there's no need to wait for the build up layer to set before laying the bedding layer and laying slabs?
Regards
Gary

[Tony McC]

Correct - as long as it's well compacted, you can start laying immediately.

[GaryN]

Thanks Tony
Regards
Gary