The Brew Cabin

Posted: **Sat Nov 14, 2020 10:55 am**

A while back I laid a 5m x 6m slab for a workshop, 8" thick at the edges, in formwork with DPM carried up the sides, a bit like a shallow paddling pool filled with concrete.

Now that it's cured, should I remove the formwork and backfill earth keeping the DPM in position, or should I remove the upstand and backfill earth onto the edge of the slab?

I wondered which would keep the slab drier. If the upstand is left, I guess the "paddling pool" could trap water. If I remove it, damp earth will be in contact with the slab's edge.

Which would you do, please?

Posted: **Sun Nov 15, 2020 10:36 am**

I'd keep the DPM in place, if at all possible.

The original role for the DPM was to maintain the water/cement ratio of the plastic concrete while it cured, but now that the concrete is set, the role of the DPM changes to being one of protecting the concrete slab against aggressive (strongly acidic or alkaline) groundwater. This may or may not be an issue in your area (generally more of a problem in mining towns and/or areas with granite bedrock) so that could influence your decision. If there is any suggestion of aggressive groundwater, then I'd definitely leave the DPM in place; if no such issue, I'd leave it in place unless it would be a nuisance to do so.

The Brew Cabin

Cut off dpm upstand after slab installed? - Question on damp in cured slab.