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Bruce[_8_] Bruce[_8_] is offline
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Default How long does concrete take to dry?

On Sun, 27 Dec 2009 02:52:04 -0800 (PST), Paul
wrote:
We are in the course of some fairly substantial building works. At the
start of November, we had some drains installed across the lounge
floor. Trenches were dug, drains installed, the trench filled with
shingle, and then concrete added on top of that to a depth of about
100mm below floor level. This was then blackjacked and another layer
of cement added to build it up to floor level.

Not knowing any better, we covered this with carpet, and a week ago
carpet layers were unable to lay a new carpet because it was
(unsurprisingly) too damp. Since then we have, of course, exposed it,
but it has not dried as quickly as expected.

If we have an ordinary fan play across it all night, it appears to
dry. But when the fan is removed, within 60 minutes it is damp again.
Oddly (or perhaps not), it is bone dry at the ends. Last night we
dried it with a hair drier, and the damp doesn't appear to have come
back so fast.

It is as if we can dry the top (say) 1mm, and then the damp comes back
from underneath.

If left to its own devices, how long will it take to dry please, and
how can we speed it up substantially please?



This sounds like it has nothing to do with concrete drying, or curing
(they are two different things). Rather, it sounds like the builder
had breached the damp proof membrane (DPM) under your ground floor
slab and has not done anything to remedy the breach, merely filled the
hole with some gravel (which allows water to flow unhindered) and
covered up the job with some concrete. It isn't going to dry out.

You have damp rising from below and around the new drains that were
installed. The damp is rising because you no longer have a competent
damp proof membrane (DPM) since it was breached by digging the
trenches for the pipes. Unless and until measures are taken to remedy
the breach(es), there will always be a damp problem.

It is unusual, to say the least, to run drains underneath a lounge
floor. Who designed the installation? Was there an architect or
structural engineer involved? If so, they should be your first point
of contact.

If this was something just done by a builder - with no involvement of
a construction professional - then you should contact the builder.