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How long does concrete take to dry?



 
 
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  #1  
Old December 27th 09, 11:52 AM posted to uk.d-i-y
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Posts: 19
Default How long does concrete take to dry?

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?

Many thanks,

Paul.
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  #2  
Old December 27th 09, 01:09 PM posted to uk.d-i-y
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Posts: 183
Default How long does concrete take to dry?

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?

Many thanks,

Paul.


What do you mean by 'blackjacked'?

The concrete below is still wet, and will remain wet for a long time,
probably permanently, if you haven't installed a plastic membrane between
the top 100mm and this concrete, it will never dry out, as the moisture in
the ground will continue to soak into the concrete and up through the top
screed

--
Phil L
RSRL Tipster Of The Year 2008


  #3  
Old December 27th 09, 01:19 PM posted to uk.d-i-y
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Posts: 897
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.

  #4  
Old December 27th 09, 01:32 PM posted to uk.d-i-y
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Posts: 19
Default How long does concrete take to dry?

Thanks both. Blackjack is a tar-like bitumen that I assume acts as a
waterproof barrier between the two layers of concrete. To be fair, the
two ends of the run are dry, and where it wasn't covered with carpet
it is now dry, so I assumed that his method worked OK, and it was just
concrete under the carpet that was not dry as it was covered.

The original plans had drains going around the house, but this would
have meant the gradient was to shallow as the run was too long. In
order to do what we wanted, they had to go across the lounge (and this
was with the approval of the Building Inspector).
  #5  
Old December 27th 09, 02:08 PM posted to uk.d-i-y
Rod
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Posts: 2,893
Default How long does concrete take to dry?

On 27/12/2009 12:32, Paul wrote:
Thanks both. Blackjack is a tar-like bitumen that I assume acts as a
waterproof barrier between the two layers of concrete. To be fair, the
two ends of the run are dry, and where it wasn't covered with carpet
it is now dry, so I assumed that his method worked OK, and it was just
concrete under the carpet that was not dry as it was covered.

The original plans had drains going around the house, but this would
have meant the gradient was to shallow as the run was too long. In
order to do what we wanted, they had to go across the lounge (and this
was with the approval of the Building Inspector).


Many years ago my mother had a small extension built and the floor
construction called for something similar to Blackjack. The lousy
builders tried to get away with a single coat - spec. called for two
layers with sanding between them. Complaint got second layer applied.

Did you check precisely what they did?

--
Rod
  #6  
Old December 27th 09, 02:24 PM posted to uk.d-i-y
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Posts: 19
Default How long does concrete take to dry?

Hi Rod, no I didn't check as I am not a builder and I wasn't watching
- I only saw the finished result.

What I am trying to get a feel for here is how long a 100mm depth of
concrete inside should take to dry indoors. If it takes longer than it
should, there is a problem, but I am not sure I have a problem right
now. That is what I need to find out before I have him dig it all up
again.



  #7  
Old December 27th 09, 02:39 PM posted to uk.d-i-y
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Posts: 71
Default How long does concrete take to dry?

Paul wrote:
Hi Rod, no I didn't check as I am not a builder and I wasn't watching
- I only saw the finished result.

What I am trying to get a feel for here is how long a 100mm depth of
concrete inside should take to dry indoors. If it takes longer than it
should, there is a problem, but I am not sure I have a problem right
now. That is what I need to find out before I have him dig it all up
again.



You need to be sure that the damproof course is working before laying
the carpet if I understand you correctly?

One way to do this is to lay a sheet of dry newspaper on the concrete
and cover it with a slightly larger sheet of polythene and press down
the edges e.g. four pieces of wood or similar. Each day, take up the
polythene and see if the paper is still dry. If it is damp/limp/tears
easily, then put in a fresh piece under the polythene ideally in a
different place for another day and repeat until the paper is reliable
left dry. only then consider laying carpet.

If water is still coming out of the concrete for whatever reason you
will need to ventilate the room.

I had read of drying/curing times of 1 week per 1" quoted but this has
been more about reach full strength than readiness for carpeting.

hth

Bob

  #8  
Old December 27th 09, 02:43 PM posted to uk.d-i-y
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Posts: 897
Default How long does concrete take to dry?

On Sun, 27 Dec 2009 04:32:11 -0800 (PST), Paul
wrote:

Thanks both. Blackjack is a tar-like bitumen that I assume acts as a
waterproof barrier between the two layers of concrete.



The problem is, it doesn't connect with the breached membrane to
provide a replacement continuous damp proof membrane.


To be fair, the
two ends of the run are dry, and where it wasn't covered with carpet
it is now dry, so I assumed that his method worked OK, and it was just
concrete under the carpet that was not dry as it was covered.



They are dry because the rising moisture is ventilated, whereas under
the carpet ...


The original plans had drains going around the house, but this would
have meant the gradient was to shallow as the run was too long. In
order to do what we wanted, they had to go across the lounge (and this
was with the approval of the Building Inspector).



The approval of the Building Inspector would have depended on
effective measures being taken to provide effective damp proofing that
you don't appear to have. Did the Building Inspector personally
witness the installation of the drains and any attempt at providing
effective damp proofing? No, I thought not.

The lesson to be learned here is that any work that breaches a damp
proof membrane should include carefully designed and installed
measures to ensure the continuity of that damp proof membrane in the
finished job. The word "ensure" implies that the work should be
supervised by a construction professional, especially the critical
elements of the work that involve re-establishing the damp-proofing by
effectively connecting any new damp proof membrane to the existing.

No doubt several people on here (the usual suspects) will froth at the
mouth at the thought of spending money to employ a professional -
usually an architect or structural engineer - to supervise the work.
However, your experience is a prime example of why saving a relatively
small amount of money by placing your complete trust in a builder is
perhaps not the most sensible option.

It might be OK, if only builders could be more easily held to account,
and therefore tended to be competent. However, the fact is that there
are very, very few cases where builders have been successfully sued by
property owners for the cost of putting right their incompetent work.
So the incompetence continues. And ever more property owners are
faced with paying the cost of having the work done again to the
standard it should have been done to in the first place.

  #9  
Old December 27th 09, 02:45 PM posted to uk.d-i-y
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Posts: 8,323
Default How long does concrete take to dry?

Paul wrote:
Hi Rod, no I didn't check as I am not a builder and I wasn't watching
- I only saw the finished result.

What I am trying to get a feel for here is how long a 100mm depth of
concrete inside should take to dry indoors. If it takes longer than it
should, there is a problem, but I am not sure I have a problem right
now. That is what I need to find out before I have him dig it all up
again.


Hire a dehum for a week which should dry it out, then see if the damp come
back.


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk


  #10  
Old December 27th 09, 03:13 PM posted to uk.d-i-y
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Posts: 19
Default How long does concrete take to dry?

Hi Bruce, thanks for your comprehensive reply.

I have no reason to think the builder is incompetent, in fact as
builders go I think he is very competent. I also do not think there
was actually a DPC under the existing (very old) concrete floor. If
rising damp was the problem, then we would be seeing signs of dampness
on the existing floor by now, I am sure. So I am reasonably confident
that we only need to wait until the 100mm of concrete dries (which it
has not been allowed to do), which is why I am trying to get an answer
to this question specifically. If and when it does not appear to be
drying properly, then we will have to look at what he has done.

 




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