How long does concrete take to dry?

[Paul]

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.

[Phil L]

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

[Bruce]

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.

[Paul]

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).

[Rod]

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?

[Paul]

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.

[Bob Minchin]

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

[Bruce]

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

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

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.

[The Medway Handyman]

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

[Paul]

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.

[cynic]

How much water was used in the mix? I would not expect such a long drying out time especially if the new bit is in contact with old dry concrete. You may have a leak in the new drain or perhaps surface water ingress from outdoors along the run of the trench. Give it a couple of weeks exposed with normal air movement and temperature and if the problem remains its time to start opening up at the worst part to look for water.

[Bruce]

You're welcome. I am sorry that I can offer very little comfort.

If you have a competent, reputable builder, it is to be hoped that the firm will want to retain their good reputation and put this work right for you at no cost.

I am surprised that neither you nor the builder have established this.

Concrete fully cures (by hydration of the cement) typically in four to six weeks. Only in conditions of extreme cold would it take any longer. All the water that was in the original concrete mix reacts chemically with the cement and, by the end of that period, the concrete will not shed any moisture.

That means that any moisture you are seeing now is almost certainly coming up from underneath, either from the ground or from leaks in the pipes themselves.

There is another possibility, that the gravel surrounding the drain pipes is acting as a conduit for water entering from outside the house. Pipe surrounds using pea gravel can be very frustrating because the gravel quickly transfers water from the smallest leak over the whole length of any trench.

I recall one project where a pipe coupling failed and the only sign of any leak was water gushing out of the pea gravel pipe surround some

300 metres away. It took a lot of excavating to find the leak, which was massive, with the pipe being 600 mm in diameter, running at 4 bar pressure and unable to be turned off for more than a couple of hours. That's all the time the repair took to complete, but it took several days to locate the leak. :-(

[Paul]

OK, thanks for the answers, if it dries (which it now appears to be doing over 50% of the area), all well and good. If it doesn't, he will be digging it up again. Watch this space....

[The Natural Philosopher]

6-8 weeks is normal in these conditions less if you heat it.

Juts be patient and leave the carpet up for now.

[Tim W]

Paul wibbled on Sunday 27 December 2009 10:52

On top of what others have said, concrete dries out about 1" per week in good warm summer conditions (jn a shady part of the house) by my observation of watching 4" go off.

This time of year it may take a considerably longer time if drying is unaided. Dehumidifier would probably be best bet.

Then establish if there is a damp problem due to poor or missing DPM construction.

If so, I would simply insist the original builder fixes this.

I've had similar works done, where there was no substantive DPM in the house in that area (it varies room by room in the limited areas where I have made holes in the floor). Other places are blackjacked.

In one pair of rooms I finished with an SBR modified screed and that's seems a good enough vapour block for my needs.

In another room where I wasn't rescreeding, I applied an epoxy 2 part DPM to the surface of the screed (F Ball F75) then primed with Stopgap P131 and finished with 3mm of Stopgap 300 HD levelling compound (required as the F75 is not a wearing layer). That worked.

It's not cheap but it's not that expensive for something that is comparitively labour unintensive. You personally could do a 20m2 room single handed with a little mental preparation. Laying 3mm of Stopgap 300 is a 1 person job for upto 5-10m2 and 2 person (just) for 20m2. It's another option if digging holes in the floor isn't an option.

[Paul]

Well just after my last post I had a fan heater play on the area, and surprise surprise, it went bone dry. It was still dry the next morning. And despite all the rain, is still completely dry five days later.

So I suspect that it had mainly dried despite the carpet covering, and the carpet had drawn the moisture up and heating the surface was enough to get rid of the last of it. Otherwise I am sure we would have started to see patches of damp again now.

Thanks for all the input.

[Tim W]

Paul wibbled on Friday 01 January 2010 16:23

The final test would be the taping some polythene over it that was mentioned a while back. If that doesn't accumulate moisture after a few more days, I'd say teh job's a good 'un. But what you've observed sounds encouraging.

[Paul]

Yes, Tim, good idea