Rain on fresh poured concrete

[Megabuck]

I originaly asked this question in the Flooring forum, but didn't get an answer - perhaps it's more of a Building question...

Just over two weeks ago, builders poured a concrete slab for a large shed alongside my house - approximate dimensions 30' long, 13' wide at one end narrowing to 10' wide at the other. The ground was prepared with shuttering and around a 3" layer of loose gravel before the concrete was poured on top - around a 3" thickness. No damp-proof layer was used between gravel and concrete. Sorry this is all so vague, I wasn't actually around when all this happened!

Anyway, the concrete was poured in the morning, and in the afternoon it rained - a lot, so my wife tells me. The concrete was not covered. Four days later the shed was constructed on top of the slab. Obviously there was a lot of moisture coming out of the slab, so I've been trying to keep the floor clear, and leaving the doors open every day to allow it to dry. The problem is, the top surface is very soft; wearing trainers, pivoting on the ball of one foot will result in loose sand. Sweeping the floor with a stiff yard brush pulls yet more sand loose.

Will the floor stabilise, given time - and if so, how much time? Anything I can do to help it? All advice gratefully received - unless it's to start again, which would be rather inconvenient at this point!

Many thanks,
Martin

 

[theoutlaw]

It sounds like all of the aggregate has sunk to the bottom leaving what I call the cream on the top. With out seeing it and it is difficult to tell if it will be strong or not. I dont like the sound of loose gravel under it though. Any sub base should be compacted regardless of what is used.
I would have a scrape in an area that is easily repaired to see how soft it is.
That is also a huge area to pour in one go, I would normally have some mesh in there to give it streght and deffo would have poured it to a deeper depth too. Did they section it? or is one big slab. It could/will likely break if it is one big slab with out mesh in it

Tony

 

[Megabuck]

Tony,

The area was split in two, with a shuttering board from side to side - so the rear area is probably around 13' long, tapering from 11' to 10' wide, while the front area is around 17' long, tapering from 13' to 11' wide. I am pretty certain that no mesh was used. This is for a shed, not a garage - they originally tried to convince me that a 2" depth would be fine!

I don't think the aggregate has sunk; there is some visible on the surface, across quite a large area. Certainly the top layer is still extremely soft; using a flat-bladed screwdriver I can easily scratch 1 to 2mm off the top before hitting aggregate; turning the screwdriver sidways, to get it between the aggregate, I can go down 5mm without any real effort. At that depth what is scratched out is darker than the top layer, so presumably not yet dried out.

Many thanks,
Martin

 

[theoutlaw]

Hi Martin
Normally the chemical reaction in the cement dries it out, concrette can get very hot in the centre during curing. I wonder if they have used old cement?
Sounds strange for it not to have gone off yet, no dpm under it wont help though.

Tony

 

[r896neo]

i agree with outlaw regarding the depth. 3" is not a lot never mind for that size.

I imagine there are three big factors here.

1. Lazy builder can't be bothered to rake it about much and got it poured too wet.

2. Loose gravel as a sub-base is not ideal but is a BAD idea for a slab as thin as yours as the loose holes between the stones suck out moisture badly.

3. The heavy rain will have further wet what was probably a weakened overly wet up mix.

Overly wet up concrete forms as said that cream on the top which is soft and sandy so I would agree re. a test patch and chisel out a small hole. I'd imagine you will find that it is harder under the top 10mm and if so you may paint some dustproofer/floorpaint on it and quietly pray the slab doesn't crack up.

Sorry to be the prophet of doom.

 

[noseall]

there is nothing wrong with the concrete (apart from the 3" thickness).

there has been no separation of aggregate.

it has not been poured to wet.

the rain will have made little difference to the core mixture.

all that has really happened is that the concrete has not been troweled properly under covered conditions. the rain has simply added to the 'dusting' problem.

to get a tight finish on any concrete surface, requires the installer to trowel up the concrete at very specific times. preferably late on in the curing process.

the slab would require tamping then a rough trowel. later on as the concrete has dried out a bit, trowel again. it can be worked several hours later and you can achieve almost a polish.

unless you follow a strict trowelling process you will lose the surface and dusting will always occur.

 

[noseall]

all of the above is based on the fact that the installer mixed at a ratio of no less than 6:1.

 

[Megabuck]

Well, thank you to all of you, for taking the time to reply - it is much appreciated. So, assuming noseall is right (which I'm prone to do, since his is the least doom-laden verdict ), where do I go from here? I have a shed floor with a dusting problem - is there some way to resolve it? Do I simply need to keep brushing it until there's no more dust? Apply some sort of binder to the surface?

Many thanks,
Martin

 

[theoutlaw]

Come on Noseall, what do we do next?

Tony

 

[stuart45]

Heavy rain caused some damage to the surface of a concrete path on a job I worked on many years ago. It looked like an exposed aggregate finish. The concrete finisher just mixed up some sharp sand/ cement and trowelled it in. I dont know how long it lasted.

 

[noseall]

unfortunately, because the thicknesses we're talking are only a few mm, then there a few hard wearing applications that can be used.

floor levelling compound springs to mind but i don't know how long it would last and if it is designed to take pedestrian traffic.

all the ones i have used are only suitable as a sub-layer, i.e. below tiles/carpet/flooring, and do not come into direct contact with the user.

there are flooring resins that are two-part mixed but this is specialist stuff.

i have to think about this one.

 

[noseall]

you could screed it but this would add a thickness greater than 25mm.

 

[Megabuck]

I can't add too much depth - that would take the floor level too close to the house DPC. The shed is a lean-to construction.

Since we're meant to be having work done inside the house in the next few months, and then completely redecorating, I guess I could recycle the large areas of 20-year-old carpet, and not worry about the state of the floor underneath...

Many thanks for all the advice,
Martin

 

[DIYedboy]

Watco make a product called Flowtop which might be your answer, look thm up at www.watco.co.uk, it's suitable for forklifts to go over, n all you have to do is pour it to a depth of 3mm

 

[Megabuck]

DIYedboy,

Thanks for pointing that out - it looks ideal for what I want. Unfortunately, it's not cheap - for the surface area I'm dealing with, it would cost somewhere in the region of £630. It would actually be cheaper to buy some cheap vinyl flooring! Not as suitable as Flowtop, admittedly. I shall have to look at my finances...

Regards,
Martin