Concrete problem ?!!!

[paulm]

A bit off topic for a woodworking forum, apologies !

I've recently had a new concrete base poured by builders for a large shed, poured about two or three weeks ago when the weather was pretty cold and wet. They laid the course of bricks on top after a few days and have put the shed up (it's about 20' x 12'), and it's been up for a week or more now.

I'm a bit concerned about the concrete though as it still seems quite soft and crumbly, easily marked and brushing it off to get rid of some muddy footprints and the like seems to produce a noticable amount of material from the surface.

I'm trying to get hold of the builder to come back and take a look at it but wondered if anyone knew if it may have been poured when too cold or possibly a mix problem, or maybe it just needs longer to cure ?

They were experienced groundworkers doing it for the builder and mixed up the concrete with a small mixer on site over the course of a day, the slab was about 6" deep I think and the mix colour looked okay to me as far as I could tell. I did ask about the temperature conditions as I was a bit surprised at them pressing ahead with it but I was told it would be okay.

Not sure of any solutions either other than take the shed down again, break up the base and start again ?!!!

Any experienced input before the builder comes round would be appreciated so I have a better idea how to proceed ?

Cheers, Paul

 

[Phil Pascoe]

That's a whole load of concrete to dry in three weeks of cold, wet weather. It should have a DPC, which won't help. I wouldn't worry for a few weeks yet.

 

[Steveshj]

Hi Paul,

You must remember that correctly designed and mixed concrete does not "dry out" as such.

The water in concrete is there to hydrate the cement , if water is lost from the surface the cement in the mix will not be fully hydrated and will remain as simply a sand cement paste at best.

The most important step in producing good concrete is the curing, 28 days under sealed polythene sheet is what should be done. It sounds like no curing was carried out at all, which would explain the surface condition where the majority of water was lost by evaporation.

Concreting in cold weather is not usually too much of a problem provided the water used is not full of ice.

Hope this helps.

Steve

 

[RobinBHM]

The most important part of concrete strength is water content and curing. Concrete made too wet will be weak and allowed to dry too quickly may be crumbly. Knocked up by hand, the ratio wont be very accurate, but it would have to be widely out for a problem. It would generally be C20 for a base which is about 6:1 ratio.

I wouldnt worry about the surface being soft and crumbly, at worst it may not have been covered afterwards and a windy day could have drawn out some moisture from the surface, it doesnt mean it will be weak all the way through.

 

[Bigdanny]

Cold weather will slow down curing a lot. I did some concreting , internal room but no heating at start of December. Was close to 0 C and it stayed soft for over a week until I managed to get the room sealed. I could still walk on it after 2 days though. I Had to build a wall and cast/ shutter a concrete window sill. The window ledge and some other concrete on a garden wall I used an antifreeze as it was more exposed.

For the rooms concrete I wasnt too worried as I wasnt going to be doing further worker to repair after window installation and not much traffic would be going through it.

When I continued finishing in the New Year, areas which had nylon DPM hanging over were still damp but solid. Once trimmed up all had dried out within a week with some fan heaters. I waited 5 weeks before removing the shuttering from the windowsill just to be safe.

I would try to not walk on it until you can see it light/ dried grey if possible. If its 6inch deep then structurally it should be ok but the surface as you say may be liable to break up. If you can try to get the guy back and suggest/ask it needs a large air-heater in there at his cost. And also what kind of finished would be acceptable to you.

Hope it all turns out OK
Danny

 

[tomf]

They will have put too much water in the mix so the top 3-8mm will be weak. Scrape/grind it off then it will be fine.

On a side note you should not pour concrete if the temperature is too low, you are just asking for problems.

 

[paulm]

Thanks for the feedback guys, hopefully just needs more time to cure by the sounds of it.

I'll be drilling some 16mm diameter holes tomorrow to strap the shed down to the base, it'll be interesting to see if it's cured any more further into the slab, or maybe I shouldn't be using expanding bolt fittings in it just yet (five or so inches in from the perimeter of the slab I guess) ?

Cheers, Paul

 

[Phil Pascoe]

If I had to fix that close in new concrete, I'd be tempted to resin studs in. The resin and studs probably wouldn't cost any more than expanding bolts.

 

[Myfordman]

+1 for resin anchors

 

[paulm]

I'll check out the resin and studs approach in the morning, might well be more sensible. Thanks guys.

Cheers, Paul

 

[mindthatwhatouch]

Perhaps the builder can supply some self levelling (put it down when its a bit warmer) to improve the finish.
Stop brushing it, if the loose material on top is an issue a coat of PVA watered down will help keep the dust down.
Good luck with the anchors. +1 on the resin option, last thing you want to do if its a bit suspect is put in an expanding anchor. Go for a decent 2 part resin.
Just a thought are galvanised straps a better option for you?