Concrete floor - mix advice

[DerbyBorn]

Having some building work done and am concered about the concrete for the floor.

After 9 days it looks very sandy and sweeping it brings up lots of loose sand. An electrician either dropped something or put his steps on part of it and the "crust" broke showing a crater that looked like a poor quality beach. My gut impression is that there is was enough cement in the mix.

The bottom slab and the kingspan insulation and DPC were inspected by the Building Inspector.

The builder will be here on Sat morning after being off site for 2 days. I have told him on the phone that there is a problem.

What should the top layer spec be? What do you think is wrong?

Thanks

[Tim Watts]

On Friday 14 June 2013 21:46 DerbyBorn wrote in uk.d-i-y:

Is it actually concrete or screed?

Assuming concrete:

I usually use a C20 mix for ad hoc jobs where a strongish mix is needed. That's a 4:2:1 of ballast:sharp-sand:cement

I would normally use a 10mm ballast for structural work.

Now - you can get the surface being a little dusty/sandy sometimes - I had that problem, and came here for advice (which was diluted PVA - worked a treat).

However, yours sounds like something has gone badly wrong.

Was it mised in a mixer?

Fresh cement - or at least unopened new type plastic bags (Mastercrete brand IIRC - that stuff keeps extremely well, even outside)?

It should not look like a poor wuality beach - that sounds like an incorrect mix or bad mixing.

Although concrete takes a relatively long time to fully set (weeks) it should have achieved a high level of strength after 9 days and heavy metal tools should be bouncing off it.

I'd invite the Building Inspector for a look - although it's a hassle challenging a builder, you really do not want a bad slab - everything depends on it.

If screed, it shouldn't have any ballast in it so reall should not be looking like a "beach".

[The Natural Philosopher]

there is not. because its not concrete, its screed, and it will be dusty and it will crater if you drop something on it

nothing. you should lay dilute PVA on it to stop it dusting and seal it

Or paint it if its to stay being 'concrete

[Mr Fuxit]

I reckon that the mix dried before it cured (in warm, dry situations, it is necessary to keep concrete or cement mixes from drying as it is a chemical reaction which needs the presence water to harden correctly.)

[DerbyBorn]

Thanks - I wasn't in when it was done. The builder was planning to skim it with a self levelling coat (of something) - but only up to the legs of the units as he is hoping to install the units tomorrow. Worried about the washing machine causing a problem

[DerbyBorn]

DerbyBorn wrote in news:XnsA1DFEF284489johnplant90ntlworldc@81.171.92.236:

Update: Builder has been over it with a hammer and claims the problem is just a patch possibly due to poor mixing. He is going to replace a patch.

[polygonum]

How the hell did he manage to wrongly mix one little bit? And make sure it was all together in one little area? If it is anything else, then a small patch is not adequate remedy, in my ignorant opinion.

[Jim K]

nt90ntlworldc@81.171.92.236:

why don't you "go over with a hammer" and see what you think?

drop some electrician type things on it and see what happens elsewhere?

remember taking the piss, fobbing you off and looking exasperated & "hard done by" is part of their "aprrentissheep".....

Jim K

[The Natural Philosopher]

very easily.

It takes a long time to lay a screed, possibly more than a day in a big area, and its a lot of mixing in batches. one mix is probably a couple of barrow loads and that's enough for 3-4 square meters at the most, If its deep, its less.

If the screeders mate is sloppy and forgets to open the next bag of cement, or just cant be arsed and is trying to make the bags last, then some bits will be of low cement content.

Unsurprisingly, mixing concrete or screed is one of the lowest skilled jobs on the site, and its not given to people with any huge commitments to quality or deep knowledge of chemistry. Nor are they paid much.

[polygonum]

Appreciate the achievement of a bad area is not difficult. Precise confinement to one patch a bit more difficult - even if whole barrow-load dumped in one spot it is likely to spread and mix a bit. I should have expanded - how did they manage that and yet are utterly confident that no other part is adversely affected? And, further, are they able to re-assure you and warrant appropriately that the rest is fine?

[Tim Watts]

On Saturday 15 June 2013 09:22 Jim K wrote in uk.d-i-y:

I agree. I would be all over that with a medium hammer - firm 4" swings to tap it, not violent arm lenght thwacking. 9 day well laid anything should ring solid. If parts sound "dull" that could be a string indicator of inconsistency.[1]

I would also consider taking some core samples if I happened to have an SDS and diamond core bit handy.

I would certainly be bringing the building inspector straight in - but it is helpful if he can see the evidence.

It is very hard to see how, with a cement mixer, he managed to get "a bit mixed badly". Sometimes, dry mix gets stuck to the back of the mixer and you get a blob that does not blend - but this is obvious when you empty it into the barrow. When that (infrequently) happened to me, the mix went back in the mixer.

No excuse for that.

[1] Just on a side - I had to lay some screed that was only 25mm thick to old concrete. This is normally not recomended. However, it is possible with an SBR modified screed and applying a bonding layer SBR followed by SBR+cement slurry.

Anyway, did all that and 3 days later, I was checking and found a couple of very local high spots. No problem I thought - I'll just scrape them off. No... Ended up with a sharp chisel in an SDS and even that was barely touching this stuff - it was basically iron.

[The Natural Philosopher]

My kitchen screed was done over two days and was in one area dreadful. A huge lump shrunk, cracked, and crumbled away from the rest, and sounded hollow when tapped. It actually moved. when trodden on. "Too much water and not enough cement' was the verdict.

I poured, on the advice of the chippies, half a gallon of PVA down the crack.

Three days later it was rock solid. PVA is more expensive than cement, but it seems like sand and PVA is not a bad screed in its own right :-)

Its been slated over for 12 years now, and its still just fine.

[Rick Hughes]

Fair comment it can happen ....

Suggest going over floor with a concrete surface hardener will help ... water it in form a watering can.

such as:

|W5fPyZGt|pcrid|7531858372|match||plid|&gclid=CKTpquz65bcCFefKtAodgn8AmQ

Not that I buy from Wickes

[Tim Watts]

On Saturday 15 June 2013 12:29 Rick Hughes wrote in uk.d-i-y:

To be honest, if taking that route - use diluted SBR and pour it on by the bucket load until the floor absorbs no more. It has the best penetrating abilities of any mix I know. I used this on a layer of sandy weak screed I found on one of my floors and it bonded it up solid. But then this was old screen and it would have been me taking it up.

The OP's screen/concrete/whaetever sounds like it needs more than "surface hardening".

But in thi scase, I think it would be more correct to take a 3rd party opinion and possibly make the builder take it up and do it a again.