What's wrong with my concrete?

[JTM]

Hello,

Last Friday I mixed up some concrete. Each mix was about 1/3 bag cement (35k bags) with aggregate and water with a splash of washing up liquid. All seemed well, but the last load was well into darkness so I was working by security light with lots of shadow.

The final load was the top inch or three of the footings, so I levelled and tamped it and covered it up for the night. It stayed covered for a few days to protect from sun, but when I looked it was soft enough to push my finger in!

The top few inches are affected, lower down seems solid, so :

1 What went wrong ? 2 Can I scrape the soft stuff off and simply replace with another batch, perhaps overlapping the solid stuff sideways?

John

[Kipper at sea]

You DO NOT add washing up liquid to concrete. Cement could be old or to weaker mix. Looks like a complete dig out, as footing foundation should be poured in one.

[The Natural Philosopher]

it dried out.

No: water it.

[Cash]

A basic structural concrete mix should be 1:2:4 (1 part cement, 2 parts sand and 4 parts Chipping's) with only sufficient water added to allow the concrete to be properly worked [1] (I have not given an amount for water figure the wetness of the sand will alter that amount) - do *NOT* add "washing up liquid).

Footings should always be poured in on session - and not added to at a later date as the 'old' and 'new' pour will not adhere to each other.

What are the footings for?

If it's for a new building or suchlike, then it would be advisable to take the lot out and start anew - laying in one continuous pour.

If it's simply for a garden wall, single brick garage or small shed, then remove the to 'soft' bit and start the brickwork/blockwork from the top of the 'solid' concrete.

If it's for any thing else - let the group know, so that the proper advice can be given.

NOTE: That the advice given is general, and *will* alter for specific concrete use.

[1] Too much or too little water will dramatically weaken the concrete, as will certain additives.

Cash

[Cash]

Erratum

Please note that the above paragraph should have read:

A basic structural concrete mix should be 1:2:4 (1 part cement, 2 parts sand and 4 parts chipping's) with only sufficient water added to allow the concrete to be properly worked [1]. Note that I have not given a specific amount for the water figure, as the moisture in the sand can and will alter that amount) - do NOT add"washing up liquid).

Sorry for that :-)

Cash

[Tim S]

Cash coughed up some electrons that declared:

I had a friable surface on a concrete slab due to excess water in the mix. It's solid further down (I had "dammed" a section out around a rising pipe, so I removed the dam and checked). I soaked the top in PVA. Not bothered here as it's only a 3m2 slab for a shower room and it's 4" thick.

As mentioned, it all depends on what the application is - JTM?

Cheers

Tim

[JTM]

[snip]> >>

Thank you, everyone, for the replies.

The project was a late start on a pizza oven mentioned in threads earlier in the year. About 1.5m sq and the solid bit seems to be about 3 - 4 inches thick, reinforced with 10mm rebar.

Regards

John

[Stuart Noble]

Eh? Haven't I seen BCOs inspecting the footings at the halfway point?

Anyway, he HAS added washing up liquid, as I do, for the simple reason that it dramatically reduces the amount of water required. Plasticisers perform the same function except that the dosage is easier to measure accurately, and they don't foam if you overdo it a little.

As NP says, try re-wetting it before doing anything more drastic.

[Recyclist]

And plasticisers don't weaken he concrete or mortar like washing up liquid does.

T

[Recyclist]

I'd just leave it, as it will probably harden eventually. You could have put too much washing up liquid in...

I always use 1:6 cement to ballast (pre mixed aggregate) and have never had a problem. I have often got the ratios wrong, added too much water, failed to cover in frosty conditions, etc. The concrete has always gone off (sometimes slowly), but then I have never added washing up liquid!

Let me guess - you were using fairy concentrated liquid and gave a generous squirt?

T

[Stuart Noble]

Depends how much you use. An excess of either isn't desirable. Plasticisers are usually very small salts (lignosulphonates?), heavily diluted for easier measuring. Fairy is a surfactant with a foaming problem that makes it more difficult to handle. The purpose of both is to get between the particles, which water alone isn't very good at. I'd go with a plasticiser for anything serious, but a squirt of Fairy is fine for odd jobs IME.

[The Natural Philosopher]

Footings are mainly under compression, and if you have any rebar at the base, completely under compression.

There really is no need to worry. All a footing is, is a way to distribute a high force at the base of a wall over a larger area, sufficiently far below ground to avoid surface movement due to humidity changes etc. and using a structure that is inherently free of voids where it contacts the subsoil - i.e. poured concrete.

And to provide a levelish base to begin laying block or brick.

If the loose top persists, scrape it off and lay a layer of blocks on their sides to make up the difference, but unless the mixture was so watery that the sand came to the top while the cement sunk - and if that was the case. you didn't have enough cement anyway - it will be good enough.

Block houses really stay up because the weight of the blocks above stops them sliding about. Mortar is almost unecessesary.

As long as the footings are relatively solid, a crumby top is no big deal.

[JTM]

Well it leaves my hands soft as my face ;-)

John, just off to buy another 1/4 meter pre-mixed aggregate.

Ta