Concrete foundations will they cure if under water?

[Tim]

I have a builder building a 2.5m x 2m porch (two skins of brick and a pitched roof). The plans for the foundation are I've been told a little excessive, 900mm deep with 600 x 200mm of concrete. The builders have been digging the trench for a couple of days in the pouring rain. The ground is sodden, the sides are caving in and the trench fills quickly. I bailed out 50cm of water at the weekend. I've come home to find they've poured the concrete and it's now under a couple of inches of water. The builder has told me that it will set better under water and not to bail it out, but from what I understand water dramatically affects the strength. Not only that it looks like it will get down to 0 or -1C tonight.

Is this likely to be an issue?

Thanks,

Tim

[Palindrome]

IANA concrete expert. Although I have used lots of the stuff, with no problems, yet. Including using it under water.

What mix did they use? What additives? When are the coming back to start building on it?

Freezing is very bad news for setting concrete. However, if the air temperature is only going to drop to -1 and there is a couple of inches of water over the concrete, the concrete itself isn't going to get to freezing point. Apart from anything else, if it is still setting, it is still going to be mildly exothermic. It generally doesn't need a lot of covering to protect setting concrete from an overnight chill.

Too wet a mix is bad news - as it can end up with pockets of water forming, plus all that extra water has to get out, somehow. However, IME, dumping water on top of a mix already well on the way to setting isn't going to be a problem. It isn't going to get significantly below the surface.

Personally, I'd be tempted to see what it is like in the morning. Find the odd lump where a bit spilled over and belt it one with a brick. If it takes that, I wouldn't be too worried about the foundations. I might ask the builder what mix and what additives - but I'd probably get that look which means, shouldn't I be in doing the housework... However, with any luck, he will give you chapter and verse on the additives used and why. If the stuff came pre-mixed and they just poured it from the truck, I'd be even less worried.

[Rob graham]

The setting of concrete is a chemical reaction, not a drying out of water. Although too much water weakens the concrete (the correct amount is actually too little to allow the mixture to be used easily) that is only if it is mixed in, not if it lies on top or around it. Let's face it, concrete is used under the sea.

Re the frost issue, although the air temperature may get that low the ground as a whole won't - not in one night. And anyway, the water in the mix will not freeze until it's below the normal freezing point of water (like having antifreeze in your car).

Don't worry about it.

Rob Graham

[Tim]

I believe it was ordered as a pre-mix job, probably poured from a truck and barrowed in. I think you mentioned below that it's probably less of an issue if this is the case.

It didn't feel like a cold night in the house, so I thought I'd got away with it until I found my car well and truly frozen. The water on the surface hadn't turned to ice, so I'm assuming that the concrete was above freezing.

I think the mix was put in at 11:00ish, I got home at 17:00. So it's taken a little while for the water to get on the surface. However where the sides have caved in the concrete can be pushed through with a metal rod. I guess the sides must have fallen in during the pouring process. I hope additional concrete can be poured in these locations.

Last time I checked the hole it was 800mm deep. It's now about 500mm deep, so unless they backed filled. There could be up to 300mm of concrete. I guess the surface is just going to be no good. So I'll brush this off after it stops raining.

At the end of the day it's a 2.5m x 2m porch, double skin with a pitch roof and a concrete floor (which I guess is independent of the brick work/foundations). A window either side and a large front door. The porch should be tied into the house as well, so I guess it can't weight that much.

Thanks for the tip.

Regards,

Tim

[Tim]

I've calmed down a little now. Most people seem to say it probably wasn't a clever idea, but no real harm has been done. The next issue to address are the areas where the surrounding earth has caved in. I don't know if concrete adheres well to existing cured concrete.

Regards,

Tim

[Rob graham]

The next issue to

The existing cured concrete is likely to be pretty rough (i.e. not as smooth as if it were the surface of render) so I would expect new concrete to adhere extremely well. Try getting set cement off a smooth shovel!

Rob

[Al]

OK, as long as the concrete wasn't poured into a pond of standing water and swished about it isn't a problem. The technical term is the Free water to Cement ratio. If this gets too high the concrete will be weakened severely. i.e. if the amount of water in the mix is too high. In this case the water isn't in the mix, it's on it (and around it)

I'm a bridge engineer by trade, and I certainly wouldn't allow concrete to be poured into standing water under normal circumstances. However, given that this isn't a bridge foundation, I doubt it will be a problem.

In fact if the water came after the concrete was laid, it may well aid the curing (hardening) process. When we use concrete on site for structures, the contractor will make cubes. These are cured under water, albeit temperature controlled water in a tank, and crushed at set intervals to determine the strength of the concrete laid. For a structures engineer, it's generally the 28 day cube strength that counts. The water you had provided you with your own curing tank, albeit not temperature controlled.

Regarding the frost, one cold night was probably better survived by concrete underwater than if the concrete hadn't been covered. You should always protect curing concrete from the elements, be they cold or hot, to allow the curing process to succeed. The 2 inches of water very probably saved you from a crumbling surface, or laitance.

In short, what they said, don't worry.

Al

[Cicero]

================================== The old concrete is often 'grouted off' before pouring the new concrete. This involves pouring a very runny mix of water and cement powder (about the consistency of milk) on to the old concrete immediately before adding the new. Sometimes a special bonding agent is used but probably not necessary for your kind of job.

Cic.