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?
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
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.
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
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
Thanks for the tip.
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.
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.
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!
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.
=================================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.
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