[Towertalk] RE: Hit water digging hole - advice?

[Jim White]

Mike,

I appreciate your zeal but for a hole which is a couple meters on a side
tops it sounds like you are engineering for the rebuild of the World Trade
Center!

I tend to overengineer myself but in this case I cannot see that this is
called for - sure a sump pump is an inexpensive peace of mind measure but
really how much of the water actually does damage to the bottom of the pour
and seconds later that water is on its way to the top of the hole, no?

73,

Jim, K4OJ


----- Original Message -----
From: "Mike Newman" <mnewman@clear.net.nz>
To: <towertalk@contesting.com>
Sent: Tuesday, July 09, 2002 6:23 PM
Subject: [Towertalk] RE: Hit water digging hole - advice?


> Never freefall concrete into standing water, and never place any
structural
> concrete into flowing water.
>
> The problem with pouring free-falling concrete into water is that the
cement
> paste in the mix is washed out of the wet concrete as it hits the water.
> The result is that the aggregate is cleaned of the cement paste, leaving a
> heap of aggregate resting on the bottom of the excavation at the point
where
> the concrete is deposited.  This heap of aggregate lacks strength from
> cement bonding, and since it contains voids, provides no protection to the
> reinforcement laid in the excavation.
>
> If you can get a trough or flume into the base of the excavation
containing
> standing water then you could flow the concrete down the trough to the
base
> of the excavation, provided the depth of water is shallow - inches not
feet.
>
> It is often useful to dig the excavation with a fall to a small sump in
one
> corner of the excavation away from the start of the pour, and keep a pump
> working in the excavation removing water displaced by concrete poured on
the
> opposite side of the excavation.
>
> Proven methods of concreting underwater are to use a tremie bucket or to
> pump through a pipeline and hose, keeping the end of the bucket or hose
> submersed at all times (that way the concrete is prevented from
freefalling
> into water), or to fill the excavation with an aggregate (without the
fines)
> and then to inject cement grout (may need to contain bentonite) into the
> aggregate from the bottom.  The grout will displace the water upwards, and
> the water can be removed from the top surface when the grout appears at
the
> top of the aggregate,  This is called pre-packed concrete.  Be sure to
> remove the depth of laitance and weak concrete from the surface.
>
> Some of the advice given in the past few days has directly contradicted
good
> practice.
>
> The difficulty with just pouring the concrete without taking precautions
or
> using proper techniques is that the no one gets to see the defects since
the
> evidence is buried .  The defects can only be revealed by taking concrete
> cores -an expensive process, and one which I have called on on several
> occasions when the techniques used have been questionable.   It's a
wake-up
> call when you see the voids in a core sample where the concrete is
weakened
> or only loose aggregate exists.  Often there is no practicable remedy
other
> than grout injection, an expensive process.
>
> The rule is - get it right, first time.
>
> Mike  ZL1BNB
>
>
>
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