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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