Civil Engineering Reference
In-Depth Information
supervision. The basic principles for normal concrete
work in the dry apply, with common sense, to underwater
concreting. The following special points, however, should
be observed:
The slump of the concrete should be specified at 150
to 230 mm (6 to 9 in.) and the mixture should have a max-
imum water-cementitious materials ratio of 0.45. Gen-
erally, the cementitious materials content will be 390
kg/m
3
(600 lb/yd
3
) or more.
It is important that the concrete flow without segrega-
tion; therefore, the aim in proportioning should be to
obtain a cohesive mixture with high workability. Anti-
washout admixtures can be used to make concrete cohe-
sive enough to be placed in limited depths of water, even
without tremies. Using rounded aggregates, a higher per-
centage of fines, and entrained air should help to obtain
the desired consistency.
The current in the water through which the concrete
is deposited should not exceed 3 m (10 ft) per minute.
Methods for placing concrete underwater include the
following: tremie, concrete pump, bottom-dump buckets,
grouting preplaced aggregate, toggle bags, bagwork, and
the diving bell.
A tremie is a smooth, straight pipe long enough to
reach the lowest point to be concreted from a working
platform above the water. The diameter of the tremie pipe
should be at least 8 times the diameter of the maximum
size of aggregate. A hopper to receive the concrete is
attached to the top of the pipe. The lower end of the tremie
should be kept buried in the fresh concrete to maintain a
seal below the rising top surface and to force the concrete
to flow in beneath it by pressure. Placing should be con-
tinuous with as little disturbance to the previously placed
concrete as possible. The top surface should be kept as
level as possible. See
ACI Committee 304 (2000)
for addi-
tional information.
Mobile concrete pumps with a variable radius boom
makes easy work of placing concrete underwater. Because
the flexible hose on a concrete pump is similar to a tremie,
the same placement techniques apply.
With the grouting preplaced aggregate method, the
forms are first filled with clean coarse aggregate, then the
voids in the coarse aggregate are filled with a grout to
produce concrete. Grouting preplaced aggregate has
advantages when placing concrete in flowing water.
Concrete can be placed more quickly and economically
than by conventional placement methods. However, the
method is very specialized and should only be performed
by qualified experienced personnel.
Sand bags half full of plastic concrete can be used for
small jobs, filling gaps, or temporary work. The tied end
should face away from the outside.
are described in
Chapter 18
. No matter what method is
used, the basics of mixing, placing, consolidating, and
curing apply to all portland cement concretes.
CONSOLIDATING CONCRETE
Consolidation is the process of compacting fresh concrete;
to mold it within the forms and around embedded items
and reinforcement; and to eliminate stone pockets, honey-
comb, and entrapped air (Fig. 11-9). It should not remove
significant amounts of intentionally entrained air in air-en-
trained concrete.
Consolidation is accomplished by hand or by mechan-
ical methods. The method chosen depends on the consis-
tency of the mixture and the placing conditions, such as
complexity of the formwork and amount and spacing of
reinforcement. Generally, mechanical methods using either
internal or external vibration are the preferred methods of
consolidation.
Workable, flowing mixtures can be consolidated by
hand rodding, that is, thrusting a tamping rod or other
suitable tool repeatedly into the concrete. The rod should
be long enough to reach the bottom of the form or lift and
thin enough to easily pass between the reinforcing steel
and the forms. Low-slump concrete can be transformed
into flowing concrete for easier consolidation through the
use of superplasticizers without the addition of water to
the concrete mixture.
Spading can be used to improve the appearance of
formed surfaces. A flat, spadelike tool should be repeat-
edly inserted and withdrawn adjacent to the form. This
forces the larger coarse aggregates away from the forms
and assists entrapped air voids in their upward move-
ment toward the top surface where they can escape. A
mixture designed to be readily consolidated by hand
methods should not be consolidated by mechanical
methods; otherwise, the concrete is likely to segregate
under intense mechanical action.
SPECIAL PLACING TECHNIQUES
Concrete may be placed by methods other than the usual
cast-in-place method. These methods, such as shotcreting,
Fig. 11-9. Honeycomb and rock pockets are the results of
inadequate consolidation. (50207)
