Civil Engineering Reference
In-Depth Information
chutes, over 6 meters (20 ft), or those not meeting slope
standards must discharge into a hopper before distribu-
tion to point of need.
Alternatively, a concrete pump can move the concrete
to its final position (Fig. 10-15). Pumps must be of ade-
quate capacity and capable of moving concrete without
segregation. The loss of slump caused by pressure that
forces mix water into the aggregates as the mix travels
from pump hopper to discharge at the end of the pipeline
must be minimal—not greater than 50 mm (2 in). The air
content generally should not be reduced by more than
2 percentage points during pumping. Air loss greater than
this may be caused by a boom configuration that allows
the concrete to fall excessively. In view of this, specifica-
tions for both slump and air content should be met at the
discharge end of the pump. Pipelines must not be made of
aluminum or aluminum alloys to avoid excessive entrain-
ment of air; aluminum reacts with cement alkali hydrox-
ides to form hydrogen gas which can result in serious
reduction in concrete strength.
Belt conveyors are very useful for work near ground
level. Since placing concrete below ground is frequently a
matter of horizontal movement assisted by gravity, light-
weight portable conveyors can be used for high output at
relatively low cost.
Fig. 10-16. The screw spreader quickly spreads concrete
over a wide area to a uniform depth. Screw spreaders are
used primarily in pavement construction. (69895)
See
Panarese (1987)
for extensive information on methods
to transport and handle concrete.
Choosing the Best Method
The first thing to look at is the type of job, its physical size,
the total amount of concrete to be placed, and the time
schedule. Studying the job details further will tell how
much of the work is below, at, or above ground level. This
aids in choosing the concrete handling equipment neces-
sary for placing concrete at the required levels.
Concrete must be moved from the mixer to the point
of placement as rapidly as possible without segregation or
loss of ingredients. The transporting and handling equip-
ment must have the capacity to move sufficient concrete
so that cold joints are eliminated.
Work Above Ground Level
Conveyor belt, crane and bucket, hoist, pump, or the ulti-
mate sky-hook, the helicopter, can be used for lifting con-
crete to locations above ground level (Fig. 10-17). The
tower crane (Fig. 10-12) and pumping boom (Fig. 10-18)
are the right tools for tall buildings. The volume of con-
crete needed per floor as well as boom placement and
length affect the use of a pump; large volumes minimize
pipeline movement in relation to output.
The specifications and performance of transporting
and handling equipment are being continuously
improved. The best results and lowest costs will be real-
ized if the work is planned to get the most out of the
equipment and if the equipment is flexibly employed to
reduce total job cost. Any method is expensive if it does
not get the job done.
Panarese (1987)
is very helpful in
deciding which method to use based on capacity and
range information for various methods and equipment.
Work At and Below Ground Level
The largest volumes of concrete in a typical job usually are
either below or at ground level and therefore can be
placed by methods different from those employed on the
superstructure. Concrete work below ground can vary
enormously—from filling large-diameter bored piles or
massive mat foundations to the intricate work involved in
basement and subbasement walls. A crane can be used to
handle formwork, reinforcing steel, and concrete.
However, the crane may be fully employed erecting form-
work and reinforcing steel in advance of the concrete, and
other methods of handling the concrete may have to be
used to place the largest volume in the least time.
Possibly the concrete can be chuted directly from the
truck mixer to the point needed. Chutes should be metal
or metal lined. They must not slope greater than 1 vertical
to 2 horizontal or less than 1 vertical to 3 horizontal. Long
