Civil Engineering Reference
In-Depth Information
high cement contents speed up hydration excessively.
Planning should eliminate or minimize any variables that
would allow the concrete to stiffen to the extent that full
consolidation is not achieved and finishing becomes diffi-
cult. Less time is available during conditions that hasten
the stiffening process, such as hot and dry weather, use of
accelerators, and use of heated concrete.
Segregation.
Segregation is the tendency for coarse
aggregate to separate from the sand-cement mortar. This
results in part of the batch having too little coarse aggre-
gate and the remainder having too much. The former is
likely to shrink more and crack and have poor resistance
to abrasion. The latter may be too harsh for full consolida-
tion and finishing and is a frequent cause of honey-
combing. The method and equipment used to transport
and handle the concrete must not result in segregation of
the concrete materials.
Methods and Equipment for
Transporting and Handling Concrete
Table 10-1 summarizes the most common methods
and equipment for moving concrete to the point where
it is needed.
Table 10-1. Methods and Equipment for Transporting and Handling Concrete
Type and range of work for
Equipment
which equipment is best suited
Advantages
Points to watch for
Belt conveyors
For conveying concrete
Belt conveyors have adjustable
End-discharge arrangements
horizontally or to a higher or
reach, traveling diverter, and variable
needed to prevent segregation
lower level. Usually positioned
speed both forward and reverse.
and leave no mortar on return belt.
between main discharge point
Can place large volumes of concrete
In adverse weather (hot, windy)
and secondary discharge point.
quickly when access is limited.
long reaches of belt need cover.
Belt conveyors
For conveying concrete to a
Conveying equipment arrives with the
End-discharge arrangements
mounted on
lower, horizontal, or higher
concrete. Adjustable reach and
needed to prevent segregation
truck mixers
level.
variable speed.
and leave no mortar on return belt.
Buckets
Used with cranes, cableways,
Enables full versatility of cranes, cable-
Select bucket capacity to conform
and helicopters for construction
ways, and helicopters to be exploited.
to size of the concrete batch and
of buildings and dams. Convey
Clean discharge. Wide range of
capacity of placing equipment.
concrete directly from central
capacities.
Discharge should be controllable.
discharge point to formwork or
to secondary discharge point.
Chutes on
For conveying concrete to a
Low cost and easy to maneuver. No
Slopes should range between 1 to
truck mixers
lower level, usually below
power required; gravity does most of
2 and 1 to 3 and chutes must be
ground level, on all types of
the work.
adequately supported in all posi-
concrete construction.
tions. End-discharge arrangements
(downpipe) needed to prevent
segregation.
Cranes and
The right equipment for work
Can handle concrete, reinforcing steel,
Has only one hook. Careful sched-
buckets
above ground level.
formwork, and sundry items in bridges
uling between trades and opera-
and concrete-framed buildings.
tions is needed to keep crane busy.
Dropchutes
Used for placing concrete in
Dropchutes direct concrete into form-
Dropchutes should have sufficiently
vertical forms of all kinds.
work and carry it to bottom of forms
large, splayed-top openings into
Some chutes are one piece
without segregation. Their use avoids
which concrete can be discharged
tubes made of flexible rubber-
spillage of grout and concrete on rein-
without spillage. The cross section
ized canvas or plastic, others
forcing steel and form sides, which is
of dropchute should be chosen to
are assembled from articulated
harmful when off-the-form surfaces
permit inserting into the formwork
metal cylinders (elephant
are specified. They also will prevent
without interfering with reinforcing
trunks).
segregation of coarse particles.
steel.
Mobile batcher
Used for intermittent production
A combined materials transporter and
Trouble-free operation requires
mixers
of concrete at jobsite, or
mobile batching and mixing system for
good preventive maintenance pro-
where only small quantities
quick, precise proportioning of spe-
gram on equipment. Materials must
are required.
cified concrete. One-man operation.
be identical to those in original mix
design.
