Civil Engineering Reference
In-Depth Information
soils so they maintain sufficient bearing capacity to sup-
port structural loads.
Chutes and dropchutes are used to move concrete to
lower elevations without segregation and spattering of
mortar on reinforcement and forms. Properly designed
concrete has been allowed to drop by free fall into caissons.
Results of a field test to determine if concrete could be
dropped vertically 15 meters (50 ft) into a caisson without
segregation proved that there was no significant difference
in aggregate gradation between control samples as deliv-
ered and free-fall samples taken from the bottom of the
caisson (
Turner 1970
). More recent field studies indicate
that free fall of concrete from heights of up to 46 m
(150 ft)—directly over reinforcing steel or at a high
slump—does not result in segregation of the concrete
ingredients nor reduce compressive strength (
Suprenant
2001
). However, if a baffle is not used to control the flow of
concrete onto sloped surfaces at the end of an inclined
chute, segregation can occur.
Concrete is sometimes placed through openings,
called windows, in the sides of tall, narrow forms. When a
chute discharges directly through the opening without
controlling concrete flow at the end of the chute there is
danger of segregation. A collecting hopper should be used
outside the opening to permit the concrete to flow more
smoothly through the opening; this will decrease the ten-
dency to segregate.
When concrete is placed in tall forms at a fairly rapid
rate, some bleed water may collect on the top surface, espe-
cially with non-air-entrained concrete. Bleeding can be
reduced by placing more slowly and by using concrete of a
stiffer consistency, particularly in the lower portion of the
form. When practical, concrete should be placed to a level
300 mm to 400 mm (about a foot) below the top of tall
forms and an hour or so allowed for the concrete to par-
tially set. Placing should resume before the surface hardens
to avoid formation of a cold joint. If practical to work
around vertical reinforcing steel, it is good practice to over-
fill the form by 25 mm (an inch) or so and cut off the excess
concrete after it has stiffened and bleeding has ceased.
In monolithic placement of deep beams, walls, or
columns, to avoid cracks between structural elements,
concrete placement should stop (usually about 1 hr) to
allow settlement of the deep element before concreting is
continued in any slabs, beams, or girders framing into
them. The delay should be short enough to allow the next
layer of concrete to knit with the previous layer by vibra-
tion, thus preventing cold joints and honeycombing (
ACI
Committee 304 2000
). Haunches and column capitals are
considered part of the floor or roof slab and should be
placed integrally with them.
Fig. 11-7. Curb machines continuously extrude low-slump
concrete into a shape that immediately stands without
support of formwork. (69937)
Fig. 11-8. Concrete should be placed as near as possible to
its final position. (70009)
from placing freshly mixed concrete on concrete past initial
set. Layers should be about 150 to 500 mm (6 to 20 in.) thick
for reinforced members and 380 to 500 mm (15 to 20 in.)
thick for mass work; the thickness will depend on the width
between forms and the amount of reinforcement.
To avoid segregation, concrete should not be moved
horizontally over too long a distance as it is being placed
in forms or slabs. In some work, such as placing concrete
in sloping wingwalls or beneath window openings in
walls, it is necessary to move the concrete horizontally
within the forms, but this should be kept to a minimum.
Where standing water is present, concrete should be
placed in a manner that displaces the water ahead of the
concrete but does not allow water to be mixed in with the
concrete; to do so will reduce the quality of the concrete.
In all cases, water should be prevented from collecting at
the ends, in corners, and along faces of forms. Care
should be taken to avoid disturbing saturated subgrade
PLACING CONCRETE UNDERWATER
Concrete should be placed in the air rather than under-
water whenever possible. When it must be placed under-
water, the work should be done under experienced
