Civil Engineering Reference
In-Depth Information
bond with the next placement. As long as no laitance (a
weak layer of concrete), dirt, or loose particles are present,
newly hardened concrete requires little preparation prior
to placing freshly mixed concrete on it. When in service
for a period of time, old hardened concrete usually
requires mechanical cleaning and roughening prior to
placement of new concrete. The subject of placing freshly
mixed concrete on hardened concrete is discussed in more
detail under the sections entitled “
Placing on Hardened
Concrete
” and “
Construction Joints
.”
Forms should be accurately set, clean, tight, ade-
quately braced, and constructed of or lined with materials
that will impart the desired off-the-form finish to the hard-
ened concrete. Wood forms, unless oiled or otherwise
treated with a form-release agent, should be moistened
before placing concrete, otherwise they will absorb water
from the concrete and swell. Forms should be made for
removal with minimum damage to the concrete. With
wood forms, use of too large or too many nails should be
avoided to facilitate removal and reduce damage. For
architectural concrete, the form-release agent should be a
nonstaining material. See
Hurd (1979)
and
ACI Commit-
tee 347 (1999)
for more information on formwork.
Reinforcing steel should be clean and free of loose rust
or mill scale when concrete is placed. Unlike subgrades,
reinforcing steel can be colder than 0ºC (32ºF) with special
considerations. See “
Concreting Aboveground
” in
Chapter 14 for more details. Mortar splattered on rein-
forcing bars from previous placements need not be
removed from steel and other embedded items if the next
lift is to be completed within a few hours; loose, dried
mortar, however, must be removed from items that will be
encased by later lifts of concrete.
All equipment used to place concrete must be clean
and in good working condition. Standby equipment
should be available in the event of a breakdown.
Fig. 11-4. Wheelbarrows are used to place concrete in areas
that are not easily accessed by other placement methods.
(69929)
DEPOSITING THE CONCRETE
Fig. 11-5. The swing arm on a conveyor belt allows fresh
concrete to be placed fairly evenly across a deck. (70002)
Concrete should be deposited continuously as near as pos-
sible to its final position without objectionable segregation
(Figs. 11-4, 11-5, 11-6, 11-7, and 11-8). In slab construction,
placing should be started along the perimeter at one end
of the work with each batch discharged against previously
placed concrete. The concrete should not be dumped in
separate piles and then leveled and worked together; nor
should the concrete be deposited in large piles and moved
horizontally into final position. Such practices result in
segregation because mortar tends to flow ahead of the
coarser material.
In general, concrete should be placed in walls, thick
slabs, or foundations in horizontal layers of uniform thick-
ness; each layer should be thoroughly consolidated before
the next is placed. The rate of placement should be rapid
enough so that previously placed concrete has not yet set
when the next layer of concrete is placed upon it. Timely
placement and adequate consolidation will prevent flow
lines, seams, and planes of weakness (cold joints) that result
Fig. 11-6. Dump trucks deposit concrete ahead of a slip-
form paver that places the entire width of a street in one
pass. Epoxy coated dowels on metal chairs are positioned
at a joint and spiked down to the base course just ahead of
the paver. (69936)
