Civil Engineering Reference
In-Depth Information
rial should be installed full depth in the saw cut, without
a backer rod, and flush with the floor surface.
Isolation joints are intended to accommodate move-
ment; thus a flexible, elastomeric sealant should be used to
keep foreign materials out of the joint.
eral, for concrete temperatures above 10°C (50°F), the side
forms of reasonably thick, supported sections can be
removed 24 hours after concreting. Beam and floor slab
forms and supports (shoring) may be removed between 3
and 21 days, depending on the size of the member and the
strength gain of the concrete. For most conditions, it is
better to rely on the strength of the concrete as determined
by in situ or field-cured specimen testing rather than arbi-
trarily selecting an age at which forms may be removed.
Advice on reshoring is provided by
ACI Committee 347
.
For form removal, the designer should specify the
minimum strength requirements for various members. The
age-strength relationship should be determined from rep-
resentative samples of concrete used in the structure and
field-cured under job conditions. It should be remembered,
however, that strengths are affected by the materials used,
temperature, and other conditions. The time required for
form removal, therefore, will vary from job to job.
A pinch bar or other metal tool should not be placed
against the concrete to wedge forms loose. If it is necessary
to wedge between the concrete and the form, only
wooden wedges should be used. Stripping should be
started some distance away from and move toward a pro-
jection. This relieves pressure against projecting corners
and reduces the chance of edges breaking off.
Recessed forms require special attention. Wooden
wedges should be gradually driven behind the form and
the form should be tapped lightly to break it away from
the concrete. Forms should not be pulled off rapidly after
wedging has been started at one end; this is almost certain
to break the edges of the concrete.
UNJOINTED FLOORS
An unjointed floor, or one with a limited number of joints,
can be constructed when joints are unacceptable. Three
unjointed floor methods are suggested:
1. A prestressed floor can be built through the use of
post-tensioning. With this method, steel strands in
ducts are tensioned after the concrete hardens to pro-
duce compressive stress in the concrete. This com-
pressive stress will counteract the development of
tensile stresses in the concrete and provide a crack-
free floor. Large areas, 1000 m
2
(10,000 ft
2
) and more,
can be constructed in this manner without inter-
mediate joints.
2. Large areas—a single day of slab placement, usually
800 to 1000 m
2
(8000 to 10,000 ft
2
)—can be cast with-
out contraction joints when the amount of distributed
steel in the floor is about one-half of one percent of the
cross-sectional area of the slab. Special effort should
be made to reduce subgrade friction in floors without
contraction joints.
Farny (2001)
discusses use of dis-
tributed steel in floors.
3. Concrete made with expansive cement can be used to
offset the amount of drying shrinkage to be antici-
pated after curing. Contraction joints are not needed
when construction joints are used at intervals of 10 to
35 meters (40 to 120 ft). Large areas, up to 2000 m
2
(20,000 ft
2
), have been cast in this manner without
joints. Steel reinforcement is needed in order to pro-
duce compressive stresses during and after the expan-
sion period—this is a form of prestressing.
PATCHING, CLEANING, AND FINISHING
After forms are removed, all bulges, fins, and small pro-
jections can be removed by chipping or tooling. Undesired
bolts, nails, ties, or other embedded metal can be removed
or cut back to a depth of 13 mm (
1
⁄
2
in.) from the concrete
surface. When required, the surface can be rubbed or
ground to provide a uniform appearance. Any cavities
such as tierod holes should be filled unless they are
intended for decorative purposes. Honeycombed areas
must be repaired and stains removed to present a concrete
surface that is uniform in color. All of these operations can
be minimized by exercising care in constructing the form-
work and placing the concrete. In general, repairs are
easier to make and more successful if they are made as
soon as practical, preferably as soon as the forms are
removed. However, the procedures discussed below
apply to both new and old hardened concrete.
REMOVING FORMS
It is advantageous to leave forms in place as long as pos-
sible to continue the curing period. However, there are
times when it is necessary to remove forms as soon as pos-
sible. For example, where a rubbed finish is specified,
forms must be removed early to permit the first rubbing
before the concrete becomes too hard. Furthermore, it is
often necessary to remove forms quickly to permit their
immediate reuse.
In any case, shoring should not be removed until the
concrete is strong enough to satisfactorily carry the
stresses from both the dead load of the structure and any
imposed construction loads. The concrete should be hard
enough so that the surfaces will not be injured in any way
when reasonable care is used in removing forms. In gen-
Holes, Defects, and Overlays
Patches usually appear darker than the surrounding con-
crete; therefore, some white cement should be used in
mortar or concrete for patching where appearance is
