Civil Engineering Reference
In-Depth Information
important. Samples should be applied and cured in an
inconspicuous location, perhaps a basement wall, several
days in advance of patching operations to determine the
most suitable proportions of white and gray cements. Steel
troweling should be avoided since this darkens the patch.
Bolt holes, tierod holes, and other cavities that are
small in area but relatively deep should be filled with a
dry-pack mortar. The mortar should be mixed as stiff as is
practical: use 1 part cement, 2
1
⁄
2
parts sand passing a 1.25
mm (No. 16) sieve, and just enough water to form a ball
when the mortar is squeezed gently in the hand. The
cavity should be clean with no oil or loose material and
kept damp with water for several hours. A neat-cement
paste should be scrubbed onto the void surfaces, but not
allowed to dry before the mortar is placed. The mortar
should be tamped into place in layers about 13 mm (
1
⁄
2
in.)
thick. Vigorous tamping and adequate curing will ensure
good bond and minimum shrinkage of the patch.
Concrete used to fill large patches and thin-bonded
overlays should have a low water-cement ratio, often with
a cement content equal to or greater than the concrete to
be repaired. Cement contents range from 360 to 500 kg per
cubic meter (600 to 850 lb per cubic yard) and the water-
cement ratio is usually 0.45 or less. The aggregate size
should be no more than
1
⁄
3
the patch or overlay thickness.
A 9.5-mm (
3
⁄
8
-in.) nominal maximum size coarse aggregate
is commonly used. The sand proportion can be higher
than usual, often equal to the amount of coarse aggregate,
depending on the desired properties and application.
Before the patching concrete is applied, the sur-
rounding concrete should be clean and sound (Fig. 11-32).
Abrasive methods of cleaning (sandblasting, hydrojetting,
waterblasting, scarification, or shotblasting) are usually
required. For overlays, a cement-sand grout, a cement-
sand-latex grout, or an epoxy bonding agent should be
applied to the prepared surface with a brush or broom (see
the earlier section “
Bonding New to Previously Hardened
Concrete
”). Typical grout mix proportions are 1 part
cement and 1 part fine sand and latex or epoxy admix-
tures. The grout should be applied immediately before the
new concrete is placed. The grout should not be allowed
to dry before the freshly mixed concrete is placed; other-
wise bond may be impaired. The concrete may be dry or
damp when the grout is applied
but not wet
with free-
standing water. The minimum thickness for most patches
and overlays is 20 mm (
3
⁄
4
in.). Some structures, like bridge
decks, should have a minimum repair thickness of 40 mm
(1
1
⁄
2
in.). A superplasticizer is one of many admixtures
often added to overlay or repair concrete to reduce the
water-cement ratio and to improve workability and ease
of consolidation (
Kosmatka 1985a
).
Honeycombed and other defective concrete should be
cut out to expose sound material. If defective concrete is
left adjacent to a patch, moisture may get into the voids; in
time, weathering action will cause the patch to spall. The
edges of the defective area should be cut or chipped
straight and at right angles to the surface, or slightly
undercut to provide a key at the edge of the patch. No
featheredges should be permitted (Fig. 11-33). Based on
the size of the patch, either a mortar or a concrete patching
mixture should be used.
Shallow patches can be filled with a dry-pack mortar
as described earlier. This should be placed in layers not
more than 13 mm (
1
⁄
2
in.) thick, with each layer given a
scratch finish to improve bond with the subsequent layer.
The final layer can be finished to match the surrounding
concrete by floating, rubbing, or tooling, or on formed sur-
faces by pressing a section of form material against the
patch while still plastic.
Deep patches can be filled with concrete held in place
by forms. Such patches should be reinforced and doweled
to the hardened concrete (
Concrete Manual
1981
). Large,
(a) Incorrectly installed patch. The feathered edges can break down
under traffic or weather away.
(b) Correctly installed patch. The chipped area should be at least 20 mm
(
3
/
4
in.) deep with the edges at right angles or undercut to the surface.
Fig. 11-32. Concrete prepared for patch installation. (69972)
Fig. 11-33. Patch installation.
