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Geotechnical News •June 2015
THE GROUT LINE
Concrete
The most common voids found in con-
crete are cracks, which can of course
be observed on the surface. Not visible
are interior faults which can be in the
form of rock pockets (honeycomb) that
result from the large aggregate becom-
ing separated from the mortar fraction
during placement, or completely empty
voids usually resulting from interfer-
ence of the reinforcing steel or other
embedments to the placement flow and
vibration compaction during original
casting. As in geomaterials, location
of internal deficiencies in concrete is
difficult, and similar methods such as
coring, and seismic wave propagation
are used. Where defective zones are
confirmed or suspected, providing a
grid of regularly spaced grout holes is
common.
Honeycomb is easily repaired by sim-
ply drilling holes to intersect it or if
on the surface, placing a tight fitting
form over it as shown in Figure 2. A
stable cement grout is then injected so
as to fill all the voids. While it is very
common practice to simply chip out
the porous concrete followed by fill-
ing with mortar or drypack, because
it is often very difficult to remove all
the culprit honeycomb, such does not
always provide the best repair. Pressure
grouting has the advantage that all of
the voids can be filled, so long as suf-
ficient venting of any entrapped air is
provided. This is not much of a prob-
lem with most concrete as it is typically
somewhat permeable to air. Further, it
is extremely difficult to construct an air
tight form, so ample venting is virtu-
ally always available.
Treating non visible voids in the inte-
rior of mass concrete is much more dif-
ficult in that the exact location of the
suspected voids is seldom known, but
must be established. This might be by
way of coring or non-destructive seis-
mic techniques. In many cases howev-
er, voids can be so extensive that a reg-
ularly spaced grid of grout holes over
the suspect area is best. It is important
that the holes not completely penetrate
the section however as grout travel
must be contained within the concrete
to be effective. Small diameter holes,
normally no larger than 38 mm (1.5
in.) are spaced over the suspected area.
Spacing is typically much tighter than
in geomaterials, commonly 1 to 1.2
m, (3 to 4 ft), although initial grouting
may be at greater spacing, using typical
split spacing to treat suspect areas.
All holes should be wash cleaned of
drill cuttings and water pressure tested,
Figure 3. Those holes that freely take
water or communicate to adjacent
holes or the concrete surface, Figure 4,
should be indicated for pressure grout-
ing, Figure 5. Holes that do not readily
take water, but experience a pressure
decay indicating small voids and/or
lack of venting of entrapped air, should
be identified for vacuum grouting, Fig-
ure 6. Prior to grouting, all holes should
be blown free of water, Figure 7.
Unlike in rock, even very small voids
in concrete can have negative influence
on the capacity of a concrete member,
if adjacent to the reinforcing steel or in
a bearing area. For such work, a stable,
cementitious grout with strength simi-
lar to the substrate concrete should be
used. A pre-blended bagged grout con-
forming to the Post-Tensioning Insti-
tute
PTI M55.1-12 Specification for
Grouting of Post-Tensioned Structures
is a good choice for most applications.
In earlier times, post tensioning ten-
don ducts were typically filled with
unstable cement-water grouts which
often resulted in voids in the upper eleva-
tions. Because, the purpose of the grout
is to protect the enclosed tendons from
corrosion, complete filling is neces-
sary, and any voids found, should be
grout filled. Because post-tension ducts
and the surrounding concrete are usu-
ally very tight, they seldom provide for
proper venting of entrapped air; filling
is thus best accomplished with vacuum
grouting.
Grouting of cracks is typically for one
of three principal reasons, to stop the
flow of water, weld the section back
to a monolithic mass, or prevent intru-
sion of foreign or deleterious elements
therein. An essential consideration is
Figure 3. Pressure testing grout hole using line pressure
of facility system.
Figure 4. Leak on the surface in an area where no
problem was previously visible.
