The success of a waterproofing application depends on,
among other things, the type, smoothness, and cleanliness of
the concrete surface being coated.
- Form release agents, such as oil, grease, wax, and
silicones, will transfer from the forms to the surface of the
concrete during casting. These will cause poor adhesion of
- Concrete curing compounds may contain waxes, resins,
chlorinated rubber, or film formers of various types. If such
materials must be used, the specifier should be certain that the
materials can be completely removed.
- Admixtures such as water-immiscible chemical curing
agents are sometimes used in concrete. These should be
- Laitance, dust and dirt, moisture, and grease, and oil can
inhibit or impair the adhesion of the waterproofing system. These
must be removed
- Moisture contained within the matrix of the concrete as
well as surface moisture present from curing or precipitation
events can impair adhesion of the waterproofing system.
Where possible, the concrete surface to receive waterproofing
should be protected from precipitation for a minimum of 48 h
prior to application of the waterproofing.
Repair of Surface Defects
- Surface defects that may impair adhesion
include honeycomb, fins, “snots,” tie holes, “bug holes,” sharp
offsets from displaced forms, rutted cracks, ragged corners,
deviations in the surface plane, and other similar concrete
defects, along with spalling and delaminations of the concrete
- Fins, protrusions, or similar irregularities should be cut
back to the surface by chipping, grinding, bushhammering, needle gunning, or wire brushing. Avoid polishing of the concrete surface by these techniques
- Sharp offsets in the surface, such as those caused by
formwork misalignment, should be cut back to an even surface
by chipping, grinding, bushhammering, needle gunning, wire brushing, or transitioning with grout or patching mortar.
- Defective concrete areas should be removed down to
sound concrete, preferably by chipping; if grinding is
necessary, care must be taken to avoid “polishing” the surfaces.
If sizable areas or amounts of unsound concrete are found, a
structural engineer shall specify appropriate corrective action.
- Cracks exceeding 2 mm [1⁄16 in.] should be investigated to
determine whether they are still active. Such cracks should be
chipped or routed out before being patched, and edges should
be undercut slightly as recommended by the manufacturer of the patching
material. No feathered edges shall be permitted. Tie holes and
“bug holes” larger than 16 mm [5⁄8 in.] in diameter or deeper
than 3 mm [1⁄8 in.], or both, should be prepared similarly for
- The areas to be patched, along with a band at least 150
mm [6 in.] wide surrounding it, should be dampened before the
patching application to prevent rapid absorption of water from
the bonding grout and the patching mortar,
- Immediately after the surface water has evaporated
from the dampening step, a bonding grout or bond coat,
consisting of approximately one part of cement to one part of
fine sand passing No. 30 mesh sieve with an amount of water
sufficient to obtain a consistency of thick cream, should be
brushed thoroughly into the surface,
- The patching mortar should be made from the same
materials and in the same proportions as the concrete, with the
exception of coarse aggregate. (In any case, use no more than
one part cement to 2.5 parts sand.) Mix the patching mortar
thoroughly with an amount of water sufficient only to obtain
the stiffest consistency that will permit placement. Apply the
mortar as soon as the bonding coat begins to lose the water
- Chemical cleaning may be necessary prior to blast
cleaning or acid etching, in order to remove surface contaminants such as oil, grease, and dirt. Solutions of 10 % caustic
soda or trisodium phosphate may be used as well as
proprietary detergents specially formulated for use on concrete.
They should be applied with vigorous scrubbing, followed by
flushing with water to remove all traces of both the detergent
and the contaminant, until the surface is neutral or only slightly
alkaline, as indicated by litmus or pH paper. Solvents must be
avoided because they dissolve the oil, grease, etc., and spread
the contamination over a larger area.
Mechanical preperation of Surface Defects
- Scarification by a mechanical impacting device is used
on concrete surfaces to remove thick overlays of dirt, previous
waterproofing membranes, or weak surface material. After
scarification, water or sand-blasting is required to remove
aggregates weakened by mechanical impacting. If the scarification process produces a surface too coarse or too uneven for
proper adhesion, it should be made smooth with patching
- Blast cleaning is an effective method for the removal
of laitance, dirt, efflorescence, and weak surface material.
There are three types of blast cleaning methods: dry
sandblasting, wet sandblasting, and high-pressure water jetting.
Dry and wet sandblasting are usually the most effective.
Abrasive cleaning techniques can also be used, provided the
abrasive used does not contaminate the surface being prepared.
Oils or greases should be removed prior to blast cleaning. If
compressed air is used, it must be clean and free of water or oil.
Care must be exercised to ensure that clean water is used for
wet-blast cleaning. The concrete surface should be abraded in
the blasting step to the extent that small aggregate particles are
exposed. The amount removed should be sufficient to provide
a strong, sound substrate suitable for the application of
- Acid Etching:
6.5.1 General—Acid etching procedures may be used in
lieu of mechanical cleaning procedures; however, mechanical
abrading methods are preferred to acid etching because of
possible chloride ion contamination (and resultant deterioration) of the concrete unless extreme care is taken and because
acid etching is not as dependable as mechanical abrasion.
Because of these factors and the potential hazards involved
with the use of acid, as well as cleanup and environmental
considerations, such cleaning should be used only where no
alternative means of cleaning are possible and should be
performed only by those experienced in its use.
- Etching Procedure—A 10/90 to 20/80 dilution of
commercial-grade hydrochloric acid in water is typically
prepared and applied to the concrete at a rate of 1.0 L/m2 [1
]. The surface to be etched is normally pre dampened to
obtain a more uniform acid etch. The acid solution is scrubbed
into the surface. Foaming will subside in 3 to 5 min, after
D5295/D5295M − 14
which the surface should be flushed thoroughly with fresh
water while scrubbing with stiff bristle brushes, in order to
remove the salts formed by the acid reaction and to dislodge
loose particles. A second treatment may be necessary to obtain
a reasonably clean surface. When the presence of chlorides is
inadvisable (due to effects on concrete or reinforcing steel), a
15 % phosphoric acid solution should be used. The application
and washing techniques are the same. The efficacy of the
flushing operation should be verified at a number of points
within the area treated by placing litmus or pH paper on the wet
surface to determine that the surface is reasonably free of acid
and chloride residues.