Civil Engineering Reference
In-Depth Information
and not harmful. If possible, identify the characteristics of
the discoloration because some treatments are more effec-
tive than others in removing certain materials.
Water cleaning
methods include low-pressure washes,
moderate-to-high-pressure waterblasting, and steam.
Low-pressure washing is the simplest, requiring only that
water run gently down the concrete surface for a day or
two. The softened dirt then is flushed off with a slightly
higher pressure rinse. Stubborn areas can be scrubbed with
a nonmetallic-bristle brush and rinsed again. High-pres-
sure waterblasting is used effectively by experienced oper-
ators. Steam cleaning must be performed by skilled
operators using special equipment. Water methods are the
least harmful to concrete, but they are not without poten-
tial problems. Serious damage may occur if the concrete
surface is subjected to freezing temperatures while it is still
wet; and water can bring soluble salts to the surface,
forming a chalky, white deposit called efflorescence.
Chemical cleaning
is usually done with water-based
mixtures formulated for specific materials such as brick,
stone, and concrete. An organic compound called a surfac-
tant (surface-active agent), which acts as a detergent to wet
the surface more readily, is included in most chemical
cleaners. A small amount of acid or alkali is included to
separate the dirt from the surface. For example,
hydrochloric (muriatic) acid is commonly used to clean
masonry walls and remove efflorescence. There can be
problems related to the use of chemical cleaners. Their acid
or alkaline properties can lead to reaction between cleaner
and concrete as well as mortar, painted surfaces, glass,
metals, and other building materials. Since chemical
cleaners are used in the form of water-diluted solutions,
they too can liberate soluble salts from within the concrete
to form efflorescence. Some chemicals can also expose the
aggregate in concrete. Chemicals commonly used to clean
concrete surfaces and remove discoloration include weak
solutions (1% to 10% concentration) of hydrochloric, acetic,
or phosphoric acid. Diammonium citrate (20% to 30%
water solution) is especially useful in removing discol-
oration stains and efflorescence on formed and flatwork
surfaces. Chemical cleaners should be used by skilled
operators taking suitable safety precautions. See
Greening
(1966)
and
PCA (1988)
for more information.
Mechanical cleaning
includes sandblasting, shot-
blasting, scarification, power chipping, and grinding.
These methods wear the dirt off the surface rather than
separate it from the surface. They, in fact, wear away both
the dirt and some of the concrete surface; it is inevitable
that there will be some loss of decorative detail, increased
surface roughness, and rounding of sharp corners.
Abrasive methods may also reveal defects (voids) hidden
just beneath the formed surface.
Chemical and mechanical cleaning can each have an
abrading effect on the concrete surface that may change
the appearance of a surface compared to that of an adja-
cent uncleaned surface.
Fig. 11-34. Good curing is essential to successful patching.
This patch is covered with polyethylene sheeting plus rigid
insulation to retain moisture and heat for rapid hydration
and strength gain. (40434)
shallow vertical or overhead repairs may best be accom-
plished by shotcreting. Several proprietary low-shrinkage
cementitious repair products are also available.
Curing Patches
Following patching, good curing is essential (Fig. 11-34).
Curing should be started immediately to avoid early
drying. Wet burlap, wet sand, plastic sheets, curing paper,
tarpaulins, or a combination of these can be used. In loca-
tions where it is difficult to hold these materials in place,
an application of two coats of membrane-curing com-
pound is often the most convenient method.
Cleaning Concrete Surfaces
Concrete surfaces are not always uniform in color when
forms are removed; they may have a somewhat blotchy
appearance and there may be a slight film of form-release
agent in certain areas. There may be mortar stains from
leaky forms or there may be rust stains. Flatwork can also
become discolored during construction. Where appear-
ance is important, all surfaces should be cleaned after con-
struction has progressed to the stage where there will be
no discoloration from subsequent construction activities.
There are three techniques for cleaning concrete sur-
faces: water, chemical, and mechanical (abrasion). Water
dissolves dirt and rinses it from the surface. Chemical
cleaners, usually mixed with water, react with dirt to sep-
arate it from the surface, and then the dirt and chemicals
are rinsed off with clean water. Mechanical methods—
sandblasting is most common—remove dirt by abrasion.
Before selecting a cleaning method, it should be tested
on an inconspicuous area to be certain it will be helpful
