Civil Engineering Reference
In-Depth Information
Further advice on cleaning methods is given in several standards and
other publications (CRA, 1992; Dept of Transport BD43/90, 1990, Dept
of Transport BA33/90, 1990 and Dept of Transport BD27/86, 1986).
Additionally, a range of plaques has been produced that replicate the typical
profiles given by various methods of preparation. These specimens are
also linked to the use of coatings/treatments of different thickness, see the
ACI Concrete Repair Manual (ACI, 1999). However, it is recommended
here that this correlation should be used for general guidance, rather than
prescriptively, as supplier's recommendations may differ and should take
precedence.
Relatively simple washing techniques may not be effective where
contaminants such as oil and grease have been absorbed by the concrete.
They will also be ineffective if the contamination is merely spread further.
Following wet cleaning, thorough washing with fresh water should be carried
out, as some detergent and biocide residues can have an adverse effect on
adhesion. Similar problems can arise if chemical strippers, used to remove
old coatings, are allowed to dry - washing off all stripping residues can then
become difficult.
Readily available proprietary products that contain biologically active
micro-organisms can rapidly degrade hydrocarbon residues and greatly assist
removal.
Acid etching is now used infrequently, due mainly to the associated health
and safety risks. Additionally, the absorption, via porous areas/cracks, or
surface retention of certain ionic species, e.g. chlorides, can be detrimental.
Wet grit-blasting and vacuum dry-blasting are commonly favoured
because they are effective techniques that allow good control of the cleaning
process, while generally having no significantly detrimental effects on the
surface. Health, safety and environmental risks are also reduced, compared
with 'open dry-blasting', although the wet process may create containment
and disposal problems. Many typical concrete surfaces require no more than
very light blasting ('sweep-blasting') prior to coating.
High-pressure water jetting is an effective method for dealing with many
contaminated or unsound surfaces. Equipment is available that allows good
control of the process, giving an acceptable profile, and recycling of the
jetting water.
Power driven mechanical methods, such as scarifying, needle gunning
and scabbling, are widely used and can be very effective for the removal of
defective surface layers and firmly adhering or deeply absorbed contaminants.
However, they can also be too aggressive, producing micro-cracking of the
aggregate, which may be detrimental to bond strength, and an overly deep
texture. Excessive power wire-brushing can lead to undesirable polishing of
the surface.
Milling, which is also a very aggressive method, can remove an appreciable
depth of concrete, but the large, heavy equipment is only suited to horizontal
surfaces.
