Civil Engineering Reference
In-Depth Information
Flame blasting is used infrequently. It fractures and removes the concrete
surface by superheating the pore water, thus generating expansive forces.
It is essential that the surface is thoroughly washed with clean water and/
or vacuumed following preparation, thus ensuring that all loose material
is fully removed. A simple 'water droplet' test may also be used to provide
an indication of surface cleanliness - contamination will tend to prevent
spreading and absorption of the water.
Void filling and levelling
Durability (for other than permanently dry conditions) and aesthetics require
that coatings form a continuous film, free of pinholes and with an adequate
thickness over the entire surface. Consequently, any significant unevenness
must be dealt with following initial preparation.
With some thicker products, this may be achieved by direct application
to a prepared surface. In other cases, however, small voids and 'blow-holes'
(which usually become more exposed and enlarged during initial preparation)
must be filled and the surface must be levelled prior to coating. Filling and
re-profiling is carried out by 'bagging-in', by use of a 'scrape/skim coat' or by
application of a (typically) 1-2 mm thick 'fairing coat'.
Cementitious slurries and proprietary products, generally polymer-
modified, are most commonly used, although pastes or fine mortars based
on a reactive resin, e.g. an epoxy, can also be useful. Compatibility with the
coating system and any other specific requirements should be checked with
the supplier.
17.5.2 Application
It must always be ensured that the coating/treatment is suitable for use
on an alkaline concrete surface - some products are readily degraded and
debonded by alkalies, particularly in moist conditions.
Excessively absorbent or weak and friable/dusty surfaces may be brought
to a satisfactory condition with a sealer/stabiliser. The over-application
of some products must be avoided and, in general, the sealer and other
components of the system should be obtained from the same supplier to
ensure compatibility.
The required age of new concrete prior to coating varies, but a minimum
of 21-28 days under reasonable ambient conditions is not uncommon.
Normally, this allows for both early shrinkage effects and drying. Particular
attention must be given to new floor slabs and screeds, as very long periods
can be required to attain a moisture content that will not cause premature
failure of a coating (FerFa, 2000). Specific products - surface damp-proof
membranes - are available for circumventing this problem.
More generally, for mature concrete elements, the tolerable moisture
level varies according to the particular coating system - its ability to displace
