Civil Engineering Reference
In-Depth Information
made for on-site temperatures that differ significantly from those given on
the data sheet). An excessively long delay between coats can lead to poor
intercoat adhesion because the surface of the initial coat becomes less
receptive to bonding as cure progresses.
The period recommended for the attainment of full cure/drying, which
can be as long as 10-14 days, must be allowed prior to imposing aggressive
service conditions, for example: chemical contact, immersion in water or
heavy trafficking. Where a data sheet refers only to a single cure temperature,
such as 20°C, a significantly longer period is likely to be required under
colder, more variable site conditions.
17.5.3 Quality control
The many coating variations available differ in their tolerance of surface
and ambient conditions. Consequently, an appropriate process for coating
application must be specified and followed for any given set of circumstances.
Good workmanship, supervision and inspection are essential. A variety of
proprietary instruments provide applicators, supervisors and inspectors with
objective means of assessing the quality of coating work on concrete.
Inspection and supervision should ensure that surfaces are well prepared
and suitable for application of the coating/treatment. In-situ strength
measurement (e.g. by pull-off testing) may be used to ensure that the
substrate is mechanically sound and receptive to bonding. The principle of
the test is simply to apply a tensile load to a circular metal dolly, bonded to
the surface, until failure occurs. The test has been very widely used for many
years and is described in numerous national standards and elsewhere (EN
1504-2, EN1504-10 and BS EN 1542: 1999, 'Products and Systems for the
Protection and Repair of Concrete Structures, Test Methods: Measurement
of Bond Strength by Pull-Off ').
Various designs of pull-off adhesion tests are available from many
equipment suppliers. The dollies, usually of steel or aluminium, are
commonly bonded to the surface using a rapid-curing two-pack epoxy resin,
allowing a same/next day test. Because most concrete has coarse aggregate,
it is generally advisable to use dollies with a minimum diameter of 50 mm.
Determination of the moisture level in the concrete is often advisable;
in the case of floor slabs/screeds, this measurement is usually required by
specification (see, for example, the application guide for resin flooring
(FerFa, 2000)).
Particular attention should be given to the limitations of surface
measurements: low moisture levels may not reflect the content at depth.
While initial adhesion of a coating may be quite satisfactory, failure may
occur once the relatively impermeable covering allows a high moisture level
to be established at the bonded interface.
On floor slabs/screeds, this potential problem is avoided by stipulating in
various codes that moisture content is determined only by the 'hygrometer
