Civil Engineering Reference
In-Depth Information
/ sealed box' method - essentially determination of the equilibrium relative
humidity in an enclosed air space formed above the concrete (see BS 8204-
6, 'Screeds, Bases and In Situ Floorings, Part 6: Synthetic Resin Floorings
- Code of Practice' and BS 8203, 'Code of Practice for Installation of
Resilient Floor Coverings'). Equipment developed for this test is available
commercially.
Application of the correct coating thickness is normally essential for
satisfactory appearance and durability. It can be monitored approximately by
recording the consumption rate over known areas - with due allowance for
the effects of surface roughness and wastage. Wet film gauges can be useful
for carrying out rapid spot-checks during application, but their accuracy is
limited unless the surface is reasonably smooth.
Dry film thickness is more difficult to check on concrete than on metallic
substrates, although direct reading instruments based on ultrasonic methods
are now available. Monitoring may be carried out via metallic coupons
attached to the surface prior to coating, thus allowing use of the more
common electronic thickness gauges.
Depending on the substrate/coating texture, dry film thickness can also
be measured by cutting or boring through the coating using a tool whose
tip has a defined angular geometry (for example, the very well-known
paint inspection gauge). The total film thickness or that of individual
layers is then read off from a microscope graticule scale. Instruments using
a borer are generally better suited than the linear cutters when harder,
more brittle coatings are examined.
Figure 17.3
shows a typical dry film
thickness gauge.
In some circumstances, it may be necessary to take cored specimens so that
the coating thickness along sawn edges can be determined with a laboratory
microscope. Core specimens can also be used to determine the penetration
depth of hydrophobic impregnants and pore-blocking sealers.
Most coating systems require the application of at least two coats and, in
some cases, different colours can assist in achieving the correct build-up of
successive layers.
Once the coating has fully dried/cured, adhesion can be checked by pull-
off tests using one of the many instruments available commercially, or by a
cross hatch cutter (
Figure 17.4).
It is often recommended or specified that
the test area is isolated by coring through the coating and into the concrete.
This provides a well-defined test area and eliminates restraint from the
coating adjacent to the dolly.
Procedural details, the extent of testing and performance criteria should
be agreed beforehand. It is generally advisable to agree to test locations, the
level of replication, and performance criteria at the outset. The simplest
requirement is that failure should occur only (or predominantly) in a
cohesive mode, in either the concrete or the coating, and not adhesively at
the substrate or between coats. A required failure stress is usually expressed
as an average together with a minimum for any one result.
