Civil Engineering Reference
In-Depth Information
(i) Alkali-silica reaction (ASR)
The hypothesis has often been advanced that above-ground structures
subject to ASR can be protected by surface waterproofing, thus maintaining
the concrete in a sufficiently dry state to inhibit or arrest deterioration.
The results of various laboratory studies seemingly support this hypothesis.
However, there appears to be little, if any, documented evidence for the
effectiveness of this approach on real structures. Field trials, and more
general considerations, have also led to the conclusion that there are serious
practical difficulties in attaining the ideal conditions within the concrete for
the inhibition of ASR.
These difficulties may be influenced by a variable micro-climate in the
vicinity of the affected concrete and by moisture ingress from other parts
of the structure. Furthermore, many coatings and related treatments that
provide excellent barriers against liquid water ingress have a relatively low
resistance to moisture vapour transmission. Consequently, for this form of
deterioration in particular, the simple extrapolation of laboratory results to
in-service conditions is likely to be very speculative.
(ii) Waterproofing and carbonation
Concerns have arisen over the use of pore-lining penetrants, such as silanes,
that eliminate periods of high internal moisture content while offering little, if
any, protection against carbonation. However, the view that carbonation may
be encouraged by the maintenance of an optimum internal moisture state (50-
70% RH) does not appear to be supported by experience from real structures.
(iii) Control of reinforcement corrosion by the limitation of oxygen
ingress
Oxygen is required both to sustain the electrochemical corrosion reaction at
the steel reinforcement and also for the formation of the expansive rust that
causes cracking and spalling of the concrete cover zone.
The potential use of surface coatings to control reinforcement corrosion
by limiting oxygen ingress is noted in
Table 17.2
(Principle 9 of BS EN 1504-
9: 2008). However, in general, this approach is most unlikely to be effective
because, as noted earlier, only very small levels of oxygen are required to
fuel the corrosion process, while even the most impermeable coatings are
not completely resistant to oxygen transmission. Furthermore, without total
encapsulation, oxygen is likely to reach the reinforcement via other pathways.
17.4 Materials for surface coating/treatment
Many proprietary products are available for the surface treatment of
concrete. They can be classified conveniently according to the main generic
component, as shown in
Figure 17.1.
