Page 6 - OpenAccess-April-2021

TECHNICAL PAPER

Figure 2: Graphite-based organic coating applied to the surface of a concrete cross beam (left) and decorative overcoating (right)

Progress in achieving these requirements has been rapid since of 5 to 15 years when properly applied and operated at current
the earliest reported successful application of CP to a reinforced densities below 20 mA/m in an appropriate environment . They
[9]
2
concrete bridge deck in the USA in 1974 . In this and other do not withstand conditions of continuous wetting or abrasion
[8]
similar cases, the anode system consisted of high silicon cast and undergo gradual degradation at a rate dependent on the
iron primary anodes in a thick conductive asphalt overlay. A applied current density. Reported failure mechanisms involve
non-conductive asphalt wearing course was applied over the disbondment, blistering and flaking related to degradation
top to prevent wear. The added weight, as well as difficulties of of the cementitious matrix by anodic acidity and oxidation
applying the conductive medium in vertical surfaces, effectively of the organic binder [11] . The carbon present in the coating is
prevented the system from being used for structures other consumed by the anodic reaction so that enough carbon for the
than bridge decks and car parks. In the early 1980’s, many desired lifetime should be incorporated into the coating. They
alternative anode systems were developed which enabled CP are relatively easy to reapply and are commonly overcoated
to be applied to reinforced concrete in a much wider range of with grey or white paint to improve appearance, to provide UV
situations, including highway bridge substructures, buildings protection and to limit heat gain to the structure (Figure 2).
[9]
and marine structures . A number of these systems have now
been used quite extensively for decades and are considered Thermally sprayed zinc may be applied to reinforced concrete
sufficiently well-proven to be described in some detail in by arc or flame spraying (Figure 3) and shows greater tolerance
ISO EN 12696: 2016 [10] . The Standard recognizes that further new than that of organic conductive coatings to conditions of
and effective anode systems are likely to become available for moisture at the time of application or during service. Poor
CP of atmospherically exposed reinforced concrete. adhesion of the zinc coating to the concrete substrate is often
experienced, , however, if the coating thickness exceeds about
Amongst the anode systems described by the Standard are the 0.4 mm [10] . Calculated rates of zinc anode consumption suggest
following: that sustained operation at 20 mA/m would oxidise completely
2
i. organic conductive coatings, normally applied to a dry film
thickness in the range 0.25 to 0.5 mm,
ii. metallic conductive coatings of thermally sprayed zinc,
normally applied to a thickness in the range 0.2 to 0.4 mm,
iii. activated titanium, in various forms such as mesh mounted
on the surface of the structure and embedded in a
cementitious overlay, thin ribbon buried in continuous slots
cut in the cover concrete and lengths of fine mesh rolled in
the form of a tube which can be buried in drilled holes in
the concrete,
iv. ceramic conductive titanium oxide tubes inserted in drilled
holes in the concrete.

Organic conductive coatings containing graphite have shown
Figure 3: Thermally sprayed zinc anode applied to the surface of a steel
variable performance but have a range of effective service lives reinforced concrete element

THE INDIAN CONCRETE JOURNAL | APRIL 2021 63