cathodic protection of steel in concrete pdf

Steel and concrete have become the most common materials for manmade structures
over the last hundred or so years with the use of the composite material, concrete
reinforced with steel, becoming one of the most popular methods for civil construction. The historical reasons for steel-reinforced concrete’s popularity are not hard to find: its cheapness, high structural strength, mouldability, fire resistance and its supposed imperviousness to the external environment, while requiring little or no maintenance, provide a virtually unbeatable combination.

In order to harness these properties, both national and international standards have been developed. The standards for both concrete and steel were initially defined principally by compositional limits and strength, and this has continued to be the primary means of quality control to date.

Until the 1950s it was assumed that when steel was encased in the alkaline
concrete matrix, neither would suffer from any degradation for the indefinite future.
However, evidence of degradation was noted as early as 1907 (Knudsen, 1907) where
it was observed that chlorides added to concrete could allow sufficient corrosion of
the steel to crack the concrete. The implicit assumption to this day by many civil
engineers of reinforced concrete’s virtually infinite durability has proven to be true in
several cases, with structures reaching their design lives without any evidence of
structural degradation.

However, it is now evident that in areas where there is an aggressive atmosphere, the concrete can be damaged or the steel can corrode in a dramatically shorter time period than that specified as a design life. For UK highways the current design life was originally set at 120 years and despite all the evidence of highway structures showing significant problems after a short time period it is still set at this extremely hopeful figure even though no corrosion design life analysis is
required.

This head-in-the-sand approach can be contrasted with the reality illustrated by research (Bamforth, 1994) showing that the estimated time to corrosion activation of steel reinforcement in modern concrete with the designated cover can be as low as five and a half years at the 0.4% chloride level with modern concrete. These research findings are in good accordance with site investigations. A substantial number of structures have been found to have their steel reinforcement sufficiently corroded within 20 years of construction to be structurally unsound.