CATHODIC Protection of Steel in Concrete by P. M. Chess
Book Details :
LanguageEnglish
Pages204
FormatPDF
Size4.88 MB

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CATHODIC Protection of Steel in Concrete by P. M. Chess



INTRODUCTION:

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.


Even after the publicity surrounding the large number of structures exhibiting acute signs of distress 25 or so years into a designed 120-year life-span, there is still a body of engineers who believe that all that is required to achieve any specified design life in a hostile environment is to provide a higher concrete grade with the same design and maintenance of the structure. This contention does not correspond with the facts and means that publications such as this book will not only be dealing with civil engineering miscalculations of the past but also those perpetrated in the future.

The traditional use of cathodic protection has been to prevent corrosion of steel objects in the ground or water and this is still its most common application. It is now almost universally adopted on ships, oil rigs and oil and gas pipelines. Over the last 50 years cathodic protection has advanced from being a black art to something approaching a science for these applications.

Over the past 30 or so years there has been a steady increase in the use of cathodic protection for the rehabilitation of reinforced concrete structures which are exhibiting signs of distress. The most common damage mechanism is chloride-induced corrosion of the steel reinforcement and this is normally what cathodic protection systems are intended to stop.


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