ABSTRACT

Concrete structures located in aggressive environments where high temperatures and high chloride levels exist in Both natural soils and water are subject to premature deterioration from corrosion of the reinforcement. Traditional methods of corrosion protection, such as concrete admixtures and passive barrier systems, may not be sufficient to provide the level of corrosion control needed for the intended design life. As a solution to this problem the use of cathodic protection (or cathodic prevention as it is called) at the time of construction is proposed. Although cathodic protection has been used as a rehabilitation method on existing salt-contaminated concrete structures for over 20 years, its application to new reinforced concrete structures is relatively new. It has been shown, however, that the cathodic current density necessary to maintain a passive layer on the reinforcing steel before the concrete is contaminated with chlorides is relatively low and that the chloride ion tends to migrate towards the anode. The application of cathodic prevention during new construction can have lower initial cost than conventional methods and significant long-term benefits. Anode materials such as catalyzed titanium ribbon, which are cast monolithically in the new concrete, can provide effective corrosion control for the life of the structure (i.e. >75 years).

INTRODUCTION

It is well known that good quality concrete provides a very beneficial environment for reinforcing steel. At the high alkalinity of concrete, embedded steel forms a thin protective oxide layer at its surface. This oxide film causes the surface of the steel to passivate, effectively preventing further corrosion, It is also recognized that the introduction of chloride ions destroys this passive layer, thereby initiating corrosion. Since corrosion products can occupy several times the volume of the original steel, tremendous tensile stresses are exerted on the surrounding concrete as the steel corrodes, When these stresses exceed the tensile strength of the concrete. cracking and delamination develop, resulting in deterioration of the concrete. This then leads to the intrusion of further chloride ions, thereby accelerating the overall corrosion process.

Reinforced concrete structures located in aggressive environments are especially prone to chloride induced corrosion. In general, aggressive environments can be characterized as those that may have the following conditions:

l High levels of chlorides and sulfates in native soils

l High levels of chlorides in the natural waters

l High ambient temperatures, i.e. >40° C

l High relative humidity, i.e. > 90?%0

l Atmospheres containing large amounts of airborne salt and sulfur compounds

When combined with other conditions that effect the quality of concrete, such as inadequate cover and high permeability, premature deterioration will certainly result, Such is the case on many concrete structures located in the Arabian Gulf region, Latin America and the Pacific Rim.

The Arabian Gulf seaboard is considered to be one of the most corrosive environments for reinforced concrete in the world. A survey by Matta of more than 100 reinforced concrete structures in the Arabian Gulf, found that the typical service life to be 10-20 years, considerably less than the 50+ year design life.1Castro et al. assert that Mexico?s infrastructure is corroding largely unchecked. In the Yucatan Peninsula, where aggregates are rough and angular and the water/cement ratios are consequently high, there is much corrosion due to the high permeability of concrete 2 In Hong Kong, the Kamnon Tunnels, which have been designed for a 120 year design life, leaked so

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