Standard: NACE 01105
SACRIFICIAL CATHODIC PROTECTION OF REINFORCED CONCRETE ELEMENTS— A STATE-OF-THE-ART REPORT
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Over the past several decades the corrosion of steel reinforcement embedded in concrete structures has received considerable worldwide attention. In theory, concrete and reinforcing steel are very compatible. They have similar coefficients of thermal expansion. Concrete, because of its highly alkaline nature, creates a protective environment for the steel. Studies have shown that corrosion activity and damage result when critical quantities of aggressive ions penetrate through the concrete pore structure by diffusion and other transport phenomena and reach the embedded steel reinforcement. At this time, the naturally occurring passive film developed by highly alkaline concrete becomes saturated with these ions, eventually breaking down this protective layer. In regions of low resistance, aggressive ions, mostly in the form of salts, attack the passive film and develop localized anodic sites (pits) on the surface of the steel. Immediately adjacent to these anodic sites are oxygen-rich regions that cathodically "fuel" the corrosion reaction. As active corrosion proceeds, the lower pH in and around the anodic sites reduces the passive layer in greater proportions, making it more prone to iron oxide (Fe2O3) development. Because Fe2O3 (rust) is much more voluminous than solid steel, it imparts considerable tensile forces within the concrete matrix and eventually leads to cracking of the concrete cover.
There are several approaches that have been used to rehabilitate concrete structures suffering from the effects of corrosion damage. The most widely used approach typically involves removing the damaged concrete in and around the affected area and replacing it to the original dimension. The principal intent of this remove-and-replace approach is to return the form and function of the structure. Although this strategy is widely used, it rarely incorporates the complete removal of contaminated areas that surround the damaged region, and is sometimes regarded as only a short-term solution. Modifications to this technique include expanding the area excavated to include sound but chloride-contaminated or carbonated concrete, or to include areas where the steel-reinforcement potential is more negative than a defined threshold.
The remove-and-replace approach is normally broken into two general categories. The first is patch repair, and the second is rehabilitation. Patch repair is a short-term solution that makes no attempt to extend the structure life, but merely restores concrete back to dimension. The rehabilitation technique attempts to return the distressed area to uniformity with the pre-existing conditions and normalizes any conditions of further distress. The rehabilitation technique carries with it some expectation of an increase in service life. In some applications, a corrosion inhibitor is included either as an additive to the repair/replacement concrete mix or as a post treatment. These methods of concrete repair are much more involved than discussed in this report and are well integrated into most structure owner agencies and the civil engineering community.
Alternatively, CP applies electrochemistry to halt the corrosion process or reduce it to levels below engineering significance. Cathodic protection is an electrochemical technique used to reduce the corrosion of metallic materials. This is accomplished through the addition of a cathodic current to the metal-electrolyte system so as to increase the rate of the cathodic reaction (the formation of hydrogen [H2] or hydroxide [OH-]) on the metal being protected, and at the same time decrease the rate of the anodic reaction (metal dissolution). The source of this cathodic current is immaterial to the protection process, and can come from direct current (DC), alternating current (AC), or galvanic sources
|Document Number:||nace 01105|
|Most Recent Revision:||YES|