Cathodic protection of prestressed concrete

Extreme caution must be used when applying cathodic protection to prestressing steel or to elements that include prestressing steel. This is for 

1 Because of the risk that if the potential exceeds the hydrogen evolution potential hydrogen embrittlement could occur with potentially 

2 Because if the prestressing steel has corrosion pits in it, those pits are potential stress concentrators and there is a risk of either under protection in the bottom of the pit which could lead to continued corrosion and then to failure of the stressed strands, or of overprotection 

In the United States trials of impressed current cathodic protection have been conducted on pre-tensioned structures (Bennett and Schue, 1998). Many pre-tensioned bridge piles have been cathodically protected in Florida using galvanic anodes. This is unlikely to cause overprotection and hydrogen evolution. 

Thus it can bee seen that each prestressed structure must be evaluated to determine the feasibility of cathodic protection current reaching the steel that needs protection, whether galvanic cathodic protection can be applied and if not using qualification criteria such as those in NACE 01102 (2002) to determine if the structure is suitable for impressed current cathodic protection. 

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The NACE standard states explicitly in its title that it refers to atmospherically exposed reinforced concrete. NACE also has a standard on cathodic protection of prestressed concrete cylinder pipe RPOlOO-2000, also reaffirmed in 2005 and undergoing revision. 

NACE RPOl 00-2000 Cathodic Protection of Prestressed Concrete Cylinder Pipelines. NACE Standard Recommended Practice. 2000 March. 

The NACE standard was the first to be produced (1990) and has been revised since the original. Its scope states that it excludes prestressed concrete. There is a separate NACE report on cathodic protection of prestress NACE 01102 (2000). This gives valuable information on applying CP to prestressing but there has been no pressure to convert this to a standard. A list of NACE recommended practice documents, test methods and reports for all electrochemical techniques is given at the end of this chapter. 

The technology of cathodic protection of reinforced concrete is to elaborate its own protection criteria, other than for steel structures corroding in different environments than concrete. They are still mainly potential criteria, used carefully so as not to lead to overprotection and as a consequence liberation of hydrogen and weakening of the adhesion of concrete to steel. In prestressed structures, the presence of hydrogen can lead to embrittlement. John and Messham (1989) exhaustively describe the following protection criteria of reinforced concrete ... 

Most standards state that cathodic protection should not be applied to prestressed concrete structures. This is because of the risk that if the potential exceeds the hydrogen evolution potential, hydrogen embrittlement could occur with potentially catastrophic failure of the steel These problem, were discussed in Sections 6.2.1 and 6,5. High strength steel may trap atomic hydrogen which weakens grain boundaries and the crystalline structure. There are several issues that can be considered when considering cathodic protection of prestre,ssed concrete structures ... 

Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is positive experience in the case of a direct connection without coated pipes this is protection of buried prestressed concrete pipelines by zinc anodes [38], Stability against H-induced stress corrosion in high-strength steels with impressed current has to be tested (see Section 2.3.4). 

Trials. The effectiveness of chloride extraction depends on characteristics of individual structures, such as the concrete composition, the actual chloride-penetration profile and the depth of cover. So, it may be useful to carry out a trial on an area (about 1 to 10 m ), which must be representative of the structure to be treated and should last at least 4 to 8 weeks. The results of such a trial in terms of the chloride profile before, during and after chloride extraction gives an indication of the duration required and can be used to show that chloride-extraction treatment of the particular structure will be effective under field conditions. Trials are most certainly recommended if prestressed structures are to be treated with chloride extraction. Careful monitoring of the potential of the prestressing steel should be carried out to establish the risk of hydrogen embrittlement. As a safe criterion, the potential should not become more negative than -900 mV SCE, as apphes for cathodic protection [13]. 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

There are now many national, international and proprietary specifications for impressed current cathodic protection for steel in concrete. It can only be applied with great care if there is prestressing in the structure. Careful investigation is recommended in the presence of ASR. There are additional installation costs if there are large numbers of electrically unconnected reinforcing bars. 

It is therefore advisable to set an limit of -1100 mV vs. CSE for the instant off potential for all systems. This is more as a convenient upper limit for reinforced concrete structure.s,but as a rigid limit in the case of prestressing steel if exposed to the cathodic protection current. 

The interactions between reinforced concrete, prestressed concrete and the marine environment are very complex. From the corrosion standpoint, the primary consideration is protection of the embedded steel from attack by the aggressive marine environment. This protection may rely solely on the ability of the ordinary concrete cover to protect the embedded steel or may be augmented by the addition of corrosion inhibitors to the concrete, by coating of the steel, by cathodic protection, or a combination of one or more of these methods. 

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