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Federal Highway Administration > Publications > Research > CORROSION PROTECTION - CONCRETE BRIDGES

Publication Number: FHWA-RD-98-088
Date: September 1998

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Chapter 6. Summary

Salt-induced reinforcing steel corrosion in concrete bridges has undoubtedly become a considerable economic burden to many state and local transportation agencies. Since the iron in the steel has a natural tendency to revert eventually to its most stable oxide state, this problem will, unfortunately, still be with us for the next few decades—although likely in a lesser extent because of some of the corrosion protection measures that came into practice in the last two decades in building new concrete bridges. There is no doubt that adoption of corrosion protection measures, such as use of sensible construction designs, adequate concrete cover depth, low-permeability concrete, corrosion-inhibiting admixtures, and coated reinforcing steel, in new construction will help in significantly prolonging the occurrence of reinforcing steel corrosion in the new bridges.

The use of good construction design and procedures, adequate concrete cover depth, corrosion-inhibiting admixture, and low-permeability concrete alone will not abate the problem, because concrete has a tendency to crack inordinately. In fact, it has been observed lately that the new low permeability concrete or high-performance concrete (made from partial substitution of Portland cement with silica fume or fly ash) has an even more pronounced tendency than conventional concrete to crack—potentially trading a normally slow intrusion of chloride ions into the concrete (by the diffusion process) for a potentially faster gravity-assisted flow of salt-laden water. Even corrosion-inhibiting admixture for concrete would likely not be of use when the concrete cracked. This situation essentially leaves the reinforcing steel itself as the last line of defense against corrosion. For this very reason, the use of a barrier system on the reinforcing steel, such as epoxy coating or other organic or even other possible metallic coatings, is even more critical in abating this costly corrosion problem.

It is likely that there may never be any organic coating that can hold up to the extreme combination of constant wetting and high temperature and high humidity that reinforcing steel is exposed to in the marine environment in Florida, especially in the splash zone, and that either steel bars coated with a sufficiently-stable metallic coating or a type of corrosion-resistant solid metal bars would have to be used in conjunction with the use of sound construction designs and concrete. However, as discussed in an earlier section, there were very convincing reports of good corrosion resistance performance shown by epoxy-coated steel bars in concrete bridge decks, where unlike in coastal bridges in Florida, the concrete does not remain constantly wet and the other exposure condition is not as severe. And, just recently, good performance by epoxy-coated bars has been observed in bridge decks surveyed in Pennsylvania and New York by researchers from a corrosion engineering firm in Virginia (101) and in cracked and uncracked concrete by researchers from the University of New Brunswick (102). It must also be mentioned that unfavorable performance by epoxy-coated bars has recently been claimed, albeit unconvincingly (103,104). In one latter case, the poor performing epoxy-coated rebars were located mostly in noise walls, where the quality of the concrete was known to be poor, and in concrete expansion dams (beside expansion joints), where it is suspected that the coated rebars may have been cleaned by abrasive blasting before pouring of the concrete.

The many successful performance of embedded epoxy-coated steel bars in places outside of Florida and possibly other similar locations, indicates that when used in exposure conditions that do not keep the concrete constantly wet, the epoxy coating will provide a certain degree of protection to the steel bars and, thereby, delay the initiation of corrosion. The recent claims of poor performance of epoxy-coated rebars serve, at most, to indicate that the corrosion protection provided by ECR (more accurately, the old generation of ECR) is not permanent and also to raise the question: For how long does the use of ECR, in a particular exposure condition, delays the initiation of steel corrosion in the concrete? And, for a prospective user, the next question is: Is the savings in maintenance and traffic control costs resulting from this extra time worth the initial extra cost of using ECR instead of black steel bars? Unfortunately, accurate determination of the actual field performance of ECR in a particular state or region or exposure condition is extremely difficult, if not impossible, since many contributing factors are involved and have to be accounted for (98). Needless to say, the recent improvement of specifications for ECR by the industry and the tightening of requirements on proper storage and handling of ECR at construction sites will ensure good corrosion protection.

The ongoing research study on steel bars coated with new organic and metallic coatings and alternative solid metal bars should result into identification of more corrosion-resistant and, hopefully, cost-effective alternative reinforcement for future use in concrete bridges.

For construction of new prestressed concrete bridge members, the use of a corrosion-inhibiting admixture in the concrete or the grout, in conjunction with use of good construction designs and practices, would provide some corrosion protection. However, the long-term effectiveness of the commercial inhibitor admixtures has not been verified yet.

For existing chloride-contaminated concrete bridge decks, impressed-current cathodic protection—using titanium mesh anodes—provides the ultimate and permanent solution to stopping reinforcing steel corrosion in the structures, as long as associated rectifiers and electrical wiring are properly maintained. Electrochemical chloride extraction provides an alternative rehabilitation method for stopping steel corrosion in contaminated concrete, albeit less permanently. This alternative has the advantage of having no rectifier or wiring to maintain after the treatment.

These same corrosion protection methods are applicable for existing inland concrete substructures. However, in cathodic protection of such concrete members, the appropriate anodes to use include the arc- or flame-sprayed zinc coating and the water-based conductive paints. Although electrochemical chloride extraction is applicable to concrete sub structures, it is more difficult to conduct this treatment efficiently on these bridge members than on bridge decks, because it is relatively difficult to set up the necessary treatment system on vertical concrete surfaces to keep them wet during the entire treatment.

For various prestressed concrete bridge members, either impressed-current or galvanic cathodic protection or both modes can be applied, depending on the types of the bridge members and their surrounding environments. However, before CP is applied to any of these concrete members, it should be qualified first following the proposed guidelines discussed earlier. And, to ensure that no prestressed concrete member is overprotected, which may lead to hydrogen embrittlement of the high-strength prestressed steel strands, the use of electronic remote monitoring systems should be incorporated with any installed CP system.

For prestressed concrete bridge members, such as beams, box beams, etc., impressed-current cathodic protection—using arc-sprayed zinc or titanium as the anode—is the most suitable mode. However, when applying CP, measures must be taken to ensure that the drainage system on a bridge is always properly maintained, so that there would not be any uneven electrical resistivity across the concrete members that would lead to uneven distribution of protection current and subsequent overprotection of the wet areas. Otherwise, use of CP on such members has to be excluded.

For prestressed concrete piles in marine environment, either mode of CP can be applied—with the impressed-current mode allowing for control of the current density being applied on the prestressed steel strands. The use of the zinc-hydrogel and the arc-sprayed Al-Zn-In coating as a galvanic anode in this application appeared to be promising. However, the long-term durability of these galvanic anodes is still being evaluated.

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