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Revolutionizing Bridge Rehabilitation
The Ontario Ministry of Transportation's Materials Engineering and Research Office is currently testing an innovative new method of corrosion prevention that has the potential to transform modern bridge rehabilitation. For decades, the MTO and other jurisdictions have used cathodic protection to arrest the corrosion of reinforcing steel within bridge decks. The implementation of new sacrificial anode systems could dramatically improve and simplify the rehabilitation of some structures suffering from corrosion.
MTO is responsible for 2,500 bridges located throughout the province, and is constantly working to ensure proper maintenance measures. The deterioration of concrete bridge decks, caused by corrosion induced by the use of de-icers, has always been a primary concern to bridge personnel. Cathodic protection (CP) has proved to be an effective tool in the fight against corrosion. The theory of CP is to apply sufficient electrical current to the surface of the reinforcement so that corrosion does not occur. In order for CP to function properly the reinforcement must be continuous (electrically connected); this is normally the case for structures with black steel reinforcement. For structures constructed with epoxy-coated reinforcement, the reinforcement may or may not be continuous depending on the condition of the coating. If not, the reinforcement is made continuous before CP is applied. There are two types of CP systems: impressed current, in which an external source of AC power is required to provide electrical current to the embedded reinforcing steel through an anode placed on the concrete surface, and sacrificial anode systems, in which electrical current is generated by the potential difference between the anode and reinforcing steel, and no external source of current is required. Since 1974, only impressed current systems have been installed in Ontario bridge decks.
Impressed current systems require an uninterrupted AC power supply and ongoing monitoring and maintenance if they are to provide effective corrosion protection. This requires dedicated personnel and has significant ongoing operating costs. The recent development and commercial availability of new sacrificial anode technologies prompted the MTO to install and evaluate three new sacrificial anode systems to determine their potential for future ministry use.
Three trial systems were installed in September 2003 at the North Otter Creek Bridge on Highway 9, in Southwestern Region. The anodes were installed by the anode suppliers' representatives to ensure proper handling. The three proprietary systems utilizing zinc anodes - Galva shield mesh and strip anodes and a Galvostrip anode - all operate on the same principle: two dissimilar metals are electrically connected and current flows from the more "active" metal to the more "passive" due to the difference in their electrical potential. In these systems, electrons pass from the more active zinc metal of the anodes to the less active steel of the reinforcing bars. This arrangement polarizes the steel and protects it from corrosion.
A previously conducted survey to assess the condition of the North Otter Bridge indicated that 88% of the concrete deck had readings suggesting a high probability of corrosion; this made it an ideal demonstration site for the new sacrificial anode systems. Multiple element probes (MEPs) were embedded in the concrete deck to evaluate the performance of the sacrificial anodes. The early readings, although preliminary, are encouraging. More than sufficient current was being supplied by each of the anodes to meet the criteria generally accepted as indicative of effective corrosion protection. Monitoring will continue over a 4-year period, at the end of which the Concrete Section will prepare a report including evaluation and recommendation of the sacrificial anodes for future ministry use.
Based on their potential to reduce the costs of long-term monitoring and system maintenance and to enhance the durability of Ontario's bridges, sacrificial anode systems may eventually replace impressed current systems as the standard for cathodic protection throughout the province.