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Prestressed Concrete Bridges

Whereas some of the methods discussed for conventional reinforced-concrete bridges are applicable to prestressed concrete components (e.g., high-performance concrete and corrosion inhibiting admixtures), special consideration for corrosion prevention of prestressed reinforced-concrete bridges is required.

Most of these bridges are relatively new and their numbers are relatively low; therefore, the overall economic impact is not as significant as for conventional reinforced-concrete bridges. However, failure of the high-strength prestressing steel can compromise the integrity of the prestressed concrete bridge (corrosion-related deterioration compromising the structural integrity of a conventional concrete structure is highly unlikely). This makes close attention to construction details and subsequent monitoring and inspection of the prestressed concrete bridges critical.

Corrosion prevention of pretensioned structures is primarily accomplished through the use of high-performance concretes or the addition of corrosion-inhibiting admixtures. Remedial measures such as cathodic protection are possible as long as care is taken to prevent overprotection that can lead to hydrogen-induced cracking of the high-strength steel. Other measures such as electrochemical chloride removal cannot be used for prestressed concrete structures because of the relatively large amounts of hydrogen produced at the steel surface during the removal process.

Recent failures of post-tensioned structures have underscored the importance of maintaining void-free grouting of the tendons, especially near the anchorage. Maintaining the integrity of the post-tensioned tendon starts with ensuring the integrity of the duct (typically polyethylene), followed by the application of a good-quality grout that is continuous around the strands. Placement of the grout is often more difficult when low water-cement ratio mixes and/or mineral admixtures are employed. Improved grouting practices are continuing to be developed. In addition, the use of corrosion-inhibiting admixtures can provide added protection against corrosion of the prestressing steel strands. Note that in August 2001, the American Segmental Bridge Institute conducted a 3-day training school for certifying grouting specialists. This training school will be held in the future once or twice a year. (reference)