Evaluation of Concrete Bridges by Impact-Echo

by Al Ghorbanpoor, Univ of Wisconsin, Milwaukee, United States,
Y. P. Virmani, Univ of Wisconsin, Milwaukee, United States,
G. R. Fatemi, Univ of Wisconsin, Milwaukee, United States,

Abstract: Strength of post-tensioned bridge structural members primarily depends on the tensile capacity of their post-tensioning steel reinforcement. To prevent corrosion, the post-tensioning steel is normally encased in plastic or galvanized metal ducts filled with a grout mix. However, corrosion of the steel can result if the ducts are not filled completely. In addition, use of an improper grout mix may result in partial filling of the ducts and/or grout bleeding, which is the separation of water from the cement paste. When bleeding of the grout takes places, the bleed water is usually absorbed back into the hardened grout and a bleed void, sometimes as long as the entire length of the duct, is left in the duct. The bleed voids could be a major factor in the corrosion of post-tensioning steel by facilitating penetration and movement of salt water and other harmful solutions inside of the ducts from the end anchorage areas or cracks, or other flaws, in the surrounding concrete. This study has examined the application of and further development of a non-destructive evaluation technique, the Impact-Echo, for determination of location and extent of voids in the ducts of post-tensioned bridge structural members. Theoretical, laboratory, and field studies have been performed and it has been demonstrated that it is possible to detect, with high reliability, the location and extent of voids in the post-tensioning ducts of concrete structures. It was found that information corresponding to the location of voids within concrete members may readily be obtained by examining the wave reflection frequency values, from an induced impact at the surface of the member, due to the incidents at the free boundaries of these voids.

Subject Headings: Duct | Concrete bridges | Tension | Voids | Concrete | Bridge tests | Grouting | Corrosion

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