A reliable method to determine the remaining prestressing force is often needed in routine inspection of bridges, during rehabilitation of structures, or when retrofitting for increased vehicle capacity. This article reports on a study that was undertaken to determine the remaining tendon force of a large-scale (17 meters long), 38-year-old prestressed concrete bridge girder. The authors used a semi-destructive test method and compare measurements of the prestressing force losses determined by their method with calculations methods based on fib guidelines and Swiss code. They found that calculated and measured prestressing force losses were generally in good agreement. A long-term prestressing force loss of about 20% was determined. The authors conclude that their material tests on the prestressing steel showed that 38 years of constant stress had no discernable effect on its tensile strength. This article reports on the first part of an extensive test program that will be carried out on five similar test beams.
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Czaderski et al. (2006) studied this question.