Fatigue strength of prestressing steel tendons embedded in concrete of an aged highway bridge

Current requirements for assessing fatigue behavior in existing prestressed concrete bridges include determining the bridges built-in steel tendons fatigue strength. Standardized S-N curves are normally used for fatigue design in new bridges, which display the tendons' fatigue strength in relation to the number of load cycles under a specific stress range. However, because older design requirements for road bridges did not demand fatigue design, specific curves for older steel types are not available. Yet the structural health of older road bridges must be assessed to determine their safety and viability of further usage. Therefore, the research presented here was undertaken to determine whether standardized S-N curves could be applied to the older prestressing steel found in older road bridges. For this testing, it was determined that some prestressing steel patterns might be reclaimed from an older bridge along the German Autobahn A1 south of the city of Dortmund (Bridge # 67, called Station Westhofen, built in 1957) during the bridge's demolition. Because the steel tendons of this old bridge were not entirely covered by concrete due to bonding failure, some strands were easily obtained. These tendons did not display any signs of damage or indications of pre-damage caused by fatigue. Interestingly, the strands' surfaces showed only traces of rust, which were removed carefully before the tests were begun. Overall, five concrete beams with curved ducts for the steel tendons, which were prestressed with the reclaimed strands, were subjected to fatigue loads. Varied stress ranges were used in order to obtain a full S-N curve for the older steel. The results of these tests were then compared to standardized S-N curves for current prestessing steel. These comparisons indicate that older prestressing steel fulfills the present fatigue requirements so that current standardized S-N curves may also be used to assess the fatigue strength of older steel tendons.