Effect of unbonded prestressing steel in terms of serviceability

Abstract One of the methods of strengthening bridges is strengthening using unbonded post-tensioned prestressing steel e.g. strands type monostrand. The individual strands, protected with grease and HDPE coating, are routed polygonally. Trajectory changes take place in deviators, where the smallest possible radii of curvature are often chosen for layout reasons. Due to the lack of bond between prestressing steel and concrete, unbonded prestressing steel requires different procedures and approaches compared to bonded prestressing steel. During its service life, the structures is subjected to various types of loads, including unsymmetrical or dynamic loads, which may affect the ultimate behaviour of unbonded prestressing steel, as these affects may result in, among other things, a redistribution of stresses in prestressing steel. This may be caused by the friction of the prestressing steel in the deviators being overcome and the displacement in the HDPE coating, which may also lead to the penetration of the HDPE coating itself and the loss of primary corrosion protection. All of these factors can have a major impact on the service life of the structure, not only in terms of ultimate limit states but also serviceability limit states. This article aims to highlight the possible risks of using unbonded prestressing steel to strengthen bridges and to give examples of thinning of HDPE coating.