External posttensioning or unbonded prestressing was found to be a powerful tool for retrofitting and for increasing the life extension of existing structures. Since the 1950s, this technique of reinforcement was applied with success to bridge structures in many countries, and was found to provide an efficient and economic solution for a wide range of bridge types and conditions. Unbonded prestressing is defined as a system in which the post-tensioning tendons or bars are located outside the concrete cross-section and the prestressing forces are transmitted to the girder through the end anchorages, deviators, or saddles. In response to the demand for a faster and more efficient transportation system, there was a steady increase in the weight and volume of traffic throughout the world. Besides increases in legal vehicle loads, the overloading of vehicles is a common problem and it must also be considered when designing or assessing bridges. As a result, many bridges are now required to carry loads significantly greater than their original design loads; and their deck results still deteriorated by cracking of concrete, corrosion of rebars, snapping of tendons, and so forth. In the following, a case study about a railway bridge retrofitted by external posttensioning technique will be illustrated. 1. Introduction In Europe, the motorway and railway networks were built about 30 to 40 years ago [1]; they have a large number of prestressed concrete bridges and viaducts. Many reinforced concrete bridges in highway systems are deteriorated and/or distressed to such a degree that structural strengthening of the bridge or reducing the allowable truck loading on the bridge by load posting is necessary to extend the service life of the bridge [2]. Besides, in motorway networks, over the last few decades, there was a rapid increase in traffic volume and weight of heavy vehicles and contemporarily there was a rapid increase in transit speed on railway networks. Many bridges, which were built with now obsolete design standards, are not able to carry on the recent traffic requirements and they require either weight or speed restriction, the strengthening, or, even, the total replacement [2]. Various methods for the rehabilitation of bridges are currently available including the addition of structural reinforcement components as steel rebar or reinforced concrete (RC) jackets and bonded steel plates. Two methods are currently proving to be very useful in increasing the cross-section capacity of the bridge beam: the strengthening by fibres reinforced polymer (FRP) or
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