Upgrading concrete bridges : post-tensioning for higher loads

Sammanfattning: There are a great number of old structures around the world, some of which were designed for completely different purposes than in their current application. Swedish railway bridges were for example only designed for maximum axle loads of 200 kN in the beginning of the 20th century, while the highest axle loads of today are twice as high. The traffic intensities have also increased dramatically and the velocities are now higher than ever before. Reinforced concrete trough bridges were typically designed and built in the mid-20th century and it is still one of the most frequent railway bridge types in Sweden. The trough bridges were normally designed for traffic loads which were smaller than the loads today and in order to maintain an old structure as the loads increases, structural upgrading of the load bearing capacity might be necessary. Upgrading the load carrying capacity can be performed in two ways, namely administrative upgrading or strengthening. Administrative upgrading refers to refined design calculations, using real material data, geometry and loads, which provides a higher capacity than the original design and the bridge can thereby be upgrading with minor physical impact. Upgrading by strengthening on the other hand, refers to, often, larger physical alteration of the structure in order to enhance the original load carrying capacity.Upgrading methods for increased flexural resistance of concrete trough bridges has been developed and tested previously, but strengthening methods for increased shear resistance in the bridge deck are still absent. The objective of this thesis is therefore to find an existing- or develop a new strengthening method which can be applied in order to enhance the shear resistance of concrete trough bridge decks. The difficulties associated to strengthening of existing railway bridges include traffic during the strengthening work and concrete surfaces concealed by the ballast.The State-of-the-Art indicated that none of the existing strengthening techniques were sufficient for this application and internal unbonded post-tensioning in the transverse direction was nominated as the most promising method. The research was thereafter focused on testing the possibilities and strengthening effects of post-tensioning. Two laboratory investigations were performed during the research project and the method was finally tested in a field test on a 50 years old trough bridge in Haparanda, Sweden. The strengthening procedure of internal unbonded post-tensioning consists of four consecutive steps:1.Transverse drilling of the horizontal holes through the bottom slab.2.Installation of the prestressing system.3.Post-tensioning of the system.4.Sealing of the prestressing system.The laboratory and field tests were successful and the results proved that the internal steel reinforcement within the concrete was compressed when the trough bridge was post-tensioned. Due to the compression, a higher load could be carried by the bridge deck before the tensile reinforcement yields and the bridge fails. In other words, the flexural capacity of the bridge deck was increased. The field test actually showed that eight steel bars, post-tensioned with 430 kN per bar on the Haparanda Bridge, completely counteracted the tensile stresses caused by a train with 215 kN axle loads. The effect on the shear resistance was however not as easy to measure, but the laboratory test recorded a significant strain reduction in the tensile reinforcement which was bent up at the transition zone between the bridge deck and the main girders. The reduced strain might be interpreted as lower shear stresses and post-tensioning can thereby be considered to have a positive effect on the shear resistance of the bridge deck. Shear design according to the protocol of Eurocode 2 or BBK was however found to be restrictive in predicting the post-tensionings effect on the shear capacity and further research is proposed in chapter 8.