Behaviour of partially prestressed beams with external tendons

The behaviour of prestressed concrete structures with unbonded tendons differs significantly from that of those with bonded tendons after the working stage. To assess the safety of such structures, it is necessary to carry out full-range analysis with particular attention to the post-peak behaviour. This paper reports the initial findings of experimental investigations. A number of simply supported externally prestressed concrete beams with either steel or aramid fibre-reinforced polymer tendons are tested to failure, so as to study the effect of external prestressing on the ductility. The specimens tested all have T-sections prestressed by two external tendons with one on each side. In the experiments, different factors are taken into consideration. They include the level of prestressing force, area of non-prestressed steel reinforcement and partial prestressing ratio and so on. It is found that friction may vary significantly when a beam is subjected to large deformations. Comprehensive analysis of internal forces in selected specimens has been carried out. While the tendons play a key role in supporting loads during the initial stage of loading, their contribution decreases with further loading. At the same time, the role of bottom non-prestressed reinforcement increases and remains predominant after the apparent yielding of the specimen.