EXTERNAL PRESTRESSING OF RC T-BEAMS WITH CFRP TENDONS

As the acceptance for non-stressed FRP (Fibre Reinforced Polymers) increase steadily in the construction industry, the industries susceptibility for new ideas in the material’s usage increase. Improving the performance of a bridge, a parking garage or a multistory building by the use of appropriate FRPs is about to become common practice. Several guidelines on the subject have been presented during the last decade, e.g. [1-5]. Next step is to further utilize the excellent behaviour of the advanced material, in particular CFRP (Carbon Fiber Reinforced Polymers). CFRPs have mechanical properties that fit the purpose of prestressing very well. The material is in addition corrosion resistant, experience only limited creep and relaxation over time and is reported to sustain other problems related to the durability well. If FRP could be used as a reliable prestressing material not only the ultimate capacity could be increased, with a carefully applied compressive stress existing open cracks in concrete may be partly closed and it might also be possible to reduce large deflections. In that way the durability of the entire RC (Reinforced Concrete) -structure can be increased. Application of FRPs as prestressing materials does however require a new design philosophy. FRPs are in contradiction to high strength prestressing steel linearly elastic until failure and they are, in their nature, also generally orthotropic. The behaviour of a RC-structure prestressed with FRP might therefore differ from a structure prestressed with high strength steel although both the ultimate tensile capacity and the modulus of elasticity in some cases are the same. A number of guidelines for the usage of FRP have been published within the last years with [6] as a good example. To verify design principles stated in [6] and future guidelines, a database of tested structures to compare with is necessary. Such a database is for example used in [7] for verification of FRP shear strengthening models and is in that case composed of more than 200 tested beams with different material and geometrical properties. The experimental study contained in this paper should serve as a contribution to a similar database for RC-structures prestressed with FRP materials. As such it should be possible to combine with tests performed in a wide range of publications, [8-10]. It should also give researchers involved in the project an introduction to problems involved with the use of prestressing with FRP and highlight differences in the behaviour of the structure compared to one prestressed with steel strands.