Concrete reinforced with FRP rebars : Evaluation of durability and behaviour in the Service Limit State (SLS)

Abstract: One of the most common building materials is concrete and it has been for a long time. To overcome its low tensile capacity concrete structures are normally reinforced with steel rebars. The use of FRP (Fibers Reinforced Polymers) bars in concrete structures has emerged as an alternative to conventional steel reinforcement, due to the corrosion of steel in aggressive environments. FRP has been used as internal reinforcement for more than 30 years, bridges and parking garages are examples of structures in harsh environments where FRP is a good replacement for steel reinforcement. This due to the higher strength of FRP compared to steel and non-corrosive properties, however FRP as internal reinforcement is not commonly used in Scandinavia. This work has been divided into four parts, a Literature survey, a Literature study on durability, structural behaviour in the serviceability limit state and a FE analysis of previously carried out laboratory tests. In the literature survey the material FPR is described with its components, manufacturing process, history and various applications. A literature study was done to determent the long-term durability of GFRP by accelerated laboratory tests for durability, then compared to field tests on durability of GFRP rebars. The accuracy of FRP design international standards has been evaluated in terms of serviceability limit stat, such as ACI 440.1R-15, ISIS and a variant of Eurocode 2 (EC2). The design models for deflection available for these standards were compared to a database of experimental studies collected by the author. The stiffness of structures reinforced with FRP is such an important parameter so a non-linear calculation using ATENA software was conducted. Results were compared to laboratory tests performed at Denmark Technical University (DTU). In several accelerated laboratory experiments where bare FRP bars were exposed to different harsh environments the degradation of strength was significant, where an alkaline solution at elevated temperature was the harshest environment for the GFRP bars. When GFRP rebars are embedded in concrete the degradation was significantly lower (around 40 percentage points), the concrete protects the GFRP rebars considerably. The largest rate of degradation on GFRP rebars is in the initial state, in comparison to steel which starts to corrode when carbonation and/or chloride penetration critical levels reaches the reinforcement. In field studies there were small signs of degradation of the GFRP rebars, mainly in tropical climates. De-icing salts have a limited effect on the degradation. Laboratory experiments are very conservative with unrealistic harsh environments compared to the natural harsh environments. Therefore, after 20 years of service in harsh environment there were no or small signs of degradation on the GFRP rebars which indicates the validity of GFRP. All three standards evaluated had a large spread on the predicted deflection compared to the experiments, with ACI 440.1R-15 as the most conservative standard with a mean value of the deflection ratio at 0.81. The mean value of the deflection ratio when using ISIS was 0.87, slightly less conservative but with the same spread as ACI 440.1R-15. The calculation using a variant of EC2 had the most spread of results, but with a mean value of the deflection ratio at 0.93, this excluding 11 beams that had an unrealistic prediction due to the wrong prediction of the crack moment. The FEM model created had a similar stiffness as compared to the experiment from DTU, which indicates that the use of Atena was accurate for calculating the deflection of the beams. Although the ultimate load was not well predicted, probably due to the failure mode crushing of concrete in the compressive zone. Despite this, there are many structural parts where FRP could be beneficial, for example in splash zones, in edge beams and slabs etc. This could bring down the costs for maintenance and also prolong the life span of the structure.