Ultimate limit state of MF-FRP beams

Usually, FRP materials are adhesively bonded to the structural substrate. An innovative technique application is based on the use of mechanical fastening (MF-FRP) by means of steel anchors to attach FRP laminates with enhanced bearing strength to concrete substrate. The benefit of MF-FRP, compared to adhesive bonding for FRP flexural strengthening is due to speed of installation using unskilled labor, minimal or absent surface preparation under any meteorological condition and immediate use of the strengthened structures. Some of the potential shortcomings are: brittle failure modes for members strengthened with the MF-FRP without a proper design; possible concrete damage during the drilling and dense internal reinforcement of the members that could limit the installation. Laboratory testing and a number of field applications have shown the effectiveness of such method. In this paper an analytical model is discussed for reinforced concrete (RC) beams strengthened by using MF-FRP strips. The model accounts for equilibrium, compatibility and constitutive relationships of materials, in particular, it accounts explicitly for the slip between the surface of the substrate and the FRP strip due to the behavior of fasteners. The proposed flexural model, coupled with an appropriate computation algorithm, is able to predict the fundamentals of flexural behavior of RC members strengthened with MF-FRP strips in terms of ultimate limit state. A comparison between the analytical predictions and the experimental results has been performed to validate the proposed model. The comparison shows a good agreement between the analytical predictions and the experimental results.