Performance under fire situations of concrete members reinforced with FRP rods: bond models and design nomograms

Intuitively, the fire endurance of concrete members reinforced with fiber reinforced polymer (FRP) bars is related to the decrease in the mechanical properties of the materials concerned, especially resin. Large-scale fire tests recently performed on nine concrete slabs reinforced with glass FRP bars demonstrated the importance of bond between FRP and concrete for performance under fire situations. The experimental results showed that (1) the length of the FRP bars in the zone of slabs not directly exposed to fire (namely, anchoring length in fire situations) can be much more relevant to fire endurance than the concrete cover in the zone directly exposed to fire; and (2) the shape of the bar, for instance bent at the end, allows a reduction in anchoring length. From a design point of view, evaluating the necessary anchoring length through a bond model seems to be a key aspect. Full-scale test results, extensively presented elsewhere, are used in this paper to investigate the bond behavior of FRP bars embedded in concrete at high temperature and to assess a procedure to predict bond stress, slip, and load transfer at elevated temperature, based on both the results of numerical thermal analysis and the predictions of a bond theoretical model adjusted for fire situations. The design procedure outlined for calculating the minimal required anchoring length proves a valuable approach for the practicing engineer and stands together with the experimental and numerical results presented earlier. Finally, design nomograms are shown as examples of application of the procedure.