The bearing capacity of reinforced concrete (RC) beams can be increased thanks to FRP strengthening techniques such as externally bonded reinforcement (EBR) or near surface mounted (NSM) reinforcement. In EBR and NSM, FRP is connected to the RC beams by means of adhesive material (epoxy or grout) which allows the mutual interaction between FRP and the concrete section. FRP strengthening system may certainly represent a versatile solution when existing concrete members need to be adapted to new load conditions. Although FRP strengthening has revealed great performance at ambient condition, in case of potential elevated temperature exposure, such as fire, further issues have to be discussed concerning the bond interaction, thermal sensitivity of the adhesive and design approach.
The first part of this research work has involved an extended literature study on the bond interaction between FRP and concrete at elevated temperature in order to understand the influence of the temperature exposure on the bond strength, bond shear stress-slip and effective transfer length. Afterwards, the need to define a bond model for the temperature dependent FRP-concrete interaction has led to focus on the investigation of experimental bond shear stress-slip curves obtained by double bond shear tests at elevated temperature performed by authors, such as Blontrock, Leone, Palmieri and Cassaert. These studies have allowed to show the influence of the adhesive (epoxy and grout), FRP type (glass and carbon) and FRP configuration (EBR and NSM) on the bond behaviour at high temperatures.
The previous preliminary study has represented the first step of this research work in order to address, in the following, the thermal response of the epoxy adhesive under several environmental scenarios and the analysis of the flexural behaviour of RC FRP strengthened beam exposed to fire.
Literature provides several formulas to estimate the bearing capacity of FRP strengthened RC beams at room temperature, considering different failure aspects. In case of elevated temperatures, the efficiency of the FRP strengthening system is mostly affected by the degradation of the bond interaction FRP-concrete because of the change of state of the epoxy adhesive. Indeed, at room temperature, epoxy shows a stiff and glassy behaviour but, as soon as the glass transition zone is reached, it starts being soft and rubbery. This change is a gradual and progressive but, commonly, a single value is used to represent the temperature dependency of the epoxy: the glass transition temperature (Tg). Because of the significant influence of the Tg of the epoxy on the bond interaction between FRP and concrete member, changes of the glass transition temperature can potentially impact on the structural behaviour of the RC FRP strengthened beam exposed to elevated temperature. For this reason, an experimental investigation was carried out with the purpose to study the influence of different environmental scenarios on the Tg of three different epoxies currently used for strengthening applications.
The fire design strategy for RC FRP strengthened beams is a complex issue which may depend on the sizing of the original un-strengthened beam and the amount of FRP applied for the strengthening. In addition, because of the need to consider the degradation of the bond interaction, the study of the FRP strengthened beam exposed to fire may require the use of numerical tools. The last part of this research work was dedicated to the study of different fire design approaches for RC FRP strengthened beams on the basis of the existing recommendation for un-strengthened RC beams. In case of RC FRP strengthened beam exposed to fire, the flexural bearing capacity was studied by means a new numerical tool developed during this research study. Internal temperature distribution, material property degradation, bond adhesive degradation and thermal strains were fully considered by the proposed numerical approach.
Fig.1 - A beam reinforced with NSM (a) and a beam subjected to fire action (b)