R/C Structures Exposed to Fire: Damage Assessment and Efficient Structural Repairing with FRP

As is well known, reinforced concrete structures are very performant in case of fire, due to the low conductivity of concrete compared to steel. However, heat shock leads to a damage distribution in the structure which is related to the duration and exposure, so that after fire, structural elements invariably present graded load bearing capacity reduction, although no collapse takes place. Recently, the application of FRP reinforcement systems has become one of the most used techniques in structural repair and retrofit, also for the case of fire damaged elements. As usual, FRP laminas or fabrics are glued on the external surface of the beam or column, which is the most deteriorated zone. As a consequence, it is possible that the force transmission across the concrete interface is too high to be resisted by concrete in relation to its after cooling modified capacitiy. Therefore this common restoration procedure has to be analysed more in detail in order to define optimal strengthening strategies. In this paper, according to EC2, the damaged beam is modelled through two concrete zones, with different mechanical properties related to the temperature peak raised up under fire event. The subdivision boundary is located on the 500°C isotherm. An analytical method is explained which allows to analyse damaged beam after repair, including the deformability of the bonding interface between concrete and FRP. It is pointed out that in case FRP reinforcement extended to the whole beam span, the collapse appears in a flexural way, with internal delamination of the composite, while shear stresses are very low in relation to the resistance of materials. In fact the load bearing capacity of the beam with FRP directly bonded to the damaged concrete, can be lower than the bare resistance of the naked damaged beam, if the surface tensile strength is too low with respect to the initial one. Following the above explanation, an efficient solution in improving the performance of the beam is to remove the external layer of the concrete, and to replace it with a high strength mortar layer, on which the FRP reinforcement can be subsequently applied. This restoration technique is as efficient as stronger the grade of the mortar replacement is, but its economical effectiveness is confined only to the cases of heavily damaged massive elements.