Numerical simulation of the fire behaviour of thermally insulated reinforced concrete beams strengthened with EBR-CFRP strips

This paper presents numerical investigations on the fire behaviour of reinforced concrete (RC) beams flexurally strengthened with externally bonded reinforcement (EBR) carbon fibre reinforced polymer (CFRP) strips, simultaneously subjected to a service load and the ISO 834 standard fire. Two dimensional (2D) finite element (FE) models were developed in order to simulate previous fire resistance tests on CFRP-strengthened RC beams comprising different fire protection schemes, with a thinner insulation layer along the bottom soffit of the beams and a thicker one in the CFRP anchorage zones. The variation with temperature of the thermal and mechanical properties of the constituent materials was considered and the CFRP-concrete interaction was modelled by means of bi-linear bond-slip laws previously calibrated for different temperatures. The numerical results were compared with experimental temperatures, midspan deflections and time for CFRP debonding. The models provided accurate predictions of the thermo-mechanical fire response of the beams, confirming that it is possible to exploit the CFRP mechanical contribution through a cable mechanism, provided that a thicker insulation is applied in the CFRP anchorage zones. The models predicted the CFRP debonding when the average temperature at the CFRP-concrete interface along the anchorage zones ranged from 1.1 to 1.4 times the adhesive glass transition temperature, which also compared well with test measurements. The numerical study presented in this paper, besides providing useful data to understand in further depth the structural effectiveness of CFRP strengthening systems during fire (namely, the strains/stresses in the materials and CFRP-concrete interface during fire exposure), allowed validating the global bond-slip laws for the CFRP-concrete interaction previously proposed by the authors, showing their adequacy for simulating the behaviour of EBR-CFRP strengthening systems under fire exposure, and a strategy for the design of fire protection systems that is based on the fulfilment of two main requirements: (i) the CFRP temperature along the central zone must remain below a certain critical temperature, avoiding its tensile rupture; and (ii) the temperature of the CFRP-concrete interface along the anchorage length must be kept below the adhesive Tg of the adhesive in order to prevent the CFRP debonding for the required time of fire exposure.