Fire behaviour of thermally insulated RC beams strengthened with NSM-CFRP strips: Experimental study

This paper presents the results of fire resistance tests on reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) strips installed according to the near surface mounted (NSM) technique using two different adhesives. The beams were simultaneously subjected to a service load and the ISO 834 standard fire. Different fire protection schemes were studied, comprising a thinner insulation layer along the bottom soffit of the beams and a thicker one at the CFRP anchorage zones. The main objectives of this paper were (i) to understand in further depth the fire behaviour of NSM-strengthened RC beams, in particular the structural effectiveness of the strengthening system during fire, (ii) to evaluate the efficiency of the above-mentioned fire protection strategy in extending the CFRP mechanical contribution during fire, and (iii) to compare the fire performance of the NSM-strengthening system with that of the alternative externally bonded reinforcement (EBR) technique, recently investigated under similar test conditions. The results obtained showed that using the adopted insulation schemes (i.e., thicker insulation at the anchorage zone and thinner insulation in the current zone), even after the CFRP-concrete bond is highly damaged in the central zone of the beams, the strengthening system is able to retain its structural effectiveness through a cable mechanism: for insulation thicknesses of 25 mm (current zone) and 50 mm (anchorage zones), the fire resistance of the strengthening system was extended up to 114 min. The loss of effectiveness of the CFRP system occurred when the average temperature in the adhesive at the CFRP anchorage zones attained values ranging from 2.2 to 5.6 times its glass transition temperature (Tg). The comparison with the EBR-strengthened beams confirmed the much better performance of the NSM strengthening.