Fire behaviour of reinforced concrete structures strengthened with CFRP strips

João Pedro Lage da Costa Firmo

Key information

Authors:

João Pedro Lage da Costa Firmo (João Pedro Lage da Costa Firmo)

Supervisors:

João Pedro Ramôa Ribeiro Correia (João Pedro Ramôa Ribeiro Correia); Mário Rui Tiago Arruda (Mário Rui Tiago Arruda)

Published in

05/05/2015

Abstract

Carbon fibre reinforced polymer (CFRP) materials have been successfully used to strengthen civil engineering structures over the past 25 years, due to the advantages they present over traditional materials (concrete or steel), such as high strength-to-weight ratio, corrosion resistance and easy installation. Although the structural effectiveness of CFRP strengthening techniques at ambient temperature has been widely confirmed in numerous investigations, their performance at elevated temperatures continues to hinder a more widespread application in buildings, where the fire action has to be considered at design. In fact, the strength and stiffness of CFRPs are severely deteriorated at moderately elevated temperature, namely when approaching the glass transition temperature (Tg) of the polymer matrix, which typically varies between 55 ºC and 120 ºC. The bond between CFRPs and the concrete substrate, which is critical to maintain the structural effectiveness of the strengthening system, is even more affected by temperature, since it is usually ensured by epoxybased adhesives cured at ambient temperature, with lower Tg (typically ranging from 40 °C to 80 °C) than that exhibited by CFRPs. The available studies about this subject suggest that adequate fire protection systems need to be developed to allow extending the structural use of CFRP systems in buildings, pointing out that extensive research is needed to fill all the gaps in current knowledge. The investigations developed in this thesis had two main objectives: (i) to obtain in-depth understanding about the behaviour at elevated temperature and under fire exposure of reinforced concrete (RC) structures strengthened with CFRP materials, and (ii) to develop a design method for the definition of tailored fire protection systems, thus allowing a widespread use of these strengthening systems in buildings. To achieve these goals, the research was conducted along the following two domains: (i) bond behaviour of CFRP-concrete interfaces at elevated temperature and (ii) fire behaviour of RC beams flexurally strengthened with CFRP strips. The research on the bond behaviour of CFRP-concrete interfaces comprised double-lap shear tests at elevated temperatures (up to 150 °C) on four different types of concrete blocks strengthened with CFRP strips installed as follows: (i) as externally bonded reinforcement (EBR technique) using a current epoxy adhesive, (ii) EBR using a current epoxy adhesive and mechanically anchored to concrete with bolted steel plates, (iii) near surface mounted (NSM technique) using a current epoxy adhesive, and (iv) NSM using a mixed epoxy-cement adhesive. These tests allowed quantifying the stiffness and strength degradation with temperature for the most representative CFRP strengthening techniques (EBR and NSM), obtaining temperature-dependent “bond stress vs. relative slip” laws and evaluating the influence of applying mechanical anchorages in EBR-CFRP strips and using different adhesives in NSM-CFRP strips on their bond performance with temperature. Additionally, numerical models of the above mentioned tests were developed, allowing deriving global bond vs. slip laws as a function of temperature for the CFRP-concrete interaction. Regarding the second topic, the fire behaviour of RC beams strengthened with CFRP strips, in a first stage a preliminary methodology for the design of fire protection systems was proposed. The geometry of fire insulation schemes was optimized based on results of numerical thermal models and consisted of thicker insulation boards in the CFRP anchorages zones and thinner ones along the remaining length of the CFRP systems. This insulation strategy was based on the possibility of exploiting the mechanical contribution of the CFRP strengthening system through a cable behaviour (observed in previous tests), in which the CFRP system retains its structural effectiveness during fire even after the CFRP-concrete interaction is destroyed along its central length. Additional experimental and numerical studies about the flexural behaviour of partially bonded CFRP-strengthened RC beams at ambient temperature were performed to understand in further depth such cable behaviour. In a next stage fire resistance tests on insulated RC beams flexurally strengthened with CFRP systems (similar to those used in the double-lap shear tests) were performed. These tests, besides providing a better understanding of the structural effectiveness of CFRP strengthening systems in fire, validated the above-mentioned strategy for the design of fire protection systems. With the insulation schemes proposed, it was possible to exploit the CFRP mechanical contribution during fire exposure, in some case for more than 90 min, thus fulfilling stringent building code requirements. Finally, these fire resistance tests were numerically simulated, providing further validation to that insulation strategy as well as to the temperature dependent global bond vs. slip laws proposed for the CFRP-concrete interaction, showing they are adequate for simulating the behaviour of EBR-CFRP strengthening systems under fire exposure.