CarboFire – Fire Protection of Reinforced Concrete Structural Elements Strengthened with CFRP Composite Systems
National Science Foundation (FCT) Project PTDC/ECM/118271/2010
CFRP (carbon fibre reinforced polymer) strengthening systems, now widely accepted by the research and technical communities, are being essentially used in bridge structures, in which fire resistance is not usually a primary design consideration. For buildings, although CFRP systems also present great potential, widespread application is being delayed due to concerns regarding their performance at elevated temperature. In fact, the strength, stiffness and bond properties of CFRPs are severely deteriorated at moderately elevated temperatures (65-120 °C) – in several international scientific committees, the fire behaviour of FRPs has been unanimously recognized as one of the top priorities in terms of critical research needs. The main objectives established for this project were two-fold: (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. This implied coupling an extensive experimental research programme with the development of supporting numerical modelling tools.
The research was carried out in the following two main domains: 1. Bond behaviour of CFRP-concrete interfaces at elevated temperature; 2. Fire behaviour of RC beams/slabs flexurally strengthened with CFRP strips.
Regarding the first topic, bond degradation with temperature of CFRP-concrete interfaces, the following objectives were defined: (i) to assess and quantify the stiffness and strength degradation with temperature for the most representative CFRP strengthening techniques – externally bonded reinforcement (EBR), where CFRP strips are bonded (usually with epoxy adhesive) to the surface of the concrete member that is to be strengthened), and near surface mounted (NSM), where the CFRP strips are installed in slits made in the concrete cover (typically filled also with epoxy adhesive); (ii) to obtain temperature-dependent “bond stress vs. relative slip” laws for both EBR and NSM techniques; (iii) to evaluate the influence of applying mechanical anchorages in EBR-CFRP strips and using different adhesives in NSM-CFRP strips on their bond performance with temperature.
To achieve the aforementioned goals, an experimental campaign was conducted at elevated temperatures on four different types of concrete blocks strengthened with the following systems: (i) EBR-CFRP strips bonded to concrete with a current epoxy adhesive, (ii) EBR-CFRP strips bonded with a current epoxy adhesive and mechanically anchored to concrete with bolted steel plates, (iii) NSM-CFRP strips bonded to concrete with a current epoxy adhesive, and (iv) NSM-CFRP strips bonded to concrete with a mixed epoxy-cement adhesive. Numerical models of these tests were developed, allowing to derive (based on an inverse analysis) global bond vs. slip laws as a function of temperature for the CFRP-concrete interaction. In what concerns the second topic, fire behaviour of RC beams strengthened with CFRP strips, the following objectives were defined: (i) to assess and compare the performance under fire of EBR and NSM-strengthened RC beams; (ii) to evaluate the influence on the fire response of applying mechanical anchorages in EBR-CFRP strips and using different bonding adhesives in NSM-CFRP strips; (iii) to develop a methodology for the design of fire protection systems for RC members strengthened with CFRP strips.
Fire resistance tests were performed on insulated RC beams flexurally strengthened with CFRP strips installed according to the following techniques (and bonding adhesives): (i) EBR with a current epoxy adhesive, (ii) EBR with a current epoxy adhesive and mechanically anchored to concrete with bolted steel plates, (iii) NSM with a current epoxy adhesive, and (iv) NSM with a mixed epoxy-cement adhesive. The beams were thermally insulated with different fire protection schemes, comprising thicker insulation boards in the CFRP anchorages zones and thinner ones along the remaining length of the CFRP systems.
These fire resistance tests were numerically simulated, providing a further validation to the methodology of fire protection adopted; these models also provided further validation to the global bond vs. slip laws proposed for the CFRP-concrete interaction and allowed evaluating the accuracy of those laws in the simulation of the structural response of CFRP-strengthened RC members under fire.
Finally a simplified procedure for the design of fire protection systems for CFRP strengthening systems was proposed.
The objectives set for this project were largely fulfilled. From a scientific point of view, it was possible to obtain in-depth understanding about (i) the bond between concrete and CFRP-strengthening systems at elevated temperature, and (ii) the fire resistance behaviour of CFRP-strengthened members, with different strengthening techniques and materials. In addition, the numerical models developed in the project were able to capture the main behavioural aspects observed in the extensive experimental campaign, thus constituting reliable tools to support the fire analysis and design of this type of elements. From a practical point of view, rational fire protection systems were proposed, together with a simplified design procedure that allows the explicit consideration of the CFRP system contribution in fire. These developments, which were extensively disseminated within the scientific and technical communities, will contribute to increase the safe (and economic) use of CFRP systems in the strengthening of reinforced concrete building components.
J.P. Firmo, M.R.T. Arruda, J.R. Correia, “Numerical simulation of the fire behaviour of thermally insulated RC beams strengthened with EBR-CFRP strips”, Composite Structures, Vol. 126, pp. 360-370, 2015.
J.P. Firmo, J.R. Correia, L. Bisby, “Fire behaviour of FRP-strengthened reinforced concrete structural elements: a state-of-the-art review”, Composites Part B: Engineering, Vol. 80, pp. 198-216, 2015.
J.P. Firmo, J.R. Correia, D. Pitta, C. Tiago, M.R.T. Arruda, “Experimental characterization of the bond between externally bonded reinforcement (EBR) CFRP strips and concrete at elevated temperatures”, Cement and Concrete Composites, Vol. 60, pp. 44-55, 2015.
J.P. Firmo, J.R. Correia, D. Pitta, C. Tiago, M.R.T. Arruda, “Bond behavior at high temperatures between near surface mounted (NSM) CFRP strips and concrete”, Journal of Composites for Construction, Vol 19, No. 4, 2015.
J.P. Firmo, M.R.T. Arruda, J.R. Correia, C. Tiago, “Flexural behaviour of partially bonded CFRP strengthened concrete beams: application to fire protection systems design”, Materials and Design, Vol. 65, pp. 1064-1074, 2015.
J.P. Firmo, M.R.T. Arruda, J.R. Correia, “Contribution to the understanding of the mechanical behaviour of CFRP-strengthened RC beams subjected to fire: experimental and numerical assessment”, Composites Part B: Engineering, Vol. 66, pp. 15-24, 2014.
J.A. Teixeira de Freitas, C. López, P.T. Cuong, Rui Faria, “Hybrid finite element thermal modelling of fire protected structural elements strengthened with CFRP laminates”, Composite Structures, Vol. 113, pp. 396-402, 2014.
J.P. Firmo, J.R. Correia, P. França, “Fire behaviour of reinforced concrete beams strengthened with CFRP laminates: Protection systems with insulation of the anchorage zones”, Composites Part B: Engineering, Vol. 43, No. 3, pp. 1545-1556, 2012.