This paper presents a study on the load carrying capacity of reinforced concrete (RC) beams strengthened with externally bonded (EB) carbon fiber reinforced polymer (CFRP) strips prestressed up to 0.6% in strain. At the strip ends, the innovative gradient anchorage is used instead of conventional mechanical fasteners. This method, based on the epoxy resin’s ability to rapidly cure under high temperatures, foresees a sector-wise heating followed by a gradual decrease of the initial prestress force towards the strip ends. The experimental investigation shows a promising structural behavior, resulting in high strip tensile strains, eventually almost reaching tensile failure of the composite strip. Additionally, ductility when considering deflection at steel yielding and at ultimate load is satisfying, too. From a practical point of view, it is demonstrated that premature strip grinding in the anchorage zone is not beneficial. In addition, a non-commercial 1D finite element code has been enlarged to an EB reinforcement with prestressed composite strips. A bilinear bond stress-slip relation obtained in earlier investigations is introduced as an additional failure criterion to the code. The numerical code is able to almost perfectly predict the overall structural behavior. Furthermore, the calculations are used for comparison purposes between an initially unstressed and a prestressed externally bonded composite reinforcement. The increase in cracking and yielding load, as well as differences in structural stiffness are apparent.
References
[1]
Meier, U. Strengthening of structures using carbon fibre/epoxy composites. Constr. Build. Mater.？1995, 9, 341–351, doi:10.1016/0950-0618(95)00071-2.
[2]
Bakis, C.; Bank, L.; Brown, V.; Cosenza, E.; Davalos, J.; Lesko, J.; Machida, A.; Rizkalla, S.; Triantafillou, T. Fiber-reinforced polymer composites for construction—State-of-the-art review. J. Compos. Constr.？2002, 6, 73–87, doi:10.1061/(ASCE)1090-0268(2002)6:2(73).
[3]
Meier, U.; St？cklin, I.; Terrasi, G. Civil Engineers can Make Better Use of the Strength of Fibrous Materials. In Proceedings of the International Conference on Composites in Construction (CCC), Porto, Portugal, 10–12 October 2001.
[4]
Pellegrino, C.; Modena, C. Flexural Strengthening of real-scale RC and PRC beams with end-anchored Pretensioned FRP laminates. ACI Struct. J.？2009, 106, 319–328.
[5]
El-Hacha, R.; Wight, R.; Green, M. Innovative System for prestressing fiber-reinforced polymer Sheets. ACI Struct. J.？2003, 100, 305–313.
[6]
El-Hacha, R.; Green, M.; Wight, R. Strengthening Concrete Structures with Prestressed FRP Systems. In Proceedings of the 4th International Conference on Construction Materials (CONMAT), Nagoya, Japan, 24–26 August 2009.
[7]
Fran？a, P.; Costa, A. Behavior of Flexural Strengthened Beams with Prestressed CFRP Laminates. In Proceedings of the FRPRCS-8, Patras, Greece, 18 July 2007.
[8]
Fernandes, P.; Sena-Cruz, J. Flexural Response of HSC Girders Strengthened with 2 Non- and Prestressed CFRP Laminates. In Proceedings of the FRPRCS-11, Guimaraes, Portugal, 26–28 June 2013.
[9]
St？cklin, I.; Meier, U. Strengthening of Concrete Structures with Prestressed and Gradually Anchored CFRP Strips. In Proceedings of the FRPRCS-5, Cambridge, UK, 6–18 July 2001.
[10]
Czaderski, C.; Martinelli, E.; Michels, J.; Motavalli, M. Effect of curing conditions on strength development in an epoxy resin for structural strengthening. Compos. B Eng.？2012, 43, 398–410, doi:10.1016/j.compositesb.2011.07.006.
[11]
Czaderski, C. Strengthening of Reinforced Concrete Members by Prestressed Externally Bonded Reinforcement with Gradient Method. Ph.D. Thesis; ETH Zürich: Zürich, Switzerland, April 2012. Available online: http://www.dx.doi.org/10.3929/ethz-a-007569614.
[12]
Michels, J.; Czaderski, C.; El-Hacha, R.; Br？nnimann, R.; Motavalli, M. Temporary bond strength of partly cured epoxy adhesive for anchoring prestressed CFRP strips on concrete. Compos. Struct.？2012, 94, 2667–2676, doi:10.1016/j.compstruct.2012.03.037.
[13]
Michels, J.; Czaderski, C.; El-Hacha, R.; Br？nnimann, R.; Motavalli, M. Partly Cured Epoxy Adhesive for Anchoring Prestressed CFRP Strips on Concrete. In Proceedings of the 3rd Asia-Pacific Conference on FRP in Structures (APFIS), Sapporo, Japan, 2–4 February 2012.
[14]
Czaderski, C.; Motavalli, M. 40-Year-old full-scale concrete bridge girder strengthened with prestressed CFRP plates anchored using gradient method. Compos. B Eng.？2007, 38, 878–886, doi:10.1016/j.compositesb.2006.11.003.
[15]
Kotynia, R.; Walendziak, R.; St？cklin, I.; Meier, U. RC slabs strengthened with prestressed and gradually anchored CFRP strips under monotonic and cyclic loading. J. Compos. Constr. (ASCE)？2011, 15, 168–180, doi:10.1061/(ASCE)CC.1943-5614.0000081.
[16]
Motavalli, M.; Czaderski, C.; Pfyl-Lang, K. Prestressed CFRP for strengthening of reinforced concrete structures: Recent developments at Empa, Switzerland. J. Compos. Construct. (ASCE)？2011, 15, 194–205, doi:10.1061/(ASCE)CC.1943-5614.0000125.
[17]
Michels, J.; Czaderski, C.; Br？nnimann, R.; Motavalli, M. Gradient Anchorage Method for Prestressed CFRP Strips-Principle and Application. In Proceedings of the Bridge Maintenance, Safety, Management, Resilience and Sustainability-Proceedings of the Sixth International Conference on Bridge Maintenance, Safety and Management, Safety, Management, 8–12 July 2012; pp. 1981–1986.
[18]
Michels, J.; Cruz, J.S.; Czaderski, C.; Motavalli, M. Structural strengthening with prestressed CFRP strips anchored with the gradient method. J. Compos. Construc. (ASCE)？2013, 17, 651–661, doi:10.1061/(ASCE)CC.1943-5614.0000372.
[19]
Michels, J.; Czaderski, C.; Martinelli, E.; Motavalli, M. RC Beams Strengthened with Prestressed and Gradually Anchored CFRP Strips. In Proceedings of the Second Middle East Conference on Smart Monitoring, Assessment and Retrofitting of Structures (SMAR), Istanbul, Turkey, 9–11 September 2013.
[20]
Fédéation internationale du béton. Structural Concrete—Textbook on Behaviour, Design and Performance: Updated Knowledge of the CEB/FIP Model Code (1990); FIB - Féd. Int. du Bton: Lausanne, Switzerland, 1999.
S&P-Clever-Reinforcement-Company-AG. S&P Resin 220 Epoxy Adhesive - Technical Data Sheet; S&P Clever Reinforcement Company AG: Seewen, Switzerland, 2012.
[23]
Subramaniam, K.; Carloni, C.; Nobile, L. Width effect in the interface fracture during shear debonding of FRP sheets from concrete. Eng. Fract. Mech.？2007, 74, 1241–1249.
[24]
Oudah, F.; El-Hacha, R. A new ductility model of reinforced concrete beams strengthened using fiber reinforced polymer reinforcement. Compos. B Eng.？2012, 43, 3338–3347, doi:10.1016/j.compositesb.2012.01.071.
[25]
Faella, C.; Martinelli, E.; Nigro, E. Formulation and validation of a theoretical model for intermediate debonding in FRP strengthened RC beams. Compos. B Eng.？2008, 39, 645–655, doi:10.1016/j.compositesb.2007.06.002.
[26]
Martinelli, E.; Faella, C.; di Palma, G. Shear-flexible steel-concrete composite beams in partial interaction: Closed form “exact” expression of the stiffness matrix. J. Eng. Mech. (ASCE)？2012, 138, 151–163, doi:10.1061/(ASCE)EM.1943-7889.0000320.
[27]
Martinelli, E.; Napoli, A.; Nunziata, B.; Realfonzo, R. A 1D finite element model for the flexural behaviour of RC beams strengthened with MF-FRP strips. Compos. Struct.？2014, 107, 190–204, doi:10.1016/j.compstruct.2013.07.058.
[28]
Harmanci, Y. Prestressed CFRP for Structural Retrofitting-Experimental and Analytical Investigation. Master’s Thesis, ETH Zurich, Zürich, Switzerland, 2013.
