Flexural Performance of Unbonded Posttensioned Rectangular Concrete Filled FRP Tube Beams
Publication: Journal of Composites for Construction
Volume 24, Issue 5
Abstract
This paper experimentally investigates the effect of using the posttensioning (PT) steel strands on the flexural performance of rectangular concrete filled fiber-reinforced polymer (FRP) tube (CFFT) beams. Eight PT beams were tested to failure in four-point bending. The main parameters are the effect of prestressed reinforcement, the glass fiber-reinforced polymer (GFRP) tube, the tube thickness, the magnitude of the prestress level, and the number of strands. The test results indicate that rectangular PT CFFT beams showed superior load–deflection behaviors and higher flexural strength compared with the non-PT CFFT designed with identical structural laminate and tube thickness. The results show that PT CFFT beams performed substantially better than PT concrete beams with the same cross-sectional dimensions and prestressing. The flexural behavior of the tested PT CFFT beams is highly dependent on the FRP tube confinement and thickness and, to a lesser extent, on the magnitude of the prestress level and the number of strands. A strain compatibility model has been developed considering the partially confined and unconfined concrete models and verified against the experimental results. The proposed model successfully predicts the flexural moment capacity of the tested beams with satisfactory accuracy on average of 1.05 ± 0.03 and 1.21 ± 0.04, respectively. In addition, it was found that neglecting concrete confinement highly underestimates the flexural strength. More investigations, however, are needed to assess the effect of a wide range of key influencing parameters to better model and understand the flexural behavior of rectangular PT CFFT beams.
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Acknowledgments
The authors wish to acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation, and the Fonds de recherche du Québec—Nature et Technologies—(FRQNT). The assistance of the technical staff (Mr. Claude, Sebastien, Eric, and Steven) of the Structural and Materials Laboratory in the Department of Civil and Building Engineering at the University of Sherbrooke is also greatly appreciated.
Notation
The following symbols are used in this paper:
- Abar
- cross-sectional area of steel bar, (mm2);
- Ast
- cross-sectional area of steel strand, (mm2);
- b
- cross section width, (mm);
- c
- neutral axis depth, (mm);
- d
- effective depth, (mm);
- Elfc
- elastic modulus of tube compression flange in axial direction, (GPa);
- Elft
- elastic modulus of tube tension flange in axial direction, (GPa);
- Elwc
- elastic modulus of tube compression web in axial direction, (GPa);
- unconfined concrete compressive strength, (MPa);
- ffu
- tensile strength of the tube in the longitudinal direction, (MPa);
- flfc
- compression stress of tube flange in axial direction, (MPa);
- flft
- tensile stress of tube flange in axial direction, (MPa);
- flwc
- compression stress of tube web in axial direction, (MPa);
- fsu
- ultimate tensile stress of the steel strands, (MPa);
- fy
- yield tensile stress of the steel rebars, (MPa);
- h
- member total height, (mm);
- Mcr
- moment at the first crack, (kN · m);
- Mu
- ultimate moment capacity of prestressed CFFT beams, (kN · m);
- My
- yield moment, (kN · m);
- Pcr
- load at the first crack, (kN);
- Pu
- ultimate load, (kN);
- Py
- yield load, (kN);
- tf
- thickness of FRP tube, (mm);
- Δcr
- deflection at cracking;
- Δu
- deflection at ultimate;
- Δy
- deflection at yielding;
- ɛc
- ultimate strain in compression surface;
- ɛlfc
- compression strain of tube flange in axial direction;
- ɛlft
- tensile strain of tube flange in axial direction;
- ɛlwc
- compression strain of tube web in axial direction;
- ɛlwt
- tensile strain of tube web in axial direction;
- ɛt
- ultimate strain in tension surface;
- λ
- ductility factor;
- ρbar
- steel rebars reinforcement ratio, (=Abar/bdbar);
- ρf
- FRP tube reinforcement ratio, (=Atube/Aconcrete);
- ρst
- steel tendons reinforcement ratio, (Ast/bdst);
- ωbar
- reinforcement index of steel rebars;
- ωf
- reinforcement index of FRP tube;
- ωst
- reinforcement index of steel tendons; and
- ωt
- total reinforcement index.
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Published In
Journal of Composites for Construction
Volume 24 • Issue 5 • October 2020
Copyright
© 2020 American Society of Civil Engineers.
History
Received: Jun 13, 2019
Accepted: May 20, 2020
Published online: Aug 7, 2020
Published in print: Oct 1, 2020
Discussion open until: Jan 7, 2021
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