Behavior of GFRP-Reinforced Concrete Members under Combined Bending Moment and Low Axial Load
Publication: Journal of Composites for Construction
Volume 28, Issue 4
Abstract
This study examined nine concrete members reinforced with glass fiber–reinforced polymer (GFRP) bars, exploring the impact of combined bending moment and low axial loads. Three reinforcement ratios (1.7%, 2.5%, and 3.3%) were considered under various axial loads (0, 125, and 250 kN). In the absence of a standardized test method for determining the compressive properties of rebars, a simple approach was adopted. GFRP bars demonstrated a compressive modulus of elasticity that was roughly equivalent to their tensile modulus, along with a compressive strength reaching approximately 70% of their tensile strength. The main tests on members showed that increasing the reinforcement ratio to 3.3% resulted in a 4% reduction in bending resistance with a 2% axial load, while a ratio of 2.5% led to a 7% decrease; conversely, a lower ratio of 1.7% showed a 2% increase in bending resistance under the same load. An analytical model incorporating GFRP bar compression contributions was developed for cross-sectional analysis. It was verified against experimental and literature data, to conduct parametric studies on the impact of reinforcement ratio, concrete strength, GFRP modulus, and strength on the interaction diagram shape under low axial loads. The results demonstrated that there are two major cases of interaction diagram slope in the proximity of the pure moment axis. At higher reinforcement ratios, the moment resistance diminishes at low axial loads when compared with pure moment conditions. However, more research is needed to verify the repeatability of the test results and draw conclusive empirical evidence.
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Data Availability Statement
The data or codes that support the findings of this study are available from the corresponding author upon reasonable request.
Notation
The following symbols are used in this paper:
- Af
- longitudinal reinforcement total cross-sectional area, mm2;
- Ag
- beam–column cross-sectional area, mm2;
- b
- width of beam–column cross section, mm;
- Cc
- total concrete compressive force of beam section, kN;
- c
- neutral axis of the section, mm;
- cp
- center of plastic of the section, mm;
- d
- beam–column effective depth, mm;
- di
- depth of each reinforcement layer, mm;
- Ec
- modulus of elasticity of concrete, MPa;
- Ef
- modulus of elasticity of GFRP, MPa;
- Efc
- modulus of elasticity of GFRP in compression, MPa;
- Eft
- modulus of elasticity of GFRP in tension, MPa;
- e
- eccentricity from section’s center of plastic, mm;
- eN.A
- load eccentricity from the neutral axis, mm;
- concrete compressive strength, MPa;
- fcj
- concrete stress corresponding to axial strain of concrete fiber, MPa;
- fcr
- cracking strength of concrete, MPa;
- ffcu
- compressive strength of GFRP bars, MPa;
- ffi
- axial stress of GFRP, MPa;
- fftu
- tensile strength of GFRP bars, MPa;
- h
- height of beam–column cross section, mm;
- Mc
- total bending moment due to axial force in concrete fiber, kN · m;
- Mf
- total bending moment due to axial forces in the GFRP bars, kN · m;
- Mn
- total bending moment, kN · m;
- nf
- ratio of modulus of elasticity of FRP bars to modulus of elasticity of concrete (−);
- Pn
- total compressive force of beam–column section, kN;
- sd
- distance between the strain gauges in the top and bottom reinforcements, mm;
- y
- distance from neutral axis in cross-sectional analysis, mm;
- β1
- factor for equivalent stress block depth (−);
- Δy
- thickness of concrete segment, mm;
- ɛcu
- ultimate concrete strain, μm/m;
- ɛfc
- average compressive strain of top GFRP bar in the section, μm/m;
- ɛft
- average tensile strain of bottom GFRP bar in the section, μm/m;
- ρf
- FRP reinforcement ratio (−);
- φ
- resistance factor (−); and
- ψm
- curvature of the beam–column at midspan, m−1.
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Published In
Journal of Composites for Construction
Volume 28 • Issue 4 • August 2024
Copyright
© 2024 American Society of Civil Engineers.
History
Received: Dec 4, 2023
Accepted: Mar 22, 2024
Published online: May 17, 2024
Published in print: Aug 1, 2024
Discussion open until: Oct 17, 2024
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