Behavior of Short and Slender RC Columns with BFRP Bars under Axial and Flexural Loads: Experimental and Analytical Investigation
Publication: Journal of Composites for Construction
Volume 28, Issue 1
Abstract
The recently published ACI 440.11-22 code permits the use of glass fiber–reinforced polymer (GFRP) bars as compression reinforcement in concrete columns subjected to combined axial and flexural loads. However, owing to a lack of experimental investigations, there are no current codes specifically for basalt fiber–reinforced polymer (BFRP) reinforced concrete structures. In this study, the behavior of short and slender concrete columns under axial and flexural loads was investigated. Twelve concrete columns were constructed and tested. Test variables were column slenderness ratio (short versus slender columns), eccentricity-to-depth ratio, and reinforcement type (BFRP versus steel). The test results showed the effectiveness of BFRP bars as internal reinforcement in short and slender concrete columns subjected to concentric and eccentric loadings. To generate axial load–moment interaction diagrams, the columns were tested under four different levels of load eccentricity-to-depth ratios of 0, 22.2%, 44.4%, and 66.7%. Regardless of the reinforcement type or slenderness ratio, columns tested under concentric or small eccentricity loadings exhibited a brittle compression-controlled material failure mode, while that tested under high eccentricity loads showed a tension-controlled failure mode. The developed second-order moment is higher in BFRP RC columns than in steel RC columns, this can be attributed to the lower modulus of elasticity of BFRP bars. Subsequently, the research program was expanded to develop an analytical model considering the second-order effects, and then the predicted axial load–moment interaction diagrams by the developed model were verified against experimental test results in the current study and available tested columns in the literature, the verification proved the accuracy of the new model. The findings of this study are an important step toward establishing code guidelines for employing BFRP bars and ties as internal reinforcement in concrete columns.
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Data Availability Statement
All data, models, and code generated or used during the study appear in the published article.
Notation
The following symbols are used in this paper:
- Ag
- gross area of column cross section (mm2);
- c
- distance from outermost compression fiber to the neutral axis of the section (mm);
- c/d
- neutral-axis-to-depth ratio;
- D
- diameter of circular concrete columns (mm);
- d
- distance from the outermost compression fiber to the centroid of tension reinforcement (mm);
- e
- load eccentricity (mm);
- e/h
- load eccentricity-to-width ratio (%);
- concrete compressive strength (MPa);
- fcu
- concrete compressive strength corresponding to the maximum usable concrete strain at extreme compression layer (MPa);
- ffu
- ultimate tensile strength BFRP bars (MPa);
- fy
- yield tensile strength of steel (MPa);
- h
- total width of the columns (mm);
- Kn
- normalized axial load;
- kl
- effective length of the column;
- l
- column length (mm);
- M1st
- first-order moments due to initial applied eccentricity;
- M2nd
- moments due to the second-order effects;
- Mmax
- bending moment corresponding to the Pmax. (kN·m);
- Mn
- nominal bending moment capacity (kN·m);
- Pfailure
- column load-carrying capacity at failure (kN);
- Pmax.
- maximum load-carrying capacity of columns;
- Pn
- nominal axial load-carrying capacity (kN);
- Rn
- normalized bending moment;
- r
- radius of gyration of the cross section about the weak axis;
- t
- strip thickness;
- Δaxial
- axial displacements at peak load;
- Δlat.
- lateral displacements at peak load;
- strains in longitudinal bars on the left side (tension) at peak load;
- strains in longitudinal bars on the right side (compression) at peak load;
- strains in concrete on the left side (tension) at peak load;
- strains in concrete on the right side (compression) at peak load;
- concrete strain corresponding to the maximum compressive strength;
- maximum usable concrete strain at extreme compression layer;
- ultimate tensile strain of FRP bars;
- strain in ties at peak load;
- ɛy
- yield tensile strain of steel bars;
- µ
- ductility;
- ρ
- longitudinal reinforcement ratio; and
- ψc
- column curvature.
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Journal of Composites for Construction
Volume 28 • Issue 1 • February 2024
Copyright
© 2023 American Society of Civil Engineers.
History
Received: Jul 18, 2023
Accepted: Nov 4, 2023
Published online: Dec 5, 2023
Published in print: Feb 1, 2024
Discussion open until: May 5, 2024
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