Newswise — Experimental Behavior of Glass Fiber-Reinforced PolymerReinforced Concrete Columns under Lateral Cyclic Load
American Concrete Institute ACI Structural Journal March 2018
by Mohammed G. Elshamandy, Ahmed Sabry Farghaly, and Brahim Benmokrane, Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Quebec, Canada
In practice, column figures are among the structural elements that can be exposed to severe environmental conditions (Fig. 1). These elements are exposed to aggressive environments in northern climates, which can cause the steel bars to corrode. Researchers, therefore, have examined the use of fiber-reinforced polymer (FRP) instead of steel as internal reinforcement in columns. Fiber-reinforced polymer bars show linear stress–strain behavior up to failure without any ductility, which differs from steel bars.
Feasibility study of FRP
Due to lack of experimental data, ACI 440.1R (2015) design guidelines do not recommend the use of FRP bars as longitudinal reinforcement in compression members. CSA S806 (2012), however, states that the compressive contribution of FRP longitudinal reinforcement is negligible. An extensive experimental study was conducted in the Department of Civil Engineering at the University of Sherbrooke in Quebec, Canada, to examine the applicability of using FRP bars in reinforcing lateral resisting systems. The previously tested FRP-reinforced shear walls at the University of Sherbrooke had showed adequate stiffness and acceptable levels of dissipated energy and deformability until ultimate capacity for resisting lateral loads induced by wind or earthquakes. Therefore, the main objective of this study was to demonstrate the feasibility of using glass-FRP (GFRP) bars as longitudinal and transverse reinforcement in concrete columns subjected to combined axial and cyclic lateral loads. The objective relied on a comprehensive experimental program involving full-scale GFRP-reinforced columns with different detailing configurations, longitudinal reinforcement ratios, transverse volumetric ratios, and axial load ratio (Figs. 2 and 3).
The response was essentially linear–elastic for all tested columns up to development of the first crack. The GFRP-reinforced columns exhibited lower initial stiffness than the steel-reinforced ones. The behavior of all columns was dominated by flexural response. By applying further cyclic load, vertical splitting cracks typically appeared in the columns at the compressed side of the steel- and GFRP-reinforced columns. Longitudinal bars in the steel-reinforced columns buckled during the displacement cycle before the concrete cover spalled. With further cyclic loading, excessive steel-bar buckling was observed until the axial load was lost due to the concrete core crushing (Fig. 4).
To the contrary, the GFRP bars kept their integrity with no observed degradation until one or two loading cycles before failure. The interlaminar degradation of the compressed longitudinal GFRP bars (Fig. 5) occurred at a various drift levels with a minimum drift reaching more than 3.7 percent for all the GFRP-reinforced columns, which is higher than the 2.5 percent drift recommended by the National Building Code of Canada (NBCC 2010) and CSA S806 (2012). In contrast, the steel bars lost their integrity at early drift levels of approximately 2.4 percent. When the displacement increased, all the columns lost the axial load due to the concrete core crushing. Failure of the GFRP-reinforced columns was associated with fracturing of compressed longitudinal GFRP bars and rupture of GFRP rectilinear spirals and ties.
Increasing the axial load ratio was found to result in faster deterioration of the concrete core represented by the larger plastic hinge and reduced ductility capacity of the columns. Increasing the transverse-reinforcement ratio by decreasing the spacing significantly enhanced the ductility and yielded higher strength. Closer transverse-reinforcement spacing resulted in better confinement of the concrete core and delayed deterioration of either the longitudinal reinforcement or the concrete core. This could indicate that the maximum spacing requirement by CSA S806-12 is restrictive. Maximum spacing for the specimens is controlled by 6 db, where db is the longitudinal bar diameter.
Clause 12.7 in CSA S806 (2012) gives complete detailing and limitations for designing lateral-resisting systems reinforced solely with FRP bars. This information was examined based on outcomes of the GFRP-reinforced columns tested in this study. The required area (Ash) of the rectilinear spirals and crossties provided in the tested GFRP-reinforced columns was calculated as shown in Fig 0.
All the tested GFRP-reinforced columns were designed using this equation to achieve either 2.5 percent or 4 percent drift. Generally, all the columns achieved much higher drift than the estimated values, confirming the effectiveness of using GFRP bars in lateral-resisting systems.
Study results
According to Ahmed Sabry Farghaly, PhD, postdoctoral researcher in the Department of Civil Engineering at the University of Sherbrooke, “The design stress level in FRP transverse reinforcement (fFh) is limited to the least stress corresponding to a strain of 0.006, or the stress corresponding to the failure of the rectilinear spirals or crossties.” This statement was based on the experimental results of his study on the concentrically-loaded GFRP-reinforced concrete columns. The strain limitation (0.006) is usually the predominant parameter in defining stress level due to the high tensile strength of FRP. The strain values in rectilinear spirals and crossties were less than the strain limit of 0.006, confirming that the use of GFRP transverse reinforcement based on the equation effectively confined the concrete core in the post-peak stages.
Moreover, it was found that, the longitudinal reinforcement ratio has a clear effect on estimating the transverse reinforcement ratio, which is not included in the equation. Therefore, it is of significance to include the longitudinal reinforcement ratio effect in calculating the required Ash, either as bar diameter or area, as well as the number of bars. Moreover, this point confirms the capability of using GFRP bars to carry axial load combined with lateral load.
Through this research, the effect of the different parameters on the performance of GFRP-reinforced concrete columns under simulated earthquake loading was assessed based on the experimental results of the conducted experimental program. According to Brahim Benmokrane, a professor of civil engineering at University of Sherbrooke and Tier-1 Canada Research Chair in Advanced Composite Materials for Civil Structures and NSERC Chair in FRP Reinforcement for Concrete Structures, “The GFRP-reinforced columns could attain acceptable strength and deformation capacity, and, therefore, GFRP reinforcement could be used in lateral resisting systems, although further research is needed to implement adequate design guidelines and recommendations for such structural elements.”
The research can be found in a paper titled, “Experimental Behavior of GFRP-Reinforced Concrete Columns under Lateral Cyclic Load,” published in ACI Structural Journal, March 2018.
RSS