FRP-confined concrete core-encased rebar for RC columns: Concept and axial compressive behavior

Abstract

In the ultimate failure mode of columns in RC structures under seismic load, the longitudinal reinforcement generally suffers from severe buckling due to the axial compressive load, spalling of concrete cover and dilation of concrete core. Proper configuration of RC columns to postpone or prevent the buckling phenomenon can enhance the seismic performance and make full use of the strength of steel reinforcement. In this paper, a new concept and configuration of FRP-confined concrete core-encased rebar (FCCC-R) for RC columns is proposed. The concrete surrounded the longitudinal rebar is confined by an FRP tube, which can prevent the rebar from premature buckling. The axial compressive behavior of the hybrid bar considering different parameters, including the slenderness ratio, FRP tube diameter, mortar strength, and bar position was studied experimentally. Furthermore, finite element (FE) simulation was carried out and calibrated by test results. Based on the FE model, additional discussion and a parametric study was performed. Finally, the minimum FRP tube diameter required for the FCCC-R considering the compressive behavior enhancement effect was calculated, and design curves for 3 types of steel bars with nominal yield strengths of 400 MPa, 800 MPa, and 1100 MPa were proposed.

Introduction

Buckling of longitudinal rebar is a common phenomenon in the ultimate failure mode of RC columns during earthquakes, as shown in Fig. 1(a). For the RC column with either insufficient lateral reinforcement or large axial compression ratio, the longitudinal rebar is most likely to buckle in the plastic hinge region due to the spalling of concrete cover and the dilation of concrete core under cyclic lateral load (see Fig. 1(b)) [1].

On the one hand, the collapse of RC structural columns subjected to seismic loading is linked to the instability of the longitudinal bars, which experience excessive cyclic deformation after buckling. Early fracture of the steel bars is likely to occur due to the low-cycle fatigue, resulting in the loss of the lateral load capacity and the final collapse of the RC column. Besides, buckling of the longitudinal rebar in RC columns also causes the rapid loss of steel strength under compression, which has a significant influence on the column’s seismic performance, including ductility, energy dissipation capacity and stiffness degeneration progress [2], [3], [4], [5], [6]. Thus, traditional stress-strain relationship under tension should be modified to be adopted in seismic design or analysis of RC structures [7], [8], [9], [10], [11].

On the other hand, buckling behavior also brings some disadvantages to the utilization of high strength steel bars. At a high compressive strain levels over 0.002 in RC columns, spalling and crushing of the concrete surrounded the longitudinal bar occurs, while the longitudinal bar with a yield strength above 400 MPa still remains within the elastic stage. The steel bar without the support of outside concrete is most likely to buckle without reaching its yield strength and can no longer sustain the axial compressive load (see Fig. 1(b)). In this way, design strength of longitudinal bars might be limited in a certain range due to this effect. And the utilization of high-strength steel bar as longitudinal rebar in RC columns is unnecessary. In addition, when adopting high-strength steel bars, smaller bar diameter and larger hoop spacing can also exacerbate the buckling phenomenon of longitudinal reinforcement, as suggested by Su et al. [4]. Besides, the initial imperfection of steel bars may also cause premature buckling before the yield strength [12].

To prevent the premature buckling of the longitudinal reinforcement, many approaches have been proposed, among which the proper arrangement of adequate transverse reinforcement was widely studied [13], [14], [15], [16] and accepted by the design codes for concrete structures [17], [18], [19], [20]. However, additional transverse reinforcement can make the steel reinforcement too crowded and inconvenient for construction in the joint area and plastic hinge area. Yielding and opening of the hoops will also cause the loss of this confinement effect. Moreover, a hybrid bar configuration called a rebar-restraining collar (RRC) was developed to prevent the buckling of longitudinal rebar. The behavior of an RRC with different thicknesses of steel tubes and lengths of internal bars was evaluated and showed an improvement in postbuckling behavior [21]. Experimental studies on the seismic performance of RC columns retrofitted with an RRC have also been carried out [6], [21], [22]. However, this device shows an obvious enhancement effect for columns with slender longitudinal rebar but a limited effect for normal RC columns.