FRP-confined concrete core-encased rebar for RC columns: Concept and axial compressive behavior
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.
Section snippets
Concept and configuration of FCCC-R
In this study, a new concept of FRP-confined concrete core-encased rebar (FCCC-R) and the corresponding composite column is proposed, as shown in Fig. 2(a). Many studies have revealed that the slender profile steel members [23], [24], [25], [26] and FRP bars [27] inserted in FRP tube confined concrete generally illustrated a significant compressive behavior enhancement. In this paper, a similar concept was adopted and used for restraining the longitudinal rebar in RC columns from buckling.
For a
Specimen details
Fig. 3 illustrates the configuration of the FCCC-R specimen. For all hybrid bars, an internal steel bar was placed in an FRP tube, and mortar was filled between the FRP tube and the steel bars. The length of the FRP tube was defined as the restrained length Lr. A 20 mm length of the steel bars remained outside of the FRP tube at both ends for all the FCCC-R specimens to guarantee that the axial load was applied only on the internal steel bars. Screwed thread with a length of 60 mm was
Failure mode and test observations
Fig. 5(a) shows the overall failure patterns of all the FCCC-R specimens tested. All the specimens failed in a similar pattern, with a typical buckling shape and several horizontal cracks in the FRP tube.
Axial load-displacement curve of group D40/22-400-C2 under compression is shown in Fig. 6. At the initial stage of the test, the different components of the hybrid bar generally remain elastic, and the relationship between axial load and displacement remains linear. With the increase of the
Numerical simulation and analysis
The finite element (FE) model of FCCC-R specimens was developed using the commercial finite element software ABAQUS and was calibrated by the test results. An additional parametric study was performed with a wider range of parameter values. In addition, influence of outside concrete on the compressive behavior of FCCC-R in RC columns was also assessed and discussed based on FE simulation. Finally, the relationship between the required area ratio and slenderness ratio for the FCCC-R was
Conclusion
A new concept and configuration of FRP-confined concrete core-encased rebar (FCCC-R) and a corresponding hybrid RC column were proposed in this paper. Performance of FCCC-R under axial compression with different parameters, including slenderness ratio, FRP tube diameter, mortar strength, and bar position, was studied experimentally. FE simulation was then performed and calibrated by the test results. Based on the FE model, axial performance of FCCC-R in RC columns was discussed. An additional
Acknowledgments
This research was supported by the National Key Program of China (2017YFC0703005) and the National Natural Science Foundation of China (No. 51661165016, No. 51522807 and No. 51778330)
References (34)
Fundamental principles of the reinforced concrete design code changes in Chile following the Mw 8.8 earthquake in 2010
Eng Struct
(2013)- et al.
Influence of reinforcement buckling on the seismic performance of reinforced concrete columns
Eng Struct
(2015) - et al.
Effects of the compressive reinforcement buckling on the ductility of RC beams in bending
Eng Struct
(2012) - et al.
Enhancement of seismic performance of reinforced concrete columns with buckling-restrained reinforcement
Eng Struct
(2011) - et al.
Modeling of reinforcement global buckling in RC elements
Eng Struct
(2014) - et al.
Cyclic stress-strain model incorporating buckling effect for steel reinforcing bars embedded in FRP-confined concrete
Compos Struct
(2017) - et al.
Buckling modeling of reinforcing bars with imperfections
Eng Struct
(2009) - et al.
New configuration of transverse reinforcement for improved seismic resistance of rectangular RC columns: concept and axial compressive behavior
Eng Struct
(2016) - et al.
Required tie spacing to prevent inelastic local buckling of longitudinal reinforcements in RC and FRC elements
Eng Struct
(2018) - et al.
Buckling of piecewise member composed of steel and high-strength materials in axial compression
Thin-Wall Struct
(2017)