Experimental investigation on axial compressive behavior of fiber reinforced polymer-reinforced concrete columns confined with external fiber reinforced polymer jackets

Abstract

An innovative glass fiber reinforced polymer (GFRP) closed-type winding (GFRP-CW) tie was developed to eliminate the bond slip failure and make full use of the tensile strength of ties compared with conventional pultruded fiber reinforced polymer (FRP) rod ties. Although better confinement effect of GFRP-CW ties, however after spalling of concrete cover, the compressive longitudinal FRP bars in the plastic hinge regions of columns are most likely to crush or buckle. External FRP jackets can effectively restraint damage to concrete cover. Against this background, a novel FRP-reinforced concrete column confined with external FRP jackets and the internal GFRP-CW ties were proposed to prevent the FRP bars from premature buckling or crushing. In this article, twelve square new columns were constructed and tested to characterize the axial compressive behavior. The test parameters included FRP wrapping type (GFRP or carbon fiber reinforced polymer (CFRP)), FRP wrapping layers, and spacing of ties. Test results confirmed that FRP-reinforced concrete columns with external FRP jackets had significantly larger ductile behavior and exhibited higher load-carrying capacity than their counterparts FRP-reinforced concrete columns due to the contribution of longitudinal GFRP bars and the concrete cover. The test results also suggested reasonable spacing of ties and layers of GFRP jackets for an expected moderate confinement behavior.

Keywords

concrete columns innovative glass fiber reinforced polymer closed-type winding ties longitudinal glass fiber reinforced polymer bars external fiber reinforced polymer jackets axial compression

Introduction

Corrosion of steel reinforcement is a common phenomenon in concrete structures under harsh environmental conditions during the serviceable design life. Therefore, the use of fiber reinforced polymer (FRP) bars and ties as a replacement of conventional steel reinforcement in the new construction of concrete structures has got more and more attentions due to their good corrosion resistance and high tensile strength. During the past two decades, the steel longitudinal reinforcement was first replaced by different types of FRP pultruded bars (Dong et al., 2019; Habeeb and Ashour, 2008; Tomlinson and Fam, 2015; Teng et al., 2018). Then, the pultruded FRP ties as lateral reinforcement were fabricated from various shapes like U, L, and C types to improve the confinement efficiency of the core concrete and restrain the longitudinal bars from bucking. Whereas, the pultruded FRP ties generally have two shortcomings: (1) the kinking of innermost fibers during the bending polymerized fabrication process cause the tensile strength of the bent part significantly less than that of the straight part (Ahmed et al., 2010; Lee et al., 2014; Spadea et al., 2017); (2) the premature bond slip failure at the overlapping region of FRP ties result in low confinement efficiency when the splice length is insufficient or the crush of concrete cover starts. (Mohamed et al., 2014).

Dong et al. (2018), the same research group of the authors, proposed an innovative rectangular cross-sectional glass fiber reinforced polymer closed-type winding (GFRP-CW) tie to overcome the aforementioned shortcomings of conventional pultruded FRP ties. The GFRP-CW ties were made by cutting the tube manufactured in a fully automated process of continuous filament winding. Axial compression tests conducted on the GFRP-CW ties reinforced concrete columns with steel longitudinal bars revealed that columns reinforced with GFRP-CW ties exhibited significant improvement in ductility as compared to the columns reinforced with conventional pultruded ties; besides, the bond slip failure was not observed.

The above forms of GFRP-CW ties, despite improving in both load-carrying capacity and ductility, still have some limitations. On the one hand, the unconfined concrete cover of the FRP-reinforced concrete columns might be damaged seriously in the plastic hinge area under the seismic load. (Ali and El-Salakawy, 2016; Deng et al., 2018; Dong, 2020; Elshamandy et al., 2018). The compressive longitudinal FRP bars in the plastic hinge regions are most likely to crush and buckle after concrete cover spalling, which has a significant influence on the seismic performance and post-earthquake reparability of columns. On the other hand, when the FRP-reinforced concrete columns reached high drift levels, the bond performance degradation between FRP bars and damaged concrete cover is severely dramatic, which reduce the contribution of FRP bars to improve post-yield stiffness and decrease residual displacement (Abdallah and El-Salakawy, 2021; Ali and El-Salakawy, 2016; Dong, 2020). Many studies have revealed that RC columns confined with external FRP jackets generally displayed a significant enhancement on compressive, bond, and ductile behavior (Hamad et al., 2004; Lam and Teng, 2003; Teng et al., 2009; Wang et al., 2012, 2017). Ibrahim et al. (2016) proposed an FRP-steel reinforced concrete system with external FRP jackets to realize a stable post-yield stiffness and enhance the displacement ductility, and Ding et al. (2013) used near surface mounted technique with external FRP jackets to enhance the overall seismic performance of RC columns.

In this study, a similar concept is proposed that FRP-reinforced concrete columns confined with external FRP jackets were only provided in the potential plastic hinge regions of the columns, as shown in Figure 1. This configuration of FRP-reinforced concrete columns confined with external FRP jackets aimed to enhance the load capacity and displacement ductility due to avoiding the concrete cover spalling and retard the crushing or buckling behavior of the longitudinal FRP bars. This article presents the experimental study on the FRP-reinforced concrete columns confined with external FRP jackets under axial compression load. The effect of confining reinforcement material type (GFRP and carbon fiber reinforced polymer (CFRP)), FRP jacket layers, and spacing of GFRP-CW ties are discussed in detail.

Figure 1.

Concept and structure compositions: (a) FRP-reinforced concrete column; (b) FRP-reinforced concrete column confined with external FRP jackets in plastic region.

Experimental program

Sample preparation

The presented experimental program is part of a series concerning novel FRP ties reinforced concrete columns in Harbin Institute of Technology (Dong et al., 2018; Tahir et al., 2019). A total of 12 specimens (300 × 300 × 900 mm) were prepared and tested under monotonic axial compression load. All columns were rounded into an identical corner radius of 40 mm. The cross-sectional configurations for these specimens are presented in Figure 2, and the detailed information of the test matrix is listed in Table 1.

Figure 2.

Details of FRP-reinforced concrete specimens (Units: mm): (a) assembled GFRP-CW ties; (b) GFRP cage; and (c) elevation view. GFRP-CW: GFRP closed-type winding.

Table 1.

Specimens’ details.

Specimen	Spacing (mm)	ρ_v (%)	FRP type	FRP jacket layer	ACI 440.2R-2017f_lj ^a/f_co > 0.08	GFRP tief_lt ^b/f_co	GFRP jacket and GFRP tief_lm/f_co
G75-G0	75	1.63	—	0	—	—	—
G75-G3	75	1.63	GFRP	3	0.030	0.118	0.148
G75-G6	75	1.63	GFRP	6	0.061	0.118	0.179
G100-G0	100	1.22	—	0	—	—	—
G100-G3	100	1.22	GFRP	3	0.030	0.079	0.109
G100-G6	100	1.22	GFRP	6	0.061	0.079	0.139
G100-G9	100	1.22	GFRP	9	0.091	0.079	0.170
G150-G0	150	0.81	—	0	—	—	—
G150-G3	150	0.81	GFRP	3	0.030	0.040	0.071
G150-G6	150	0.81	GFRP	6	0.061	0.040	0.101
G150-G9	150	0.81	GFRP	9	0.091	0.040	0.132
G150-C2	150	0.81	CFRP	2	0.058	—	—

^aNo hoop contribution is considered.

^bModel is referenced to Dong (2020).

The columns were longitudinally reinforced with 16-mm-diameter GFRP bars and transversely reinforced with GFRP-CW ties in the form of an inner circular hoop enclosed by the outer square one, which could confine concrete core more effective than those with conventional rectangular inner ties (Dong et al., 2018). The GFRP-CW ties were manufactured by a fully automated process at Harbin FRP Institute. This article does not present a detailed methodology of GFRP-CW ties and readers are referred to the existing publications (Dong et al., 2018).

As for all specimens, three columns without FRP jackets were control specimens to compare behavior with specimens confined with FRP jackets. Eight columns were wrapped with GFRP jackets (3, 6, or 9 layers) and one column was wrapped with CFRP jackets (2 layers) to examine the impact of FRP types and amounts. All columns with external FRP jackets were designed with respect to the confinement requirements lower than the adequate FRP confinement ratio f_lj/f_co equal to 0.08 accord to ACI 440.2R-2017 (f_co, the strength of unconfined concrete; f_lj, lateral confinement pressure provided by FRP jackets), except that FRP-reinforced concrete columns wrapping with nine layers of GFRP sheets. The overlapping zone was 200 mm to avoid FRP debonding of the wrapping end. Both the bottom and top end regions of each specimen were additionally strengthened by two layers of CFRP sheet with a height of 150 mm to ensure failure would occur in the instrumented region. Each specimen was identified with two capital letters and two numbers. The first capital letters G represents the reinforcement material with GFRP and the first number (75, 100, or 150) indicates the center-to-center spacing of GFRP-CW ties. The second capital letter G or C represents the external jacket material (GFRP or CFRP), and the second number is the number of FRP layers.

Material properties

The flat coupons were obtained by cutting the straight portions of the GFRP-CW ties and then tested to failure to determine the material properties according to the ASTM D7205 test method. The columns were cast with ready-mixed concrete by a local supplier. Six 150 × 300 mm standard concrete cylinders were prepared by the same batch of concrete used in the columns and then tested under axial compression at the time of actual test. The material properties are listed in Table 2. GFRP bars, with a rib spacing of 12.8 mm and a rib height of 1 mm, were manufactured and supplied by an international company. The tensile strength of GFRP bars was conducted according to ASTM D7205. However, there exists no standard test method to characterize the properties of GFRP bars in compression. A novel test method, proposed by AlAjarmeh et al. (2019), was imitated and improved by applying pure compression load on the GFRP bar samples. Figure 3(a) shows the schematic diagram of the test samples with the unbraced length of 100 mm. Two thick acrylic plates were drilled with concentric holes of the required diameter to hold the steel pipes (Figure 3(b)); both ends of the GFRP bars were inserted into steel pipes filled with high-stress grout and four ring acrylic plates were used to ensure the GFRP bars under vertical centering. A linear variable differential transducer (LVDT) was attached at the mid-height of the GFRP bars to capture the stress–strain behavior (Figure 3(c)). As shown in Figure 3(d), the crushing failure mode of GFRP bars in compression test was observed; the compressive strength of GFRP bars was only 38% of tensile strength. Also, the elastic modulus of GFRP bars in compression was 33% lower than the tensile elastic modulus.

Table 2.

Material properties.

Material	Diameter (d) or thickness (t) (mm)	Ultimate strength (MPa)	Elastic modulus (GPa)	Ultimate strain (%)
GFRP bar	16	1022 (389)	48.6 (32.4)	2.1 (1.2)
GFRP-CW tie	4 × 14	1096	55	1.99
GFRP sheet	0.169	1500	72	2.1
CFRP sheet	0.167	4340	244	1.78
Concrete	150 × 300	43.8	33.1	—

Note: () –compression properties of GFRP bars.

GFRP-CW: GFRP closed-type winding.

Figure 3.

Material properties of GFRP bars (Units: mm): (a) schematic diagram of the test specimen; (b) fabricating bar samples in acrylic plates; (c) setup; and (d) Stress–strain behavior in tension and compression.

Test setup and instrumentation

The instrumentation layout and test setup are shown in Figure 4. For all the specimens, four GFRP bars were instrumented with electric strain gauges at mid-height, and the GFRP-CW ties in the same level were also plastered with six strain gauges, two at straight parts and two at the bends of the out square tie, the rest at the inner circular tie. For specimens with external FRP jackets, a total 22 strain gauges were installed to monitor strains in the mid-height level of the FRP jacket, four of them are arranged vertically, as shown in Figure 4(b). Four LVDTs were attached on four corners of the aluminum frame at a gauge length of 300 mm to measure the axial strain of columns (i.e. LVDT1 ∼4 in Figure 4(a)); the other two LVDTs were diagonally mounted between the machine platens to monitor the full-height axial shortening (i.e. LVDT5 ∼6 in Figure 4(a)).

Figure 4.

(a) Test setup; (b) location of strain gauges.

The columns were tested under axial compression on a 10,000 kN hydraulic servo pressure machine in Harbin Institute of Technology. The specimens were carefully physically centered and then preloaded with an axial load 500 kN and then up to 1000 kN; the mechanical-centered specimens were obtained when the standard deviation of four displacement increments of LVDT1 ∼4 was less than 0.01 mm. After the preloaded test, a displacement-controlled rate of 0.5 mm/min was adopted until failure of the test specimens due to FRP jacket rupture or a large drop in load-carrying capacity.

Test result and discussion

General behavior and failure modes

The typical failure modes for each specimen are listed in Figures 5–8. For the FRP-reinforced concrete columns without external FRP jackets, the failure started with the cracking of corner concrete cover near the region of mid-height of the column. The extended spalling of the concrete cover was presented after the peak load was attained and rapidly developed until the end of the tests. Afterward, the crushing concrete cover was removed; the rupture of GFRP-CW ties was detected due to the core concrete dilation. The GFRP longitudinal bars suffered from premature crushing or a combination of crushing and buckling due to concrete cover spalling, as shown in Figure 5.

Figure 5.

Appearance of FRP-reinforced concrete columns after failure.

Figure 6.

Damage process of external FRP jackets (G75-G6).

Figure 7.

Appearance of FRP-reinforced concrete columns with external FRP jackets after failure.

Figure 8.

The enlarged view of failure modes of FRP-reinforced concrete columns confined with external FRP jackets.

The specimens with external FRP jackets failed when the load capacity dropped suddenly with the rupture of FRP jackets. The progress of damage of the specimens was generally similar to that of the RC column with FRP strengthening (Triantafyllou et al., 2015; Wang et al., 2012), as shown in Figure 6. Some debonding sounds of epoxy were first heard, followed by a local plasticization of the FRP jacket within the mid-height region. The local bulging of the FRP jacket occurred with the increasing axial deformation. Finally, as shown in Figure 7, the fracture of the FRP jacket initiated at or near the rounded corner which followed the bulged FRP region, and then the explosive sound of FRP rupture indicated the end of the test at the significantly larger axial deformation than that of the FRP-reinforced concrete columns without FRP jackets. This could be attributed to the dual confinement action of the GFRP-CW ties and the external FRP jackets. Also, removal of the ruptured FRP jacket and the crushing concrete cover after the test revealed that the columns with external FRP jackets had more severe rupture of GFRP-CW ties, especially in the inner circular ties, and the crushing of longitudinal bars than that of the counterparts FRP-reinforced concrete columns (Figure 8).

Axial load–strain responses

The key test results are listed in Table 3. In this table, P_max and ε_c1 are defined as the peak load and the corresponding strain, respectively. However, for the specimen G75-G6, which experienced a descending post-peak and then a second ascending part. P_max and ε_c1 are defined as the first peak load and corresponding strain, respectively. ε_bar1 is defined as the axial strain of longitudinal GFRP bars corresponding to the peak load, respectively. P_bar1 was the total bearing capacity of longitudinal GFRP bars corresponding to P_max. A linear elastic stress–strain behavior mentioned above was assumed for the longitudinal GFRP bars. ε_0.01 and P_0.01 were the axial strain of longitudinal GFRP bar and the load corresponding to the axial strain of 1%, respectively. ε_c2 was axial strain corresponding to 85% of P_max or FRP jacket fracture. ε_st and ε_sj were the rupture strain of the GFRP-CW tie and the FRP jacket. μ = ε_c2/ε_c1 was the displacement ductility ratio.

Table 3.

Key test results.

Specimen	P_max (kN)	ε_c1 (με)	ε_bar1 (με)	P_bar1/P_max (%)	ε_0.01 (με)	P_0.01 (kN)	ε_c2 (με)	ε _st	ε _sj	μ
G75-G0	4708	2375	3441	3.8	—	—	6142	4139	—	2.6
G75-G3	5407	2791	3819	3.7	7382	4929	14,220	5230	7217	5.1
G75-G6	5245	3300	3420	3.4	7816	5153	29,700	8925	7864	9.0
G100-G0	4456	2286	4310	5.0	—	—	5600	7827	—	2.4
G100-G3	4934	2092	2654	2.8	5846	4196	10,350	6431	7397	4.9
G100-G6	5271	2683	4343	4.3	7631	5020	14,380	4997	5122	5.4
G100-G9	5412	4211	5331	5.1	13,675	5312	27,744	7030	7553	6.6
G150-G0	3487	1950	2687	4.0	—	—	4433	5980	—	2.3
G150-G3	4402	2125	3849	4.6	4898	—	8175	6900	4093	3.8
G150-G6	4759	4550	6548	7.2	3475	4671	25,525	8783	6590	5.6
G150-G9	5344	3100	3843	3.7	6617.0	4972	24,300	8771	8094	7.8
G150-C2	5704	2058	3292	3.0	8527.0	4386	5217	7978	7479	2.5
Average	—	2793	3961	4.2	—	—	—	—	—	4.8

The comparisons of the axial load–strain curves of each specimen with different section configurations are shown in Figure 9, where the axial strain was the average of the strains recorded by four LVDTs (i.e. LVDT1 ∼4 in Figure 4). In general, the initial ascending part of the specimens was identical until the load corresponding to the unconfined concrete strength. Afterward, the FRP-reinforced concrete columns exhibited a descending part due to spalling of concrete cover. However, the FRP-reinforced concrete columns confined with external FRP jackets presented a gentle drop and then slightly fluctuated tendency with the increase of axial strain. The axial load–strain curves displayed some intermittent and sudden drops of load capacity owing to the crushing of the longitudinal GFRP bars or the rupture of GFRP-CW ties.

Figure 9.

Axial load–axial strain curves of all specimens.

FRP-reinforced concrete columns without external FRP jackets.

The spacing or volumetric ratio of ties has a significant effect on the behavior of the traditional steel ties columns and conventional pultruded FRP rod ties columns (Afifi et al., 2013; De Luca et al., 2010; Karim et al., 2016; Tobbi et al., 2014; Tu et al., 2019). The only difference of FRP-reinforced concrete columns (G75-G0, G100-G0, and G150-G0) was the spacing of the GFRP-CW ties, which is 75 mm, 100 mm, and 150 mm. As for 75 mm spacing, it is the maximum spacing according to CSA(2012) (one-quarter of the minimum cross dimension). Their axial load–axial strain curves are compared in Figure 10 to consider the effect of tie spacings.

Figure 10.

Axial load–axial strain curves of FRP-reinforced concrete columns.

Generally, the test results showed a fast rate of load decay after the spalling of concrete cover when the spacing of ties was equal to the maximum spacing limit (75 mm) or exceeding this limit (100 mm or 150 mm), and decreasing the spacing of GFRP-CW ties increased the load peak. Table 2 shows a slight decrease of peak load from 4708 kN to 4456 kN with increased tie spacing from 75 mm to 100 mm. The specimen with the largest tie spacing (150 mm) showed a noticeable decrease to 3487 kN, this is mainly attributed to the fact that a combination of crushing and buckling failure mode of longitudinal GFRP bars occurred after spalling of concrete cover. AlAjarmeh et al. (2019) state that the compressive GFRP bars will fail by a combination of crushing and buckling when the unbraced length is more than 8 times of the diameter of GFRP bars (8 × 16 = 128 mm). Therefore, the spacing of the GFRP-CW ties can properly widen from 75 mm to 100 mm outside the plastic hinge region; additional tests are also required to consider the effect of tie spacing. As expected, the ultimate axial strain of FRP-reinforced concrete columns ranged from 0.657% to 1.157%, which was less than the ultimate compressive strain of longitudinal GFRP bar of the coupon test (1.2%), resulting in low efficiency in utilizing the full available strength of longitudinal GFRP bar.

FRP-reinforced concrete columns confined with external FRP jackets

Existing studies have shown conclusively that the behavior of RC columns with external FRP jackets depends on the amount and types of wrapping FRP jackets (Ilki et al., 2008; Spoelstra and Monti, 1999; Wang et al., 2012). For FRP-reinforced concrete columns with different external FRP jackets, the purpose of these comparisons is to identify, mainly from a qualitative standpoint, the main aspects of the confinement action mechanisms and to compare the relative effectiveness of the two composite materials (CFRP and GFRP). This effect can be examined using the test results in the present study with a similar confinement ratio as presented in Figure 11(a) and Table 3. It is evident that the G150-G6 specimen maintains stability at peak load until the rupture of the GFRP jacket, and significantly surpassed the 1% limit of usable strain, which was introduced by the Concrete Society (2012) and the ACI (2017) to prevent excessive cracking and the resulting loss of concrete integrity. Under the level of strain, the strength of concrete was sufficiently developed and provided to resist the axial and bending forces or/and shear (Triantafyllou et al., 2015), and brittle failure mode was avoided. The axial load–strain curve of the G150-C2 specimen decays on a fast softening branch after reaching the peak load; the load corresponding to the 1% limit of usable strain was only 77% of the peak load. G150-G6 specimen shows almost 2.5 times the ultimate axial strain of the G150-C2 specimen, although the peak load of the latter is roughly 20% larger. This result is in accordance with the finding by Spoelstra and Monti (1999) for the RC columns confined with GFRP and CFRP jackets.

Figure 11.

Load versus axial strain behavior of columns: (a) confining reinforcement material types; (b)–(d) layers of FRP jackets.

Figure 11(b)–(d) show the axial load–axial strain behavior of the FRP-reinforced concrete columns confined with external FRP jackets and their counterpart columns (G75-G0, G100-G0, and G150-G0) in order to investigate the effect of the different number of layers on the compressive performance. The test results indicated that the columns with external GFRP jackets showed significantly larger ductile behavior and exhibited higher load-carrying capacity than their counterparts. For instance, the G75-G6 specimen had an ultimate axial strain up to 0.0297 compared to G75-G0 with an extremely low ultimate axial strain value of 0.0084. The axial ductility of specimens G75-G6, G100-G6, and G150-G6 were 3.5, 2.2, and 2.5 times that of specimens G75-G0, G100-G0, and G150-G0, respectively.

Besides, the peak load of G75-G6, G100-G6, and G150-G6 was increased by 11%, 18%, and 36%, respectively, compared to specimens G75-G0, G100-G0, and G150-G0. When all other parameters are the same, the ductility and peak load of FRP-reinforced concrete columns confined with external GFRP jackets increase as the number of GFRP layers increases. Furthermore, the ultimate axial strain of FRP-reinforced concrete columns with external GFRP jackets, except the G100-G3 and G150-G3 specimens, significantly surpassed the 1% limit of usable strain. It is evident from Figure 11(b)–(d) that the curves of the columns confined with external GFRP jackets are similar under the usable strain. These axial load–strain curves of FRP-reinforced concrete columns confined with external GFRP jackets fluctuated slightly from the peak load to the P_0.01, which were ranged between 85% and 95% of the peak load (Table 3). It is also evident that these specimens maintain a large load-bearing capacity and integrity due to the external GFRP jackets confinement to the inner concrete and longitudinal GFRP bars, despite the axial strain more than 1% limit of strain.

A new effective confinement ratio (f_lm/f_co) is proposed to evaluate the effective confinement of external GFRP jackets and inner GFRP ties, which is similar in form to one proposed by Wang et al. (2012) for RC columns with external CFRP jackets. The f_lm is the combination of the f_lj and the f_lt (f_lm = f_lj + f_lt); the term f_lt is the lateral confinement pressure provided by the GFRP ties referenced to Dong (2020)

f_{l t} = \frac{\sum_{i = 1}^{n} A_{t} E_{t} ε_{fe}}{b_{c} s} k_{es} k_{v}

(1)

where n is the number of ties; A_t is the total cross-sectional area of single rectangular GFRP-CW ties; E_t is the elastic of modulus of GFRP-CW ties; ε_fe is the effective rupture strain of GFRP-CW ties; b_c and s are the side length of concrete core and spacing of GFRP-CW ties, respectively; and k_es and k_v are used to estimate the effective confinement provided by the GFRP-CW ties reinforcement in the lateral and vertical directions, respectively. As depicted in Table 1, the effective confinement ratios ranged among 0.071 and 0.179 for FRP-reinforced concrete columns with external GFRP jackets. In Wang et al. (2012) model, the effective confinement ratios 0.09 ≤ f_lm/f_co ≤ 0.17 were featured as moderate confined. In general, the FRP-reinforced concrete columns with external FRP jackets, with a spacing of 75 mm, possessed an expected moderate confinement behavior with three layers of GFRP jackets. As for the spacing of 100 mm or 150 mm, six layers of GFRP jackets were required.

Figure 12 shows a typical distribution of hoop strain over the perimeter of the GFRP jackets for the selected specimens at mid-height. In general, hoop strain values were small and relatively uniform at low load levels. As the load increased over the peak load, the hoop strains also increased a quite large variation as a result of concrete core dilatation and buckling of longitudinal GFRP bars. Besides, the hoop strains at the rounded corner regions were smaller than those at the middle of the sides. The same phenomenon has also been reported by other experiments about FRP-confined steel hoop columns (Wang et al., 2012).

Figure 12.

Typical hoop strain distribution on GFRP jackets at column mid-height.

Longitudinal GFRP bar strain profiles and strength contribution

To fully understand the compressive behavior of longitudinal GFRP bars, some results of strain gauges applied on the longitudinal GFRP bars versus the total axial compressive strain were plotted in Figure 13.

Figure 13.

Axial strain of longitudinal GFRP bars versus axial strain.

The ultimate axial strain of longitudinal GFRP bars in the FRP-reinforced concrete columns experienced an average of 90% with respect to the crushing strain of the bars from the results of the coupon tests (approximately 12,000 με). The FRP-reinforced concrete columns with external GFRP jackets extended the strain of longitudinal GFRP bars to an average of 104% of the crushing strain of the bars. It shows the effectiveness of the external GFRP jackets to prevent premature crushing or buckling of the GFRP bars and make full use of the compressive strength of the GFRP bars.

The axial strain–strain curves of GFRP bars in specimens (G75-G0, G100-G0, and G150-G0) showed an almost linear ascending branch up to around an axial strain level of 5000 με followed by crushing or buckling of GFRP bars, which indicated the complete spalling of concrete cover. In Figure 13(a), (c), and (e), the average compressive strain of GFRP bars were 12,612 με, 9344 με, and 7055 με, respectively. These results clearly indicated that the longitudinal GFRP bars in the specimen with 75 mm spacing of GFRP-CW ties extended the axial strain slightly higher than the crushing strain of the GFRP bars from the results of the coupon tests with 100 mm unbraced length (approximately 12,000 με). The GFRP bars in the specimens with 100 mm or 150 mm spacing of GFRP-CW ties experienced under the crushing strain of the GFRP bars. It can be concluded that closer spacing of GFRP-CW ties results in a smaller slenderness ratio to prevent local buckling and then improve a strain level a little above the crushing strain of longitudinal GFRP bars.

For FRP-reinforced concrete columns with six layers of GFRP jackets as shown in Figure 13(b), (d), and (f), at the same compressive strain level of 5000 με, the average strain of longitudinal GFRP bars were 10,530 με, 8903 με, and 5367 με, respectively. It can be seen that the strains of FRP bars exhibited lower than that in FRP-reinforced concrete columns without FRP jackets. This indicated that the concrete cover still maintains the integrity and provided a contribution to the load-bearing capacity, thereby reducing the contribution of FRP bars. In Figure 13(b), (d), and (f), different from the columns without FRP jackets, the compressive strains of FRP bars showed a linear ascending branch up to an axial strain level of 7500 με, then followed a slight drop and a gradual descending branch until the final completely crushing of longitudinal GFRP bars. This phenomenon proved that the external FRP jackets were conducive to increasing the compressive behavior and contribution of FRP longitudinal bars. On the other hand, at the 1% limit of usable axial strain level, the average strain of longitudinal GFRP bars in specimen with external GFRP jackets (G75-G6, G100-G6, and G150-G6) still retained 7816, 7631, and 3475 με, respectively. This is due to the use of external FRP jackets, which prevented spalling of concrete cover and then delayed the premature buckling or crushing failures of the inner longitudinal GFRP bars. At the peak load level, the load carried by the longitudinal GFRP bars (computed by multiplying the total cross-sectional area of longitudinal GFRP bars by the average axial strain and modulus of elasticity of the bars in compression) ranged between approximately 2.8% and 7.2% of the peak load (Table 3). Therefore, neglecting the contribution of longitudinal FRP bars to the capacities of FRP-reinforced concrete columns underestimated the columns’ capacities; this result is consistent with the research on conventional pultruded FRP rod ties–reinforced concrete columns (Afifi et al., 2013; Mohamed et al., 2014; Tu et al., 2019).

Conclusions

This article presented a new concept and configuration of FRP-reinforced concrete columns confined with external FRP jackets. A series of axial compression tests were conducted to confirm some of the expected advantages. The test variable included the confining reinforcement material type (GFRP and CFRP), FRP jacket layers, and spacing of GFRP-CW ties. Based on the experimental results and discussion presented in the article, the following conclusions can be drawn:

The buckling or crushing of longitudinal GFRP bars easily occurred in FRP-reinforced concrete columns due to spalling of concrete cover.

The compressive ductility of FRP-reinforced concrete columns can be significantly enhanced with external GFRP jackets, 2.2–3.5 times compared to their counterparts. The increasing of the peak load was moderate from a range between 11% and 36%.

The tie spacing of 75 mm possessed an expected moderate confinement behavior with three layers of GFRP jackets; six layers of GFRP jackets were required with the 100 mm or 150 mm tie spacing.

FRP jackets maintained the dual confinement effect provided together with internal GFRP-CW ties and then extended the contribution of longitudinal GFRP bars and the concrete cover.

The feasibility of FRP-reinforced concrete columns confined with external FRP jackets has already been verified with tests of square sections. Further research is necessary to investigate rectangular columns with a new configuration of GFRP-CW ties under axial cyclic compressive and lateral cyclic loading.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to thank the financial support received from the National Natural Science Foundation of China (No. 51478143, No. 51878224) and the National Key Research and Development Program of China. (No. 2017YFC0703001).

ORCID iDs

Chuanxiang Chen

Zhenyu Wang

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