Innovative hybrid reinforcement constituting conventional longitudinal steel and FRP stirrups for improved seismic strength and ductility of RC structures

Abstract

The use of fiber reinforced polymer (FRP) reinforcement is becoming increasingly attractive in construction of new structures. However, the inherent linear elastic behavior of FRP materials up to rupture is considered as a major drawback under seismic attacks when significant material inelasticity is required to dissipate the input energy through hysteretic cycles. Besides, cost considerations, including FRP material and construction of pre-fabricated FRP configurations, especially for stirrups, and probable damage to epoxy coated fibers when transported to the field are noticeable issues. The current research has proposed a novel economical hybrid reinforcement scheme for the next generation of infrastructures implementing on-site fabricated FRP stirrups comprised of FRP sheets. The hybrid reinforcement consists of conventional longitudinal steel reinforcement and FRP stirrups. The key feature of the proposed hybrid reinforcement is the enhanced strength and ductility owing to the considerable confining pressure provided by the FRP stirrups to the longitudinal steel reinforcement and core concrete. Reinforced concrete beam specimens and beam-column joint specimens were tested implementing the proposed hybrid reinforcement. The proposed hybrid reinforcement, when compared with conventional steel stirrups, is found to have higher strength, stiffness, and energy dissipation. Design methods, structural behavior, and applicability of the proposed hybrid reinforcement are discussed in detail in this paper.

Graphical abstract

Keywords

FRP / ductility / confinement / seismic / shear

1 Introduction

The effects of aging and the annual direct cost of corrosion in infrastructures combined is estimated to be $22.6 billion in the United States [1]. Approximately 15% of the bridges currently in use throughout the United States are structurally deficient due to corroded steel and steel reinforcement. This deficiency incurs $8.3 billion total cost annually [1]. Structural strength loss in reinforced concrete (RC) structures, including bridges, exposed to a corrosive environment is thus a major issue. Stress corrosion cracking (SCC) is defined as the growth of cracks in a continuous, harshly corrosive environment. Corroded steel reinforcement in concrete exerts pressure on its surrounding concrete, causing the cover concrete to spall [2]. Since stirrups have a smaller diameter and are nearer to the outside environment, they are more vulnerable to corrosion, acting as a corrosion initiator bridges when defects exist in cover concrete. Imperfections, such as reduced thickness of cover concrete and/ or honeycombs, often occur when the reinforcement cage is congested. Such construction issues would make stirrups the first point of corrosive attack due to less protection by cover concrete.

Fiber reinforced polymer (FRP) reinforcement has recently been used to inhibit corrosion-related issues mentioned above in RC members. FRP reinforcement (in the form of internal and external, longitudinal and transverse reinforcement) is currently being utilized in buildings and bridges [3]. Several design guides and research results provide design provisions in this area [4−8]. FRP reinforcement has been used in different configurations and techniques to utilize the material effectively. The use of FRP sheets as an externally bonded reinforcement is widely recognized as an effective method for strengthening and improving the behavior of reinforced concrete members [9−16]. Another innovative strengthening technique is the near-surface mounted (NSM) technique. This technique consists of placing FRP reinforced bars or strips into grooves pre-cut into the cover concrete in the tension region of the strengthened concrete member [17−21]. FRP reinforcing bars have also been used as internal flexural and/or shear reinforcement in concrete members, and many studies [4−8] have examined the development of design equations for FRP reinforced concrete cross-sections. While different types of FRP materials are available (e.g., carbon FRP (CFRP), glass FRP (GFRP), basalt FRP (BFRP), and aramid FRP (AFRP)), GFRP bars are generally the most economical. Experimental studies on beams reinforced with internal GFRP reinforcing bars revealed that current design provisions predict the flexural behavior reasonably well up to service load [22]. More analytical and experimental research on RC structures reinforced with FRP reinforcing bars indicated that the general behavior of FRP and steel reinforced flexural members is almost identical, except for the failure mode [23,24]. The FRP reinforced concrete members exhibited brittle failure mode, whereas the steel reinforced concrete members exhibited ductile failure [23,24]. Bentz et al. [25] conducted a study on the shear strength of large-scale beams with internal GFRP longitudinal and shear reinforcement. They used two overlapped prefabricated “U” shaped GFRP reinforcing bars to form the stirrup configuration. Based on tensile test results of the GFRP bars, they noted the strength and stiffness to be “significantly lower than the values advertised by the manufacturer” [25]. Full-scale bridge systems and large-scale RC columns using FRP bars have also been deployed in past studies [26−28].

The use of internal FRP reinforcing bars in concrete members has also been the topic of several research projects conducted to better understand long-term durability and serviceability issues. Experimental and analytical studies on conventional steel and FRP used as longitudinal reinforcement in flexural members proved that the flexural stiffness after cracking of the latter is significantly lower for identical reinforcement ratios [26−29]. The Canadian Highway Bridge Design Code, CSA 2000 [30] does not allow the use of GFRP reinforcing bars as the primary reinforcement because the durability of these bars over time in the alkaline environment of concrete is still unknown. Further, if the formation of plastic hinges is assumed, FRP reinforcement bars cannot be utilized [26]. The Japan design code, JSCE 1997 [7], ignores the presence of FRP compression reinforcement due to its unreliable compressive strength [26]. Another considerable issue that has been raised in the literature is the reduction in the shear carried by concrete attributed to low modulus elasticity of glass fiber reinforcement bars [26,31].

Although the fabrication of straight longitudinal reinforcing bars comprised of FRP is straightforward, the same does not apply to bent bars or to stirrups. The production of FRP stirrups involves a more elaborate process that incurs additional costs. The production of continuous spirals comprised of FRP reinforcing bars has a tedious fabrication process. Transporting and storing these bars increases the possibility of damaging the factory produced epoxy-coated fibers. Thus, use of these high-strength, lightweight, corrosion-free composite materials may not be economically justifiable.

In the current research, a novel reinforcement scheme is proposed in which FRP stirrups made from FRP sheets are incorporated with conventional longitudinal steel reinforcing bars. This combination of steel and FRP reinforcement is referred to in this paper as “hybrid” reinforcement and is aimed to address some of the constructability and durability related issues mentioned above. The structural performance of RC beams and beam-column joints with this hybrid reinforcement scheme is examined in terms of strength, stiffness, and energy dissipation.

2 Research significance

The brittle failure nature of FRP reinforcement materials lessens their application in seismic regions, especially as the primary reinforcement. Review of existing literature reveals that studies on new applications of FRP materials for the next generation of infrastructures featuring durability, serviceability, strength, and resilience, while still being economical and applicable in the field, deem to be necessary for structural design. It is essential to offer economical and constructible methods to make the most of FRP materials, which characterize high strength-to-weight ratios with prolific properties considering corrosion and durability in civil engineering applications. The proposed stirrups fabricated from FRP sheets could be applied as a continuous spiral on any longitudinal reinforcement configuration, with the stirrup width and thickness according to the design requirements. The current study investigates the behavior of RC members with steel-FRP hybrid reinforcement. Experimental tests on beam and beam-column (i.e.,, joint) specimens were implemented to compare the monotonic and hysteretic behavior of the steel-FRP hybrid members with that of conventional steel reinforced members.

The hybrid reinforcement concept in RC members is illustrated in Fig. 1. For the hybrid reinforcement, conventional longitudinal steel reinforcing bars are utilized, while the stirrups are constructed from FRP sheets. These FRP stirrups could be also advantageous when used as continuous inclined stirrups resisting shear reversals under earthquake excitations (Fig. 1(b)). For this purpose, half of the stirrups could be wrapped with an angle of+ 45 degrees relative to the longitudinal axis of the beam/column, and the other half are wrapped with an angle of −45 degrees to form an “X,” - with each stirrup crossing another- and confining the longitudinal reinforcement and core concrete within. The schematic of this application is illustrated in Fig. 1.

3 Experimental program

3.1 Overview

This study used handmade FRP stirrups comprised of FRP sheets. These sheets are typically used with an epoxy resin, in the wet lay-up method, for the external strengthening of RC structures. First, the required length and width of the FRP strip was cut from the FRP sheet roll and then impregnated in epoxy. This strip was then mounted on and wrapped around longitudinal reinforcement to form a closed stirrup by overlapping itself.

To study the behavior of the proposed stirrups, three RC beam specimens and three RC beam-column joint specimens were tested. All beam and joint specimens were built with conventional steel reinforcement bars as longitudinal reinforcement, and the type of stirrups (i.e.,, steel or FRP) was the primary test variable.

The reference beam with vertical steel stirrups was the first beam tested. The second and third beams had vertical and inclined FRP stirrups, respectively. Three half-scale concrete joints were tested to study the hysteretic performance of such composite construction. The first joint specimen designed was a reference specimen with seismically detailed steel stirrups. The second and third joint specimens each contained the proposed FRP stirrups. For the second joint specimen, seismic ductile detailing in determination of stirrup spacing was considered (ductile joint). On the contrary, for the third joint specimen the stirrups spacing requirements for seismic detailing was not considered (non-ductile joint). The stirrup spacing in both the first and second joint specimens was identical. Stirrup spacing in the third specimen was doubled. The design method, experimental observations, results, and analyses for the beam and joint specimens implementing the proposed FRP stirrups are presented in this study.

3.2 Material properties

The concrete used in this study included Type II portland cement and had a maximum aggregate size of 10 mm. The measured slump was 100 mm. The measured concrete compressive strength of the beam specimens was 38 MPa at 28 days. The concrete compressive strength of the joint specimens was 30 MPa.

The steel stirrups used in the beam and the joint specimens had a specified yield strength of 260 MPa and an ultimate strength of 290 MPa. The longitudinal reinforcement bars on the tension face of the beam specimens had a specified yield strength and ultimate strength of 480 MPa and 510 MPa, respectively. The specified yield strength and ultimate strength of the longitudinal reinforcement bars on the compression face, were 330 MPa and 360 MPa, respectively. The joint specimen longitudinal bars had a specified yield strength of 354 MPa and ultimate strength of 542 MPa.

Woven-textured sheets with unidirectional CFRP fibers were used to construct the FRP stirrups. The tensile strength, modulus of elasticity, and ultimate rupture strain of the fibers as reported by the manufacturer were 3550 MPa, 235 GPa, and 1.5%, respectively. The mechanical properties of the fibers are summarized in Table 1.

3.3 Construction and implementation of the proposed stirrups comprised of FRP sheets

The FRP fibers used to construct the stirrups were comprised of carbon fiber reinforced polymer (CFRP) sheets as a woven-textured laminate. A sample of the CFRP sheet is illustrated in Fig. 2. First, the longitudinal bars were held together at the right location by using two temporary steel stirrups to form the reinforcing steel cage, and then the FRP stirrups were applied. The required width and length of each strip was sheared from the CFRP roll based on the required design cross section. The CFRP strips were then impregnated in epoxy and mounted in marked locations on the longitudinal reinforcement. The end of the strip was wrapped twice around one of the longitudinal reinforcement bars to ensure it could be pulled while the stirrup was wrapped around the longitudinal reinforcement bars to form the stirrup as tight as possible. This method was straightforward and left no gap between FRP stirrup and the flexural reinforcement. The construction of such FRP stirrups, when compared with conventional FRP reinforcement bar stirrups, would be more versatile. For factory-produced FRP reinforcement bar stirrups, on the other hand, cut and bends are required to form the stirrups based on the required detailing, where no modifications could be implemented in the field, if so required. The proposed FRP stirrups could be applicable to any cross-section geometry with any reinforcement detailing at any location. The use of such FRP stirrups is also useful for the placement of concrete in formwork as it reduces the volume of steel present when a congested reinforcement cage is utilized. This point is particularly valuable for ductile stirrup seismic detailing at potential plastic hinge zone with closely spaced steel stirrups. This eases the placement of concrete and reduces the chances of flaws (i.e.,, insufficient compacting, honeycombs, and poor bond between concrete and the reinforcement). This method also allows implementation of different types of FRP rather than CFRP only.

3.4 Specimen design

3.4.1 Beam specimen design

Three beam specimens were designed to fail in shear. The beams had a rectangular cross section that was 200 mm wide and 250 mm high. The beams were simply supported with a concentrated load at midspan. The spacing between the supports was 1050 mm (Fig. 3). Three beam specimens were BSV, BFV and BFI, where B= Beam, S= Steel stirrup, F = FRP stirrup, V= Vertical stirrup, and I= Inclined stirrup. For example, BFV represents the beam specimen with FRP stirrups oriented in the vertical direction. In specimen BFI, the FRP stirrups had a 45-degree inclination angle with respect to the longitudinal axis of the beam with a 3 mm² corresponding cross-sectional area. Beam specimen reinforcement details are illustrated in Fig. 3. It should be noted that in specimen BFV, the steel stirrups on the left half side were Ø6@50 mm. The stirrups on the right half side of the beam were vertical FRP stirrups spaced at 115 mm with an area equal to 6 mm² (Fig. 3). Such stirrup configuration would ensure the formation of shear failure through FRP stirrups.

3.4.2 Joint specimen design

An 8-story RC moment frame was designed as the prototype structure for the joint specimen (Fig. 4). The structure’s plan constitutes 4 equidistant bays of 5 m length in the X and Y directions (defined in Fig. 4). Each storey was assumed to be 3 m in height. The middle frame, as specified in Fig. 4, was selected for the experimental studies. Dead and live loads were assumed to be 550 and 350 kg/m², respectively. The base shear was calculated for a maximum ground motion of 0.3g (PGA). The moment resisting frame to serve as the lateral load resisting system was designed according to provisions in the ACI 318-08 code [32]. In Fig. 4, {\rho }_{\mathrm{t}}, {\rho }_{\mathrm{s}} and {\rho }_{\mathrm{s}}^{\prime } represent the reinforcement ratios of the column longitudinal bars, the tension reinforcement, and the compression reinforcement in the beam, respectively. The b, h, and d represent the width, height, and effective depth of each cross-section, respectively. The exterior joint, located on the fourth floor, was chosen as the reference joint to be studied (Fig. 4). The frame (shown in Fig. 4 at a 1/2 scaling factor) was scaled down according to the Buckingham theorem [33,34]. Three joint specimens labeled JSD, JFD and JFN were tested with the corresponding designation: J= Joint, S= Steel stirrup, F = FRP stirrup, D= Ductile stirrup, and N = Non-ductile stirrup (Fig. 5). The ductile stirrup design corresponded with the design according to the seismic provisions in the ACI 318-08 code [32] leading to closely spaced transverse reinforcement. In the non-ductile joint specimen such provisions were not considered to reflect the FRP stirrups spacing effect. All three joint specimens were cast with longitudinal steel reinforcement. The JSD specimen served as the reference specimen with ductile steel stirrups detailing. The JFD and the JFN were constructed with the proposed FRP stirrups by including seismic detailing requirements on stirrups spacing and by excluding it, respectively. The studied joint specimen geometry, reinforcement and strain gages locations are detailed in Fig. 5.

4 Design of proposed manually constructed FRP stirrups

4.1 Beam stirrup design

For FRP stirrup design, it was important to use the existing reinforced concrete- and FRP material-related codes. The design of beams with FRP stirrups (BFV and BFI) was based on equating the shear capacity equal to the shear capacity of the reference beam with steel stirrups (BSV). The longitudinal bars were designed to preclude flexural failure while satisfying both the minimum and the maximum tensile reinforcement requirements of ACI 318-08 [32]. The shear design was implemented using the shear provisions from the ACI 318-08 code [32]. The member shear strength was calculated from the following relation:

(1)V_{\mathrm{n}}=V_{\mathrm{c}}+V_{\mathrm{s}},

where V_c and V_s are given in Eqs. (2) and 3, respectively:

(2)V_{\mathrm{c}}=\mathrm{2}\sqrt{f_{\mathrm{c}}^{\prime }}{b}_{\mathrm{w}}d,

(3)V_{\mathrm{s}}={\displaystyle \frac{A_{\mathrm{v}\mathrm{s}}{f}_{\mathrm{y}\mathrm{s}}{d}_{\mathrm{s}}}{s_{\mathrm{s}}}}(\sin {\alpha }_{\mathrm{s}}+\cos {\alpha }_{\mathrm{s}}),

where b_w and d are the web width and the effective depth of the section, respectively, f_{\mathrm{c}}^{\prime } is the concrete compressive strength, A_vs is the cross-sectional area of the steel stirrup, f_ys is the yield strength of the steel stirrup, s_s is the steel stirrup spacing, and α_s is the inclination angle of the steel stirrups with respect to the longitudinal axis of the beam. Equation (3) was rearranged into Eqs. (4) and 5 by equalizing the shear carried by the FRP stirrups to the shear carried by steel stirrups obtained in terms of the ACI 440-06 [4] guide.

(4){\displaystyle \frac{A_{\mathrm{v}\mathrm{s}}{f}_{\mathrm{ys}}{d}_s}{s_{\mathrm{s}}}}(\sin {\alpha }_s+\cos {\alpha }_s)={\displaystyle \frac{A_{\mathrm{vf}}(\mathrm{0.004}{E}_f)d_f}{s_f}}(\sin {\alpha }_f+\cos {\alpha }_f),

(5){\displaystyle \frac{A_{\mathrm{vf}}}{s_f}}=\mathrm{250}{\displaystyle \frac{A_{\mathrm{sv}}{f}_{\mathrm{ys}}{d}_s}{s_s{E}_f{d}_f}}\left({\displaystyle \frac{\sin {\alpha }_s+\cos {\alpha }_s}{\sin {\alpha }_f+\cos {\alpha }_f}}\right).

Here, A_vf is the cross-sectional area of the FRP stirrup, E_f is the modulus of elasticity for the FRP material, s_f is the FRP stirrup spacing, and α_f is the inclination angle of the FRP stirrups with respect to the longitudinal axis of the beam. The required area and spacing of steel and FRP stirrups for the beam specimens are listed in Table 2.

4.2 Joint stirrup design

The required area of the FRP stirrups was calculated using the ACI 440-06 guide [4]. In the design, the steel stirrups were replaced by stirrups comprised of FRP material. According to ACI 440-06 [4] recommendations on the FRP strain, a value of effective strain of 0.4% was assumed and was used in Eq. (6) to calculate the required FRP stirrup area. The results are tabulated in Table 3.

(6)A_{\mathrm{vs}}{f}_{\mathrm{ys}}=A_{\mathrm{vf}}\times \mathrm{0.004}{E}_f\Rightarrow A_{\mathrm{vf}}=\mathrm{250}{\displaystyle \frac{A_{\mathrm{vs}}{f}_{\mathrm{ys}}}{E_f}}.

5 Experimental test setup

5.1 Beam specimens test setup

The load frame setup was assembled with one 500 kN jack and one 1000 kN load cell so that the point load could be applied at the beam midspan. The beams were tested as simply supported beams on hinge and roller supports 225 mm from each end of the beam (Fig. 6). Linear variable differential transformers (LVDTs) and strain gauges (connected to the data acquisition system) were used to measure deflection and strains during the test, respectively. Strain gages were mounted to the stirrups along the path following the anticipated shear crack (see Fig. 3). Strain gauges were also mounted on the longitudinal tension reinforcement and on the top surface of the beam at midspan. Load was applied incrementally up to failure. Load, deflection, and strains were monitored, and the cracks were marked.

5.2 Joint specimens test setup

Details of the test setup used for the joint specimens are given in Fig. 7. The panel zone was instrumented using LVDTs and embedded strain gauges to determine the applied actions. All instrumentation was connected to the data acquisition system so that values of load, deflection and strains could be carefully monitored during the test. Due to the test frame arrangement, the joint assembly was tested with the column oriented horizontally and the cantilevered beam oriented vertically (see Fig. 7). Two 200 kN jacks were used to apply cyclic lateral displacement reversals at the beam tip. A 500 kN jack was used to apply the axial load to the column. An axial load of 410 kN, which corresponds to 20% of the nominal axial capacity (P_n) of the column cross section according to Eq. (5), was applied and kept constant during the test.

(5)P_n=\mathrm{0.85}{f}_c^{\prime }(A_g-A_s)+A_s{f}_y,

where A_g, A_s and f_y are the gross cross-sectional area, the longitudinal steel reinforcement area, and the yield strength of the longitudinal steel reinforcement, respectively.

The distance between the applied horizontal load at the beam tip and the column face was 1250 mm. The horizontal load was applied to the beam as cyclic displacement reversals. Each cycle was repeated for three cycles with increasing amplitudes (Fig. 8). The applied load began with three elastic cycles at 0.5% lateral drift corresponding to the displacement at the first flexural crack. Applied drifts were used in increments of 1%. Three cycles were repeated at each drift level, reaching 7% drift, to simulate the hysteretic behavior.

6 Experimental test results

6.1 Beam specimens test results

As expected, all three beam specimens exhibited a shear dominant failure when diagonal tension cracks developed in the compression concrete zone under the loading plate (illustrated in Fig. 9). Vertical flexural cracks that occurred at the midspan were succeeded by inclined shear cracks. These cracks formed through the compression strut, beginning at the support and reaching the midspan with concrete crushing. Strain gauges mounted on the longitudinal reinforcement verified that no tensile reinforcement yielding occurred at failure. However, the diagonal cracking in beams with FRP stirrups was more severe in terms of crack width and cover damage, thus corresponding to the higher level of stress reached in the FRP stirrups as compared to the steel stirrups. This observation implies that a marginal limit exists on the strength of this type of manually made FRP stirrup that could contribute since concrete failure prior to attainment of FRP rupture strain would be the governing failure mode. However, failure due to concrete crushing prior to FRP rupture is preferred in which the latter is more brittle [4,26,28,29]. Specimen BFI with inclined FRP stirrups experienced a more distributed crack pattern than either specimen BSV or BFV with vertical stirrups. This is caused by the higher clamping effect provided by the inclined stirrups which prevented the widening of 45-degree shear cracks

The load-midspan displacement relationship of the beams is depicted in Fig. 10. All three beam specimens exhibited an identical initial elastic stiffness, prior to the formation of diagonal tensile cracks, where stirrups were almost intact. However, when shear cracks initiated, the FRP stirrups performed better than the steel stirrups in delaying crack growth and preventing crack widening. Such performance increased the strength dramatically by increasing aggregate interlock and keeping more of the compression concrete uncracked for resisting shear, thus a higher level of strength was obtained.

The shear crack angle, relative to the longitudinal axis of the beam, for both specimens BSV and the BFI was approximately 45-degrees. This angle was less steep for specimen BFV (approximately 36-degrees). Figure 11 illustrates the applied load versus the concrete compressive strain, recorded from the strain gage mounted on surface of the concrete. The maximum measured compressive strain in the concrete for specimens BSV, BFV, and BFI was 0.264%, 0.292% and 0.375%, respectively. Specimens with FRP stirrups experienced larger concrete compressive strain, verifying the greater contribution from the concrete compression zone for specimens with FRP stirrups, resulting in additional shear resistance. The stirrups were instrumented with strain gauges along an inclined path connecting the mid-span to the support (Fig. 3). The applied load versus tensile strains measured in the stirrup adjacent to the support, where diagonal tensile shear cracks initiated, are shown in Fig. 12. The peak tensile strain in the stirrup measured in specimens BSV, BFV, and BFI was 0.61%, 0.79%, and 0.82%, respectively. This corresponds to a stress of 1880 MPa in the FRP stirrups, indicating that the strain values reached approximately twice the recommended values for the effective strain (0.004).

Moment-curvature results for the beam specimens (given in Fig. 13) reveal the attainment of higher plastic curvature in beams with FRP stirrups compared with the reference specimen with steel stirrups. The ultimate curvature for specimens BFV and the BFI was 0.032 and 0.028 radians, which are 52% and 33% higher than the reference specimen, respectively.

When FRP stirrups were utilized, either vertically or inclined, the shear capacity of beams compared with the reference beam was increased. The cracking load, ultimate load, and the corresponding displacements are presented in Table 4. As expected, no deviation in the results occurred prior to cracking, whereas a considerable increase in strength occurred prior to concrete crushing failure, as explained earlier.

The experimental results were also compared to values obtained from the ACI 440-06 guide [4] and are presented in Table 5. According to this table, the predicted values for the reference beam with steel stirrups deviated by 8%. The experimental results for beams with FRP stirrups, however, varied significantly from the predicted values. An attainment of tensile strain near 0.8% justified such significant deviation between the experimental results and the predicted values. This level of strain is almost two times higher than the effective strain, thus underestimating shear carried by FRP stirrups. The beam test results proved this hybrid reinforcement comprised of longitudinal steel and FRP stirrups is practically feasible and reliable in terms of strength, stiffness, and attainment of higher plastic curvature due to the shear resisting mechanism provided by FRP composites.

6.2 Joint specimen test results

All joint specimens had identical flexural reinforcement. The flexural moment capacity for specimens JSD, JFD, and JFN was 26.6, 31.3, and 27.9 kN·m, respectively. All specimens exhibited a hardening load-displacement behavior up to 7% drift. Even specimen JFN performed reasonably well up to 7% drift, showing stable hysteretic behavior. This specimen was constructed with sparsely spaced FRP stirrups ignoring seismic detailing requirements on transverse reinforcement spacing. The moment-drift relations for the three joint specimens are given in Fig. 14. Specimen JFD had the highest moment capacity, which was 18% and 12% more than that of specimens JSD and JFN, respectively.

Both load and displacement at first yield and displacement ductility are tabulated in Table 6. Displacement ductility ({\displaystyle \frac{\mathrm{\Delta }u}{\mathrm{\Delta }y}}) were defined as the ratio of ultimate displacement (Δu) to yield displacement (Δy). The ultimate limit state (failure) corresponds to a load level not less than 85% of the peak load.

The first flexural cracks occurred at 0.5% drift. Yielding of the longitudinal reinforcement bars was initiated at 1%−1.5% drift. Figure 15 illustrates cracks forming at 1% drift. These cracks, which were followed by flexural cracks in the beam, were succeeded by diagonal tensile cracks in the joint panel zone. The initiation and formation of flexural cracks for all specimens were almost identical. Although all joints had the same geometry and flexural reinforcement, specimen JFD, with closely spaced FRP stirrups, exhibited not only the highest ductility but also the highest flexural capacity. This could be addressed due to the higher strength of FRP stirrups and the higher corresponding induced strain. The FRP stirrups were successfully diminishing flexural deformations, making longitudinal reinforcement reaching to yield, and making the most of the inelastic capacity of reinforcement bars in the post-yield behavior of joint specimens. This point is especially critical for the seismic behavior of structures when material inelasticity associated with geometric nonlinearity exists at large drift levels.

The applied load cycles caused previous cracks to become widened and closed, increasing crack depth. At 7% drift, all specimens exhibited flexural cracks in the beams with diagonal shear cracks that extended into the panel zone. One interesting point during testing specimen JFD was the severity of crack formation. At 7% drift, suddenly a large portion of cover concrete spalled off the back of column on the opposite side of the beam-column connection (see Fig. 16).

Strains determined by the LVDTs that were located on two sides of the column during applied hysteresis are illustrated in Fig. 17. It should be noted that the strain was calculated from the displacement measured by the LVDTs over the distance between them on the two sides and are therefore considered average strains. Strain for specimens JSD, JFD, and JFN reached up to 1, 1.23, and 2% in compression and 1.65, 2.05, and 1.76% in tension, respectively.

Many strain gauges were installed; however, only peak strains measured in the stirrups are presented here for brevity. Moments versus strain of the first stirrup in the beam, in the panel zone, and in the column are depicted in Figs. 18, 19, and 20, respectively. Figure 18 reveals that the peak strain in the steel stirrup was 0.083%, while the peak strain measured in the FRP stirrups in specimens JFD and JFN were 0.088% and 0.061%, respectively. The peak strain values are listed in Table 7. In specimen JFD, the strain in the column stirrup (0.051%) was less than the corresponding strain in specimen JSD (0.068%).

6.3 Effect of FRP stirrups on sectional behavior

A moment-curvature analysis was conducted so that the effect of hybrid reinforcement consisting of longitudinal steel and transverse FRP could be better understood (see Fig. 21). The maximum sectional curvature for specimens JSD, JFD, and JFN was 0.050, 0.058, and 0.057 radians, respectively. The proposed FRP stirrups were effective in keeping the ultimate curvature 16% higher than that of the steel stirrups. This reveals the adequacy of FRP stirrups in confining the core concrete and preventing longitudinal reinforcement from buckling, allowing attainment of higher curvature of the cross-section.

6.4 Energy absorption and stiffness degradation

The joint specimens were analyzed in terms of their ability to dissipate input energy through hysteretic cycles. Energy absorption could be obtained through the summation of hysteresis loop area at the drift in which the load was not less than 85% of the peak load. The absorbed energy for the studied joints is depicted in Fig. 22. The dissipated energy up to 1% drift was similar among all specimens. However, at a higher level of drift, specimen JFD specimen with closely spaced FRP stirrups was superior in dissipating energy through hysteretic cycles. This point is consistent with the damage observed during testing. Specimen JSD (with closely spaced steel stirrups) and specimen JFN (with sparsely spaced FRP stirrups) exhibited almost an identical energy dissipating capacity up to failure.

Results of stiffness degradation of the joint specimens were also computed and are presented in Fig. 23. All specimens at 35 mm lateral displacement had same degraded stiffness, with no significant deviation up to 7%.

7 Applicability of the proposed FRP stirrups

This study found that it is feasible to use the proposed, manually, hand-wound stirrups made of FRP sheets. The efficacy of this system proved to be satisfactory in terms of ductility, strength, stiffness, and structural performance. The results prove that this system is applicable at different locations, and with different longitudinal reinforcement detailing in various cross-sectional geometries. The primary goal of this work was to propose that a continuous spiral made of FRP sheets that could be either perpendicular to the axis of the structural member or inclined (i.e.,,±45). This inclination should be an economically applicable option for the utilization of inclined shear reinforcement when shear reversals exist under seismic forces. These FRP stirrups would also eliminate the need to overlap prefabricated FRP reinforced bars to form the desired geometry of stirrups, allowing for versatility in shear reinforcement design. The combination of longitudinal steel and transverse FRP incorporates the enhanced corrosion properties of FRP while still exhibiting a ductile behavior. Beam specimen test results demonstrated the enhanced efficacy of the proposed hybrid reinforcement in resisting the incremental monotonic shear forces. The joint specimen results proved the promising behavior of the hybrid reinforcement in exhibiting stable post-elastic hardening behavior under cyclic loading, typical of earthquake loading.

In this study, FRP strips were wrapped directly around longitudinal reinforcement while the epoxy remained uncured. However, either cardboard or any light formwork that represents a similar configuration of longitudinal reinforcement could be used. Once the epoxy is cured (almost 12−24 h), the inside formwork could be taken out and longitudinal reinforcement (either steel or FRP) would be inserted into the transverse reinforcing cage.

8 Conclusions

In this study, a hybrid reinforcing system comprised of FRP stirrups and conventional longitudinal steel reinforcing bars was proposed and experimentally investigated. Tests under both monotonic loads and quasi-static cyclic loads on beam and joint specimens were conducted to study the behavior. The following conclusions are drawn from the results of this study:

The hybrid reinforcement exhibited an enhanced performance considering ductility under monotonic and cyclic load reversals.

Existing analytical design equations could be utilized to design the FRP stirrups for the proposed hybrid reinforcement system.

Peak induced strains in the FRP stirrups reached up to two times the recommended value for the effective strain (0.4%). Thus, the design equations underestimated the shear carried by the FRP stirrups.

The load carrying capacity of the specimens with FRP stirrups under both monotonic loading and cyclic displacement conditions was higher than the reference specimen with conventional steel stirrups.

The ductility of the FRP reinforced specimens, when used as stirrups, proved to be higher than the steel reinforced specimens. Sectional analysis also revealed the attainment of higher curvature for such hybrid reinforcement consisting of FRP stirrups and longitudinal steel.

The energy dissipation capability of specimens with FRP stirrups was larger than that of specimens with steel stirrups under cyclic loading.

Sparsely spaced FRP stirrups exhibited nearly similar behavior in terms of strength and ductility when compared to specimens with closely spaced steel stirrups conforming to seismic design detailing provisions.

FRP stirrups spaced in accordance with seismic detailing requirements exhibited ductile behavior with stable hysteretic cycles. However, the rate of stiffness degradation for this system was not necessarily less than that of conventional steel reinforced concrete.

Experimental results under lateral cyclic loading proved that members with similar longitudinal reinforcement and either FRP or the steel stirrups experienced a similar level of member rotation.

Cross sections including FRP materials both as longitudinal and transverse reinforcement, exhibit less rotation and curvature as compared to steel reinforced member with FRP stirrups. This is due to the higher strength of the longitudinal FRP materials, providing higher strength with sudden strength loss at peak (i.e., concrete crushing or FRP rupture).

The results from this study indicate the sound structural performance of the proposed FRP stirrups. This study, however, was an initial attempt to prove the concept, using existing design equations for design, and its applicability to shed light on this topic. Large scale beam and column specimens incorporating discrete and continuous FRP stirrups proposed in this paper need to be implemented to further study and lessen the potential size effect. These set of tests would provide a reliable experimental baseline for design and implementation of this proposed method in practice.

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