Shear behavior of RC beams with CFRP-steel strip hybrid stirrups

Abstract

This study proposes a novel carbon fiber reinforced polymer (CFRP)-steel strip hybrid stirrup to enhance the ductility of reinforced concrete (RC) beams. To investigate its effectiveness, three-point bending tests on 11 RC beams were conducted, comparing the proposed hybrid stirrups with conventional steel and CFRP strip stirrups. The effects of stirrup spacing and number of CFRP layers were also examined. Results showed obvious advantages of the proposed hybrid stirrups. Compared to the beam reinforced with only CFRP strip stirrups, the inner steel strip of the hybrid stirrups enhanced the rigidity and crack control, enhancing the reinforcing efficiency by up to 45%. In comparison with the beam reinforced with only steel strip stirrups, the outer CFRP strip of the hybrid stirrups can maintain the load in the later loading stages, thereby effectively improving the ultimate deflection by 1.7 times. For beams with CFRP strip stirrups, neither decreasing the stirrup spacing nor increasing the layers of CFRP contributed obviously to the shear capacity and ductility. However, for beams with hybrid stirrups, while the ductility increased significantly as the layers of CFRP, both shear capacity and ductility decreased with the stirrup spacing. Finally, a formula to calculate the shear capacity of the beams with hybrid stirrups was proposed and the calculated results agreed well with the experimental findings, which validated its applicability.

Keywords

concrete beams CFRP hybrid strip stirrups shear performance ductility

Introduction

Fiber-reinforced polymers (FRP) composites are widely applied in strengthening of concrete structures due to its advantages of lightweight, high strength and good corrosion resistance (El-Mogy et al., 2011; Fan et al., 2021; Hao et al., 2021; Liu et al., 2024). In addition to the externally bonding strengthening (Wang et al., 2024; Wang et al., 2024; Wang et al., 2025; Xiao et al., 2025), researchers also attempted to substitute the steel reinforcements with FRP to improve the ductility and corrosion resistance of concrete members under harsh conditions (Capozucca, 2007; Nasrollahzadeh and Aghamohammadi, 2018; Said et al., 2016; Zeng et al., 2021). Although FRP longitudinal reinforcements are well studied (Gravina and Smith, 2008; Li et al., 2022; Tran et al., 2021) and even relevant guidelines and standards have been issued (ACI 440.1R, 2001; IStructE, 1999; Machida, 1997), the studies on FRP stirrups are still limited. It is difficult to make closed round section FRP stirrups under the condition that ensuring the strength currently, so the main types of FRP rod stirrups are lapped stirrups and spiral stirrups. The former involves a substantial amount of FRP for the lapped sections, which leads to various degrees of material waste. Besides, the lapped anchoring is not stable and the sliding of FRP rod stirrups could occur (Al-Hamrani and Alnahhal, 2021; Bentz et al., 2010; Whitehead and Ibell, 2005). While the latter ensures higher material utilization ratio and more stable performance (Ahmed et al., 2010), it also poses significant construction challenges to produce the spiral shape. Thus, due to the complex process in the manufacturing of FRP rod stirrups, the FRP rods stirrups are hard to be widely applied in practice.

For the FRP rod stirrups, some studies found that the bending section is prone to the fracture (Deifalla et al., 2014; Issa et al., 2016; Krall and Polak, 2019). This is primarily because that the FRP strength usually decreases during the bending process as it needs to heat and press the straight FRP bars (Deifalla et al., 2014). Besides, the FRP is an anisotropic material, under loading, it will be subjected to a multi-axial stress state in the bending section and could cause shear cracks (Krall and Polak, 2019). Under such conditions, the FRP rod stirrups are susceptible to brittle fracture, and its utilization ratio is only 30-80% of that of the straight FRP bars (Ahmed et al., 2010; Shehata et al., 2000). To overcome the drawbacks, researchers have attempted using FRP strips (Hakeem et al., 2024; Lu et al., 2024). In addition to avoiding strength loss from bending, FRP strips generally exhibit superior bond properties with cementitious materials compared to FRP rods (Weiwen et al., 2025). A complete closed-type CFRP strip stirrup with a rectangular section (CFRPRS) and overlaid extra CFRP strips on the bending sections were incorporated to impede immature damage was developed (Chen et al., 2022; Fakharifar et al., 2016; Lee et al., 2010). From the tensile tests on the bending corner of CFRPRS (Lee et al., 2014), it was found that with equal cross-sectional area and bending radii, the strength reduction of the FRP in rectangular section was notably less than that of the FRP in circular section. However, similar to the FRP rod stirrups, when the FRP strip stirrups were used in RC beams, it also experienced a brittle rupture along the path of the main diagonal crack. Additionally, this type of FRP strip stirrup has a thin thickness and is easy to deform laterally. Since the manufactured FRP strip lacks sufficient rigidity, it requires further measures to form the designed shape during construction and thus bringing inconvenience for the in-site concrete casting. Owing to the high strength and linear behavior of FRP material, its failure strain significantly exceeds the yield strain of steel reinforcements. Compared to the conventional RC beams, RC beams with FRP strip stirrups perform more substantial deformation and thus longer and wider diagonal cracks (Lee et al., 2010; Lu et al., 2024), and impacting the aesthetic appearance.

Taking consideration of the shortcomings of FRP stirrups, some researchers found that the combination of FRP and steel could be a better option. Researchers developed a steel-CFRP stirrup by placing an FRP strip between two layers of steel strips and examined the shear performance of concrete beams reinforced with these composite stirrups (Uriayer and Alam, 2015). However, this combination not only did not take advantage of using FRP to protect steel strips from harsh environments but also requires specialized clamps to ensure that no air exists between the epoxy resin-coated CFRP and the steel strips. The method of combination can be further improved. Steel-carbon fiber composite bars (SFCBs) by consisting of an inner steel core hybridized in its longitudinal direction with FRP composites was introduced by researchers (Hao et al., 2008; Saikia et al., 2005; Wu et al., 2010a, 2010b). Due to the addition of inner steel bar, the elastic modulus of SFCBs can be greatly improved and SFCBs exhibit great ductility (Zhao et al., 2020). However, the relative slippage occurred between the steel and the FRP layer, as well as the interface between the FRP layer and the concrete. When the SFCBs are used as stirrups and longitudinal reinforcements, the crack inhibition effect is not as effective as that of steel bars (Wang et al., 2022). To further enhance the bond strength between externally spiraled FRP sheet and steel bars, researchers artificially welded ribs on the surface of the steel bars (Fahmy et al., 2017). However, for the above comparison, the dimension of steel in steel-FRP stirrups was the same as that in steel stirrups, which means that the improvement of capacity was largely due to the additional CFRP sheet. Since their shear reinforcement ratios are different, the improvement efficiency of the hybrid stirrups remains unclear. Therefore, while several attempts have been made to combine steel and CFRP as hybrid stirrups, existing methods have not yet resulted in a simple, efficient, and well-quantified hybrid stirrup design, highlighting the need for further development.

Hence, this paper developed a new type of CFRP-steel strip hybrid stirrup by winding the epoxy resin-coated CFRP strips around the steel strip. This hybrid stirrup not only simplified the fabrication process but also integrated the advantages of both materials. To understand the shear behavior of RC beams reinforced with the hybrid stirrups, experiments were carried out on RC beams with different types of stirrup configurations. The effects of stirrup spacing and number of CFRP layers were also considered. Finally, a modified calculation method for shear capacity of RC beam reinforced with CFRP-steel strip hybrid stirrups was developed and verified against the test results. The results in this study could provide reference for the design and application of the RC members with hybrid stirrups.

Experimental program

Specimen arrangement and preparation

The experimental flow chart is shown in Figure 1. The program was intentionally designed to systematically investigate the influence of key design variables on the shear behavior of RC beams. A total of 11 specimens, each with cross-sectional dimensions of 200 mm × 350 mm and a clear span of 1400 mm, were prepared. This sample size enabled the evaluation of different stirrup materials (steel, CFRP, and the proposed hybrid), stirrup spacings (160 mm and 110 mm), and stirrup ratios (0.5-7.27‰), while also including reference specimens for clear comparison. Four types of beams were considered: Type I‒reference beams without and with conventional stirrups; Type II‒beams reinforced with steel strip stirrups; Type III‒beams reinforced with CFRP strip stirrups; and Type IV‒beams reinforced with hybrid stirrups. The parameters such as sectional size, stirrup spacing and stirrup reinforcement ratios for each specimen are presented in Table 1. All specimens were loaded to failure, and the results were compared to assessing the structural performance of the hybrid stirrups.

Figure 1.

Experimental flow chart of this study.

Table 1.

Test parameters.

Specimen No.	Stirrup type	Sectional size	Stirrup spacing s (mm)	Stirrup reinforcement ratio (‰)	Normalized stirrup capacity
Specimen No.	Stirrup type	∅_d or t_v×b_v (mm)	Stirrup spacing s (mm)	Stirrup reinforcement ratio (‰)	S_c (MPa)
NS	—	—	—	—	—
RS	Steel bar	ϕ10	160	4.91	1.694
SS5	Steel strip	5×16	160	5.00	1.553
C3	CFRP strip	0.501×16	160	0.50	1.805
H3.5 + 1	Hybrid strip	Steel 3.5×16 &	160	3.67	1.72
H3.5 + 1	Hybrid strip	CFRP 0.167×16	160	3.67	1.72
C6	CFRP strip	1.002×16	160	1.00	3.615
SS3.5	Steel strip	3.5×16	160	3.5	1.12
SS5b	Steel strip	5×16	110	7.27	2.26
C3b	CFRP strip	0.501×16	110	0.73	2.63
H3.5 + 1b	Hybrid strip	Steel 3.5×16 &	110	5.33	2.505
H3.5 + 1b	Hybrid strip	CFRP 0.167×16	110	5.33	2.505
H3.5 + 3b	Hybrid strip	Steel 3.5×16 &	110	5.82	4.255
H3.5 + 3b	Hybrid strip	CFRP 0.501×16	110	5.82	4.255

Note: Specimens were denoted using an alphanumeric designation: NS‒no stirrups; RS‒steel rebar stirrups, SS‒steel strip stirrups, C‒CFRP strip stirrups, H‒hybrid CFRP-steel strip stirrups. The number after SS and C indicates the steel strip thickness and the CFRP strip layers, respectively. For hybrid stirrups, the number before and after the “+” indicates the steel strip thickness and the number of CFRP layers, respectively. The letter “b” indicates that the stirrup spacing is 110 mm. For example, H3.5 + 1b denotes a hybrid stirrup with a 3.5 mm thick steel strip and 1 layer of CFRP strip, with a stirrup spacing of 110 mm. t_v and b_v are the thickness and width, respectively, of steel strip stirrups (or CFRP strip stirrups).

To comprehensively compare the shear performance, it is not rational to compare the reinforcement ratio simply because there was a large difference between the CFRP and steel regarding the strength and cost. Instead, to be more objective, both material strength and volume should be taken into consideration. Therefore, in this study, the concept of normalized stirrup capacity (S_c, see equation (1)) was adopted to evaluate the stirrups with varying material properties. Although the S_c should be the same for each case with different stirrup types, in practice, it was difficult to achieve so due to the accuracy of producing the material with small thickness. In this case, standardized thickness of materials was used, and a similar S_c was adopted for comparison in this study. To compare the effects of different types of stirrups, the experiment designed a comparison of CFRP strips, steel strips, and hybrid strips with a similar S_c. Specimens C3, SS5, and H3.5 + 1 had a similar S_c, specimens C3b, SS5b, and H3.5 + 1b had a similar S_c. Hence, to better evaluate the utilization ratio of stirrups in enhancing shear performance, the shear resistance efficiency of different stirrups can be quantified via the coefficient φ, as shown in equation (2).

S_{c} = (A_{sv} f_{y v} + A_{f v} f_{f}) / b s

(1)

φ = V_{s} s / [(A_{sv} f_{y v} + A_{f v} f_{f}) h_{0}]

(2)

where S_c is normalized stirrup capacity, A_sv and f_yv are the cross-sectional area and yield strength of the steel strip stirrups, A_fv and f_f are the cross-sectional area and tensile strength of the CFRP strip stirrups, b is the width of the concrete beam, s is the spacing of the stirrups, h₀ is the effective height of the section, and V_s is the shear capacity of the test beams.

According to Chinese code (GB 50010, 2010), the cross-sectional dimension of all beams are 200 mm × 350 mm (b × h), as shown in Figure 2. The full length and clear span of the beams were 1600 mm and 1400 mm, respectively. All specimens had the same configurations of longitudinal rebars, where 225 and 325 rebars were placed at compression and tension zone respectively, and the thickness of the concrete cover was 25 mm (from concrete surface to the surface of longitudinal rebars). Also, for all specimens, a structural stirrup ϕ6 was set at each end to support the rebar cage and, while the arrangement of stirrups was set in accordance with the experimental design for the other areas. The end structural stirrups were essential for maintaining the geometry of the reinforcement cage during fabrication, especially in beams with flexible CFRP strip stirrups. Besides, they also helped prevent premature anchorage failure at the supports. As these stirrups are located outside the critical shear span, they did not affect the shear behavior of the beams. The specifications of stirrups are shown in Table 1. In addition, to understand the location of the stirrups, the stirrups were numbered from the support to the mid-span (1-4). Steel bar stirrups, CFRP strip stirrups, steel strip stirrups and CFRP-steel strip hybrid stirrups were represented by r, f, s and h, respectively. For example, the numbers of stirrups in specimen H3.5 + 1 were h1∼h4 from the support to the mid-span, as shown in Figure 2.

Figure 2.

Dimensions of specimens with hybrid stirrups and reinforcement arrangement.

The material properties in this study are presented in Table 2. After casting and demolding, the specimens were cured at room temperature for 40 days until testing. The beams were cast using commercial ready-mix concrete of grade C30, with a mix proportion (by weight) of cement: sand: stone: water = 1 : 0.65: 3.17 : 4.58. The average compressive strength of concrete was obtained from six standard 150 mm cubes in accordance with GB/T 50081 (2002), while the mechanical properties of the reinforcing bars were determined according to GB/T 228.1 (2021). A type of CFRP named UD300CF/1 was used for the CFRP strips, and the thickness was 0.167 mm for each layer. The mechanical properties of CFRP were obtained based on GB/T 3354 (2014). In addition, the epoxy resin adhesive was used to impregnate the CFRP, and its type was JN-C3P. According to the manufacturer, its ultimate strength and elastic modulus were 55 MPa and 3.5 GPa respectively, and the elongation was 3%.

Table 2.

Mechanical properties of materials.

Materials	Type	Sectional size (mm)	Yield strength (MPa)	Ultimate strength (MPa)	Ultimate tensile strain ( $μ ε$ )	Elastic modulus (GPa)	Elongation (%)
Concrete	C30	--	--	39.8	--	--	--
Rebar	HRB400 (longitudinal bar)	25	483	634	--	210	--
	HRB235 (stirrup)	ϕ10	345	522	--	210	--
	Q235b (Steel strip)	20×5.0	311	440	--	203	--
	Q235b (Steel strip)	20×3.5	320	483		205
CFRP	UD300CF/1	--	--	3614	15,700	230	--
Epoxy resin adhesive	JN-C3P	--	--	55	--	3.5	3

Hybrid stirrups manufacturing and tensile test on CFRP stirrups

Hybrid stirrups manufacturing

The fabrication process of the hybrid stirrups was as follows: (a) The steel strip was bent with a bending radius of 30 mm and welded in the middle of the short side to form a closed rectangular stirrup, as shown in Figure 3(a). Then, the steel surface was polished and cleaned to facilitate the wrapping of the CFRP strip. (b) The CFRP strip was then impregnated with the adhesive and wrapped around the steel strip stirrup, as shown in Figure 3(b). During the wrapping process, it was important to ensure that the CFRP strip bonds closely to the steel and between each CFRP layer to avoid air bubbles. After the wrapping, it was cured at room temperature for a week. Note that the CFRP strips in this hybrid stirrup are only wrapped around the exterior of the steel strips, which was a measure adopted for the sake of convenience in manual laboratory fabrication. This arrangement was intended for the preliminary exploration of the shear behavior and ductility enhancement of the hybrid stirrup. On this basis, to improve the durability of the hybrid stirrups, further investigations will be carried out in future research.

Figure 3.

Types of stirrups.

The manufacturing process of the CFRP strip stirrups was similar to the hybrid stirrups. The difference was that there was a layer of Saran wrapping around the steel strip before the wrapping of CFRP strip, as shown in Figure 3(c). In this case, when the CFRP strip is well cured, it not only can keep a similar shape as the steel strip stirrup but also can be easily demolded from that.

Tensile test on CFRP stirrups

Due to the weakness in the corner section of CFRP strip stirrups, the end of the CFRP strips overlapped at the lower corner of the stirrups during manufacturing to have additional reinforcement, exceeding the corner by 30 mm. To test the effectiveness of the reinforcement in corner with a single-layer CFRP, tensile tests of CFRP strip stirrups were conducted in accordance with ACI 440.3R (2012), as shown in Figure 4(a). The failure mode of the specimens was fiber bundle rupture in the middle of the CFRP strip, as shown in Figure 4(b). The average CFRP ultimate stress of the four specimens was approximately 3300 MPa. Taking into account the effects of eccentricity errors caused by the placement of the jack and loading plates, it can be concluded that the CFRP strip reached its ultimate tensile strength and ruptured in non-bending sections. Therefore, it was evident that simply adding an extra layer of CFRP strip at the corner can effectively improve strength of the CFRP stirrup in bending locations and avoid the rupture of CFRP in the corner.

Figure 4.

Tensile test on CFRP stirrup: (a) Loading diagram; (b) Failure mode.

Measurement and loading

Prior to the testing, the deformation measurement was prepared for the specimens. As shown in Figure 2, the strains gauges were attached to the longitudinal rebar and concrete surface on the top of the beam in the mid-span and the surface of stirrups which may intersect with the diagonal cracks, while the linear variable differential transformers (LVDTs) were placed at the midspan and two supports.

To understand the shear performance of the RC beams reinforced with the hybrid stirrups, three-point bending tests were performed in this study, as shown in Figure 5. The loading was applied via a load cell with a capacity of 1000 kN at the mid-span of the beam and the lengths of shear segments were 700 mm. The load control mode was adopted for the testing in the first stage. First of all, the pre-loading (up to 20 kN) was conducted to assure that the testing devices were working well. After that, the formal loading started and the strain and displacement were collected by data logger every 20 kN/level during loading. When approaching the cracking load, yielding load and ultimate load, the data acquisition frequency was properly adjusted from 20 kN to 10 kN per level. Upon reaching the ultimate load, the loading moved to the second stage and began with a mode of deflection control (1 mm/level) until the failure of specimen. To clearly observe the phenomena and cracking behavior, the loading was sustained for at least 5 minutes for each load level.

Figure 5.

Test setup and loading system.

Results and analysis

From the three-point loading test, the test results of the beams are summarized in Table 3. The shear behaviors of the tested beams such as ultimate load, shear resistance efficiency of stirrups, ultimate deflection, maximum crack width, and failure mode were compared and discussed, as follows.

Table 3.

Summary of test results.

Specimen no.	Normalized stirrup capacity S_c (MPa)	Ultimate load (kN)	Increase ratio ζ(%)	Shear resistance efficiency $φ$	Ultimate deflection (mm)	Maximum crack width (mm)	Failure mode
NS	—	273	—	—	5.7	4.50	SC
RS	1.694	511	87.2	1.12	9.2	5.50	SC
SS5	1.553	481	76.2	1.07	7.3	4.00	SC
C3	1.805	480	75.8	0.92	8.8	4.50	CF
H3.5 + 1	1.72	498	82.4	1.05	8.6	3.50	SC
C6	3.615	526	92.7	0.56	10.2	3.50	CF
SS3.5	1.12	404	48.0	0.94	4.9	1.50	DT
SS5b	2.26	590	116.1	1.12	8.8	1.80	SC
C3b	2.63	523	91.6	0.76	9.1	4.00	CF
H3.5 + 1b	2.505	619	126.7	1.10	14.6	2.20	HF
H3.5 + 3b	4.255	634	132.2	0.68	31.0	2.60	--

Note: Values in each row are from a single specimen test. For the failure modes, SC, CF, DT, and HF are shear compression failure, CFRP strip fracture, diagonal tension failure, and hybrid stirrup fracture, respectively. Although the specimen H3.5 + 3b can still hold the load after concrete crushing, the loading stops when its deflection reached 1/50 l₀ as it has far exceeded the normal service limit state of the RC beam.

Failure modes and cracking behavior

The failure mode for each specimen is given in Table 3 and shown in Figure 6. Four typical failure modes were observed in the test: shear compression, CFRP fracture, diagonal tension, and hybrid stirrups fracture, as shown in Figure 7. Shear compression failures occurred in specimens NS, RS, SS5, H3.5 + 1, and SS5b, as shown in Figure 7(a). After the critical diagonal crack developed, the concrete in the compression zone crushed, causing a significant load decrease and failure. Specimens C3, C6, and C3b exhibited failure due to CFRP fracture, as shown in Figure 7(b). After concrete crushing, these beams could still maintain or even slightly increase the load until the CFRP strips experienced excessive deformation and fractured, resulting in a sharp load drop and beams failure. Due to the lower S_c, diagonal tension failure occurred in specimen SS3.5, as shown in Figure 7(c). Diagonal cracks formed early, stirrups yielded rapidly, and the crack extended to the loading point, splitting the beam diagonally into two parts. Specimen H3.5 + 1b experienced concrete crushing, followed by the fracture at the hybrid stirrup welding point. Even so, the beam could still maintain the load, until the concrete crumbled due to excessive overall deformation, leading to the fracture of the hybrid stirrups, as shown in Figure 7(d).

Figure 6.

Crack distribution of test beams:(a) NS; (b) RS; (c) SS5; (d) C3; (e) H3.5 + 1; (f) C6; (g) SS3.5; (h) SS5b; (i) C3b; (j) H3.5 + 1b; (k) H3.5 + 3b.

Figure 7.

Typical failure modes: (a) Shear compression failure; (b) CFRP strip fracture; (c) Diagonal tension failure; (d) Hybrid strip fracture.

After failure, steel strip stirrups remained intact, while CFRP strip stirrups were prone to brittle fracture, especially at corners where multi-axial stress concentrated due to diagonal cracks. In hybrid stirrups with a single CFRP layer, diagonal cracks development intensified the CFRP deformation, leading to debonding and fracture. However, increasing the number of CFRP layers effectively controlled the stirrups deformation, inhibiting the development of inclined cracks. Even after the concrete crushing, they can continue to deform gradually and the hybrid stirrups can remain intact, which avoided the brittle shear failure in case with only CFRP strip stirrups. Corner reinforcement can effectively prevent early failure of CFRP, and no damage was observed when additional CFRP layers were used. Moreover, for hybrid stirrups, even if CFRP fractured, steel strips can still prevent concrete explosive spalling due to the sudden energy release. Even so, it is necessary to pay attention to the treatment of steel strip welding points in engineering practice.

The maximum crack width of beams with different strip stirrups all decreased with the stirrup spacing, as shown in Figure 8. Reducing the stirrup spacing differently affected the crack width, depending on the stirrup type. For steel strip stirrup, closer spacing enhanced the number of stirrups intersecting the critical crack. Consequently, the confinement was enhanced, the crack opening was slowed, and the crack propagation was effectively limited, resulting in a 55.5% reduction in maximum crack width. For hybrid stirrups, the ductile steel yielded while the CFRP strip continued to carry the force. Thereby, reducing the stirrup spacing could provide additional confinement and force transfer, leading to a 37.1% reduction in crack width. In contrast, the beam reinforced with only CFRP strip stirrups were brittle and could not redistribute stress after cracking. Even with denser stirrup spacing, the crack development was not effectively suppressed, resulting in only 11.1% reduction in maximum crack width. This comparison highlights that crack width control is most effective when the stirrup can redistribute stress, as in steel strip or hybrid stirrups.

Figure 8.

Crack width of test beams: (a) Maximum crack width; (b) Load-width curve of main inclined cracks.

Shear behavior

The load-deflection curves are shown in Figure 9. As observed, all specimens showed a similar load-deflection curve and exhibited a linear manner during the initial stages of loading. At around 200 kN, the slope of each curve started to decrease as the occurrence of shear inclined cracks leading to the reduction of beam stiffness. Thereafter, the stirrups played a major role in crack inhibition and the differences in the load-deflection curves gradually appeared. For beams with CFRP strip stirrups, as CFRP shows linear elastic behavior, the load and deflection can increase synchronously until the fracture of CFRP strip. For beams with steel strip stirrups, the deflection increased rapidly once the steel strip yielded, and the load–deflection response typically shows limited post-yield hardening. Near ultimate load, a sharp drop occurred as concrete in the compression zone was crushed. For the beams with the hybrid stirrups, the initial stiffness was similar to the beams with steel strip at a comparable normalized stirrup capacity (S_c), because early response was governed mainly by flexure and concrete cracking. However, after the steel strip yielded, the beam with hybrid stirrups continued to carry the load, providing additional shear resistance and confinement, which delayed shear failure and allowed larger deflection to develop before ultimate. After the concrete was crushed, while the load can be sustained, the deflection increased significantly. This indicates that the steel strip and CFRP strip in the hybrid stirrup achieved good joint working and can largely improve the shear performance of the RC beams.

Figure 9.

Load-deflection curves of tested beams.

The ultimate load, stirrup resistance efficiency (φ) and ultimate deflection of each beam are presented in Table 3. The specimens RS and SS5, with similar stirrup reinforcement ratios and S_c, had cross-sections of ϕ10 circular and 5 mm × 16 mm rectangular respectively, and their φ values were quite comparable. This suggested that variations in cross-sectional shapes had no obvious impact on the utilization ratios of stirrups. Compared to specimen SS3.5, specimen H3.5 + 1 with merely an additional layer of CFRP strip experienced a nearly 72% enhancement in ultimate load contribution from the stirrups. This is mainly because the steel strip yielded first, providing ductility, while the CFRP strip continued to carry additional tensile force across the shear cracks, delaying the shear failure. This implies that the CFRP strip in specimen H3.5 + 1 was effectively utilized and that the CFRP strip and steel strip worked together efficiently.

At a comparable S_c, the φ of specimens C3, SS5 and H3.5 + 1 were 0.92, 1.05, and 1.07, and the ultimate deflections at mid-span of them were 8.8 mm, 7.3 mm and 8.6 mm respectively. The φ of specimen C3 was significantly lower than the others, which was only 0.92. It was primarily due to the similarity between the elastic modulus of steel and CFRP, coupled with the fact that the CFRP strip stirrup had the smallest cross-sectional area among the three types of strip stirrups. As a result, it had the largest deformation and showed the most severe cracking. This led to the crushing of concrete in the shear compression zone, and the CFRP strip stirrups fractured due to substantial deformation. The difference of the specimens SS5 and H3.5 + 1 in φ was minor, only 0.02, revealing that the shear capacity of beams with steel strip stirrups and beams with hybrid stirrups were similar. And regarding the ultimate deflections at mid-span, due to the presence of CFRP strips, beams with hybrid stirrups were superior to that of beams with steel strip stirrups.

After reducing the stirrup spacing, the S_c increased 45.6%, the φ of specimens C3b, SS5b and H3.5 + 1b were 0.76, 1.10 and 1.12, and the ultimate deflections at mid-span of them were 9.1 mm, 8.8 mm and 14.6 mm respectively. For beams with CFRP strip stirrups, as the shear capacity increased by 8.9% only, the φ decreased from 0.92 to 0.76. In contrast, the capacities of beams with steel strip stirrups and one-layer CFRP- hybrid stirrups improved by 22.7% and 24.3% respectively, the φ in both cases had a growth of 0.05. Due to the small cross-sectional area of CFRP strip stirrups, after reducing the stirrup spacing, large deformation and severe cracking can still occur, thereby the increase of shear capacity was not as obvious as the other cases. Besides, the beam with CFRP strip stirrups had the smallest increase in ultimate deflection, followed by the beam with steel strip stirrups. The specimen with hybrid stirrups exhibited the greatest increase in ultimate deflection, reaching as high as 6.0 mm. This was primarily because the outer CFRP strip can maintain the load in the later loading stages by constraining the substantial deformation of the internal steel strip, thereby effectively improving the ductility of the beam. Also, after reducing the stirrup spacing, the φ of the beam with hybrid stirrups was higher than that of the beam with CFRP strip stirrups, with an increase of 45%, while the ultimate deflection was 1.7 times than that of the beam with steel strip stirrups. This suggests that by reducing the stirrup spacing, the shear performance of hybrid stirrups became notably superior compared to individual CFRP strip stirrups and steel strip stirrups. Furthermore, the ultimate deflection of specimen H3.5 + 3b reached up to 31 mm, indicating that increasing the number of CFRP layers in the hybrid stirrups can significantly enhance its deformation capacity.

Since the poor shear performance of beam with CFRP strip stirrups was due to the less rigidity of the CFRP, the shear resistance was further examined by increasing the number of CFRP layers. The results showed that, after increasing the number of layers, S_c increased from 1.805 to 3.615. However, the shear capacity increased by only 9.6% and the φ decreased to 0.56, while the ultimate deflection increased by 15.9%. The improvement in shear capacity and ductility from increasing the number of CFRP layers was also very limited. Hence, neither decreasing the stirrup spacing nor increasing the layers of CFRP contributed obviously to the shear capacity and ductility.

Stirrup strain behavior

From the measurement, the strain distributions in different stirrups at shear span were obtained respect to the loading levels, as shown in Figure 10. During the initial stages of loading, the shear force was mainly resisted by the concrete, resulting in relatively small stress in stirrups. With the development of inclined cracks, the strain of stirrups increased steadily to withstand higher stress. The differences in the strain curves of beams with different types of stirrups gradually became apparent. For specimens with steel strip stirrups SS3.5, SS5 and SS5b, the strain increment of the stirrups was relatively small during the early loading stage due to their high rigidity, as shown in Figure 10(a)-(c). After steel stirrups yielding, the strain increased sharply, and the cracks developed rapidly. For the specimens with CFRP strip stirrups C3, C3b and C6, because of the linear elastic behavior of CFRP, the strains of the stirrups grew steadily, as shown in Figure 10(d)-(f). Even though the concrete was crushed, the beams with CFRP strip stirrups can still stably bear loads in the later loading stage, and there was a slight increase in load. In hybrid stirrups, the stirrup strain was situated between that of the steel strip stirrups and the CFRP strip stirrups, and the strain distributions in outer CFRP strip and inner steel strip were consistent, as shown in Figure 10(g)-(l). Taking specimen H3.5 + 1 as an example (Figure 10(g) and (j)), the strain curves of both materials in the same hybrid stirrup aligned closely when the load was below 400 kN but diverged in the later stages. This divergence may be due to the debonding or relative slippage between CFRP and steel, which required further investigation in the future.

Figure 10.

Stirrup strain distribution of beams:(a) SS5; (b) SS5b; (c) SS3.5; (d) C3; (e) C3b; (f) C6; (g) H3.5 + 1; (h) H3.5 + 1b; (i) H3.5 + 3b; (j) H3.5 + 1; (k) H3.5 + 1b; (l) H3.5 + 3b.

Shear capacity calculations

For the calculation of the shear capacity of concrete beams reinforced with FRP stirrups, it was generally assumed that the shear resistance mechanism of FRP reinforced concrete beams was similar to that of beams reinforced with conventional steel bars. The shear capacity was the sum of the shear contributions from concrete (including longitudinal rebars) V_c, and internal FRP stirrups V_f. In this study, Zsutty’s formula (Zsutty, 1968, 1971) was used to calculate the shear capacity of concrete (including longitudinal rebars). The formula was developed based on regression analysis of a large number of experimental data, where the shear resistance was attributed primarily to aggregate interlock and longitudinal rebar dowel action. While effective for predicting the concrete contribution, it does not account for shear reinforcement, and thus in this study it was applied only to isolate the concrete capacity. The shear capacity of concrete (including longitudinal rebars) V_c can be calculated as follows:

V_{c} = 2.17 Ψ_{c} {(\frac{f_{c} ρ}{λ})}^{1 / 3} b h_{0}

(3)

where

Ψ_{c}

is the adjustment coefficient, when the shear span ratio of test beam

λ

≥ 2.5,

Ψ_{c}

= 1.0, when

λ

< 2.5,

Ψ_{c}

= 2.5/

λ

;

ρ

is the ratio of longitudinal rebars;

f_{c}

is the compressive strength of concrete.

The calculation method for the shear capacity of FRP stirrups (V_f) varied according to different codes, including the JSCE (1997), ACI 440.1R (2015), GB50608 (2020) and CSA S806 (2017). The detailed equations of these codes are given in Table 4. Also, these codes limited the strain in FRP stirrups to the range from 0.0025 to 0.005, and assumed a linear relationship between V_f and ρ_fv E_fv. Since these limitations were very conservative, Nehdi et al. (2006, 2007) modified the equation by using artificial neural networks and genetic algorithms, as presented in Table 4. In addition, the shear capacity of concrete beams reinforced with steel strip stirrups was calculated by using the conventional method for RC beams (GB 50010, 2010), which had a good agreement with the experimental results. The method is not discussed here.

Table 4.

Shear design equations for FRP-reinforced concrete beams.

Method	Equations
JSCE (1997)	$V_{f} = \frac{A_{f v} E_{f v} ε_{f v} h_{0}}{1.15 s}$
JSCE (1997)	$ε_{f v} = \sqrt{f_{m c}^{'} \frac{ρ_{l} E_{l}}{ρ_{f v} E_{f v}}} (1 + \frac{{2 σ}_{N}^{'}}{f_{m c}^{'}}) \times 10^{- 4} \leq \frac{f_{f b}}{E_{f}}, f_{f b} = (0.05 \frac{r_{b}}{d_{b}} 0.3) f_{f u}$
ACI 440.1R (2015)	$V_{f} = \frac{A_{f v} σ_{f v} h_{0}}{s}$
ACI 440.1R (2015)	$σ_{f v} = 0.004 E_{f v} \leq f_{f b}, f_{f b} = (0.05 \frac{r_{b}}{d_{b}} + 0.3) f_{f u}$
GB50608 (2020)	$V_{f} = \frac{A_{f v} σ_{f v} h_{0}}{s}$
GB50608 (2020)	$σ_{f v} = \min {0.004 E_{f v}, f_{f b}}, f_{f b} = (0.05 \frac{r_{b}}{d_{b}} + 0.3) f_{f u}$
CSA S806 (2017)	$V_{f} = \frac{A_{f v} σ_{f v} d_{v} \cot θ}{s}$
CSA S806 (2017)	$σ_{f v} = \min {0.005 E_{f v}, 0.4 f_{f u}}, θ = 30 ° + 7000 ε_{l}, ε_{l} = \frac{M + V d_{v}}{2 d_{v} A_{l} E_{l}}$
Nehdi et al. (2007)	$V_{f} = 0.74 {(ρ_{f v} f_{f u})}^{0.51} b h_{0}$

Note: $ε_{fv}$ is the strain in the FRP stirrup; $E_{fv}$ is the elastic modulus of FRP stirrup; $f_{mc}^{'}$ is the specified compressive strength of concrete taking into account member size, $f_{mc}^{'} = {(\frac{h}{0.3})}^{‐ 0.1} f_{c}^{'}$ ; $ρ_{l}$ is the ratio of longitudinal bar; $E_{l}$ is the elastic modulus of longitudinal bar; $ρ_{fv}$ is the ratio of FRP stirrup; $σ_{N}^{'}$ is axial compressive stress; $f_{fb}$ is the tensile strength of bent portion of FRP stirrup; $f_{fu}$ is the ultimate strength of FRP stirrup; $r_{b}$ is radius of the bend; $d_{b}$ is diameter of FRP stirrup; $σ_{fv}$ is the stress level in the FRP stirrup; θ is the angle of the diagonal compressive stress, $30 ° \leq θ \leq 60 °$ ; d_v is effective shear depth, d_v = 0.9h₀; $ε_{l}$ is the longitudinal strain at mid-depth of the section; M and V are the factored shear force and factored moment, respectively; $A_{l}$ is the area of longitudinal bar.

Beam with CFRP strip stirrups

The calculation of the shear capacity from CFRP strip stirrups referenced the aforementioned calculation method. All codes uniformly provided a calculation formula for the tensile strength of bent portion of FRP stirrup $f_{f b}$ , which was initially provided by the JSCE (1997), but it was only applicable to FRP bars with a circular cross-section. Lee et al. (2014) proposed a method for calculating the equivalent cross-sectional diameter d_f, by converting the thickness t_f of strip stirrup with the rectangular cross-section, and modified the coefficient in the $f_{f b}$ , as follows:

f_{f b} = (0.02 \frac{r_{b}}{d_{f}} + 0.47) f_{f u} \leq f_{f u}

(4)

d_{f} = \frac{2 t_{f}}{\sqrt{π}}

(5)

where t_f is the thickness after the FRP was impregnated and solidified. In this experiment, the CFRP strip stirrups were handmade, so it was impossible to accurately control the amount of adhesives in the stirrups. Combined with the actual thickness of the stirrups after shaping, the thickness of the CFRP strip after curing was measured and the value was 0.8 mm per layer.

V_{c a} = V_{f} + V_{c}

(6)

The shear capacity of CFRP strip stirrups was calculated using the aforementioned formula in Table 4 and equation (4). The shear capacity of the beams with CFRP strip stirrup was calculated using equation (6). The comparison of the calculations of the shear capacity of beams with CFRP strip stirrup by different formulas is presented in Table 5. The ACI 440.1R (2015) and GB50608 (2020) provided identical calculation formulas, resulting in equal calculations, so the results of these two codes were no longer presented separately.

Table 5.

Comparison between the experimental and calculated values of shear capacity of beams with CFRP strip stirrups.

Specimen	V_ex (kN)	V_c (kN)	JSCE (1997)		ACI 440.1R (2015)/GB 50608 (2020)		CSA S806 (2017)		Nehdi et al. (2007)
Specimen	V_ex (kN)	V_c (kN)	V_f (kN)	V_ca∕V_ex	V_f (kN)	V_ca∕V_ex	V_f (kN)	V_ca∕V_ex	V_f (kN)	Vca∕Vex
C3	240	140.3	29.2	0.71	28.8	0.70	15.7	0.65	62.5	0.85
C3b	261.5	140.3	35.2	0.67	41.8	0.70	22.2	0.62	75.7	0.83
C6	263	140.3	41.4	0.69	57.5	0.75	30.4	0.65	89.1	0.87
Average value				0.69		0.72		0.64		0.85
COV				0.021		0.034		0.021		0.022

Note: V_ex and V_ca refer to the experimental and calculated values of the shear force of the test beam respectively.

The current formulas generally underestimate the shear capacity of the CFRP strip stirrups due to the brittle failure of FRP, which is too conservative for design, as presented in Table 5. In addition, according to Nehdi’s equation, the shear capacity of the CFRP strip stirrups V_f was not linearly related to the S_c ( $ρ_{f} f_{f}$ ) but had a better linear correlation with $\sqrt{ρ_{f} f_{f}}$ . Although the calculated results were consistent with the experimental results, this equation still yielded conservative results. Considering the material characteristics of FRP, these results were safe, and such a degree of deviation was acceptable.

Beam with hybrid stirrups

For hybrid stirrups, the shear capacity can be considered as the superposition contributions from steel strip (V_sv) and CFRP strip (V_fv). As observed from the experimentation, the shear capacity of hybrid stirrups was primarily provided by steel strips, while the CFRP strips in hybrid stirrups mainly enhanced the ductility of the beams by continuously sustaining the loading at later stages. The CFRP strip in hybrid stirrups cannot achieve the same effect of shear capacity enhancement as pure CFRP stirrups, and the larger amount of CFRP strip in hybrid stirrups, the smaller shear capacity enhanced. Therefore, the coefficient η was introduced to reduce the shear capacity of CFRP strip in hybrid stirrups, where the coefficient η was related to the ratio of steel and CFRP in hybrid stirrups. V_sv was calculated based on GB 50010 (2010), and V_fv was calculated by equation proposed by Nehdi et al. (2007). The shear capacity of the hybrid stirrups (V_sf) can be calculated as follows:

V_{s f} = V_{s v} + V_{f v}

(7)

V_{f v} = 0.74 η {(ρ_{f v} f_{f u})}^{0.51} b h_{0} \leq ρ_{f v} f_{f u} b h_{0}

(8)

η = A_{s v} f_{y v} / (A_{s v} f_{y v} + A_{f v} f_{f})

(9)

V_{c a} = V_{s f} + V_{c}

(10)

The shear capacity of the beams with hybrid stirrup was calculated using equation (10), as presented in Table 6.

Table 6.

Comparison between the experimental and calculated values of shear capacity of beams with hybrid stirrups.

Specimen	Vsf (kN)	Vc. (kN)	Vca (kN)	Vex (kN)	Vca/Vex
H3.5 + 1	93.3	140.3	233.6	249	0.94
H3.5 + 1b	129.9	140.3	270.2	309.5	0.87
H3.5 + 3b	130.4	140.3	270.7	317	0.85
Average value					0.89
Cov					0.040

Note: V_ex and V_ca refer to the experimental and calculated values of the shear force of the test beam respectively.

In Table 6, the calculated shear capacity of the specimens was conservative, with V_ca/V_ex of 0.89 and and COV of 4.0%. Due to the brittle fracture characteristic of CFRP upon failure, a certain safety margin must be reserved. It is feasible to use the proposed models to calculate the shear capacity of beams reinforced with hybrid stirrups. Even so, there were too few calculation samples for the beams with hybrid stirrups. Further research is required to investigate the effect of factors such as stirrup width and the ratio of steel to FRP in hybrid stirrups.

Conclusions

In this paper, a novel CFRP-steel strip hybrid stirrup was proposed, which effectively combined the material advantages of both steel and CFRP. Experimental tests were conducted to compare the shear behavior of beams reinforced with steel strip stirrups, CFRP strip stirrups, and hybrid stirrups. In addition, a modified shear prediction method was developed for practical design guidance. Overall, this work contributes to RC beam design by providing a validated hybrid stirrup concept and analytical insights for its application. From the results and discussions, the following conclusions can be reached.

(1) The shear performance of the proposed hybrid stirrups was superior to that of the individual CFRP strip stirrups and steel strip stirrups. Reinforced with similar tensile capacity of stirrups, the shear capacity of the beam with hybrid stirrups was higher than that of the beam with CFRP strip stirrups, with an increase of 45% in reinforcing efficiency. Also, the ultimate deflection of the beam with hybrid stirrups was 1.7 times than that of the beam with steel strip stirrups, demonstrating a clear advantage in ductility.

(2) The inner steel strip in the hybrid stirrups can enhance the rigidity of the stirrups and effectively inhibit the crack development, primarily bearing the shear force before the steel reinforcement yielding. In contrast, the outer CFRP strip can maintain the load in the later loading stages by constraining the substantial deformation of the internal steel strip, thereby effectively improving the ductility of the beam.

(3) For beams with hybrid stirrups, both shear capacity and ductility decreased with the stirrup spacing. When the stirrup spacing was decreased from 160 mm to 110 mm, the shear capacity and ultimate deflection of the beam increased by 24.3% and 67.8%, respectively. As the layers of CFRP increased from one to three layers, the mid-span deflection of specimen reached up to 31 mm, increasing by 112%. However, for beams with CFRP strip stirrups, both decreasing the stirrup spacing and increasing the layers of CFRP contributed insignificantly to the shear capacity (increase by 9.0% and 9.6%, respectively) and ductility (increase by 3.4% and 15.9%, respectively).

(4) A modified calculation method for predicting the shear capacity of beams with hybrid stirrups was developed, in which a coefficient was introduced to reduce the shear contribution of CFRP in the hybrid stirrup. The calculation results were in good agreement with the experimental results, which validated the proposed calculation method. However, to propose more rational design method, further research is required in the future to investigate the effect of factors such as stirrup width and the ratio of steel to FRP in hybrid stirrups.

Footnotes

ORCID iDs

Ridho Surahman

Yi Wang

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (project No. 52178308), Central South University Innovation-Driven Research Programme, China (project No. 2023CXQD051), and National Key Research and Development Program of China (project No. 2022YFC3800902).

Declaration of conflicting interests

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

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