Behaviour of stainless steel wire mesh strengthened reinforced concrete flanged beams under pure torsion

Abstract

Strengthening existing concrete structures contributes to reduced reconstruction efforts and conservation of natural resources. Advanced composite materials are increasingly employed to strengthen concrete structures to satisfy changes in serviceability characteristics, modified analysis, and design philosophies. Among the various options, Stainless Steel Wire Mesh (SSWM) has emerged as a promising material for strengthening Reinforced Concrete (RC) structural elements. The present experimental study investigates the effectiveness of SSWM for enhancing the torsional performance of RC flanged beams. 12 RC flanged beam specimens made of M25 grade concrete are prepared. The cross-section of each T-shape RC beam specimen has a 225 mm wide and 50 mm deep flange and a 100 mm wide and 150 mm deep web, with a total length of 1300 mm. Ten T-beams are strengthened using five different SSWM wrapping configurations, while two unstrengthened beams are considered as control specimens and tested under pure torsional load. The torsional moment and angle of twist at the initial crack and peak torque stages, strain at critical locations, energy absorption capacity, strength index and torsional ductility index of SSWM strengthened specimens are compared with that of the control specimen. The results show that beams strengthened with SSWM, particularly specimens with corner and vertical strip above the stirrups configurations, exhibited significantly enhanced peak torque, corresponding twist and improved failure behaviour compared to control specimens and specimens strengthened with other SSWM wrapping configurations.

Keywords

stainless steel wire mesh reinforced concrete torsional behaviour flanged beam strengthening

Introduction

Reinforced Concrete (RC) structural elements are often subjected to torsional loading, specifically in complex geometries and loading conditions such as flanged beams curved in plan, inverted T-shaped precast beams supporting slab on both sides, and box girder carrying vehicular loads. The flexural and shear behaviour of RC elements has been extensively studied, while torsional behaviour remains relatively unexplored, despite its critical role in ensuring structural integrity under eccentric loads. The torsional strength and stiffness of RC specimens under eccentric transverse loading are of major concern due to the increased risk of cracking and damage.

To address torsional deficiencies, various rehabilitation and strengthening methods such as fibre-reinforced polymer (FRP) and concrete jacketing are widely adopted to restore or enhance the structural performance of RC beams (Alabdulhady et al., 2017; Behera et al., 2016; Chalioris, 2008; Jayasree et al., 2016). These methods help in eliminating the necessity of demolishing and reconstructing the damaged structural elements or, in some instances, the entire structure. However, concrete jacketing is generally a cumbersome and time-consuming process that increases the cross-sectional area and may lead to localised damage in the jacketed zone. The other innovative materials, such as aluminium honeycomb panels and high-strength geopolymer grout have shown potential for flexure strengthening of RC beams but lack application for torsional strengthening (Chen et al., 2024; Kantarci et al., 2023; Kantarci and Maraş, 2022).

Recent studies have significantly improved the understanding of torsional strengthening of RC members through experimental, analytical and numerical approaches. Majeed et al. (2017) reported experimental work on torsional strengthening of a multi-cell box-section RC girder with externally bonded Carbon Fibre Reinforced Polymer (CFRP) strips. They also carried out numerical simulations using non-linear finite element method and found good agreement of torque-twist behaviour and crack propagation with experimental results. Majed et al. (2020) developed an analytical model based on the softened truss model (STM) to predict the torsional behaviour of RC beams strengthened with FRP. The model was able to predict the torsional behaviour of the FRP-strengthened RC beams before and after cracking stages with reasonable accuracy. Allawi et al. (2023) assessed torsional behaviour of non-damaged and pre-damaged RC multi-cell box girder specimens externally retrofitted by CFRP through experimental and numerical studies. They found that the CFRP strips started to work more effectively and contributed together with the transverse steel reinforcement to resist torsional load when the girder specimen was cracked. Majed et al. (2021) investigated numerically the behaviour of FRP-strengthened rectangular RC beam subjected to pure torsion and validated results of analysis with existing data from the literature. They considered parameters such as number of FRP plies, concrete compressive strength, FRP strip orientation and compared torque-twist behaviour, response of reinforcements and crack patterns. Further, a comprehensive FE parametric study conducted by Altaee et al. (2020) employed damage plasticity models to assess the torsional behaviour of FRP-strengthened rectangular beams. However, these contributions largely focus on beams with rectangular or hollow cross sections and FRP strengthening systems. Limited research has addressed flanged RC beams and the use of materials alternative of FRP. Additionally, previous studies have not compared the effectiveness of multiple wrapping configurations under torsional loading. Moreover, FRP strengthening has several drawbacks, including brittle failure, debonding from concrete surfaces, poor fire resistance and being expensive (Abdoli et al., 2024; Behera et al., 2016; Chalioris, 2007; Jayasree et al., 2016; Jiang et al., 2017).

To overcome the limitations of FRP and concrete jacketing, researchers have explored alternative strengthening materials. One such material, stainless steel wire mesh (SSWM), was explored by Kumar and Patel, (2016a, 2016b), which is locally available, cost-effective, and exhibits good fire and corrosion resistance. SSWM was first employed for the strengthening of circular concrete columns by Kumar and Patel (2016a). Significant improvement in the axial load-carrying capacity of the circular columns with different concrete grades and aspect ratios strengthened with different numbers of SSWM layers was observed by the authors (Kumar and Patel, 2016a). A finite element-based numerical simulation of SSWM-strengthened circular columns was presented by Kumar and Patel (2016b). They found close agreement between experimental results and numerical results in terms of the load-displacement behaviour of SSWM-strengthened concrete columns. The use of SSWM was further explored for torsion and shear strengthening of RC beams due to the availability of material, cost-effectiveness, and resistance to fire and corrosion (Patel et al., 2018; Raiyani et al., 2020; Raiyani and Patel, 2023). The authors (Patel et al., 2018; Raiyani and Patel, 2023) performed an experimental study to explore the torsional behaviour of SSWM strengthened RC square solid and hollow beams. The use of SSWM significantly enhanced the ductility of RC beams and reduced cracks. The results of the study also revealed that the bond between the concrete surface and the SSWM is crucial for transferring loads at the interface. SSWM has good bond with the concrete surfaces when SSWM is applied with Sikadur 30LP epoxy. Results of experimental study also showed higher strength and ductility of RC elements wrapped with a circumferential SSWM configuration.

The current research on torsional strengthening of RC beams using SSWM is largely focused on rectangular, square solid and hollow cross-sections. The application of SSWM for torsional strengthening of RC flanged beams remains largely unexamined, particularly regarding the influence of different wrapping configurations. In addition, international design guidelines, such as FIB-CEB Bulletin 14 (2001) and ACI 440 - 2R (2017) related to externally bonded strengthening systems for RC elements are concerned about premature debonding, anchorage issues, and stress transfer under torsion, highlighting the need for an alternative method or material to improve bond behaviour and ductility.

The present study addresses these research gaps by experimentally investigating 12 RC flanged beam specimens strengthened with five distinct SSWM wrapping configurations and subjected to pure torsional loading. Key performance parameters evaluated during experimental investigation include torque–twist response, cracking and peak torque, energy absorption, and failure mechanisms. This experimental study fills a critical void in both the geometric application of torsional strengthening to RC flanged cross-sections and in expanding beyond traditional FRP materials. The findings contribute valuable insights for the development of practical, durable, and cost-effective retrofitting strategies for RC beams subjected to pure torsion.

Experimental program

The experimental program includes the preparation of test specimens, evaluation of material properties, test setup for applying pure torsion, and instrumentation for measuring the response of the specimen during testing.

Details about flanged beam specimens

The study aims to evaluate the behaviour of SSWM-strengthened RC flanged beams subjected to pure torsional load. For this purpose, 12 RC beams of flanged (T-shape) sections are prepared. The cross-section of the RC beam specimen consists of a flange 225 mm wide and 50 mm deep, and a web 100 mm wide and 150 mm deep. The total length of each specimen is 1300 mm, which includes a test length of 1000 mm. The design and reinforcement detailing of T-shaped RC beam are carried out according to IS 456 (2000). The longitudinal reinforcement of RC beam specimens includes four numbers of 10 mm diameter bars in the flange and two numbers of 10 mm diameter bars in the tension zone of the web. Two-legged 8 mm diameter stirrups at equal longitudinal spacing of 150 mm c/c are provided in the transverse direction, as shown in Figure 1(a). A total of five different SSWM wrapping configurations are used for the strengthening of the RC flanged beam. Table 1 and Figure 1 present the details of different wrapping configurations employed for the strengthening of flanged beams. Two specimens for each un-strengthened and SSWM-strengthened beam are prepared to ensure the reliability of experimental results.

Figure 1.

Schematic illustration of various SSWM wrapping configurations applied to Flanged RC Beams (dimensions in mm). (a) Cross-section, longitudinal section, and reinforcement details of flanged RC beams (TCON). (b) Flanged (T shape) beam with 100 mm Strip wrapping Above the Stirrups (T100SAS). (c) Flanged (T shape) Beam with COrner strip and 100 mm Strip wrapping Above the Stirrups (TCO&100SAS). (d) Flanged (T shape) beam with 100 mm Strip wrapping In between the Stirrups (T100SIS). (e) Flanged (T shape) beam with COrner strip and 100 mm Strip wrapping In between the Stirrups (TCO&100SIS). (f) Flanged (T shape) beam with three sides in U shape Wrapping Full Wrapping (TUW).

Table 1.

Nomenclature of beam specimen and strengthening scheme.

Specimen Identification	Description of SSWM wrapping configuration	Fig. No.
TCON	Flanged (T-shape) beam with no SSWM wrapping (CONtrol specimen)	Figure 1
T100SAS	Flanged (T-shape) beam with 100 mm Strip wrapping Above the Stirrups	Figure 2(a)
TCO&100SAS	Flanged (T-shape) beam with COrner strip and 100 mm Strip wrapping Above the Stirrups	Figure 2(b)
T100SIS	Flanged (T-shape) beam with 100 mm Strip wrapping In between the Stirrups	Figure 2(c)
TCO&100SIS	Flanged (T-shape) beam with COrner strip and 100 mm Strip wrapping In between the Stirrups	Figure 2(d)
TUW	Flanged (T-shape) beam with three faces of web in U-shape Wrapping	Figure 2(e)

Materials and their properties

12 RC flanged beam specimens for the experimental study are prepared from M25 grade concrete. The concrete mix is designed in accordance with Indian Standard IS 10262 (2019). Prior to mix preparation, all constituent materials are tested individually as per relevant Indian standards to ensure compliance with quality requirements. In accordance with IS 516 ((Part 1/Sec 1), 2021), three concrete cubes are cast and tested to determine the compressive strength. The average 28-day compressive strength of the cubes is observed to be 33.17 MPa, which complies with Indian Standard IS 10262 (2019).

The SSWM is selected in alternate of commonly used strengthening material due to its ductility and better bonding with the concrete surfaces when applied with epoxy-based bonding material Sikadur 30LP (Kumar and Patel, 2016a; Patel et al., 2018; Raiyani et al., 2020; Raiyani and Patel, 2023). The SSWM consists of stainless-steel wires woven with square openings of 0.365 mm in orthogonal directions. The SSWM used in the study is made from SS304 grade stainless-steel wire cords with bidirectional plain weaves with 40 wires per inch and conforming to 32 standard wire gauge specifications, with a nominal wire diameter of 0.27 mm (Banaraswala Metal Crafts (P) Ltd, 2016). The mechanical properties, including the modulus of elasticity and the tensile strength of SSWM, are obtained through coupon tensile test reported by Kumar and Patel (2016a), Patel et al. (2018) and Raiyani et al. (2020); Raiyani and Patel (2023). Two coupon specimens of SSWM, each measuring 100 mm × 500 mm, are prepared as per the standard geometry recommended by ASTM D3039/D3039M-17 (2017). The specimens are tested using a Universal Testing Machine. The initial cracking in the specimens occurred at a stress of 450.88 MPa at the top near the mid-length, with the corresponding strain of 0.0227. The average ultimate tensile strength is recorded as 601.3 MPa with an average rupture strain of 0.085.

The composite used for strengthening the RC flanged beams in this study consists of SSWM and Sikadur 30LP as the bonding agent. Sikadur 30LP is a two-part epoxy adhesive consisting of a resin and hardener. It is characterised by a long pot life, minimal shrinkage during curing, and excellent adhesion between the SSWM and concrete surfaces (Sika Product Data Sheet, 2016).

Prior to applying SSWM to a flanged beam, it is essential to conduct a bond test to evaluate the adhesion between the SSWM and the concrete surface. The double shear lap bond test is carried out for this purpose, as reported by previous studies (Kumar and Patel, 2016a; Patel et al., 2018; Raiyani et al., 2020; Raiyani and Patel, 2023). During the bond test, failure was observed due to wire mesh tearing, but no SSWM debonding occurred. Based on tensile strength and bond characteristics, the composite of 40 × 32 type SSWM bonded on a concrete surface using Sikadur 30LP epoxy is selected to strengthen the RC specimens. The volumetric ratio of SSWM is kept varied to investigate the role of SSWM and strip spacing on the behaviour of the RC flanged beam.

Strengthening procedure

SSWM strips are applied on the flanged beam surface after 28 days of wet curing of the specimen. The procedure for strengthening flanged beam specimens with SSWM using Sikadur 30LP is as follows:

• Grinding the surface: A Smooth and clean surface of the concrete enhances the bond between SSWM and concrete. So, in the first step, grinding is carried out for smoothening the surface and rounding of corners of the specimen, as shown in Figure 2(a). The surface of the specimen is then cleaned by removing dust and dirt using an air blower.

• Cutting of SSWM strips: The SSWM strips are cut carefully to ensure that the SSWM wires along the edge are not damaged during the cutting. Strips of the required width are cut from the roll of SSWM as per the required wrapping configuration. A scale is used to measure the required length of SSWM, as shown in Figure 2(b). Marking on the flanged beam specimen is made for applying SSWM as per the required wrapping configuration.

• Mixing of adhesive: Sikadur 30LP components – resin (Part A) and hardener (Part B) are mixed in a 3:1 proportion using a spindle attached to a slow-speed electric stirrer. The mixture is stirred for about 3 minutes until a smooth, uniform grey colour adhesive is achieved, as illustrated in Figure 2(c).

• Applying the first layer of adhesive and wrapping of SSWM: Apply a 2 mm thick layer of Sikadur 30LP and place the pre-cut SSWM strip on the fresh Sikadur 30LP adhesive with pressure. Finally, apply 1 mm thick layer of Sikadur 30LP to cover the SSWM strip, as shown in Figure 2 (d).

Figure 2.

Step-by-step procedure for strengthening of RC specimen. (a) Grinding of the concrete surface. (b) Cutting of SSWM Strips. (c) Mixing of Sikadur 30 LP adhesive. (d) Wrapping of SSWM on the specimen.

Test setup and instrumentation

The experimental investigation on the flanged beam under pure torsion is conducted at the Structural Engineering Laboratory of Nirma University, India. The test setup is developed by the authors (Raiyani and Patel, 2023) to apply pure torsion to horizontally placed beam specimens by means of a vertical load applied (P) through a hydraulic jack. The setup used in the experimental study is illustrated in Figure 3 and has been described in detail by the authors (Raiyani and Patel, 2023).

Figure 3.

Schematic diagram and actual photograph of torsion test setup and instrumentation. (a) Actual photograph of torsion test setup and instrumentation. (b) Schematic diagram of torsion test setup and instrumentation. (c) Enlarge the render view of the torsion test setup.

The applied torque is calculated as the product of half of the applied load (P) using a hydraulic jack (transferred through the lever arm) and eccentricity (a), as expressed in Equation (1), and illustrated in Figures 3 and 4(a).

T o r q u e (T) = (\frac{P}{2} \times a) k N m

(1)

Figure 4.

Line diagram for load transfer mechanism and angle of twist measurement of specimen. (a) Load transfer mechanism and location of LVDT. (b) Angle of twist measurement.

The vertical load applied to the specimen at the time of testing is recorded with the help of a load cell. Two LVDTs are kept at the bottom of the free end of the lever arm at each end of the specimen to measure the vertical displacement resulting from the applied load, as shown in Figures 3 and 4(a).

The angle of twist at each end of the specimen is calculated using the vertical downward displacement and the eccentricity (a), as described in equation (2), and depicted in Figure 4(b).

θ_{L} = θ_{R} = \tan^{- 1} (\frac{y}{a}) d e g r e e

(2)

The total twist angle ( $θ$ ) per unit length of the specimen is obtained by summing the twist angles at both ends and dividing by the effective test length L of specimen, as given in equation (3).

θ = \frac{θ_{L} + θ_{R}}{L} d e g r e e / m

(3)

Here,

θ_{L}

and

θ_{R}

represent the twist angle at the left and right end of the specimen, respectively; ‘y’ is the measured vertical downward displacement at the end of lever arm; ‘a' is an eccentricity (375 mm), and L is the effective test length of the specimen.

To assess the contribution of concrete, steel reinforcement, and SSWM during testing, strain gauges are affixed to each specimen at critical locations, as shown in Figure 5. Load, displacement, and strain data are recorded at every second using a Data Taker DT80 system and stored on a connected computer. The test continued until the specimen failed and could no longer sustain torsional loading, after which the specimen is removed from the setup.

Figure 5.

Location of strain gauges.

Experimental results and discussion

During the experiment, the twist angle is monitored at intervals of torque to prepare a torque v/s twist graph for examining the behaviour of the flanged beam specimen under pure torsion. To analyse the effectiveness of SSWM strengthening for flanged RC beams, the torque at the first crack, the peak torque, the associated angle of twist, and the failure pattern of beam specimens are also studied. Two specimens are tested for each category of RC beams, and the average torque and twist values are considered for the comparison of the torque-twist graph of controlled and SSWM-strengthened flanged beams.

Torque-twist behaviour of test specimens

Figure 6 presents the comparison of twist per unit length at an interval of torque for the unstrengthened control and SSWM-strengthened RC flanged beam (T-beam) specimens. The corresponding test results are summarised in Table 2, which includes the cracking torque (T_cr) and corresponding twist (θ_cr), as well as peak torque (T_p) and corresponding twist (θ_p) for each specimen. The SSWM strengthened specimens exhibit significantly improved torsional performance under pure torsion as compared to the unstrengthened control specimen. Initially, all specimens demonstrate similar torsional stiffness, indicating the contribution of uncracked concrete to torsional resistance. However, once cracking occurred, the SSWM-strengthened RC flanged beam specimens resisted higher cracking torque and exhibited a higher corresponding angle of twist due to the confinement effect provided by the SSWM warping on concrete.

Figure 6.

Experimental results of the torque-twist response of control and SSWM strengthened RC flanged beam specimens.

Table 2.

Summary of experimental results of RC flanged beam specimens.

Flanged beam	First cracks				Peak				Strength index	Torsional ductility index
Flanged beam	T_cr (kNm)	$\frac{T_{c r}}{T_{c r, c}}$	$θ_{c r}$ (degree/m)	$\frac{θ_{c r}}{θ_{c r, c}}$	T_p (kNm)	$\frac{T_{p}}{T_{p, c}}$	$θ_{p}$ (degree/m)	$\frac{θ_{p}}{θ_{p, c}}$	Strength index	Torsional ductility index
TCON (Control)	1.51	---	0.66	---	3.31	---	4.67	---	--	1.72
T100SAS	2.71	1.79	1.72	2.60	4.01	1.21	5.64	1.21	1.21	1.70
TCO&100SAS	2.71	1.79	1.49	2.25	5.21	1.57	5.99	1.28	1.57	1.27
T100SIS	4.71	3.12	3.58	5.40	5.52	1.67	6.67	1.43	1.67	1.97
TCO&100SIS	5.38	3.56	3.17	4.79	6.53	1.97	7.57	1.62	1.97	2.65
TUW	3.83	2.54	1.71	2.59	4.39	1.33	4.29	0.92	1.33	1.89

Subscript ‘c’ indicates the control specimen.

The control RC flanged beam specimen (TCON) displays an almost linear torque-twist behaviour until the first crack appears at a torque of 1.51 kNm and a corresponding twist of 0.66°/m. Subsequently, the twist of the specimen increases at a faster rate towards the peak torque of 3.31 kNm and a corresponding twist of 4.67°/m. After reaching the peak, the torque decreases while the twist continues to increase, indicating progressive damage and degradation of torsional resistance. The loading of the specimens is stopped when the beam exhibits instability due to excessive cracking.

In contrast, the T100SAS specimen, strengthened with vertical SSWM strips, exhibits a linear torque-twist response up to a cracking torque of 2.71 kNm and a corresponding twist of 1.72°/m. After the first crack, the torque-twist response became nonlinear, with the specimen reaching a peak torque of 4.01 kNm and corresponding twist of 5.64°/m. After reaching peak torque, the specimen follows a typical softening behaviour where the torque reduces with increasing twist.

The specimen TCO&100SAS, strengthened with both the corner and vertical SSWM strips, exhibited superior torsional performance. It shows an initial linear response up to a cracking torque of 2.71 kNm with a twist of 1.49°/m. Post-cracking, the twist increases nonlinearly, reaching a peak torque of 5.21 kNm and a corresponding twist of 5.99°/m. Similar to other specimens, the torque reduces while the twist continues to rise beyond the peak torque, indicating ductile post-peak behaviour.

For specimens T100SIS and TCO&100SIS, the torque-twist response is initially linear up to first crack, which occurs at 4.71 kNm and 5.38 kNm, respectively. The corresponding twist angles at cracking are 3.58°/m for T100SIS and 3.17°/m for TCO&100SIS specimens. After cracking, both specimens exhibit nonlinear torque-twist behaviour. The T100SIS specimen reaches a peak torque of 5.52 kNm with a corresponding twist of 6.67°/m, while the TCO&100SIS specimen attains a higher peak torque of 6.53 kNm with a twist of 7.57°/m.

As observed in Table 2, experimental results indicate that the T100SAS and T100SIS specimens resist 1.21 times and 1.67 times higher peak torque, respectively, as compared to the control specimen (TCON). The TCO&100SIS specimen demonstrates a maximum enhancement of 1.97 times increase in peak torque compared to TCON specimen. The vertical SSWM strips contribute to torsional strength by preventing torsional shear cracks and functioning similarly to transverse reinforcement. The placement of SSWM strips between stirrups enhances concrete confinement, leading to improved torsional resistance of T100SIS and TCO&100SIS specimens. However, in the case of T100SAS and TCO&100SAS specimens, SSWM strips coinciding with transverse reinforcement of beam lead to cracks in the concrete at lower torque and reduced resistance to peak torque. The addition of the corner SSWM strips, for the strengthening of specimen, contributes to the improvement of peak torque resistance.

The torque-twist behaviour of an RC flanged beam with U wrapping (TUW) is characterised by its linearity up to the first crack, which occurs at a torque of 3.83 kNm with a corresponding twist of 1.71°/m. The peak torque for TUW specimen is 4.39 kNm with a corresponding angle of twist of 4.29°/m. A significant increase in cracking torque of the TUW specimen, almost 2.54 times, is observed in comparison of the control specimen (TCON), as depicted in Table 2. Specimen TUW exhibits a lesser twist angle as compared to the control specimen at the peak torque, indicating improved torsional stiffness likely due to more external reinforcement. However, beyond cracking, the torque-twist curve becomes nonlinear, with a decline in stiffness. The increase in twist without significant improvement in torque suggests a redistribution of forces from the concrete to reinforcement after initial cracking. In the post-cracking phase, SSWM-strengthened specimens show ductile behaviour, supported by both active presence of internal reinforcement and external SSWM wrapping. While comparing the torque-twist behaviour of TUW with TCO&100SIS, it is evident that continuous SSWM wrapping is less effective than discrete SSWM strips configuration. This may be caused by improper bonding of SSWM on concrete surfaces due to its large area, which compromises confinement efficiency along the length.

Figure 7(a) compares the cracking torque of SSWM-strengthened RC flanged beams with the control specimen. All SSWM strengthened RC flange beam specimens show improved cracking resistance. The TCO&100SIS specimen exhibits a maximum improvement with a 256.3% increase in cracking torque as compared to TCON specimen. However, the increase in cracking torque for specimens T100SAS and TCO&100SAS is 79.5%. The specimen TCO&100SIS resisted more cracking torque compared to the TCO&100SAS specimen due to improved confinement of the concrete due to SSWM strip in between the stirrups. Almost a 154% increment in cracking torque is observed for the TUW specimen.

Figure 7.

Comparison of Cracking and Peak torque for flanged beam specimens strengthened using SSWM. (a) Cracking torque. (b) Peak torque.

A comparison of peak torque resisted by unstrengthened control and SSWM strengthened specimens obtained from the experimental study is presented in Figure 7(b). According to experimental results, specimen TCO&100SIS shows a significant increase of 97.3% in peak torque as compared to the control specimen (TCON). The difference between T100SAS and TCO&100SAS is 36.2%, while the difference between T100SIS and TCO&100SIS is 30.5%, indicating that the contribution of corner strips is approximately 30% to the increase in peak torque. Additionally, an increase in peak torque for specimen TUW is observed to be 32.6% as compared to the control specimen.

Energy absorption, strength, and ductility index

The energy absorption, strength, and ductility are crucial parameters for assessing the structural performance of RC flanged beam specimens when subjected to torsional loads. The energy absorption capacity is determined as the area under the torque-twist curve up to failure, reflecting the ability of a specimen to dissipate energy during loading. A comparative analysis of energy absorption in the pre-cracking stage, post-cracking stage and total energy absorption for all the flanged RC beam specimens is presented in Figure 8. It is evident that the energy absorption values in all three stages are higher for SSWM strengthened specimens as compared to TCON specimen, demonstrating the effectiveness of various SSWM configurations in enhancing torsional resistance. Among the tested strengthened specimens, T100SAS and TCO&100SAS exhibited nearly identical pre- and post-cracking energy absorption values, suggesting the inclusion of corner strips in TCO&100SAS does not significantly contribute to energy dissipation in either stage. Conversely, T100SIS and TCO&100SIS displayed higher energy absorption in both pre- and post-cracking phases, indicating the superior performance of vertical strip wrapping between the stirrups, especially when combined with corner strips. It shows the effectiveness of corner and strip wrapping of SSWM in between the stirrups of flanged beams as compared to other wrapping configurations.

Figure 8.

Comparison of Energy absorption of flanged beam specimens.

The strength index (SI) is defined as the ratio of the peak torque of the SSWM-strengthened beam (T_s) to that of the unstrengthened control beam (T_c). As presented in Table 2, the strength index values of all SSWM-strengthened RC flanged beams are greater than unity, confirming the enhancement in peak torque capacity due to strengthening. The highest strength index of 1.97 is observed for TCO&100SIS wrapping configuration, confirming the effectiveness of combining corner strips with 100 mm vertical SSWM strips placed between the stirrups of beam.

The torsional ductility index ( $μ_{θ}$ ) is defined as the ratio of the area under the torque twist curve up to the point where torque drops to 20% of its peak torque value, to the area under the curve up to peak torque. This ductility index reflects the post-peak energy dissipation capacity and deformability of beam. As shown in Table 2, the ductility index of T100SAS is nearly equal to that of the control specimen (TCON), suggesting that transverse SSWM strips placed over stirrups do not significantly contribute in enhancing ductility, despite improving strength. In contrast, specimens TCO&100SIS and T100SIS show considerable improvements in ductility index, with TCO&100SIS achieving a maximum increase of 54% over the control specimen. The TUW specimen shows a 9% improvement in ductility index in comparison with control specimen TCON. This limited improvement is likely due to development of microcracks after initial cracking, which induce passive confinement and allow for increased torque resistance but with constrained deformation, limiting ductility gains.

These findings, along with the torque-twist responses and energy absorption behaviour illustrated in Figures 5 and 8, clearly demonstrate that corner strips in combination with vertical SSWM strips placed between stirrups are the most effective configuration for enhancing both strength and ductility under pure torsional loading.

Strain measurements of test specimens

The torque-strain behaviour of the control and SSWM strengthened RC flanged beam specimens is evaluated under increasing torsional loading. Figure 9 illustrates that the representative torque-strain curves are recorded at key locations: RE (reinforcement on east face), RW (reinforcement on west face), CE/SE (concrete or SSWM surface on east face), and CW/SW (concrete or SSWM surface on west face). In this notation, R denotes reinforcement, C represents the concrete surface, S indicates the SSWM surface, E and W refer to the east and west faces of the beam, respectively.

Figure 9.

Torque-strain response of flanged beam specimens. (a) TCON. (b) T100SAS. (c) TCO&100SAS. (d) T100SIS. (e) TCO&100SIS. (f) TUW.

The control specimen (TCON) demonstrates the least ductile response, with low cracking strain values of 0.00015 at RE and 0.00021 at RW, increasing slightly at peak torque to 0.00291 at RE and 0.00409 at RW. The peak strain values at CE and CW are also modest, recorded as 0.00252 and 0.00132, respectively (Table 3). The torque-concrete strain curve for the specimen (Figure 9(a)) shows a sharp rise to peak torque followed by a sudden drop, indicating the limited ductility and energy dissipation capacity of the beam. In contrast, the specimens TCO&100SAS and TCO&100SIS, which are strengthened with corner SSWM strips, exhibited significantly higher strain capacities. Among them, TCO&100SIS demonstrated the most ductile response, as cracking strain values at SE and SW are 0.01004 and 0.01734, respectively, while peak torque strain increased to 0.05886 and 0.03245. These are the highest strain values among all specimens (Table 3), indicating excellent deformation capacity and efficient stress transfer between the concrete and SSWM. The torque-strain response (Figure 9(e)) of TCO&100SIS shows a smooth post-peak descent, confirming the enhanced energy absorption. The beams T100SAS and T100SIS demonstrate the improved performance compared to the control specimen (TCON), but lower strain capacities than the flanged beam with corner SSWM strips (TCO&100SAS and TCO&100SIS) as shown in Figure 9. In T100SIS, the peak SSWM strain at SE is 0.00151, and at SW it is 0.00208, which are only a fraction of those strains observed in TCO&100SIS, clearly highlighting the effectiveness of corner SSWM strips placement. The TUW specimen shows a controlled and consistent torque-strain response (Figure 9(f)). Though peak strain values are lower than TCO&100SIS, still considerably higher than the control specimen, particularly with a strain of 0.01848 at SE, signifying enhanced ductility and strain distribution through effective SSWM confinement.

Table 3.

Strain at first cracking torque and peak torque for flanged beam specimens strengthened with SSWM.

Flanged beam		TCON^a (control)	T100SAS	TCO&100SAS	T100SIS	TCO&100SIS	TUW
Strain at first crack	RE	0.00015	0.00075	0.00414	0.00154	0.00247	0.00008
	RW	0.00021	0.00393	0.00173	0.00017	0.00332	0.00017
	CE/SE	0.00020	0.00164	0.00292	0.00066	0.01004	0.00286
	CW/SW	0.00028	0.00127	0.00076	0.00078	0.01734	0.00156
Strain at peak torque	RE	0.00291	0.00421	0.01287	0.01318	0.00941	0.00395
	RW	0.00409	0.00999	0.01224	0.00098	0.01613	0.00062
	CE/SE	0.00252	0.00493	0.01200	0.00151	0.05886	0.01848
	CW/SW	0.00132	0.00302	0.00925	0.00208	0.03245	0.00339

^aRepresent strain on the concrete surface only. R – represents reinforcement, S – represents SSWM surface, C – represents concrete surface, E – represents East face, W – represents West face.

From Table 3, a comparative examination of peak strain value across specimens reveals that the maximum strain of 0.05886 at peak torque is recorded on the west face of the TCO&100SIS specimen, demonstrating significant utilisation of the tensile strength of SSWM. The peak strain values on SSWM in TCO&100SAS and TCO&100SIS are 0.01200 and 0.05886, respectively, both significantly exceeding the values recorded in the control specimen. Even in specimens T100SAS and T100SIS, the maximum strain on SSWM at peak torque is 0.00493 and 0.00208, respectively, demonstrating the participation of SSWM in torsional stress resistance.

A comparative assessment of peak strain values highlights clear differences between the SAS and SIS wrapping configurations. The SIS configuration, where SSWM strips are placed in between the stirrups, effectively prevents the direct crack propagation through the transverse mesh. This is attributed to a stronger interfacial bonding between the wire mesh and the concrete surface, which allowed torsional shear stresses to be transferred deeper into the concrete core. On the other hand, the SAS configuration allows crack propagation through unconfined zones between SSWM strips, leading to wider crack spacing and lower torsional capacity. The SIS configuration, by effectively reducing the centre-to-centre distance between transverse steel and SSWM strips, enhances confinement and contributes to more uniform stress distribution. The strain distribution in TCO&100SIS is observed to be more uniform across both concrete and SSWM, leading to reduced crack spacing and width, and an increased number of fine cracks. This improved cracking pattern directly contributes to enhanced energy dissipation and torsional capacity. The torsional strength of the SIS configuration specimens is found to be approximately 25% higher than that of the TCO&100SAS specimen, reinforcing the advantage of placing SSWM strips between stirrups rather than directly above the stirrups.

Cracking patterns and modes of failure

Figure 10 illustrates the failure patterns observed in each flanged beam specimen during the experiment. In the control specimens (TCON), the initial diagonal crack forms on the east face within the web region and subsequently propagates into the flanges. These diagonal cracks appear at an inclination of approximately 44° in both the web as well as in flange regions. Additionally, concrete spalling is observed at the top of the flange and bottom of the web faces, accompanied by the twisting of the longitudinal reinforcement bars, as shown in Figure 10(a). This behaviour is indicative of brittle torsional failure with minimal post-crack ductility. In specimens T100SAS and TCO&100SAS, failure is partially delayed compared to the control specimens, as shown in Figure 10(b) and (c), respectively. However, diagonal torsional cracks eventually occur and propagate in the unwrapped concrete region between adjacent SSWM strips, suggesting that transverse SSWM placed directly over stirrups provides limited confinement of concrete between transverse stirrups.

Figure 10.

Failure pattern of flanged beam specimens. (a) Failure of TCON RC flanged beam. (b) Failure of T100SAS flanged beam. (c) Failure of TCO&100SAS flanged beam. (d) Failure of T100SIS flanged beam. (e) Failure of TCO&100SIS flanged beam. (f) Failure of TUW flanged beam.

In T100SIS and TCO&100SIS, diagonal cracks also emerge on the concrete surface between SSWM strips, as shown in Figure 10(d) and (e). In T100SIS, the first crack appears in the web region (in an unstrengthened area) at cracking torque of 4.71 kNm and a corresponding twist of 3.58°/m. A second crack forms at the mid-span of the web, which subsequently widens, causing concrete spalling and reinforcement bar twisting. Beyond this, the torque-twist curve becomes nonlinear, leading to a peak torque of 5.52 kNm with an accompanying twist of 6.67°/m. Thereafter, torque decreases while twist continues to increase. The use of SSWM strip in between stirrups (SIS) and the strip above stirrups (SAS) prolongs the failure process after initial cracking, indicating improved post-cracking behaviour and ductility. However, SIS strengthening configuration is more effective in delaying failure as compared to SAS configuration.

In the TUW specimen, the first diagonal crack at a 41° inclination is observed on the concrete surface in the flange section, specifically in areas not covered by SSWM strips, as illustrated in Figure 10(f). Concrete spalling is observed at the top flange under the peak loading conditions. The initial diagonal crack is noticed at a torque of 3.83 kNm at the flange portion, experiencing an angle of twist of 1.71°/m in the TUW beam specimen. The second crack forms mid-span of the flange, following which the beam exhibits reduced torsional stiffness. At a peak torque of 4.39 kNm and twist of 4.29°/m, tearing of the SSWM is observed, highlighting the active role of SSWM in torsional resistance. All the SSWM-strengthened specimens show ductile failure characteristics, primarily due to the deformability of SSWM, which delays the brittle failure. In no case, debonding of SSWM from the surface of the specimen is observed. At the peak twisting of specimen, stretching of wires is observed prior to failure of SSWM in all the strengthened specimens. This observation confirms the utilisation of the full capacity of SSWM when used for the strengthening of RC flange beams.

Conclusion

In this paper, the experimental study carried out on the control and SSWM strengthened RC flanged beams under the torsional load is presented. Five different types of SSWM wrapping configurations are considered to enhance the torsional resistance of RC beams. The significant outcomes based on the experimental results are as follows:

When a corner strip of SSWM is provided in addition to a vertical strip of SSWM, the torsional behaviour of the RC beam is improved. This is evident from peak torque resisted by flanged beam specimen strengthened with only vertical strips and with corner & vertical strips. Specimen TCO&100SAS resisted 36.2% higher peak torque as compared to T100SAS. While specimen TCO&100SIS resisted 30.5 % higher peak torque as compared to T100SIS.

In the case of corner and vertical strip configuration of SSWM wrapping, the torsional behaviour of the RC beam is improved when a vertical strip is provided between the transverse stirrups rather than a vertical strip provided at the location of stirrups. The cracking torque of the TCO&100SIS specimen is 177% higher than that of TCO&100SAS, while the peak torque of TCO&100SIS is 40% higher than that of TCO&100SAS. When SSWM strips are provided between the stirrups, more volume of concrete is confined due to SSWM and steel stirrups. As a result, the torsional resistance of the RC specimen with SIS configuration of SSWM strengthening is increased as compared to SAS configuration.

SSWM wrapping configurations such as corner strip and 100 mm strip wrapping in between the stirrups (TCO&100SIS) and 100 mm strip wrapping in between the stirrups (T100SIS) contribute to an average peak toque carrying capacity of 82.1% compared to unstrengthened RC beam specimen and even found to be more efficient in enhancement of energy absorption capacity as compared to other wrapping configurations.

TCO&100SAS wrapping configuration shows less energy absorption before cracking compared to TCO&100SIS due to the early stage of cracking occurring in an unwrapped portion of a beam between two reinforcement stirrups or SSWM strips.

Concrete crushing governs the failure of the unstrengthened control beam, whereas SSWM rupture, followed by concrete crushing and preceded by longitudinal & transverse reinforcement yielding, governs the failure of SSWM strengthened RC beam specimens.

Torsional failure of TUW beams occurs by tearing of SSWM wrapping, whereas in beams with T100SAS, TCO&100SAS, T100SIS and TCO&100SIS SSWM wrapping, cracks are formed in the concrete surface before the tearing of SSWM. No debonding of SSWM from the concrete surface is observed, indicating full utilisation of the tensile strength of SSWM for torsion resistance.

The analysis of the strength index and ductility index indicates that the RC beam strengthened with T100SIS and TCO&100SIS SSWM wrapping configuration demonstrates superior strength and ductility capability.

Future studies can extend the present work by considering combined loading scenarios, including torsion, bending, and shear, representing more realistic conditions. Additionally, investigations into long-term durability under environmental exposure and cyclic loading would provide valuable insights into the real-time performance of the SSWM strengthening system.

Footnotes

ORCID iDs

Sunil D Raiyani

Paresh V Patel

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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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