Behaviour of Stainless-Steel Fibre-Reinforced Exterior Beam-Column Joints under Reverse Cyclic Loading

Te objective of this study is to evaluate the fexural behaviour of stainless-steel fbre-reinforced concrete beam-column (BC) joints under reverse cyclic loading. Based on the properties of concrete with various percentages of fbre, the optimized volume fraction was obtained as 0.75% of stainless-steel fbre. In the present work, two sets of beam-column joints with and without fbres were cast and tested under reverse cyclic loading. Te beam-column joints were loaded up to fve cycles, to study their behaviour, and examine the failure pattern of the joint. Based on test results, parameters such as ductility and the energy absorption capacity characteristics were evaluated. It is concluded that the inclusion of stainless-steel fbre improves the overall seismic resistance of RC beam-column joints.


Introduction
Concrete is economical in the long haul as compared to other engineering materials. As the concrete matrix is poor in tension and ductility, it has little resistance to cracking. When the concrete hardens, microcracks are formed, and these microcracks start developing along the planes, which may experience relatively low tensile strain with the application of load. To overcome these difculties, enhancing the structural properties of concrete becomes important [1,2]. Research, to overcome the above defciencies, led to the development of various special structural concrete. Fibre-reinforced concrete is one such development in special structural concrete, which performs better where plain reinforced concrete has certain limitations and exhibits higher structural strength and cohesion due to the presence of fbres [3]. Fibre-reinforced concrete characteristics can be changed by altering the quantity, fbre substance, geometric confguration, dispersal, orientation, and fbre concentration. Te addition of fbres to a great extent improves the tensile strength, crack resistance, and toughness of reinforced concrete [4]. When fbres are added comparatively to a small amount, they create a unique reinforcement in the cement matrix and eradicate the problem of crack development during plastic shrinkage [5]. Fibres are selected based on cost, availability, and fbre properties. Te most regularly utilized fbres are steel, glass, asbestos, polypropylene, and polyester [6]. In addition to the above fbres, stainless steel fbre is a promising development that makes the reinforced concrete more durable as stainless-steel fbre contains chromium that avoids corrosion of fbres [7]. Stainless steel does not readily corrode, but under low oxygen, it also corrodes [8].
Beam-column joints in RC buildings are defned as the zone of connection of beams and columns and are vulnerable to seismic forces. Te load-carrying capacity of joints is limited to the strength of the constituent materials. In most cases, beam-column joints are designed to resist earthquakes, as the earthquake forces, larger than the structural design, causes irreversible damage to the buildings [9,10]. Earthquake-resistant structures should have good ductility and energy absorption capacity when subjected to lateral loads and should deform laterally [11]. Te combined efect of micro and macro steel fbres on the hinged zone of RC beams enhances the fexural capacity [12]. Te addition of 1% of steel fbres and 0.5% of polypropylene fbres improves the energy dissipation capacity and stifness degradation [13]. Te strength of deep beams is infuenced by the amount of discrete fbres in the web reinforcement [14]. Steel fbres, when used in concrete, increase the tensile, fexural, and impact strength and thereby reduce cracks and shrinkage [15]. Flexural strength of concrete increases by 70%, 45%, and 35% for hybrid fbres of steel-polypropylene, steel-glass, and steel-nylon fbres [16]. Te addition of hook fbres by 2% increases the modulus of elasticity by 58% and toughness by 19% [17]. Concrete with 10% of alccofne fbres increases compressive strength by 34.5% [18]. Te seismic performance of beam-column joint reinforced with basalt fbres is about 71.6% when compared to conventional beamcolumn joints [19]. A maximum peak load of 11.05 kN is achieved for concrete reinforced with 0.75% of steel fbres [20]. Te use of steel hook fbres shows signifcant improvement in pre and post behaviour of beam-column joints [21]. Fibre-reinforced concrete possesses higher strength, better ductility, and energy absorption capacity. Steel fbrereinforced concrete is obtained by adding steel fbres in concrete while mixing creates a homogeneous reinforcement. Te use of steel fbres in reinforced concrete results in the corrosion of steel fbres when the structure is exposed to aggressive environments [22]. Hence, to reduce this problem, stainless steel fbres are used which have excellent potential in resisting corrosion.

Experimental Investigation
Te experimental study is divided into two portions: namely, the performance of the control beam-column joint (CC) and the stainless steel fbre-reinforced concrete beam-column joint (SSFBC) with 0.75% of fbres by volume fraction.

Materials Used.
OPC 53 grade conforming to IS 12269-2013 [23] with a specifc gravity of 3.17 was used, and river sand having a specifc gravity of 2.60 was used as fne aggregate. A coarse aggregate of 20 mm size with a specifc gravity of 2.65 conforming to zone II as per IS 10262 2019 [24] was used. Concrete having a compressive strength of 30 N/mm 2 grade concrete mix was designed as per IS 10262-2019 [24] guidelines. Te specimens were cast and cured for 28 days and were tested immediately after the required curing period. Te stainless-steel fbre used in this study was round crimped fbre which is shown in Figure 1, and its properties are shown in Table 1.

Test Setup & Test
Procedure. For this study, an exterior beam-column joint with a beam of 120 mm × 170 mm and a column of 120 mm × 230 mm was used. Te column height and length of the beam are 700 mm and 450 mm, respectively. Te reinforcement provided for the beam-column joint used in the present study is shown in Figure 2. Te specimens were cast in two diferent test series one without fbres and another with stainless steel fbres. For the study on beam-column joints, a loading frame of 100 tonnes capacity was used, and the axial load was applied using a screw jack of 50 tonnes capacity. Te load was applied cyclically at the end of the beam at regular intervals, and the defection was measured under the load. Te applied loads were recorded, and corresponding defections were measured. Downward and upward displacements are measured using dial gauges and linear variable diferential transformer (LVDT). Te load setup of the beam-column joint is shown in Figure 3.

Load Sequence.
Quasistatic reverse cyclic loading simulating earthquake load is applied on the exterior beam-column joints. Te load was applied manually, and for each increment of load, the corresponding defection was recorded. Te load was increased and decreased in stages up to the fnal failure of the specimen. Te maximum load in each cycle was increased by 10 kN. Once the maximum load is reached in each cycle, the loading will be reversed by placing the jack and proving ring on the bottom face of the beam and the dial gauge on the top face of the beam. Te ultimate load of the CC specimen was observed during the fourth cycle whereas the ultimate load of SSFBC specimens was observed during the ffth cycle of loading. Te load sequence history for various specimens is shown in      Advances in Civil Engineering subjected to cyclic loading which is the case with seismic loads. Even though the load-carrying capacity of fbrereinforced concrete BC joints increased marginally by 4.5 kN from reinforced concrete beam-column joints, there was a better ductile behaviour seen in fbre-reinforced concrete BC joints. Te load-displacement behaviour of all the BC joints is shown in Figures 6 and 7. Due to the presence of fbres, the SSFBC specimen exhibited better crack resistance than the CC beam-column joint.

Ductility Behaviour.
From the load-defection response ductility which is assumed as bilinear, the ductility factor can be calculated as the ratio of ultimate deformation to yield deformation as shown in the following equation:

Ductility �
Maximum deflectionat any load level, ∆ max First yield deflection, ∆ y . (1) Te frst yield defection of the CC specimen is 2 mm for the forward cycle and 2.8 mm for the reverse cycle, and for the SSFBC specimen, it is 1.8 mm in the forward and 1.5 mm in the reverse cycle. Te variation of ductility factor for forward and reverse cycles is shown in Figures 8  and 9. Cumulative ductility is an important earthquakeresistant parameter for a structure subjected to reverse cyclic loading and is obtained by adding the ductility at maximum load for each cycle. Te cumulative ductility for CC beam-column joint increased from 0.72 in the frst cycle of loading to 9.8 during the fourth cycle of loading, and for the SSFBC beam-column joint, it increases from 1.30 in the frst cycle to 20.52 in the ffth cycle of loading. Te cumulative ductility factor for several cycles is presented in Figures 10 and 11. Te ductility of stainless steel BC joints shows better results than conventional reinforced concrete BC joints. Also, it was observed that improved integrity of fbre-reinforced concrete in the failure zone prevents the buckling of compression bars         Advances in Civil Engineering and provides better ductility to the fbre-reinforced concrete BC joint specimen.

Relative and Cumulative Energy Absorption Capacity.
Energy is absorbed in each cycle in BC joints when they are subjected to reverse cyclic loading similar to an earthquake. Relative energy absorption capacity is found by adding the areas under load-defection behaviour's hysteresis loop for each load cycle, and the cumulative energy absorption capacity is obtained as the sum of the energy absorption capacity of the joint in each cycle. For the CC specimen, the relative energy absorption capacity (shown in Figure 12) varies from 12 kN-mm in the frst load cycle to 60 kN-mm in the fourth load cycle whereas for the SSFBC specimen, the relative energy absorption capacity (shown in Figure 13) varies from 12 kN-mm to 126 kN-mm in the ffth cycle of loading. Te cumulative energy absorption capacity for the CC specimen (shown in Figure 14) is 216 kN-mm and for the SSFBC specimen (shown in Figure 15) is 468 kN-mm. Tables 2 and 3 show the experimental results of CC and SSFBC beam-column joints, respectively. Te seismic energy injected into a structure during an earthquake must be absorbed by the structure to a greater extent, and then, the structure can resist earthquake forces. Te energy absorption by the structural members before failure is an essential structural parameter, and from the results of relative and cumulative energy absorption studies, it is evident that the stainless-steel fbre-reinforced concrete BC joint absorbs energy much higher than the reinforced concrete beamcolumn joint.

Behaviour and Mode of Failure.
Te beam-column joints were loaded for up to fve cycles to study their behaviour and failure pattern. All the specimens failed by crack propagation exactly at the intersection of the BC joint. Figures 16 and 17 show a crack pattern of CC and SSFBC, respectively.

Advances in Civil Engineering
Both the BC joint specimens displayed similar linear loading patterns from the initial load to the frst crack load. Te inclusion of fbres deferred the development of the initial crack in the fbre-reinforced concrete BC joint. Also, when the load was increased, more numerous cracks formed at the beam-column intersection for the CC joint when compared with the fbre-reinforced BC joint. Te eventual load-carrying capacity of fbre-reinforced BC joint improved considerably due to the addition of fbres. Tere was also a considerable reduction in the spalling of concrete for fbrereinforced BC joint specimens. It was also observed that the fbres present in the specimens acted as a secondary reinforcement and reduced the crack propagation thereby enhancing the ductile behaviour. Te fbres acted as crack arrestors and enhanced the load-carrying capacity, energy absorption, ductility, and behaviour under all stages of loading.

Conclusion
Te current study focuses on the behaviour of fbrereinforced exterior beam-column joints under reverse cyclic loading with stainless-steel fbre. Te following conclusions were drawn from the study: (1) Te maximum load-carrying capacity for the beamcolumn joints with 0.75% of stainless-steel fbres by volume fraction (SSFBC) was 15% higher than that of the control specimen (CC). Also, the cumulative energy absorption capacity of the BC joints with 0.75% of stainless steel fbres by volume fraction (SSFBC) was 100% higher than the control specimen (CC), which is a notable improvement. Tis further demonstrates the signifcance of steel fbres as a means of enhancing the strength at the structural joints. (2) Te SSFBC joint undergoes large displacements without developing wider cracks when compared with the CC beam-column joint indicating that stainless steel fbres impart higher ductility to the stainless-steel fbre-reinforced beam-column joint    which is one of the essential properties for a beamcolumn joint. Moreover, the fbres played a vital role in delaying the crack formation and crack propagation thereby enhancing the behaviour of the beamcolumn joints during seismic forces.
(3) Te cumulative ductility value of the SSFBC specimen was twice as high as that of the control specimen (CC). (4) It should also be noted that stainless steel is better known for corrosion resistance, and this also adds to the durability performance of the beam-column joints than the one with mild steel fbres. Overall, the addition of stainless steel fbre in concrete has better load-carrying capacity and better crack resistance and has high energy absorption capacity and better ductility which justifes its use in BC joints that are vulnerable to seismic forces.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.

Authors' Contributions
Palaniappan Prasath contributed conceptualization, data curation, and investigation. Balaji Shanmugam collected resources, wrote the article, and reviewed and edited the article. Paul Awoyera developed the methodology and did project administration. All authors have read and approved the fnal manuscript.