Cold rolling techniques in mechanical splices: Experimental investigations
الموضوعات :
mohamad reza shokrzadeh
1
(Department of civil engineering, Islamic Azad University, Science and Research Branch, Tehran, Iran)
Fariborz Nateghi Allahe
2
(Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology, Tehran, Iran)
taleb sadeghian
3
(Department of Earthquake Engineering, Shahid Ashrafi Esfahani University, Esfahan, Iran)
الکلمات المفتاحية: Mechanical threaded splice, Ductile Members, Cold rolling, Modifying threaded,
ملخص المقالة :
This study presents a comprehensive evaluation of the performance of oversize threaded splices under cyclic loading conditions. The research includes monotonic tensile testing and cyclic loading experiments to investigate the seismic behavior of the splices. The experimental results demonstrate that the splices exhibit lower values of εu (strain at peak load) in cyclic loading compared to monotonic tensile testing. This suggests that the cyclic response can serve as a conservative lower bound for the mechanical performance of the splices. The findings highlight the importance of considering cyclic loading conditions when determining conservative lower bounds for the design and evaluation of threaded splices. Understanding the behavior and performance of threaded splices under cyclic loading is crucial for ensuring their reliable and safe operation in seismic regions.
Cold rolling techniques in mechanical splices: Experimental investigations
Taleb Sadeghian1, Mohamad Reza Shokrzadeh2, F. Nateghi-Alahi3
ABSTRACT
The problem of overcrowding at the junction of the rebars is very significant, particularly for seismic details. Mechanical couplers can thus offer an appealing solution that eliminates the disadvantages of traditional reinforcement splicing. In this study, by modifying the method of making a threaded splice, one type of patch is introduced that can be used in the plastic hinge areas of ductile members in seismic areas. The splice area in the suggested method is oversized. To enlarge the splice area, the technique of cold rolling is used. In total, 72 samples (including three repeated samples of each type) were tested. Threaded couplers (TC), oversize-threaded couplers (OTC), and non-spliced reference specimens underwent uniaxial tensile and cyclic testing with and without concrete (NS). The sensitivity to bar diameter, as well as bar strength, ductility, energy absorption, and failure mode performance, were evaluated. In terms of strength, ductility, energy absorption, and failure mode, OTC performed the best and is suited for usage in high seismic zones. Additionally, the TC is appropriate for use in low-to-medium seismic zones.
Keywords: Mechanical threaded splice, Ductile Members, Cold rolling, Modifying threaded
1. Introduction
Due to bar length limits, splicing of reinforcing bars is unavoidable in reinforced concrete (RC) structures and may alter the overall behavior of structures under static and dynamic stresses [1,2]. Splicing methods introduced and explored thus far can be divided into three categories: lap, welded, and mechanical splices, each with advantages and disadvantages [3–7]. Lap splicing is the traditional way of splicing that involves arranging a suitable length of connecting bars side by side and can be characterized as contact or non-contact [8–14]. The increased length of the steel bars may produce congestion and may increase the cost due to the higher steel amount. When they are placed in locations with inelastic deformations, it also reduces their strength or displacement capacity [1,15,16]. More importantly, the performance of the lap splice is strongly dependent on the concrete strength. This means that even if the lap splice is correctly constructed and operated, it may fail due to low-strength concrete [17–20]. Gas pressure welding (GPW) is another splicing technology that was introduced in the 1930s in the United States and Japan [5,21].Rails, steel pipes, and reinforcing bars can all be joined using this technique, which is also known as the forging method. By heating the bars using acetylene and oxygen gases, bars can be joined together using this technique. When they are close to the plastic range, pressure is applied to crimp them together head-to-head [22–24]. The main benefits of this approach are that it can be applied to medium- to large-diameter bars, that it produces splices with acceptable behavior, and that it is quick and affordable. It should be remembered that the effectiveness of this approach depends greatly on the operator's skills; therefore, the price and time required to operate this splice may be comparable to those of a mechanical splice [1,21,22,25]. In the mechanical splice method, couplers are rigid components that are used to join reinforcement bars together. Couplers can be broadly divided into five kinds based on how much stress is transferred between the bars and the couplers: shear screw couplers, headed bar couplers, threaded couplers, grouted couplers, and swaged couplers [2]. Tensile stress in a mechanically spliced bar is transferred from one bar to the other through the coupler and its parts [24,26]. Fast installation, ecologically friendly application, and acceptable performance are all advantages of using mechanical methods [2,8,27,28]. Bar couplers are categorized as Type 1 or Type 2 by ACI 318 [29]. The strength that a coupler can create serves as the basis for this classification. For instance, a Type 1 coupler is one that can withstand more than 1.25 times the splicing bar's yield strength. According to their strain capacity, "Service" and "Ultimate" couplers are categorized by Caltrans SDC [30]. Couplers can only be used if they can develop a minimum strength of 1.25 times the yield strength of the bar, according to AASHTO [31]. According to the EC8 [32], the use of mechanical couplers for splicing reinforcing bar in the inelastic deformation zones brought on by earthquakes must be tested to ensure that the conditions are consistent with the ductility class that is selected (i.e. medium ductility: DCM, or high ductility: DCH). Current bridge and building design rules forbid the use of mechanical bar splices in the plastic hinge regions of ductile elements in high seismic zones, even though couplers are typically permitted [30,31,33]. Studies done on the performance of mechanical splices can be broken down into three categories: (a) application (with and without concrete), (b) applied load (cyclic or monotonic), and (c) loading rate. All of these studies have come to the same conclusion: splicing all the bars in one area may lead to poor behavior under cyclic load. Steel bars that have been mechanically spliced may fail in the coupler or in the bond between the coupler and the bar [1,2,8,27,34–40]. The first kind of failure might have been influenced by the fragile material of the couplers. In this instance, the couplers crack and fail when the spliced bars are subjected to monotonic or cyclic loads. The second type of failure occurs when the bars or sleeves are not properly prepared. Bond failure may be caused by parameters such as thread depth and length in both bars and sleeves (in threaded couplers), insufficient pressure and bar-sleeve lock (in swaged couplers), and incorrect screws in shear screw couplers [1,2,8,18,27,34–37,39]. The authors of the studies believe that the most effective parameters for grouted splices are embedded length and sleeve geometry (diameter, length, and thread). An embedded length of 6 db and a sleeve length of 16 db might produce acceptable performance by increasing the bond capacity [41–48]. The paper is organized as follows: By modifying the method of making a mechanical bar splice, one type of patch can be introduced that can be used in the plastic hinge areas of ductile members in seismic areas. The splice area in the suggested method is oversized. To enlarge the splice area, one technique—cold rolling—is used. This study conducts uniaxial tensile and cyclic with and without concrete testing on threaded couplers (TC) and oversize-threaded couplers (OTC) reinforcement bar diameters of 16 mm and 20 mm, as well as non-spliced (NS) reference specimens. Strength, ductility, energy absorption, and failure mode performance were evaluated. A thorough explanation of the seismic criteria for the bar splices based on various design standards is also provided in this article for practical use.
The behavior of threaded couplers was investigated using uniaxial and cyclic loading. Monotonic static tensile, tension, and compression tests in with and without concrete were carried out on threaded couplers that join steel bars with different configurations. The tests were performed in the Structures Laboratory at the University of IIEES (the International Institute of Earthquake Engineering and Seismology in Tehran, Iran). Using the Instron Universal Testing Machine (UTM) with a maximum capacity of 600 kN in the static state and a maximum of 500 kN in the dynamic state. The objective was to evaluate the tensile and cyclic behavior of the spliced bars, identify their cause of failure, modify the method of making a mechanical bar splice and combine it with rotary friction welding (two types of patches are introduced that can be used in the plastic hinge areas of ductile members in seismic areas), and use an analytical model to predict the ultimate tensile strength of the threaded splices while taking threaded couplers into consideration. These models are useful for designing RC columns with plastic hinge regions that employ threaded couplers.
2.1. Specimen details
A total of 72 specimens were prepared for the tensile loads and cyclic loads, considering the practical requirements of the plastic hinge areas of ductile members in seismic areas. Two types of tension-compression couplers, namely threaded couplers (TC) and oversize-threaded couplers (OTC), as well as non-spliced (NS) reference specimens, were selected for detailed assessment (as illustrated in Fig. 1) with diameters of 16 mm and 20 mm, respectively. Details of the specimen are shown in Fig. 1 and Table 1. To obtain a detailed insight into the with and without concrete response of mechanical splices, uniaxial monotonic and cyclic tests were carried out (Table 2). Specimen ID is broken down into three parts. The first section determines whether the response is without concrete (A) or with concrete (C). The second part refers to the specimen that represents the non-spliced (NS), threaded couplers (TC), and oversize-threaded couplers (OTC). The last part identifies the bar size as well as the test protocol (monotonic (tensile test, M) or cyclic (alternating tension and compression test for large plastic strains in mechanical splice, C1, or alternating tension and compression test for high stresses in mechanical splice, C2) (Table 2).
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Fig. 1. Details of threaded coupler specimens for TC, OTC, and RFWTC without concrete and with concrete.
Specimen | db (mm) | L (mm) | LS (mm) | LC (mm) | LT (mm) | LW (mm) | LCon (mm) | d1 (mm) | d2 (mm) | d3 (mm) | D (mm) | D1 (mm) |
Non-spliced (NS) | 16 20 | 700 700 | - - | - - | - - | - | 600 600 | - - | - - | - - | - - | 121 151 |
Threaded couplers (TC) | 16 20 | 700 700 | 350 350 | 42 50 | 21 25 | - - | 600 600 | 16 20 | - - | 2.5 2.5 | 23 30 | 121 151 |
Oversize-threaded coupler (OTC) | 16 20 | 700 700 | 350 360 | 46 54 | 23 27 | - - | 600 600 | 18 22 | 18 22 | 2.5 2.5 | 28 33 | 121 151 |
Table. 1. Details of test specimens.
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| With concrete tests |
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Sample | Specimen ID | Test protocol | specimen | Test protocol | ||
Non-spliced (NS) | A-NS-16M A-NS-16C1 A-NS-20M A-NS-20C1 | Monotonic Cyclic C1 Monotonic Cyclic C1 | C-NS-16M C-NS-16C2 C-NS-20M C-NS-20C2 | Monotonic Cyclic C2 Monotonic Cyclic C2 | ||
Threaded couplers (TC) | A-TC-16M A-TC-16C1 A-TC-20M A-TC-20C1 | Monotonic Cyclic C1 Monotonic Cyclic C1 | C-TC-16M C-TC-16C2 C-TC-20M C-TC-20C2 | Monotonic Cyclic C2 Monotonic Cyclic C2 | ||
Oversize-threaded coupler (OTC) | A-OTC-16M A-OTC-16C1 A-OTC-20M A-OTC-20C1 | Monotonic Cyclic C1 Monotonic Cyclic C1 | C-OTC-16M C-OTC-16C2 C-OTC-20M C-OTC-20C2 | Monotonic Cyclic C2 Monotonic Cyclic C2 | ||
Table. 2. Monotonic and cyclic test matrix for threaded splice bar specimens.
2.2. Construction and materials
In the TC method, threads are cut into the rebar on both sides. Half of the coupler's length will be the depth of these threads. The assembly is then finished by rotating the rebar (Figs. 2.a and 2.c). A special cold rolling method was used to fabricate the OTC specimens; the machine first applied hydraulic pressure to the rebar. The new, bigger thread area allows for a one-size increase in threading size for each rebar. For instance, a 20-rebar after oversizing will have a 22-thread (Figs. 2.b). The without concrete specimens exposed to monotonic loading had a distance of 700 mm between the testing machine jaws. The with concrete specimens were created using a vertical plastic frame, as shown in Fig. 2c. The frame's vertical posture was maintained by simple props. The with concrete specimens were constructed using splices that were 700 mm long, with 600 mm of that length implanted in a concrete cylinder. The external diameter of the concrete cylinder was Dc = 121 mm for the splices made of db = 16 mm rebars and Dc = 151 mm for the splices made of db = 20 mm rebars (Table 1). The concrete cover has a significant impact on the bond characteristics and crack pattern. The concrete cover was 52 mm at the rebar, 49 mm at the coupler (TC) and 47 mm at the coupler (OTC) for members incorporating 16 mm bars, and 65 mm at the rebar, 60 mm at the coupler (TC) and 59 mm at the coupler (OTC) f (Fig.1). The concrete cover was chosen to take into consideration the experimental restrictions, minimize the occurrence of splitting failure along the implanted bars, and maintain the same cover-to-rebar ratio (c/d) for all with concrete specimens. For both rebar diameters, c/d = 3.3 is above the limit value at which splitting is anticipated to happen when there is no or minimal transverse confinement (c/d = 1) [35,49] and above the limit at which cover splitting can be entirely disregarded (c/d = 3) [35,50]. The compressive strength of three 150 × 150 × 150 mm3 cubic samples of concrete was evaluated. They had been tested under compressive force after being submerged in water for 28 days. Table 3 provides the compressive strength of the cubic samples and the equivalent compressive strength of the cylinder samples. It should be noted that the equivalent compressive strength of a cylinder is determined in accordance with BS 1881: Part 120:1983 and is based on the assumption that a cylinder's strength is 0.8 times that of a cube's strength [51].
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Fig. 2. Construction process of specimens TC and OTC (a) TC, (b) OTC, (c) with concrete specimens.
Sample | Compressive strength of cubic samples (MPa) | f’c (MPa) | εc0 (%) | εcu (%) | Weight (Kg) | Mass (kg/m3) |
A | 50.42 | 43.14 | 0.230 | 0.386 | 7.20 | 2150 |
B | 51.23 | 44.28 | 0.225 | 0.376 | 7.30 | 2160 |
C | 50.95 | 43.86 | 0.241 | 0.401 | 7.25 | 2140 |
Average values | 50.86 | 43.34 | 0.232 | 0.388 | 7.25 | 2150 |
Standard deviation | 0.20 | 0.16 | 0.212 | 0.338 | 7.30 | 2.160 |
Table. 3. Properties of concrete.
2.3. Instrumentation and testing procedures
A static universal testing machine, its hydraulic system, controller, and a test specimen with an extensometer for without concrete specimens and two strain gauges for concrete specimens (one for the coupler and one for the rebar) are shown in Fig. 3 as the test setup for mechanical bar splices. A sample's maximum length of 1092 mm might be accommodated by the all-purpose testing device. The machine had a 178-mm overall stroke. The machine could produce a force of up to 500 kN in the dynamic state and 600 kN in the static state. Furthermore, the accuracy of the loads and head displacements provided by this universal testing equipment is 1.0 N and 0.0001 mm, respectively. The sampling frequency for machine data was 10 Hz. For all test specimens, a consistent geometry was required to reduce variability in the outcomes. Figure 4 displays the chosen geometry for reference non-spliced bars (per ASTM E8 [52]) and spliced specimens, which were created in accordance with the specifications outlined in [53]. Based on the dimensions of the bar and the length of the mechanical bar splice (Ls), the total specimen length (L) was calculated. The coupler length plus α times the bar diameter (αdb) from each side of the coupler ends is known as the coupler region length (Lcr). In the present study, alpha was more than twice the bar diameter [53]. The bar length from outside the coupler region to the grip was at least 16 times the bar diameter to avoid any localized failure. For regular bar testing, ASTM E8 and ISO ISO/DIS 15835 [52,53] require at least 5 db grip-to-grip length. Extensometers were used to measure the strains of non-spliced and spliced specimens, respectively. The bar extensometer had 100-mm stroke and could measure strains until the fracture of the bar. In the monotonic testing of the without concrete mechanical splices, three cycles between zero and 60% yield strength of the non-spliced counterpart were used to evaluate elastic slip at the threads. The identical specimens were then exposed to an axial displacement that increased monotonically until fractur. For without concrete or with concrete specimens, the yield displacement Δy was derived from the test data and utilized to define the key parameters for the cyclic loading technique after getting the whole stress-displacement σ-Δ curve from the monotonic test. The C1 low-cycle reverse elastic-plastic loading pattern, as specified by ISO 15835-2:2009 [53] and schematically shown in Fig. 4a, was applied to the cyclic without concrete tests. The loading process involves applying displacements ranging from zero up to 2×Δy (yield displacement) in tension, followed by a reversal corresponding to fifty percent of the yield strength in compression, and repeating this process four times. The applied force is then raised from zero to five times in tension, reversed to 50% of the yield strength in compression, and repeated four times. Following the cycling, the test specimen is subjected to a technique that entails applying increasing tension until failure. The with concrete tests were put through a C2 low cycle loading test, in which the sample was originally put through 20 cycles ranging from 90% of its yield strength in tension to 50% of its yield strength in compression, as schematically shown in Fig. 4b. The process entails raising the test specimen's tension once the cycling is finished until failure.[53]
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Fig. 4. Testing configuration for without concrete specimens and with concrete specimens.
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Fig. 4. An illustration of the loading methods in schematic form: a: C1 Alternating tension and compression tests for mechanical splices with substantial plastic strains b: C2 Alternating tension and compression tests for mechanical splices with high stresses [53].
3. Results and Discussion
3.1. Without concrete specimens
In this section, monotonic loading and cyclic C1 loading were used to evaluate 24 mechanical bar splices and 12 non-spliced bars made up of 16 mm and 20 mm splices. These bar sizes were specifically selected since they are available in markets using either SI or Imperial units. Two different types of couplers (TC and OTC) consisting of three different products were included in this experimental program. Two spliced specimens were tested per product, and at least one non-spliced bar was tested per product as the reference sample. The non-spliced samples' minimum tensile yield strength fy was 511 MPa for 16 mm and 510 MPa for 20 mm, respectively, while their ultimate strength fu was 618 MPa and 654 MPa, respectively. Both fy and fu were calculated by dividing the recorded load by the nominal bar area. The minimal ultimate mean strain u, calculated by dividing the measured displacement by the clear length of the specimen, was εu = 0.090 for 16 mm bars and εu = 0.090 for 20 mm bars. Table 4 shows the test findings in terms of yield force Fy and strength fy, ultimate force Fu and strength fu, mean strain at yield y and ultimate mean strains u, and a ductility factor calculated as the ratio of ultimate-to-yield mean strains εu/εy. The stress-strain response of the monotonic and cyclic tests on non-spliced and connected rebars is depicted in Fig. 5. All responses, as can be seen from these curves, are within comparable ranges, with εu between 0.09 and 0.130, and εy being almost identical for each set of tests (16 mm and 20 mm). The slight discrepancies at the end may be related to regular material fluctuations that are inherent. Notably, OTC and OTC coupling systems function effectively under monotonic and cyclic loading. Between the examined configurations, εu consistently decreases, as seen by the cyclic loading tests (C1) in Fig. 5. The highest εu values were found in the NS and OTC, in the range of 0.13. TC has the greatest reduction in ductility, with an εu of 0.09. The production method of mechanical splices with compact couplers has increased the cross-section of the rebar at the threads, which has a positive effect on the strain distribution over the length of the splice with minimal stresses at the coupler region. Some strain localization occurs at the threads in the elastic slip response depicted, as well as at the coupler to rebar interface in the inelastic regime, which ultimately promoted a failure at the coupler region for TC couplers. The εu reductions indicated above occur at the splice level and may not characterize the coupler response. Because the coupler has a larger cross-section than the rebar, the weaker segment is transmitted outside of the coupler. As a result, increased strain is created at the rebar, particularly when employing TC couplers, resulting in shorter rebar regions and premature failure near the coupler-to-rebar interface (Fig. 6). The decrease in εu between splices may become proportionally less important as total specimen length increases. This must also be carefully examined for bending elements with relatively large couplers, as the moment gradient and probable concentration of plasticity in dissipative zones may contribute to ductility reduction [1,2,7,27].
Specimen | Fy (kN) | Fu (kN) | fy (MPa) | fu (MPa) | εy (mm/mm) | εu (mm/mm) | με (εusp/ εub)
| μ (εu/ εy)
| Ru (%) | Ry (%) |
A-NS-16M-1 A-NS-16M-2 A-NS-16M-3 | 102 106 105 | 122 127 125 | 510 530 525 | 623 647 638 | 0.0041 0.0038 0.0042 | 0.122 0.116 0.126 |
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Average | 104±1.7ab | 126±2.1a | 520±8.5a | 636±9.9a | 0.0040±0.00017a | 0.122±0.004a | 1.00 | 30.40 | - | - |
A-TC-16M-1 A-TC-16M-2 A-TC-16M-3 | 102 100 106 | 121 116 128 | 530 525 535 | 618 592 653 | 0.0041 0.0038 0.0042 | 0.100 0.096 0.102 |
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Average | 103±2.5a | 122±4.9a | 530±4.1a | 622±25a | 0.0040±0.00017a | 0.098±0.003a | 0.80 | 24.50 | 119.60 | 101.53 |
A-OTC-16M-1 A-OTC-16M-2 A-OTC-16M-3 | 105 109 108 | 125 128 126 | 509 530 520 | 643 653 637 | 0.0040 0.0038 0.0038 | 0.103 0.101 0.108 |
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Average | 107±1.7ab | 127±1.2a | 519±8.6a | 644±6.6a | 0.0039±0.00017a | 0.111±0.003a | 0.91 | 28.46 | 125.57 | 99.81 |
A-NS-16C1-1 A-NS-16C1-2 A-NS-16C1-3 | 103 104 104 | 123 124 123 | 515 535 520 | 629 630 627 | 0.0044 0.0044 0.0043 | 0.130 0.131 0.134 |
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Average | 104±0.5df | 123±0.5de | 524±8.5d | 628±1.3de | 0.0044±0.00005de | 0.132±0.002de | 1.00 | 30.00 | - | - |
A-TC-16C1-1 A-TC-16C1-2 A-TC-16C1-3 | 100 099 106 | 120 121 122 | 512 511 525 | 612 619 621 | 0.0040 0.0036 0.0041 | 0.090 0.086 0.094 |
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Average | 102±3.1d | 121±0.80e | 516±6.4d | 618±3.8e | 0.0038±0.00020e | 0.090±0.003e | 0.68 | 23.68 | 117.94 | 98.47 |
A-OTC-16C1-1 A-OTC-16C1-2 A-OTC-16C1-3 | 108 106 112 | 131 124 132 | 520 504 509 | 668 632 673 | 0.0039 0.0038 0.0040 | 0.097 0.092 0.103 |
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Average | 109±2.5fg | 129±3.6d | 511±6.7d | 658±18.3d | 0.0039±0.0001d | 0.097±0.004d | 0.74 | 25.00 | 123.45 | 97.52 |
A-NS-20M-1 A-NS-20M-2 A-NS-20M-3 | 157 161 168 | 192 196 197 | 550 510 539 | 692 690 689 | 0.0048 0.0038 0.0047 | 0.122 0.126 0.121 |
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Average | 162±4.5h | 195±2.1hi | 533±16h | 691±1.2h | 0.0044±0.00045h | 0.124±0.002h | 1.00 | 28.20 | - | - |
A-TC-20M-1 A-TC-20M-2 A-TC-20M-3 | 165 160 163 | 192 186 189 | 522 528 517 | 670 650 663 | 0.0041 0.0040 0.0040 | 0.094 0.090 0.091 |
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Average | 163±2.1h | 189±2.4h | 522±4.5h | 660±8.3i | 0.0040±0.00005hi | 0.091±0.002i | 0.73 | 22.70 | 123.80 | 97.94 |
A-OTC-20M-1 A-OTC-20M-2 A-OTC-20M-3 | 162 167 169 | 193 199 192 | 517 533 539 | 676 697 672 | 0.0039 0.0038 0.0037 | 0.096 0.097 0.100 |
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Average | 166±2.9h | 195±3.1hi | 530±9.3h | 683±11h | 0.0038±0.00008i | 0.097±0.002i | 1.01 | 25.52 | 128.14 | 99.50 |
A-NS-20C1-1 A-NS-20C1-2 A-NS-20C1-3 | 160 161 163 | 195 197 196 | 506 517 518 | 682 689 687 | 0.0046 0.0043 0.0047 | 0.126 0.126 0.132 |
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Average | 161±1.2j | 196±0.8j | 514±5.4j | 686±2.9jl | 0.0046±0.00016j | 0.128±0.003j | 1.00 | 27.80 | - | - |
A-TC-20C1-1 A-TC-20C1-2 A-TC-20C1-3 | 160 157 162 | 189 184 186 | 512 500 517 | 640 640 645 | 0.0040 0.0036 0.0041 | 0.090 0.086 0.094 |
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Average | 160±2.1j | 187±2.1k | 510±7.1j | 641±3.6k | 0.0040±0.0002k | 0.090±0.003k | 0.70 | 22.5 | 124.50 | 99.22 |
A-OTC-20C1-1 A-OTC-20C1-2 A-OTC-20C1-3 | 168 166 172 | 195 194 197 | 530 528 544 | 683 679 690 | 0.0040 0.0038 0.0040 | 0.096 0.101 0.092 |
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Average | 169±2.5k | 196±1.3j | 534±7.2k | 686±4.5j | 0.0039±0.0001k | 0.097±0.004k | 1.00 | 25.01 | 133.46 | 103.90 |
*Different letters in the same column indicate significant differences (P < 0.05).
** Rebar fracture
Table. 4. Test results of without concrete rebar tests*.
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Fig. 5. Without concrete test σ-ε relationships for monotonic and cyclic specimens NS, TC and OTC (16 mm and 20 mm).
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Fig. 6. Failure locations of investigated specimens NS, TC and OTC (16 mm and 20 mm), a) Without concrete specimens, b) With concrete specimens.
3.2. With concrete specimens
Mechanical bar splices and non-spliced bars with 16 mm and 20 mm splices were tested utilizing monotonic and cyclic C2 loads. This experimental program includes two distinct types of couplers (TC and OTC). Three spliced specimens were evaluated each product, with at least one non-spliced bar tested as a control sample. Table 5 summarizes the key findings from the monotonic and cyclic testing on embedded mechanical splices in terms of yield strength Fy and ultimate strength Fu. The table also provides the matching overall yield εy and ultimate εu mean stresses, which are calculated for qualitative comparisons by dividing the recorded machine displacement by the member clear length, which includes the concrete region and free rebar area. A ductility ratio μ is also supplied, which is calculated from the ultimate-to-yield mean strain ratio εu/εy. With concrete members combining non-spliced and spliced 16 mm and 20 mm rebars, as shown in Fig. 7, there is a consistent reduction in εu with splice type and type of loading. It is also worth mentioning that the uniaxial tests revealed lower εu for with concrete members than for without concrete specimens, implying that the with concrete response might be considered a conservative lower bound for splice performance [7,35,54]. Bompa [35] developed four performance parameters for comparative analysis to qualitatively investigate the strength, deformation capacity, and size influence of mechanical splices: (a) the ratio of the splice's ultimate tensile strength to the yield strength of the non-spliced rebar, σy,sp/σy,b; (b) the ratio of the splice's ultimate tensile strength to the rebar's ultimate tensile strength, σu,sp/σub; (c) the ultimate strain ratio between spliced and non-spliced εu,sp/εu,b, (d) a size factor calculated as the product of the coupler's diameter and length (dc × Lc) and the length of the coupler region (Lcr = Lc + 4 × db), where db is the original.
Specimen | Fy (kN) | Fu (kN) | fy (MPa) | fu (MPa) | εy (mm/mm) | εu (mm/mm) | με (εusp/ εub)
| μ (εu/ εy)
| Ru (%) | Ry (%) |
C-NS-16M-1 C-NS-16M-2 C-NS-16M-3 | 106 102 105 | 127 122 125 | 531 510 525 | 647 623 640 | 0.0042 0.0038 0.0041 | 0.113 0.107 0.111 |
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Average | 104±1.7ab | 125±2.1a | 520±8.8a | 637±10a | 0.0040±0.00017a | 0.110±0.003ad | 1.00 | 27.50 | - | - |
C-TC-16M-1 C-TC-16M-2 C-TC-16M-3 | 100 102 106 | 117 122 125 | 515 520 525 | 590 617 650 | 0.0039 0.0040 0.0042 | 0.079 0.083 0.085 |
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Average | 103±2.5b | 122±3.3a | 520±4.1a | 619±25a | 0.0041±0.00012a | 0.083±0.003b | 0.75 | 2025 | 119.03 | 100.00 |
C-OTC-16M-1 C-OTC-16M-2 C-OTC-16M-3 | 107 105 106 | 126 122 124 | 520 518 517 | 646 643 645 | 0.0045 0.0043 0.0043 | 0.097 0.093 0.103 |
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Average | 106±0.8a | 124±1.6a | 518±1.3a | 645±1.3a | 0.0044±0.00001b | 0.097±0.004c | 0.88 | 22.00 | 124.03 | 100 |
C-NS-16C2-1 C-NS-16C2-2 C-NS-16C2-3 | 105 104 105 | 124 125 126 | 542 535 542 | 625 630 635 | 0.0045 0.0046 0.0044 | 0.101 0.096 0.098 |
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Average | 105±0.5e | 125±0.8e | 538±3.3e | 630±4.1e | 0.0045±0.00008e | 0.098±0.002e | 1.00 | 21.80 | - | - |
C-TC-16C2-1 C-TC-16C2-2 C-TC-16C2-3 | 103 103 106 | 119 120 121 | 512 512 525 | 619 619 620 | 0.0042 0.0037 0.0039 | 0.094 0.087 0.092 |
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Average | 104±1.4e | 120±0.8f | 517±6.4f | 619±0.5e | 0.0039±0.00021f | 0.091±0.003f | 0.92 | 23.30 | 115.06 | 96.09 |
C-OTC-16C2-1 C-OTC-16C2-2 C-OTC-16C2-3 | 108 109 108 | 130 131 126 | 518 520 506 | 669 673 635 | 0.0040 0.0039 0.0040 | 0.092 0.090 0.088 |
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Average | 108±0.5f | 129±2.1g | 514±6.7f | 659±17.0f | 0.0040±0.00005f | 0.090±0.002f | 0.92 | 22.50 | 122.50 | 95.54 |
C-NS-20M-1 C-NS-20M-2 C-NS-20M-3 | 161 157 168 | 199 197 198 | 543 517 557 | 694 689 698 | 0.0047 0.0040 0.0045 | 0.114 0.104 0.110 |
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Average | 162±4.5h | 198±0.8h | 539±17h | 693±3.7h | 0.0043±0.00029h | 0.109±0.004h | 1.00 | 25.35 | - | - |
C-TC-20M-1 C-TC-20M-2 C-TC-20M-3 | 165 160 163 | 190 185 188 | 522 530 518 | 672 653 661 | 0.0044 0.0046 0.0043 | 0.086 0.080 0.082 |
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Average | 163±2.1h | 187±2.1i | 523±5.0h | 662±7.8i | 0.0044±0.00012h | 0.083±0.003i | 0.76 | 18.90 | 122.82 | 97.03 |
C-OTC-20M-1 C-OTC-20M-2 C-OTC-20M-3 | 164 167 165 | 197 197 194 | 525 532 528 | 690 697 692 | 0.0038 0.0043 0.0038 | 0.091 0.086 0.082 |
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Average | 165±1.2hi | 195±1.4h | 528±2.9h | 693±2.9h | 0.0040±0.00024h | 0.086±0.004i | 1.01 | 21.50 | 128.57 | 98.00 |
C-NS-20C2-1 C-NS-20C2-2 C-NS-20C2-3 | 165 162 165 | 198 197 199 | 525 517 520 | 689 685 689 | 0.0048 0.0046 0.0049 | 0.112 0.106 0.109 |
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Average | 164±1.4jk | 198±0.8j | 521±3.3j | 688±1.9jl | 0.0048±0.00013jk | 0.110±0.002j | 1.00 | 2.300 | - | - |
C-TC-20C2-1 C-TC-20C2-2 C-TC-20C2-3 | 162 158 162 | 185 182 185 | 517 515 517 | 650 648 650 | 0.0037 0.0040 0.0039 | 0.084 0.078 0.081 |
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Average | 161±1.9j | 184±1.4k | 516±0.9j | 649±1.0k | 0.0039±0.00013l | 0.081±0.002k | 0.73 | 20.75 | 124.00 | 99.04 |
C-OTC-20C2-1 C-OTC-20C2-2 C-OTC-20C2-3 | 168 168 167 | 197 197 196 | 530 530 529 | 685 687 683 | 0.0043 0.0035 0.0040 | 0.090 0.086 0.086 |
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Average | 168±0.5kl | 197±0.5j | 530±0.5k | 685±1.6j | 0.0039±0.00033l | 0.088±0.002l | 1.00 | 22.60 | 131.48 | 101.73 |
*Different letters in the same column indicate significant differences (P < 0.05).
Table. 5. Test results of concrete members*.
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Fig. 7. With concrete test F-ε relationships: monotonic and cyclic specimens NS, TC, OTC, and RFWTC (16 mm and 20 mm).
3.3. Ductility and energy absorption
The μ and με of each sample in Table 4 were determined using Fig. 8 and Eq. (1) respectively.
(1)
The ultimate-to-yield mean strain ratio can be used to calculate a ductility ratio [35]. Additionally, the ratio of the ultimate strain (Ɛusp) of the spliced bar to the ultimate strain (Ɛub) of the non-spliced bar can be used to assess ductility. Here, Ɛusp stands for the ultimate strength of the spliced bar [55]. The ductility ratio (Ɛusp/Ɛub), which is over 0.65, can satisfy the EC2 [56] and EC8 [32] requirements. When the bar class C is utilized [53], the ductility ratio (Ɛusp/Ɛub), which is above 0.65, can satisfy the requirements of the EC2 [56] and EC8 [32] codes for medium ductility. However, the splice bar, which has a ductility ratio (Ɛusp/ Ɛub) less than 0.65, would seem undesirable for members that are subjected to significant inelastic deformations [55]. The above conditions should be confirmed by the splice bar's high ductility ratio, which is necessary for this investigation. According to the recommendation, the ductility of the spliced bar (μsp) should also be at least as high as that of the unspliced bar (μb). To employ splice bars in structural components that can bear significant seismic stresses, the ratio (μsp/μb) must be larger than or equal to 1.0. The ductility of the specimens was also assessed using the (Ɛusp/Ɛub) ductility ratio recommended by the earlier study [55]. To see the outcomes According to Eq. (1), Tables 4 and 5 show the average ductility values of deformed bars (non-splice bars), splice bars, and all specimens combined. It is advised that the OTC specimen is appropriate for use in structural members with high inelastic deformation since their higher ductility value exceeds the ductility of the distorted bar, allowing them to be employed for members in seismically active areas. To withstand low-to-moderate seismic loads, TC specimens can be employed as structural elements.
Fig. 8. yield and ultimate displacements definition[55,57]
3.4. Effect of loading mode on failure
The cyclic tension-tension loading path with a stress ratio greater than zero described in ISO 15835-1:2009 [53] is typically used in the fatigue test for the mechanical coupler failure investigation. Rebars are primarily used in RC structures to support tension stress in order to compensate for the concrete's low tensile strength. For this to be the optimal ultimate failure state of a concrete structure, the rebars in the tension zone must be destroyed at the same time that the concrete in the compression zone is damaged under compression. Therefore, only the reinforcement's tensile strength is taken into account while designing RC structures. In fact, the rebars with mechanical couplers in important RC structural components or connections are repeatedly subjected to the tension-compression load rather than the tension-tension load for RC structures under high earthquake excitation. The failure of mechanical splices under cyclic tension-compression loads, which has received less attention in the past, must obviously be studied. When the splices are exposed to compression loading, however, modest lateral displacement of the splices can be noticed, which significantly impacts the deformation of the mechanical splices under cyclic stress (Tables 4 and 5). As a result, even if only the strength and deformation properties of mechanical couplers are strictly inspected according to ISO 15835-1:2009 [53], they are still significantly reduced under cyclic loading, implying that mechanical splices of reinforcements in RC structures are potentially dangerous under strong earthquake excitation. To assure the safety of RC structures subjected to strong seismic excitation, it is required to evaluate the performance of mechanical splices both "without concrete" and embedded with concrete. Experimental research into the effects of loading mode on the failure of TC and OTC splices is presented in this paper. To ensure the safety and dependability of RC structures under the action of disasters like strong earthquakes, it is crucial to promote more in-depth experimental research based on the actual engineering situation and splicing type when novel mechanical couplers are adopted in new and important structures or in structures subjected to unusual loads.
4. Evaluation of the mechanical behavior of thread couplers
The grade of the reinforcement bars in this investigation is Grade 80 in accordance with ACI 318-19 [33]and Class C in accordance with EC8 [32]. It was discovered in thread couplers that the mechanism of the threaded bar and coupler on the bar had adequate interlocking strength to prevent slip displacement. The embedded thread diameter, on the other hand, is critical in ensuring the high performance of threaded couplers. Due to the high engagement strength of the strong connector in the threaded section, no slip displacement in the side of the threaded bar was detected. To observe stronger bonding between the thread and couplers, the thread position's cross-section area should be larger. In the event of a larger cross-sectional area of the bar, the bonding stresses will be uniform on the bar surface. ACI 318-19 [29], ACI 439 [58], and AC-133 [59], as well as the ISO 15835-Part 1: 2018 and ISO 15835-Part 2: 2018 standards [53], all indicate the recommended conditions for a mechanical splice utilizing a coupler. The ultimate tensile strength of the mechanical splice should be greater than 1.25 times the bar yield strength, according to BS-8110 [60] and ACI 318-19 [29] specifications. Thus, it is crucial to assess each sample's ultimate strength ratio (Ru). In this study, Ru stands for the ratio of the thread coupler sample's ultimate tensile strength to the average yield tensile strengths of the specimens of deformed bar (non-splice bar). In the case of OTC couplers, they satisfy ACI 318 specifications (Tables 4 and 5). Unfortunately, some bars do not have the potential to be oversize, or, in other words, after the rebar is oversized, the hardness of the threading area increases significantly, making threading problematic. There is no forming process. Furthermore, the yield strength ratio (Ry) was calculated; Ry signifies the ratio of the thread coupler sample's yield tensile strength to the average yield tensile strength of the deformed bars (Tables 4 and 5). While the average Ry ratios in OTC and TC are less than 1.0, In comparison to other couplers, OTC couplers perform best in terms of strengths (Ru and Ry), ductility, energy dissipation, and failure mode. Due to their improved performance, the equal splicing of RFWTC and OTC samples makes them ideal for use in high seismic zones. Additionally, the TC's performances in terms of strength, energy dissipation, and failure mode have met the standards. The structural part can withstand low-to-medium earthquake loads thanks to the ductility value of Therefore.
5.Conclusions
In this study, by modifying the method of making a threaded splice, one type of patch is introduced that can be used in the plastic hinge areas of ductile members in seismic areas. The splice area in the suggested method is oversized. In this study, more than 72 threaded couplers and oversize-threaded couplers were tested under uniaxial tensile and cyclic "without concrete" and "with concrete" conditions on NC, TC, and OTC reinforcement bars with diameters of 16 mm and 20 mm. Specimens to determine the influence of the threaded diameter on strength, ductility, and energy absorption. The following judgments were reached:
1. In the elastic cycle test, the OTC coupler exhibited somewhat equal stresses to the non-spliced reference bar, with no noticeable slide at the threads. Cyclic loading also had a negative influence on the without-concrete response, with strain at fracture reductions of up to 18% on average when compared to monotonic examples. The detailed strain measurements revealed that the enlarged rebar cross-section near the threads of couplers shifts the weak area away from the coupler region.
2. The oversize threads can improve the performance of the embedded couple with concrete members. The behavior of the OTC meets the good performance requirements for the structural member subjected to the cyclic loading test and meets the seismic zone standards. Due to its improved performance, the equal splicing of the OTC sample makes it ideal for use in high seismic zones. Additionally, the TC's performances in terms of strength, energy dissipation, and failure mode have met the standards. The structural part can withstand low-to-medium earthquake loads thanks to the ductility value of Therefore.
3. One key factor that may be utilized to assess the behavior of couplers is their energy absorption. For increased energy absorption compared to a non-splice bar, the OTC requires the threading size be increased by one size. The ultimate tensile load capacity of the couplers will increase with an increase in the thread area. The embedded bar length in the OTC shows the best performance. The OTC's ductility ratio was higher than the non-splice bar. According to practical design codes, the strength of the OTC specimens is greater than 125% of the bar yield strength.
4. The yield and ultimate strengths of OTC are comparable to those of NC, and they can also fulfill the strength requirement in the alternating tension and compression test with high stresses. Considering the outstanding connection efficiency and ease of OTC, the mechanical connection of rebars has substantially higher benefits.
Acknowledgements
The authors would like to thank the logistical support provided by the IIEES (International Institute of Earthquake Engineering and Seismology in Tehran, Iran). Support is also gratefully acknowledged from the Cuboid Construction Company (Dr. Bahrani).
Nomenclature
Ac Cross-sectional concrete
Aco Cross-sectional area
D Coupler Diameter
D1 Concrete Diameter
Etc Elastic modulus of the coupler
F Load
Fc Load of the concrete
Fy Yield load
Fu Ultimate load/peak load
Fus Thread splice sample's load-carrying capacity
Fut Ultimate tensile load of the threaded area in the bar and coupler
Fuc Tensile load resistance of the concrete
K Stress concentration factor
L Specimen length
LCon Concrete Length
LC Coupler length
LS Splice length
LT Thread Length
LW Welding Length
Ry Yield strength ratio
Ru Ultimate strength ratio
εtc Strain in the coupler
εc0 Concrete strain at peak stress
εcu Concrete ultimate strain
εusp Ultimate strain of the splice bar
εub Ultimate strain of the non-splice bar
εco Strain of the coupler
εc0 Concrete strain at peak stress
εcu Concrete ultimate strain
εf Failure strain of steel bars
εy Yield strain of steel bars
εu Ultimate strain of steel bars
σtc Stress of the coupler
σco Determine the coupler's design transverse tensile stress
σmax Maximum stress
σnom Nominal stress
fu Ultimate strength
μ Ductility
με Ductility ratio
β Coefficient based on the bar type
db Steel bar Diameter
d1 Thread area
d2 Bar oversize
d3 Thread pitch
f’c Equivalent compressive strength of cylinder sample
fcr Compressive concrete strength
fy Yield strength
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