Tensioning device and application thereof

Abstract

The invention discloses a tensioning device and application thereof, belongs to the technical field of building material performance testing, and solves the problems that the deviation between the actual stress ratio and the theoretical stress ratio of a test piece is large, and the stress level is too high due to the fact that the bolt pretightening force is not easy to control too tight during unloading, and the stress level is insufficient due to the fact that the bolt pretightening force is too loose. The tensioning device comprises base plates, twisted steel bars and loading frames, a concrete cylinder test piece is arranged between the two symmetrical base plates, the two twisted steel bars are symmetrically distributed on the two sides of the concrete cylinder test piece, the two ends of the twisted steel bars penetrate through the base plates, the loading frames are arranged on one base plate, the twisted steel bars penetrate through the loading frames, and the loading frames are arranged on the other base plate. Self-locking assemblies are arranged on the back surfaces of the loading frame and the other base plate to realize self-locking of the twisted steel, and centering assemblies are arranged on the front surfaces of the two base plates to center and reinforce the concrete cylinder test piece. Different stress levels are applied to the concrete cylinder test piece by utilizing the tensioning device, so that dynamic mechanical property testing is carried out.

Description

Technical Field

[0001] The invention belongs to the technical field of building material performance testing, and in particular relates to a tensioning device and application thereof. Background Art

[0002] In recent years, the engineering sector has faced significant challenges. Premature failure of engineering structures due to poor durability has become increasingly common. Among the many factors that deteriorate concrete performance, freeze-thaw cycles are particularly critical. Furthermore, my country is located in a seismically active region, subject to frequent earthquakes. These natural disasters significantly weaken the seismic performance of buildings, including hysteresis, energy dissipation, ductility, and stiffness. These conditions pose a serious threat to the safety and stability of engineering structures.

[0003] Given the significant impact of concrete durability and seismic effects on building structures, there is an urgent need for extensive experimental research and theoretical analysis of the seismic performance of existing concrete structures under freeze-thaw conditions. Our team found that varying freeze-thaw cycles lead to uneven degradation of concrete mechanical properties at different depths. We investigated the stress-strain relationship of compressed concrete, considering varying freeze-thaw cycles. We analyzed the influence of relative freeze-thaw depth, number of freeze-thaw cycles, and stress level on the concrete stress-strain curve. During this study, we used a tensioning device to tension the concrete stress-strain curve. We found that the anchors loosened during the tensioning of the threaded rebar, causing stress loss and an increased deviation between the actual and theoretical stress ratios of the concrete. This difficulty arises from the difficulty in controlling the bolt preload during unloading—overtightening results in excessive stress levels in the specimens, while overtightening results in insufficient stress levels. This increased deviation is also due to problems with concrete alignment. Secondly, determining the mechanical properties of concrete structures under repeated loading can more accurately estimate the seismic capacity of the structure and avoid damage under repeated compressive loads, thereby improving the service life and safety of the structure. Summary of the Invention

[0004] In response to the above problems, the present application provides a tensioning device and its application, which uses the tensioning device to apply different stress levels to the concrete cylindrical specimen. After the specimen with the loading device is corroded in a set freeze-thaw cycle environment, the loading device is disassembled, and the specimen is cut and cored considering the uneven damage caused by freeze-thaw, and then repeated pressure tests are carried out.

[0005] In the first aspect, the present invention provides a tensioning device, including a pad, a threaded steel bar and a loading frame. A concrete cylindrical specimen is placed between two symmetrical pads. Two threaded steel bars are symmetrically distributed on both sides of the concrete cylindrical specimen, and both ends of the threaded steel bars pass through the pads. A loading frame is provided on one of the pads, and the threaded steel bar passes through the loading frame. Self-locking components are provided on the back of the loading frame and the other pad for self-locking the threaded steel bar, and centering components are provided on the front of the two pads to center and reinforce the concrete cylindrical specimen.

[0006] In some preferred examples of this aspect, the self-locking component includes: A base having a through hole at its center and a boss with a hole on its surface; The first gear ring is slidably arranged on the outer side of the boss, and a receiving ring is arranged above the first gear ring, and the receiving ring and the first gear ring are connected by a column; A first driver is provided on the base, and a driving gear is provided on the driving shaft thereof, and the driving gear is engaged with the first gear ring; The guide frame is arranged between the first gear ring and the receiving ring, and the two ends of the guide frame are movably connected to the first gear ring and the receiving ring respectively; A plurality of rotary clamping members are arranged at equal intervals around the through hole of the base. The opposite ends of the plurality of rotary clamping members are hinged to the base, and the opposite ends of the plurality of rotary clamping members face the threaded steel bar.

[0007] In some preferred examples in this aspect, a cover body is also provided on the base, an opening for threaded steel bars to pass through is provided on the top of the cover body, a plurality of equally spaced auxiliary locking parts are provided inside the cover body along the circumferential direction of the opening, and the top of the auxiliary locking parts is hinged to the inner top wall of the cover body; an auxiliary braking assembly is provided between the cover body and the receiving ring, and the auxiliary locking parts are driven by the auxiliary braking assembly to lock the threaded steel bars.

[0008] In some preferred examples in this aspect, the auxiliary braking assembly includes a cylinder, the top of the cylinder is connected to the inner top wall of the cover, and a movable ring is provided inside the cylinder, the movable ring is threadedly connected to the cylinder, and the inner wall of the movable ring abuts against the outer wall of the auxiliary locking part; a fixed column is provided on the receiving ring, and the movable ring is movably sleeved on the fixed column.

[0009] In some preferred examples in this aspect, an annular groove is provided on the pad, a closed-loop sunken area and a breakpoint ring sunken area are provided on the bottom wall of the annular groove, and a plurality of equally spaced sunken holes are provided at the bottom of the breakpoint ring sunken area, a sunken cavity is provided below each sunken hole, and both ends of the sunken cavity are respectively connected to the sunken hole and the closed-loop sunken area; the centering component includes a plurality of centering arc plates, and the plurality of centering arc plates are distributed at equal intervals along the axial direction of the threaded steel bar, and a movable plate is provided on the outer wall of each centering arc plate, and the movable plate realizes circumferential movement or radial movement through the centering drive component.

[0010] In some preferred examples of this aspect, the centering drive assembly includes: a second driver, which is arranged in the sunken area of the breakpoint ring, and a first section of rope is wound around a driving shaft of the second driver; Inverted L-shaped shaft, each centering arc plate corresponds to an inverted L-shaped shaft, the horizontal end of each inverted L-shaped shaft is set in the moving groove on the side wall of the moving plate, and the vertical end of each inverted L-shaped shaft contacts the bottom wall of the sunken area of the breakpoint ring, and a guide tooth is set on the vertical arm of the inverted L-shaped shaft, which meshes with the rack on the moving plate; A connecting ring is sleeved on the vertical arm of each inverted L-shaped shaft. Adjacent connecting rings are connected by a second section of rope. The connecting ring close to the second drive is connected to the first section of rope.

[0011] In some preferred examples in this aspect, the centering drive assembly also includes a second gear ring, which is arranged in a closed-loop sunken area, and a third driver is arranged in the closed-loop sunken area. A braking tooth is arranged on the driving shaft of the third driver, and the braking tooth is engaged with the second gear ring. A plurality of movable columns are arranged below the second gear ring, and a limiting plate is provided at the bottom of each movable column, and the limiting plate is located in the sunken cavity; a movable seat is provided in the sunken cavity, and a first special-shaped groove and a second special-shaped groove are respectively provided at both ends of the top of the movable seat, and the limiting plate is located in the second special-shaped groove; a starting head is provided in each sunken hole, and the guide plate of the starting head is located in the sunken cavity and in the first special-shaped groove; a first extrusion member and a second extrusion member are respectively provided on the rod body of the starting head and the movable column.

[0012] In a second aspect, the present invention provides a method for testing the dynamic mechanical properties of confined concrete taking into account uneven freeze-thaw damage, comprising the following steps: Step 1: Prepare concrete cylindrical specimens; Step 2: Apply different stress levels to the concrete cylindrical specimen using the tensioning device described above; Step 3: The concrete cylinder specimen treated in step 2 is placed in an environmental chamber for a freeze-thaw cycle test, and then the concrete cylinder specimen is stretched; Step 4: Cut and core the concrete cylindrical specimen after being stretched in step 3, and record it as the test specimen; Step 5: Repeat the compression test on the test specimen obtained in step 4 to obtain a stress-strain curve.

[0013] In some preferred examples of this aspect, in step 2, the specific process of applying different stress levels to the concrete cylindrical specimen includes: Before the tension test begins, first stick the strain gauge on the threaded steel bar. After sticking, apply a small amount of silicone rubber on the surface of the strain gauge, connect the strain gauge to the prepared wire, and finally wrap the sticked strain gauge with epoxy resin. Then, the two threaded steel bars are tensioned simultaneously until they reach 20% of the controlled stress and the load is maintained for 2 minutes. Then, the two threaded steel bars were tensioned to 50% of the controlled stress and held for 2 minutes; Continue to tension the two threaded steel bars to 80% of the controlled stress and hold the load for 2 minutes; Continue to tension the two threaded steel bars to 110% of the controlled stress and hold the load for 2 minutes; Remove the through-type jack and loading frame. After tensioning is completed, cut off the excess threaded steel bars and monitor the changes in the prestressing force of the threaded steel bars in real time.

[0014] In some preferred examples of this aspect, in step 3, the specific process of performing a freeze-thaw cycle test on a concrete cylinder specimen is as follows: The concrete cylinder specimens together with the self-locking components, centering components, pads and threaded steel bars were laid under the sprinkler in the environmental chamber, and the freeze-thaw cycles were set to 0, 100, 200 and 250 times respectively; Each freeze-thaw cycle is divided into a cooling section, a low-temperature constant temperature section, a heating section, a high-temperature constant temperature section, and a spraying section. The durations of the cooling section, the low-temperature constant temperature section, the heating section, the high-temperature constant temperature section, and the spraying section are 2 hours, 2 hours, 0.5 hours, 1.5 hours, and 5 minutes, respectively. The temperature of the low-temperature constant temperature section is set at -20°C, and the temperature of the high-temperature constant temperature section is set at 20°C. When the predetermined number of freeze-thaw cycles is reached, the temperature rise and fall system of the environmental chamber is turned off, and the two first actuators are driven to flip in sequence to unload the two threaded steel bars.

[0015] In some preferred examples of this aspect, in step 5, the cut test specimen is placed on a loading platform, the resistance strain gauges are attached to the test specimen at symmetrical positions on both sides of the surface, and the displacement meter is fixed; Turn on the strain gauge, select the corresponding force and displacement acquisition channels, set and clear the displacement meter and strain gauge acquisition channels, and capture the local stress-strain curve of the test specimen; Before formal loading, perform a preload test at a preload rate of 0.5 mm / min. When the load reaches 5 kN, check whether the displacement meter value is close to the strain reading on the strain acquisition instrument. If the difference between the two values is greater than the threshold, it means that the test specimen is not centered and bias occurs. Adjust the position of the test specimen and reset the data of the strain acquisition instrument and displacement meter. During the loading phase, the loading speed is 0.1 mm / min, the displacement increment is 0.1 mm, and the unloading rate is 0.1 mm / min. The displacement meter can measure the vertical displacement of the concrete and calculate the stress-strain curve of the test specimen. Repeated loading is performed according to the loading system. After the first loading reaches the predetermined displacement value, the test specimen is loaded for 5 seconds and then unloaded until the load reaches 0.1 kN and then loaded for another 5 seconds. The loading and unloading cycle is repeated until the load-displacement curve stabilizes or the specimen is damaged, and then loading is stopped.

[0016] Compared with the prior art, the present invention has the following advantages: 1. The present invention applies different stress levels to the concrete cylindrical specimen through a self-made tensioning device. By observing the hydraulic gauge reading and loading step by step, the required stress level can be accurately applied to the concrete cylindrical specimen.

[0017] 2. The present invention also designs a self-locking component, which can realize self-locking of the threaded steel bar in the current position, avoiding stress loss caused by loosening of the nut or improper control of the pre-tightening force when stress is applied, causing excessive deviation between the actual stress ratio and the theoretical stress ratio.

[0018] 3. The present invention also designs a centering component, which is used to center and reinforce the concrete cylindrical specimen. At the same time, the centering component can be used to achieve stress unloading when unloading threaded steel bars to avoid danger.

[0019] 4. The present invention considers the coupling between concrete bearing and freeze-thaw damage, and also considers the uneven distribution of freeze-thaw damage inside the specimen, and can obtain the repeated compressive stress-strain relationship of freeze-thaw damaged bearing concrete at different depths. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the mold; Figure 2 This is a schematic diagram of the tensioning device when the centering component is not opened; Figure 3 is a schematic diagram of the tensioning device with the centering component opened; Figure 4 is a perspective view of a self-locking assembly; Figure 5 A schematic diagram of the self-locking assembly after the cover is removed; Figure 6 Schematic diagram of the self-locking assembly after removing the cover and the cylinder; Figure 7 Schematic diagram of the connection relationship between the first out-loop, the rotating clamping member, the driving gear and the guide frame; Figure 8 It is a cross-sectional view of the self-locking component; Figure 9 is a schematic diagram of the centering component; Figure 10 This is a schematic diagram of the centering component after it is opened; Figure 11 for Figure 10 Enlarged view of point A in the middle; Figure 12 It is a partial schematic diagram of the misalignment between the second gear ring and the guide gear; Figure 13 is a partial schematic diagram of the second gear ring meshing with the guide gear; Figure 14 A schematic diagram of a mobile seat; Figure 15 is a cross-sectional view of the pad; Figure 16 The first number in the above naming indicates the stress ratio, and the second number indicates the number of freeze-thaw cycles. The concrete cylinder specimens here are all measured for their prestress changes at room temperature, and the number of freeze-thaw cycles mentioned in the naming is only used to distinguish the concrete cylinder specimens.

[0021] Figure 17 is a schematic diagram of the displacement meter clamping device; Figure 18 Schematic diagram of cutting concrete cylinder specimen; Figure 19 Schematic diagram of coring of the concrete cylinder specimen after cutting; Figure 20 Schematic diagram of the repeated compression loading platform; Figure 21 Schematic diagram of repeated compression loading system; Figure 22Stress-strain curves of the test specimens under 0 freeze-thaw cycles; (a) C0-0-1; (b) C0-0-2; (c) C0-0-3; (d) C0-0-4; (e) C0.2-0-1; (f) C0.2-0-2; (g) C0.2-0-3; (h) C0.2-0-4; (i) C0.3-0-1; (j) C0.3-0-2; (k) C0.3-0-3; (l) C0.3-0-4; (m) C0.4-0-1; (n) C0.4-0-2; (p) C0.4-0-3; (q) C0.4-0-4; Figure 23 Stress-strain curves of the test specimens under 100 freeze-thaw cycles; among them, (a) C0-100-1; (b) C0-100-2; (c) C0-100-3; (d) C0-100-4; (e) C0.2-100-1; (f) C0.2-100-2; (g) C0.2-100-3; (h) C0.2-100-4; (i) C0.3-100-1; (j) C0.3-100-2; (k) C0.3-100-3; (l) C0.3-100-4; (m) C0.4-100-1; (n) C0.4-100-2; (p) C0.4-100-3; (q) C0.4-100-4; Figure 24 Stress-strain curves of the test specimens under 200 freeze-thaw cycles; among them, (a) C0-200-1; (b) C0-200-2; (c) C0-200-3; (d) C0-200-4; (e) C0.2-200-1; (f) C0.2-200-2; (g) C0.2-200-3; (h) C0.2-200-4; (i) C0.3-200-1; (j) C0.3-200-2; (k) C0.3-200-3; (l) C0.3-200-4; (m) C0.4-200-1; (n) C0.4-200-2; (p) C0.4-200-3; (q) C0.4-200-4; Figure 25 Stress-strain curves of the test specimens under 250 freeze-thaw cycles; among them, (a) C0-250-1; (b) C0-250-2; (c) C0-250-3; (d) C0-250-4; (e) C0.2-250-1; (f) C0.2-250-2; (g) C0.2-250-3; (h) C0.2-250-4; (i) C0.3-250-1; (j) C0.3-250-2; (k) C0.3-250-3; (l) C0.3-250-4; In the names of each sub-figure, C represents the test specimen, the first number represents the stress ratio, the second number represents the number of freeze-thaw cycles, and the third number represents the relative freeze-thaw depth, where 1 represents 0.1, 2 represents 0.3, 3 represents 0.5, and 4 represents 0.8.

[0022] In the figure, 1, PVC pipe; 2, plug; 3, pad; 301, annular groove; 302, closed loop sunken area; 303, breakpoint ring sunken area; 304, sunken hole; 305, sunken cavity; 4, loading frame; 5. Self-locking assembly; 501. Base; 502. Cover; 503. First gear ring; 504. Receiving ring; 505. First driver; 506. Driving gear; 507. Guide frame; 508. Rotating clamp; 509. Auxiliary locking member; 510. Cylinder; 511. Moving ring; 512. Fixed column; 6. Through-type jack; 7. Concrete cylindrical specimen; 8. Threaded steel bar; 9. Centering assembly; 901. Centering arc plate; 902. Moving plate; 903. Inverted L-shaped shaft; 904. Guide gear; 905. Second gear ring; 906. First rope section; 907. Second rope section; 908. Third driver; 909. Braking gear; 910. Moving column; 911. Limiting plate; 912. Moving seat; 913. First special-shaped groove; 914. Second special-shaped groove; 915. Starting head; 916. Guide plate; 917. First extrusion member; 918. Second extrusion member. 10. Anchor rod; 11. Displacement meter clamping frame; 12. Displacement meter; 13. Loading platform; 14. Resistance strain gauge; 15. Fixing plate; 16. Test specimen. DETAILED DESCRIPTION

[0023] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the protection scope of the present invention.

[0024] Example 1: This embodiment provides a tensioning device, such as Figure 2-Figure 15 As shown, the homemade tensioning device includes a pad 1, and the concrete cylindrical specimen 7 is located between two symmetrical pads 1. A centering component 9 is provided on the front of the two pads 1. The centering component 9 is used to center and reinforce the concrete cylindrical specimen 7, and at the same time, stress unloading can be achieved when unloading the threaded steel bars 8 to avoid danger.

[0025] A threaded steel bar 8 passes through each side of the two symmetrical pads 1, the concrete cylindrical specimen 7 is located between the two threaded steel bars 8, and a self-locking component 5 is provided on one side of the two threaded steel bars 8 on the back of the pads 1. The self-locking component 5 can realize self-locking of the current position of the threaded steel bars 8, and will not loosen during the stress loading stage.

[0026] In this embodiment, the self-locking assembly 5 includes a base 501 with a central hole for threaded rebar 8 to pass through. The bottom of the base 501 is welded or bolted to the backing plate 1. A boss with a hole is provided on the surface of the base 501. A first gear ring 503 is disposed on the outer side of the boss. The first gear ring 503 is slidably connected to the boss via a bearing. A receiving ring 504 is provided above the first gear ring 503. The receiving ring 504 is fixedly connected to the first gear ring 503 via a column.

[0027] A guide frame 507 is also provided between the first gear ring 503 and the receiving ring 504. There are at least three guide frames 507, and the three guide frames 507 are evenly distributed on the circular surface of the first gear ring 503. The two ends of the guide frame 507 are movably connected to the first gear ring 503 and the receiving ring 504 respectively. Each guide frame 507 corresponds to a rotary clamping member 508. The rod of the rotary clamping member 508 passes through the guide frame 507. The opposite ends of the three rotary clamping members 508 are hinged to the base 501, and the opposite ends of the three rotary clamping members 508 face the threaded steel bar 8.

[0028] A first driver 505 is also provided on the base 501, and a drive shaft thereof is provided with a drive tooth 506, which meshes with the first ring gear 503. After the first driver 505 is started, the drive tooth 506 meshes with the first ring gear 503 and rotates (because the rotation amplitude of the guide frame 507 is relatively small, a speed reducer is provided between the first driver 505 and the drive tooth 506 to prevent the first ring gear 503 from rotating too quickly. The connection method of the first driver 505, the speed reducer, and the drive tooth 506 is conventional and will not be described in detail here). The first ring gear 503 drives the guide frame 507 to rotate, forcing the rotary clamping member 508 to rotate. The spacing between the opposite ends of the three rotary clamping members 508 is reduced, so that the opposite ends of the three rotary clamping members 508 abut against the threaded steel bar 8, and the first self-locking is achieved through the meshing of the first ring gear 503 and the drive tooth 506.

[0029] A cover 502 is disposed outside the first gear ring 503 and the first driver 505. The cover 502 is bolted to the base 501. An opening is provided at the top of the cover 502 for the threaded rebar 8 to pass through. At least three equally spaced auxiliary locking members 509 are disposed within the cover 502 along the circumference of the opening. The tops of the auxiliary locking members 509 are hingedly connected to the inner top wall of the cover 502. An auxiliary brake assembly is disposed between the cover 502 and the receiving ring 504. The auxiliary brake assembly includes a cylinder 510, the top of which is connected to the inner top wall of the cover 502. A movable ring 511 is disposed within the cylinder 510, threadedly connected to the cylinder 510, and the inner wall of the movable ring 511 abuts the outer wall of the auxiliary locking member 509. A fixed post 512 is disposed on the receiving ring 504. The movable ring 511 is sleeved on the fixed post 512 and can move along the fixed post 512.

[0030] When the receiving ring 504 rotates synchronously with the first gear ring 503, the moving ring 511 rotates with the receiving ring 504. However, since the moving ring 511 and the inner wall of the cylinder 510 are threadedly connected, the relationship between the moving ring 511 and the cylinder 510 is that of a screw and a nut. The moving ring 511 is the screw, and the cylinder 510 is fixed. Then, the moving ring 511 moves upward while rotating, and the free ends of the three auxiliary locking parts 509 gather together and abut against the threaded steel bar 8, and the second self-locking is achieved through the threaded connection between the moving ring 511 and the cylinder 510.

[0031] In order to prevent the movable ring 511 from getting stuck with the cylinder 510 and being unable to rotate upward when rotating synchronously with the receiving ring 504, the parameters of the threaded connection between the movable ring 511 and the cylinder 510 are: the pitch is 10mm and the helix angle is 6.7°.

[0032] Double self-locking is achieved by the meshing of the driving teeth 506 and the first gear ring 503 and the threaded connection between the moving ring 511 and the cylinder 510, effectively avoiding stress loss caused by loosening of the anchor.

[0033] The two pads 1 are each provided with an annular groove 301 surrounding the concrete cylindrical specimen 7 on one side thereof, a closed-loop sunken area 302 and a breakpoint ring sunken area 303 are provided on the bottom wall of the annular groove 301, and a plurality of equally spaced sunken holes 304 are provided at the bottom of the breakpoint ring sunken area 303, a sunken cavity 305 is provided below each sunken hole 304, and both ends of the sunken cavity 305 are respectively connected to the sunken hole 304 and the closed-loop sunken area 302.

[0034] The centering component 9 is arranged in the annular groove 301, which includes a centering arc plate 901. Five centering arc plates 901 are arranged around the annular groove 301. A movable plate 902 is arranged on the outer wall of each centering arc plate 901. A movable groove is arranged on the side wall of each movable plate 902, and a rack is arranged below the movable groove. A movable inverted L-shaped shaft 903 is provided in the movable groove, and a guide tooth 904 is provided on the vertical arm of the inverted L-shaped shaft 903, which engages with the rack, and the end of the vertical arm of the inverted L-shaped shaft 903 contacts the bottom wall of the breakpoint ring sunken area 303, and a second driver is provided in the breakpoint ring sunken area 303. The first section of rope 906 is wound around the driving shaft of the second driver, and a connecting ring is respectively provided on the vertical arm of each inverted L-shaped shaft 903, and the connecting ring is located below the guide tooth 904. Adjacent connecting rings are connected by a second section of rope 907, and the connecting ring close to the second driver is connected to the first section of rope 906.

[0035] Taking the position of the second driver as the head, the vertical end of the inverted L-shaped shaft 903 located at the tail of the breakpoint ring sunken area 303 is fixedly connected to the breakpoint ring sunken area 303 .

[0036] The second section of the rope 907 can all use a coil spring (the width of the coil spring can be controlled), have memory ability, and can be reeled in when the external force is lost. The first section of the rope 906 can be an ordinary rope. Multiple second sections of the rope 907 are respectively wound around the third connecting ring, the fourth connecting ring and the fifth connecting ring. In the initial state, when the multiple second sections of the rope 907 are reeled in, the five centering arc plates 901 are close to each other, and the first section of the rope 906 is straightened.

[0037] When the second drive is started, the first section of the rope 906 is wound, and the first connecting ring drives the current centering arc plate 901 to move toward the second drive. Since the vertical end of the inverted L-shaped axis 903 at the tail of the breakpoint ring sunken area 303 is fixedly connected to the breakpoint ring sunken area 303, the three middle arc plates in the middle are unfolded in turn, so that they are evenly spaced on the outer periphery of the concrete cylindrical specimen 7.

[0038] A second gear ring 905, a fixed ring, and a third driver 908 are provided in the closed-loop sunken area 302. The fixed ring is located inside the second gear ring 905 and is connected to the second gear ring 905 via a bearing. The fixed ring and the second gear ring 905 are both arranged around the closed-loop sunken area 302. The inner wall of the fixed ring is attached to the outer wall of the closed-loop sunken area 302, and a plurality of movable columns 910 are provided below the fixed ring. A limit plate 911 is provided at the bottom of each movable column 910, and the limit plate 911 is located in the sunken cavity 305. A movable seat 912 is provided in the sunken cavity 305. The top ends of the movable seat 912 are respectively provided with a first special-shaped groove 913 and a second special-shaped groove 914. The limit plate 911 is located in the second special-shaped groove 914. The cooperation between the second special-shaped groove and the limit plate 911 can cause the second gear ring 905 to be misaligned with the guide gear 904.

[0039] Compression springs are provided between the two ends of the movable seat 912 and the two side walls of the sunken cavity 305 , and the two compression springs have the same specifications to keep the movable seat 912 centered.

[0040] The drive shaft of the third driver 908 is provided with brake teeth 909, which mesh with the spur teeth of the second ring gear 905. The first driver 505, the second driver, and the third driver 908 all use motors, and the output shafts of the motors are connected to the reducer via couplings. The output shaft of the reducer is respectively connected to the drive teeth 506, the first rope section 906, and the brake teeth 909.

[0041] In this embodiment, five centering arc plates 901 correspond to four sunken holes 304 (i.e., n centering arc plates 901 correspond to n-1 lower limit holes), and a starting head 915 is provided in each sunken hole 304. A guide plate 916 is provided at the bottom of the starting head 915. The guide plate 916 is located in the sunken cavity 305 and in the first special-shaped groove 913.

[0042] The rod body of the actuating head 915 and the movable column 910 are respectively provided with a first extrusion member 917 and a second extrusion member 918. In this embodiment, the first extrusion member 917 and the second extrusion member 918 are both compression springs.

[0043] When the five centering arc plates 901 are positioned adjacent to each other, a sunken hole 304 appears on the top of the starting head 915, the guide plate 916 is positioned above the first special-shaped groove 913, the limit plate 911 is positioned within the second special-shaped groove 914, and the second gear ring 905 is sunken and misaligned with the guide gear 904. When the second actuator is activated and the five centering arc plates 901 are evenly spaced around the circumference of the concrete cylindrical specimen 7, the four starting heads 915 are simultaneously pressed downward, the guide plate 916 moves into the first special-shaped groove 913, squeezing the inclined surface of the first special-shaped groove 913, the movable seat 912 moves away from the second gear ring 905, and the limit plate 911 moves to the notch of the second special-shaped groove 914. Under the action of the second extruder 918, the multiple limit plates 911 simultaneously push the fixing ring upward, and the helical teeth of the second gear ring 905 engage with the helical teeth of the guide gear 904.

[0044] Then the third driver 908 is started, and the brake gear 909 drives the second gear ring 905 to rotate, and the guide gear 904 (the guide gear 904 is connected to the inverted L-shaped shaft 903 through a bearing) rotates synchronously. Since the movable plate is engaged with the guide gear 904 through the rack, the movable plate 902 moves horizontally, carrying the centering arc plate 901 toward the concrete cylindrical specimen 7, and the concrete cylindrical specimen 7 is centered and clamped.

[0045] The above-mentioned centering component 9 has the following advantages: Uniform circumferential force: The first and second ropes ensure that the five centering arc plates 901 are synchronously and equidistantly distributed, avoiding eccentricity or local stress concentration of the specimen due to uneven force. This is particularly suitable for mechanical property testing of brittle materials such as concrete, ensuring that the test data truly reflects the material characteristics.

[0046] Reduced manual intervention: Traditional manual clamping methods rely on operator experience and are prone to large discrete test results due to uneven force or centering deviation. This embodiment uses mechanical automation to strictly reproduce the clamping process, improve the repeatability and comparability of test data, and meet the requirements of standardized testing processes.

[0047] Regardless of how the diameter of the specimen changes, the synchronous transmission of the guide teeth 904 and the rack ensures that each centering arc plate 901 always moves radially at the same speed, forcing the axis of the concrete cylindrical specimen 7 to automatically align with the center of the mechanism through the symmetrical force field.

[0048] The specific process of tensioning using the above-mentioned tensioning device includes: first, sticking a strain gauge on the threaded steel bar, and after sticking, applying a small amount of silicone rubber on the surface of the strain gauge. After air-drying for two hours, a protective film can be formed on the surface of the strain gauge to reduce the influence of external factors on the strain gauge. Use a welding gun to weld the strain gauge to the prepared wire, and finally use gauze dipped in an appropriate amount of epoxy resin to wrap the pasted strain gauge. The threaded steel bars 8 at both ends are tensioned at the same time. First, the two threaded steel bars 8 are tensioned to 20% of their controlled stress and held for 2 minutes. Then, the two are tensioned to 50% of their controlled stress and held for 2 minutes. In the same way, they are tensioned to 80% and 110% of their controlled force and held for 2 minutes. Remove the through-type jack 6 and the loading frame 4, and finally complete the tensioning of the specimen. After the tensioning is completed, cut off the excess threaded steel bars 8 and monitor the changes in the prestress of the threaded steel bars in real time, such as Figure 16 As shown in the figure, after optimization, the difference between the maximum and minimum values of the strain-history curve decreases, indicating that the two self-locking effects have taken effect, reducing the prestress loss of threaded rebar 8 and bringing the stress level closer to the design level after optimization. (The difference in initial values is due to errors in steel material properties; the strain-history curve shows a rapid initial decline and a gradual flattening later in the process. This initial decline is primarily due to the significant combined effects of concrete shrinkage and creep and stress relaxation of threaded rebar 8; the latter stage gradually stabilizes as the effects of these two factors weaken.)

[0049] Example 2: See also Figure 1 、 Figures 17-25 As shown, this embodiment provides a method for testing the dynamic mechanical properties of confined concrete taking into account the freeze-thaw damage gradient, comprising the following steps: Step 1: Prepare a concrete cylindrical specimen 7; Step 2: Using the self-made tensioning device in the embodiment to apply different stress levels to the concrete cylindrical specimen 7; Step 3: The concrete cylindrical specimen 7 treated in step 2 is placed in an environmental chamber for a freeze-thaw cycle test, and then the concrete cylindrical specimen 7 is opened; Step 4: Cut and core the concrete cylindrical specimen 7 processed in step 3, and record it as test specimen 16; Step 5: Repeat the compression test on the test specimen 16 to obtain a stress-strain curve.

[0050] Specifically, in step 1, the present invention first makes a homemade mold. The homemade mold uses a PVC pipe 1 with an inner diameter of D1, an outer diameter of D2, and a height of h. The error is controlled within ±2 / h. The plug 2 is tightly connected to the PVC pipe 1 at the bottom. Then, the homemade mold is used to make a concrete cylindrical specimen 7. A layer of release agent is applied to the inner side 1 of the PVC pipe and the plug 2. The release agent is vaseline. The four molds are tied together with wire to prevent tilting during the pouring process. The configured concrete is pumped into the mold from top to bottom. Layered vibration is used to pour 1 / 3 of the overall mold first. An inserted vibrating rod is used to vibrate until the concrete surface no longer bubbles. Then, concrete is poured until 2 / 3 of the mold height. The pouring of the overall specimen is completed in this order.

[0051] After initial setting, the concrete cylinder specimen 7 was covered with a tarpaulin and steam-cured at 55±5°C, with a heating rate of no more than 10°C per hour. After 8 hours, the formwork was removed and the specimen was placed in a natural environment for curing for 28 days. After curing, the specimen was completed.

[0052] Specifically, the operation of step 2 includes: first, pasting the strain gauge on the threaded steel bar, and then applying a small amount of silicone rubber on the surface of the strain gauge. After air drying for two hours, a protective film can be formed on the surface of the strain gauge to reduce the influence of external factors on the strain gauge. The strain gauge is welded to the prepared wire using a welding gun, and finally, gauze is dipped in an appropriate amount of epoxy resin to wrap the pasted strain gauge. The threaded steel bars 8 at both ends are tensioned at the same time. First, the two threaded steel bars 8 are tensioned to 20% of their controlled stress and held for 2 minutes. Then, the two are tensioned to 50% of their controlled stress and held for 2 minutes. In the same way, they are tensioned to 80% and 110% of their controlled stress and held for 2 minutes. The through-type jack 6 and the loading frame 4 are removed, and the tensioning of the specimen is finally completed. After the tensioning is completed, the excess threaded steel bars 8 are cut off and the prestressing changes of the threaded steel bars are monitored in real time.

[0053] In step 3, the specimens are placed in an environmental chamber for a freeze-thaw cycle test, and the specimens are evenly spread under the environmental chamber nozzle to ensure that each surface is evenly frozen. The number of freeze-thaw cycles is: 0 times, 100 times, 200 times, and 250 times; heating and cooling mechanism: A freeze-thaw cycle is divided into a cooling stage, a low-temperature constant temperature section, a heating section, a high-temperature constant temperature section, and a spraying section. The duration of each stage is: 2h, 2h, 0.5h, 1.5h, and 5min, for a total of 6 hours and 5 minutes. The low-temperature constant temperature section is set to -20℃ to ensure that the specimen can complete the freezing process at this temperature. The high-temperature constant temperature section is set to 20℃. After the specimen is melted, the environmental chamber system automatically sprays to replenish water to the specimen.

[0054] At 100, 200, and 250 freeze-thaw cycles, the heating and cooling system was turned off, and the two first actuators 505 were driven in reverse to unload the two threaded steel bars. To protect the specimen from damage during the unloading process, a wooden board of a certain height was laid underneath.

[0055] In step 4, the specimens are cut. To eliminate the effects of complex stresses at the ends, each concrete cylindrical specimen 7 is cut off at a certain length along its length, forming several cylindrical specimens. Drill bits with diameters of 1 / 5, 2D1 / 5, 3D1 / 5, or 4D1 / 5 are used to core the specimens. The core size lines are drawn, and the specimens are placed on the coring machine table and secured with a clamp. The cooling water flow rate is controlled at 3-5 L / min, and drilling is complete.

[0056] In step 5, if Figure 17 、 Figure 20 As shown, the cut test specimen is placed on a loading platform 13. Resistive strain gauges 14 are attached to the test specimen 16 (displacement gauge holders 11 are fixed to the outer wall of the test specimen 16 via anchor rods 10) at symmetrical locations on both sides of the surface. Displacement gauges 12 are secured to the self-made displacement gauge holders 11 and connected to the holders 11 via fixing plates 15. The strain gauge is then turned on, and the corresponding force and displacement acquisition channels are selected. The acquisition channels for the displacement gauges 12 and the resistive strain gauges 14 are set and reset to capture the stress-strain curve of the concrete. Before formal loading, a preload test is performed at a preload rate of 0.5 mm / min. When the load reaches approximately 5 kN, the two displacement gauges 12 on the acquisition instrument are checked for close proximity to the strain readings. If the relative error between the two readings exceeds a threshold (the threshold is 10%), this indicates that the specimen is not centered and bias is present. Adjustments should be made to the specimen until the data error is within a reasonable range. After the preload process is complete, the data is reset to zero.

[0057] This test used a constant displacement loading method with a loading rate of 0.1 mm / min, a displacement increment of 0.1 mm, and an unloading rate of 0.1 mm / min. The displacement meter measured the vertical displacement of the test specimen during loading, and the stress-strain curve of the concrete component was calculated using the compressive stress and compressive strain formulas.

[0058] The calculation formula for the compressive stress of the test specimen is: ; Where: To test the compressive stress of the specimen, F is the load on the test specimen (kN), A is the pressure-bearing area of the test specimen (mm); The calculation formula of the compressive strain of the test specimen is: ; Where: is the longitudinal compressive strain of the uniformly compressed area of the test specimen; Δ L is the arithmetic mean of the deformation of the test specimen (mm); L is the measuring gauge length of the test specimen (mm).

[0059] The stress-strain curve of the test specimen was calculated according to the above formula.

[0060] The computer section instrument loading is set according to the repeated loading system. After the first loading reaches the predetermined displacement value, the specimen is loaded for 5 seconds and then unloaded. The unloading is carried out until the load reaches about 0.1kN and then the load is held for 5 seconds. The loading and unloading cycle is repeated until the load-displacement curve becomes stable or the specimen is damaged. The loading is stopped. The loading system is as follows: Figure 21 shown.

[0061] Example 3: 1. Preparation of concrete cylindrical specimens: The mold shell is constructed from a PVC tube (1) with an outer diameter of 200mm and an inner diameter of 194mm. The PVC tube is cut to a height of 1000mm with an error of ±2mm. Both ends are smoothed with an angle grinder, and a plug (2) is placed at the bottom of the PVC tube. The formwork is tied together in groups of four using wire. Concrete is then prepared according to the mix ratios in Table 1. Concrete is poured into the mold using a pump. Due to the high height of the specimen, layered vibrating is used. First, pour the concrete to one-third of the mold. Vibrate with an insertable vibrator, inserting and withdrawing the concrete rapidly until the surface stops bubbling. Then, pour the concrete to two-thirds of the mold height. This sequence completes the entire specimen pouring. After the specimen is poured, smooth the surface. The mold is removed, and curing is complete. The concrete cylinder specimen is complete.

[0062] Table 1 Concrete mix ratio: ; 2. Use a homemade tensioning device to apply different stress levels to the concrete cylinder specimens: The tensioned concrete cylindrical specimens were tensioned using a self-designed and processed tensioning device. In order to prevent local damage to the specimens during the test, a pad with a thickness of 300 mm was used.

[0063] Specific steps for tensioning prestressed threaded steel bars: During tensioning, the threaded steel bars at both ends are tensioned simultaneously. First, the two threaded steel bars are tensioned to 20% of their controlled stress and held for 2 minutes. Then, the two are tensioned to 50% of their controlled stress and held for 2 minutes. In the same way, the two are tensioned to 80% and 110% of their controlled stress and held for 2 minutes. Remove the through-type jack and the loading frame to complete the tensioning of the specimen. After the tensioning is completed, cut off the excess threaded steel bars and monitor the changes in the prestress of the threaded steel bars in real time. Figure 16 The optimized curve in the figure. The effective strain of the threaded steel bar and the hydraulic gauge reading during loading are shown in Table 2. Table 2 Design strain of threaded steel bars and hydraulic gauge readings: ; 3. Freeze-thaw cycle test The freeze-thaw cycle test was carried out on the specimens by the air freezing and air thawing method. The number of freeze-thaw cycles was 0, 100, 200 and 250 times. The threaded steel bars were unloaded in sequence by fixing one end and unloading the other end. The tensioning was completed. 4. Cut the concrete cylindrical specimen 7 along its length to divide it into 5 equal sections. To eliminate the influence of complex stress at the ends, the three sections in the middle are used as core specimens. Figure 18 After the cutting is completed, the core is taken with a drill bit of φ150mm, φ112mm, and φ63mm as the test specimen 16. The core sample size line is drawn. The core specimen is placed on the coring machine table and fixed with a clamp. The water flow rate for cooling is controlled at 3-5L / min. The drilling is completed (such as Figure 19 ).

[0064] Due to the wall thickness of the diamond drill bit (4mm) and the gap in the equipment, the radius of the cored specimen was measured. x n , define the relative depth λ of the specimen n = x n / x, x Represents the radius of the concrete before cutting. The relative depth λ of the axial compression specimens is shown in Table 5.

[0065] Table 5 Relative depth of axial compression specimens: ; Plotted as stress-strain curve Figure 22 Middle (a) - Figure 22 Middle (q), Figure 23 Middle (a) - Figure 23 Middle (q), Figure 24 Middle (a) - Figure 24 Middle (q) and Figure 25 Middle (a) - Figure 25As shown in (l), there is no experimental data for the 0.4 stress level under 250 freeze-thaw cycles, because the number of freeze-thaw cycles is too high, the greater the stress level, the more serious the damage to the specimen, and the core sampling of the 0.4 stress level specimen failed.

[0066] The above descriptions are merely a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application discloses the preferred embodiments as above, they are not intended to limit the present application. Any technical personnel familiar with the present profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A tensioning device, comprising a pad, a threaded steel bar, and a loading frame. A concrete cylindrical specimen is placed between two symmetrical pads. Two threaded steel bars are provided, symmetrically distributed on both sides of the concrete cylindrical specimen, and both ends of the threaded steel bars pass through the pads. A loading frame is provided on one of the pads, and the threaded steel bars pass through the loading frame. The device is characterized in that: Self-locking components are provided on the back of the loading frame and the other pad to achieve self-locking of the threaded steel bars, and centering components are provided on the front of the two pads to center and reinforce the concrete cylindrical specimen.

2. A tensioning device according to claim 1, characterized in that: Self-locking components include: A base having a through hole at its center and a boss with a hole on its surface; The first gear ring is slidably arranged on the outer side of the boss, and a receiving ring is arranged above the first gear ring, and the receiving ring is connected to the first gear ring through a column; A first driver is provided on the base, and a driving gear is provided on the driving shaft thereof, and the driving gear is engaged with the first gear ring; The guide frame is arranged between the first gear ring and the receiving ring, and the two ends of the guide frame are movably connected to the first gear ring and the receiving ring respectively; A plurality of rotary clamping members are arranged at equal intervals around the through hole of the base. The opposite ends of the plurality of rotary clamping members are hinged to the base, and the opposite ends of the plurality of rotary clamping members face the threaded steel bar.

3. A tensioning device according to claim 2, characterized in that: A cover is also provided on the base, and an opening is provided on the top of the cover for threaded steel bars to pass through. A plurality of equally spaced auxiliary locking members are provided inside the cover along the circumference of the opening, and the top of each auxiliary locking member is hinged to the inner top wall of the cover; An auxiliary braking component is provided between the cover body and the receiving ring, and the auxiliary locking piece is driven by the auxiliary braking component to lock the threaded steel bar.

4. A tensioning device according to claim 3, characterized in that: The auxiliary brake assembly includes a cylinder, the top of the cylinder is connected to the inner top wall of the cover, and a movable ring is provided inside the cylinder, the movable ring is threadedly connected to the cylinder, and the inner wall of the movable ring abuts against the outer wall of the auxiliary locking member; A fixed column is arranged on the receiving ring, and the movable ring is movably sleeved on the fixed column.

5. The tensioning device according to claim 1, characterized in that: An annular groove is provided on the pad, a closed-loop sunken area and a breakpoint ring sunken area are provided on the bottom wall of the annular groove, and a plurality of equally spaced sunken holes are provided at the bottom of the breakpoint ring sunken area, a sunken cavity is provided below each sunken hole, and both ends of the sunken cavity are respectively connected to the sunken hole and the closed-loop sunken area; The centering assembly includes multiple centering arc plates, which are evenly spaced along the axial direction of the threaded steel bar. A movable plate is provided on the outer wall of each centering arc plate, and the movable plate realizes circumferential movement or radial movement through the centering drive assembly.

6. A tensioning device according to claim 5, characterized in that: The centering drive assembly includes: a second driver, which is arranged in the sunken area of the breakpoint ring, and a first section of rope is wound around a driving shaft of the second driver; Inverted L-shaped shaft, each centering arc plate corresponds to an inverted L-shaped shaft, the horizontal end of each inverted L-shaped shaft is set in the moving groove on the side wall of the moving plate, and the vertical end of each inverted L-shaped shaft contacts the bottom wall of the sunken area of the breakpoint ring, and a guide tooth is set on the vertical arm of the inverted L-shaped shaft, which meshes with the rack on the moving plate; A connecting ring is sleeved on the vertical arm of each inverted L-shaped shaft. Adjacent connecting rings are connected by a second section of rope. The connecting ring close to the second drive is connected to the first section of rope.

7. A tensioning device according to claim 6, characterized in that: The centering drive assembly also includes a second gear ring, which is arranged in the closed-loop sunken area. A third driver is arranged in the closed-loop sunken area. A brake tooth is arranged on the drive shaft of the third driver. The brake tooth is engaged with the second gear ring. A plurality of movable columns are arranged below the second gear ring. A limit plate is arranged at the bottom of each movable column. The limit plate is located in the sunken cavity. A movable seat is provided in the sunken cavity, and a first special-shaped groove and a second special-shaped groove are provided at both ends of the top of the movable seat respectively, and the limiting plate is located in the second special-shaped groove; A starting head is provided in each sunken hole, and a guide plate of the starting head is located in the sunken cavity and in the first special-shaped groove; A first extrusion component and a second extrusion component are respectively provided on the rod body and the moving column of the starting head.

8. A method for testing the dynamic mechanical properties of pressure concrete taking into account uneven freeze-thaw damage, characterized in that: The following steps are involved: Step 1: Prepare a concrete cylindrical specimen; Step 2: applying different stress levels to the concrete cylindrical specimen using the tensioning device according to any one of claims 1 to 7; Step 3: The concrete cylinder specimen treated in step 2 is placed in an environmental chamber for a freeze-thaw cycle test, and then the concrete cylinder specimen is stretched; Step 4: Cut and core the concrete cylindrical specimen after being stretched in step 3, and record it as the test specimen; Step 5: Repeat the compression test on the test specimen obtained in step 4 to obtain a stress-strain curve.

9. A method for testing dynamic mechanical properties of pressure-bearing concrete considering freeze-thaw uneven damage according to claim 8, characterized in that: In step 2, the specific process of applying different stress levels to the concrete cylinder specimen includes: Before the tensile test begins, first stick the strain gauge on the threaded steel bar. After sticking, apply silicone rubber on the surface of the strain gauge, connect the strain gauge to the prepared wire, and finally apply an appropriate amount of epoxy resin to wrap the sticked strain gauge. Then, the two threaded steel bars are tensioned simultaneously until they reach 20% of the controlled stress and the load is maintained for 2 minutes. Then, the two threaded steel bars were tensioned to 50% of the controlled stress and held for 2 minutes; Continue to tension the two threaded steel bars to 80% of the controlled stress and hold the load for 2 minutes; Continue to tension the two threaded steel bars to 110% of the controlled stress and hold the load for 2 minutes; Remove the through-type jack and loading frame. After tensioning is completed, cut off the excess threaded steel bars and monitor the changes in the prestress of the threaded steel bars in real time.

10. The method for testing dynamic mechanical properties of pressure concrete considering freeze-thaw uneven damage according to claim 8, characterized in that: In step 3, the specific process of conducting freeze-thaw cycle test on concrete cylinder specimen is as follows: The concrete cylinder specimens together with the self-locking components, centering components, pads and threaded steel bars were laid under the sprinkler in the environmental chamber, and the freeze-thaw cycles were set to 0, 100, 200 and 250 times respectively; Each freeze-thaw cycle is divided into a cooling section, a low-temperature constant temperature section, a heating section, a high-temperature constant temperature section, and a spraying section. The durations of the cooling section, the low-temperature constant temperature section, the heating section, the high-temperature constant temperature section, and the spraying section are 2 hours, 2 hours, 0.5 hours, 1.5 hours, and 5 minutes, respectively. The temperature of the low-temperature constant temperature section is set at -20°C, and the temperature of the high-temperature constant temperature section is set at 20°C. When the concrete cylindrical specimen reaches a predetermined number of freeze-thaw cycles, the two first drivers 505 are driven in sequence to rotate in opposite directions to unload the two threaded steel bars.

11. The method for testing dynamic mechanical properties of confined concrete considering freeze-thaw uneven damage according to claim 8, characterized in that: In step 5, the test specimen is placed on the loading platform, the resistance strain gauges are attached to the symmetrical positions between the two sides of the test specimen surface, and the displacement meter is fixed; Turn on the strain gauge, select the corresponding force and displacement acquisition channels, set and clear the displacement meter and strain gauge acquisition channels, and capture the local stress-strain curve of the test specimen; Before formal loading, perform a preload test at a preload rate of 0.5 mm / min. When the load reaches 5 kN, check whether the displacement meter value is close to the strain reading on the strain acquisition instrument. If the difference between the two values is greater than the threshold, it means that the test specimen is not centered and bias occurs. Adjust the position of the test specimen and reset the data of the strain acquisition instrument and displacement meter. During the loading phase, the loading speed was 0.1 mm / min, the displacement increment was 0.1 mm, and the unloading rate was 0.1 mm / min. The vertical displacement of the test specimen during loading was measured by a displacement meter, and the stress-strain curve of the test specimen was calculated. Repeated loading is performed according to the loading system. After the first loading reaches the predetermined displacement value, the test specimen is loaded for 5 seconds and then unloaded until the load reaches 0.1 kN and then loaded for another 5 seconds. The loading and unloading cycle is repeated until the load-displacement curve stabilizes or the specimen is damaged, and then loading is stopped.

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