In-situ detection device and method for shear and tensile strength of new and old concrete joint surface

Abstract

The application belongs to the technical field of building engineering quality detection and structure reinforcement monitoring, and provides a new and old concrete joint surface shear and tensile strength in-situ detection device and method to solve the problems of high cost and inconvenience caused by the need for two completely different devices to obtain shear and tensile indexes.The device comprises a base, a counter-force frame, a universal self-locking holder and a loading unit.The universal self-locking holder is used to clamp a concrete core sample to be measured, and is provided with a vertical connecting port.When the loading unit is installed at a first installation position, the output end of the loading unit is connected with the vertical connecting port of the universal self-locking holder to apply a tensile force perpendicular to the new and old concrete joint surface to the concrete core sample to be measured.When the loading unit is installed at a second installation position, the output end of the loading unit abuts against a lateral pressure bearing surface of the universal self-locking holder to apply a shear force parallel to the new and old concrete joint surface to the concrete core sample to be measured.The device can realize the switching of shear and tensile modes, and reduce the equipment cost and the field operation intensity.

Description

Technical Field

[0001] This invention belongs to the field of building engineering quality testing and structural reinforcement monitoring technology, and particularly relates to an in-situ testing device and method for shear and tensile strength of the interface between new and old concrete. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] In the reinforcement and renovation of existing buildings (such as the enlarged cross-section method and the replacement concrete method), the bond quality between the old and new plain concrete surfaces or the chopped fiber reinforced concrete surfaces is a key factor determining the reinforcement effect. The stress state of plain concrete surfaces or chopped fiber reinforced concrete surfaces typically includes both shear and tension, with shear failure being the most common failure mode.

[0004] Currently, the main methods for testing the strength of the bonding surface are in-situ pull-out tests and laboratory direct shear tests. Laboratory direct shear tests require a core sample to be taken from the core using a core drill and brought back to the laboratory for cutting and processing before testing. Furthermore, vibration and transportation during sampling can easily cause secondary damage (disturbance) to the fragile bonding surface, leading to distorted laboratory test data. While in-situ pull-out tests overcome the shortcomings of laboratory direct shear tests, existing in-situ pull-out testing equipment can only detect tensile strength and cannot directly reflect the shear resistance of the bonding surface. To obtain both shear and tensile strength indicators, two completely different sets of equipment must be purchased, which is costly and inconvenient to carry. Summary of the Invention

[0005] To address the technical problems mentioned above, this invention provides an in-situ testing device and method for the shear and tensile strength of the interface between new and old concrete, which can switch between shear and tensile modes, significantly reducing equipment costs and on-site work intensity.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of the present invention provides an in-situ testing device for the shear and tensile strength of the interface between new and old concrete.

[0007] An in-situ testing device for shear and tensile strength of the interface between new and old concrete includes: a base, a reaction frame, a universal self-locking clamp, and a loading unit; The base is provided with a concrete core sample to be tested, which includes the interface between new and old concrete; the reaction frame is a U-shaped structure, which is set on the base and located outside the concrete core sample to be tested; the universal self-locking clamp is used to clamp the concrete core sample to be tested, and the universal self-locking clamp is provided with a vertical connection port and a lateral bearing surface on the side wall. The reaction frame is provided with a first mounting position; the base is provided with a second mounting position; when the loading unit is installed in the first mounting position, its output end is connected to the vertical connection port of the universal self-locking clamp, and a tensile force perpendicular to the interface between the new and old concrete is applied to the concrete core sample to be tested; when the loading unit is installed in the second mounting position, its output end abuts against the lateral bearing surface of the universal self-locking clamp, and a shear force parallel to the interface between the new and old concrete is applied to the concrete core sample to be tested.

[0008] In one embodiment, the universal self-locking clamp includes a cylindrical shell, shear-resistant force-bearing blocks, and locking bolts. The shear-resistant force-bearing blocks are evenly distributed along the circumference of the cylindrical shell. By tightening the locking bolts, the shear-resistant force-bearing blocks are pushed down the inclined surface of the inner wall of the cylindrical shell and contracted towards the center to achieve self-centering locking of the side wall of the concrete core sample to be tested.

[0009] In one embodiment, the shear-resistant block is a wedge-shaped clamping block.

[0010] As one implementation, the lower edge of the cylindrical shell of the universal self-locking clamp is provided with a depth limiting scale or an adjustable limiting ring to control the depth at which the universal self-locking clamp fits into the concrete core sample to be tested, ensuring that the clamping position is located at a preset distance above the interface between the old and new concrete.

[0011] In one implementation, the first mounting position is located at the center of the crossbeam of the reaction frame.

[0012] In one embodiment, the second mounting position is provided with a loading unit mounting base.

[0013] In one implementation, the loading unit is a hydraulic loading unit.

[0014] As one implementation, the in-situ testing device for shear and tensile strength of the interface between new and old concrete further includes a displacement monitoring component; the displacement monitoring component includes a first displacement sensor and a second displacement sensor, the first displacement sensor being used to collect in real time the displacement of the concrete core sample to be tested relative to the matrix during the tensile testing of the interface between new and old concrete; the second displacement sensor being used to collect in real time the displacement of the concrete core sample to be tested relative to the matrix during the shear testing of the interface between new and old concrete.

[0015] In one implementation, the displacement monitoring component is connected to a processor, which calculates the shear / tensile strength based on the force applied by the loading unit and the displacement of the concrete core sample relative to the matrix during the detection process.

[0016] A second aspect of the present invention provides an in-situ method for detecting the shear and tensile strength of the interface between new and old concrete.

[0017] An in-situ method for testing the shear and tensile strength of the interface between new and old concrete includes: Samples were taken from the interface between the old and new concrete to be tested to obtain concrete core samples. Insert the universal self-locking clamp onto the new concrete portion of the concrete core sample to be tested and lock it in place; Select the mode based on testing requirements: If testing tensile strength, the loading unit is installed in the first mounting position, and its output end is connected to the vertical connection port of the universal self-locking clamp. A tensile force perpendicular to the interface between the new and old concrete is applied to the concrete core sample to be tested until it is damaged. If testing shear strength, the loading unit is installed in the second mounting position, with its output end abutting against the lateral bearing surface of the universal self-locking clamp, and a shear force parallel to the interface between the new and old concrete is applied to the concrete core sample to be tested until it is destroyed. The shear / tensile strength is calculated based on the force applied by the loading unit during the failure process and the displacement of the concrete core sample relative to the matrix.

[0018] The beneficial effects of this invention are: This invention designs a universal self-locking clamp to hold the concrete core sample to be tested. The universal self-locking clamp has a vertical connection port and a lateral bearing surface on its side wall. When the loading unit is installed in the first mounting position on the reaction frame, its output end is connected to the vertical connection port of the universal self-locking clamp, applying a tensile force perpendicular to the interface between the old and new concrete to the concrete core sample. When the loading unit is installed in the second mounting position on the base, its output end abuts against the lateral bearing surface of the universal self-locking clamp, applying a shear force parallel to the interface between the old and new concrete to the concrete core sample. By changing the installation position of the loading unit, the same universal self-locking clamp is used to switch between shear and tensile modes, significantly reducing equipment costs and on-site work intensity.

[0019] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 This is a schematic diagram of the in-situ testing device for shear and tensile strength of the interface between new and old concrete according to an embodiment of the present invention, in tensile testing. Figure 2This is a schematic diagram of the in-situ testing device for shear and tensile strength of the interface between new and old concrete according to an embodiment of the present invention, in the context of shear testing. Figure 3 This is a cross-sectional view of the universal self-locking gripper according to an embodiment of the present invention; Figure 4 This is a top view of the universal self-locking clamp according to an embodiment of the present invention; Figure 5 This is a stress-displacement curve of the shear and tensile strength of the interface between new and old concrete according to an embodiment of the present invention; Figure 6 The in-situ true strength, core-drilled strength, and strength loss rate of the in-situ testing device for shear and tensile strength of the interface between new and old concrete in this embodiment of the invention are measured.

[0022] Among them, 1. reaction frame; 2. universal self-locking clamp; 3. loading unit; 4. base; 5. concrete core sample to be tested; 21. cylindrical shell; 22. shear force-bearing block; 23. locking bolt; 24. threaded hole; 25. flat bearing block. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0026] In this invention, terms such as "upper," "lower," "left," "right," "front," "back," "vertical," "horizontal," "side," and "bottom" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used only to facilitate the description of the structural relationships of the various components or elements of this invention and do not specifically refer to any component or element in this invention. They should not be construed as limiting the invention.

[0027] In this invention, terms such as "fixed connection," "connected," and "linked" should be interpreted broadly, indicating a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can determine the specific meaning of these terms in this invention based on the specific circumstances, and they should not be construed as limitations on the invention.

[0028] The concrete bonding surface of this invention refers to either plain concrete bonding surface or short-cut fiber reinforced concrete bonding surface.

[0029] like Figure 1 and Figure 2 As shown, in this embodiment of the invention, an in-situ testing device for shear and tensile strength of the interface between new and old concrete is provided, comprising: a base 4, a reaction frame 1, a universal self-locking clamp 2, and a loading unit 3; The base 4 has a concrete core sample 5 to be tested, which includes the interface between the old and new concrete. The reaction frame 1 has a U-shaped structure and is set on the base 4 and located outside the concrete core sample 5. The universal self-locking clamp 2 is used to clamp the concrete core sample 5. The universal self-locking clamp 2 has a vertical connection port and a lateral bearing surface on its side wall. The reaction frame 1 has a first mounting position. The base 4 has a second mounting position. When the loading unit 3 is installed in the first mounting position, its output end is connected to the vertical connection port of the universal self-locking clamp 2 and applies a tensile force perpendicular to the interface between the old and new concrete to the concrete core sample 5. When the loading unit 3 is installed in the second mounting position, its output end abuts against the lateral bearing surface of the universal self-locking clamp 2 and applies a shear force parallel to the interface between the old and new concrete to the concrete core sample 5.

[0030] This invention enables the loading unit to quickly switch between vertical and horizontal modes, integrates in-situ detection of shear and tensile strength, and utilizes the same universal self-locking clamp and loading unit to achieve switching between shear and tensile modes, significantly reducing equipment costs and on-site work intensity.

[0031] like Figure 3 and Figure 4 As shown, in this embodiment, the universal self-locking clamp 2 includes a cylindrical outer shell 21, shear-resistant force-bearing blocks 22, and locking bolts 23. The shear-resistant force-bearing blocks 22 are evenly distributed along the circumference of the cylindrical outer shell 21. By tightening the locking bolts 23, the shear-resistant force-bearing blocks 22 are pushed down the inclined surface of the inner wall of the cylindrical outer shell 21 and contract towards the center, thereby achieving self-centering locking of the sidewall of the concrete core sample 5 to be tested. In this embodiment, the shear-resistant force-bearing block 22 is a wedge-shaped clamping block.

[0032] The universal self-locking clamp 2 of this invention is designed with an internal expansion structure, which can be placed into the annular space left by drilling. The top of the universal self-locking clamp 2 is provided with a threaded hole 24 for connecting a pull rod, and the side wall is provided with a flat pressure block 25 for receiving horizontal thrust.

[0033] like Figure 3 As shown, the universal self-locking clamp 2 has three wedge-shaped blocks distributed at 120° inside. When the top locking bolt 23 is tightened downwards, the wedge-shaped blocks slide down the inclined plane and retract inwards, tightly engaging the core sample surface. One side of the outer wall of the universal self-locking clamp 2 is milled with a flat surface as a stress point to resist shear thrust.

[0034] In other embodiments, the lower edge of the cylindrical outer shell 21 of the universal self-locking clamp 2 is provided with a depth limiting scale or an adjustable limiting ring to control the depth at which the universal self-locking clamp 2 fits into the concrete core sample 5 to be tested, ensuring that the clamping position is located at a preset distance above the interface between the new and old concrete.

[0035] The universal self-locking clamp in this embodiment has a self-locking and anti-slip function. It adopts a wedge-shaped clamping block structure with an internal 120° distribution. According to the principle of inclined plane friction, the greater the applied vertical tensile force, the stronger the tendency of the wedge-shaped clamping blocks to retract downward and inward along the inclined plane of the inner wall, and the greater the radial clamping force (normal pressure) generated. This realizes that the clamping force increases adaptively with the load, completely solving the problem of slippage when testing high-strength concrete surfaces that are smooth and prone to slippage during high-limit load testing. The circumferentially distributed wedge-shaped clamping blocks retract synchronously under the push of the locking bolt 23, which can automatically find the geometric center of the core sample, ensuring that the pull-out force is absolutely perpendicular to the mating surface, avoiding the eccentric tension phenomenon that is very easy to occur in traditional manual pasting and pulling of blocks (eccentricity will lead to lower test strength).

[0036] This embodiment of the universal self-locking clamp not only features a threaded hole at the top for connecting the pull rod to resist tension, but also has a specially milled flat bearing block on the side wall for shear resistance. This dual-purpose structural design is the direct basis for achieving multi-mode switching of a single device. This embodiment of the invention uses a wedge-shaped locking block structure of the universal self-locking clamp to achieve self-locking, replacing the traditional epoxy resin bonding method. This eliminates the waiting time for adhesive curing, and the clamping force increases with the load, solving the problem of easy slippage of the concrete core sample being tested. Traditional methods use high-strength structural adhesives such as epoxy resin to bond the pull block, requiring 24-48 hours for the adhesive to fully cure before testing, and are greatly affected by temperature and humidity. This embodiment of the universal self-locking clamp uses mechanical anchoring, allowing for immediate installation and testing, reducing test preparation time from days to minutes. Simultaneously, its internal expansion structure is directly placed into the annular groove left by the hollow drill bit, without additional damage to the surrounding concrete, perfectly fitting in-situ micro-destructive testing.

[0037] In practice, the first mounting position is located at the center of the crossbeam of the reaction frame 1. The second mounting position is equipped with a mounting base for the loading unit 3.

[0038] In this embodiment, the loading unit 3 is a hydraulic loading unit. The hydraulic loading unit includes a miniature hydraulic jack and a manual / electric pump station. The mounting base of the loading unit 3 is provided with a guide hole that allows the piston rod of the hydraulic loading unit to pass horizontally.

[0039] In other alternative embodiments, the loading unit 3 may also employ a servo motor drive mechanism, which will not be described in detail here.

[0040] In one or more embodiments, the in-situ testing device for shear and tensile strength of the interface between new and old concrete further includes a displacement monitoring component; the displacement monitoring component includes a first displacement sensor and a second displacement sensor, the first displacement sensor being used to collect in real time the displacement of the concrete core sample 5 to be tested relative to the matrix during the tensile testing of the interface between new and old concrete; the second displacement sensor being used to collect in real time the displacement of the concrete core sample 5 to be tested relative to the matrix during the shear testing of the interface between new and old concrete.

[0041] The displacement monitoring component is connected to the processor, which is used to calculate the shear / tensile strength based on the force applied by the loading unit 3 and the displacement of the concrete core sample 5 relative to the matrix during the detection process.

[0042] In conjunction with a high-precision displacement sensor, this invention can plot a complete stress-displacement curve. Figure 5 (a) in the figure is a tensile stress versus displacement curve of the interface between old and new concrete in an embodiment of the present invention. Figure 5 (b) is a shear stress and displacement curve of the interface between new and old concrete in an embodiment of the present invention.

[0043] In one or more embodiments, a method for in-situ testing of the shear and tensile strength of the interface between new and old concrete includes: Step 1: Take samples at the interface between the old and new concrete to be tested to obtain 5 concrete core samples; For example, a hollow drill bit is used to drill an annular groove at the interface between new and old concrete to be tested, forming a cylindrical core sample with the root intact, and the drilling depth penetrates the interface. In this way, only the annular groove needs to be drilled out without breaking and removing the core sample, and shear loading can be performed directly in situ, avoiding damage to the interface caused by sampling disturbance, realizing in-situ non-destructive / minimally destructive testing, and the data is more accurate and reliable.

[0044] Figure 6 This invention relates to an in-situ testing device for the shear and tensile strength of the interface between new and old concrete, based on the actual strength, core-drilled strength, and strength loss rate of the device. Figure 6As can be seen, the in-situ testing device for shear and tensile strength of the interface between new and old concrete in this embodiment of the invention only requires drilling out an annular groove without breaking and removing the core sample, and directly performs shear loading in situ, avoiding damage to the interface caused by sampling disturbance, realizing in-situ non-destructive / micro-destructive testing, and the data is more authentic and reliable.

[0045] Step 2: Insert the universal self-locking clamp 2 into the new concrete part of the concrete core sample 5 to be tested, and lock it in place; Step 3: Select the mode according to the testing requirements: If testing tensile strength, load unit 3 is installed in the first installation position, and its output end is connected to the vertical connection port of universal self-locking clamp 2. A tensile force perpendicular to the interface between the new and old concrete is applied to the concrete core sample 5 to be tested until it is destroyed. If testing shear strength, load unit 3 is installed in the second installation position, and its output end abuts against the lateral bearing surface of universal self-locking clamp 2. A shear force parallel to the interface between the new and old concrete is applied to the concrete core sample 5 to be tested until it is destroyed. Step 4: Calculate the shear / tensile strength based on the force applied by the loading unit 3 during the failure process and the displacement of the concrete core sample 5 relative to the matrix.

[0046] Specifically, the shear strength test process is as follows: Drilling: Use a diamond thin-walled drill bit with an inner diameter of 50mm or 75mm to drill vertically into the concrete surface, penetrating to a depth of about 20mm through the interface between the old and new concrete.

[0047] Installation: Clean the dust from the groove and place the universal self-locking clamp 2 into the annular groove. Adjust the height so that the bottom surface of the universal self-locking clamp 2 is about 5mm higher than the mating surface (to avoid shearing old concrete), and tighten the bolts to lock the universal self-locking clamp 2.

[0048] Assembly: Cover the reaction frame 1 externally, and horizontally insert the loading unit 3 into the mounting hole on the side of the base 4, aligning its top rod with the lateral bearing surface of the universal self-locking clamp 2. Install a displacement sensor to monitor the horizontal displacement of the core sample.

[0049] Loading: Start the hydraulic pump and apply a horizontal thrust at a rate of 0.05 MPa / s until shear failure occurs at the joint surface (core sample misalignment or sudden drop in force).

[0050] Calculation: Record the maximum destructive force Shear strength: ; The concrete core sample 5 to be tested is a cylinder. Let be the radius of the cylinder.

[0051] Tensile strength test method: The same drilling and installation steps as described above are used.

[0052] Move loading unit 3 onto the top crossbeam of reaction frame 1.

[0053] One end of a high-strength tie rod is connected to the piston of a hydraulic cylinder, and the other end is screwed into the threaded hole at the top of the universal self-locking clamp 2.

[0054] Apply a vertically upward tensile force until the joint breaks, and record the maximum tensile force. And calculate the tensile strength The concrete core sample 5 to be tested is a cylinder. Let be the radius of the cylinder.

[0055] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An in-situ testing device for the shear and tensile strength of the interface between new and old concrete, characterized in that, include: Base, reaction frame, universal self-locking clamp and loading unit; The base is provided with a concrete core sample to be tested, which includes the interface between new and old concrete; the reaction frame is a U-shaped structure, which is set on the base and located outside the concrete core sample to be tested; the universal self-locking clamp is used to clamp the concrete core sample to be tested, and the universal self-locking clamp is provided with a vertical connection port and a lateral bearing surface on the side wall. The reaction frame is provided with a first mounting position; the base is provided with a second mounting position; when the loading unit is installed in the first mounting position, its output end is connected to the vertical connection port of the universal self-locking clamp, and a tensile force perpendicular to the interface between the new and old concrete is applied to the concrete core sample to be tested; when the loading unit is installed in the second mounting position, its output end abuts against the lateral bearing surface of the universal self-locking clamp, and a shear force parallel to the interface between the new and old concrete is applied to the concrete core sample to be tested.

2. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 1, characterized in that, The universal self-locking clamp includes a cylindrical shell, shear-resistant force-bearing blocks, and locking bolts. The shear-resistant force-bearing blocks are evenly distributed along the circumference of the cylindrical shell. By tightening the locking bolts, the shear-resistant force-bearing blocks are pushed down the inclined surface of the inner wall of the cylindrical shell and contracted towards the center to achieve self-centering locking of the side wall of the concrete core sample to be tested.

3. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 2, characterized in that, The shear-resistant block is a wedge-shaped clamping block.

4. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 1, characterized in that, The lower edge of the cylindrical shell of the universal self-locking clamp is provided with a depth limiting scale or an adjustable limiting ring, which is used to control the depth at which the universal self-locking clamp fits into the concrete core sample to be tested, ensuring that the clamping position is located at a preset distance above the interface between the old and new concrete.

5. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 1, characterized in that, The first mounting position is located at the center of the crossbeam of the reaction frame.

6. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 1, characterized in that, The second mounting position is provided with a loading unit mounting base.

7. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 1, characterized in that, The loading unit is a hydraulic loading unit.

8. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 1, characterized in that, The in-situ testing device for shear and tensile strength of the interface between new and old concrete also includes a displacement monitoring component; the displacement monitoring component includes a first displacement sensor and a second displacement sensor, the first displacement sensor being used to collect in real time the displacement of the concrete core sample to be tested relative to the matrix during the tensile testing of the interface between new and old concrete; the second displacement sensor being used to collect in real time the displacement of the concrete core sample to be tested relative to the matrix during the shear testing of the interface between new and old concrete.

9. The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in claim 8, characterized in that, The displacement monitoring component is connected to the processor, which is used to calculate the shear / tensile strength based on the force applied by the loading unit and the displacement of the concrete core sample relative to the matrix during the detection process.

10. A method for in-situ testing of shear and tensile strength at the interface between new and old concrete, characterized in that, The in-situ testing device for shear and tensile strength of the interface between new and old concrete as described in any one of claims 1-9 includes: Samples were taken from the interface between the old and new concrete to be tested to obtain concrete core samples. Insert the universal self-locking clamp onto the new concrete portion of the concrete core sample to be tested and lock it in place; Select the mode based on testing requirements: If testing tensile strength, the loading unit is installed in the first mounting position, and its output end is connected to the vertical connection port of the universal self-locking clamp. A tensile force perpendicular to the interface between the new and old concrete is applied to the concrete core sample to be tested until it is damaged. If testing shear strength, the loading unit is installed in the second mounting position, with its output end abutting against the lateral bearing surface of the universal self-locking clamp, and a shear force parallel to the interface between the new and old concrete is applied to the concrete core sample to be tested until it is destroyed. The shear / tensile strength is calculated based on the force applied by the loading unit during the failure process and the displacement of the concrete core sample relative to the matrix.

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