A concrete constant compressive stress fracture testing device and method

Abstract

The application discloses a concrete constant compressive stress fracture testing device and method, which comprises double-layer slidable steel plates, two flexible pads are fixed on both sides of a prefabricated notch of a concrete sample containing the prefabricated notch, and load sensing patches are symmetrically pasted at two ends of the side of the concrete sample where the prefabricated notch is located; a double-layer slidable steel plate is fixedly connected below each flexible pad, the double-layer slidable steel plate controls self-locking and separation between upper and lower layers through an electromagnet, the upper layer is fixed with the lower end of the flexible pad, the lower layer is fixed on a bearing pad, the lower end of the bearing pad is arranged on an experimental platform of a three-point bending fracture experiment of the concrete, and two fixed supports on the experimental platform are located below two load sensing patches respectively. The application solves the problem that the concrete sample is not damaged along the prefabricated notch during the experiment, and realizes cracking of the concrete at the prefabricated notch through the load sensing patches and the double-layer slidable steel plates.

Description

Technical Field

[0001] This invention relates to the field of fracture mechanics testing technology, and in particular to a test device and method for concrete constant compressive stress fracture. Background Technology

[0002] For concrete, crack propagation is closely related to the direction of the load. Taking road concrete as an example, the weight of a vehicle is transmitted through the tires, generating cyclic parallel compressive stress on the crack surface, causing the crack to continuously open and propagate. For structures such as bridges, industrial and civil buildings, reinforced concrete columns or prestressed members often develop longitudinal or diagonal cracks under seismic loading. In this case, the compressive stress parallel to the crack surface can approach the compressive strength of the concrete. Related research and engineering practice show that the influence of parallel crack compressive stress on the fracture performance of concrete cannot be ignored and has become a focus of attention in structural safety assessment.

[0003] Chinese patent CN113092265A discloses a device for biaxial compressive loading of concrete under load-environment coupling, which can provide a stable and effective continuous load. Chinese patent CN211292318U discloses a biaxial compressive loading device for concrete, which can ensure that the applied biaxial load is fully and uniformly applied to the concrete specimen, thus improving the stability of the test. The devices and methods involved in the above patents are mainly applicable to studying the mechanical properties of concrete materials under biaxial stress. However, in the study of concrete fracture characteristics, there is currently a lack of an effective method and supporting device to accurately evaluate the fracture performance of concrete under constant compressive stress constraint.

[0004] Based on the above analysis, it is necessary to propose a concrete fracture testing device and method under constant compressive stress constraint to study the fracture performance of concrete materials under actual stress conditions, and to provide some guidance for scientific research and engineering practice. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a concrete constant compressive stress fracture testing device and method for the fracture failure of concrete under constant compressive stress, so as to effectively study the fracture characteristics of concrete materials under parallel crack compressive stress constraint conditions.

[0006] The technical solution adopted by the present invention to solve the aforementioned technical problem is as follows: In a first aspect, the present invention provides a concrete constant compressive stress fracture testing device, comprising a double-layer sliding steel plate, two flexible pads fixed on both sides of a precast notch in a concrete sample containing a precast notch, and load sensing patches symmetrically attached to both ends on the side of the precast notch in the concrete sample; a double-layer sliding steel plate is fixedly connected below each flexible pad, the double-layer sliding steel plate is controlled by an electromagnet to self-lock and separate between the upper and lower layers, the upper layer of the double-layer sliding steel plate is fixed to the lower end of the flexible pad, and the lower layer of the double-layer sliding steel plate is fixed on a bearing pad, the lower end of the bearing pad is set on the experimental platform of the concrete three-point bending fracture test, and two fixed supports on the experimental platform are respectively located below the two load sensing patches; Initially, the two load sensing patches do not contact the fixed support. After the flexible pad 7 is compressed to the yield stage, the two load sensing patches can contact the fixed support. Two rigid pads are symmetrically placed at the upper end of the concrete specimen along the coaxial position of the precast notch, and the testing machine applies pressure to the rigid pads through the loading head.

[0007] Secondly, the present invention provides a method for testing the constant compressive stress fracture of concrete. The method involves attaching a load-sensing patch to a concrete specimen with a precast notch, and setting up a flexible pad and a double-layered sliding steel plate to ensure that crack failure develops along the precast notch. Using the aforementioned device, the method includes the following steps: Step 1: Install the fixed support, pressure pad, double-layer sliding steel plate, flexible pad and rigid pad in sequence. At the same time, fix an industrial camera directly in front of the precast notch of the concrete sample to record the entire process of crack initiation, steady-state propagation and instability penetration in the mid-span area when the external load is applied. Step 2: Start the testing machine. The loading head compresses the flexible pad at a pre-set loading rate. When the flexible pad is compressed to the yield stage, the strain continues to increase but the stress remains basically unchanged. The two sides of the precast notch of the concrete sample will be subject to a constant constraint parallel to the notch direction, thus achieving the stable application of constant compressive stress. Step 3: After the concrete specimen comes into contact with the fixed support, the movement of both ends of the specimen in the loading direction is restricted. After the load sensing patch senses the presence of force, it automatically unlocks the double-layer sliding steel plate. As the concrete specimen bends, the upper layer of the double-layer sliding steel plate translates accordingly, causing the crack to expand at the preset position. The test is terminated when the expanding crack reaches the top of the concrete specimen.

[0008] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention can provide a precise compressive stress field, which can effectively solve the bottleneck of existing equipment being unable to study the fracture performance of concrete under constant compressive stress constraints. It provides a stable and feasible test method for evaluating the fracture behavior of concrete and has good prospects for scientific research and engineering application.

[0009] This application proposes a concrete fracture testing device with constant compressive stress constraint to solve the problem that concrete specimens do not break along the precast notch during the experiment. The device achieves concrete cracking at the precast notch by using a load-sensing patch and a double-layer sliding steel plate. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of one embodiment of the concrete constant compressive stress fracture testing device of the present invention; Figure 2 This is a schematic diagram of the uniaxial compressive stress-strain curve of a flexible pad. Figure 3 This is a schematic diagram of the load-deflection curve of a concrete specimen under constant compressive stress constraint. Figure 4 Schematic diagram of fracture failure of a precast concrete beam without double-layered sliding steel plates; Figure 5 This is a schematic diagram illustrating the fracture failure of a concrete three-point bending beam containing a double-layered sliding steel plate according to the present invention. Figure 6 This is a schematic diagram of the structure of one embodiment of the double-layer sliding steel plate of the present invention; In the figure, 1 is the loading head, 2 is the rigid pad, 3 is the concrete sample, 4 is the mid-span area, 5 is the precast notch, 6 is the load sensing patch, 7 is the flexible pad, 8 is the fixed support, 9 is the double-layer sliding steel plate, and 10 is the pressure-bearing pad. Detailed Implementation

[0011] To make the objectives and technical solutions of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments of this invention are not limited thereto; any equivalent modifications and improvements made by those skilled in the art without substantial innovation should be within the protection scope of this invention.

[0012] This invention provides a concrete constant compressive stress fracture testing device. Based on the traditional concrete three-point bending fracture test, the device ensures that crack failure develops along the precast notch by attaching load-sensing patches to concrete specimens with precast notches and by setting flexible pads and double-layer sliding steel plates.

[0013] The specific steps for the testing method are as follows: Step 1: As Figure 1As shown, the fixed support 8, the pressure pad 10, the double-layer sliding steel plate 9, the flexible pad 7, the concrete sample 3 and the rigid pad 2 are installed in the following order from bottom to top. The upper side of the flexible pad 7 is fixed to both sides of the precast notch 5 with strong glue. The lower side of the flexible pad 7 is fixed to the upper layer of the double-layer sliding steel plate 9. The lower layer of the double-layer sliding steel plate 9 is fixed to the pressure pad 10. At the same time, an industrial camera is fixedly installed in front of the precast notch of the concrete sample 3 to record the entire process of crack initiation, steady-state propagation and instability penetration in the mid-span region 4 when the external load is applied. Step 2: Start the testing machine. Loading head 1 will compress the flexible pad 7 at a preset loading rate. After the flexible pad 7 is compressed to the yield stage, the strain continues to increase, but the stress remains basically unchanged (see...). Figure 2 The precast notch 5 of concrete specimen 3 will be subject to constant constraint on both sides parallel to the direction of the precast notch, so as to achieve stable application of constant compressive stress. Step 3: When the concrete specimen 3 comes into contact with the fixed support 8, the movement of both ends of the specimen in the loading direction will be restricted, forming a standard three-point bending form with constant compressive stress constraint. After the load sensing patch 6 synchronously senses the existence of the force, it will automatically unlock the double-layer sliding steel plate 9 connected by the wire. As the external load is further applied, the concrete specimen 3 bends, and the upper layer of the double-layer sliding steel plate 9 moves accordingly, causing the crack to expand at the preset position. The test can be terminated when the crack expands to the top of the concrete specimen 3. This invention is particularly suitable for fracture testing of beam specimens with low strength, enabling them to propagate along precast notches.

[0014] The rigid pad 2 can be made of high-strength steel and is required to have good compressive strength (far exceeding the compressive strength of the concrete sample 3) to ensure that it does not deform during the application of load; The width of the flexible pad 7 (defined) Figure 1 The width of the horizontal axis must be the same as that of the rigid pad 2. The lengths of the rigid pad 2 and the flexible pad 7 (in the horizontal direction) Figure 1 The length of the flexible pad 7 (perpendicular to the plane of the paper) is consistent with that of the concrete sample 3. The flexible pad 7 can be made of materials such as polyethylene or polypropylene, and the compressive stress-strain curve (…) is required to… Figure 2 After reaching the yield stage, a constant compressive stress constraint is generated on the concrete specimen 3 in the direction of load application.

[0015] The compression stress-strain curve of the flexible pad 7 must be obtained using the same loading rate as the fracture test to ensure high consistency with the constant compressive stress constraint condition.

[0016] After the flexible pad 7 is compressed to yield, there is a possibility that the two sides of the concrete sample 3 may not be in contact with the fixed support 8. Since the yield stage of the compressive stress-strain curve has a certain length, the influence caused by this can be ignored.

[0017] The height of the flexible pad 7 (defined) Figure 1 The value of the vertical direction (its height) can be referenced from the compressive stress-strain curve of flexible pad 7. Figure 2 The strain value corresponding to the beginning of the yielding stage is determined, thereby ensuring that the flexible pad 7 can contact the fixed support 8 on both sides of the concrete sample 3 shortly after entering the yielding stage, so as to achieve the loading of the standard three-point bending form.

[0018] The bottom of the flexible pad 7 is provided with a double-layered sliding steel plate 9, which is constructed using a double-layered sliding electromagnet (see [reference]). Figure 6 The lower layer is connected to the load sensing patch 6 via a wire. When the load sensing patch senses the load, the electromagnet is de-energized, and the upper and lower layers unlock directly. During the experimental preparation stage, the flexible pad 7 is fixed to the upper layer of the sliding steel plate, while the lower layer is fixed to the bearing pad 10. Before the load sensing patch 6 senses the load, the two layers of electromagnets are tightly fitted together (i.e., self-locking), which can effectively prevent the concrete sample 3 from moving suddenly in the horizontal direction. After the load sensing patch 6 contacts the fixed support 8, the magnetism of the two layers of sliding electromagnets disappears. The lower layer remains fixed to the bearing pad 10, while the upper layer slides left and right in the horizontal direction as the concrete sample breaks, ensuring that the movement of the flexible pad 7 in the horizontal direction is unrestricted.

[0019] The thickness of the load sensing patch 6 is generally less than 0.1 mm, which will not affect the normal contact between the sample and the fixed support. The double-layer sliding steel plate 9 will only be automatically unlocked after both patches on both sides sense the presence of force. The pressure-bearing pad 10 is made of the same material as the rigid pad 2, and its height can be determined according to the stress-strain curve of the flexible pad 7. Figure 2 The strain value at the beginning of the yield stage is adjusted to ensure that the initial distance between the load sensing patch 6 and the fixed support 8 is the yield deformation value of the flexible pad 7, which is the yield stress × yield strain.

[0020] Two rigid pads, two flexible pads, and a loading head are all arranged coaxially with pre-made notches. In order to facilitate the loading head to apply the same uniform force on the two rigid pads, a rigid transition piece is provided on the two rigid pads. The projected area of ​​the rigid transition piece on the sample covers the area where the two rigid pads are located.

[0021] The invention can also include a controller, which is electrically connected to an industrial camera, an electromagnet with a double-layer sliding steel plate, and a load sensing patch. The controller can adjust the current flowing through the electromagnet.

[0022] Example 1: This embodiment uses a concrete specimen with a precast notch, whose three-dimensional dimensions are 100 mm, 100 mm, and 440 mm, respectively, and the length of the precast notch is 30 mm. The three-point bending fracture test was conducted on a universal testing machine with a range of 300 kN, at a constant loading rate of 0.3 mm / min. The measured load-crack opening displacement curve is shown below. Figure 3 As shown, the first half of the curve corresponds to the flexible pad (a polyethylene flexible pad was used in this experiment) from the beginning of compression to yielding, while the second half is the three-point bending fracture failure process under constant compressive stress constraint.

[0023] To improve the contrast between cracked and uncracked areas and to effectively observe cracks on the surface of concrete samples, a layer of matte paint was pre-applied to the mid-span area (camera observation surface). Figure 4 and Figure 5 Schematic diagrams of concrete surface crack propagation are provided for both cases without the double-layer sliding steel plate and cases including the double-layer sliding steel plate of this invention. Figure 4 It can be seen that cracks occurred along both sides of the prefabricated joint (outside the polyethylene flexible pad) when the sample finally failed. Figure 5 A schematic diagram of the fracture failure obtained using the device of this invention is provided, showing that the concrete sample cracked at the precast notch, achieving the expected result. This invention effectively improves the success rate of fracture tests on concrete with constant compressive stress constraint, and is suitable for studying concrete containing materials such as mortar, coarse aggregate, or fibers in the matrix. It can be widely applied in scientific research and engineering fields.

[0024] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A concrete constant compressive stress fracture testing device, characterized in that, The test includes a double-layered sliding steel plate. Two flexible pads are fixed on both sides of the precast notch of the concrete specimen containing the precast notch, and load-sensing patches are symmetrically attached to both ends of the side where the precast notch of the concrete specimen is located. A double-layered sliding steel plate is fixedly connected below each flexible pad. The double-layered sliding steel plate is controlled by an electromagnet to lock and separate the upper and lower layers. The upper layer of the double-layered sliding steel plate is fixed to the lower end of the flexible pad, and the lower layer of the double-layered sliding steel plate is fixed to a pressure pad. The lower end of the pressure pad is set on the test platform of the three-point bending fracture test of concrete. Two fixed supports on the test platform are located below the two load-sensing patches respectively. Initially, the two load sensing patches do not contact the fixed support. After the flexible pad is compressed to the yield stage, the two load sensing patches can contact the fixed support. Two rigid pads are symmetrically placed at the upper end of the concrete specimen along the coaxial position of the precast notch, and the testing machine applies pressure to the rigid pads through the loading head.

2. The apparatus according to claim 1, characterized in that, The device also includes an industrial camera and a controller. The industrial camera is used to record the entire process of crack initiation, steady-state propagation and instability penetration in the mid-span region when an external load is applied. The controller is electrically connected to an industrial camera, an electromagnet with a double-layered sliding steel plate, and a load-sensing patch.

3. The apparatus according to claim 1, characterized in that, The double-layer sliding steel plate uses a double-layer sliding electromagnet. The lower layer is connected to the load sensing patch via a wire. During the experimental preparation stage, a flexible pad is fixed to the upper layer of the sliding steel plate, while the lower layer is fixed to the bearing pad. Before the load sensing patch senses the load, the upper and lower layers are tightly attached together, effectively preventing sudden movement of the concrete sample in the horizontal direction. After the load sensing patch contacts the fixed support, the magnetism of the double-layer sliding electromagnet disappears. The lower layer remains fixed to the bearing pad, while the upper layer slides left and right in the horizontal direction as the concrete sample breaks, ensuring that the movement of the flexible pad in the horizontal direction is unrestricted.

4. The apparatus according to claim 1, characterized in that, The height of the flexible pad is chosen to ensure that the two sides of the concrete sample can contact the fixed support shortly after the flexible pad enters the yielding stage, thus achieving loading in the form of standard three-point bending. The pressure-bearing pad is made of the same material as the rigid pad. Its height is adjusted according to the strain value at the beginning of the yield stage of the stress-strain curve of the flexible pad to ensure that the initial distance between the load sensing patch and the fixed support is the yield deformation value of the flexible pad, which is the yield stress × yield strain.

5. A method for testing the fracture of concrete under constant compressive stress, characterized in that, The method, using the apparatus described in any one of claims 1-4, includes the following steps: Load-sensing patches are adhered to concrete specimens with precast notches, and flexible pads and double-layered sliding steel plates are provided to ensure that crack failure propagates along the precast notches. Step 1: Install the fixed support, pressure pad, double-layer sliding steel plate, flexible pad and rigid pad in sequence. At the same time, fix an industrial camera directly in front of the precast notch of the concrete sample to record the entire process of crack initiation, steady-state propagation and instability penetration in the mid-span area when the external load is applied. Step 2: Start the testing machine. The loading head will compress the flexible pad at a pre-set loading rate. When the flexible pad is compressed to the yield stage, the strain continues to increase but the stress remains basically unchanged. The two sides of the precast notch of the concrete sample will be subjected to a constant constraint parallel to the notch direction, so as to achieve the stable application of constant compressive stress. Step 3: After the concrete specimen comes into contact with the fixed support, the movement of both ends of the specimen in the loading direction is restricted. After the load sensing patch senses the presence of force, it automatically unlocks the double-layer sliding steel plate. As the concrete specimen bends, the upper layer of the double-layer sliding steel plate translates accordingly, causing the crack to expand at the preset position. The test is terminated when the expanding crack reaches the top of the concrete specimen.

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