Test device and evaluation method for durability of concrete under coupling action of constraint stress and freeze-thaw cycle

Abstract

The invention discloses a testing device and an evaluation method for the durability of concrete under the condition of coupling action of constraint stress and freeze-thaw cycle, and the device comprises a concrete load applying device, a freeze-thaw testing machine and a constraint clamp containing a concrete test piece, the concrete load applying device comprises a concrete axial load applying device and a concrete lateral load applying device which have the same structure and are mutually inserted to form a nested structure; an acoustic emission probe electrically connected with the computer is arranged in the constraint clamp; lateral and axial loads are applied to the constraint clamp through the concrete load applying device, the loads are accurately measured through the pressure sensor, the constraint clamp after the loads are applied is frozen or melted through the freeze-thaw testing machine, and the acoustic emission probe monitors the internal damage development condition of concrete under the load and freeze-thaw coupling effect. According to the invention, the actual working state of concrete can be simulated, and the internal damage development condition of the concrete under the load and freeze-thaw coupling effect can be monitored.

Description

Technical Field

[0001] The invention belongs to the technical field of concrete performance measurement, and particularly relates to a test device and an evaluation method for concrete durability under the conditions of coupled action of restraint stress and freeze-thaw cycles. Background Art

[0002] In northern regions, due to large temperature fluctuations, structures such as concrete dams face various challenges during their service life, such as continuous loads and freeze-thaw cycles. These problems seriously affect the durability of concrete. In particular, when the water level of the dam fluctuates, the upstream dam surface is prone to freeze-thaw damage. At the same time, axial stress will be generated inside the concrete due to the deadweight of the upper concrete, and the circumferential direction will be affected by the thermal expansion and contraction of the surrounding concrete and water pressure. During the discharge of water from the overflow dam surface, the concrete will also bear stress in the axial direction caused by the gravity of the water, and continue to be affected by stress in the circumferential direction caused by the thermal expansion and contraction of the surrounding concrete. Faced with the above situation, how to effectively test and evaluate the service performance and service life of concrete under actual service conditions has become one of the key tasks that urgently need to be broken through in the field of concrete durability.

[0003] Over the past two decades, a new trend in concrete durability research, both domestically and internationally, has been to consider concrete's actual service conditions and gradually develop more in-depth and effective studies of freeze-thaw damage behavior and mechanisms. On the one hand, research has shifted from focusing solely on freeze-thaw to coupling freeze-thaw with multiple environmental factors, or even freeze-thaw with load. On the other hand, a focus has been placed on the differences between the restraint state of concrete specimens in freeze-thaw tests and that of actual service concrete, striving to obtain freeze-thaw behavior that better reflects actual service conditions. In traditional freeze-thaw tests, concrete specimens are typically in a free-deformation state. In actual service, however, a single unit in a concrete component is not only subject to freeze-thaw forces caused by alternating positive and negative temperatures but, more importantly, to the restraint of the surrounding concrete. Currently, research on the coupled freeze-thaw and load conditions of concrete is limited by the availability of apparatus specifically designed for freeze-thaw testing under restraint. Furthermore, most existing apparatuses only consider the axial loads experienced by concrete during actual service, failing to fully account for the lateral restraints experienced in actual applications and the internal damage development of concrete under load and freeze-thaw coupling. Summary of the Invention

[0004] The purpose of the present invention is to provide a concrete durability test device and evaluation method under the conditions of restraint stress and freeze-thaw coupling, which can simulate the actual working state of concrete and monitor the internal damage development of concrete under the action of load and freeze-thaw coupling.

[0005] To achieve the above-mentioned object, the present invention provides the following technical solution: a test device for the durability of concrete under the coupled action of restraint stress and freeze-thaw cycles, comprising a concrete load applying device, a freeze-thaw testing machine, and a restraining fixture containing a concrete specimen, wherein the concrete load applying device comprises a concrete axial load applying device and a concrete lateral load applying device, which are identical in structure and interlaced with each other to form a nested structure;

[0006] The concrete lateral load applying device and the concrete axial load applying device both include support frames, each of which is connected to a pressure sensor, which is electrically connected to a digital display. The two support frames are interspersed with each other to form a nested structure, and a constraint fixture connected to the pressure sensors of the concrete lateral load applying device and the concrete axial load applying device, respectively, is provided in one of the support frames. An acoustic emission probe electrically connected to a computer is provided in the constraint fixture. Lateral and axial loads are applied to the constraint fixture by the concrete load applying device, and the magnitude of the load is accurately measured by the pressure sensor. After the load is applied, the constraint fixture is frozen or thawed by a freeze-thaw testing machine, and the acoustic emission probe monitors the internal damage development of the concrete under the coupling of load and freeze-thaw.

[0007] Furthermore, the constraint fixture includes a lateral constraint steel mold and an axial constraint steel mold, wherein the axial constraint steel mold includes a No. 1 steel mold and a No. 2 steel mold which are arranged up and down and are rectangular, and the lateral constraint steel mold includes a No. 3 steel membrane and a No. 4 steel membrane which are arranged on the left and right sides of the lateral constraint steel mold and are circular, and an acoustic emission probe electrically connected to a computer is arranged in the No. 3 steel membrane and the No. 4 steel membrane; the No. 1 steel mold and the No. 2 steel mold, as well as the No. 3 steel membrane and the No. 4 steel membrane, are fixedly connected by through bolts and nuts.

[0008] Furthermore, a 3mm gap is reserved between the No. 1 steel mold and the No. 2 steel mold.

[0009] Furthermore, the support frame includes a top plate and a bottom plate, two groups of spiral columns are connected between the top plate and the bottom plate and fixed by nuts, and the constraint clamp is connected to the top plate and the bottom plate by bolts between the two groups of spiral columns.

[0010] The evaluation method of the concrete durability test device under the conditions of coupled restraint stress and freeze-thaw cycle comprises the following steps:

[0011] Step 1: Sample preparation

[0012] Prepare and mix concrete according to the test raw materials and mix ratio, put the concrete into the mold for molding and curing to obtain concrete specimens;

[0013] Step 2: Sample loading

[0014] Place the concrete specimen in the restraint fixture. After installing and debugging the restraint fixture, connect the fixture to the concrete load application device. Rotate the nuts of the concrete axial load application device and the concrete lateral load application device to apply axial and lateral loads respectively. The restraint fixture transfers the load to the concrete specimen. When the set load is reached, rotate the nut on the restraint fixture until the digital display reads zero. Then remove the concrete load application device and the restraint fixture.

[0015] Step 3: Freeze-thaw test

[0016] The restraint fixture is placed in a rubber box with the side exposed to the concrete specimen facing downward. A freeze-thaw medium is injected into the rubber box to completely cover the concrete restraint fixture. The rubber box is placed in a freeze-thaw testing machine and a freeze-thaw test is performed according to the set freeze-thaw parameters. When the number of freeze-thaw cycles is reached, the specimen is removed, the mass and dynamic elastic modulus of the specimen are tested, and the mass loss rate and dynamic elastic modulus loss rate of the specimen before and after freeze-thaw are calculated. The test is stopped when the concrete mass loss rate is ≥5% or the dynamic elastic modulus loss rate is ≥40%. The height and diameter of the specimen at different points are measured, and the average of the diameter and height measurements is taken to calculate the frost heave strain. The ring count, amplitude, and energy parameters inside the concrete are measured using an acoustic emission probe to analyze the damage condition of the specimen under the coupling of load and freeze-thaw.

[0017] Step 4: Evaluation of concrete durability

[0018] When the loss rate of concrete dynamic elastic modulus is ≥40% or the mass loss rate is ≥5%, the corresponding number of load and freeze-thaw coupling is the critical value of concrete failure of the concrete test specimen under the coupling of the test load and freeze-thaw parameters.

[0019] Furthermore, the height of the freeze-thaw medium in the rubber box in step 3 is 5-10 mm higher than the restraining fixture.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention can effectively simulate the restraint stress suffered by concrete under actual service conditions through the concrete load application device, and can not only apply lateral loads but also axial loads. The restraint fixture after the load is applied is placed in a freeze-thaw testing machine to simulate the coupling effect of load and freeze-thaw on the concrete. The device and method of the present invention can quickly and effectively simulate the durability of concrete under the coupling effect of load and freeze-thaw cycles, which is similar to the service environment conditions of actual concrete (structures) serving in cold areas. It can more accurately design the durability of the corresponding concrete, and can better grasp the service performance of the concrete under the service conditions, overcoming the shortcomings of the previous concrete antifreeze performance test method that only considers the single effect of freeze-thaw cycles and the application of loads in a single direction.

[0022] The device of the present invention uses an acoustic emission probe to monitor the ringing count, amplitude, and energy parameters of concrete under the action of restraint stress and freeze-thaw coupling. By combining the traditional mass loss rate and dynamic elastic modulus loss rate, it is helpful to understand the evolution law of concrete performance and evaluate the durability of concrete more scientifically and effectively. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a structural schematic diagram of the concrete load applying device of the present invention;

[0024] Figure 2 Schematic diagram of the connection structure of the concrete load lateral application device and the restraint clamp of the present invention;

[0025] Figure 3 Schematic diagram of the connection structure of the concrete load axial application device and the restraint fixture of the present invention;

[0026] Figure 4 is a schematic diagram of the concrete restraint fixture of the present invention;

[0027] Figure 5 is a cross-sectional schematic diagram of the concrete restraint fixture of the present invention;

[0028] Figure 6 Schematic diagram of the freeze-thaw test of the present invention.

[0029] In the figure: 1-constraint fixture; 11-No. 1 steel mold; 12-No. 2 steel mold; 13-No. 3 steel membrane; 14-No. 4 steel membrane; 2-concrete load applying device; 21-concrete axial load applying device; 22-concrete lateral load applying device; 3-support frame; 31-top plate; 32-bottom plate; 33-screw column; 34-nut; 35-bolt; 4-pressure sensor; 5-digital display; 6-concrete specimen; 7-acoustic emission probe; 8-computer; 9-rubber box; 10-freeze-thaw testing machine. DETAILED DESCRIPTION

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] The concrete durability test device under the conditions of constraint stress and freeze-thaw coupling described in this embodiment includes a concrete load applying device 2, a freeze-thaw testing machine 10, and a constraint fixture 1 containing a concrete specimen 6, such as Figure 1 As shown, a restraining fixture 1 is provided in the concrete load applying device 2, and the concrete load applying device 2 comprises a concrete axial load applying device and a concrete lateral load applying device of the same structure and interlaced with each other to form a nested structure. Figure 2 、3 As shown, both the concrete lateral load applying device and the concrete axial load applying device include a support frame 3, within which a pressure sensor 4 is connected. The pressure sensor 4 is electrically connected to a digital display 5. The two support frames 3 are interlaced into a nested structure, and within one of the support frames 3 is a restraint fixture 1 connected to the pressure sensors 4 of the concrete lateral load applying device and the concrete axial load applying device, respectively. The support frame 3 includes a top plate 31 and a bottom plate 32. Two sets of spiral columns 33 are connected between the top plate 31 and the bottom plate 32 and fixed by nuts 34. Bolts 35 are used between the two sets of spiral columns 33 to connect the restraint fixture 1 to the top plate 31 and the bottom plate 32. Load is applied to the restraint fixture 1 via the support frame 3, and the two pressure sensors 4 are used to accurately measure the magnitude of the lateral and axial loads, respectively.

[0032] like Figure 4 、 5 As shown, the restraint fixture 1 includes lateral and axial restraint steel molds. The axial restraint steel mold comprises a rectangular No. 1 steel mold 11 and a No. 2 steel mold 12, arranged one above the other. Through-bolts are used to securely connect the No. 1 and No. 2 steel molds 11 and 12. A 3mm gap is provided between the No. 1 and No. 2 steel molds 11 and 12 to ensure a complete fit between the concrete specimen 6 and the restraint fixture 1 during pressurization. The lateral restraint steel mold comprises circular No. 3 and No. 4 steel molds 13 and 14, arranged on either side of the mold. These two molds are also secured together by through-bolts and nuts. Acoustic emission probes 7, electrically connected to a computer 8, are located within the No. 3 and No. 4 steel molds 13 and 14.

[0033] like Figure 6 As shown, when conducting a freeze-thaw experiment, the constraint fixture 1 after the load is applied is completely mounted in a rectangular freeze-resistant rubber box 9. The rubber box 9 is filled with a freeze-thaw medium. The freeze-thaw medium completely covers the concrete constraint fixture 1, and the height of the freeze-thaw medium is 5-10 mm higher than the constraint fixture 1. The rubber box 9 is placed in a freeze-thaw testing machine 10 for freezing or thawing. At the same time, during the test, the ringing count, amplitude and energy parameters inside the concrete are measured by the acoustic emission probe 7 and transmitted to the computer 8 to analyze the damage condition of the concrete specimen 6 under the coupling of load and freeze-thaw.

[0034] The evaluation method of the concrete durability test device under the conditions of restraint stress and freeze-thaw coupling includes the following steps:

[0035] Step 1: Sample preparation

[0036] Mixed concrete was prepared according to the test raw materials and mix ratio. The concrete components were configured in the following mass ratio: P.O42.5 ordinary silica cement: mineral powder: water: medium sand: 5-25 mm crushed stone: superplasticizer = 1:0.30:0.50:1.8:2.8:0.006. The design strength grade was C30. The measured compressive strength was 38.5 MPa, the slump was 160 mm, and the air content was 3.0%. The concrete was placed in a mold for molding and curing to obtain concrete specimen 6.

[0037] Step 2: Sample loading

[0038] Place the concrete specimen 6 in the restraint fixture 1. After installing and debugging the restraint fixture 1, connect the fixture to the concrete load applying device 2. Rotate the nuts 34 of the concrete axial load applying device and the concrete lateral load applying device respectively to apply axial and lateral loads. The restraint fixture 1 transfers the load to the concrete specimen 6. When the set load is reached, rotate the nut on the restraint fixture 1 until the value on the digital display 5 is zero. Then remove the concrete load applying device 2 and take out the restraint fixture 1.

[0039] Step 3: Freeze-thaw test

[0040] The restraint fixture 1 is placed in the rubber box 9 with the side exposed to the concrete specimen 6 facing downward, a freeze-thaw medium is injected into the rubber box 9 to completely cover the concrete restraint fixture 1, and the rubber box 9 is placed in the freeze-thaw testing machine 10, and a freeze-thaw test is performed according to the set freeze-thaw parameters; when the number of freeze-thaw cycles is reached, the specimen is removed, the mass and dynamic elastic modulus of the specimen are tested, and the mass loss rate and dynamic elastic modulus loss rate of the specimen before and after freeze-thaw are calculated; the test is stopped when the concrete mass loss rate is ≥5% or the dynamic elastic modulus loss rate is ≥40%; the frost heave strain is calculated by measuring the height and diameter of different points of the specimen and taking the average of the diameter and height measurements; the ringing count, amplitude, and energy parameters inside the concrete are measured by the acoustic emission probe 7 to analyze the damage condition of the specimen under the coupling of load and freeze-thaw.

[0041] Freeze-thaw parameters are determined according to the actual service conditions of concrete and the provisions of GB / T 50082-2009. The freeze-thaw cycle is a 2-4 hour freeze cycle. During the freeze-thaw process, the minimum temperature at the center of the specimen is controlled at -20°C and the maximum temperature is controlled at 10°C. Therefore, the freeze-thaw cycle in this embodiment is a 2-hour freeze cycle and a 1.5-hour thaw cycle. During the freeze-thaw process, the minimum temperature at the center of the specimen is controlled at -20°C and the maximum temperature is controlled at 10°C.

[0042] Step 4: Evaluation of concrete durability

[0043] When the loss rate of concrete dynamic elastic modulus is ≥40% or the mass loss rate is ≥5%, the corresponding number of load and freeze-thaw coupling is the critical value of concrete failure of the concrete test specimen under the coupling of the test load and freeze-thaw parameters.

[0044] The concrete specimens of this embodiment were subjected to freeze-thaw cycle coupling tests under the condition of applying a load of 20% of their ultimate strength, and the critical value of failure of the prepared concrete was 250 times.

Claims

1. Test device for durability of concrete under the conditions of coupled action of restraint stress and freeze-thaw cycles, characterized in that: The invention comprises a concrete load applying device (2), a freeze-thaw testing machine (10), and a restraining fixture (1) containing a concrete specimen (6); the concrete load applying device (2) comprises a concrete axial load applying device and a concrete lateral load applying device, which have the same structure and are interlaced with each other to form a nested structure; The concrete lateral load applying device and the concrete axial load applying device both comprise a support frame (3), a pressure sensor (4) is connected in each support frame (3), the pressure sensor (4) is electrically connected to a digital display (5), the two support frames (3) are interlaced with each other to form a nested structure, and a constraint fixture (1) is provided in one of the support frames (3) and is connected to the pressure sensors (4) of the concrete lateral load applying device and the concrete axial load applying device respectively, and an acoustic emission probe (7) is provided in the constraint fixture (1) and is electrically connected to a computer (8); lateral and axial loads are applied to the constraint fixture (1) by the concrete load applying device (2), and the magnitude of the load is accurately measured by the pressure sensor (4); after the load is applied, the constraint fixture (1) is frozen or thawed by a freeze-thaw testing machine (10), and the acoustic emission probe (7) monitors the internal damage development of the concrete under the coupling action of load and freeze-thaw.

2. The test device for concrete durability under the coupling conditions of restraint stress and freeze-thaw cycles according to claim 1 is characterized in that: The restraining fixture (1) includes a lateral restraining steel mold and an axial restraining steel mold, wherein the axial restraining steel mold includes a No. 1 steel mold (11) and a No. 2 steel mold (12) which are arranged on the upper and lower sides and are rectangular, and the lateral restraining steel mold includes a No. 3 steel film (13) and a No. 4 steel film (14) which are arranged on the left and right sides of the lateral restraining steel mold and are circular, and an acoustic emission probe (7) electrically connected to a computer (8) is arranged in the No. 3 steel film (13) and the No. 4 steel film (14); the No. 1 steel mold (11) and the No. 2 steel mold (12) and the No. 3 steel film (13) and the No. 4 steel film (14) are fixedly connected by through bolts and nuts.

3. The test device for concrete durability under the conditions of coupled restraint stress and freeze-thaw cycles according to claim 2, characterized in that: A gap of 3 mm is reserved between the No. 1 steel mold (11) and the No. 2 steel mold (12).

4. The test device for concrete durability under the conditions of coupled restraint stress and freeze-thaw cycles according to any one of claims 1 to 3, characterized in that: The support frame (3) comprises a top plate (31) and a bottom plate (32), two groups of spiral columns (33) are connected between the top plate (31) and the bottom plate (32) and fixed by nuts (34), and the constraint clamp (1) is connected to the top plate (31) and the bottom plate (32) by bolts (35) between the two groups of spiral columns (33).

5. A method for evaluating the durability of concrete under the conditions of coupled restraint stress and freeze-thaw cycles as claimed in claim 4, characterized in that: The following steps are involved: Step 1: Sample preparation Prepare and mix concrete according to the test raw materials and mix ratio, put the concrete into a mold, shape it, and cure it to obtain a concrete specimen (6); Step 2: Sample loading Place the concrete specimen (6) in the restraint fixture (1), install and debug the restraint fixture (1), and then connect the fixture to the concrete load applying device (2); rotate the nuts (34) of the concrete axial load applying device and the concrete lateral load applying device respectively to apply axial and lateral loads, and the restraint fixture (1) transfers the load to the concrete specimen (6); when the set load is reached, rotate the nut on the restraint fixture (1) until the value of the digital display 5 is zero, then remove the concrete load applying device (2) and take out the restraint fixture (1); Step 3: Freeze-thaw test The restraining fixture (1) is placed in a rubber box (9) with the side exposed to the concrete specimen (6) facing downward, a freeze-thaw medium is injected into the rubber box (9) to completely cover the concrete restraining fixture (1), and the rubber box (9) is placed in a freeze-thaw testing machine (10), and a freeze-thaw test is performed according to the set freeze-thaw parameters; when the number of freeze-thaw cycles is reached, the specimen is taken out, the mass and dynamic elastic modulus of the specimen are tested, and the mass loss rate and dynamic elastic modulus loss rate of the specimen before and after freeze-thaw are calculated; when the concrete mass loss rate is ≥5% or the dynamic elastic modulus loss rate is ≥40%, the test is stopped; by measuring the height and diameter of different points of the specimen, the average of the diameter and height measurement values is taken to calculate the frost heave strain; the ring count, amplitude, and energy parameters inside the concrete are measured by an acoustic emission probe (7) to analyze the damage condition of the specimen under the coupling of load and freeze-thaw; Step 4: Evaluation of concrete durability When the loss rate of concrete dynamic elastic modulus is ≥40% or the mass loss rate is ≥5%, the corresponding number of load and freeze-thaw coupling is the critical value of concrete failure of the concrete test specimen under the coupling of the test load and freeze-thaw parameters.

6. The evaluation method of the concrete durability performance test device under the conditions of coupled restraint stress and freeze-thaw cycles according to claim 5 is characterized in that: In step 3, the height of the freeze-thaw medium in the rubber box (9) is 5-10 mm higher than the restraining fixture (1).

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