Apparatus for determining compressive stress-strain curves of concrete at high temperatures and method of use

Abstract

This invention relates to an apparatus and method for determining the compressive stress-strain curve of concrete under high temperature. It belongs to the field of concrete mechanical property testing technology, specifically relating to an apparatus and method for determining the compressive stress-strain curve of concrete under high temperature. The apparatus includes a heating system, a loading system, and a deformation measurement system. The heating system includes a high-temperature furnace, a support frame, thermocouples, a temperature controller, and wires. The loading system includes a reaction frame, a jack, an oil pump, oil pipes, a force sensor, a first metal indenter, and a second metal indenter. The deformation measurement system includes a first screw, a second screw, a first steel washer, a second steel washer, a third screw, a fourth screw, a first extended nut, a second extended nut, a displacement gauge, and a steel plate. The various systems work together to simultaneously heat and load the concrete specimen, collect the deformation of the concrete specimen under high temperature, and obtain the compressive stress-strain curve. This effectively solves the problem of difficulty in testing the compressive stress-strain curve of concrete under high temperature.

Description

Technical Field

[0001] This invention belongs to the field of concrete mechanical property testing technology, specifically relating to a device for measuring the compressive stress-strain curve of concrete at high temperature and its usage method. Background Technology

[0002] Fire, as an extremely frequent type of disaster, poses a serious threat to human life and property. Once a fire breaks out, the high-temperature environment it generates is a "killer" of concrete structures. Under the scorching heat, concrete structures will suffer severe damage, and in severe cases, they may even collapse, causing irreparable losses.

[0003] Beyond sudden fires, concrete structures operating in specific industrial settings are also subject to the continuous effects of high temperatures. For example, nuclear power plant containment vessels experience consistently high internal temperatures during operation, subjecting their concrete structures to prolonged exposure to heat. Similarly, in smelting plants, high-temperature workshops maintain consistently high temperatures due to production processes, placing their concrete structures under such conditions for extended periods. Chimneys, as conduits for exhaust gases, are constantly subjected to the scouring action of hot gases, keeping their concrete structures at a high temperature. Even the roller conveyors transporting hot steel billets are affected by the significant heat emitted from the billets. Prolonged exposure to high temperatures gradually degrades the mechanical properties of concrete, directly reducing its load-bearing capacity and posing a threat to the safe operation of related facilities. Therefore, a thorough understanding of the mechanical properties of concrete at high temperatures is crucial for ensuring the safety and stability of various concrete structures.

[0004] In numerical analysis of the mechanical properties of concrete structures at high temperatures, the compressive stress-strain curve is an indispensable key element. This curve provides a direct understanding of the strain of concrete under different stress states, thus offering accurate data support for structural safety assessment and design optimization. However, obtaining the compressive stress-strain curve of concrete at high temperatures is no easy task. According to testing requirements, the concrete specimen must be heated while being loaded to simulate the actual pressure exerted on concrete under high-temperature conditions. However, in reality, the limitations of existing testing instruments make this testing work extremely difficult.

[0005] Temperatures generated by fires can reach up to 1000℃, and concrete structures operating in hot environments can experience temperatures up to 350℃. When studying the mechanical properties of concrete at high temperatures, the typical test temperature range is 20℃ to 800℃. At room temperature, displacement gauges or strain gauges are generally used to test concrete deformation. Ordinary displacement gauges can accurately measure the deformation of concrete at room temperature. Typical high-temperature displacement gauges can only withstand temperatures up to 250℃ and are expensive. They cannot be used at higher temperatures. Similarly, typical high-temperature strain gauges can only withstand temperatures up to 250℃; they will fail at higher temperatures. For studying concrete deformation at temperatures above 250℃, ordinary high-temperature displacement gauges and strain gauges are clearly insufficient. Even more challenging is that when the compressive stress-strain curve of concrete enters the descending phase, cracks appear inside the concrete, at which point its deformation increases dramatically. In such cases, strain gauges may detach from the concrete or be damaged due to excessive deformation, making it impossible to accurately measure the concrete strain and hindering research on the mechanical properties of concrete at high temperatures. Therefore, there is an urgent need to develop a new technology for testing concrete deformation at high temperatures.

[0006] In conclusion, to enable simultaneous heating and loading of concrete specimens and accurate testing of their deformation at high temperatures, the development of a novel device and its application method for determining the compressive stress-strain curve of concrete at high temperatures is urgently needed. This will not only contribute to the in-depth research on the mechanical properties of concrete materials under high-temperature environments but also provide a solid theoretical basis and technical support for the design, construction, and maintenance of various concrete structures that may face high-temperature environments. Summary of the Invention

[0007] To address the above-mentioned problems, this invention provides an apparatus and a method for measuring the compressive stress-strain curve of concrete at high temperatures.

[0008] The present invention provides an apparatus for determining the compressive stress-strain curve of concrete under high temperature, comprising a heating system, a loading system, and a deformation measurement system;

[0009] The heating system includes a high-temperature furnace, a support frame, thermocouples, a temperature controller, and wires; the high-temperature furnace is mounted on the support frame, and a first round hole, a second round hole, and a third round hole are pre-drilled in the side wall of the high-temperature furnace; the thermocouple is inserted into the high-temperature furnace through the first round hole; the thermocouple and the temperature controller are connected by wires.

[0010] The loading system includes a reaction frame, jacks, an oil pump, oil pipes, a force sensor, a first metal indenter, and a second metal indenter. The reaction frame includes an upper crossbeam, a lower crossbeam, a left column, a right column, and diagonal braces. The two ends of the upper crossbeam are connected to the upper parts of the left and right columns, respectively, and the two ends of the lower crossbeam are connected to the lower parts of the left and right columns, respectively. The angle between the beam and column is reinforced by diagonal braces. The jacks are suspended on the upper crossbeam and connected to the oil pump via oil pipes. The first metal indenter is placed on the lower crossbeam, and the second metal indenter is positioned above the first metal indenter. The force sensor is positioned at the upper end of the second metal indenter. The concrete specimen is placed between the first and second metal indenters.

[0011] The deformation measurement system includes a first screw, a second screw, a first steel washer, a second steel washer, a third screw, a fourth screw, a first extended nut, a second extended nut, a displacement gauge, and a steel plate. The sidewall of the concrete specimen is machined with a first pre-drilled hole and a second pre-drilled hole. A first steel washer is placed on the surface of the first pre-drilled hole, and a second steel washer is placed on the surface of the second pre-drilled hole. One end of the first screw is embedded in the first pre-drilled hole, and the other end is connected to the third screw via the first extended nut. One end of the second screw is embedded in the second pre-drilled hole, and the other end is connected to the fourth screw via the second extended nut.

[0012] The third screw and the fourth screw are led out of the furnace through the second and third round holes reserved in the side wall of the high-temperature furnace, respectively; the third screw is provided with a vertical section, on which a displacement gauge is fixed; the end of the fourth screw is welded with a steel plate, and the iron core of the displacement gauge is perpendicular to the steel plate.

[0013] The specific steps for using the above-mentioned device for measuring the compressive stress-strain curve of concrete at high temperatures are as follows:

[0014] Step 1: Place the first metal indenter on the lower crossbeam of the reaction frame, and then place the concrete specimen, the second metal indenter, and the force sensor in sequence from bottom to top;

[0015] Step 2: The concrete specimen has dimensions of 100mm × 100mm × 400mm and a deformation measurement gauge length of 200mm. A first pre-drilled hole is machined 100mm from the top surface of the specimen at the upper end of the gauge length, and a second pre-drilled hole is machined 100mm from the bottom surface of the specimen at the lower end of the gauge length. A first steel washer is placed on the surface of the first pre-drilled hole, and a second steel washer is placed on the surface of the second pre-drilled hole. One end of the first screw is embedded in the first pre-drilled hole, and the other end is connected to the third screw through the first extension nut. One end of the second screw is embedded in the second pre-drilled hole, and the other end is connected to the fourth screw through the second extension nut.

[0016] Step 3: Lead the third screw and the fourth screw out of the furnace through the second and third round holes reserved in the side wall of the high-temperature furnace, respectively;

[0017] Step 4: Fix the displacement gauge to the vertical section of the third screw, weld a steel plate to the end of the fourth screw, and make the iron core of the displacement gauge perpendicular to the steel plate;

[0018] Step 5: Wrap the part of the screw located inside the high-temperature furnace with aluminum silicate fiber cotton;

[0019] Step 6: Connect the force sensor and displacement gauge to the static data acquisition unit, and connect the static data acquisition unit to the computer;

[0020] Step 7: Use the software that comes with the static data acquisition instrument on the computer to acquire data, and set the acquisition frequency to 5Hz;

[0021] Step 8: Start the jack, open the oil pump inlet valve, apply an initial load of 1kN to the concrete specimen, make all components contact each other, and check whether the force sensor and displacement gauge are working properly.

[0022] Step 9: Cover the top of the high-temperature furnace with aluminum silicate fiber cotton and seal the bottom of the high-temperature furnace;

[0023] Step 10: Set the target heating temperature on the temperature controller, start the high-temperature furnace, and heat the concrete specimen;

[0024] Step 11: Monitor the furnace temperature using a temperature controller. Once the furnace temperature reaches the target temperature, stop heating and maintain a constant temperature.

[0025] Step 12: After the constant temperature time reaches the target time, start the oil pump and drive the jack to apply compressive stress to the concrete specimen in a force-controlled manner.

[0026] Step 13: During the loading process, simultaneously collect load data from the force sensor and displacement data from the displacement gauge;

[0027] Step 14: When the compressive stress reaches 80% of the estimated peak stress, reduce the oil pump's oil inlet flow rate;

[0028] Step 15: Load until the specimen fails, close the oil pump inlet valve, open the return valve, and control the jack piston to retract;

[0029] Step 16: Remove the aluminum silicate fiber cotton from the upper and lower openings of the high-temperature furnace. After the concrete specimen has cooled naturally to room temperature, remove it to end the test.

[0030] Step 17: Based on the collected load and displacement data, plot the compressive stress-strain curve of the concrete specimen.

[0031] The beneficial effects of this invention are:

[0032] This invention inserts a screw into a concrete specimen and extends it outside the furnace, testing the relative displacement of the two screws outside the furnace. This transforms high-temperature deformation testing into room-temperature testing. This testing method allows the displacement gauge to be unaffected by the high temperature inside the furnace, meeting the deformation testing requirements for concrete in the temperature range of 20℃ to 800℃. This invention converts high-temperature deformation testing into room-temperature deformation testing, avoiding displacement gauge failure due to high temperatures, and accurately measuring concrete deformation from 20℃ to 800℃, effectively solving the problem of difficult high-temperature deformation testing. By increasing the stiffness of the reaction frame, sudden failure of the specimen after reaching peak stress can be effectively prevented, ensuring a complete compressive stress-strain curve of the concrete is obtained. Attached Figure Description

[0033] Figure 1 A schematic diagram of the overall structure of the device for measuring the compressive stress-strain curve of concrete at high temperature;

[0034] Figure 2 This is a schematic diagram of the concrete specimen in the embodiment;

[0035] Figure 3 This is a schematic diagram of the concrete specimen after the screws are installed in the embodiment;

[0036] Figure 4 This is a schematic diagram of the displacement measuring device on the concrete specimen after installation in the embodiment.

[0037] Figure 5 The compressive stress-strain curves were plotted for the example. Detailed Implementation

[0038] The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any combination of the specific embodiments.

[0039] Specific Implementation Method 1: This implementation method is described in conjunction with the accompanying drawings. This implementation method is a device for measuring the compressive stress-strain curve of concrete at high temperature, which includes a heating system, a loading system, and a deformation measurement system.

[0040] The heating system includes a high-temperature furnace 1, a support 2, a thermocouple 3, a temperature controller 4, and wires 5; the high-temperature furnace 1 is mounted on the support 2, and a first round hole 1-1, a second round hole 1-2, and a third round hole 1-3 are pre-drilled on the side wall of the high-temperature furnace 1; the thermocouple 3 is inserted into the high-temperature furnace 1 through the first round hole 1-1; the thermocouple 3 is connected to the temperature controller 4 through the wires 5.

[0041] The loading system includes a reaction frame 6, a jack 7, an oil pump 8, an oil pipe 9, a force sensor 10, a first metal pressure head 11, and a second metal pressure head 12. The reaction frame 6 includes an upper crossbeam 6-1, a lower crossbeam 6-2, a left column 6-3, a right column 6-4, and a diagonal brace 6-5. The two ends of the upper crossbeam 6-1 are connected to the upper parts of the left column 6-3 and the right column 6-4, respectively. The two ends of the lower crossbeam 6-2 are connected to the lower parts of the left column 6-3 and the right column 6-4, respectively. The angle between the beam and the column is reinforced by the diagonal brace 6-5. The jack 7 is suspended on the upper crossbeam 6-1 and is connected to the oil pump 8 through the oil pipe 9. The first metal pressure head 11 is placed on the lower crossbeam 6-2, and the second metal pressure head 12 is positioned above the first metal pressure head 11. The force sensor 10 is positioned on the upper end of the second metal pressure head 12. The concrete specimen 23 is positioned between the first metal pressure head 11 and the second metal pressure head 12.

[0042] The deformation measurement system includes a first screw 13, a second screw 14, a first steel washer 15, a second steel washer 16, a third screw 17, a fourth screw 18, a first extended nut 19, a second extended nut 20, a displacement gauge 21, and a steel plate 22. The sidewall of the concrete specimen 23 is machined with a first pre-drilled hole 23-1 and a second pre-drilled hole 23-2. A first steel washer 15 is placed on the surface of the first pre-drilled hole 23-1, and a second steel washer 16 is placed on the surface of the second pre-drilled hole 23-2. One end of the first screw 13 is embedded in the first pre-drilled hole 23-1, and the other end is connected to the third screw 17 via the first extended nut 19. One end of the second screw 14 is embedded in the second pre-drilled hole 23-2, and the other end is connected to the fourth screw 18 via the second extended nut 20.

[0043] The third screw 17 and the fourth screw 18 are led out of the furnace through the second round hole 1-2 and the third round hole 1-3 reserved in the side wall of the high-temperature furnace 1, respectively. The third screw 17 is provided with a vertical section, and a displacement gauge 21 is fixed on the vertical section. The end of the fourth screw 18 is welded with a steel plate 22, and the iron core of the displacement gauge 21 is perpendicular to the steel plate 22.

[0044] This method indirectly tests concrete deformation at high temperatures outside the furnace. The displacement gauge will not fail due to the furnace temperature. The screw and displacement gauge used are ordinary and inexpensive commercial products. Expensive high-temperature resistant displacement gauges are not required. The method can complete the concrete deformation test at high temperatures. It is simple to operate, low in cost, and has high measurement accuracy.

[0045] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the high-temperature furnace 1 has an open structure at both the top and bottom, and resistance wire is wound around the side wall of the high-temperature furnace 1 as a heating element. Everything else is the same as in Specific Implementation Method One.

[0046] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One in that the upper crossbeam 6-1, lower crossbeam 6-2, left column 6-3, right column 6-4, and diagonal brace 6-5 are connected by welding. Everything else is the same as in Specific Implementation Method One.

[0047] Specific Implementation Method Four: This implementation method differs from Specific Implementation Method One in that stiffening ribs are provided on the upper crossbeam 6-1, lower crossbeam 6-2, left column 6-3, and right column 6-4. Everything else is the same as in Specific Implementation Method One.

[0048] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method One in that the first metal pressure head 11 and the second metal pressure head 12 are made of Inconel 718 nickel-based alloy. Everything else is the same as in Specific Implementation Method One.

[0049] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method One in that the axes of the upper crossbeam 6-1, jack 7, force sensor 10, second metal pressure head 12, concrete specimen 23, and first metal pressure head 11 are all on the same reference axis. Everything else is the same as in Specific Implementation Method One.

[0050] This implementation method aligns the axes of each component to prevent eccentric loading.

[0051] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method One in that the stiffness of the reaction frame 6 is greater than the sum of the stiffnesses of the jack 7, force sensor 10, concrete specimen 23, first metal indenter 11, and second metal indenter 12. Everything else is the same as in Specific Implementation Method One.

[0052] Specific Implementation Method Eight: The method of using the device for measuring the compressive stress-strain curve of concrete at high temperature according to this implementation method is as follows:

[0053] Step 1: Place the first metal indenter 11 on the lower crossbeam 6-2 of the reaction frame 6, and then place the concrete specimen 23, the second metal indenter 12, and the force sensor 10 from bottom to top in sequence;

[0054] Step 2: The concrete specimen 23 has dimensions of 100mm×100mm×400mm and a deformation measurement gauge length of 200mm. A first pre-drilled hole 23-1 is machined 100mm from the top surface of the specimen at the upper end of the gauge length, and a second pre-drilled hole 23-2 is machined 100mm from the bottom surface of the specimen at the lower end of the gauge length. A first steel washer 15 is placed on the surface of the first pre-drilled hole 23-1, and a second steel washer 16 is placed on the surface of the second pre-drilled hole 23-2. One end of the first screw 13 is embedded in the first pre-drilled hole 23-1, and the other end is connected to the third screw 17 through the first extension nut 19. One end of the second screw 14 is embedded in the second pre-drilled hole 23-2, and the other end is connected to the fourth screw 18 through the second extension nut 20.

[0055] Step 3: Lead the third screw 17 and the fourth screw 18 out of the furnace through the second round hole 1-2 and the third round hole 1-3 reserved in the side wall of the high-temperature furnace 1, respectively;

[0056] Step 4: Fix the displacement gauge 21 to the vertical section of the third screw 17, and weld the steel plate 22 to the end of the fourth screw 18, so that the iron core of the displacement gauge 21 is perpendicular to the steel plate 22.

[0057] Step 5: Wrap the portion of the screw located inside the high-temperature furnace 1 with aluminum silicate fiber cotton;

[0058] Step 6: Connect the force sensor 10 and displacement gauge 21 to the static data acquisition unit, and connect the static data acquisition unit to the computer;

[0059] Step 7: Use the software that comes with the static data acquisition instrument on the computer to acquire data, and set the acquisition frequency to 5Hz;

[0060] Step 8: Start jack 7, open the oil inlet valve of oil pump 8, apply an initial load of 1kN to the concrete specimen, make all components contact each other, and check whether the force sensor 10 and displacement gauge 21 are working properly.

[0061] Step 9: Cover the top of the high-temperature furnace 1 with aluminum silicate fiber cotton and seal the bottom of the high-temperature furnace 1;

[0062] Step 10: Set the target heating temperature on the temperature controller 4, start the high-temperature furnace 1, and heat the concrete specimen 23;

[0063] Step 11: Monitor the furnace temperature using temperature controller 4. Once the furnace temperature reaches the target temperature, stop heating and maintain a constant temperature.

[0064] Step 12: After the constant temperature time reaches the target time, start the oil pump 8 and drive the jack 7 to apply compressive stress to the concrete specimen 23 in a force-controlled manner.

[0065] Step 13: During the loading process, the load data of the force sensor 10 and the displacement data of the displacement gauge 21 are collected simultaneously;

[0066] Step 14: When the compressive stress reaches 80% of the estimated peak stress, reduce the oil inlet flow of oil pump 8;

[0067] Step 15: Load until the specimen fails, close the oil inlet valve of oil pump 8, open the return valve, and control the jack piston to retract;

[0068] Step 16: Remove the aluminum silicate fiber cotton from the upper and lower openings of the high-temperature furnace 1. After the concrete specimen 23 has cooled naturally to room temperature, remove it to end the test.

[0069] Step 17: Based on the collected load and displacement data, plot the compressive stress-strain curve of concrete specimen 23.

[0070] During the test in this embodiment, when the compressive stress is close to the expected peak value, the oil pump inlet valve is appropriately reduced to decrease the strain development rate and ensure that a complete compressive stress-strain curve is obtained.

[0071] In this embodiment, a set of deformation measurement systems can be set on each side of the specimen, and the average value of the two sets of test data can be taken as the final result.

[0072] This embodiment simulates a real high-temperature and high-pressure environment. The high-temperature furnace 1 in the heating system is stably erected via a support 2. Thermocouples 3 sense the temperature inside the furnace, and wires 5 transmit the temperature signal to a temperature controller 4, thereby achieving precise temperature control. The reaction frame 6 of the loading system has a robust structure, consisting of an upper crossbeam 6-1, a lower crossbeam 6-2, a left column 6-3, a right column 6-4, and diagonal braces 6-5, ensuring it can withstand enormous pressure without deformation during loading. The two systems work together, allowing the concrete specimen 23 to withstand gradually increasing pressure while being heated to the target temperature. This highly simulates the real state of concrete structures under high temperature and pressure in actual fire or high-temperature industrial scenarios, solving the previous problem of simultaneously heating and loading specimens. The robust structure of the reaction frame 6 ensures that high temperature will not cause deformation or displacement during loading, affecting loading accuracy. The stable heating environment of the high-temperature furnace does not interfere with the normal operation of the loading system. The two systems work together to provide a stable foundation for accurately determining the compressive stress-strain curve of concrete at high temperatures.

[0073] The concrete specimen 23 has a first pre-drilled hole 23-1 and a second pre-drilled hole 23-2 machined on its sidewall. The displacement gauge 21 is mounted outside the furnace via components including a first screw 13, a second screw 14, a first steel washer 15, a second steel washer 16, a third screw 17, a fourth screw 18, a first extension nut 19, and a second extension nut 20. Specifically, the first screw 13 and the second screw 14 are embedded in the pre-drilled holes and connected to the third screw 17 and the fourth screw 18 outside the furnace via extension nuts. The third screw 17 has a vertical section for fixing the displacement gauge 21, and the end of the fourth screw 18 is welded with a steel plate 22 that perpendicularly abuts against the iron core of the displacement gauge 21. This design avoids direct exposure of the displacement gauge to the high-temperature environment, allowing it to operate normally without being affected by the high temperature inside the furnace and accurately measure the deformation of the concrete specimen under pressure at temperatures ranging from 20℃ to 800℃.

[0074] When the compressive stress-strain curve of concrete enters the descending phase, cracks appear inside the concrete, and deformation increases sharply. In this embodiment, because the displacement gauge is placed outside the furnace and connected to the concrete specimen through connecting components such as screws, even if cracks appear inside the concrete specimen, causing a significant increase in deformation, the displacement gauge will not detach or be damaged like traditional strain gauges that are directly pasted onto the specimen surface. This allows for continuous and accurate measurement of deformation, solving the problem of measuring concrete deformation at the cracking stage under high temperatures.

[0075] The heating system, loading system, and deformation measurement system work together to simultaneously heat and load the concrete specimen 23, while the displacement gauge 21 and force sensor 10 collect deformation and load data respectively. The force sensor 10 monitors the pressure applied to the concrete specimen in real time, and the displacement gauge 21 simultaneously measures the specimen's deformation. These data are transmitted to a computer via a static data acquisition instrument and collected using the accompanying software at a set acquisition frequency of 5Hz. Finally, based on the collected load and displacement data, the compressive stress-strain curve of the concrete specimen 23 is plotted. Throughout the process, the three systems work collaboratively, from specimen heating and loading to deformation measurement and data acquisition and analysis, each step is interconnected, completely solving the problem mentioned in the background technology of simultaneously heating and loading concrete specimens and accurately testing their deformation at high temperatures, providing a reliable technical means for in-depth research on the mechanical properties of concrete at high temperatures.

[0076] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Eight in that the thickness of the aluminum silicate fiber cotton is 100mm. Everything else is the same as in Specific Implementation Method Eight.

[0077] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Eight in that it adjusts the dimensions of the reaction frame 6 and the high-temperature furnace 1 and increases the number of test components accordingly to test multiple concrete specimens. Everything else is the same as in Specific Implementation Method Eight.

[0078] The beneficial effects of the present invention are verified using the following embodiments:

[0079] Example: The method of using a device for measuring the compressive stress-strain curve of concrete at high temperature is specifically carried out according to the following steps:

[0080] Step 1: Place the first metal indenter 11 on the lower crossbeam 6-2 of the reaction frame 6, and then place the concrete specimen 23, the second metal indenter 12, and the force sensor 10 from bottom to top in sequence;

[0081] Step 2: The concrete specimen 23 has dimensions of 100mm×100mm×400mm and a deformation measurement gauge length of 200mm. A first pre-drilled hole 23-1 is machined 100mm from the top surface of the specimen at the upper end of the gauge length, and a second pre-drilled hole 23-2 is machined 100mm from the bottom surface of the specimen at the lower end of the gauge length. A first steel washer 15 is placed on the surface of the first pre-drilled hole 23-1, and a second steel washer 16 is placed on the surface of the second pre-drilled hole 23-2. One end of the first screw 13 is embedded in the first pre-drilled hole 23-1, and the other end is connected to the third screw 17 through the first extension nut 19. One end of the second screw 14 is embedded in the second pre-drilled hole 23-2, and the other end is connected to the fourth screw 18 through the second extension nut 20.

[0082] Step 3: Lead the third screw 17 and the fourth screw 18 out of the furnace through the second round hole 1-2 and the third round hole 1-3 reserved in the side wall of the high-temperature furnace 1, respectively;

[0083] Step 4: Fix the displacement gauge 21 to the vertical section of the third screw 17, and weld the steel plate 22 to the end of the fourth screw 18, so that the iron core of the displacement gauge 21 is perpendicular to the steel plate 22.

[0084] Step 5: Wrap the portion of the screw located inside the high-temperature furnace 1 with aluminum silicate fiber cotton;

[0085] Step 6: Connect the force sensor 10 and displacement gauge 21 to the static data acquisition unit, and connect the static data acquisition unit to the computer;

[0086] Step 7: Use the software that comes with the static data acquisition instrument on the computer to acquire data, and set the acquisition frequency to 5Hz;

[0087] Step 8: Start jack 7, open the oil inlet valve of oil pump 8, apply an initial load of 1kN to the concrete specimen, make all components contact each other, and check whether the force sensor 10 and displacement gauge 21 are working properly.

[0088] Step 9: Cover the top of the high-temperature furnace 1 with aluminum silicate fiber cotton and seal the bottom of the high-temperature furnace 1;

[0089] Step 10: Set the target heating temperature on the temperature controller 4, start the high-temperature furnace 1, and heat the concrete specimen 23;

[0090] Step 11: Monitor the furnace temperature using temperature controller 4. Once the furnace temperature reaches the target temperature, stop heating and maintain a constant temperature.

[0091] Step 12: After the constant temperature time reaches the target time, start the oil pump 8 and drive the jack 7 to apply compressive stress to the concrete specimen 23 in a force-controlled manner.

[0092] Step 13: During the loading process, the load data of the force sensor 10 and the displacement data of the displacement gauge 21 are collected simultaneously;

[0093] Step 14: When the compressive stress reaches 80% of the estimated peak stress, reduce the oil inlet flow of oil pump 8;

[0094] Step 15: Load until the specimen fails, close the oil inlet valve of oil pump 8, open the return valve, and control the jack piston to retract;

[0095] Step 16: Remove the aluminum silicate fiber cotton from the upper and lower openings of the high-temperature furnace 1. After the concrete specimen 23 has cooled naturally to room temperature, remove it to end the test.

[0096] Step 17: Based on the collected load and displacement data, plot the compressive stress-strain curve of concrete specimen 23; as shown... Figure 5 As shown, this device can measure the complete compressive stress-strain curve of concrete at high temperatures. By increasing the stiffness of the reaction frame 6, the sudden failure of the concrete specimen 23 after reaching the peak compressive stress can be effectively avoided.

Claims

1. Apparatus for determining the compressive stress-strain curve of concrete at high temperatures, characterised in that The apparatus for determining the compressive stress-strain curve of concrete at high temperatures includes a heating system, a loading system, and a deformation measurement system. The heating system includes a high-temperature furnace (1), a support (2), a thermocouple (3), a temperature controller (4), and wires (5); the high-temperature furnace (1) is mounted on the support (2), and a first round hole (1-1), a second round hole (1-2), and a third round hole (1-3) are pre-drilled on the side wall of the high-temperature furnace (1); the thermocouple (3) is inserted into the high-temperature furnace (1) through the first round hole (1-1); the thermocouple (3) is connected to the temperature controller (4) through the wires (5); The loading system includes a reaction frame (6), a jack (7), an oil pump (8), an oil pipe (9), a force sensor (10), a first metal pressure head (11), and a second metal pressure head (12); the reaction frame (6) includes an upper crossbeam (6-1), a lower crossbeam (6-2), a left column (6-3), a right column (6-4), and a diagonal brace (6-5). The two ends of the upper crossbeam (6-1) are connected to the upper parts of the left column (6-3) and the right column (6-4), respectively. The two ends of the lower crossbeam (6-2) are connected to the upper parts of the left column (6-3) and the right column (6-4), respectively. -4) is connected to the lower part, and the angle between the beam and column is reinforced by diagonal bracing (6-5); the jack (7) is suspended on the upper crossbeam (6-1), and the jack (7) is connected to the oil pump (8) through the oil pipe (9); the first metal pressure head (11) is placed on the lower crossbeam (6-2), the second metal pressure head (12) is set above the first metal pressure head (11), and the force sensor (10) is set at the upper end of the second metal pressure head (12); the concrete specimen (23) is set between the first metal pressure head (11) and the second metal pressure head (12); The deformation measurement system includes a first screw (13), a second screw (14), a first steel washer (15), a second steel washer (16), a third screw (17), a fourth screw (18), a first extended nut (19), a second extended nut (20), a displacement gauge (21), and a steel plate (22). The sidewall of the concrete specimen (23) is machined with a first pre-drilled hole (23-1) and a second pre-drilled hole (23-2). A first steel washer (15) is placed on the surface of the first pre-drilled hole (23-1), and a second steel washer (16) is placed on the surface of the second pre-drilled hole (23-2). One end of the first screw (13) is embedded in the first pre-drilled hole (23-1), and the other end is connected to the third screw (17) through the first extended nut (19). One end of the second screw (14) is embedded in the second pre-drilled hole (23-2), and the other end is connected to the fourth screw (18) through the second extended nut (20). The third screw (17) and the fourth screw (18) are led out of the furnace through the second round hole (1-2) and the third round hole (1-3) reserved on the side wall of the high-temperature furnace (1), respectively. The third screw (17) is provided with a vertical section, and a displacement meter (21) is fixed on the vertical section. The end of the fourth screw (18) is welded with a steel plate (22), and the iron core of the displacement meter (21) is perpendicular to the steel plate (22).

2. A device for determining the stress-strain curve of concrete under compression at high temperatures according to claim 1, characterized in that The high-temperature furnace (1) has an open structure at the top and bottom, and resistance wire is wound around the side wall of the high-temperature furnace (1) as a heating element.

3. A device for determining the stress-strain curve of concrete under compression at high temperatures according to claim 1, characterized in that The upper crossbeam (6-1), lower crossbeam (6-2), left column (6-3), right column (6-4), and diagonal brace (6-5) are connected by welding.

4. The apparatus for determining the compressive stress-strain curve of concrete at high temperatures according to claim 1, characterized in that The upper crossbeam (6-1), lower crossbeam (6-2), left column (6-3), and right column (6-4) are all equipped with stiffening ribs.

5. The apparatus for determining the compressive stress-strain curve of concrete at high temperatures according to claim 1, characterized in that The first metal indenter (11) and the second metal indenter (12) are made of Inconel 718 nickel-based alloy.

6. The apparatus for determining the compressive stress-strain curve of concrete under high temperature according to claim 1, characterized in that... The axes of the upper crossbeam (6-1), jack (7), force sensor (10), second metal indenter (12), concrete specimen (23), and first metal indenter (11) are on the same reference axis.

7. The apparatus for determining the compressive stress-strain curve of concrete under high temperature according to claim 1, characterized in that... The stiffness of the reaction frame (6) is greater than the sum of the stiffnesses of the jack (7), force sensor (10), concrete specimen (23), first metal indenter (11), and second metal indenter (12).

8. The method of using the apparatus for measuring the compressive stress-strain curve of concrete at high temperature as described in claim 1, characterized in that... The specific steps for using the device to determine the compressive stress-strain curve of concrete at high temperatures are as follows: Step 1: Place the first metal indenter (11) on the lower crossbeam (6-2) of the reaction frame (6), and then place the concrete specimen (23), the second metal indenter (12), and the force sensor (10) from bottom to top. Step 2: The concrete specimen (23) has dimensions of 100mm×100mm×400mm and a deformation measurement gauge length of 200mm. A first pre-drilled hole (23-1) is machined 100mm from the top surface of the specimen at the upper end of the gauge length, and a second pre-drilled hole (23-2) is machined 100mm from the bottom surface of the specimen at the lower end of the gauge length. A first steel washer (15) is placed on the surface of the first pre-drilled hole (23-1), and a second steel washer (16) is placed on the surface of the second pre-drilled hole (23-2). One end of the first screw (13) is embedded in the first pre-drilled hole (23-1), and the other end is connected to the third screw (17) through the first extension nut (19). One end of the second screw (14) is embedded in the second pre-drilled hole (23-2), and the other end is connected to the fourth screw (18) through the second extension nut (20). Step 3: Lead the third screw (17) and the fourth screw (18) out of the furnace through the second round hole (1-2) and the third round hole (1-3) reserved on the side wall of the high-temperature furnace (1), respectively; Step 4: Fix the displacement gauge (21) on the vertical section of the third screw (17), weld the steel plate (22) to the end of the fourth screw (18), and make the iron core of the displacement gauge (21) perpendicular to the steel plate (22); Step 5: Wrap the part of the screw located inside the high-temperature furnace with aluminum silicate fiber cotton; Step 6: Connect the force sensor (10) and displacement meter (21) to the static data acquisition instrument, and connect the static data acquisition instrument to the computer; Step 7: Use the software that comes with the static data acquisition instrument on the computer to acquire data, and set the acquisition frequency to 5Hz; Step 8: Start the jack (7), open the oil inlet valve of the oil pump (8), apply an initial load of 1kN to the concrete specimen, make each component contact each other, and check whether the force sensor (10) and displacement gauge (21) are working properly. Step 9: Cover the top of the high-temperature furnace (1) with aluminum silicate fiber cotton and seal the bottom of the high-temperature furnace (1); Step 10: Set the target heating temperature on the thermostat (4), start the high-temperature furnace (1), and heat the concrete specimen (23); Step 11: Monitor the furnace temperature using the temperature controller (4). Once the furnace temperature reaches the target temperature, stop heating and maintain a constant temperature. Step 12: When the constant temperature time reaches the target time, start the oil pump (8) and drive the jack (7) to apply compressive stress to the concrete specimen (23) in a force-controlled manner; Step 13: During the loading process, load data from the force sensor (10) and displacement data from the displacement gauge (21) are collected simultaneously; Step 14: When the compressive stress reaches 80% of the estimated peak stress, reduce the oil flow rate of the oil pump (8); Step 15: Load until the specimen fails, close the oil inlet valve of the oil pump (8), open the return valve, and control the jack piston to retract; Step 16: Remove the aluminosilicate fiber cotton from the upper and lower openings of the high-temperature furnace (1), and take out the concrete specimen (23) after it has cooled to room temperature naturally, thus ending the test; Step 17: Based on the collected load and displacement data, plot the compressive stress-strain curve of the concrete specimen (23).

9. The method of using the apparatus for determining the compressive stress-strain curve of concrete at high temperature according to claim 8, characterized in that... The thickness of the aluminum silicate fiber cotton is 100mm.

10. The method of using the apparatus for determining the compressive stress-strain curve of concrete at high temperature according to claim 8, characterized in that... Multiple concrete specimens were tested by adjusting the size of the reaction frame (6) and the high-temperature furnace (1) and increasing the number of test components accordingly.

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