A method and device for testing three-directional strain of a hollow cylinder specimen of asphalt mixture

Abstract

A kind of asphalt mixture hollow cylinder specimen three-way strain test method and device, it mainly includes radial deformation sensor, ring deformation sensor and the protection tooling consisting of upper top plate, lower top plate, metal support and fixed cross. Combined with the independent research and development of internal and external double surrounding pressure air bag triaxial test device for joint test. The device can more accurately test the three-way strain of mixture and avoid the damage of sensor due to axial pressure. It has great significance for studying the mechanical properties of asphalt mixture under complex stress state, reasonably designing asphalt mixture and preventing and treating asphalt pavement damage.

Description

Technical Field

[0001] This invention relates to the field of road engineering, specifically to a method and apparatus for testing the triaxial strain of asphalt mixtures. Background Technology

[0002] Over 90% of China's existing expressways are constructed with asphalt pavement. With the significant increase in highway traffic volume, the service life of some asphalt pavements is far below their design life. This is attributed to a disconnect between material design and asphalt pavement structural design. Researching triaxial stress-strain testing techniques and devices for asphalt mixtures is crucial for objectively characterizing the mechanical behavior of asphalt concrete materials under vehicle loads and for the scientific and rational design of pavement structures.

[0003] Under traffic loads, pavement structures exhibit significant three-dimensional stress and strain. Current conventional one-dimensional or two-dimensional stress state tests of asphalt mixtures, such as tensile, compressive, bending, shear, and torsion tests, cannot accurately simulate the actual service conditions of pavements, and therefore cannot objectively reflect the three-dimensional stress-strain characteristics of pavement materials under service conditions. Currently, commonly used methods for indoor strain measurement of asphalt mixtures include resistance strain gauge testing, LVDT displacement sensor testing, and displacement gauge testing integrated into material testing systems such as MTS. However, these tests are mostly conducted under simple stress states (tensile, compressive, bending, and shear tests), making it difficult to measure the triaxial strain of cylindrical or cubic specimens. This is because radial and circumferential strain testing of asphalt mixture specimens is challenging for cylindrical specimens; furthermore, during triaxial strain testing, the measurements of radial, vertical, and circumferential deformations interfere with each other, making it difficult to perceive the material's triaxial strain in real time. Furthermore, stress concentration easily occurs at the corners of cubic specimens, so triaxial strain testing of asphalt mixture specimens is less common. Therefore, it is essential to conduct triaxial strain tests on asphalt mixture specimens, which is of great significance for objectively characterizing the mechanical behavior of pavement materials under complex stress conditions.

[0004] To address this, this invention develops a triaxial strain testing device and method for hollow cylindrical asphalt mixture specimens. Combining this with existing biaxial compression triaxial testing technology, it achieves real-time sensing of triaxial stress and strain. Simultaneously, a protective fixture for the sensor is developed, which protects the sensor during testing, facilitates its positioning and installation, and prevents damage due to operational errors or overload. This enables the testing and sensing of triaxial deformation in hollow cylindrical asphalt mixture specimens. Summary of the Invention

[0005] The present invention aims to provide a triaxial strain testing device for hollow cylindrical asphalt mixture specimens, comprising: a circumferential deformation sensor; a radial deformation sensor; and a protective fixture. The circumferential and radial deformation sensors are disposed within the protective fixture. The protective fixture includes an upper protective plate, a lower protective plate, a metal bracket, and a sensor fixing frame. The metal bracket is disposed on and connected to the circumferential surfaces of the upper and lower protective plates. The sensor fixing frame is disposed inside the upper and lower protective plates and the metal bracket for fixing the radial deformation sensor. The circumferential deformation sensor is placed at a lower height than the radial deformation sensor. When the triaxial strain testing device for hollow cylindrical asphalt mixture specimens is disposed within the hollow cylindrical asphalt mixture specimen, the circumferential deformation sensor measures the circumferential deformation of the hollow cylindrical asphalt mixture specimen, and the radial deformation sensor measures the radial deformation of the hollow cylindrical asphalt mixture specimen.

[0006] Optionally, there are two radial deformation sensors, which are installed at the center of the sensor holder and located at 1 / 3 and 1 / 2 of the height of the hollow cylindrical specimen of asphalt mixture, respectively. The circumferential deformation sensor is installed at the bottom of the hollow cylindrical specimen of asphalt mixture.

[0007] Optionally, the upper and lower protective plates are made of high-strength titanium alloy to resist axial loads during the experiment, thereby protecting the radial and circumferential deformation sensors. The dimensions of the upper and lower protective plates are slightly smaller than the inner diameter of the empty cylindrical specimen.

[0008] Optionally, the number of metal brackets is four, which are made of high-strength titanium alloy and are interconnected with the upper and lower protective plates. They are symmetrically distributed at the quarter points of the circumference of the upper and lower top plates, providing support for the entire protective fixture.

[0009] Optionally, the metal bracket has slots near the third point and the midpoint to facilitate the installation and fixation of the sensor mounting bracket with the radial deformation sensor.

[0010] Optionally, the sensor mounting bracket is a cross-shaped mounting bracket, which consists of two mutually perpendicular high-strength alloy frames on which the sensor is placed. The entire protective fixture is equipped with multiple pairs of fixing crosses at the slots of the metal bracket and fixed with bayonets to directly measure the radial deformation of the asphalt mixture.

[0011] Optionally, a hook is provided on the lower surface of the upper protective plate, and the hook is provided with a buckle for fixing the radial deformation sensor, so as to facilitate the measurement of the radial strain of the hollow cylindrical specimen of asphalt mixture. The circumferential deformation sensor is installed at the bottom of the hollow cylindrical specimen of asphalt mixture, and an adjustable speed ultra-quiet rotating motor is provided below the circumferential deformation sensor to measure the circumferential strain of the hollow cylindrical specimen of asphalt mixture.

[0012] Optionally, the triaxial strain testing device for hollow cylindrical asphalt mixture specimens is combined with a triaxial testing device for asphalt mixtures for joint testing. The outer airbag assembly and the inner airbag assembly of the triaxial testing device are provided with adjustable air pressure by a pneumatic control console. The outer airbag of the outer airbag assembly is positioned on the outside of the hollow cylindrical asphalt mixture specimen and wraps around its outer cylindrical surface. The inner airbag of the inner airbag assembly is installed in the inner hole of the hollow cylindrical asphalt mixture specimen and is in close contact with it. The upper and lower protective plates are fixed to the upper and lower surfaces of the hollow cylindrical asphalt mixture specimen, respectively. The lower top plate is connected to the support base of the MTS material testing machine, and the upper protective plate is connected to the telescopic arm of the MTS material testing machine. The vertical deformation of the hollow cylindrical asphalt mixture specimen is measured using an MTS displacement sensor. With the protective fixture, this constitutes a triaxial strain testing device, enabling high-precision real-time measurement of the triaxial strain of the asphalt mixture. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the triaxial strain testing device for hollow cylindrical specimens of asphalt mixture according to the present invention.

[0014] Figure 2 This is a schematic diagram of the sensor arrangement and protective fixtures of the triaxial strain testing device for hollow cylindrical specimens of asphalt mixture according to the present invention.

[0015] Figure 3 This is a top view of the tooling used in this invention.

[0016] Figure 4 This is a schematic diagram illustrating the application of the generalized Hooke's Law.

[0017] Figure 5 This is the axial stress-strain curve of the AC-13 asphalt mixture plane isobaric / axial compression test of the present invention.

[0018] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Please see Figure 1-3 This invention provides a triaxial strain testing device for hollow cylindrical specimens of asphalt mixture (hereinafter referred to as specimens), comprising sensors (e.g., radial deformation sensors, circumferential deformation sensors, and vertical deformation sensors) and protective fixtures. In this invention, radial refers to the radial direction of the specimen, circumferential refers to the circumferential direction of the specimen, and vertical, also known as axial, refers to the length direction of the specimen. The radial deformation sensor is used to measure the radial deformation of the specimen under external force. The circumferential deformation sensor is used to measure the circumferential deformation of the specimen under external force. The vertical deformation sensor is used to measure the vertical deformation of the specimen under external force. The sensors are housed within the protective fixtures. The protective fixtures are used for positioning and installing the sensors during the test, protecting the sensors, preventing damage due to operational errors or overload, and enabling the testing and sensing of the triaxial deformation of the specimen.

[0021] The protective fixture consists of an upper protective plate, a lower protective plate (also referred to as an upper top plate and a lower top plate, respectively), metal supports, and a sensor mounting bracket. The upper and lower protective plates are each a single circular, high-strength sheet material (e.g., high-strength titanium alloy). Metal supports are evenly arranged and connected to the circumference of the upper and lower protective plates. The number of metal supports can be two, three, four, or six. The sensor mounting bracket is located inside the upper and lower protective plates and the metal supports, used to fix at least part of the sensor. In a specific embodiment, the protective fixture is a plate-column structure, consisting of two high-strength, circular upper and lower protective plates and four high-strength metal supports evenly distributed at 90° intervals on the circumference of the protective plates. The metal supports are made of high-strength titanium alloy, capable of withstanding an ultimate load of 40-80 MPa, preferably 50 MPa, to prevent damage to the sensor due to excessive axial load or operational errors. The mounting bracket is a cross-shaped mounting bracket. The cross-shaped fixing frame consists of two mutually perpendicular high-strength alloy frames, on which at least some of the sensors (e.g., radial deformation sensors) are placed.

[0022] In some embodiments, the circular upper and lower protective plates are a set structure, which can be fitted according to different inner diameters of the specimen. The metal bracket is available in various heights to ensure that specimens of various sizes can be tested. The size of the upper and lower protective plates is slightly smaller than the inner diameter of the specimen by a certain size (e.g., 1-2 mm) to ensure that the protective fixture can be installed smoothly and to prevent the protective fixture from being unable to fit into the inner diameter of the specimen due to reasons such as specimen core tilting. In addition, in some embodiments, a small level bubble is installed on the lower protective plate to ensure that the protective fixture can be installed horizontally; a hook is provided on the lower surface of the upper protective plate, and the hook is provided with a buckle for fixing the radial deformation sensor to facilitate the measurement of the radial strain of the hollow cylindrical asphalt mixture specimen. The circumferential deformation sensor is installed at the bottom of the hollow cylindrical asphalt mixture specimen, and an adjustable speed ultra-quiet rotating motor is provided below the circumferential deformation sensor to measure the circumferential strain of the hollow cylindrical asphalt mixture specimen.

[0023] According to the above embodiment, there are a total of four metal supports, evenly distributed at 90° intervals on the circumference of the protective plate. The metal supports are interlocked with the upper and lower protective plates, providing support for the entire protective fixture. Specifically, the metal supports are interconnected with the upper and lower protective plates and symmetrically distributed at the quarter points of the circumference of the upper and lower top plates, providing support for the entire protective fixture. Sensor mounting slots are provided near specific positions on each metal support (e.g., quarter points, midpoints, or other suitable positions) to facilitate the installation and fixation of the cross-shaped fixing brackets containing the radial deformation sensors. Multiple pairs of cross-shaped fixing brackets can be installed in the slots of the metal supports as needed and fixed with clamps, facilitating direct measurement of the radial deformation of the asphalt mixture. The length of the cross-shaped fixing brackets is within a certain range. For example, the length of the cross-shaped fixing brackets is sufficient to fix the sensors. In some embodiments, the radial deformation sensors positioned on the cross-shaped fixing brackets can be arranged at vertical intervals of 3–5 mm.

[0024] Radial and circumferential deformation sensors are used to measure the radial and circumferential strains generated in the specimen under external force, respectively. Radial strain is measured using a linear displacement sensor. For example, a Donghua displacement sensor with a range of 200 mm and an accuracy of ±3 μm can be used; other radial deformation sensors that meet the test range and accuracy requirements can also be used. Circumferential strain is measured by a laser displacement sensor driven by a spindle. This laser displacement sensor can be conveniently installed in a set position on a protective fixture. For example, a KEYENCE laser displacement sensor with a test radius of 70 mm and an accuracy of ±2 μm can be used; other circumferential deformation sensors that meet the test range and accuracy requirements can also be used. Circumferential strain can also be measured by strain gauges or other methods, provided the test requirements are met.

[0025] In this embodiment, the radial deformation sensor is mounted on the central slot of the cross-shaped fixing bracket. Two radial deformation sensors are used, positioned at predetermined heights of 1 / 3 and 1 / 2 of the distance from the top surface of the specimen. One circumferential deformation sensor is mounted on the bottom of the specimen. The circumferential deformation sensor measures the entire inner surface of the hollow cylinder. The sensor is secured with a bayonet to prevent it from falling off with light shaking and to ensure its position remains fixed during testing. In some embodiments, the circumferential deformation sensor is placed at a lower height than the radial deformation sensor to prevent interference from the radial deformation sensor. In some embodiments, the circumferential deformation sensor is positioned on the upper surface of the lower protective plate. The radial and circumferential deformation data measured by the radial and circumferential deformation sensors can be transmitted to the control computer in real time.

[0026] This invention combines a self-developed triaxial testing device and method for asphalt mixtures (application number CN2012105837297) for joint testing. The outer and inner airbag assemblies are supplied with adjustable air pressure by a pressure control console. The outer airbag of the outer airbag assembly is positioned on the outside of the specimen and wraps around its outer cylindrical surface. The inner airbag of the inner airbag assembly is installed in the inner hole of the specimen and in close contact with it. An upper pressure plate and a lower pressure plate are fixed to the upper and lower surfaces of the specimen, respectively. The lower top plate is connected to the support base of the MTS material testing machine, and the upper top plate is connected to the telescopic arm of the MTS material testing machine. The vertical deformation of the specimen is measured using an MTS displacement sensor, and with the addition of protective fixtures, this constitutes a triaxial strain testing device, enabling high-precision real-time measurement of the triaxial strain of asphalt mixtures.

[0027] The entire device is mainly composed of the aforementioned components, enabling relatively accurate measurement of the radial and circumferential strain of the specimen, achieving real-time sensing of biaxial strain. The triaxial testing device for asphalt mixtures provided in application number CN2012105837297 achieves real-time sensing of triaxial stress and strain. Simultaneously, the protective fixture prevents damage to the deformation sensor due to excessive axial displacement or operational errors during the experiment, ensuring the smooth progress of the test.

[0028] According to other embodiments of this application, a method for testing the triaxial strain of a hollow cylindrical specimen of asphalt mixture (hereinafter referred to as the specimen) is provided, comprising the following steps:

[0029] Step a: Form a cylindrical asphalt mixture, drill the core to obtain the specimen required for the test, place the specimen in a temperature-controlled chamber and keep it at the set temperature, and then place the specimen on the test bench of the MTS material testing machine.

[0030] Step b: Place the test protective fixture inside the specimen, ensuring that the radial deformation sensor test positions are at 1 / 3 and 1 / 2 of the specimen height, or other set heights, and that the level bubble on the protective fixture is centered. After the installation position is stable, tighten the slot to ensure that the protective fixture and sensor holder will not move when gently shaken.

[0031] Step c: Adjust the radial and circumferential deformation sensors. Place the radial deformation sensor in the pre-drilled slot on the metal bracket, ensuring its measuring end contacts the inner wall of the specimen. After fixing it securely, record its initial reading. Install the circumferential deformation sensor at the bottom of the specimen and tighten the slot after adjustment.

[0032] Step d: Check that there are no other obstructions within the scanning range of the circumferential deformation sensor, and that it can rotate at the set speed driven by the spindle to ensure normal testing.

[0033] Step e: Depending on the test requirements, install internal and external double airbags, a single airbag, or no airbags on the specimen.

[0034] Step f: Control the air pressure of the inner and outer airbags, set different stress states according to the test requirements, and apply an axial load to the surface of the specimen using the MTS testing machine. Under the action of the axial load, or the combined action of the axial load and the air pressure of the airbags, the specimen deforms under a three-dimensional complex stress state. The axial strain is measured by the loading axis of the MTS testing machine, and the radial and circumferential strains of the specimen are measured by radial and circumferential deformation sensors, respectively, and transmitted to a computer connected to the strain acquisition instrument.

[0035] Step g: After the computer performs parallel tests on the acquired triaxial strain data, it obtains the triaxial strain data of the asphalt mixture.

[0036] In step a, the specimen can be a hollow cylinder of asphalt mixture with an outer diameter of 150 mm, an inner diameter of 100 mm, and a height of 150 mm, obtained by core drilling after static or rotary compaction. Other dimensions can also be used as needed. The specimen is suitable for loading on a general MTS testing machine. Furthermore, before drilling the inner hole, a centering cap is placed on the surface of the asphalt mixture cylinder to ensure the drill hole is centered.

[0037] In step f, the data acquisition instrument can acquire air pressure data, with a loading rate of 2 mm / min, a temperature of 15℃, a test time of 5 minutes, and a data acquisition density of 10 points / s. Other test conditions can also be selected as needed.

[0038] Example 1: Determination of triaxial deformation of hollow cylindrical specimen by uniaxial compression test

[0039] Uniaxial compression test

[0040] Step 1-1: Prepare asphalt mixture cylinders with a diameter and height of 150 mm using static pressure or rotary compaction methods. The asphalt mixture cylinders should ideally have a finer gradation. Select a circular positioning and centering cap with an outer diameter of 151 mm and an inner diameter of 101 mm to ensure the drill hole is centered. Core sampling yields specimens with an outer diameter of 150 mm, an inner diameter of 100 mm, and a height of 150 mm.

[0041] Steps 1-2: The test temperature is 15℃. Before the test, the specimen is controlled at temperature for more than 4 hours using a temperature control chamber, with the temperature control accuracy to ±0.5℃. The axial load is applied using an MTS testing machine at a loading rate of 2mm / min.

[0042] Steps 1-3: Install the indenter and base on the MTS testing machine, and apply lubricant to the indenter, base, and the upper and lower surfaces of the specimen. Pre-install the protective fixture, radial deformation sensor, and circumferential deformation sensor. Adjust the radial and circumferential deformation sensors so that the measuring end of the radial deformation sensor contacts the inner wall of the specimen. After fixing, record the initial reading. Check that there are no obstructions within the scanning range of the circumferential deformation sensor, ensuring it can rotate at the set speed driven by the spindle. Then, mount the specimen, with multiple circumferential strain gauges attached to its inner surface at the same height and connected end-to-end, onto the protective fixture containing the sensors. Place the specimen under the indenter and place a friction-reducing pad to reduce the constraint effect caused by surface friction. Adjust the position of the indenter to contact the upper surface of the specimen, while simultaneously subjecting the specimen to pre-compression. Then observe the readings of the radial and circumferential deformation sensors to ensure they can detect data.

[0043] Steps 1-4: The loading shaft of the MTS testing machine is uniformly loaded onto the specimen at a loading rate of 2 mm / min until failure. After obtaining the peak load, the load is unloaded, the triaxial strain value of the specimen is obtained, and the uniaxial compressive strength of the specimen is calculated.

[0044] Steps 1-5: According to the "Specifications for Design of Asphalt Pavement of Highway" (JTG D50-2017), the Poisson's ratio of asphalt mixture μ = 0.25. Following the test requirements of T0713-2000 in the "Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering" (JTG E20-2011), the compressive resilient modulus E of the specimen is measured to be 2052 MPa. Then, combined with the commonly used generalized Hooke's law (such as...)... Figure 4 (As shown) Calculate the principal strain values ​​in the radial and circumferential principal stress directions. After conducting parallel tests, the calculated and measured triaxial strain values ​​of the asphalt mixture under uniaxial compressive stress are shown in Table 1. The calculated triaxial strain value of the asphalt mixture under uniaxial compressive stress can be obtained from the following formula:

[0045]

[0046]

[0047]

[0048] Where σ1, σ2, and σ3 represent radial stress, circumferential stress, and axial stress, respectively, arranged in numerical order as the first principal stress, second principal stress, and third principal stress, all in MPa. ε1 calculation, ε2 calculation, and ε3 calculation represent the calculated strain values ​​corresponding to the three principal stresses, while ε1 test, ε2 test, and ε3 test represent the measured strain values, with strain units of 10⁻⁶. -3 In the experiment, compression was defined as positive and tension as negative.

[0049] Table 1. Triaxial strain test results of hollow cylindrical specimens of asphalt mixture in uniaxial compression test.

[0050]

[0051] As shown in Table 1, the measured values ​​of triaxial strain deviate from the theoretical values ​​by less than 15%, indicating that the test results are relatively accurate. Furthermore, the results of strain gauge testing also verified the accuracy of the circumferential strain test results.

[0052] Example 2: Planar Isobaric / Axial Compression Test

[0053] Step 2-1: Prepare asphalt mixture cylinders with a diameter and height of 150 mm using static pressure or rotary compaction methods. A finer gradation is preferable for the asphalt mixture cylinders. Select a circular positioning and centering cap with an outer diameter of 151 mm and an inner diameter of 101 mm to ensure the drill hole is centered. Core sampling yields specimens with an outer diameter of 150 mm, an inner diameter of 100 mm, and a height of 150 mm.

[0054] Step 2-2: The test temperature is 15℃. Before the test, the specimen is controlled at temperature for more than 4 hours using a temperature control chamber, with the temperature control accuracy to ±0.5℃. The axial load is applied using an MTS testing machine at a loading rate of 2mm / min.

[0055] Steps 2-3: A dual-enclosure-pressure airbag triaxial testing apparatus is used to conduct plane isobaric / axial compression failure characteristic tests on the specimens. The asphalt mixture triaxial testing system provided in application number CN2012105837297 mainly consists of a material testing machine, an airbag assembly, an air pressure control system, and a data acquisition system. The airbag assembly includes an inner airbag assembly and an outer airbag assembly. The air pressure control system regulates the inner and outer dual airbags to apply equal air pressure to the inner and outer surfaces of the specimen, respectively. Then, the MTS loading plate applies an axial compressive load to the specimen, thereby forming a plane isobaric / axial compression complex stress state.

[0056] Steps 2-4: Before conducting the planar isobaric / axial compression test, clean the contact surface between the specimen and the pressure plate, and place a friction-reducing pad between the specimen and the upper and lower protective plates. Then, install the thin inner and outer double airbags, and install the protective fixture, radial deformation sensor, and circumferential deformation sensor inside the inner airbag. Adjust the radial and circumferential deformation sensors so that the measuring end of the radial deformation sensor contacts the inner wall of the specimen. After fixing, record the initial reading. Check that there are no other obstructions within the scanning range of the circumferential deformation sensor to ensure that it can rotate at the set speed driven by the spindle. Then, mount the specimen with multiple circumferential strain gauges attached to the inner surface at the same height and connected end to end on the protective fixture containing the sensors. Then, the pneumatic control system applies equal confining pressure to the inner and outer surfaces of the hollow cylindrical specimen through the flexible airbag, ensuring that the measuring ends of the radial deformation sensor contact the surface of the inner airbag and deform as the inner airbag expands and contracts. Finally, the MTS loading device applies an axial compression load until the specimen fails under oblique shear. The MTS testing machine directly acquires axial strain data, while the sensor acquires radial and circumferential strain data of the asphalt mixture and uploads them synchronously to the computer.

[0057] The stress-strain curve of asphalt mixture under plane isobaric / axial compressive stress state is as follows: Figure 5 As shown, therefore, the failure principal strain can be calculated from the failure principal stress according to the generalized Hooke's law.

[0058] This paper selects an experimental temperature of 15℃ and a loading rate of 2 mm / min. Under these conditions, the asphalt mixture approximates an elastic body. The stress-strain curve of the asphalt mixture under planar isobaric / axial compressive stress is shown below. Figure 5 As shown. Therefore, the linear elastic limit point A is defined as the failure point. The asphalt mixture remains within the linear elastic range before failure, and the generalized Hooke's law applies. Therefore, the failure principal strain can be calculated from the failure principal stress according to the generalized Hooke's law.

[0059] Steps 2-5, Verification of Experimental Results: As described in Example 1, during the experiment, an MTS displacement sensor was used to collect axial strain values, a radial deformation sensor was used to test radial strain values, and a circumferential deformation sensor was used to test circumferential strain values. On the other hand, the triaxial principal strains ε1, ε2, and ε3 were calculated according to the generalized Hooke's law. The strain values ​​calculated using the generalized Hooke's law and the circumferential strain gauge readings were compared with the measured triaxial strain values ​​obtained by the device of this invention. The calculated triaxial strain values ​​and test results of AC-13 asphalt mixture under complex stress conditions of plane isobaric / axial compression are shown in Table 2.

[0060] Table 2. Triaxial strain test results of AC-13 asphalt mixture in plane isobaric / axial compression tests.

[0061]

[0062] As shown in Table 2, the measured values ​​of triaxial strain deviate from the theoretical values ​​by less than 15%, indicating that the test results are relatively accurate. Furthermore, the results of strain gauge testing also verified the accuracy of the circumferential strain test results.

[0063] By employing the aforementioned apparatus structure and testing method, a complex stress environment was constructed for the specimens, expanding the research scope of asphalt mixture strength and deformation characteristics from one-dimensional to three-dimensional. This testing apparatus and method have the following advantages:

[0064] (1) A high-precision laser rotation sensor is installed inside the specimen to measure the circumferential strain, and a radial deformation sensor is installed to test the radial strain in real time. Combined with the existing axial strain testing technology of the double confining pressure triaxial test, the real-time sensing of triaxial stress and strain is realized.

[0065] (2) Combined with the triaxial test device and method for asphalt mixture provided in application number CN2012105837297, the test effectively simulated the three-dimensional complex stress-strain state inside the pavement structure under wheel load. The triaxial strain failure criterion of asphalt mixture can be studied as a resistance calculation model for asphalt pavement. The triaxial strain development law during fatigue process can also be studied. Torque can be applied to the upper surface of the specimen to form a torsional shear triaxial stress state to consider the change of principal stress axis.

[0066] (3) The protective fixture is a high-strength aluminum alloy structure of column plate type, which can play a role in fixing and protecting the sensor during the test process, preventing damage to the sensor due to operational errors or overload, and increasing the practicality of the device and method.

[0067] (4) The test device is easy to assemble, the test method is simple and easy to implement, and it is easy to promote.

[0068] In summary, the method and apparatus provided by this invention have the advantages of being simple, easy to implement, easy to promote, and having high measurement accuracy. They fill the gap in the current ability to directly measure the triaxial strain of asphalt mixtures; the innovative design of sensor protection fixtures prevents damage to the sensors under load, which is of great significance for studying the stress-strain characteristics and other mechanical behaviors of asphalt mixtures under complex stress states, for rational asphalt mixture design, and for preventing damage to asphalt pavements.

[0069] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection defined by the claims submitted herein.

Claims

1. A test method for a triaxial strain testing device for hollow cylindrical specimens of asphalt mixture, characterized in that, The triaxial strain testing device for hollow cylindrical specimens of asphalt mixture includes: Circumferential deformation sensor; Radial deformation sensor; and A protective fixture is provided, in which the circumferential deformation sensor and the radial deformation sensor are disposed. The protective fixture includes an upper protective plate, a lower protective plate, a metal bracket, and a sensor mounting bracket. The metal bracket is disposed on and connected to the circumferential surfaces of the upper and lower protective plates. The sensor mounting bracket is disposed inside the upper and lower protective plates and the metal bracket, and is used to fix the radial deformation sensor. The height at which the circumferential deformation sensor is placed is lower than the height at which the radial deformation sensor is placed. The dimensions of the upper and lower protective plates are slightly smaller than the inner diameter of the hollow cylindrical specimen. The triaxial strain testing device for hollow cylindrical specimens of asphalt mixture is combined with a triaxial testing device for asphalt mixture for joint testing; The triaxial testing device for asphalt mixture consists of a material testing machine, an airbag assembly, an air pressure control system, and a data acquisition system. The airbag assembly includes an inner airbag assembly and an outer airbag assembly. The outer and inner airbag assemblies are supplied with adjustable air pressure by an air pressure control console. The outer airbag of the outer airbag assembly is positioned on the outside of the hollow cylindrical specimen of asphalt mixture and wraps around the outer cylindrical surface of the specimen. The inner airbag of the inner airbag assembly is installed in the inner hole of the hollow cylindrical specimen of asphalt mixture and is in close contact with the inner hole of the specimen. The testing method includes the following steps: Step a: Form an asphalt mixture cylinder, drill the core to obtain the specimen required for the test, place the specimen in a temperature control chamber and keep it at the set temperature, and then place the specimen on the test bench of the MTS material testing machine. Step b: Place the test protective fixture inside the specimen, ensuring that the radial deformation sensor test positions are at 1 / 3 and 1 / 2 of the specimen height, and that the level bubble on the protective fixture is centered; after the installation position is stable, tighten the slot to ensure that the protective fixture and sensor mounting bracket will not move when gently shaken. Step c: Adjust the radial deformation sensor and the circumferential deformation sensor; Place the radial deformation sensor in the pre-reserved slot of the metal bracket, so that its measuring end contacts the inner wall of the specimen, and record its initial reading after fixing it. Install a circumferential deformation sensor at the bottom of the specimen, and tighten the slot after adjustment; Step d: Check that there are no other obstructions within the scanning range of the circumferential deformation sensor, and that it can rotate at the set speed under the drive of the spindle; Step e: Install inner and outer dual airbags on the specimen; Step f: Control the air pressure of the inner and outer airbags, set different stress states according to the test requirements, and apply axial load to the surface of the specimen by the MTS testing machine. Under the action of axial load, or axial load and airbag pressure, the specimen will deform under a three-dimensional complex stress state. The axial strain is measured by the loading shaft of the MTS testing machine, the radial strain of the specimen is measured by the radial deformation sensor, and the circumferential strain of the specimen is measured by the circumferential deformation sensor. The data are then transmitted to a computer connected to the strain acquisition instrument. Step g: After the computer performs parallel tests on the acquired triaxial strain data, it obtains the triaxial strain data of the asphalt mixture.

2. The test method of the triaxial strain testing device for hollow cylindrical asphalt mixture specimens as described in claim 1, characterized in that, The radial deformation sensor is of two types, and the circumferential deformation sensor is installed at the bottom of the hollow cylindrical specimen of the asphalt mixture.

3. The test method of the triaxial strain testing device for hollow cylindrical asphalt mixture specimens as described in claim 1, characterized in that, The upper and lower protective plates are made of high-strength titanium alloy.

4. The test method of the triaxial strain testing device for hollow cylindrical specimens of asphalt mixture as described in claim 1, characterized in that, The number of metal supports is four, which are made of high-strength titanium alloy. They are interconnected with the upper and lower protective plates and are symmetrically distributed at the quarter points of the circumference of the upper and lower top plates, providing support for the entire protective fixture.

5. The test method of the triaxial strain testing device for hollow cylindrical specimens of asphalt mixture as described in claim 1, characterized in that, The metal bracket has slots near the third point and the midpoint to facilitate the installation and fixation of the sensor mounting bracket with the radial deformation sensor.

6. The test method of the triaxial strain testing device for hollow cylindrical asphalt mixture specimens as described in claim 1, characterized in that, The sensor mounting bracket is a cross-shaped bracket, which consists of two mutually perpendicular high-strength alloy frames on which the sensor is placed. The entire protective fixture is equipped with multiple pairs of cross-shaped brackets at the slots of the metal bracket and is fixed with bayonets to directly measure the radial deformation of the asphalt mixture.

7. The test method of the triaxial strain testing device for hollow cylindrical specimens of asphalt mixture as described in claim 1, characterized in that, The lower surface of the upper protective plate is provided with a hook, and the hook is provided with a buckle for fixing the radial deformation sensor, so as to facilitate the measurement of the radial strain of the hollow cylindrical specimen of asphalt mixture; an adjustable speed ultra-quiet rotating motor is provided below the circumferential deformation sensor to measure the circumferential strain of the hollow cylindrical specimen of asphalt mixture.

8. The test method of the triaxial strain testing device for hollow cylindrical specimens of asphalt mixture as described in claim 1, characterized in that, The upper and lower protective plates are respectively fixed to the upper and lower surfaces of the hollow cylindrical specimen of asphalt mixture. The lower protective plate is connected to the support base of the MTS material testing machine, and the upper protective plate is connected to the telescopic arm of the MTS material testing machine. Combined with the MTS displacement sensor, the vertical deformation of the hollow cylindrical specimen of asphalt mixture is measured. With the protective fixture, it is a triaxial strain testing device.

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