A fatigue test device and method for a rigid-flexible composite pavement asphalt mixture

Abstract

The present invention discloses a fatigue test device and method for asphalt mixture of rigid-flexible composite pavement. The fatigue test device includes an intermediate fixture I, an intermediate fixture II and two end fixtures. Loading heads are provided on the two intermediate fixtures. The two intermediate fixtures are arranged separately from each other. The loading heads are rigidly connected to the intermediate fixtures. The two loading heads are respectively connected to different loading devices. Displacement sensors are provided at the center point of the beam and on the two intermediate fixtures respectively, which are used to measure the displacement of the beam center and the displacements of the two intermediate fixtures. During the test, different loads are applied through the two loading heads to simulate the stress changes of the asphalt pavement at the joint. In view of the stress characteristics of the rigid-flexible composite pavement structure, by improving the four-point bending fatigue test device and the waveform of the cyclic load, the present invention can simulate the stress process of the asphalt layer when the actual vehicle load passes through the joint, thereby effectively improving the prediction accuracy of the fatigue life and providing a reference for the structural design of the rigid-flexible composite pavement.

Description

Technical Field

[0001] The present invention relates to the fatigue performance detection of asphalt mixtures, and particularly to a fatigue test device and method for asphalt mixtures of a rigid-flexible composite pavement. Background Art

[0002] The four-point bending fatigue test method in the "Specifications for Tests of Bitumen and Bituminous Mixtures for Highway Engineering" is commonly used to detect the fatigue performance of asphalt mixtures. The corresponding four-point bending fatigue test device includes four clamps, where the two outer clamps are fixed and the two middle clamps are movable; however, the two middle clamps are connected, and the loading heads on the two clamps are connected to the same pneumatic or hydraulic loading device. During the test, the two clamps are subjected to the same load and produce the same displacement. Such a setting can generate uniform bending and tensile stresses to simulate the fatigue performance of asphalt concrete beams under bending and tensile loads, but it cannot simulate the working conditions of shear stresses.

[0003] For a rigid-flexible composite pavement structure, the fatigue failure of its asphalt layer under load is the result of the combined action of bending and tensile and shear stresses. Specifically, due to the shrinkage characteristics of cement concrete, when such materials are used as the base layer, a continuous structure is usually not formed, but contains a large number of joints. Generally, the joints cannot bear tensile stresses and have weak shear stress-bearing capacity. Therefore, the asphalt surface layer on the top of the joints is extremely vulnerable to damage and cracking occurs under traffic loads. Such cracks are the result of the combined action of bending and tensile stresses and shear stresses. As Figure 1 shown: When the load approaches the joint, relative displacements occur on both sides of the joint, which in turn cause large shear stresses in the asphalt surface layer; when the load is above the joint, there is no relative displacement or a small relative displacement between the adjacent two plates. At this time, the asphalt surface layer mainly bears the action of bending and tensile stresses; finally, when the load leaves the joint, the surface layer bears shear stresses opposite to those when the load approaches the joint; during the whole process, the asphalt surface layer will be subjected to 2 times of shear and 1 time of bending and tensile actions.

[0004] In summary, the existing four-point bending fatigue test device and method can only simulate bending and tensile loads and cannot reflect the shear action, so they cannot fully reflect the characteristics of load-type fatigue of rigid-flexible composite pavements. It is necessary to propose a more reasonable fatigue test device and method according to the mechanical characteristics of rigid-flexible composite pavement structures. Summary of the Invention

[0005] Object of the Invention: The object of the present invention is to propose a fatigue test device and method for asphalt mixtures that comprehensively consider the actions of bending and tensile stresses and shear stresses according to the mechanical characteristics of rigid-flexible composite pavement structures.

[0006] Technical solution: The fatigue test device for the rigid-flexible composite pavement asphalt mixture of the present invention includes an intermediate fixture one, an intermediate fixture two, and two end fixtures. Loading heads are provided on the two intermediate fixtures. The two intermediate fixtures are arranged separately from each other. The loading heads are rigidly connected to the intermediate fixtures, and the two loading heads are respectively connected to different loading devices; displacement sensors are provided at the center point of the beam and on the two intermediate fixtures respectively for measuring the displacement of the beam center and the displacements of the two intermediate fixtures.

[0007] Further, the four fixtures are arranged at equal intervals of one-third on the beam.

[0008] The present invention also protects a fatigue test method for the rigid-flexible composite pavement asphalt mixture. Using the above fatigue test device, it includes the following steps:

[0009] (1) Referring to the requirements of the "Test Regulations for Bitumen and Bituminous Mixtures for Highway Engineering", cyclic loading is applied to the beam specimen. When the termination condition is reached, the loading is stopped and the fatigue life of the beam specimen is recorded;

[0010] (2) Calculate the fatigue test performance indicators, including:

[0011] The maximum flexural tensile stress, the calculation formula is where P is the peak load; L represents the beam span, that is, the distance between the two end fixtures; w represents the beam width, and h represents the beam height;

[0012] The maximum flexural tensile strain, the calculation formula is where δ0 represents the maximum displacement at the center of the beam;

[0013] The flexural stiffness modulus, using the calculation method in the "Test Regulations for Bitumen and Bituminous Mixtures for Highway Engineering";

[0014] When reaching the state of the maximum shear force, the maximum shear stress at the center of the beam, the calculation formula is

[0015] When reaching the state of the maximum shear force, the maximum shear strain at the center of the beam, the calculation formula is where μ represents the Poisson's ratio of the beam, δ1 represents the displacement of the intermediate fixture one when reaching the state of the maximum shear force, and δ2 represents the displacement of the intermediate fixture two when reaching the state of the maximum shear force.

[0016] Further, the expression within a single cycle of the cyclic load in step (1) is:

[0017]

[0018]

[0019] Wherein, P1 is the load applied on the first intermediate fixture, P2 is the load applied on the second intermediate fixture, and T is the loading period.

[0020] Furthermore, during actual loading, a strain-controlled loading mode is used, and the magnitude of the target flexural tensile strain is set according to requirements; the loading frequency is 10 Hz; the test termination condition is the number of cyclic loadings corresponding to the flexural stiffness modulus of the beam specimen being reduced to 50% of the initial value.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages:

[0022] Aiming at the mechanical properties of the rigid-flexible composite pavement structure, by improving the four-point bending fatigue test device and the waveform of the cyclic load, the present invention can simulate the stress process of the asphalt layer when the actual vehicle load passes through the joint, thereby effectively improving the prediction accuracy of the fatigue life and providing an important reference for the structural design of the rigid-flexible composite pavement. Description of the Drawings

[0023] Figure 1 is the fatigue crack mechanism diagram of the rigid-flexible composite pavement under traffic load;

[0024] Figure 2 is the structural schematic diagram of the fatigue test device;

[0025] Figure 3 is Figure 2 the front view of

[0026] Figure 4 is the cyclic load waveform diagram applied in the fatigue test;

[0027] Figure 5 is the curve of the change in shear force and bending moment at the center of the beam caused by the cyclic load. Detailed Embodiments

[0028] The present invention will be further described in detail below with reference to the accompanying drawings.

[0029] As Figure 2 and Figure 3As shown in the figure, a fatigue test device for a rigid-flexible composite pavement asphalt mixture includes an intermediate fixture 1, an intermediate fixture 2, and two end fixtures. The four fixtures are arranged at equal intervals on the beam in a three-equal-part manner. There is a loading indenter rigidly connected to the two intermediate fixtures, enabling it to provide two types of loads: tensile and compressive. The two intermediate fixtures are separated from each other, that is, the connection between the two intermediate fixtures is released. The two loading indenters are respectively connected to different pneumatic or hydraulic loading devices, and the two loading devices are controlled by the same control system. By distributing different forces on the two intermediate fixtures, the stress change process of the asphalt pavement when the load passes through the joint is simulated. A displacement sensor is set on the surface of the center point of the beam, and at the same time, one displacement sensor is additionally installed on each of the two intermediate fixtures. The three displacement sensors are respectively used to measure the displacement of the beam center and the displacements of the two intermediate fixtures. In this embodiment, the displacement sensor uses an LVDT displacement sensor.

[0030] The following introduces the method for conducting the fatigue test of asphalt mixture using the above fatigue test device, including the following steps:

[0031] Step 1: Test preparation.

[0032] Cut the beam specimen with reference to the requirements of the "Test Procedures for Bitumen and Bituminous Mixtures for Highway Engineering". The size of the beam specimen should meet the requirements of a length of 380mm ± 5mm, a height of 50mm ± 6mm, and a width of 63mm ± 6mm. Inspect the size parameters and volume parameters of the beam specimen, and cure it according to the specification requirements. After curing, place the beam specimen in the fatigue test device, fix it with fixtures, and place displacement sensors, one on the surface of the center of the beam specimen and the other two on the surfaces of the two intermediate fixtures. Adjust the sensor readings to be as close to zero as possible.

[0033] Step 2: Selection of test parameters.

[0034] The waveform of the cyclic load applied in the test is as Figure 4 shown. The expression of this cyclic load within a single period is as follows:

[0035]

[0036]

[0037] where P is the load peak value, P1 is the load applied on the intermediate fixture 1, P2 is the load applied on the intermediate fixture 2, and T is the loading period. Figure 5 The change curves of the shear force and bending moment at the center of the beam caused by the cyclic load are given. It can be seen that within one period, A→B is from zero loading to the maximum shear force, B→C is from the maximum shear force to the bending moment peak value, C→D is from the bending moment peak value to the maximum reverse shear force, and D→E represents the unloading stage.

[0038] During actual loading, a strain-controlled loading mode is used, and the magnitude of the target flexural tensile strain, the loading frequency, and the test termination condition are set according to requirements. In this embodiment, the test temperature is selected as 15 °C, the loading frequency is set at 10 Hz, and the test termination condition is the number of cyclic loadings corresponding to the flexural stiffness modulus of the beam specimen being reduced to 50% of the initial value.

[0039] Step 3: Conduct cyclic loading with reference to the requirements of the "Test Procedures for Bitumen and Bituminous Mixtures for Highway Engineering".

[0040] First, conduct 50 cycles of preloading at the target strain level to determine the initial reference stiffness modulus. Then, start the formal test. The testing machine adjusts to the level of the target flexural tensile strain within 50 cycles and monitors and records the test parameters and results according to the selected loading cycle interval. When the specimen reaches the termination condition, stop loading and record the fatigue life.

[0041] The calculation methods for the main indicators involved in the fatigue test are as follows:

[0042] The maximum flexural tensile stress, and the calculation formula is where P is the peak load; L represents the beam span, that is, the distance between the two end clamps; w represents the beam width, and h represents the beam height.

[0043] The maximum flexural tensile strain, and the calculation formula is where δ0 represents the maximum displacement at the center of the beam.

[0044] The flexural stiffness modulus, and the calculation formula is

[0045] When reaching the state of the maximum shear force, the maximum shear stress at the center of the beam, and the calculation formula is

[0046] When reaching the state of the maximum shear force, the maximum shear strain at the center of the beam, and the calculation formula is where μ represents the Poisson's ratio of the beam, δ1 represents the displacement of the intermediate clamp 1 when reaching the state of the maximum shear force, and δ2 represents the displacement of the intermediate clamp 2 when reaching the state of the maximum shear force.

[0047] The indicators in the existing four-point bending fatigue test method include the maximum tensile stress, the maximum tensile strain, and the flexural stiffness modulus. In the present invention, shear force-related indicators are added.

Claims

1. A fatigue test method for a rigid-flexible composite pavement asphalt mixture, using a fatigue test device, characterized in that: The fatigue test device includes a first intermediate fixture (1), a second intermediate fixture (2) and two end fixtures. Loading heads are provided on the two intermediate fixtures. The two intermediate fixtures are arranged separately from each other. The loading heads are rigidly connected to the intermediate fixtures. The two loading heads are respectively connected to different loading devices. A displacement sensor is provided at the center point of the beam and on each of the two intermediate fixtures, respectively for measuring the displacement of the beam center and the displacements of the two intermediate fixtures. The fatigue test method includes the following steps: (1) Referring to the requirements of the "Test Procedures for Bitumen and Bituminous Mixtures for Highway Engineering", apply cyclic loads to the beam specimen. When the termination condition is reached, stop loading and record the fatigue life of the beam specimen. (2) Calculate the fatigue test performance indexes, including: The maximum flexural tensile stress, and the calculation formula is where P is the peak load; L represents the beam span, that is, the distance between two end fixtures; w represents the beam width, and h represents the beam height; The maximum flexural tensile strain, the calculation formula is where δ0 represents the maximum displacement at the center of the beam; The flexural stiffness modulus, using the calculation method in the "Test Procedures for Bitumen and Bituminous Mixtures for Highway Engineering". When reaching the state of maximum shear force, the maximum shear stress at the center of the beam is calculated by the formula When reaching the state of maximum shear force, the maximum shear strain at the center of the beam is calculated by the formula where μ represents the Poisson's ratio of the beam, δ1 represents the displacement of the intermediate fixture 1 at the state of maximum shear force, and δ2 represents the displacement of the intermediate fixture 2 at the state of maximum shear force.

2. The fatigue test method according to claim 1, characterized in that: The expression within a single cycle of the cyclic load in step (1) is: where P1 is the load applied on the first intermediate fixture (1), P2 is the load applied on the second intermediate fixture (2), and T is the loading period.

3. The fatigue test method according to claim 2, characterized in that: During actual loading, a strain-controlled loading mode is used, and the magnitude of the target flexural tensile strain is set according to requirements. The loading frequency is 10 Hz. The test termination condition is the number of cyclic loadings corresponding to the flexural stiffness modulus of the beam specimen decreasing to 50% of the initial value.

4. The fatigue test method according to claim 1, wherein: The four fixtures are arranged at equal intervals of one-third on the beam.

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