A method for bending test of semicircular specimen of asphalt mixture

Abstract

The present invention belongs to the technical field of structural component testing. The present invention provides a method for bending test of a semicircular specimen of asphalt mixture. The method derives a semi-analytical solution of the bending tensile stress at the bottom center of the semicircular specimen, and then obtains a new dynamic modulus calculation equation and a destructive strength calculation equation; a new set of test methods for the mechanical properties of asphalt mixture is formed. During the test, it is not necessary to prefabricate cracks in the semicircular specimen, and the original structure of the specimen will not be damaged. The road performance data of the asphalt mixture obtained by the method provided by the present invention is more comprehensive. When the method of the present invention is used in actual projects such as road design and maintenance, the scientificity and rationality of the strategy are guaranteed. More scientific technical guidance and support are provided to optimize maintenance strategies, extend the service life of pavement, improve traffic safety, and reduce accident losses. Thereby bringing significant benefits to society and the economy.

Description

Technical Field

[0001] The invention relates to asphalt mixture mechanical property detection, in particular to a method for asphalt mixture semicircular specimen bending test, and belongs to the technical field of transportation infrastructure construction. Background Art

[0002] In the construction of modern transportation infrastructure, the performance evaluation of asphalt pavement is of vital importance. With the increase in traffic volume and the complexity of pavement conditions, the traditional asphalt pavement core sample analysis method is gradually unable to meet the growing engineering needs. At present, the research work on asphalt pavement core samples is mainly focused on the integrity investigation of core samples and splitting tests. However, the data obtained by these methods are relatively limited, and there is a significant difference between the stress state of the specimens in the splitting test and the actual pavement stress state, which affects the reliability of the evaluation results.

[0003] At present, the estimation of the remaining life of asphalt pavement and the determination of the timing of maintenance and repair require the help of expert experience diagnosis. The evaluation of pavement condition has not completely solved the subjective judgment, and the subjective influence can only be weakened through multi-data analysis. The scientific evaluation of the performance of existing asphalt pavements has become an unavoidable challenge, among which the on-site core sampling and analysis of asphalt pavements is the most direct and objective; however, the traditional asphalt pavement core sample analysis method mainly focuses on the integrity investigation of core samples and splitting experiments. The data obtained by these methods are relatively limited, and there is a significant difference between the stress state of the specimens in the splitting test and the actual pavement stress state, which affects the reliability of the evaluation results.

[0004] In order to more effectively utilize the original pavement core samples to obtain more effective test data, some scholars began to use semicircular bending tests to test the crack resistance of the specimens. However, some limitations were also exposed in practical applications. The dynamic modulus and fatigue life tests were usually not considered during the experiment. Instead, prefabricated crack specimens were used to test the crack resistance of the core samples. This method destroyed the original structure of the specimens and it was difficult to truly reflect their performance. At present, domestic researchers mainly use the destructive strength and fracture energy of prefabricated cracked semicircular specimens under stable vertical displacement as performance evaluation indicators for different asphalt mixture specimens. This experimental method based on prefabricated cracks makes it impossible for existing testing methods to comprehensively and objectively evaluate the actual performance of asphalt pavements, and the amount of test data obtained is relatively small.

[0005] Therefore, a method of testing the semicircular bending dynamic modulus, fatigue life and destructive strength of semi-cylindrical specimens based on actual asphalt pavement was proposed to obtain the comprehensive road performance index of asphalt mixture specimens, which has a high reference value for the prediction and maintenance decision of asphalt pavement. Summary of the invention

[0006] In view of the problem that the existing experimental method based on prefabricated cracks in the prior art makes it impossible for the existing testing means to comprehensively, truly and effectively evaluate the actual performance of asphalt pavements, the present invention innovatively provides a method for bending test of semicircular specimens of asphalt mixtures. The method obtains the comprehensive road performance index of asphalt mixture specimens by conducting semicircular bending dynamic modulus, fatigue life and destructive strength tests on semi-cylindrical specimens with the same gradation and oil-stone ratio as the actual asphalt pavement or cylindrical asphalt mixture core samples retrieved from the actual highway. First, by deriving a bending tensile stress equation at the bottom center of a semicircular specimen, the bending tensile stress equation is used as a semi-analytical solution to obtain a new dynamic modulus calculation equation and a destructive strength calculation equation, forming a new test method for the mechanical properties of asphalt mixtures. During the test, there is no need to prefabricate cracks on the semicircular specimens, and the original structure of the specimens will not be destroyed. It is simpler and more accurate to obtain test data, further ensuring the scientificity and rationality of road design. The road performance of asphalt mixtures obtained by this method is more comprehensive, and potential problems can be identified in advance, maintenance strategies can be optimized, and the service life of the pavement can be extended. By accurately predicting the impact of traffic accidents, the maintenance frequency of asphalt pavement can be reduced, which can significantly reduce maintenance costs, improve traffic safety, and reduce accident losses, thus bringing greater benefits to society and the economy.

[0007] In order to achieve the above technical objectives, the technical solutions adopted by the present invention are as follows:

[0008] A method for a semicircular specimen bending test of an asphalt mixture, the method comprising the following steps:

[0009] I: Based on the short beam wedge effect of the semicircular specimen, the calculation equation for the bending tensile stress at the bottom center of the semicircular specimen under elastic conditions is obtained:

[0010] (1);

[0011] In formula (1), is the bending tensile stress at the bottom center of the semicircular specimen, MPa. P is the vertical load applied to the top of the semicircular specimen, N. L is the axial distance between the two supports at the bottom of the semicircular specimen, mm. b For the load P The thickness of the semicircular specimen is mm. r For the load P The radius of the semicircular specimen, mm.

[0012] It should be noted that the elastic condition refers to the situation where the semicircular specimen will be deformed when subjected to external force, and will return to its original state when the external force disappears. That is, the situation where the deformation of the semicircular specimen after being subjected to force is all recoverable deformation is under the elastic condition; conversely, when the external force is too large and causes the semicircular specimen to produce irreversible deformation, it does not fall within the scope of the elastic condition.

[0013] II: The calculation equation of the bending tensile stress at the bottom center of the semicircular specimen is used as a semi-analytical solution to derive the calculation equation of the dynamic modulus of the semicircular specimen:

[0014] (2);

[0015] In formula (2), E is the dynamic modulus of the semicircular specimen, MPa. is the test gauge length of the displacement sensor at the bottom of the semicircular specimen, mm. is the recoverable deformation within the bottom gauge length of the semicircular specimen, mm.

[0016] III: Perform a bending dynamic modulus test on any semicircular specimen to obtain the bending performance data of the semicircular specimen (including: the vertical load applied on the top of the semicircular specimen P , the axial distance between the two supports at the bottom of the semicircular specimen L , when the load is P The thickness of the semicircular specimen is b , when the load is P The radius of the semicircular specimen is r , test gauge length of displacement sensor at the bottom of semicircular specimen , the recoverable deformation within the bottom gauge length of the semicircular specimen The bending performance data obtained are substituted into the above formula (1) and formula (2) to calculate the bending tensile stress at the bottom center of the semicircular specimen and the dynamic modulus of the semicircular specimen.

[0017] Preferably, the method further comprises:

[0018] IV: The semicircular specimen is subjected to a bending failure test to obtain the failure load of the semicircular specimen, and then the calculation equation of the failure strength of the semicircular specimen is deduced by combining the calculation equation of the bending tensile stress at the bottom center of the semicircular specimen:

[0019] (3).

[0020] In formula (3), is the semicircular bending failure strength, MPa. P max is the vertical load borne by the semicircular specimen when it reaches the failure condition, N. b f For the loadP max The thickness of the semicircular specimen is mm. r f For the load P max The radius of the semicircular specimen, mm.

[0021] Perform a bending failure test on any semicircular specimen to obtain the bending failure performance data of the semicircular specimen (including: the vertical load that the semicircular specimen bears when it reaches the failure condition). P max , the axial distance between the two supports at the bottom of the semicircular specimen L , when the load is P max The thickness of the semicircular specimen is b f , when the load is P max The radius of the semicircular specimen is r f The bending failure performance data is substituted into the formula (3) to calculate the failure strength of the semicircular specimen.

[0022] Preferably, the bending failure test is conducted using a multifunctional asphalt mixture test system (model: UTM-30, brand: Australian IPC, also known as IPC UTM-30); test condition parameters: test temperature is 20±0.5°C, vertical load loading rate is 40~60mm / min; failure condition is that cracks appear in the semicircular specimen or the vertical deformation at the bottom of the semicircular specimen reaches more than 10% of the original vertical thickness of the semicircular specimen. During the experiment, vertical displacement and displacement data of the fixed area at the bottom of the semicircular specimen are collected.

[0023] Preferably, the bending dynamic modulus test is carried out using a multifunctional asphalt mixture testing system (model: UTM-30, brand: Australian IPC, also known as IPC UTM-30); test condition parameters: test temperature is 20±0.5°C, vertical load adopts dynamic cyclic loading method, and the loading waveform is a semi-sine waveform.

[0024] Preferably, the amplitude of the semi-sine waveform is such that the semicircular specimen is subjected to a vertical load of 200-400N, and the period is 5-20 Hz (eg 10 Hz).

[0025] Preferably, the magnitude of the vertical peak load is such that the vertical deformation of the bottom of the semicircular specimen is 0-1 mm, and the vertical valley load is 0.1-0.3 times the vertical peak load.

[0026] Preferably, in step I, the semicircular specimen is obtained by the following method: firstly, a cylindrical specimen of asphalt mixture is obtained, and then the cylindrical specimen is cut along its radial direction to obtain two semicircular specimens of the same size.

[0027] Preferably, the cylindrical specimen is obtained by the following method: first, a core sample is prepared by oneself according to the same gradation and oil-stone ratio as the actual asphalt pavement, or a core sample is obtained by core sampling of the actual asphalt pavement, and then the core sample is layered and cut along the axial direction to obtain at least one cylindrical specimen.

[0028] In the present invention, the diameter of the cylindrical test piece is 80-120 mm, and the column height of the cylindrical test piece is 30-70 mm.

[0029] Preferably, the diameter of the cylindrical test piece is 90-110 mm, and the height of the cylindrical test piece is 40-60 mm.

[0030] Preferably, in step I, the short beam wedge effect of the semicircular specimen is specifically as follows: taking the vertex of the semicircular specimen as the center and the radius of the semicircular specimen as the radius, a circle is drawn in the plane where the radial section of the semicircular specimen is located, thereby dividing the semicircular specimen into an upper part and a lower part; and then determining the horizontal force generated by the wedge action of the upper part of the semicircular specimen through force balance, and inferring the tensile stress and bending stress generated by the horizontal force in the middle cross section of the semicircular specimen (referring to the middle along the axial direction of the semicircular specimen).

[0031] Preferably, when performing a bending dynamic modulus test on a semicircular specimen, an auxiliary tool needs to be attached to the rectangular bottom surface of the semicircular specimen. The auxiliary tool includes a first component and a second component, and both the first component and the second component are of "F" type structure.

[0032] Preferably, the two "F"-shaped first and second components are spliced ​​together into a rectangular component, the solid part of the "F"-shaped auxiliary tool contacts the semicircular specimen, and the displacement sensor is placed in the gap between the two "F"-shaped first and second components.

[0033] In the present invention, when the semicircular specimen is subjected to the bending dynamic modulus test, the semicircular specimen is subjected to the bending dynamic modulus test. Figure 2 The method shown in the figure is to place the load bearing rod, pressure strip, roller, roller support, etc. in accordance with the standard (BS EN 12697-44:2019) equipment. It further includes auxiliary tools (including the first component and the second component), as shown in the figure. Figure 3As shown in the figure, the first component is F-shaped and has a preset circular hole for the displacement sensor to pass through. The second component is also F-shaped but has no circular hole and is used to support the displacement sensor. During the test, the two components are attached to the fixed position at the bottom of the semicircular bend specimen and then the displacement sensor is inserted. During the test, the semicircular bend specimen cracks and the distance between the two components increases. The displacement sensor stretches to measure In addition, it should be noted that the carrier rods, layering strips, rollers, roller supports, etc. are all equipment listed in the specification (BS EN 12697-44:2019) and can be purchased on the market.

[0034] In the present invention, the flexural dynamic modulus test is performed in accordance with BS EN 12697-44:2019 standard.

[0035] In the present invention, the bending dynamic modulus test of the semicircular asphalt mixture specimen includes: cutting the cylindrical asphalt mixture core sample made by ourselves or retrieved from a highway in layers (along the axial direction) (the specific steps are shown in the attached Figure 1 After the core sample is cut, auxiliary tools need to be attached (such as Figure 3 As shown, an "F" type auxiliary tool is used. The specimen is in contact with the surface of the "F" type auxiliary tool, and the displacement sensor is placed in the gap of the "F" type auxiliary tool. The displacement sensor can be better placed to make the test data more accurate), and placed in an environmental box at 20±0.5℃ for a period of time (generally not less than 4h). Then, two semicircular specimens with the same core sample and the same surface layer were subjected to bending and tensile dynamic modulus tests (the semicircular bending dynamic modulus test was carried out in the environmental box of the UTM-30 multifunctional asphalt mixture test system. The test temperature was controlled at 20±0.5℃, and the vertical load adopted a dynamic cyclic loading method. The loading waveform was a semi-sine waveform. Considering that the stress at the bottom of the semicircular bending test specimen is mainly horizontal tensile stress, the vertical load is set to make the vertical deformation of the bottom of the semicircular specimen 0~1mm (or the strain is the displacement measured by the sensor divided by the test gauge length of the displacement sensor at the bottom of the semicircular specimen (for example, 50mm) multiplied by 10 6 The dimensionless microstrain ); the valley load is generally 0.1 to 0.3 times the peak load (for example, 0.1 times). Further, a semicircular specimen with a larger dynamic modulus is selected for dynamic fatigue life testing to determine its fatigue life under a specific load until a through crack appears. Finally, a static destructive test is performed on another semicircular specimen from the same core sample and the same layer (i.e., with a relatively small dynamic modulus) until a crack appears in the semicircular specimen or the vertical deformation at the bottom of the semicircular specimen reaches more than 10% of the original vertical thickness of the semicircular specimen.

[0036] In the present invention, the stress distribution at the bottom of the semicircular specimen is explored through bending test, which is of great significance for theoretical analysis and practical application. Considering that there are significant differences between the semicircular specimen and the ordinary beam component, its height-to-span ratio is greater than that of the ordinary beam, and it belongs to a short beam. The load point of the short beam is close to any point inside the specimen, so the stress will be highly concentrated and irregularly distributed. Therefore, a semi-analytical solution for the stress distribution at the bottom of the semicircular specimen is proposed to explore the stress distribution of the semicircular specimen under vertical load. P The stress distribution at the bottom center of the specimen during the test under the action of force. That is, the method for the bending test of the semicircular specimen of asphalt mixture described in the present invention is a new method for numerical simulation and testing on a derived semi-analytical solution. A semi-analytical solution for the dynamic modulus is first derived, and the reliability of this new formula is obtained through finite element simulation comparison. The data processing of the subsequent fatigue test and semicircular bending failure test are all carried out on the basis of this derived semi-analytical solution. No prefabricated cracks are required during the test, and the original structure of the specimen will not be damaged. It is simpler and more accurate to obtain the test data, and the obtained road performance of the asphalt mixture is more comprehensive.

[0037] In the present invention, the derivation process of the semi-analytical solution includes: referring to Timoshenko S. Mechanics of materials: 1st Edition [M]. WANG Yilin, Translation. 3rd Edition. Beijing: Science Press, 1964. for the calculation method of bending tensile stress in short beams, and obtaining the vertical load P The stress distribution at the bottom center of the semicircular bending and tensile specimen under the action of the short beam wedge effect is first analyzed by isolating the semicircular specimen to determine the force generated by the short beam wedge effect. F , as attached Figure 4 As shown, the vertex of the semicircular specimen is taken as the center and the radius is r The dotted arc divides the semicircular specimen into two parts, and the force balance is used to determine F The size of is as shown in formula (1-1):

[0038] (1-1).

[0039] In formula (1-1), P is the vertical load applied to the top of the semicircular specimen, N.

[0040] Furthermore, we determine the force F Tensile stress generated in the middle cross section of the semicircular specimen , bending stress As shown in formula (1-2) and (1-3) respectively:

[0041] (1-2).

[0042] (1-3).

[0043] The calculation formula of bending stress of ordinary beam is , The sum of the two is the bending tensile stress at the bottom center of the semicircular specimen considering the short beam effect. The calculation equation is as follows:

[0044] (1).

[0045] In formula (1), is the bending tensile stress at the bottom center of the semicircular specimen, MPa. P is the vertical load applied to the top of the semicircular specimen, N. L is the axial distance between the two supports at the bottom of the semicircular specimen, mm. b For the load P The thickness of the semicircular specimen is mm. r For the load P The radius of the semicircular specimen, mm.

[0046] In the prior art, modulus is the ratio of material stress to strain. Under dynamic load conditions, materials produce corresponding dynamic deformation responses under periodic or non-periodic dynamic loads. Load or stress changes with time and is a function of time; the corresponding deformation or strain response is also a function of time; the ratio of stress to strain is the dynamic modulus, and therefore also a function of time. The mathematical relationship between them is: E ( t )= σ ( t ) / ε ( t ). In the present invention, the mathematical relationship between stress and strain that defines the dynamic modulus is combined with equation (1) to obtain a new calculation equation for the dynamic modulus of the semicircular specimen of the present invention, which is as follows:

[0047] (2-1);

[0048] (2-2);

[0049] Combining equation (1), equation (2-1) and equation (2-2), we can get:

[0050] (2).

[0051] In formula (2), E is the dynamic modulus of the semicircular specimen, MPa. is the test gauge length of the displacement sensor at the bottom of the semicircular specimen, mm. is the recoverable deformation within the bottom gauge length of the semicircular specimen, mm. ε For strain.

[0052] In the present invention, a three-dimensional elastic bending test model is established by finite element software and the reliability of the semi-analytical solution (i.e., Formula 1) and the new calculation equation of the dynamic modulus (i.e., Formula 2) is verified. The three-dimensional elastic semicircular bending test model is established for verification: first, two semicircular specimens with different column heights (i.e., thicknesses) are three-dimensionally modeled. Taking the semicircular model with a column height of 40 mm as an example, as shown in the attached figure, Figures 5 and 6 As shown, the top compression rod of the model has two degrees of freedom, upper and lower, while the support bearing at the bottom is only allowed to rotate, and both are rigid bodies. During the simulation, the same load as the indoor bending test is applied to the upper surface of the compression rod at the top of the specimen, and finally we will obtain the stress and strain distribution response data of the lower surface of the semicircular specimen under the corresponding load. The present invention extracts and analyzes the deformation data within a certain range (e.g., 50mm range) of the bottom gauge length of the specimen. By comparing the finite element simulation results and the central bending tensile stress values ​​of the bottom of the specimen under different vertical loads in formula (1), it is finally found that there is good similarity between the two, and the error is relatively small (the error is between 6 and 8%).

[0053] Furthermore, under the same load conditions, we continuously adjusted the Young's modulus value in the finite element model to make the deformation at the bottom of the model specimen consistent with the indoor bending test. Through this adjustment, we found that the input Young's modulus value and the dynamic modulus value calculated by formula (2) are very similar (e.g. Figure 8 The difference between the two formulas is very small (all within 3%). This shows that the dynamic modulus value obtained by formula (2) has a high accuracy. Therefore, it can be confirmed that under elastic conditions, the bottom bending tensile stress formula and dynamic modulus formula of the semicircular bending specimen proposed in the present invention are reasonable and effective. At the same time, the dynamic modulus value data obtained by this method provides an important reference for asphalt pavement design, which will significantly improve the accuracy and reliability of the design.

[0054] In the present invention, a bending failure test is performed on a semicircular specimen of asphalt mixture to obtain the failure load of the semicircular specimen, and then the calculation equation of the semicircular specimen failure strength is deduced by combining the calculation equation of the bending tensile stress at the bottom center of the semicircular specimen, and then the calculation equation of the semicircular specimen failure strength is obtained, which is specifically shown as follows:

[0055] (3).

[0056] In formula (3), is the semicircular bending failure strength, MPa. P max is the vertical load borne by the semicircular specimen when it reaches the failure condition, N. b f For the load P max The thickness of the semicircular specimen is mm. r f For the load P max The radius of the semicircular specimen, mm.

[0057] It should be noted that the semicircular specimen destructive strength calculation equation proposed in the present invention is consistent with the destructive strength calculation formula used in the European standard (EN12697-44-2019) ( , D is the specimen diameter), the failure strength calculation formula used in the European standard does not take into account the short beam effect, and the calculated failure strength value has a larger error fluctuation than the actual value (such as Figures 15 and 16 As shown in the figure: the destructive strength value calculated by the destructive strength calculation formula used in the European standard is 0.25~1.25 times that of the formula proposed in the present invention), and the calculation formula mentioned in the European standard only considers the load size and the specimen size, while the formula proposed in the present invention can effectively consider the experimental measured data of the size of the destructive deformation in the experiment, that is, the present invention shows higher accuracy and applicability.

[0058] In the present invention, it should be noted that all the formulas in the present invention are obtained by fitting by the inventors based on experiments and engineering applications, and all calculations are numerical values ​​converted according to prescribed units, which are obtained by substituting the numerical values ​​after the conversion into the formulas (after converting the units, only the numerical values ​​are substituted into the formulas for calculation, without substituting the units, and the units are only used to adjust the size of the numerical values).

[0059] In the present invention, the bending dynamic modulus test is performed according to the BS EN 12697-44:2019 standard. The UTM-30 multifunctional asphalt mixture test system is a prior art. The equipment used for the bending performance test of the semicircular specimen (such as a load bar, a pressure strip, a roller, a roller support, etc.) are also commercially available products that comply with the existing specifications (BS EN 12697-44:2019).

[0060] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0061] 1. The new semicircular bending test method of the present invention is to derive a semi-analytical solution of the bending tensile stress at the bottom center of a semicircular specimen, thereby obtaining a new dynamic modulus calculation equation and a new destructive strength calculation equation, and then form a new test method for the mechanical properties of asphalt, which can identify potential problems in advance, optimize maintenance strategies, extend the service life of asphalt pavement, and ensure the scientificity and rationality of road design.

[0062] 2. The new semicircular bending test method of the present invention is simple to operate. During the test, there is no need to prefabricate cracks in the semicircular specimen, and the original structure of the specimen will not be damaged. The test data obtained is simpler and more accurate, and the road performance of the asphalt mixture finally obtained is closer to the actual road conditions.

[0063] 3. Through rationality analysis, the technical solution provided by the present invention can not only more realistically reflect the bearing capacity and failure mode of each layer of asphalt mixture under actual working conditions, but also provide an important basis for evaluating the stability and durability of the pavement structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 It is a schematic diagram of the preparation process of the semicircular specimen of asphalt mixture of the present invention.

[0065] Figure 2 It is a schematic diagram of the structure of the loading device for the semicircular specimen of the present invention.

[0066] Figure 3 It is a schematic structural diagram of the auxiliary tool for the loading device of the present invention.

[0067] Figure 4 It is a schematic diagram of the wedge effect generated inside the semicircular specimen during isolation analysis of the present invention.

[0068] Figure 5 Schematic diagram of the partitioning of the three-dimensional elastic bending test model of the semicircular specimen established using finite element software.

[0069] Figure 6 Schematic diagram of mesh division for establishing a three-dimensional elastic bending test model of a semicircular specimen using finite element software.

[0070] Figure 7 This is a diagram showing the maximum bending tensile stress relationship at the bottom center of the specimen obtained by the present invention using finite element simulation and formula calculation.

[0071] Figure 8 This is a dynamic modulus relationship diagram obtained by the present invention using finite element simulation and formula calculation.

[0072] Fig. 9 This is a relationship diagram between vertical load and bottom microstrain in the fatigue life test of the present invention.

[0073] Fig.10 This is a relationship diagram between the number of load actions, the vertical load, and the deformation of the bottom of the specimen in the fatigue test of the present invention.

[0074] Fig.11 for Fig.10 The enlarged view of the 996th to 1000th load application.

[0075] Fig.12 This is a relationship diagram between the dynamic modulus and the number of load cycles of different core sample specimens during fatigue testing of the present invention.

[0076] Fig.13 for Fig.12 The legend for the figure shows an enlarged image.

[0077] Fig.14 This is a comparison chart of fatigue life of surface layer specimens of different core samples of the present invention.

[0078] Fig.15 This is a comparison diagram of the European standard's destructive strength calculation formula and the present invention's destructive strength calculation equation (40 mm specimen).

[0079] Fig.16 The figure is a comparison result diagram of the European standard's destructive strength calculation formula and the present invention's destructive strength calculation equation (50mm test piece).

[0080] Fig.17 Comparison diagram of load-displacement curves of semicircular destruction test and splitting test. DETAILED DESCRIPTION

[0081] The technical solution of the present invention is illustrated below by way of example, and the scope of protection requested by the present invention includes but is not limited to the following embodiments. Example 1

[0082] I: Based on the short beam wedge effect of the semicircular specimen, the calculation equation for the bending tensile stress at the bottom center of the semicircular specimen under elastic conditions is obtained:

[0083] (1).

[0084] In formula (1), is the bending tensile stress at the bottom center of the semicircular specimen, MPa. P is the vertical load applied to the top of the semicircular specimen, N. L is the axial distance between the two supports at the bottom of the semicircular specimen, mm. b For the load P The thickness of the semicircular specimen is mm. r For the load P The radius of the semicircular specimen, mm.

[0085] II: The calculation equation of the bending tensile stress at the bottom center of the semicircular specimen is used as a semi-analytical solution to derive the calculation equation of the dynamic modulus of the semicircular specimen:

[0086] (2).

[0087] In formula (2), E is the dynamic modulus of the semicircular specimen, MPa. is the test gauge length of the displacement sensor at the bottom of the semicircular specimen, mm. is the recoverable deformation within the bottom gauge length of the semicircular specimen, mm.

[0088] III: Perform a bending dynamic modulus test on any semicircular specimen to obtain the bending performance data of the semicircular specimen. Substitute the obtained bending performance data into the above formula (1) and formula (2) to calculate the bending tensile stress at the bottom center of the semicircular specimen and the dynamic modulus of the semicircular specimen. Example 2

[0089] A method for a semicircular specimen bending test of an asphalt mixture, the method comprising the following steps:

[0090] I: Based on the short beam wedge effect of the semicircular specimen, the calculation equation for the bending tensile stress at the bottom center of the semicircular specimen under elastic conditions is obtained:

[0091] (1).

[0092] In formula (1), is the bending tensile stress at the bottom center of the semicircular specimen, MPa. P is the vertical load applied to the top of the semicircular specimen, N. L is the axial distance between the two supports at the bottom of the semicircular specimen, mm. b For the load P The thickness of the semicircular specimen is mm. r For the load P The radius of the semicircular specimen, mm.

[0093] II: The calculation equation of the bending tensile stress at the bottom center of the semicircular specimen is used as a semi-analytical solution to derive the calculation equation of the dynamic modulus of the semicircular specimen:

[0094] (2).

[0095] In formula (2), E is the dynamic modulus of the semicircular specimen, MPa. is the test gauge length of the displacement sensor at the bottom of the semicircular specimen, mm. is the recoverable deformation within the bottom gauge length of the semicircular specimen, mm.

[0096] III: Perform a bending dynamic modulus test on any semicircular specimen to obtain the bending performance data of the semicircular specimen. Substitute the obtained bending performance data into the above formula (1) and formula (2) to calculate the bending tensile stress at the bottom center of the semicircular specimen and the dynamic modulus of the semicircular specimen.

[0097] IV: The semicircular specimen is subjected to a bending failure test to obtain the failure load of the semicircular specimen, and then the calculation equation of the failure strength of the semicircular specimen is deduced by combining the calculation equation of the bending tensile stress at the bottom center of the semicircular specimen:

[0098] (3).

[0099] In formula (3), is the semicircular bending failure strength, MPa. P max is the vertical load borne by the semicircular specimen when it reaches the failure condition, N. b f For the load P max The thickness of the semicircular specimen is mm. r f For the load P max The radius of the semicircular specimen, mm. Example 3

[0100] Example 2 was repeated, except that the bending failure test was performed using the UTM-30 multifunctional asphalt mixture test system. Test condition parameters: the test temperature was 20±0.5°C, and the loading rate of the vertical load was 40~60mm / min. The failure condition was that cracks appeared on the semicircular specimen or the vertical deformation at the bottom of the semicircular specimen reached more than 10% of the original thickness of the semicircular specimen. During the experiment, the vertical displacement and the displacement data of the fixed area at the bottom of the semicircular specimen were collected. Example 4

[0101] Example 3 was repeated, except that the bending dynamic modulus test was performed using the UTM-30 multifunctional asphalt mixture test system. Test condition parameters: the test temperature was 20±0.5°C, the vertical load was dynamic cyclic loading, and the loading waveform was a semi-sine waveform. The amplitude of the semi-sine waveform was such that the semicircular specimen was subjected to a vertical load of 200~400N, and the period was 10 Hz. Example 5

[0102] Example 4 was repeated, except that under elastic conditions, the magnitude of the vertical peak load was such that the vertical deformation of the bottom of the semicircular specimen was 0.05 μm, and the vertical valley load was 0.12 times the vertical peak load. Example 6

[0103] Example 5 is repeated, except that in step I, the semicircular specimen is obtained by the following method: first, a cylindrical specimen of asphalt mixture is obtained, and then the cylindrical specimen is cut along its radial direction to obtain two semicircular specimens of the same size. Example 7

[0104] Example 6 was repeated, except that the cylindrical specimens were obtained by the following method: a core specimen was first prepared according to the same gradation and oil-stone ratio as the actual asphalt pavement, and then the core specimen was cut in layers along the axial direction to obtain a plurality of cylindrical specimens with a diameter of 100 mm and column heights of 40 mm and 50 mm, respectively. Example 8

[0105] Example 7 is repeated, except that in step I, the short beam wedge effect of the semicircular specimen is specifically as follows: with the vertex of the semicircular specimen as the center and the radius of the semicircular specimen as the radius, a circle is drawn in the plane where the radial section of the semicircular specimen is located, thereby dividing the semicircular specimen into upper and lower parts, and then the horizontal force generated by the wedge action of the upper part of the semicircular specimen is determined by force balance, and the tensile stress and bending stress generated by the horizontal force in the middle cross section of the semicircular specimen are deduced.

[0106] Application Examples

[0107] The semicircular specimen bending test was carried out on the asphalt mixture using the method described in Example 8:

[0108] The asphalt mixture core sample with a diameter of 100 mm retrieved from a highway was cut into layers (see the attached Figure 1 : First, cut the cylindrical core sample with a diameter of 100mm and a height of 180mm from top to bottom into small cylinders with heights of 40mm, 60mm, and 80mm, respectively. Then, cut off 10mm and 30mm from the small cylinders of 60mm and 80mm, respectively, to obtain two small cylinders with a height of 50mm. Finally, cut the three small cylinders with heights of 40mm, 50mm, and 50mm along their radial directions to obtain two semicircular specimens with a height of 40mm and four semicircular specimens with a height of 50mm. Then, place an "F" type auxiliary tool (such as the one attached) on each semicircular specimen. Figure 3 As shown in the figure, the "F" type auxiliary tool refers to a pair of auxiliary fixing plates with two projected geometric surfaces in the shape of "F", and a through hole for the displacement sensor to pass through is opened on one of the auxiliary fixing plates. The bottom surface of the semicircular specimen contacts the surface of the "F" type auxiliary tool, and the displacement sensor is placed in the gap of the "F" type auxiliary tool), and then placed in a 20℃ environmental box for treatment for 6 hours. After the treatment is completed, the following process is carried out:

[0109] First, the semicircular bending dynamic modulus test was carried out on each semicircular specimen using the environmental chamber of the UTM-30 multifunctional asphalt mixture test system (the semicircular specimen was installed as follows during the test). Figure 2 As shown: rollers as supports are placed on both sides of the bottom surface of the semicircular specimen, and a pressure strip in contact with the loading rod is installed on the top of the arc surface of the semicircular specimen. The bottom surface of the semicircular specimen between the two rollers is fixed with an "F" type auxiliary tool and the displacement sensor is installed on the "F" type auxiliary tool): the test temperature is controlled at 20±0.5℃, the vertical load adopts dynamic cyclic loading, the loading waveform is a semi-sine waveform, the vertical peak load is to make the bottom of the specimen produce 0.05μm deformation, and the valley load is 0.1 times the peak load. Real-time recording of test process data (the measured vertical peak load is about P =150N, the axial distance between the two supports at the bottom of the semicircular specimen L= 80mm, the test gauge length of the displacement sensor at the bottom of the semicircular specimen is =50mm, the recoverable deformation within the bottom gauge length of the semicircular specimen = 1.25×10 -3 mm).

[0110] The bending tensile stress at the bottom center of the 40 mm semicircular specimen is calculated by formula (1) to be approximately 0.158 MPa; the bending tensile stress at the bottom center of the 50 mm semicircular specimen is approximately 0.127 MPa.

[0111] The dynamic modulus of the 40 mm semicircular specimen was calculated to be 6320 MPa by formula (2), and the dynamic modulus of the 50 mm semicircular specimen was 5080 MPa.

[0112] Afterwards, the semicircular specimen with the larger dynamic modulus of the two semicircular specimens with the same core sample and the same surface layer was selected for dynamic fatigue life test: the test loading device and ambient temperature were consistent with the semicircular bending dynamic modulus test. The loading waveform adopted a semi-sine waveform with a frequency of 10Hz. The stress response results of the asphalt pavement under the action of standard vehicle load were calculated by finite element simulation. The results showed that the tensile stress at a depth of 40mm below the pavement was about 332.6kPa. In order to shorten the fatigue test time, the load value required to apply 3 times the standard vehicle load bending tensile stress at the bottom center of the specimen was determined by calculation formula (1). Finally, it was determined that the peak load of the 40mm high specimen was about 941.34N, and the peak load of the 50mm high specimen was about 1176.6N. The valley load was 0.1 times the peak load. The test end condition was the appearance of a through crack in the semicircular specimen. The test process data was recorded in real time.

[0113] Finally, a static destructive test was conducted on the semicircular specimen with the smaller dynamic modulus from the same core sample and the same surface layer: the test loading device and ambient temperature were consistent with the semicircular bending dynamic modulus test. The loading rate of the vertical load was set to 50 mm / min. The failure condition of the test was set to the vertical load dropping to 20% of the maximum load during the test. The test process data was recorded in real time.

[0114] Reliability verification of formula (1) and formula (2): Verification is carried out by establishing a three-dimensional elastic semicircular bending test model. Finite element software is used to establish a three-dimensional elastic semicircular bending test model with a thickness of 40 mm and 50 mm respectively. Figures 5 and 6 The partitioning and meshing of the 40 mm thick model are demonstrated: that is, the half-circle specimen is first divided into multiple blocks, and then each block is meshed. The top compression rod of the model has two degrees of freedom, upper and lower, while the bottom support bearing is only allowed to rotate, and both are rigid bodies. During the simulation, the same load as the indoor test is applied to the upper surface of the compression rod at the top of the specimen, and the upper surface size of the compression rod is 10 mm × 50 mm. Finally, we will obtain the stress and strain distribution response data of the lower surface of the specimen under the corresponding load. We extracted and analyzed the deformation data within the range of 50 mm at the bottom of the specimen. By comparing the finite element simulation results and the central bending tensile stress values ​​of the bottom of the specimen under different vertical loads in formula (1), it can be found that there is good similarity between the two, and the error is relatively small, ranging from 6% to 8% (for details, see Figure 7 : From the error result curve of the formula calculation of the semicircular specimen with a thickness of 50mm in the figure, it can be seen that the error fluctuation range is between 6% and 8%).

[0115] Then, under the same load conditions, the Young's modulus value in the finite element model was continuously adjusted to make the deformation of the bottom of the model specimen consistent with the indoor test. Through this adjustment, we found that the difference between the input Young's modulus value and the dynamic modulus value calculated by formula (2) was very small, both within 3% (see Figure 8 : Whether it is a 40 mm or 50 mm semicircular specimen, the difference in the results is within 3%). This shows that the dynamic modulus value obtained by formula (2) has a high accuracy.

[0116] Using the dynamic modulus calculation formula (2) for semicircular specimens proposed in this paper, semicircular bending dynamic modulus tests were carried out on field drilled core samples of real highway asphalt pavement. The dynamic modulus data of different sampling pile numbers and each layer of asphalt pavement are shown in Table 1, where "Left" and "Right" are used to indicate the direction of the lane, "Left" represents the down lane, and "Right" represents the up lane. The sampling points are all located on the driving lane.

[0117] Table 1 Bending and tensile dynamic modulus of each layer of asphalt mixture of core samples

[0118]

[0119] It can be seen that the test data can show the dynamic modulus change characteristics of each layer of the asphalt pavement, and no abnormal data appears, which also proves the rationality and effectiveness of the test method.

[0120] Application of formula (2) in fatigue life test: Referring to the dynamic compression modulus data processing method of T0738 in JTG E20-2011 "Testing Procedures for Asphalt and Asphalt Mixtures in Highway Engineering", the data curves of vertical load and horizontal strain within the gauge range of the bottom of the specimen in the last five loading cycles under dynamic fatigue life test experimental conditions are extracted and linearly fitted (e.g. Fig. 9 Shown: Goodness of fit R 2 The fitting correlation R 2 ≥0.95, the fitting curve passes through the origin, and then the dynamic modulus of the specimen is calculated using formula (2).

[0121] In the dynamic fatigue life test of the semicircular specimen, we recorded the vertical load and the deformation data of the specimen. By analyzing these data, we can gain a deeper understanding of the fatigue characteristics of the specimen under cyclic load. In the semicircular bending dynamic fatigue test experiment, the relationship between the number of load actions and the horizontal deformation of the bottom of the specimen is as follows: Figures 10-11 As shown in Figure 1. Fatigue tests are generally time-consuming and require a large amount of data. Therefore, in this test, the data of every 1000 load cycles are divided into a group, and the test data of the 996th to 1000th cycles are extracted (as shown in Figure 1. Fig.11 The following table shows the enlarged view of the test data from the 996th to 1000th cycles. The main reason for selecting the last 5 cycles in each set of data is that they can provide representative data near the end of the fatigue test. Based on the large amount of data and the processing accuracy requirements, we extracted the following key indicators in each set of data: peak load, peak specimen bottom deformation, valley load and valley specimen bottom deformation, and calculated the load difference of a single cycle Δ P , recoverable deformation , and its average value within 5 cycles. In addition, the dynamic modulus value of the semicircular specimen at this time can be calculated using formula (2). By performing the above processing on all data groups, we finally obtained the curve of the relationship between the dynamic modulus of the semicircular specimens with different surface layers and the number of load cycles during the fatigue life test (such as Fig.12As shown in the figure: the relationship curve between the dynamic modulus and the number of load cycles of the upper, middle and lower specimens of K433+910-Left, K458+700-Right, K460+020-Left, K463+520-Right, K474+260-Left, K476+770-Left, K478+925-Right, etc., which clearly reveals the relationship between the dynamic modulus and cyclic load of different specimens, fully demonstrating the significant advantages of this method. More importantly, combined with the revised dynamic modulus formula (i.e., formula (2)), this method significantly improves the calculation accuracy of the dynamic modulus value. This makes our understanding of the fatigue characteristics of the material more comprehensive, and also provides more accurate reference data for asphalt pavement design, thereby effectively improving the scientificity and reliability of the design.

[0122] In order to determine the semicircular bending fatigue life of asphalt mixture, it is necessary to give the standard of fatigue failure. Fig.12 It can be seen that the dynamic modulus of the semicircular bending fatigue test has indeed deteriorated significantly. E Down to the initial value E 0 1 / 2 of the fatigue failure standard is used, and this test will also be carried out according to this standard. Fig.11 The fatigue data in the paper were further sorted and analyzed, and the semicircular bending fatigue life of each layer of asphalt mixture of the drilled core samples with different pile numbers was obtained as shown in Table 2 (the corresponding columnar comparison chart is shown in Fig.13 as shown).

[0123] Table 2 Bending and tensile fatigue life of each layer of asphalt mixture of core samples

[0124]

[0125] Therefore, it can be confirmed that the bottom center bending tensile stress formula (1) and dynamic modulus formula (2) of the semicircular bending specimen under elastic conditions are reasonable and effective. The dynamic modulus and fatigue life data obtained by this method provide an important reference for asphalt pavement design, which can significantly improve the accuracy and reliability of the design.

[0126] Verification of formula (3) (i.e., semicircular specimen failure strength formula): Under the same asphalt mixture material conditions, the present invention compares and analyzes the newly proposed semicircular bending failure strength formula (3) with the domestic standard splitting strength. The specimens were made using a rotary compactor: 6 cylindrical specimens with a diameter of 100 mm and a height of 150 mm were made, and then the redundant parts with unstable compaction at both ends were cut off to avoid the deviation of the experimental results caused by different compaction degrees. Finally, 6 splitting specimens with a height of 63.5±0.5 mm and 12 semicircular specimens with a thickness of 40.0±0.5 mm were obtained. The asphalt mixture gradation is AC-13, and the selected materials include SBS modified asphalt, basalt aggregate, and limestone powder. The amounts of each material required to complete a rotary compaction sample are: SBS modified asphalt: 318.5g, basalt aggregate: 6149g, limestone powder: 351g; According to the specification (BS EN 12697-44:2019), a specimen with a diameter of 150mm and a height of 170 is first made, and then cut into cylindrical specimens with a diameter of 100mm and a height of 150mm by a cutting machine, and finally cut into semicircular specimens.

[0127] Table 3 shows the grading data of basalt aggregate and limestone powder

[0128]

[0129] The semicircular bending failure test and the splitting test were carried out on the splitting specimens and the semicircular specimens prepared above (the test temperature was controlled at 20°C) to obtain the load-displacement curves of the semicircular bending failure test and the splitting test (such as Fig.17 As shown: The load-displacement relationship curves of each split specimen and each semicircular specimen): The two experimental data show good agreement in the elastic stage. The semicircular specimen bending test data are processed according to formula (3), and the comparison results are shown in Table 4:

[0130] Table 4 Comparison of semicircular bending failure strength and domestic standard splitting strength (20℃)

[0131]

[0132] It can be seen that the ratio of the bending failure strength to the splitting strength of the semicircular specimens is between 3.3 and 3.6 times, with an average ratio of 3.4 and a standard deviation of 0.016, indicating that there is a significant proportional relationship between the two, further confirming the effectiveness of the new semicircular bending failure strength formula (3) as a material strength assessment method. The semicircular bending failure strength can not only more realistically reflect the bearing capacity and failure mode of each layer of asphalt mixture under actual working conditions, but also provide an important basis for evaluating the stability and durability of pavement structures.

[0133] The bending failure strength of the semicircular specimen can not only more truly reflect the bearing capacity and failure mode of each layer of asphalt mixture under actual working conditions, but also provide an important basis for evaluating the stability and durability of the pavement structure. Using the semicircular specimen bending formula (3) proposed in the present invention, the bending failure strength of the semicircular specimens of each layer of asphalt mixture of the pavement core samples with different pile numbers was tested. The test results are shown in Table 5:

[0134] Table 5 Semicircular bending failure strength of different surface layers of each core sample

[0135]

[0136] The semicircular bending failure strength of each layer of the pavement core sample can effectively analyze the damage degree of different road sections and different surface layers. The above data in Table 5 will help guide the future pavement maintenance design optimization and material selection, thereby improving the overall performance of asphalt pavement and extending its service life.

[0137] In summary, the present invention proposes a method for asphalt mixture semicircular specimen bending test through theoretical derivation, numerical analysis and comparative tests:

[0138] (1) The calculation formula for the central stress at the bottom of the semicircular specimen was derived, which provided a semi-analytical solution for the calculation of the dynamic modulus. The results were in good agreement with the finite element analysis.

[0139] (2) The bending dynamic modulus of the semicircular specimen is reduced to half of the initial bending dynamic modulus as the fatigue failure standard, which can obtain a stable bending dynamic modulus degradation law, and the fatigue life data results are reasonable and effective.

[0140] (3) A formula for the bending failure strength of a semicircular specimen was proposed, and a comparative test was used to calibrate that the bending failure strength of a semicircular specimen is approximately 3.4 times the splitting failure strength, which can be used to evaluate the tensile properties of asphalt mixtures.

Claims

1. A method for a semicircular specimen bending test of an asphalt mixture, characterized in that: The method comprises the following steps: I: Based on the short beam wedge effect of the semicircular specimen, the calculation equation for the bending tensile stress at the bottom center of the semicircular specimen under elastic conditions is obtained: （1）； In formula (1), is the bending tensile stress at the bottom center of the semicircular specimen, MPa; P is the vertical load applied on the top of the semicircular specimen, N; L is the axial distance between the two supports at the bottom of the semicircular specimen, mm; b For the load P The thickness of the semicircular specimen, mm; r For the load P The radius of the semicircular specimen, mm; II: The calculation equation of the bending tensile stress at the bottom center of the semicircular specimen is used as a semi-analytical solution to derive the calculation equation of the dynamic modulus of the semicircular specimen: （2）； In formula (2), E is the dynamic modulus of the semicircular specimen, MPa; is the test gauge length of the displacement sensor at the bottom of the semicircular specimen, mm; is the recoverable deformation within the bottom gauge length of the semicircular specimen, mm; III: Perform a bending dynamic modulus test on any semicircular specimen to obtain the bending performance data of the semicircular specimen; substitute the obtained bending performance data into the above formula (1) and formula (2) to calculate the bending tensile stress at the bottom center of the semicircular specimen and the dynamic modulus of the semicircular specimen.

2. The method according to claim 1, characterized in that: The method further includes: IV: The semicircular specimen is subjected to a bending failure test to obtain the failure load of the semicircular specimen, and then the calculation equation of the failure strength of the semicircular specimen is deduced by combining the calculation equation of the bending tensile stress at the bottom center of the semicircular specimen: （3）； In formula (3), is the semicircular bending failure strength, MPa; P max is the vertical load borne by the semicircular specimen when it reaches the failure condition, N; b f For the load P max The thickness of the semicircular specimen, mm; r f For the load P max The radius of the semicircular specimen, mm; A bending failure test is performed on any semicircular specimen to obtain the bending failure performance data of the semicircular specimen, and the obtained bending failure performance data is substituted into the formula (3) to calculate the failure strength of the semicircular specimen.

3. The method according to claim 2, characterized in that: The bending failure test is carried out using a multifunctional asphalt mixture testing system; the test condition parameters are: the test temperature is 20±0.5°C, the loading rate of the vertical load is 40~60mm / min; the failure condition is that cracks appear on the semicircular specimen or the vertical deformation at the bottom of the semicircular specimen reaches more than 10% of the original vertical thickness of the semicircular specimen; during the experiment, the vertical displacement and the displacement data of the fixed area at the bottom of the semicircular specimen are collected.

4. The method according to any one of claims 1 to 3, characterized in that: The bending dynamic modulus test is carried out using a multifunctional asphalt mixture test system; the test condition parameters are: the test temperature is 20±0.5°C, the vertical load adopts a dynamic cyclic loading method, and the loading waveform is a semi-sine waveform.

5. The method according to claim 4, characterized in that: Under elastic conditions, the magnitude of the vertical peak load is such that the vertical deformation of the bottom of the semicircular specimen is 0~1mm, and the vertical valley load is 0.1~0.3 times the vertical peak load.

6. The method according to any one of claims 1 to 3 and 5, characterized in that: In step I, the semicircular specimen is obtained by the following method: firstly, a cylindrical specimen of asphalt mixture is obtained, and then the cylindrical specimen is cut along its radial direction to obtain two semicircular specimens of the same size.

7. The method according to claim 6, characterized in that: The cylindrical specimen is obtained by the following method: first, a core sample specimen is obtained by self-production according to the same gradation and oil-stone ratio of an actual asphalt pavement or a core sample is obtained by core sampling an actual asphalt pavement, and then the core sample specimen is layered and cut along the axial direction to obtain at least one cylindrical specimen; the diameter of the cylindrical specimen is 80-120 mm; the column height of the cylindrical specimen is 30-70 mm.

8. The method according to claim 7, characterized in that: The diameter of the cylindrical specimen is 90~110mm; the column height of the cylindrical specimen is 40~60mm.

9. The method according to any one of claims 1-3, 5, 7-8, characterized in that: In step I, the short beam wedge effect of the semicircular specimen is specifically as follows: with the vertex of the semicircular specimen as the center and the radius of the semicircular specimen as the radius, a circle is drawn in the plane where the radial section of the semicircular specimen is located, thereby dividing the semicircular specimen into upper and lower parts. After that, the horizontal force generated by the wedge action of the upper part of the semicircular specimen is determined by force balance, and the tensile stress and bending stress generated by the horizontal force in the middle cross section of the semicircular specimen are deduced.

10. The method according to claim 1, characterized in that: When performing a bending dynamic modulus test on a semicircular specimen, an auxiliary tool needs to be pasted on the rectangular bottom surface of the semicircular specimen; the auxiliary tool includes a first component and a second component, and both the first component and the second component are of "F" type structure.

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