CN102072840B - Method for testing shear fatigue of asphalt mixture - Google Patents

Abstract

The invention discloses a method for testing the shear fatigue of an asphalt mixture, and provides the method for testing the shear fatigue by applying periodically repeated load of control stress to an inner steel column through an outer steel column in a fixed coaxial shear testing device by combining loading characteristics of the conventional fatigue test of the asphalt mixture and an implementation mode of shearing of the mixture in the coaxial shear test method. The method is suitable for the asphalt mixture which is newly mixed in a laboratory and can also be used for evaluating the shear fatigue performance of the asphalt mixture in an actual pavement, so road workers can conveniently further investigate a rule for producing shear flow deformation of an asphalt pavement and a mechanism for damaging the shear fatigue of the asphalt mixture under the action of vehicle repeated load, the conventional asphalt mixture performance evaluation system is perfected, and a test analysis method can also be provided for the optimal design of the asphalt mixture based on the shear fatigue resistance performance.

Description

Test Method for Shear Fatigue of Asphalt Mixtures

technical field

The invention belongs to the application technical field of testing mechanical properties of asphalt mixture, and in particular relates to a method for testing shear fatigue of asphalt mixture.

Background technique

The complex effects of climate, vehicle load and environment on asphalt concrete pavement, as well as the large-scale and heavy-duty vehicles in recent years, have caused some asphalt concrete pavement structures to be damaged to varying degrees within the design period, resulting in A lot of material and financial losses. Among the various diseases of asphalt pavement, fatigue cracking of pavement structure caused by repeated vehicle loads is one of the more common types of diseases. In order to effectively prevent and treat fatigue cracking, road workers in various countries have done a lot of research work, and believe that pavement fatigue cracking can usually be divided into three types: opening type, shear type and composite type. Among them, the opening type crack is mainly caused by the horizontal tensile stress of the pavement material under load, the shear type crack is mainly caused by the asphalt mixture being subjected to multiple shear stresses, and the compound type crack is caused by the combination of horizontal tensile stress and shear stress. result of the action.

Based on the classification of asphalt pavement fatigue cracks, people also realize that the formation mechanism of different types of cracks should be studied, and corresponding prevention measures should be put forward on this basis. At present, the research on the open fatigue crack is the most, mainly using the fatigue test in the laboratory to test the fatigue life of the asphalt mixture, and thus put forward the tensile stress or tensile strain at the bottom of the surface layer as the control index of fatigue design, At present, my country's "Code for Design of Highway Asphalt Pavement" (JTG D50-2006) is a pavement structure design that considers the fatigue fracture performance of the tensile structure based on the strength coefficient of the tensile structure. Compared with the study of opening cracks, the fatigue failure due to shear action has always been neglected by people. However, in recent years, the asphalt pavement in the intersection area of the main road in some big cities has suffered serious diseases such as congestion, pushing and rutting during the high temperature season. Basically, it needs to be milled and resurfaced every two years. After one or two summers, the bag and rutting problems are still very serious. Not only that, due to the increase of vehicle axle loads in recent years, shear cracks of varying degrees have also appeared on some expressway structures. Based on this, the shear failure of asphalt pavement has attracted more and more attention. How to evaluate the shear performance of asphalt mixture under one load and the shear fatigue performance under multiple repeated loads has become an urgent problem to be solved. question.

For the evaluation of the shear performance of asphalt mixture under one load, researchers have developed a variety of shear instruments to test the shear strength of asphalt mixture, such as coaxial shear tester, uniaxial penetration shear Shearing instrument and torsional shearing instrument, etc., through the above-mentioned equipment, the shear strength of asphalt mixture can be reasonably evaluated. Compared with the test of shear strength, the study of shear fatigue performance under multiple loads is relatively lagging behind. So far, only the SST (SuperPave Shear Tester) shear tester developed by the US SHRP program can be effectively used to compare and estimate the shear fatigue resistance of asphalt mixtures. However, the equipment is expensive and has high requirements for operators, so it has not been widely used in China at present. Therefore, on the basis of making full use of existing asphalt mixture performance testing equipment including MTS, UTM, etc., it is of great practical significance to realize the repeated shearing process of asphalt mixture and conduct shear fatigue performance testing.

Contents of the invention

Purpose of the invention: the purpose of the invention is to provide a method for testing the shear fatigue of asphalt mixture in view of the deficiencies in the prior art.

Technical solution: The asphalt mixture shear fatigue test method described in the present invention combines the fatigue loading characteristics of the UTM material testing machine and the coaxial shear test device, and proposes a fixed outer steel cylinder in the coaxial shear test device. Test method for shear fatigue of internal steel columns by subjecting them to cyclically repetitive loading of controlled stress.

The test method of asphalt mixture shear fatigue of the present invention comprises the steps:

1) For large Marshall specimens of asphalt mixture formed indoors, the specimens are cut into cylindrical specimens with a height of 50 mm, and a cylinder with a diameter of 50 mm is drilled with the center of the cylindrical specimen as the center of the circle to obtain a circular section The hollow cylinder specimen;

2) Apply high-strength glue or epoxy resin on the outer side of the cylindrical steel column with a diameter of 50mm and the inner side of the steel sleeve with an inner diameter of 150mm, and install the test piece between the inner steel column and the outer steel sleeve, so that the test piece The parts are tightly and firmly bonded to the two steel modules;

3) After the glue or resin is completely dry, put the test piece and the steel module into the environmental box at the target temperature for 2 to 4 hours; and place the test piece in the UTM material testing machine, fix the outer steel sleeve, and put it inside The steel column is applied with a load rate of 1mm/min to obtain the load when the asphalt mixture is sheared ;

作为疲劳峰值荷载，并设定荷载波形、加载频率、间歇时间等试验参数，固定外钢套，对内钢柱进行应力控制模式下10000次作用的剪切疲劳试验，获得

荷载下沥青混合料的剪切疲劳曲线，由此得到剪切疲劳寿命；疲劳应力百分比是0~100之间的数值。 4) to

As the fatigue peak load, and set the test parameters such as load waveform, loading frequency, intermittent time, etc., fix the outer steel sleeve, and carry out the shear fatigue test of 10,000 times in the stress control mode on the inner steel column, and obtain

Shear fatigue curve of asphalt mixture under load, from which shear fatigue life is obtained; fatigue stress percentage It is a value between 0 and 100.

The shear fatigue life in step 4) is the number of load actions corresponding to the inflection point between the shear stable deformation stage and the failure stage of the shear fatigue curve.

的取值，以获得完整的疲劳曲线，并得到剪切疲劳寿命。 The above step 4) For the situation that the third failure stage of the fatigue curve does not appear when the load is applied for 10,000 times, then increase

to obtain the complete fatigue curve and obtain the shear fatigue life.

Beneficial effects: the present invention is a test method for the shear fatigue life of asphalt mixture, and is also an important means for studying the law of shear flow deformation and fatigue failure mechanism of the mixture, and carrying out the optimal design of the mixture accordingly. The invention has obvious advantages in the following aspects: The advantages.

(1) The present invention can not only test the shear strength of asphalt mixture, but also obtain the shear deformation development law and shear fatigue life of asphalt mixture under a certain shear stress level, so as to comprehensively evaluate the asphalt mixture The road performance analysis method is provided.

(2) The present invention makes full use of the existing material testing machine UTM, and can conduct shear fatigue test research without adding additional test equipment, and the test steps are simple and easy for operators to master.

(3) The present invention is not only applicable to the shear performance test of the fresh asphalt mixture, but also can be used to study the shear fatigue performance of the original asphalt mixture on the road surface through the method of drilling and coring.

Description of drawings

Figure 1 is a test schematic diagram of the asphalt mixture shear fatigue test.

Figure 2 is the relationship curve between the shear deformation of asphalt mixture and the number of loads.

Among them: 1-loading pressure head; 2-inner steel column; 3-asphalt mixture ring test piece; 4-outer steel sleeve; 5-repeat

cyclic load.

Detailed ways

The test method of asphalt mixture shear fatigue of the present invention specifically comprises the steps:

, Preparation of asphalt mixture specimens

① Molding and preparation of test piece

When testing the shear fatigue performance of fresh asphalt mixture in the laboratory, the design gradation and asphalt ratio of the asphalt mixture shall be formed in accordance with the relevant test methods of "Test Regulations for Asphalt and Asphalt Mixtures in Highway Engineering" (JTJ052-2000) For a large Marshall specimen with a diameter of 152.4mm and a height of 95.3±2.5mm, compact each side 112 times, and the number of specimens in each group shall not be less than 4, and the specimen shall be ready for use after demoulding. When testing the shear fatigue performance of the asphalt mixture in the actual pavement, the method of drilling and coring is used to directly drill the specimen with a diameter of 150 mm from the pavement structure, and select a relatively complete specimen for use.

② Cutting of test piece

Professor Guo Zhongyin of Tongji University, when conducting the coaxial shear test of asphalt mixture, thought that the height of the cylinder specimen of the mixture should be 50 mm. The shear fatigue test of the present invention needs to cut the prepared Marshall test piece into a test piece with a thickness of about 50 mm, and take the center of the test piece as a circle to drill a cylinder with a diameter of 50 mm, thereby obtaining an outer diameter of 150 mm and an inner diameter of 150 mm. It is a cylindrical asphalt mixture specimen with a 50mm middle cavity and a circular cross-section.

2. Installation of coaxial shear specimen

Choose high-strength glue or epoxy resin that does not deform in an environment below 60°C, and apply high-strength glue or epoxy evenly on the outer surface of the inner steel column with a diameter of 50mm and the inner surface of the outer steel sleeve with an inner diameter of 150mm. resin, and put the annular asphalt mixture test piece into the annular space formed by the inner steel column and the outer steel sleeve. After the glue or resin is completely dry and the test piece is firmly bonded to the steel module, remove the excess epoxy outside the cylinder. resin or glue.

3. Test of the shear fatigue performance of the specimen

①Fatigue control mode

For the traditional flexural fatigue test of asphalt mixture, there are mainly two test control modes: stress control and strain control. The test time in the stress control mode is relatively short, and the complete fracture of the specimen is usually used as the failure criterion; while in the strain control mode, the stiffness of the mixture is generally reduced to 50% of the initial stiffness as the failure criterion. In comparison, the stress control method has better reproducibility, shorter test time, the specimen fractures at the end of the test, and the definition of fatigue damage is clear; the number of specimens required for the controlled stress test is small, and the degree of dispersion of fatigue data points is small. In addition, the present invention is proposed in combination with a UTM material testing machine and a coaxial shear testing device. If the strain mode is to be used, an additional servo control system needs to be installed. Accordingly, the present invention employs a stress-controlled shear fatigue mode.

(KN)作为控制目标，如需要获得沥青混合料受到的剪应力

(MPa)可根据式(1)进行计算。 It should be noted that the control mode of the present invention is to control the vertical load applied to the inner steel column by the loading link

(KN) as the control target, if it is necessary to obtain the shear stress of the asphalt mixture

(MPa) can be calculated according to formula (1).

                           (1)

(1)

值随着温度的增大而降低，在20~60℃的范围内为0.14~0.12。 in, is the shear strength coefficient of the mixture, according to the research of Professor Guo Zhongyin, the conventional asphalt mixture

The value decreases with the increase of temperature, in the range of 20~60℃ It is 0.14~0.12.

②Load type

(Km/h)，对荷载频率

(Hz)进行调整，具体方法为： The fatigue life of the material has a certain relationship with the load waveform. It is generally believed that the sine wave is relatively close to the load waveform borne by the actual road surface. The present invention also uses the sine wave load to carry out the fatigue test. A large number of calculations show that the fatigue loading frequency of 10Hz is equivalent to the vehicle speed of 60-65Km/h, and the calculation speed range of the automobile-only road stipulated in China's current "Highway Engineering Technical Standards" is 40-120Km/h, so 10Hz is used as the The frequency of load application is appropriate. Of course, if the shear fatigue of asphalt pavement in the intersection area of urban arterial roads is the research purpose, the vehicle speed of the actual local investigation can be

(Km/h), for load frequency

(Hz) to adjust, the specific method is:

                           (2)

(2)

③Testing process

Put the firmly bonded test piece together with the steel module into the environmental chamber of the UTM material testing machine, and set the temperature of the environmental chamber as the target temperature. After 2 to 4 hours of heat preservation, place the specimen in the asphalt mixture shear tester, and fix the outer steel sleeve on the fixture; column contact. According to the needs of the research, set appropriate parameters such as applied load frequency, waveform, intermittent time and peak load on the UTM testing machine, start the motor of the tester, and make the loading connecting rod act on the inner steel column according to a certain load waveform, thereby driving the ring Shearing motion of shaped asphalt mixture. During the test, the UTM loading connecting rod can not only control the peak value and waveform of the load, but also collect the displacement of the connecting rod, so that the relationship curve between the shear flow deformation of the asphalt mixture and the loading time and cycle can be obtained.

4. The failure standard of shear fatigue

From the relationship curve between shear deformation and load cycle of SST (SuperPave Shear Tester) shear testing machine, it can be seen that the shear fatigue curve is divided into three stages: compression deformation stage, shear flow stable deformation stage and failure stage. According to the fatigue curve, the inflection point between the second stage and the third stage is generally taken as the fatigue failure point, and the number of loads at this time is taken as the shear fatigue life. According to the graph drawn by AASHTO given the SST shear fatigue test cycle of 5000 times, most of the mixtures cannot show the rheological characteristics of the third stage; and according to the research of He Qiaomin and Shao Lageng, when the load is repeated After 10,000 times, there are still some asphalt mixtures that do not have an inflection point. Therefore, in order to ensure that the inflection point of failure occurs in the test process, the present invention proposes to test the shear strength of the asphalt mixture first, and then select a larger stress level to carry out the shear fatigue test, and use the number of load actions at the inflection point of the fatigue curve as the shear strength. Fatigue life; for the situation where the inflection point does not appear after 10,000 loads, the load stress level can be appropriately increased.

The technical solutions of the present invention will be described in detail below through examples in conjunction with the accompanying drawings, but the protection scope of the present invention is not limited to the examples.

Embodiment: a kind of testing method of asphalt mixture shear fatigue, comprises the steps:

(1) Preparation of asphalt mixture specimens

According to the gradation and asphalt ratio of the asphalt mixture to be evaluated, a group of 7 large Marshall specimens were formed in the laboratory. The diameter of the specimens was 152.4mm and the height was 95.3±2.5mm. Design porosity to be controlled. Saw the prepared Marshall specimen into a cylindrical specimen with a thickness of about 50 mm, and drill a hole in the center of the specimen to take out a small cylinder with a diameter of 50 mm to obtain a cylindrical specimen with a circular cross-section. After the surface is completely dry, set aside.

(2) Preparation before shear test

A coaxial shear test device is used as the mold for shear fatigue, specifically: apply high-strength glue evenly on the outer surface of a cylindrical steel column with a diameter of 50mm and the inner surface of an outer steel sleeve with a diameter of 150mm. And insert the steel column into the hollow part of the cylindrical test piece, and then put the test piece into the outer steel sleeve, so that the inner wall and outer wall of the hollow test piece are in close contact with the inner steel column and the outer steel sleeve respectively. After the glue is dry and the steel mold and the asphalt mixture specimen are completely and firmly bonded, use a blade to scrape off the excess glue exposed on the outside of the cylindrical specimen for later use.

(3) Shear fatigue test

，否则，增加试样数量进行试验，直至满足要求。接着，以

作为疲劳峰值荷载（

为疲劳应力百分比，是0~100之间的数值），并设定荷载波形、加载频率、间歇时间等试验参数，对其它4个试件进行10000次作用的剪切疲劳试验，获得

荷载下沥青混合料的剪切疲劳曲线，由此得到剪切疲劳寿命。对于当荷载作用周期达到10000次仍未出现疲劳曲线的第三破坏阶段德情形，可以适当增大

的取值。 When it is necessary to test the shear fatigue performance of asphalt mixture at 45°C, the target temperature of the temperature box is set to 45°C, and the cylindrical specimen and the two steel modules closely bonded to it are put into the UTM testing machine. In the temperature box, keep warm for 2 to 4 hours and place it on the tester. Then, select 3 specimens, conduct a shear test of the specimens under a load respectively, the shear rate is 1mm/min, and obtain the peak load when the shear failure of each asphalt mixture specimen, when the three measured values When the dispersion coefficient is within 20%, the average value is taken as the shear strength

, otherwise, increase the number of samples for testing until the requirements are met. Next, with

As the fatigue peak load (

is the percentage of fatigue stress, which is a value between 0 and 100), and set the test parameters such as load waveform, loading frequency, intermittent time, etc., and perform 10,000 shear fatigue tests on the other 4 specimens, and obtain

The shear fatigue curve of the asphalt mixture under load, from which the shear fatigue life is obtained. For the third failure stage where the fatigue curve does not appear when the load cycle reaches 10,000 times, it can be appropriately increased

value of .

(4) Fatigue test results and fatigue equation

The same sample is tested four times in parallel, and when the dispersion coefficient of the four measured values is within 20%, take the average value as the test result of shear fatigue life, otherwise, increase the number of samples for the test until the requirements are met. For the test results of fatigue life, the test conditions should be indicated, including test temperature, load stress level, load waveform, intermittent time, etc.

时的沥青混合料疲劳寿命后，可以改变4个应力水平，并获得不同应力水平下的疲劳寿命，并按照式(3)拟合得到疲劳参数

和。 When the load stress level is obtained as

After the fatigue life of the asphalt mixture at time, the four stress levels can be changed, and the fatigue life under different stress levels can be obtained, and the fatigue parameters can be obtained by fitting according to formula (3)

and .

                          （3）

(3)

为沥青混合料的剪切疲劳寿命；

为剪切应力水平；

为疲劳试验参数；

为对数坐标下疲劳曲线的斜率。 in,

is the shear fatigue life of asphalt mixture;

is the shear stress level;

is the fatigue test parameter;

is the slope of the fatigue curve in logarithmic coordinates.

Claims (3)

1. A test method for asphalt mixture shear fatigue, characterized in that the method may further comprise the steps: 1) For large Marshall specimens of asphalt mixture formed indoors, the specimens are cut into cylindrical specimens with a height of 50 mm, and a cylinder with a diameter of 50 mm is drilled with the center of the cylindrical specimen as the center of the circle to obtain a circular section The hollow cylinder specimen; 2) Apply high-strength glue or epoxy resin on the outside of the cylindrical steel column with a diameter of 50mm and the inside of the steel sleeve with an inner diameter of 150mm, and install the hollow cylindrical specimen between the inner steel column and the outer steel sleeve , so that the hollow cylinder specimen is closely and firmly bonded to the two steel modules; 3) After the glue or resin is completely dry, put the hollow cylinder test piece and the steel module into the environmental box at the target temperature for 2 to 4 hours; and place the hollow cylinder test piece in the UTM material testing machine and fix it. The outer steel jacket applies a load at a rate of 1mm/min to the inner steel column to obtain the load when the asphalt mixture shears and fails ; 4) to

As the fatigue peak load, set the load waveform, loading frequency, and intermittent time test parameters, fix the outer steel sleeve, and conduct a shear fatigue test of 10,000 times on the inner steel column under the stress control mode, and obtain

Shear fatigue curve of asphalt mixture under load, from which shear fatigue life is obtained; fatigue stress percentage

It is a value between 0 and 100. 2. The test method for asphalt mixture shear fatigue as claimed in claim 1, wherein the shear fatigue life of said step 4) corresponds to the inflection point of the shear fatigue curve shear stable deformation stage and the failure stage number of load actions. 3. The test method for asphalt mixture shear fatigue as claimed in claim 1, characterized in that, in the step 4) for the situation where the third failure stage of the fatigue curve does not appear when the load is applied 10,000 times, then increase to obtain the complete fatigue curve and obtain the shear fatigue life.

Images