1、Standard Method of Test for Determining the Fatigue Life of Compacted Asphalt Mixtures Subjected to Repeated Flexural Bending AASHTO Designation: T 321-14 American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-2d T 321-1
2、AASHTO Standard Method of Test for Determining the Fatigue Life of Compacted Asphalt Mixtures Subjected to Repeated Flexural Bending AASHTO Designation: T 321-14 1. SCOPE 1.1. This standard provides procedures for determining the fatigue life and fatigue energy of 380 mm long by 50 mm thick by 63 mm
3、 wide asphalt mixture beam specimens sawed from laboratory- or field-compacted asphalt mixtures and subjected to repeated flexural bending until failure. 1.2. This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concern
4、s associated with its use. It is the responsibility of the user of this procedure to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: PP 3, Preparing Hot Mix Asphalt (HMA) Speci
5、mens by Means of the Rolling Wheel Compactor1 T 2, Sampling of Aggregates T 40, Sampling Bituminous Materials T 168, Sampling Bituminous Paving Mixtures T 247, Preparation of Test Specimens of Hot Mix Asphalt (HMA) by Means of California Kneading Compactor T 269, Percent Air Voids in Compacted Dense
6、 and Open Asphalt Mixtures 2.2. ASTM Standards: D 3549/D 3549M, Standard Test Method for Thickness or Height of Compacted Bituminous Paving Mixture Specimens D 5361/D 5361M, Standard Practice for Sampling Compacted Bituminous Mixtures for Laboratory Testing E 29, Standard Practice for Using Signific
7、ant Digits in Test Data to Determine Conformance with Specifications 3. TERMINOLOGY 3.1. Definition: 3.1.1. failure pointthe load cycle at which the specimen exhibits a 50 percent reduction in stiffness relative to the initial stiffness. 2014 by the American Association of State Highway and Transpor
8、tation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-2 AASHTO 4. SIGNIFICANCE AND USE 4.1. The fatigue life and failure energy determined by this standard can be used to estimate the fatigue life of asphalt mixture pavement layers under repeated traffic load
9、ing. The performance of asphalt mixtures can be more accurately predicted when these properties are known. 5. APPARATUS 5.1. Test SystemThe test system shall consist of a loading device, an environmental chamber (optional), and a control and data acquisition system. The test system shall meet the mi
10、nimum requirements specified in Table 1. Table 1Test System Minimum Requirements Load measurement and control Range: 0 to 5 kN Resolution: 2 N Accuracy: 5 N Displacement measurement and control Range: 0 to 5 mm Resolution: 2 m Accuracy: 5 m Frequency measurement and control Range: 5 to 10 Hz Resolut
11、ion: 0.005 Hz Accuracy: 0.01 Hz Temperature measurement and control Range: 10 to 25C Resolution: 0.25C Accuracy: 0.5C 5.1.1. Loading DeviceThe test system shall include a closed-loop, computer-controlled loading component that, during each load cycle in response to commands from the data processing
12、and control component, adjusts and applies a load such that the specimen experiences a constant level of strain during each load cycle. The loading device shall be capable of (1) providing repeated sinusoidal loading at a frequency range of 5 to 10 Hz; (2) subjecting specimens to four-point bending
13、with free rotation and horizontal translation at all load and reaction points; and (3) forcing the specimen back to its original position (i.e., zero deflection) at the end of each load pulse. (Figure 1 illustrates the loading conditions.) 5.1.2. Environmental Chamber (Optional)The environmental cha
14、mber shall enclose the entire specimen and maintain the specimen at 20.0 0.5C during testing. An environmental chamber is not required if the temperature of the surrounding environment can be maintained within the specified limits. 5.1.3. Control and Data Acquisition SystemDuring each load cycle, th
15、e control and data acquisition system shall be capable of measuring the deflection of the beam specimen, computing the strain in the specimen, and adjusting the load applied by the loading device such that the specimen experiences a constant level of strain on each load cycle. In addition, it shall
16、be capable of recording load cycles, applied loads, and beam deflections and computing and recording the maximum tensile stress, maximum tensile strain, phase angle, stiffness, dissipated energy, and cumulative dissipated energy at load cycle intervals specified by the user. 5.2. Miscellaneous Appar
17、atus and MaterialsA suitable saw for cutting the beams and a mechanism for setting proper clamp spacing. For loading devices that require a glued nut for deformation measurement, a screw, nut (suggested size M 8 by 1) and block assembly for referencing the linear variable differential transducer (LV
18、DT) to the neutral axis of the specimen, and epoxy for attaching the nut to the specimen are also needed. 2014 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-3 AASHTO SpecimenLoadLoadReaction Rea
19、ctionDeflectionSpecimenClampReturn toOriginal PositionFree Translation and RotationFigure 1Load and Freedom Characteristics of Fatigue Test Apparatus 6. HAZARDS 6.1. Observe standard laboratory safety precautions when preparing and testing HMA specimens. 7. SAMPLING AND SPECIMEN PREPARATION 7.1. Lab
20、oratory-Mixed and Compacted SpecimensSample asphalt binder in accordance with T 40 and sample aggregate in accordance with T 2. Prepare three replicate asphalt mixture beam specimens, from slab(s) or beam(s) compacted in accordance with PP 3 or T 247. Note 1The type of compaction device may influenc
21、e the test results. It is recommended to cut beams from a large slab compacted by a vibratory roller. Note 2Normally, test specimens are compacted using a standard compactive effort. However, the standard compactive effort may not reproduce the air voids of roadway specimens measured according to T
22、269. If specimens are to be compacted to a target air void content, the compactive effort to be used should be determined experimentally. 2014 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-4 AAS
23、HTO 7.2. Plant-Mixed, Laboratory-Compacted SpecimensObtain asphalt mixture samples in accordance with T 168. Prepare three replicate asphalt mixture beam specimens, from slab(s) or beam(s) compacted in accordance with PP 3 or T 247. (See Notes 1 and 2.) 7.3. Roadway SpecimensObtain compacted asphalt
24、 mixture samples from the roadway in accordance with ASTM D 5361/D 5361M. 7.4. Saw at least 6 mm from both sides of each test specimen to provide parallel (saw-cut) surfaces to eliminate high air void sections on the specimen surface. For loading devices that require gluing a nut for deformation mea
25、surement, these cut surfaces provide smooth surfaces for mounting the measurement gauges. The final required dimensions, after sawing, of the specimens are 380 6 mm in length, 50 6 mm in height, and 63 6 mm in width. 8. PROCEDURE 8.1. Specimen MeasurementMeasure the height and width of the specimen
26、to the nearest 0.01 mm at three different points along the middle 100 mm of the specimen length in accordance with applicable sections of ASTM D 3549/D 3549M. Determine the average of the three measurements for each dimension and record the averages to the nearest 0.1 mm. 8.2. Epoxying Nut to Neutra
27、l Axis of SpecimenLocate the center of a specimen side. Apply epoxy in a circle around this center point and place the nut on the epoxy such that the center of the nut is over the center point. Avoid applying epoxy such that it fills the center of the nut. Allow the epoxy to cure before moving the s
28、pecimen. (Figure 2 illustrates a nut epoxied to the neutral axis of the specimen.) 380mm50 mm63 mmNut - M8Note: Not to scale Figure 2Nut Epoxied to the Neutral Axis 8.3. Place the specimen in an environment that is at 20.0 0.5C for 2 h to ensure the specimen is at the test temperature prior to begin
29、ning the test. 8.4. Open the clamps and slide the specimen into position (Figures 3, 4, and 5). Use the jig to ensure proper horizontal spacing of the clamps, 119 mm center-to-center. When the specimen and clamps are in the proper positions, close the outside clamps by applying sufficient pressure t
30、o hold the specimen in place. Next, close the inside clamps by applying sufficient pressure to hold the specimen in place. 2014 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-5 AASHTO Clamp A Cla
31、mp B Clamp B Clamp AClamp A Clamp B Clamp B Clamp AStep 1Step 2Clamp A Clamp B Clamp B Clamp AStep 3Figure 3Specimen Clamping Procedure 2014 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-6 AASHT
32、O Air PlenumEnvironmentalSystemFigure 4Schematic of Flexural Beam Fatigue Test Apparatus, Side View 2014 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-7 AASHTO Air OutAir InAirInPlenumAirOutPlen
33、umFigure 5Schematic of Flexural Beam Fatigue Test Apparatus, Top View 8.5. Attach the LVDT block to the specimen by screwing the screw into the nut epoxied on the specimen (see Figure 4). Clamp the LVDT into position such that the LVDT probe rests on top of the block and the LVDT is reading close to
34、 zero. 8.6. Select the desired initial peak-to-peak strain (generally in the 250 to 750 microstrain for conventional asphalt mixtures) and loading frequency, and the load cycle intervals at which test results are recorded and computed, and enter them into the recording and control components test pr
35、ogram. In some instances, with highly modified materials for specialized applications, testing has been conducted with initial peak-to-peak strains as high as 2000 microstrain. Set the loading frequency within a range of 5 to 10 Hz. Note 3The data collection sequence should ensure that at least 200
36、data points are captured within each log decade of loading and that these should enable a smooth curve of flexural stiffness versus load cycles to be obtained. The collection sequence shall include data capture at cycle 50. 8.7. Select a deflection level (peak-to-peak strain level) such that the spe
37、cimen will undergo a minimum of 10,000 load cycles before its stiffness is reduced to a condition that represents specimen failure. A minimum of 10,000 load cycles ensures that the specimen does not decrease in stiffness too rapidly. Note 4A test of 10,000 load cycles at 10 Hz will take 17 min to co
38、nduct once the test is started. A practical upper limit on the test time would correspond to around 1 day, during which approximately 1 million load applications can be achieved (around 28 h). Data in this range can be used to produce a relationship between strain and life to failure. Some trial and
39、 error may be 2014 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-8 AASHTO required to estimate the strain levels to achieve these testing times. If an estimation of endurance limit is required,
40、then additional testing beyond 1 million cycles is needed. Some methods suggest that tests as long as 10 million cycles may be required. 8.8. After selecting the appropriate test parameters, begin the test. Activate the recording and control components so that the test results at the selected load c
41、ycle intervals are monitored and recorded, ensuring that the test system is operating properly. 8.9. Determine at all load cycles the flexural stiffness and phase lag at each load cycle throughout the test while the test is being performed as follows: 8.9.1. Maximum Peak-to-Peak Stress (Pa): ( ) ( )
42、20.357 /tP bh= (1) where: P = peak-to-peak load applied by actuator, N; b = average specimen width, m; and h = average specimen height, m. 8.9.2. Maximum Peak-to-Peak Strain (m/m): ( ) ( )2212 / 3 4th La= (2) where: = maximum peak-to-peak deflection at center of beam, m; a = space between inside cla
43、mps, 0.357/3 m, (0.119 m); and L = length of beam between outside clamps, 0.357 m. 8.9.3. Flexural Stiffness (Pa): /ttS = (3) 8.9.4. Phase Angle (deg): 360 fs= (4) where: f = load frequency, Hz; and s = time lag between Pmaxand max, s. Note 5When automated testing software is used in the recording a
44、nd control component of the test system, is approximated by an algorithm contained in the automated testing software. 8.10. For each load cycle at which data are collected, compute the product of the flexural stiffness and load cycles (S n). 8.11. Terminate the data collection and stop the test afte
45、r a point where the computed S n has reduced from a peak value by 15 percent 9. CALCULATIONS 9.1. Cycles to FailureFailure is defined as the point at which the product of the specimen stiffness and loading cycles is a maximum. 2014 by the American Association of State Highway and Transportation Offi
46、cials.All rights reserved. Duplication is a violation of applicable law.TS-2d T 321-9 AASHTO Note 6The use of appropriate data smoothing methods, such as fitting six-order polynomials and cubic splines to the S n curve, have been used to smooth out any irregularities in the data collection. 10. REPO
47、RT 10.1. Asphalt Mixture DescriptionReport the binder type, binder content, aggregate gradation, and air void percentage. 10.2. Specimen DimensionsReport the specimen length, average specimen height, and average specimen width in meters to four significant figures. Note 7See ASTM E 29 for informatio
48、n on determination of significant figures in calculations. 10.3. Report the average test temperature to the nearest 0.2C. 10.4. Report the test results listed in Table 2 for each load cycle interval selected by the operator to three significant figures. Table 2Test Results Load Cycle Applied Load Be
49、am Deflection Peak-to-Peak Stress Peak-to-Peak Strain Flexural Stiffness Phase Angle S n N m Pa m/m MPa degree 10.5. Report the flexural stiffness at cycle 50 in MPa. 10.6. Report the cycles to failure. 10.7. Report the flexural stiffness at the failure cycle. 10.8. Prepare a plot of stiffness versus load cycles as shown in Figure 6 and the S n versus load cycles as shown in Figure 7. 10.9. When multiple test results are conducted at a single strain level, the average of the results shal
