1、Designation: D 7460 08Standard Test Method forDetermining Fatigue Failure of Compacted Asphalt ConcreteSubjected to Repeated Flexural Bending1This standard is issued under the fixed designation D 7460; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、 case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method provides procedures for determining aunique failure point for estimati
3、ng the fatigue life of 380 mm(14.96 in.) long by 50 mm (1.97 in.) thick by 63 mm (2.48 in.)wide asphalt concrete beam specimens sawed from laboratoryor field compacted asphalt concrete, which are subjected torepeated flexural bending.1.2 The between-laboratory reproducibility of this testmethod is b
4、eing determined and will be available on or beforeJune 2013. Therefore, this test method should not be used foracceptance or rejection of a material for purchasing purposes.1.3 The text of this standard references notes and footnoteswhich provide explanatory material. These notes and footnotes(exclu
5、ding those in tables and figures) shall not be consideredas requirements of the standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practice
6、s and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D75 Practice for Sampling AggregatesD 140 Practice for Sampling Bituminous MaterialsD 979 Practice for Sampling Bituminous Paving MixturesD 2041 Test Method for Theoretical Maximum Sp
7、ecificGravity and Density of Bituminous Paving MixturesD 3203 Test Method for Percent Air Voids in CompactedDense and Open Bituminous Paving MixturesD 3549 Test Method for Thickness or Height of CompactedBituminous Paving Mixture SpecimensD 3666 Specification for Minimum Requirements forAgen-cies Te
8、sting and Inspecting Road and Paving MaterialsD 5361 Practice for Sampling Compacted Bituminous Mix-tures for Laboratory TestingE29 Practice for Using Significant Digits in Test Data toDetermine Conformance with Specifications2.2 AASHTO Standards:3T 321 Standard Method of Test for Determining the Fa
9、tigueLife of Compacted Hot-Mix Asphalt (HMA) Subjected toRepeated Flexural BendingPP 3 Preparing Hot-Mix Asphalt (HMA) Specimens byMeans of the Rolling Wheel CompactorR30Standard Practice for Mixture Conditioning of Hot-Mix Asphalt (HMA)3. Terminology3.1 Definitions:3.1.1 beam modulusFlexural Beam S
10、tiffness, as deter-mined in 10.1.3.3.1.2 failure pointthe number of cycles to failure, Nf,which corresponds to the maximum or peak NormalizedModulus 3 Cycles (Fig. 13) when plotted versus Number ofCycles.3.1.3 initial beam modulusFlexural Beam Stiffness deter-mined at approximately 50 load cycles.3.
11、1.4 normalized modulus 3 cyclessee Rowe and Boul-din (1):4Beam Stiffness 3 Cycle Number!Initial Beam Modulus 3 Cycle of Initial Beam Modulus!4. Summary of Test Method4.1 The four-point flexural bending test method is con-ducted on compacted beam specimens to evaluate the fatigueproperties of an asph
12、alt concrete mixture. A cyclic haversine(displaced sine wave with full amplitude on tension side ofzero) displacement is applied at the central H-frame thirdpoints of a beam specimen, while the outer third points are heldin an articulating fixed position. The frequency rate rangesfrom 5 to 10 Hz. Th
13、is produces a constant bending momentover the center third (L/3) span (118.5 to 119 mm (4.66 to 4.691This test method is under the jurisdiction of ASTM Committee D04 on Roadand Paving Materials and is the direct responsibility of Subcommittee D04.26 onFundamental/Mechanistic Tests.Current edition ap
14、proved July 1, 2008. Published July 2008.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from Amer
15、ican Association of State Highway and TransportationOfficials (AASHTO), 444 N. Capitol St., NW, Suite 249, Washington, DC 20001,http:/www.transportation.org.4The boldface numbers in parentheses refer to the list of references at the end ofthis standard.1Copyright ASTM International, 100 Barr Harbor
16、Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.in.) between the H-frame contact points on the beam speci-men. The level of desired strain is pre-calculated and an inputfor the displacement control. The deflection at the mid-lengthposition (L/2) of a beam specimen is regulated by
17、 the closedloop control system.5. Significance and Use5.1 The laboratory fatigue life determined by this standardfor beam specimens have been used to estimate the fatigue lifeof asphalt concrete pavement layers under repeated trafficloading. Although the field performance of asphalt concrete isimpac
18、ted by many factors (traffic variation, speed, and wander;climate variation; rest periods between loads; aging; etc.), ithas been more accurately predicted when laboratory propertiesare known along with an estimate of the strain level induced atthe layer depth by the traffic wheel load traveling ove
19、r thepavement.NOTE 1The quality of the results produced by this standard aredependent on the competence of the personnel performing the procedureand the capability, calibration, and maintenance of the equipment used.Agencies that meet the criteria of Specification D 3666 are generallyconsidered capa
20、ble of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance withSpecification D 3666 alone does not completely assure reliable results.Reliable results depend on many factors; following the suggestions ofSpecification D 3666 or some similar ac
21、ceptable guideline provides ameans of evaluating and controlling some of those factors.6. Apparatus6.1 Test SystemThe test system shall consist of a loadframe, an environmental chamber (temperature control system)and a closed loop control and data acquisition system. The testsystem shall meet the mi
22、nimum requirements specified inTable 1. This standard specifically describes the systems of twoprimary suppliers (Cox and Sons, Inc. Cox and IndustrialProcess Controls, Ltd. IPC); however, other similar equip-ment could also be used.6.1.1 Loading DeviceThe test system shall include aclosed-loop, com
23、puter controlled loading component which,during each load cycle in response to commands from the dataprocessing and control component, adjusts and applies a loadsuch that the specimen experiences a constant level of displace-ment (and resulting strain) during each load cycle. The loadingdevice shall
24、 be capable of (1) providing cyclic haversine(= SIN2(degrees/2) loading at a frequency range of 5 to 10 Hz,(2) subjecting specimens to 4-point bending with free rotationand horizontal translation at all load and reaction points, and(3) forcing the specimen back to its original position (that is,zero
25、 deflection) at the end of each loading cycle. Fig. 1illustrates the haversine waveform. Figs. 2 and 3 show themovements of the Cox and IPC loading devices, respectively;the Cox device loads in a downward direction and the IPCloads in an upward direction. The early version of the IPCdevice does not
26、have free translation at the inner clamps;however, the newer model allows free rotation and translationat all four clamps.6.1.2 Environmental Chamber (Temperature ControlSystem)The environmental chamber shall enclose the entirespecimen and maintain the specimen at the desired testtemperature. The te
27、mperature shall be within 60.5C (60.9F)throughout the conditioning and testing times. An environmen-tal chamber is not required if the temperature of the surround-ing environment can be maintained within the specified limits.6.1.3 Control and Data Acquisition SystemDuring eachload cycle the control
28、and data acquisition system shall becapable of measuring the displacement of the beam specimen,and adjusting the load applied by the loading device such thatthe specimen experiences a constant level of displacement oneach load cycle. In addition, it shall be capable of recordingload cycles, applied
29、loads, beam displacements, and tempera-ture while computing and recording the maximum tensilestress, maximum tensile strain, phase angle, and stiffness atload cycle intervals specified by the user.6.2 Miscellaneous Apparatus and MaterialsFor the Coxdevice, an aluminum, wedge-shaped target for connec
30、ting thedisplacement sensor to the neutral axis of the specimen andcyanoacrylate (super glue) or equivalent is needed for attachingthe target to the specimen. With both the Cox and the IPCequipment, an alignment fixture and a solid aluminum beamare needed for setting the proper clamp spacing and a s
31、awsuitable for cutting the beams with parallel faces to the propertolerance.7. Hazards7.1 Observe standard laboratory safety precautions whenpreparing and testing asphalt concrete specimens.8. Sampling and Test Specimen Preparation8.1 Laboratory-Mixed and Compacted SpecimensSampleasphalt binder in a
32、ccordance with Practice D 140 and sampleaggregate in accordance with Practice D75. If a completefatigue curve is desired, prepare nine replicate asphalt concretebeam specimens, from slab(s) or beam(s) compacted in accor-dance with AASHTO PP 3. Otherwise, prepare as manyspecimens as desired for indiv
33、idual beam test results. Labora-tory prepared mixtures are typically conditioned with a short-term aging process, such as defined in AASHTO R30. Test atleast six replicate asphalt concrete beam specimens at differentstrain levels in order to develop a fatigue curve, as shown inFig. 4. The extra spec
34、imens may also be tested as desired, if thedata appears to include an outlier, or if a beam failure occursTABLE 1 Test System Minimum RequirementsLoad Measurement and Control Range: 0 to 5 kN (0 to 1124 lbf)Resolution: 2.5 N (0.56 lbf)Accuracy: 5 N (1 lbf)Displacement Measurement and Control Range:
35、0 to 5 mm (0 to 0.2 in.)Resolution: 2.5 m (9.8 3 105in.)Accuracy: 5 m (2.0 3 104in.)Frequency Measurement and Control Range: 5 to 10 HzResolution: 0.005 HzAccuracy: 0.01 HzTemperature Measurement and Control Resolution: 60.25C (60.45F)Accuracy: 60.5C (60.9F)Displacement Sensor Linear Variable Differ
36、entialTransducer (LVDT), Extensometer,or similar deviceD7460082FIG. 1 Illustration of Haversine Wave Form Relative to Sine WaveFIG. 2 Load and Freedom Characteristics of Fatigue Test Apparatus (Cox)D7460083NOTE 1Early model shown; the newer model allows free rotation and translation at all four clam
37、ps.FIG. 3 Load and Freedom Characteristics of IPC Fatigue Test ApparatusNOTEStrain levels should be adjusted for the material.FIG. 4 Example Fatigue CurveD7460084directly at a clamp.Alinear relationship on a log-log plot existsbetween Nfand the level of strain (, microstrain = strain 3106).NOTE 2The
38、 type of compaction device (linear kneading, rollingwheel, vibratory) may influence the test results, relative to representingactual construction.NOTE 3Normally test specimens are compacted using a standardcompactive effort. However, the standard compactive effort may notreproduce the air voids of r
39、oadway specimens measured according to TestMethod D 3203. If specimens are to be compacted to a target air voidcontent, the compactive effort should be determined experimentally.8.2 Plant-Mixed, Laboratory Compacted SpecimensObtain asphalt concrete samples in accordance with PracticeD 979. If a comp
40、lete fatigue curve is desired, prepare ninereplicate asphalt concrete beam specimens, from slab(s) orbeam(s) compacted in accordance with AASHTO PP 3. Oth-erwise, prepare as many specimens as desired for individualbeam test results. See Notes 2 and 3. Test at least six replicateasphalt concrete beam
41、 specimens at different strain levels inorder to develop a fatigue curve, as shown in Fig. 4. The extraspecimens may also be tested as desired, if the data appears toinclude an outlier, or if a beam failure occurs directly at aclamp.8.3 Roadway SpecimensObtain compacted asphalt con-crete samples fro
42、m the roadway in accordance with PracticeD 5361.8.4 Specimen TrimmingSaw at least 6 mm from all sidesof each compacted specimen to provide smooth, parallel(saw-cut) surfaces for mounting the measurement gages. Thefinal required dimensions of the test specimen, after sawing,are 380 6 6 mm (14.96 6 0.
43、24 in.) in length, 50 6 2 mm (1.966 0.08 in.) in height, and 63 6 2 mm (2.48 6 0.08 in.) inwidth. To minimize specimen variability, it is recommendedthat the beams be immediately labeled to ensure consistentorientation (top and sides) during testing, relative to thecompaction process.8.5 Specimen St
44、orageThe specimens should be stored ona 12.7 mm (12 in.) steel plate with a flatness of 0.127 mm(0.005 in.) across the surface of the plate from end to end. Thisflat surface keeps the beam specimens from being pre-strainedbefore testing. Limit stacking of specimens to two high onstorage racks.9. Pro
45、cedure9.1 Fixture AlignmentA solid aluminum beam, havingdimensions specified in 8.4 with tolerances to a flatness of0.051 mm (0.002 in.) across the length of the aluminum beam(measured using a straight edge and feeler gauges), is used toensure proper alignment of the beam fixture prior to testing.In
46、sert the aluminum beam into the fixture, clamping the sideclamps on the outside frame first. Clamp the top clamps on theoutside frames followed by the top clamps on the insideframes. Place the actuator in load control and remove the load.Verify that the clamps are fully seated on the aluminum beam.O
47、n the Cox frame, apply the side clamps to the inside frames.After returning to displacement control, adjust the load to thepositive side of zero; make a note of the actuator displacementsensor location and start cycling from this position. Once thiszero load position is located and used as a guide,
48、the bottom ofall the clamps will be aligned. If the top and bottom sides of thebeam test specimen are not parallel, it should not be an issuewith the clamping. The saw cuts are typically straight on allsides of the beam even if these are not parallel to each other;the top clamps will compensate for
49、the lack of parallelism,since the clamps are all independent of each other. The Coxfixture is designed to use the 3.175 mm (18 in.) tensile barcoupler for facilitating 360 movement without creating aneccentric moment, as shown in Fig. 5. The two spent 3.175 mm(18 in.) tensile bars pictured on the right-hand side of Fig. 5show the deformation that can occur due to years of fatiguetesting; any shortening of the shaft changes the stroke location.If the fixture uses this type of coupler, this initial sensorlocation should be checked after every
