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17 Table 3.2. High and intermediate temperature test Test Methods data for PG 64-22 binder. Flexural Beam Fatigue Testing Test Value, Failure kPa Temperature, Four-point beam fatigue testing was conducted according °C to AASHTO T321, "Determining the Fatigue Life of Com- DSR G*/sin , original binder at 64°C 1.702 68.4 DSR G*/sin , RTFO residue at 64°C 4.268 69.1 pacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flex- DSR G* (sin ), PAV residue at 25°C 2805 20.4 ural Bending." In this procedure, beam specimens (380-mm length, 63-mm width, 50-mm height) are loaded under strain- controlled conditions using sinusoidal loading at 10 Hz. The The PG 67-22 used at the 2003 NCAT Test Track is a non- literature indicated that beam fatigue tests were the most standard grade used in the southeastern United States. The high commonly used form of fatigue test in the United States. The temperature and intermediate temperature binder test data for literature also indicated that beam fatigue tests were sensitive the neat binder used in the 2003 NCAT Test Track are shown to material properties. in Table 3.2. The data indicates that the PG 67-22 used at the Testing was conducted in constant strain mode. Although NCAT Test Track also meets the properties of a PG 64-22. the literature indicated that constant stress tests may be more Following the procedures developed during SHRP and de- appropriate for thick pavements, it also indicated that pave- scribed in AASHTO R30, all mixtures underwent short-term ments never perform in a true constant stress manner, whereas aging for 4 h at 135°C before compaction. This short-term the performance of thick pavements can be approximated aging procedure allows for absorption of the asphalt binder by constant strain tests. Further, the stiffest mix performs the into the aggregate and simulates the aging that occurs during best in constant stress testing, but this is usually not the case production at an HMA facility. in the field. It is felt that mixture stiffness is accounted for in Sample preparation affects the measured fatigue life. To the analysis when calculating the strain at the bottom of the reduce variability, all of the samples tested in the study were HMA layer. mixed and compacted by NCAT. Individual beams were com- Each of the cells in the experimental plan (Table 3.1) was to pacted using a linear kneading compactor for beam fatigue be tested at six strain levels beginning on the high end of the testing. Samples were then wet sawed to specified dimensions. range, as follows: Cylindrical samples were compacted using the Superpave gyratory compactor for uniaxial tension testing. These sam- · 800 ms, ples were later cored and sawed to size once they reached the · 400 ms, University of New Hampshire's laboratory. Samples were care- · 200 ms, fully packed for shipping to other laboratories. · 100 ms, The air void contents of the optimum asphalt content sam- · 70 ms, and ples were targeted at 7 ± 0.5%. An experiment was conducted · 50 ms. to assess the expected reduction in air voids, using the same constant stress compaction effort that would result from the At least two replicates were tested for each cell. Once the fatigue optimum plus asphalt content. A 3.7% reduction in air voids lives of both replicates at a given strain level exceeded 50 mil- was observed, resulting in a target air voids content of 3.3 ± lion cycles, the next lower strain level was not tested. AASHTO 0.5% for the optimum plus asphalt content samples. T321 indicates typical strain levels between 250 and 750 ms. The literature suggests that the endurance limit in the labo- ratory is on the order of 70 ms (8, 36) and possibly up to 200 ms Phase II in the field (11). The 50 ms strain level was added so that at Additional testing was completed at the end of Phase I to least one strain level would be investigated that was believed examine the variability of beam fatigue testing and calcula- to be below the endurance limit. tion of the endurance limit and the affect of binder grade on Two replicate tests were performed at each strain level. the endurance limit. Two additional binder grades, PG 58-28 This provided a maximum total of 12 data points to fit the and PG 64-22, were utilized in the previously described mix- relationship between strain and cycles to failure. Ideally, the ture at optimum asphalt content. research team would have tested three replicates at each strain To date, a precision statement has not been developed for level. However, there was concern over the additional time beam fatigue testing. A full round-robin according to ASTM this would take at low strain levels. If the research team were C802 is beyond the scope of this project. A smaller scale round- assured that the log-log relationships for strain or energy con- robin was conducted to provide an estimate of the variability cepts remained a straight line at low strain levels, three repli- of beam fatigue testing. cates would have been preferable (51). However, since this
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18 study was trying to identify a break or curve in those relation- be found by utilizing the time-temperature superposition ships, it was felt that fewer points at more levels provided principle and the concept of reduced time. more information. Chehab et al. (33) demonstrated that the viscoelastic time- Testing was conducted to failure (a reduction in stiffness temperature shift factors are applicable to mixtures with grow- of 50%) or a minimum of 50 million cycles. Since the goal ing damage. Therefore, the shift factors determined from of this study was to determine the existence of an endurance complex modulus master curve construction can be used to limit, the strain levels were being altered to better define the shift the characteristic curves at various temperatures to a endurance limit. For instance, the PG 64-22 mix at optimum single reference temperature. Complex modulus (frequency asphalt content tested at 100 ms had fatigue lives in excess sweep) testing was conducted at five temperatures, -10°C, of 50 million cycles, but when tested at 200 ms, failed prior 0°C, 10°C, 20°C, and 30°C to develop the master curve. to 50 million cycles (average 20,445,922 cycles). Therefore, it Uniaxial frequency sweep testing was conducted with a was decided that it was more informative to perform tests at mean stress of zero to prevent the accumulation of perma- an intermediate strain level between 100 and 200 ms instead nent deformation. It is interesting to note that Daniel and of conducting tests at strain levels less than 100 ms. In this Kim (32) recommend the following for testing: example, 170 ms was selected as the point where the log-log relationship between strain and cycles to failure, developed at Ms levels of 5070 should be targeted at each frequency- temperature combination to ensure that the linear viscoelastic higher strain levels (800 to 200 ms), predicted a fatigue life of response is measured and that damage is not induced in the 50 million cycles. specimens. Three beam fatigue devices were used to conduct the test- ing. The study began with NCAT using a single IPC Global The ms levels noted by Daniel and Kim correspond to the beam fatigue device and the Asphalt Institute using a Cox & anticipated level of the endurance limit. Following the fre- Sons beam fatigue fixture in an Interlaken hydraulic load quency sweep tests, the same samples were loaded in monoto- frame. NCAT later added a second IPC Global beam fatigue nic tension to failure. The strain rate will be chosen to prevent device. The Asphalt Institute had some difficulties testing at the occurrence of a brittle failure. The monotonic tension tests low strain levels and testing to greater than 10 million cycles will be used to develop the characteristic curve. to failure with their Interlaken hydraulic load frame. Conse- Once the characteristic curve and viscoelastic shift factors quently, the Asphalt Institute also obtained an IPC Global are known, the behavior of the mix at other temperatures and beam fatigue device. Rutgers University also tested a PG 67-22 loading rates/amplitudes can be predicted. The number of at optimum plus asphalt content beam at 200 ms using an cycles to failure for different amplitudes and temperatures were IPC Global beam fatigue device. then predicted using the characteristic curve, and the shift In Phase II, two of the labs used a Cox & Sons fixture in a factors were determined from complex modulus testing. servo-hydraulic frame and the remaining four labs used IPC Selected continuous cycles to failure tests were performed Global's pneumatic system to conduct the beam fatigue tests. to verify the predicted values. The continuous cycles to failure Testing in Phase II was conducted at 800 and 400 ms and the test consists of a constant crosshead strain amplitude haver- strain level representing the average of the predicted endurance sine loading applied continuously to the specimen in the ten- limit for all of the labs testing a given mix. sile direction until failure occurs. Frequencies of 1 Hz and 10 Hz are used for the fatigue testing. The amplitude is chosen Uniaxial Testing to achieve failure of the specimen at a desired number of cycles based on the fact that the higher the amplitude, the faster the A methodology by which the material response under var- specimen will fail. For this study, tests at 10 Hz were used for ious uniaxial tensile testing conditions (type of loading and the verification fatigue tests to allow comparison with the beam temperature) can be predicted from the material response fatigue results. Because of machine compliance, even when obtained from a single testing condition has been proposed constant strain tests are conducted, the sample receives a mixed by Daniel and Kim (32). The basis of this methodology is in a mode of loading comparable to real pavements. characteristic curve that describes the reduction in material Due to limitations in computer memory, and the need for integrity as damage increases. The characteristic curve is gen- a reasonably fast data acquisition rate to capture the neces- erated by modeling the viscoelastic material behavior using sary information, only snapshots of data can be acquired Schapery's correspondence principle, continuum damage during the damage tests. In the continuous cyclic fatigue tests, mechanics, and work potential theory. The characteristic curve one-second snapshots of data at a rate of 100 points per cycle at any combination of temperature and loading conditions (1,000 points per second for the 10 Hz loading frequency) (cyclic versus monotonic, amplitude/rate, frequency) where were collected on a logarithmic scale up to a time increment viscoelastic behavior dominates the material response can of 2 to 10 minutes, depending upon the projected failure time
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19 of the specimen. If specimens are expected to fail in a shorter amount of time, the time between successive snapshots was reduced in an attempt to acquire data close to the actual fail- ure point and to adequately describe the changing material behavior as damage grows in the specimen. Samples 150 mm tall by 75 mm in diameter were cored from gyratory samples for testing. Prior to testing, steel end plates were glued to the specimen using Devcon Plastic Steel epoxy. A gluing jig was used to minimize any eccentricities due to unparallel specimen ends. Four loose core type linear variable differential transformers (LVDTs) were mounted to the specimen surface at 90° radial intervals using a 100-mm gage length. Additionally, two spring-loaded LVDTs were mounted 180° apart to measure the plate-to-plate defor- mations. The ram and LVDT deformations and load cell measurements were collected using a National Instruments data acquisition board and Labview software. Testing was performed using a closed-loop servo-hydraulic testing system. A 8.9 kN (2,000-lb) or 89 kN (20,000-lb) load cell was used depending upon the anticipated testing loads. The temperature was controlled with an environmental cham- ber that uses liquid nitrogen for cooling and a feedback system that maintains the temperature during testing. An example of a failed uniaxial tension fatigue sample is shown in Figure 3.1. Indirect Tensile Testing The literature indicates that parameters from the indirect tensile strength test, AASHTO T322, may be correlated with parameters related to the endurance limit. This test was con- sidered as a possible surrogate test, which could be conducted more expediently, than the long-duration beam fatigue tests. Indirect tensile tests were conducted on the Phase I mixes. Figure 3.1. Uniaxial tension fatigue sample.