Validating the Fatigue Endurance Limit for Hot Mix Asphalt (2010)

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15 Introduction Based upon the review of the literature, a controlled labo- ratory experimental plan was developed. The experimental plan was developed with the primary objective of testing the hypothesis that there is an endurance limit for HMA mixtures. As a secondary objective, some of the HMA material proper- ties that affect the endurance limit were investigated. A working definition of the endurance limit was devel- oped as a framework for testing within the experimental plan. Although the endurance limit is defined as an essentially in- finite fatigue life for metal alloys, testing for an infinite life is impractical. The endurance limit must be defined in practi- cal, usable terms if it is to have meaning. For example, the literature has defined 40 to 50 years as a reasonable lifetime to be considered as a long lasting, or perpetual, pavement. Hence, determining a strain level that results in 40 to 50 years (or even more) of pavement life is a very practical way to identify the endurance limit. The Highway Capacity Manual states that the maximum number of passenger cars per hour per lane for a freeway at a free flow speed of 65 mph is 2,350 (48). In rolling terrain, a single truck or bus would replace 2.5 passenger cars (48). Thus, one would expect a maximum of 940 trucks per hour, 22,560 trucks per day, or a maximum of 329,376,000 trucks in a 40-year period. Such a case might represent a dedicated truck lane running at capacity 24 hours a day, 7 days a week, 365 days a year, an unlikely occurrence. By comparison, the very heavily traveled section of Interstate 710 in California carried a maximum of 9,650 trucks per day in the design lane (20). By calculating the appropriate heavy-vehicle adjust- ment factor and determining its impact on traffic flows (48), mixed traffic streams with 25% and 50% trucks would pro- duce a maximum of 148,219,200 (10,152 trucks per day) and 235,118,400 trucks in a 40-year period, respectively. Consider, for example, an FHWA Class 9 vehicle or five-axle single trailer, which typically consists of two tandem axles and a single steering axle (three axle groups). Strain traces indicate that the tandem axle results in two distinct load repetitions (49). Assuming that one is designing a perpetual pavement for the tandem axle load, the steering axles would have a lower loading so, theoretically, in a perpetual pavement design they would do no damage to the pavement. Thus, each Class 9 vehicle would provide four load repetitions to the pave- ment for a maximum total of 1,317,504,000 axle load repeti- tions in a 40-year period. This represents a theoretical maxi- mum loading where every truck is fully loaded and the design lane is at maximum capacity for 24 hours a day, 7 days a week, for a 40-year period. This loading condition would be expected to be even more severe than a dedicated truck lane. A similar methodology was used by Mahoney to calculate the maxi- mum number of ESALs expected in a 40-year period (unpub- lished data). In actual mixed traffic streams, the highest percentage of trucks tends to be about 50%, which would reduce the maxi- mum number of load repetitions to 940,473,600 or a maxi- mum number of load repetitions of 592,876,800 for 25% trucks. Even the most heavily traveled highways do not maintain traffic streams at capacity 24 hours a day and not all trucks are loaded. The fact that all trucks are not fully loaded is illustrated by a Washington DOT study of 10 weigh- in-motion sites over a one-year period, which indicated that the typical number of ESALs for a Class 9 vehicle was 1.2 (50). If 1.2 “design load” axles were applied per truck for the maximum number of trucks per lane in a 40-year period (329,376,000), a total of 395,251,200 load repetitions would be applied. Also, in winter months in many parts of the country, the pavement stiffness is very high, and this results in significantly lower strains. Therefore, it is a reasonable assumption that the maximum possible number of load repetitions expected in a 40-year period is approximately 500 million. This could be considered as a practical target for evaluating parameters (strain or energy) indicating an endurance limit. C H A P T E R 3 Research Plan

Research conducted during the SHRP recommended a shift factor of 10 between laboratory beam fatigue results and field performance, equating to 10% cracking in the wheel-path (46). Considering this shift factor, laboratory testing to 50 million cycles would equate to approximately 500 million loading cycles in the field or approximately the maximum possible loading in a 40-year period. Based on these analyses, a mix that provided 50 million cycles or more of fatigue life in the laboratory was considered to be indicative of a long-life pavement. If pavements were designed to have a strain level at the en- durance limit, then all pavements, regardless of traffic, would be designed with approximately the same thickness of HMA (assuming the same underlying support). This approach is contrary to the way pavements have been designed in the past and is unlikely to be cost effective for future designs. Hence, it is more reasonable, especially for highways with low to medium traffic, to design a pavement for the expected traffic during an extended period of years (e.g., 40 to 50) than to simply design at the endurance limit. Practically speaking, highways with lower traffic levels can be designed with less pavement structure and still have long lives. Hence, the amount of traffic has to be a critical element in the design process. In the past there has been very little fatigue testing at low strains (very high cycles to failure) and this study was designed to identify the relationship between strain and cycles to failure at these very low strains (high cycles to failure). In the process of evaluating low strains, it was felt that the “endurance limit” would be better identified. Two test procedures were utilized to evaluate the existence of the endurance limit: beam fatigue tests and uniaxial ten- sion tests. Beam fatigue tests have been the most widely used method for testing fatigue in the United States. Uniaxial ten- sion tests have provided an alternative that allowed a more fundamental analysis by modeling the viscoelastic material behavior using Schapery’s correspondence principle, contin- uum damage mechanics, and work potential theory. Materials The literature indicated that the primary material proper- ties affecting fatigue life are binder content, binder stiffness, and air void content. The literature indicated that aggregate gradation, type, shape, and angularity have more limited effects. Different nominal maximum aggregate sizes, with their corresponding differing minimum VMA requirements, will tend to produce differing volumes of asphalt binder. Phase I A full-factorial experiment was conducted to evaluate the existence of an endurance limit and to identify factors affect- ing the endurance limit. Two main factors were included in the experiment. Two additional factors were fixed. These fac- tors along with their levels are as follows: • Nominal maximum aggregate size (NMAS)—19.0 mm • Aggregate type—granite • Asphalt content—optimum and optimum + 0.7% • Binder stiffness—PG 67-22 and PG 76-22 The lower lifts of part of the structural experiment from the 2003 NCAT Test Track were replicated to provide two of the mixes for the experiment. The mixes were a 19.0 mm NMAS granite mixture at optimum asphalt content with both neat PG 67-22 and SBS modified PG 76-22 binder. The average field gradations, shown in Table 3.1, were used as the target gradation for the laboratory study. Previous research on fatigue has indicated that fatigue results are relatively insen- sitive to gradation. Therefore, it was felt that the use of the average gradation would be appropriate. The base mix was placed in four lifts for sections N3 and N4 and two lifts in the other sections. The asphalt content of the base layer for section N8 was intentionally set at 0.5% above optimum. Therefore, it was not included in the average. 16 Sieve Size N1 N2 N3 - Uppe r N3 - Lower N4 - Uppe r N4 - Lower N5 N6 N7 N8 Average 1" 100 100 100 100 100 100 100 100 100 100 100 3/4" 92 93 100 90 92 88 92 90 90 92 92 1/2" 80 84 84 79 79 77 79 78 78 83 80 3/8" 71 74 75 68 66 66 66 71 71 73 70 No. 4 49 53 57 50 49 49 49 53 53 54 52 No. 8 40 43 48 44 43 42 43 44 44 45 44 No. 16 33 35 42 39 36 36 36 36 36 37 37 No. 30 24 24 33 30 26 28 26 27 27 26 27 No. 50 13 14 20 16 14 16 14 15 15 14 15 No. 100 8 9 11 9 8 9 8 9 9 9 9 No. 200 5.5 5.5 6.7 5.6 5.5 5.5 5.5 5.7 5.7 5.5 6 Asphalt Content 4.3 4.5 4.3 4.6 4.7 4.4 4.7 5.0 5.0 5.2 4.7 Table 3.1. Production gradations for base layers of 2003 NCAT Test Track structural experiment.

The PG 67-22 used at the 2003 NCAT Test Track is a non- standard grade used in the southeastern United States. The high temperature and intermediate temperature binder test data for the neat binder used in the 2003 NCAT Test Track are shown in Table 3.2. The data indicates that the PG 67-22 used at the NCAT Test Track also meets the properties of a PG 64-22. Following the procedures developed during SHRP and de- scribed in AASHTO R30, all mixtures underwent short-term aging for 4 h at 135°C before compaction. This short-term aging procedure allows for absorption of the asphalt binder into the aggregate and simulates the aging that occurs during production at an HMA facility. Sample preparation affects the measured fatigue life. To reduce variability, all of the samples tested in the study were mixed and compacted by NCAT. Individual beams were com- pacted using a linear kneading compactor for beam fatigue testing. Samples were then wet sawed to specified dimensions. Cylindrical samples were compacted using the Superpave gyratory compactor for uniaxial tension testing. These sam- ples were later cored and sawed to size once they reached the University of New Hampshire’s laboratory. Samples were care- fully packed for shipping to other laboratories. The air void contents of the optimum asphalt content sam- ples were targeted at 7 ± 0.5%. An experiment was conducted to assess the expected reduction in air voids, using the same constant stress compaction effort that would result from the optimum plus asphalt content. A 3.7% reduction in air voids was observed, resulting in a target air voids content of 3.3 ± 0.5% for the optimum plus asphalt content samples. Phase II Additional testing was completed at the end of Phase I to examine the variability of beam fatigue testing and calcula- tion of the endurance limit and the affect of binder grade on the endurance limit. Two additional binder grades, PG 58-28 and PG 64-22, were utilized in the previously described mix- ture at optimum asphalt content. To date, a precision statement has not been developed for beam fatigue testing. A full round-robin according to ASTM C802 is beyond the scope of this project. A smaller scale round- robin was conducted to provide an estimate of the variability of beam fatigue testing. Test Methods Flexural Beam Fatigue Testing Four-point beam fatigue testing was conducted according to AASHTO T321, “Determining the Fatigue Life of Com- pacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flex- 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 literature indicated that beam fatigue tests were the most commonly used form of fatigue test in the United States. The literature also indicated that beam fatigue tests were sensitive to material properties. Testing was conducted in constant strain mode. Although the literature indicated that constant stress tests may be more appropriate for thick pavements, it also indicated that pave- ments never perform in a true constant stress manner, whereas the performance of thick pavements can be approximated by constant strain tests. Further, the stiffest mix performs the best in constant stress testing, but this is usually not the case in the field. It is felt that mixture stiffness is accounted for in the analysis when calculating the strain at the bottom of the HMA layer. Each of the cells in the experimental plan (Table 3.1) was to be tested at six strain levels beginning on the high end of the range, as follows: • 800 ms, • 400 ms, • 200 ms, • 100 ms, • 70 ms, and • 50 ms. At least two replicates were tested for each cell. Once the fatigue lives of both replicates at a given strain level exceeded 50 mil- lion cycles, the next lower strain level was not tested. AASHTO 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 in the field (11). The 50 ms strain level was added so that at least one strain level would be investigated that was believed to be below the endurance limit. Two replicate tests were performed at each strain level. This provided a maximum total of 12 data points to fit the relationship between strain and cycles to failure. Ideally, the research team would have tested three replicates at each strain level. However, there was concern over the additional time this would take at low strain levels. If the research team were assured that the log-log relationships for strain or energy con- cepts remained a straight line at low strain levels, three repli- cates would have been preferable (51). However, since this 17 Test Value, kPa Failure Temperature, °C DSR G*/sin δ, original binder at 64°C 1.702 68.4 DSR G*/sin δ, RTFO residue at 64°C 4.268 69.1 DSR G* (sin δ), PAV residue at 25°C 2805 20.4 Table 3.2. High and intermediate temperature test data for PG 64-22 binder.

study was trying to identify a break or curve in those relation- ships, it was felt that fewer points at more levels provided more information. Testing was conducted to failure (a reduction in stiffness of 50%) or a minimum of 50 million cycles. Since the goal of this study was to determine the existence of an endurance limit, the strain levels were being altered to better define the endurance limit. For instance, the PG 64-22 mix at optimum asphalt content tested at 100 ms had fatigue lives in excess of 50 million cycles, but when tested at 200 ms, failed prior to 50 million cycles (average 20,445,922 cycles). Therefore, it was decided that it was more informative to perform tests at an intermediate strain level between 100 and 200 ms instead of conducting tests at strain levels less than 100 ms. In this example, 170 ms was selected as the point where the log-log relationship between strain and cycles to failure, developed at higher strain levels (800 to 200 ms), predicted a fatigue life of 50 million cycles. Three beam fatigue devices were used to conduct the test- ing. The study began with NCAT using a single IPC Global beam fatigue device and the Asphalt Institute using a Cox & Sons beam fatigue fixture in an Interlaken hydraulic load frame. NCAT later added a second IPC Global beam fatigue device. The Asphalt Institute had some difficulties testing at low strain levels and testing to greater than 10 million cycles to failure with their Interlaken hydraulic load frame. Conse- quently, the Asphalt Institute also obtained an IPC Global beam fatigue device. Rutgers University also tested a PG 67-22 at optimum plus asphalt content beam at 200 ms using an IPC Global beam fatigue device. In Phase II, two of the labs used a Cox & Sons fixture in a servo-hydraulic frame and the remaining four labs used IPC Global’s pneumatic system to conduct the beam fatigue tests. Testing in Phase II was conducted at 800 and 400 ms and the strain level representing the average of the predicted endurance limit for all of the labs testing a given mix. Uniaxial Testing A methodology by which the material response under var- ious uniaxial tensile testing conditions (type of loading and temperature) can be predicted from the material response obtained from a single testing condition has been proposed by Daniel and Kim (32). The basis of this methodology is in a characteristic curve that describes the reduction in material integrity as damage increases. The characteristic curve is gen- erated by modeling the viscoelastic material behavior using Schapery’s correspondence principle, continuum damage mechanics, and work potential theory. The characteristic curve at any combination of temperature and loading conditions (cyclic versus monotonic, amplitude/rate, frequency) where viscoelastic behavior dominates the material response can be found by utilizing the time-temperature superposition principle and the concept of reduced time. Chehab et al. (33) demonstrated that the viscoelastic time- temperature shift factors are applicable to mixtures with grow- ing damage. Therefore, the shift factors determined from complex modulus master curve construction can be used to shift the characteristic curves at various temperatures to a single reference temperature. Complex modulus (frequency sweep) testing was conducted at five temperatures, −10°C, 0°C, 10°C, 20°C, and 30°C to develop the master curve. Uniaxial frequency sweep testing was conducted with a mean stress of zero to prevent the accumulation of perma- nent deformation. It is interesting to note that Daniel and Kim (32) recommend the following for testing: Ms levels of 50–70 should be targeted at each frequency- temperature combination to ensure that the linear viscoelastic response is measured and that damage is not induced in the specimens. The ms levels noted by Daniel and Kim correspond to the anticipated level of the endurance limit. Following the fre- quency sweep tests, the same samples were loaded in monoto- nic tension to failure. The strain rate will be chosen to prevent the occurrence of a brittle failure. The monotonic tension tests will be used to develop the characteristic curve. Once the characteristic curve and viscoelastic shift factors are known, the behavior of the mix at other temperatures and loading rates/amplitudes can be predicted. The number of cycles to failure for different amplitudes and temperatures were then predicted using the characteristic curve, and the shift factors were determined from complex modulus testing. Selected continuous cycles to failure tests were performed to verify the predicted values. The continuous cycles to failure test consists of a constant crosshead strain amplitude haver- sine loading applied continuously to the specimen in the ten- sile direction until failure occurs. Frequencies of 1 Hz and 10 Hz are used for the fatigue testing. The amplitude is chosen to achieve failure of the specimen at a desired number of cycles based on the fact that the higher the amplitude, the faster the specimen will fail. For this study, tests at 10 Hz were used for the verification fatigue tests to allow comparison with the beam fatigue results. Because of machine compliance, even when constant strain tests are conducted, the sample receives a mixed mode of loading comparable to real pavements. Due to limitations in computer memory, and the need for a reasonably fast data acquisition rate to capture the neces- sary information, only snapshots of data can be acquired during the damage tests. In the continuous cyclic fatigue tests, one-second snapshots of data at a rate of 100 points per cycle (1,000 points per second for the 10 Hz loading frequency) were collected on a logarithmic scale up to a time increment of 2 to 10 minutes, depending upon the projected failure time 18

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. 19 Figure 3.1. Uniaxial tension fatigue sample.