Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures (2022)

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B-1   Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 Introduction Fatigue cracking occurs in asphalt pavements as the result of repeated bending from traffic loads. This bending results in tensile strains in the asphalt layer, which result in the formation of cracks, either at the bottom of the layer for thin pavements (classical bottom-up fatigue) or at the surface in thicker pavements (top-down fatigue). Top-down cracking is also influenced by tire-pavement interactions and by the oxidative aging of the asphalt binder used in the surface layer (Myers and Roque, 2002). Bottom-up fatigue is considered to consist of two phases. The first, crack initiation, involves the development of micro-cracks that grow from a microscopic size to a critical length of approximately 7.5 mm. During the crack propagation phase, the cracks spread from the bottom of the pavement layer to the surface (Myers and Roque, 2002). This literature review looks at three tests that have been developed to measure fatigue-cracking potential: the bending beam fatigue test (BBFT), uniaxial fatigue, and the overlay test (OT). While the studies found during the literature search provided much information about the fatigue behavior of asphalt mixtures, the focus of this literature review is placed on the relation- ships between these three tests, field cracking performance, and laboratory asphalt binder fatigue-cracking tests. Bending Beam Fatigue The BBFT has been used to characterize the fatigue behavior of asphalt concrete since the 1960s but was not standardized until the early 1990s as part of the Strategic Highway Research Program (SHRP) (Tayebali et al., 1994). The test is performed by subjecting a 380-mm by 50-mm by 63-mm beam of compacted HMA (cut from either a laboratory compacted specimen or a field sample) to four-point bending in a fixture like the one shown in Figure B-1. The beam is clamped into the fixture and placed into a temperature chamber at 20°C to simulate an intermediate pavement temperature. The test is performed by moving an actuator attached to the middle two clamps to apply a repeated load using either a sinusoidal or haversine wave form. Loading frequency ranges from 5 to 10 Hz, with 10 Hz being the most commonly used frequency. The BBF test may be performed using either a controlled stress or a controlled strain mode, with controlled strain being the most common. Strain levels may be set between 200 and 800 microstrain, which allows the user to account for pavement thickness (higher strains = thinner pavement). Test results include number of cycles to failure and cumulative dissipated energy (AASHTO T 321, ASTM D7460). The fatigue life determined from the labo- ratory BBF test may be used with a shift factor to predict the fatigue life of an asphalt pavement constructed using the same mix. A P P E N D I X B

B-2 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures The failure point for the BBF was initially defined as the number of cycles for the stiffness of the beam to fall to 50% of the original stiffness value. This value was rather arbitrary and did not always correlate well to the development of macro-cracks in the test specimen. There are two methods of determining the number of cycles to failure currently being used for beam fatigue testing. Both methods use the maximum tensile stress and strain per loading cycle to determine the flexural stiffness of the beam at each cycle. The stiffness calculated at the 50th loading cycle is typically defined as the initial stiffness of the beam. AASHTO T 321 uses an exponential model to determine the relationship between loading cycle and calculated beam stiffness and sets the failure point at a 50% reduction in the initial stiffness from the model. ASTM D7460 uses an indicator called the Normalized Stiffness X Cycles (NSC) to define the failure point. For each cycle, NSC is defined as a ratio of the product of stiffness and number of cycles to the product of the initial stiffness and initial cycle. NSC is plotted versus cycle and the point at which the maximum NSC is reached is defined as the failure point (Rowe and Bouldin, 2000). Dissipated energy is a measure of the energy used to cause damage in the BBF test specimen during loading, defined as the area under the stress strain curve. Changes in dissipated energy can be used to define the failure point and to indicate specimen behavior independent of load- ing rate. In 2000, Ghuzlan and Carpenter theorized that a sudden change in dissipated energy is an effective indication of damage accumulation and failure in BBF tests. A ratio of the change in dissipated energy between cycles (RDEC) and the dissipated energy of the previous cycle was defined. Changes in the relationship between the RDEC and the load during the BBF test can be used to indicate the point where failure occurs (Ghuzlan and Carpenter, 2000). The BBF can also be used to determine an endurance limit for fatigue in asphalt pavements. The endurance limit is the theoretical strain below which the mix will not crack and can be used as a tool for designing the thickness of asphalt pavements. In the laboratory, the endurance limit is defined as the strain level corresponding to a fatigue life of 50 million cycles in the BBF. This concept was verified by several studies (Carpenter et al., 2003; Peterson et al., 2004; Prowell et al., 2010). As part of NCHRP Project 09-38, “Endurance Limit of Hot Mix Asphalt Mixtures Figure B-1. Bending beam fatigue fixture.

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-3   to Prevent Fatigue Cracking in Flexible Pavements,” Prowell et al. (2010) verified the endurance limit concept and proposed decreasing the amount of testing time required to determine the endurance limit. A single-stage Weibull survivor function was proposed for estimating the fatigue life for BBF tests conducted as strain levels near the endurance limit. Correlation of Bending Beam Fatigue Results to Field Performance and Asphalt Binder Properties Many studies have been carried out to evaluate the use of the BBF test and to correlate it with field performance and with asphalt binder properties. Some of the studies are briefly described in this section. The BBF has been used to study the benefits of using a rich bottom layer to mitigate fatigue cracking. These layers are placed between pavement layers and provide two elements believed to extend fatigue life: increased asphalt content and decreased air voids. In 1996, Harvey and Tsai confirmed that for a typical Caltrans mix, using a rich bottom layer could increase the fatigue life 25% to 45% based on BBF results. Their results also showed an unexpected increase in fatigue life with increasing initial stiffness in the BBF. Because of the possible effect of other variables on the initial stiffness, it was suggested that initial stiffness not be included in the fatigue life models unless these relationships are clearly understood. Instead, relative changes in predicted pavement life should be used to evaluate the effect of mix variables on fatigue life (Harvey and Tsai, 1996). A study by Deacon et al. (1997) examined the new binder loss modulus value, G* sin (δ), proposed by SHRP to limit fatigue cracking and its effect on laboratory fatigue performance. While it was not clear what laboratory fatigue test was performed for this study, the proce- dure used was described as a controlled strain, flexural beam fatigue type test run on a similar size specimen as the BBF procedure. Thirty-two asphalt mixtures were evaluated using eight asphalt binders, two aggregates, and two air void contents. The results of the study showed that asphalt binder G* sin (δ) correlated well with laboratory fatigue resistance for mixtures and could identify changes in asphalt binder properties in mixes that were otherwise identical. The researchers cautioned, however, that there was not a good enough correlation between the two properties to use G* sin (δ) by itself as an indicator of fatigue resistance of asphalt mixtures (Deacon et al., 1997). In 2001, Bahia et al. (2001) used the BBF as part of NCHRP Project 09-10, “Superpave Pro- tocols for Modified Asphalt Binders,” to evaluate the effectiveness of the asphalt binder fatigue parameter G* sin (δ) (AASHTO M 320, Standard Specification for Performance-Graded Asphalt Binder). For this study, four aggregate gradations were used: coarse limestone, coarse granite, fine limestone, and fine granite. Mixtures were made using each of the gradations blended with nine different modified asphalt binders ranging in high-temperature PG grade from 58 to 82. After mixing, the mixtures were short-term oven aged for 4 hours at 135°C then tested in the BBF at a temperature corresponding to the temperature at which the asphalt binder used in the mix met the G* sin (δ) requirement of 5,000 kPa. A loading frequency of 10 Hz was used for the test and controlled strains ranging from 250 to 750 microstrain. The fatigue failure point for this testing was defined as a 50% reduction in the initial beam stiffness. Figures B-2 and B-3 show a summary of the results from the study. Figure B-2 shows that for this testing, the BBF appears to be sensitive to both binder modifica- tion type and aggregate properties, with binder type having a larger effect than aggregate type. The effect of the binder also appears to be independent of the aggregate effect. Given that the BBF test was run at temperatures that should have corresponded to an equal binder stiffness value, these results raised concerns about the validity of the asphalt binder G* sin (δ) parameter.

B-4 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures Figure B-2. Fatigue life of NCHRP 09-10 mixes with 9 modified asphalt binders (Bahia et al., 2001). Figure B-3. Correlation between asphalt binder G* sin(c) and mixture fatigue life (Bahia et al., 2001).

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-5   Figure B-3 shows the correlations between asphalt binder G* sin (δ) and mixture fatigue life. For this comparison, the binder tests were run using RTFO-aged binder at the same temperatures that gave G* sin (δ) = 5,000 kPa for the PAV-aged binders. The RTFO-aged binder was used for this comparison instead of the PAV-aged binder specified in AASHTO M 320 because the researchers felt that the RTFO-aging procedure might be a better match for the short-term aging used for the mixes. These plots show that there is little correlation between the mixture fatigue life and the binder G* sin (δ) parameter (Bahia et al., 2001). In 2002, Stuart et al. (2002) reported on the results of a study conducted using the FHWA ALF. Four lanes of the FHWA ALF experiment were used to validate the BBF and the dynamic shear rheometer (DSR) intermediate temperature parameter, G* sin (δ). Lanes 1 and 2 were constructed with a 100-mm layer of HMA over a 560-mm unbound crushed aggregate base and a prepared subgrade. Lanes 3 and 4 utilized a 200-mm layer of HMA over a 460-mm unbound crushed aggregate base and a prepared subgrade. Lanes 1 and 3 used a PG 58-34 asphalt binder, while lanes 2 and 4 used a PG 64-22 binder. ALF testing was performed at three temperatures: 10°C, 19°C, and 28°C. BBF tests were performed on laboratory-compacted ALF mixes using a strain-controlled sinusoidal load at levels of 300, 600, 900, and 1,100 microstrain at temperatures corresponding to those used in the ALF. All the mixes were tested to 40% of their initial stiffness. The researchers directly compared BBF fatigue life to the ALF fatigue life without using shift factors. Results are summarized in Table B-1. Figure B-4 shows a graphical summary of the number of ALF passes to a crack length of 50 m compared with the BBF fatigue life. Both Table B-1 and Figure B-4 show a good correlation between FHWA ALF fatigue life and BBF fatigue life, with the best comparison occurring at the 28°C test temperature. The worst Table B-1. Results of FHWA-ALF BBF comparison.

B-6 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures correlation was at 10°C. At all temperatures, the BBF did a reasonable job of quantifying the increase in fatigue life from increased pavement thickness. The BBF was also able to identify, in most cases, an increase in fatigue life owing to the softer PG 58-34 asphalt binder and the use of a thicker pavement. Figure B-5 shows the correlation between binder G* sin (δ) and FHWA ALF fatigue life. For the thinner pavements, the G* sin (δ) parameter ranked relatively well compared to the ALF fatigue life and provided the expected relationship of decreasing mix fatigue life with increasing asphalt binder stiffness. The opposite effect was seen for the thicker pavement, with fatigue life increasing with increasing binder stiffness (Stuart et al., 2002). In 2005, Tsai and Monismith presented the results of research intended to expand on Deacon’s 1997 study (Tsai and Monismith, 2005). This study, performed for the Pacific Coast Conference on Asphalt Specifications, investigated the effects of nine asphalt binders on the fatigue life of asphalt pavements. Laboratory binder characterization was done to obtain, among other param- eters, the G* sin (δ) at an intermediate temperature corresponding to the PG grade of the binder. G* sin (δ) was tested 20°C, a temperature corresponding to the BBF test temperature. PAV-aged asphalt binders were used for this determination. Other asphalt binder testing was done to determine the relaxation spectra for the binders and their molecular weight distributions. Beam fatigue testing was performed at three temperatures (10°C, 20°C, and 30°C) and two strain levels (200 and 400 microstrain). Testing was conducted until failure and the results used to obtain the ratio of stiffness at each cycle to the initial stiffness. This value was plotted against Figure B-4. Comparison of FHWA-ALF and BBF fatigue life.

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-7   the number of cycles and the number of cycles to failure defined as the cycle where the ratio was equal to 0.5. Figure B-6 shows a summary of the asphalt binder fatigue parameter results from this study compared to the laboratory fatigue results. Little correlation was found between the laboratory BBF results and the asphalt binder G* sin (δ) values (Tsai and Monismith, 2005). Johnson et al. used the same mixture and binders from Tsai and Monismith’s study to further investigate the use of a different asphalt binder fatigue test (Johnson et al., 2007). The proposed test was a strain-controlled, cyclical test performed using the DSR. Repeated cycles of shear loading were applied to a test specimen, and the decrease in complex shear modulus with time was recorded. DSR tests were performed at 10°C, 20°C, and 30°C to match the mixture fatigue tests from the earlier study. Strain levels of 1% and 2% were evaluated at a testing frequency of 10 Hz. Figure B-7 shows the results of the initial DSR testing compared with the mixture fatigue results from the BBF. The results in Figure B-7 show a good correlation between the BBF results and the DSR cyclical fatigue test. The researchers were concerned, however, about the possible effects of healing in the sample during the test. The first set of DSR tests used a “precision-sampling” method in the DSR software to maintain the strain level in the test. This method pauses the test between cycles in order to calculate the necessary stress to apply to the sample to get the desired strain. A second set of DSR tests was performed using a strain-control method that did not pause between cycles. Figure B-8 shows the results from the second set of DSR tests compared to Figure B-5. Comparison of binder G* sin(c) and FHWA ALF fatigue life.

B-8 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures Figure B-6. Comparison of PCAS asphalt binder G* sin(c) and mix Nf. Figure B-7. Initial DSR fatigue results compared to mixture fatigue from BBF. the mixture BBF results; the second set of test results did not provide a good correlation between binder and mixture fatigue results. The researchers stated that more work was needed to fully understand fatigue behavior of asphalt binders tested in the DSR (Johnson et al., 2007). The NCAT test track structural sections from the 2003 testing cycle were used by Willis and Timm to validate the use of strain thresholds in the design of asphalt pavements (Willis and Timm, 2009). This research focused on setting a horizontal strain limit that would prevent fatigue cracking in an asphalt pavement. Laboratory testing for this study included the use of BBF data to determine a fatigue endurance limit based on the results from the BBF test 20°C and multiple strain levels. Phase I of the study compared the laboratory endurance limit to a field strain value calculated using strain-temperature equations developed in an earlier part of the

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-9   study. Phase II of the study compared the magnitude of the endurance limit from the labora- tory to the measured field strain distributions in the test track sections. Phase III of the study compared the laboratory endurance limit based on the BBF and the developed field strain dis- tribution using a fatigue ratio between the measured strain values and the laboratory endurance limit. The first two phases had difficulty finding a clear relationship among laboratory results, field strains, and pavement performance. The third phase showed that there was a distinct difference in the fatigue ratios between test track sections that failed because of cracking and sections that did not, with the cracked sections having higher ratios (Willis et al., 2009). Way et al. (2012) presented the results of a study reviewing the use of the BBF test in the design of several Arizona paving projects dating from 1993 to 2012. The projects used were mostly overlay projects and represented various asphalt mix types and grades of binders. Comparisons between mixture properties showed trends similar to other studies already discussed, with the BBR showing sensitivity to mixture and binder properties. Annual field cracking surveys for the projects studied showed that the BBF results correlated reasonably well with actual field performance of the projects. A study presented in the International Journal of Fatigue in 2015 (Mannan et al., 2015) compared laboratory mixture fatigue results to asphalt binder fatigue results measured using two provisional test procedures. Two asphalt mixtures, one with 35% recycled asphalt pavement and one with no recycled asphalt pavement, were evaluated. BBF was performed on the mixes at 20°C using four strain levels (400, 600, 800, and 1000 microstrain) and three loading frequencies (1, 5, and 10 Hz). Asphalt binder was extracted and recovered from the mix and tested for fatigue resistance using the linear amplitude sweep (LAS) procedure described in AASHTO TP 101. A time-sweep procedure similar to the one used by Johnson was also used to measure the fatigue resistance of the asphalt binder (Johnson et al., 2009). Both asphalt binder procedures used for this study were performed using a DSR. The time- sweep procedure is a cyclical shear loading that looks at the degradation of the complex shear modulus (G*) of the binder over time. For this study, the time sweep was performed at 20°C and at three frequencies (1, 5, and 10 Hz) and four strain levels (4%, 6%, 8%, and 10%). The LAS procedure damages the DSR sample by applying a linearly increasing strain to the sample. For this study, the LAS was also performed at 20°C and used strains ranging from 1% to 30%. Figure B-8. DSR fatigue results using continuous loading.

B-10 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures Figures B-9 and B-10 show the results of the comparison between laboratory mixture fatigue and the asphalt binder fatigue tests used in this study. e results of this study showed that there were strong correlations between mixture fatigue life and both binder tests evaluated. e study also showed that the mixture fatigue life from the BBF test decreased with the addition of recycled asphalt pavement to the mix, as would be expected from the increased stiness caused by the recycled asphalt pavement binder (Mannan et al., 2015). Uniaxial Fatigue Testing e uniaxial fatigue test is similar to the exural fatigue test, but specimens are cylindrical in shape and are loaded in simple tension or compression rather than in exure. is has the advantage that stresses and strains are uniform across the central portion of the specimen. Data from uniaxial fatigue tests are oen analyzed using concepts from continuum damage theory, although this type of analysis is not a required part of uniaxial fatigue testing (Kim and Little, Figure B-9. Comparison of mixture and asphalt binder fatigue for 35% recycled asphalt pavement mixture. Figure B-10. Comparison of mixture and binder fatigue life (time sweep).

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-11   1990; Christensen and Bonaquist, 2012; Underwood et al., 2012). Regardless of the type of analysis used, fatigue data from uniaxial tests are most often used to evaluate the way damage accumulates in the asphalt mixture as the number of loading cycles increases. The most common way of evaluating damage growth is by plotting the reduction in modulus against a damage function, usually designed so that damage curves collected over a range of conditions collapse to a single “master curve” for fatigue damage. This is one of the main advantages of this type of testing—the rate of damage that occurs during fatigue loading can be characterized in a single damage curve with just a few parameters. These parameters can then be correlated to other factors, such as mix composition and various asphalt binder properties. As with flexural fatigue testing, uniaxial testing can be done in either stress or strain control, although properly con- structed damage curves should be independent of the manner of testing—another advantage of uniaxial fatigue tests. However, as with flexural testing, strain-controlled uniaxial fatigue testing often results in gradual damage accumulation with a difficult-to-define failure point. In fact, the emphasis in analyzing uniaxial fatigue test data has been for the rate of damage accumulation rather than for characterizing and modeling the point of failure, although more emphasis has recently been placed on developing failure criteria for these types of data (Kim and Little, 1990; Underwood et al., 2012). Along with the advantages mentioned, however, there are several disadvantages in uniaxial fatigue testing compared with flexural fatigue testing. One disadvantage is that because uni- axial fatigue testing is relatively new, there is not yet a large body of data linking data from this test to field performance. Another important disadvantage is that the vast majority of fatigue tests on asphalt mixtures to date have been done using flexural fatigue testing, and so there is a large body of knowledge revolving around this type of testing and relatively little using uniaxial fatigue testing. Most methods of flexible pavement design, including the latest AASHTO procedure (AASHTOWare Pavement ME Design) have used data based on flexural fatigue testing, and many have an option for including input from flexural fatigue tests on specific asphalt mixtures. For these reasons, most practicing pavement engineers are more familiar with flexural fatigue tests and are more comfortable interpreting flexural fatigue data; uniaxial fatigue testing is at this point largely a research tool. Although continuum damage theory is potentially a use- ful tool in characterizing the fatigue behavior of asphalt mixes and for the design of flexible pavement, it is unfortunately also complex and difficult to understand. The fact that uniaxial fatigue testing of asphalt mixes has largely been used within the context of continuum damage theory has probably increased the amount of confusion many engineers feel concerning this test and the resulting data. Although the disadvantages of uniaxial fatigue testing are signifi- cant, it does have significant advantages, including the possibility that this will become the standard method for characterizing the fatigue performance of asphalt concrete mixtures in the future. Texas Overlay Test A common method of rehabilitating old asphalt and concrete pavements involves the place- ment of an asphalt overlay. These overlays, compared with new asphalt pavements, are thin and are placed over already-damaged structures. Reflective cracking can occur in asphalt overlays at locations corresponding to existing cracks or joints in the underlying pavement. Movement in the vicinity of these joints or cracks, caused by bending or shearing action from traffic loading or temperature changes, causes high concentrations of stress to form at the bottom of the overlay. Eventually, these stresses will lead to cracks that can propagate to the surface of the pavement. Figure B-11 shows the effect of traffic loading on an asphalt overlay. Figure B-12 shows the common failure modes associated with slab movements (Elseifi and Al-Qadi, 2004).

B-12 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures The Texas OT was developed in the late 1970s at the Texas Transportation Institute (TTI) by Germann and Lytton as a means of simulating the movement of joints or cracks in a pavement, as discussed earlier (Germann and Lytton, 1979). The original apparatus, shown in Figure B-13, consists of two steel plates (Zhou and Scullion, 2003). One plate is fixed, while the other can move horizontally, thus simulating the opening and closing movements believed to cause reflective cracks. The original OT device had some limitations, mainly associated with the large test specimen size required. The 375-mm (15-in.) length of the specimen made it difficult to fabricate samples in the lab, and even more difficult to obtain field samples (Zhou and Scullion, 2005). Work by Zhou and Scullion in the early 2000s focused on upgrading the existing OT equip- ment to fully automate the test procedure and to determine a sample size that was easier to Figure B-11. Reflection cracking mechanisms. Figure B-12. Common failure modes associated with slab movements (Elseifi and Al-Qadi, 2004).

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-13   work with (Zhou and Scullion, 2003, 2005). This work came about from concerns that the asphalt mixture design procedure in Texas was focused heavily on rutting resistance, leading to the design of asphalt mixtures that had good rutting properties, but were stiff and had low asphalt contents. These stiff, dry mixes were prone to cracking. The intent of this study was to develop a procedure that could be used in conjunction with the Hamburg Wheel Tracking test for rutting resistance to ensure that the mixes produced met both rutting and cracking resistance require- ments (Zhou and Scullion, 2003). As part of the study conducted by Zhou and Scullion (2003), a smaller sample size was developed that was easier to obtain using existing Superpave gyratory compactor molds and standard size drill bits for field cores. The recommended test specimen size was a 150-mm (6-in.) by 75-mm (3-in.) wide by approximately 50-mm (2-in.) tall specimen that could be prepared from either a Superpave gyratory compactor specimen or a field core. The height of the specimen was chosen to correspond to the average 2-in. overlay thickness used in Texas. Figure B-14 shows the upgraded OT equipment (Zhou and Scullion, 2003). To run the OT, the test specimens are glued to the plates, using tape to prevent the glue from seeping into the opening between the moving and nonmoving plates and to provide a consistent gauge length. The OT is then performed by opening the space between the test plates to a width of 0.64 mm (0.025 in.), then repeatedly closing it at a rate of 1 cycle per 10 seconds until the specimen reaches the failure limit. A typical test temperature of 25°C is used. During the test, tensile load and displacement (measured by a linear variable differential transducer, or LVDT, under the fixed plate) are recorded every 0.1 seconds. Figure B-15 shows a typical set of OT results (Zhou and Scullion, 2003). As shown in Figure B-15, the OT results can be divided into three distinct phases. Phase I consists of the crack initiation and early propagation phase and is distinguished by a rapid drop Figure B-13. Original overlay tester concept (Zhou and Scullion, 2003). Figure B-14. Upgraded OT test equipment (Zhou and Scullion, 2003).

B-14 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures in test load as cracks develop and move through the test specimen. Phase II is the late propagation phase and is indicated by the slow decrease in the maximum load applied to the specimen, with the load staying in phase with the displacement. Phase III is sample failure and is distinguished by the small applied load occurring before the displacement reaches a maximum value. The sample is considered to have reached failure when the peak load during a cycle is reduced at least 93% compared with the peak load of the first cycle applied. If failure is not reached, the test is terminated at 1,200 cycles. One proposed benefit of the OT over other fatigue testing methods is its ability to measure not only the crack initiation phase but also the crack propagation phase (Zhou and Scullion, 2003). A recent upgrade of the OT equipment by IPC Global provided an OT device for use in the Asphalt Mixture Performance Tester (AMPT). In this device, the OT is performed vertically. The top plate is fixed while the bottom plate is opened and closed. Two LVDTs are used, one to measure the actuator displacement and one to measure the opening displacement. Figure B-16 shows the IPC Global device (IPC Global, 2012). Figure B-15. Typical OT results (Zhou and Scullion, 2003). Figure B-16. IPC Global AMPT OT device (IPC Global, 2012).

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-15   A study of the sensitivity of the OT showed that it was sensitive to mixture properties, including temperature, asphalt content, asphalt type, aggregate type, and air voids. The OT results were also found to be sensitive to the displacement used to run the test. Studies of repeat- ability found that the test provided results that had similar standard deviations and coefficient of variation values compared with other mixture cracking tests and that three replicates were enough for running the test. Based on the results of five case studies in Texas, the OT seemed to show good correlation with field reflective cracking performance. The OT was also found to have good correlations with the bending beam fatigue results and low-temperature performance of the mixtures. From this work, limiting criteria were proposed for the OT results on the basis of the number of cycles to failure (Nf): Nf > 750 cycles for rich bottom layers and Nf > 300 cycles for all other mixture types (Zhou and Scullion, 2005). These limits were used by Holdt and Scullion in 2006 as part of a study evaluating different materials used to mitigate joint reflection cracking in Texas and were found to be able to distinguish materials that performed well (Holdt and Scullion, 2006). In 2007, Zhou et al. reported that the 300-cycles limit was also reasonable for evaluating the fatigue cracking resistance of mixtures and proposed a method for using the OT device as a means for predicting the fatigue life of asphalt pavements using a modified version of Paris’ Law. For this analysis, Zhou and his associates used a measurement-based method to deter- mine the Paris’ Law parameters using the normalized OT load results (Zhou et al., 2007). The normalization of the load allowed a representation of the damage in the specimen during the test relative to its initial state. The traditional fatigue models used in the Mechanistic-Empirical Pavement Design Guide (MEPDG) showed poor correlation to fatigue performance measured in the FHWA Accelerated Loading Facility (ALF). This led to the desire for a better prediction model for fatigue life. The approach developed in this research differed from previous methods in that it considered not only the crack initiation stage but also the crack propagation stage. The OT tester results allow for the measurement of cracking characteristics associated with both of these stages and allow a more accurate prediction model (Zhou et al., 2007). Further studies by TTI have continued to provide improvements in the test method and to recommend its use in the design of asphalt pavements. A major area of focus has been on improving the repeatability of the test. Items of interest have been sample fabrication, testing temperature, gluing methods, and plate opening. Continuous efforts are also being made to improve the proposed fatigue-cracking model (Hu et al., 2008; Zhou et al., 2011, 2013; Walubita et al., 2012). Correlation of Overlay Tests Results to Field Performance Many studies have been done, particularly by TTI, to validate the results of the OT compared to field performance. In 2003, a validation of this work was done by taking cores from several Texas highways (Zhou and Scullion, 2003). One of the locations had little cracking after 10 years of service (overlay placed over a cracked, stabilized base) in both the control sections (SPS5 Virgin) and in a section containing recycled asphalt pavement (RAP) (SPS5 Recycled), while the other sections showed significant cracking after only a few months of service. Figure B-17 shows a comparison of the OT results from the pavement sections evaluated. The OT was able to distinguish between the sections with and without cracking. The OT was also able to distinguish between different asphalt binder sources and showed that stiffer asphalt binders had much lower cracking resistance in the OT than softer binders. Figure B-18 shows a comparison of OT results for a set of test specimens made using different binder types in a set of mixes where all other properties were the same. It can be seen from

B-16 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures Figure B-18 that the softer asphalt binders, particularly those with polymer modifiers perform better in the OT than stiffer asphalt binders (Zhou and Scullion, 2003). Further work by Zhou and Scullion (2005) analyzed additional roadways in Texas. Figure B-19 shows OT results for three more pavements, all of which showed reflective cracking within a few years of construction. In addition to the results shown, a section of the SH3 project was constructed using a crack- resistant material called Strata as an interface between the overlay and the existing pavement. OTs from this section showed a fatigue life of greater than 750 cycles. A final comparison in this study used three sections from MnRoad. Cores were taken from the three sections (15, 18, and 20) and shipped to TTI for overlay testing. Table B-2 shows the comparison between the OT results and the cracking observed in the sections. It can be seen from Table B-2 that the OT provided a good ranking of cracking resistance compared with the actual observed cracking on the test sections. On the basis of the work done in this study, an OT fatigue life limit of greater than 750 cycles was proposed for overlays using rich bottom layers from crack relief and greater than 300 cycles for all other materials (Zhou and Scullion, 2005). In 2007, cores taken from the FHWA ALF fatigue test lanes were sent to TTI for testing. The OT results were compared with the measured cracking performance in the accelerated loading Figure B-17. OT results on Texas Department of Transportation highway sections (Zhou and Scullion, 2003). Figure B-18. Comparison of OT results for mix sample with different asphalt binders (Zhou and Scullion, 2003).

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-17   facility (Zhou et al., 2007). OT testing was conducted at 19°C to match the test temperature in the FHWA ALF. The results showed an improved correlation with the actual fatigue perfor- mance of the ALF pavements compared with traditional methods of predicting fatigue life. Figure B-20 and Table B-3 show a summary of the actual cracking performance of the FHWA ALF sections and the predicted fatigue life calculated using the proposed fatigue-cracking model. The results show that the number of cycles to failure correlates well with the actual observed cracking performance of the FHWA ALF sections (Zhou et al., 2007). In 2008, Bennert and Maher used the OT as one method of evaluating the effectiveness of an asphalt interlayer-type mix, reflective crack relief interlayer (RCRI), at reducing the reflective cracking in composite pavements (Bennert and Maher, 2008). In this study, falling weight deflec- tometer (FWD) and weigh-in-motion sensors were used to determine the vertical movements at the concrete slab joints in the underlying pavement. Cores of the underlying concrete were tested to determine the coefficient of thermal expansion of the concrete, and this value was used to estimate the horizontal movements in the concrete from environmental changes. The horizontal and vertical deflections were then used in laboratory performance tests to evaluate the perfor- mance of HMA mixtures commonly used in New Jersey for overlays. Three test sections were constructed, two consisting of an RCRI overlaid by a 12.5-mm Superpave mix and the other test section consisting of a 12.5-mm Superpave mix overlaid with a 9.5-mm Superpave mix. OTs showed that the RCRI interlayer should have a fatigue life of over 46,000 cycles while the other two mixes used had fatigue lives in the range of 22 to 24 cycles. Actual performance, however, showed cracking in all sections after 6 months. Forensic analysis of the test sections attributed the poor performance of the RCRI sections to construction issues that led to poor bonding between the layers and also to inadequate compaction thickness of the RCRI layer (Bennert and Maher, 2008). Figure B-19. OT results on Texas highways (Zhou and Scullion, 2005). Table B-2. MnRoad comparison between OT and field cracking.

B-18 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures A study by Zhou et al. (2010) used the OT to analyze six different types of overlays placed at the University of California Pavement Research Center. The goal of this research was to further validate the proposed fatigue-cracking model. A 90-m test road was built at the University of California, consisting of compacted clay subgrade, a 410-mm aggregate base, and a 90-mm dense HMA surface. A heavy vehicle simulator (HVS) was used to induce fatigue damage to a density of 2.5 m/m2. Six different overlay types (control, plus five different rubber–modified binders) were placed over the damaged pavement. The HVS was again used to induce fatigue damage of 2.5 m/m2 in the overlays. Cores were taken from each section and submitted to TTI for OT testing. Table B-4 shows a summary of the results from the study. The results of the study showed good correlations between both the OT results and the fatigue life predicted by the model compared with the actual number of equivalent single-axle loads (ESALs) to failure as determined by the HVS (Zhou et al., 2010). Figure B-20. FHWA ALF test section cracking performance (Zhou et al., 2007). Table B-3. FHWA ALF cracking performance calculated using fatigue model (Zhou et al., 2007).

Evaluation and Selection of Mixture Fatigue Tests for Use in NCHRP 09-59 B-19   Mixes from five test sections located on the National Center for Asphalt Technology (NCAT) test track were used by Ma to recommend a new method of determining the failure point of the overlay test based on a normalized load cycle procedure. OT results from this study show that the procedure provided similar rankings for the materials from the test sections compared to the bending beam fatigue procedure. The OT results also correlated well with the field performance data of the five test sections seen on the test track (Ma, 2014). Mixture Cracking Tests Selected for Use in Related Project NCHRP 09-57 The objective of NCHRP Project 09-57, “Experimental Design for Field Validation of Labora- tory Tests to Assess Cracking Resistance of Asphalt Mixtures” (Zhou et al., 2016a 2016b), was closely related to NCHRP 09-59. After the Asphalt Cracking Tests Workshop organized as part of NCHRP 09-57 in February 2015, the following mixture cracking tests are recommended for including in NCHRP 09-57’s experimental design to evaluate three types of cracking: 1. Bottom-up fatigue cracking a. Flexural fatigue testing b. Semicircular bend test at intermediate temperature 2. Top-down fatigue cracking a. Semicircular bend test at intermediate temperature b. Indirect tension test with dissipated creep strain energy and energy ratio 3. Reflection cracking a. Overlay test b. Semicircular bend test at intermediate temperature c. Flexural fatigue testing The two mixture flexural tests recommended for use in NCHRP 09-57 through consultation with an expert panel were BBF and the OT (Zhou et al., 2016a, 2016b). Uniaxial fatigue testing was not recommended. This supports the use of these tests in NCHRP 09-59 for technical rea- sons, and because this suggests that findings based on these tests would have the best chance of being accepted by other pavement researchers and engineers. Section ID Fracture properties from OT Predicted ESALs to 50% reflective cracking (from model) Measured ESALs to 2.5 m/m2 cracking (from field HVS test) A n 586 2.77 × 10 − 6 4.974703 No crack propagation after 91 million ESALs None after 88 million ESALs 587 3.41 × 10 − 9 4.003453 50.4 million 60 million 588 6.10 × 10 − 10 4.9019 8.3 million 16 million 589 2.44 × 10 − 8 5.543798 No crack propagation after 66 million ESALs None after 66 million ESALs 590 6.52 × 10 − 8 5.184681 No crack propagation after 37 million ESALs None after 37 million ESALs 591 3.44 × 10 − 10 4.763871 None after 91 million ESALs None after 91 million ESALs Table B-4. Summary of OT results (Zhou et al., 2010).

B-20 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures Mixture Fatigue Test Methods Selected for Use in NCHRP 09-59 Both the BBF and OT methods have been evaluated to determine their effectiveness at pre- dicting asphalt mixture cracking resistance. The BBF test showed reasonable correlations with field performance and was able to distinguish between different mixture properties. The BBF showed mixed results when compared with the asphalt binder fatigue tests (G* sin (δ), LAS, cyclical testing). While some research showed a correlation with the binder parameter, other research was unable to show any correlation at all. This lack of correlation is likely not from an issue with the BBF procedure, but may be an indication of the inability of the binder parameter by itself to account for the effects of other mixture properties, such as asphalt content, aggregate type, and air voids, on fatigue life (Deacon et al., 1997). For the OT, the studies discussed in this literature review show that this procedure was also able to correlate reasonably well with field performance and can differentiate between mixture properties. The OT was also able to identify the benefits of reflective crack relief interlayers. The selection of BBF and the OT as preferred test methods in NCHRP 09-57 (Zhou et al., 2016a, 2016b) lends additional support for using these tests in NCHRP 09-59. Correlations between uniaxial fatigue data and field performance are not nearly as extensive as for the BBF and OT, largely because this is a relatively new procedure that has not yet been widely used by pavement engineers and researchers. Because of the rigor of this test method and the related continuum damage theory, significant resources have gone toward its development and implementation, and it will likely become more widely used in the future. After consultation with the panel, it was decided to use both bending beam flexural and uniaxial fatigue testing in NCHRP 09-59. Although limited overlay tests were performed early in the project, modifications of the work plan ultimately led to discontinuing this test. Because the available time and funding were not adequate to allow testing of all mixes using both fatigue test methods, it was decided to rely more on uniaxial fatigue testing. All 16 NCHRP 09-59 mixtures were tested using uniaxial fatigue testing, while only 9 of 16 mixes were tested using flexural fatigue. References Bahia, H. U., D. I. Hanson, M. Zeng, H. Zhai, M. A. Khatri, and R. M. Anderson. NCHRP Report 459: Character- ization of Modified Asphalt Binders in Superpave Mix Design. TRB, National Research Council, Washington, D.C., 2001. Bennert, T., and A. Maher. Field and Laboratory Evaluation of a Reflective Crack Interlayer in New Jersey. Transportation Research Record: Journal of the Transportation Research Board, No. 2084, 2008, pp. 114–123. Carpenter, S. H., K. A. Ghuzlan, and S. Shen. Fatigue Endurance Limits for Highway and Airport Pavements. Transportation Research Record: Journal of the Transportation Research Board, No. 1832, 2003, pp. 131–138. Christensen, D. W., and R. Bonaquist. 2012. Modeling of Fatigue Damage Functions for Hot Mix Asphalt and Applications to Surface Cracking. Journal of the Association of Asphalt Paving Technologists, Vol. 81, pp. 241–271. Deacon, J., J. Harvey, A. Tayebali, and C. Monismith. Influence of Binder Loss Modulus on the Fatigue Perfor- mance of Asphalt Concrete Pavements. Journal of the Association of Asphalt Paving Technologists, Vol. 66, 1997. Elseifi, M., and I. Al-Qadi. A Simplified Overlay Design Model Against Reflective Cracking Utilizing Service Life Prediction. Road Materials and Pavement Design, Vol. 5, No. 2, 2004, pp. 169–191. Germann, F. P., and R. L. Lytton. Methodology for Predicting the Reflection Cracking Life of Asphalt Concrete Overlays. Research Report FHWA/TX-79/09+207-5, College Station, Texas, 1979. Ghuzlan, K. A., and S. H. Carpenter. Energy-Derived, Damage-Based Failure Criterion for Fatigue Testing. Transportation Research Record: Journal of the Transportation Research Board, No. 1723, 2000.

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B-22 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures Zhou, F., H. Li, S. Hu, J. W. Button, and J. A. Epps. Characterization and Best Use of Recycled Asphalt Shingles in Hot Mix Asphalt. FHWA/TX-12/0-6614-2, Texas A&M Transportation Institute, College Station, TX, 2013. Zhou, F., D. Newcomb, C. Gurganus, S. Banihashemrad, E. Park, M. Sakhaeifar, and R. Lytton. Experimental Design for Field Validation of Laboratory Tests to Assess Cracking Resistance of Asphalt Mixtures. College Station, TX: Texas A&M Transportation Institute, 2016a, 251 pp. Zhou, F., D. Newcomb, C. Gurganus, S. Banihashemrad, E. Park, M. Sakhaeifar, and R. Lytton. NCHRP Research Results Digest 399: Field Validation of Laboratory Tests to Assess Cracking Resistance of Asphalt Mixtures: An Experimental Design. Transportation Research Board, Washington, D.C., 2016b. Zhou, F., S. Hu, X. Hu, T. Scullion, M. Mikhail, and L. Walubita. Development, Calibration, and Verification of a New Mechanistic-Empirical Reflective Cracking Model for HMA Overlay Thickness Design and Analysis. Journal of Transportation Engineering, Vol. 136, No. 4, 2010.