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66 HMA layers. This survivability analysis was completed to try ulus and tensile strain at failure, where both properties are to estimate a value for the endurance limit based on alligator determined at 77F (25C). Points below the line in Figure 6.12 cracking observations within the LTPP program, rather than are assumed to have inferior fatigue properties, and those just using values estimated from limited laboratory testing above the line exceed the fatigue strength/life of the standard programs. This survival analysis was a desk-top study that has mixture. Laboratory tests and field observations of alliga- yet to be formally documented. The LTPP data were used to tor cracking have been used to check the validity of this rela- determine the probability of occurrence of different amounts tionship over time. More alligator cracking has been observed of alligator cracking for different HMA thicknesses and ten- where the tensile strain at failure is less than that value from sile strains. Figure 6.12 for a specific HMA modulus value based on the The EVERSTRESS Program was used to calculate the maxi- equivalent temperature concept. mum tensile strain at the bottom of the HMA layer for each test Von Quintus used this relationship to estimate or define section using the equivalent annual temperature and equivalent the endurance limit for different HMA mixtures as 1% of the (18-kip) single-axle load concepts. The HMA modulus value tensile strain at failure measured in accordance with the test used in the calculation of tensile strain was determined using protocol from the AAMAS study (59). That definition has yet the Witczak equation (70) based on volumetric data and to be confirmed and validated. physical properties of the HMA for the equivalent annual temperature. The modulus values for the other pavement Updated Survivability Analysis and soil layers were based on resilient modulus testing per- Using LTPP Data formed in the laboratory. Figure 6.11 shows the survival curve from that limited study. A magnitude of 2% cracking The survivability analysis completed for this project included was used in this initial survival analysis because of the mea- the same test sections from the 1995 study, plus additional surement error in alligator cracking with time. A small mea- test sections within the GPS-1, GPS-2, and SPS-1 experiments. surement error could result in significant changes to this A subset of the LTPP test sections was used, which was ran- definition of the endurance limit. In summary, the endurance domly selected to cover all environmental regions, soil types, limit was determined to be 65 ms at a 95% confidence level and HMA thicknesses. The additional test sections used in the for an 18-kip single-axle load applied to the pavement at the updated survivability analysis were from the GPS-1, GPS-2, equivalent annual temperature for each LTPP site included and SPS-1 experiments--LTPP database version VR 2004.06, in the analysis. release 18.0 (2004). Figure 6.13 shows the distribution of HMA thickness for all test sections included in the updated survivability analysis. Preliminary Definition of the Endurance Figure 6.14 shows the distribution of pavement age for the Limit as an HMA Mixture Property test sections with more than 10 in. of HMA that were used to The AAMAS project sponsored by NCHRP recommended update the survival curve (Figure 6.11). As shown, the age of use of the indirect tensile strength and modulus tests to esti- 45% of the thicker test sections included in the updated study mate the fatigue strength/life of specific HMA mixtures (59). is greater than 15 years. Many of these additional test sec- Figure 6.12 illustrates that relationship between HMA mod- tions were from the SPS-1 experiment that was excluded from Fatigue Cracking <2% 100.00 95.00 90.00 85.00 Survival, % 80.00 75.00 70.00 65.00 60.00 55.00 50.00 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Tensile Strain, micro-strains Figure 6.11. Survival curve for flexible pavements developed from data included in the LTPP GPS-1 and GPS-2 experiments (undocumented study, 1995).

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67 100 Tensile Strain at Failure, mils/in. 10 1 10 100 1000 10000 Total Resilient Modulus, ksi Note: ms and mils/in. are the same unit. Figure 6.12. Relationship between modulus and tensile strain at failure to estimate the fatigue strength of HMA Mixtures at 77F (25C) (59). 110 Number of sections in each thickness layer 100 90 80 70 60 50 40 30 20 10 0 20 Number 94 65 88 101 104 92 71 88 25 Thickness Layer of HMA Layers Figure 6.13. Distribution of HMA thickness for the test sections used in the updated survivability analysis. HMA Thickness > 10 inches 70 Number of Test Sections 60 50 40 30 20 10 0 <11 11 12 13 14 15 16 17 18 19 20+ Age, years Figure 6.14. Distribution of age for those test sections with HMA layer thicknesses in excess of 10 in.

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68 the initial study to estimate the endurance limit. The reason there are an appreciable number of test sections with thicker that the SPS-1 test sections were excluded from the study in HMA layers (15 in. or more) that have levels of fatigue crack- 1995 is that most of the projects within the SPS-1 experiment ing exceeding 5%. were relatively new at that time. Figure 6.17 compares the maximum tensile strain calcu- The other important parameter in the survivability analy- lated for each section and HMA thickness. The modulus of sis is the truck traffic applied to each of these test sections. the HMA layer was determined using the equivalent temper- Without significant truck traffic, defining the endurance limit ature concept for an 18-kip ESAL, as described previously for from field observations has limited meaning. Figure 6.15 the original survivability analysis. As shown and expected, the shows the distribution of the cumulative number of 18-kip tensile strains decrease with increasing HMA layer thickness. ESALs for the test sections included in the updated surviv- Figure 6.18 compares the maximum tensile strain at the bottom ability study that have HMA thickness in excess of 10 in. The of the HMA layer and the amount of fatigue cracking observed cumulative truck traffic for these thicker test sections is con- on the LTPP test sections from the most recent distress survey sidered moderate traffic with most test sections having less included in the LTPP database. As shown and expected, the than 15 million cumulative 18-kip ESALs. test sections with the lower tensile strains have less fatigue or In summary, the test sections with the thicker HMA lay- alligator cracking. ers are not new pavements (Figure 6.14), but do have truck traffic levels that are lower than what would be considered Maximum Tensile-Strain-Based Definition heavy truck traffic (Figure 6.15). This level of truck traffic is a concern to the definition established for the endurance An updated survival curve to the one presented in Fig- limit. Much higher levels of truck traffic are needed to vali- ure 6.11 was developed for the additional alligator cracking date the endurance limit design premise with field observa- data and LTPP test sections. Figure 6.19 shows the results tions and data. from the survival analysis for a range of fatigue cracking levels. The results from the updated survival analysis are signifi- cantly different from the 1995 desk-top study. In fact, the HMA Thickness-Based Definition updated survival curve for the 1% and 2% alligator crack- The asphalt industry has proposed some maximum HMA ing levels would indicate that there is no endurance limit for thicknesses that are believed to be resistant to alligator crack- these sections. The relationships shown in Figure 6.19 for ing. The LTPP database was used to determine the level of the 1% and 2% cracking levels have a peak survival rate sig- HMA thickness at which none or little alligator cracking has nificantly less than 100% and then begin to decrease with been observed on HMA pavement surfaces. Figure 6.16 com- lower tensile strain values. pares the amount of fatigue cracking (percent of wheel-path Some of the GPS test sections that were without alligator area) from the most recent distress survey and HMA thick- cracks in 1995 now have some alligator cracks recorded in the ness. As shown and expected, the test sections with thinner LTPP database for the test sections with the thickest HMA HMA layers generally have more fatigue cracking. However, layers. Possible reasons for the significant difference in results HMA Thickness > 10 inches 120 Number of Test Sections 100 80 60 40 20 0 30 Range of Cumulative Axles, millions of ESALs Figure 6.15. Distribution of cumulative equivalent single-axle loads for the test sections used in the survivability analysis with HMA layer thicknesses in excess of 10 in.

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69 Series1 Log. (Series1) 100 Area Fatigue Cracking, % 90 80 70 60 50 40 30 20 10 0 0 5 10 15 20 25 30 HMA Thickness, inches Figure 6.16. Comparison of area fatigue cracking (area alligator cracking based on a percent of wheel-path area) and HMA layer thickness. Series1 Power (Series1) 1000 Tensile Strain, Micro-Strain 100 10 0 5 10 15 20 25 30 HMA Thickness, inches Figure 6.17. Comparison of the maximum tensile strain at the bottom of the HMA layer and HMA thickness. Fatigue Cracking Log. (Fatigue Cracking) 100 90 Fatigue Cracking, % 80 70 60 50 40 30 20 10 0 10 100 1000 10000 Tensile Strain, micro-strains Figure 6.18. Comparison of the area fatigue cracking and maximum tensile strain computed at the bottom of the HMA layer.

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70 Fatigue <1% Fatigue <2% Fatigue <4% Fatigue <8% 100.00 95.00 90.00 85.00 Survival, % 80.00 75.00 70.00 65.00 60.00 55.00 50.00 0 50 100 150 200 250 300 350 400 Tensile Strain, micro-strains Figure 6.19. Survival curves based on the maximum tensile strain at the bottom of the HMA layers of flexible pavements included in the LTPP program. or the survival curve from the one developed in 1995 are sum- It is expected that this is not the reason for the difference marized as follows: in survival curves. There was a change in the LTPP definition of longitudinal The SPS-1 projects were added to the updated analysis. It cracking in the wheel path. The change in definition defi- is expected that including the SPS-1 projects did not cause nitely could have affected the updated survival curve. Many this difference in findings, unless the fatigue cracking ini- of the previously measured longitudinal cracks that are as- tiated from some other design-site feature that would have sumed to have initiated at the surface are now recorded as a higher probable occurrence within the SPS-1 test sec- alligator cracking and are assumed to have initiated at the tions, as compared to the GPS sections. In addition, the bottom of the HMA layer. The cracking maps and video dis- study completed for the Asphalt Pavement Alliance con- tress data logs can be reviewed to segregate longitudinal cluded that there was a possibility that the GPS test sections cracks with crack deterioration along the edges from tradi- selected by the individual agencies for the LTPP program tional alligator cracks. This evaluation process is time con- are biased towards the better performing pavements. The suming. Figure 6.20 graphically presents the change in per- SPS-1 projects were built during the LTPP program and centage of survival sections as a function for varying alligator would not be biased toward better performing pavements. cracking levels for different tensile strains at the bottom of Tensile Strain=50 mils/in. Tensile Strain=75 mils/in. Tensile Strain=100 mils/in. Tensile Strain=150 mils/in. 100 95 90 85 Survival, % 80 75 70 65 60 55 50 0 1 2 3 4 5 6 7 8 9 Alligator Cracking, % of Wheel Path Figure 6.20. Percent survival of test sections for different levels of alligator cracking and tensile strain at the bottom of the HMA layer.

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71 the HMA layer. As shown in Figure 6.20, the 150 ms (mils/ An additional reason or explanation for the difference in in.) curve deviates from the other relationship. Errors in results is that there is no endurance limit for HMA mixtures. measuring small amounts of alligator cracking as well as a change in the definition for alligator cracking could have In summary, it is still believed that the endurance limit is caused this anomaly. This indicates that other types of crack- an HMA mixture property. Based on the results from the ing may be included as fatigue cracks for pavements with updated survival analysis, however, forensic investigations strain levels of 100 ms or less at the bottom of the asphalt of the test sections with the thicker HMA layers are needed layer calculated using an equivalent annual temperature to confirm the location of crack initiation and other assump- and 18-kip axle load. To determine the cause of the anom- tions used noted above in the survivability analysis. aly requires that forensic investigations be completed on these much thicker HMA sections to determine the cause of Affect of Polymer Modification the recorded alligator cracking. on Field Performance The location where alligator cracks recorded in the LTPP database initiated are assumed. As noted above, alligator Von Quintus et al. (71) conducted a study to quantify the cracks are assumed to initiate at the bottom of the HMA effects of polymer modification on pavement performance. layer and propagate to the surface. The validity of this Sites were selected from the LTPP database, NCAT Test Track, assumption would have an effect on the survival curve. FHWA Accelerated Loading Facility (ALF), and a number In addition, the maximum tensile strain at the bottom of of Canadian provinces and U.S. states with good records of the HMA layer was based on the assumption of full-bond performance and material properties. For each polymer mod- between all HMA lifts. If partial bond exists between two ified section, a control mix or two to three unmodified sites lifts near the surface, load-related cracks can initiate at that were selected for comparison. The unmodified sections were location and propagate downward as well as upward. A termed companion sites. full forensic investigation will be needed to determine the The performance of the polymer modified asphalt (PMA) location of where these cracks, recorded in the LTPP data, test sections and their companion sections was compared initiated and the mechanism (debonding between adjacent using a normalization technique that is based on computing HMA lifts) that resulted in those cracks. damage indices for each test section. This normalization tech- The initial and updated survivability analysis was performed nique uses M-E models to reduce the effect from confounding assuming that stripping or moisture damage is not present factors between projects. The M-E performance prediction within the HMA layer. Stripping and moisture damage were models were calibrated to local conditions using performance adequately identified during the initial sampling and coring data from companion sections (without any additive or mod- program for the GPS test sections. For the SPS-1 projects, ifier in the HMA mixture). This local calibration procedure stripping or moisture damage may have occurred on some was used to estimate the true effect of PMA because of the vari- of the projects and resulted in premature alligator cracking ation and errors associated with the models selected for use. for the thicker sections. This possible cause for the difference Figure 6.21 presents a comparison of the fatigue cracking in findings can be resolved with forensic investigations. predicted using the locally calibrated fatigue cracking equation Companion Sites PMA Sites Line of Equality 100 Measured Fatigue Cracking, % 90 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100 Predicted Fatigue Cracking, % Figure 6.21. Comparison between the predicted and measured fatigue cracking for neat HMA and PMA mixtures ( 71).

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72 Trend Line for Companion Sites Companion Sites PMA Sites 100 Measured Fatigue Cracking, % 90 80 70 60 50 40 30 20 10 0 0.01 0.1 1 10 Locally Calibrated Fracture Damage Index Figure 6.22. Comparison between the fracture damage index and measured fatigue cracking for neat HMA mixtures and PMA mixtures ( 71). and that actually measured for the sites. The data for the constructed with PMA can withstand a larger percentage unmodified sections fall along the line of equality, whereas of their maximum load repetitions for a given level of the data for the polymer modified section indicate that the cracking. actual cracking is less than the predicted cracking. Fig- Both Figure 6.21 and 6.22 support the findings in Chapter 4 ure 6.22 shows the damage index (DI) or ratio applied loads that indicate the polymer modified PG 76-22 should have a to allowable load before failure occurs versus measured higher endurance limit. This also indicates that the endurance fatigue cracking. Figure 6.22 also indicates that pavements limit is mixture specific.