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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 77°F (25°C). 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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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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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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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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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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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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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.
