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Publication Number:  FHWA-HRT-11-045    Date:  November 2012
Publication Number: FHWA-HRT-11-045
Date: November 2012


Performance Testing for Superpave and Structural Validation


This report provides a critical evaluation of the Superpave® binder specifications |G*|/sinδ and |G*|sinδ as controlling parameters for rutting and fatigue cracking. Previously, the SHRP program explored a variety of theoretical principles and experimental techniques and ultimately selected these parameters because they are fundamental, easily measured in a DSR, and describe the dependence on temperature, frequency, and aging, all of which are relevant to factors that influence pavement performance. However, the SHRP binder validation was largely limited to unmodified asphalts. Since the implementation of Superpave®, polymer modified asphalt binder usage has become prevalent. Key research studies identified shortcomings using |G*|/sinδ to control rutting of polymer modified asphalts; principally, the measurement of these material properties are in the small strain, linear viscoelastic region that does not mobilize polymer structures that impart beneficial performance. Research also showed shortcomings of fatigue parameter |G*|sinδ, as this seemed to be valid only for thinner, unmodified asphalt pavements.

Following exploratory laboratory studies of polymer modified asphalts (NCHRP 9-10, 90-07), a full-scale APT experiment was designed.(20,22) This ALF experiment generated performance data that provided quantitative grounds to screen weaker tests and identify discriminating binder characterization parameters to potentially replace or enhance portions of the existing Superpave® PG system. Asphalt test pavements were built with a single volumetric mix design using a variety of unmodified and polymer modified binders with practically equivalent high-temperature properties and different intermediate-temperature fatigue properties.

The experiment also included pavement sections to assess the performance of crumb rubber modified asphalt. A case study for FWD layer modulus back-calculation was provided and an assessment of emerging AMPT test equipment and NCHRP MEPDG software was made.


Binder Performance Specification Parameters

Key findings are as follows:

Binder Test to Discriminate Permanent Deformation and Rutting

Key findings are as follows:

The research literature discussed in chapter 1 documents the shortcomings of the Superpave® high-temperature PG test. Namely, the test deforms the binder to small strains and has trouble capturing the performance-related effect of polymer modification. However, the rutting performance of the polymer modified and unmodified asphalt binders in this particular full-scale APT experiment do not reflect this deficiency. The binders used in this study were designed and selected to have the same high-temperature PG (see figure 15). Therefore, these binders should have exhibited the same rutting performance, which was essentially the case (see figure 28, figure 29, table 17, and table 19). There were quantifiable differences in the mean rutting, but variability in the measured performance diminished those differences. Therefore, one interpretation of this experiment’s rutting data is that |G*|/sinδ still has its merits as a specification parameter to control rutting.

It must be strongly emphasized that there were notable numerical and statistical challenges in making defensible comparisons between the varieties of binder candidates to identify the best tests. Ideally, such analyses use more data points with larger spread to overcome scatter. Simply using R2 was not appropriate. The Kendall’s tau measure of association method was utilized because it is better suited for small datasets and quantifies the quality of a ranking. Statistical significance (probability) was computed as well. The different quantities were combined into a single composite score. Laboratory scale performance tests were integrated to strengthen the scoring because the binder contributions are reflected in laboratory tests in addition to full-scale pavement tests, which provide the analysis with a degree of balance with the full-scale ALF performance.

Some candidate binder parameters such as LSV and ZSV were found to have a strong relation to laboratory-scale and full-scale performance. However, these parameters can be susceptible to registering improved performance from additives or modification (i.e., acid), which is not as legitimate as the beneficial improvements from polymer modification used in this study. Thus, they were not recommended.

Binder Tests to Discriminate Fatigue and Cracking

There were fewer numerical and statistical challenges in the datasets to rank weaker and stronger binder tests for fatigue cracking. The strongest analysis came from the thinner, 4-inch (100-mm) ALF lanes because two of the thicker, polymer modified mixtures in the 5.8-inch (150-mm) lanes did not exhibit any fatigue cracking. Nondestructive testing and extrapolated estimates were used to provide a reasonable measure of ranking for the whole set of 5.8-inch (150‑mm) lanes.

The key finding is as follows:

Crumb Rubber Modified Asphalt

The key finding is as follows:

The CR-AZ mix was formulated using a PG58-22 base binder plus 17 percent crumb rubber by weight following ASTM D6114.(24) The estimated high and intermediate PG grades were 194 and 74.1 °F (90.1 and 23.4 °C), respectively using techniques developed in research based on DSR measurements on unaged binder. The CR-AZ binder could be characterized in the RTFO by tilting the oven back and in the 0.975-inch (25-mm) plate DSR using a 0.078-inch (2-mm) gap rather than a 0.039-inch (1-mm) gap. The aggregate gradation was gap-graded to accommodate the crumb rubber particles, and the binder content was 7.1 percent. The mix was produced using an onsite shearing mill at the asphalt plant. The rutting performance was similar to the dense-graded mixtures in the experiment, although the mixture has a higher binder content and somewhat lower mix dynamic modulus. The fatigue performance was the best of the 4-inch (100-mm) lanes. The ability to arrest the bottom-up fatigue cracks is attributable to the rich rubber modified binder and possibly because the stress concentration is in the lower control lift.

The CR-TB binder did not require onsite mills at the plant and was delivered and handled like conventional asphalt binder. The CR-TB mix performed well. The rutting performance was the best of all the 4-inch (100-mm) lanes because it had slightly stiffer Superpave® PG than the others, and the fatigue performance was slightly less than the SBS modified binder.

Fiber Reinforced HMA

Key findings are as follows:

Mixture Performance Characterization Tests

Key AMPT findings are as follows:

Key axial fatigue findings are as follows:

Other key findings are as follows:

A variety of materials characterization tests were conducted on ALF cores, plant-produced mixtures, and laboratory-produced mixtures. Some were established tests while others were emerging tests. The following tests were utilized in this research:


FWD testing revealed variation in the stiffness of the unbound layer within each binder’s test lane and between test lanes. Back-calculation programs EVERCALC and MODCOMP, having different optimization algorithms, were evaluated and found to provide the same trends. The average stiffness of the crushed stone base was between 11.9 and 9.6 ksi (82 and 66 MPa), slightly stiffer than the average subgrade stiffness ranging between 11.4 and 11.1 ksi (79 and 77 MPa). Known depth to bedrock was easily detected. EVERCALC moduli were chosen because the base was generally stiffer and the subgrade was generally softer. Predicted HMA tensile strain using laboratory-measured modulus and FWD back-calculated base and subgrade stiffness gave reasonable magnitudes compared to what was measured from embedded strain gauges.

The use of MDDs was key in evaluating the quality of the back-calculations. Forward predictions of the base and subgrade layer deflections using the back-calculated modulus revealed very good agreement on the top of the base layer, fair agreement in the center of the base layer, and less agreement at the bottom of the base layer. This indicated that the back-calculation could have been additionally optimized. However, the focus of the experiment was the HMA layers, and the back-calculated moduli were deemed sufficient given that the critical strains and stresses for the purposes of this study were located within the asphalt layers. A seasonal monitoring program enables the depth to the seasonally-affected and unaffected subgrade layer to be estimated and identified more accurately.

Mechanistic–Empirical Pavement Performance Analysis

The key finding is as follows:

The NCHRP 1-37A and NCHRP 1-40D MEPDG models are largely calibrated using data from LTPP.(1) These do not include any significant amounts of polymer modified asphalt data, whereas the ALF experiment included several different polymer modified asphalts. Consequently, the rank order and the magnitude of predicted rutting and fatigue cracking were not accurate. In fact, the actual MEPDG software was not able to be used for the predictions because it was not developed to accommodate specialized APT conditions (i.e., controlled temperatures, heavy wheel loads). A standalone application that emulates the MEDPG had to be used.

Just as with the binder parameters previously discussed, modulus measured at small strains (binder |G*| and mixture |E*|) does not mobilize the material into regions that force the particular performance aspects to be revealed, whether permanent deformation or fatigue crack resistance. Along these lines, the NCHRP 9-30A project is pursuing this type of material characterization input and recalibration of the guide for improved rutting prediction.(90) This study illustrated an approach similar to the methodologies that will come out of NCHRP 9-30A, which use flow number permanent deformation curves to yield mixture-specific coefficients for the MEPDG model framework rather than a single national calibration for all HMA. Magnitudes of the predicted rutting were improved drastically, but the ranking was still not captured. However, the statistical similarities and variability in the measured rutting bracket the improved predictions. Additional insight gained from the flow number-based rutting prediction analysis is that the value of characterization at more than one temperature would allow a better prediction of rutting over a range of temperatures.

Two aspects of damage and fatigue cracking that the MEPDG does not provide were highlighted. Seismic PSPA tests on two ALF lanes illustrated that HMA modulus decreases by as much as 50 percent before fatigue cracks show at the surface. HMA modulus in the MEPDG is not recursively updated to reflect damage and modulus reduction. The pattern in which actual bottom-up fatigue cracking develops was not captured whereby the surface remains crack free until surface cracking is initiated and propagates.

Despite these shortcomings, the MEPDG was useful in analyzing the uniformity of ALF construction. The national calibration containing many different material characteristics and structural features taken from LTPP can be assumed to adequately capture the impact variations in layer thickness, density (modulus), and base and subgrade support have on predicted rutting and fatigue. Three scenarios were evaluated: as-built, as-designed, and as-designed with fixed (average) base and subgrade stiffness. The very similar rankings revealed from the exercise indicated no significant changes in the rank order.


The following recommendations are made based on the conclusions from this research:

A number of follow-up activities are recommended, as follows:

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