The semicircular bend test was recently modified to develop the Illinois flexibility index test (I-FIT). The I-FIT test quantifies the cracking resistance of asphalt mixtures by using the flexibility index (FI), which includes the fracture energy and postpeak behavior of a mixture. This paper presents results from testing asphalt mixtures in Wisconsin. A statistical analysis approach was chosen for this study, in which the I-FIT procedure was used to differentiate between mixtures on the basis of changes in mixture composition and aging treatments. Mixtures included in this study varied in terms of the percentage of reclaimed asphalt pavement, design traffic levels, binder grades, modification levels, and aging conditions. In addition, a laboratory experiment to evaluate the effects of variability in mixture production, including asphalt content and filler content, on the I-FIT testing results was performed to provide a basis for evaluating the tolerance limits of the current production and for setting reasonable specification criteria. The statistical analysis indicated that the FI could discriminate between types of asphalt mixtures and aging conditions. The FI parameter was found to be a better parameter for capturing some of the critical changes in mixture variables and aging than the fracture energy and other parameters from the I-FIT procedure. It was also clear that the FI parameter was sensitive to the variation in production binder and filler contents within the tolerances of the current construction specifications of the Wisconsin Department of Transportation.

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Modifiers have been widely used to improve the engineering value of asphalt binders and the performance characteristics of asphalt mixtures. Recent studies have confirmed that binder modification can influence mixture performance by two mechanisms: improving the mechanical properties of the continuous phase (i.e., binder or the asphalt–mineral filler mastic) of the mixture and changing the initial or inherent aggregate structure of the mixture (i.e., aggregates’ packing). A study identifies the relative importance of these two mechanisms on the low-temperature response of asphalt mixtures by using both experimental testing and finite element model analysis. This study uses three modifications: elastomeric, plastomeric, and a combination of elastomeric and plastomeric (i.e., hybrid). The low-temperature mechanical characterization of asphalt binders and mastics are measured with the bending beam rheometer and the glass transition temperature test. Mixture performance at low temperatures is evaluated with the asphalt thermal cracking analyzer. Internal structure of the aggregates is quantified by two-dimensional imaging analysis by using the recently developed Image Processing and Analysis System. Results indicate a significant relation between indexes describing the internal structure of the aggregates and the laboratory performance indicators at low temperatures. Experimental and finite element model simulation results confirm that modification enhances mixture responses by improving the initial internal structure of the aggregates, thus allowing a more favorable distribution of thermal strains within the binder. These results suggest new opportunities to optimize mixture resistance to cracking through binder formulation.

The Hamburg wheel-tracking test (HWTT) has shown promise to predict permanent deformation resistance and moisture damage potential of asphalt mixtures. Several state agencies have implemented the test as a mixture evaluation and design tool. One aspect of the test that remains a topic of research is the testing temperature. Many studies and specifications use 50°C for all testing, but some use a test temperature that depends on the base asphalt used in the mixture. Concern exists about the use of 50°C as the sole test temperature in cooler weather regions, such as Wisconsin, because the asphalts used in such regions tend to be relatively soft (high temperature grades of PG 58 and below). This paper presents findings in support of an effort to apply the HWTT to mixtures in cold climates with the use of three test temperatures and several mixture design variables. The paper presents the effects of the mixture design traffic level, the PG of the binder, and the binder modification level on the deformation resistance, creep slope, stripping slope, and stripping inflection point (SIP). The HWTT was found to be sensitive to the factors evaluated in this study. On the basis of statistical analysis of the test data, logical trends were observed. The testing temperature was found to affect not only the response variables but also the level of significance of controlled factors. The effectiveness of the SIP to characterize the moisture sensitivity of mixtures requires more research to validate the effect of moisture damage on HWTT results.