Aaron Leavitt

Different methods were compared for analyzing Hamburg wheel tracking test (HWTT) rutting curves to evaluate the viscoplastic and stripping-induced rutting performance of asphalt mixtures. Two common methods—the sixth-degree polynomial and logarithmic-based model—were evaluated against a new two-part power (2PP) model. The 2PP formulation provides a unified representation of the rutting curve and supports adjusted stripping inflection point (SIP) calculation based on a 12.5 mm rut depth threshold. A total of 358 mixtures from the Federal Highway Administration (FHWA) Pavement Testing Facility (PTF) project and the Texas Department of Transportation database were analyzed. An open-source software tool, AutoHWTT, was developed to automate HWTT data analysis using all evaluated methods. The results show that corrected rut depth (CRD) values at 20,000 passes from the three models were highly correlated, while the polynomial model produced unreliable stripping number (SN) estimates for mixtures. The adjusted SIP showed slight improvement, in correlation with SN (74.2 versus 68.5 percent for conventional SIP) and a reduction in misclassification against the 15,000-pass threshold (26.7 versus 45.7 percent). Results from FHWA PTF mixtures show that stripping-induced rutting dominated total rut depth (TRD) variability; this rutting measurement variability was significantly reduced when using CRD as a rutting indicator. Additionally, ideal rutting test (IDEAL-RT) results followed CRD trends, but did not correlate with those of TRD. This observation demonstrates the detrimental influence of stripping on TRD when used as a rutting metric. The 2PP model offers an efficient and integrated framework for HWTT analysis, with adjusted SIP improving stripping assessment.

Validation of balanced mix design (BMD) test thresholds is essential for building confidence in performance-based asphalt mix designs. To support Texas’ BMD implementation, multiple field test sections statewide have been monitored, accumulating data from preconstruction through several years of service. These sections, exposed to traffic levels ranging from 600 to 23,000 average annual daily traffic (AADT) and up to 1.0 million equivalent single-axle loads (MESALs), provide valuable insights into test thresholds across diverse traffic and regional conditions. Complementary data from three national center for asphalt technology test track sections subjected to accelerated loading compare traditional volumetric designs and BMD specifications. Additionally, materials from the original WesTrack test track were evaluated with current cracking and rutting tests to relate historical field performance with present protocols. Despite challenges in collecting long-term field data and variability from traffic and environmental factors, key conclusions emerged. The indirect tensile asphalt cracking test and Texas overlay tester effectively gauge cracking resistance differences among mixtures, correlating well with field outcomes. The Hamburg wheel tracking (HWT) test reliably identifies mixtures prone to rutting, supported by minimal rutting observed in TxDOT sections. The IDEAL-rutting test (IDEAL-RT) consistently differentiates rutting resistance and predicts field rutting performance, underscoring its value in mix design. Early reflection cracking observed within two years exhibited strong correlation with the cracking tolerance index (CT Index), indicating higher CT Index values improve cracking resistance. Gradation also significantly influences CT Index and crack progression rate (CPR) results, emphasizing the need to incorporate gradation-specific thresholds. These findings enhance understanding of BMD test strengths and limitations, informing refinements aimed at improving pavement durability statewide.

This study evaluates the rutting performance of asphalt binders used in the construction of the Pavement Testing Facility (11 lanes) at the Federal Highway Administration’s Turner-Fairbank Highway Research Center. The binders were either sourced directly from the asphalt plant tank during construction (“tank binders”) or extracted and recovered from asphalt mixtures sampled during construction (“recovered binders”). The researchers performed laboratory tests, including high-temperature performance grade (HTPG), multiple stress creep recovery (MSCR), and frequency sweep tests. Results from the overall binder ranking, based on analyzed outputs from all tests, showed the superior rutting performance of polymer-modified binders, followed by binders recovered from recycling agent (RA)-modified high-reclaimed asphalt pavement (RAP) mixtures, control mixtures, and the no-RAP mixture. The binder rankings based on nonrecoverable creep compliance at 3.2 kPa from the MSCR test and zero shear viscosity (ZSV) from the frequency sweep test correlated most closely with overall rankings, whereas HTPG and Shenoy parameters showed limitations in effectively classifying binders. Given the applied small strains within the linear viscoelastic region during the frequency sweep test, ZSV appeared to be the most promising viscoelastic-based parameter capable of providing robust evaluations of expected rutting resistance comparable to the MSCR test. The study further validated the effectiveness of RA modifications in accommodating elevated RAP contents while maintaining high-temperature performance comparable to control mixtures with lower RAP levels. In addition, recovered binders exhibited significantly higher expected rutting resistance than tank binders, a finding that can potentially be attributed to RAP incorporation and aging effects during production, hauling, and storage.

The increased use of recycled asphalt pavements (RAP) is motivated by the need to enhance resource efficiency and reduce environmental effects. Asphalt recycling agents may also be added to high RAP mixtures to mitigate the impact of stiff, brittle RAP binder, leading to enhanced performance. This study evaluated the properties of binders and mixtures that contain high RAP (i.e., 40%) and a recycling agent. The results were compared against a mixture with 25% RAP and no recycling agent. Concerning the binder properties, the recycling agent enhanced the low-temperature properties and decreased stiffness more than it affected the high-temperature properties. However, the recycling agent increased the aging susceptibility of the asphalt binder. Mixture performance tests showed that the high RAP mixture with recycling agent had comparable or even better performance at intermediate and high temperatures than the mixture with 25% RAP. These findings demonstrate the benefits of adding the recycling agent utilized in this study to increase the RAP content in the mixture while restoring binder properties and improving cracking resistance. However, one must consider the potential impact of recycling agents on accelerating mixture aging.

This paper develops a simple asphalt pavement field aging model using IDEAL-CT data from 24 field sections from seven field projects that were monitored for 1–4 years each. Field distress surveys were conducted, and field cores were procured and tested in the laboratory approximately every 12 months for each field section. The model predicts IDEAL-CT parameter l₇₅/ m₇₅ (mm²/kN) from the post-peak load–displacement curve. An R² of 0.91 was achieved when comparing predicted values to measured values indicating that this ratio behaves predictably with field aging. A good correlation is also seen between this ratio and measured field cracking data, making it an excellent candidate for evaluating pavement cracking performance with field aging. This simple model has promising potential use cases including as a tool for mix design, pavement monitoring, and pavement forensics.

Economics and sustainability goals have driven higher use of recycled asphalt materials (RAMs), such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles, in the pavement industry. Owing to the brittle nature of aged binders found in RAMs, a mitigation strategy is usually needed to ensure adequate cracking performance. Determining a proper mitigation strategy is vital to designing long-lasting asphalt concrete (AC) pavements with balanced performance. This study explored the use of mitigation strategies on AC mixtures with intermediate and high RAP contents, using materials from four mixtures that were part of a mill and overlay project in Milford, Delaware. This project included one standard mixture and three experimental mixtures with 25%–40% RAP. This paper presents performance data for field cores, plant-mixed laboratory-compacted specimens, and laboratory-mixed laboratory-compacted specimens. The effects and benefits of using a softer binder versus a recycling agent and the effects of using both a softer binder and a recycling agent to mitigate the stiff RAP binder are explored. To understand the effects of each mitigation strategy with aging, data from one-year field-aged cores and from laboratory-aged specimens are shown. Adjusting only one variable to mitigate the increased stiffness and brittleness of recycled binders yielded the best results in this study. Additionally, some test parameters were found to be sensitive to aging, including the flexibility index from the semicircular bend test and the cracking test (CT) index from the IDEAL-CT, while others were not observed to be sensitive to aging, including the fracture energy measured by the low-temperature disk-shaped compact tension test.