State departments of transportation (DOTs) are at various stages of implementation of their balanced mix design (BMD) program. Some states have not yet started and may still be exploring the feasibility of integrating BMD within their asphalt pavement program, while others have already moved forward with implementation activities and are currently at different stages of the implementation process. The latter have valuable experience and lessons learned that could facilitate the implementation of a BMD program into practice to improve long-term pavement durability and performance. Thus, an effort was initiated to identify and put forward positive practices from state DOTs when implementing BMD and performance testing of asphalt mixtures. To accomplish this objective, information was collected through virtual site visits, and other means made necessary by the coronavirus pandemic, with seven key state DOTs. As a result of this effort, it was identified that five state DOTs out of seven use Approach A for the BMD process as defined in AASHTO PP 105-20, and one state DOT uses a combination of Approach A and Approach B. One state DOT also allows for Approach C while another state DOT allows for Approach D. Eight major tasks for the overall process for implementing BMD as part of mixture design approval and acceptance are established, and positive practices with examples for each task are provided. Those successful efforts used by state DOTs could be considered by other state DOTs in their effort to implement BMD within their asphalt pavement program.

Asphalt binders are common construction materials, however due to time-and temperature-dependence, their mechanical properties are often difficult to characterize. Several standard tests methods exist to describe their complex behavior. This paper presents an exploratory feasibility study of a flat-tip indentation testing to analyze the linear viscoelastic properties of asphalt binders. Depthsensing indentation testing has been extensively used to characterize the properties of many engineering materials, however the applications to asphalt binders are very limited. This paper presents a simple solution for the creep compliance in tension derived for flat-tipped indenter. This solution was verified with the Finite Element Analysis and then applied to the experimental results from the indentation testing performed on one typical unmodified asphalt binder. The testing was conducted at three different low temperatures and under three different creep load levels to verity the linearity of the response, and to evaluate the robustness and applicability of the indentation method. Furthermore, the creep compliance functions determined from the indentation testing were compared with a more traditional 3-point bending experiments. The results show that there is a non-uniform discrepancy between the two testing methods, most likely due to nonlinear behavior of the asphalt binder at higher temperatures and micro-damage of the binder samples at lower temperatures. Other possible sources of error between indentation and 3-point bending are problems determining the initial tip-specimen contact surface and possible tip-specimen adhesion. It is concluded that flat-tipped indentation at low temperatures should be performed at lower load levels to avoid excessive stress concentrations that leads to micro-damage and nonlinear response of asphalt binders. Alternatively, asphalt binders at low temperatures could be evaluated using different indenter geometries, such as spherical or pyramidal, using corresponding parameter interpretation procedures.

Performance testing has been recognized by state highway agencies (SHAs) in the U.S. and the asphalt paving industry as an important tool to complement volumetric properties for improving asphalt pavement performance. Thus, Maine Department of Transportation (MaineDOT) initiated a research effort in 2019 to evaluate the cracking and rutting resistance of asphalt mixtures using several performance tests, including the Hamburg wheel-tracking test (HWTT), indirect tensile cracking test (IDEAL-CT), cyclic fatigue test, and stress sweep rutting (SSR) test. These tests were conducted on reheated common plant-produced asphalt mixtures, and results were analyzed to: (1) develop baseline rutting and cracking performance; (2) evaluate the effects of mixture properties on the performance test results; and (3) verify the performance enhancement from the extended use of polymer-modified asphalt binders. Several mixture properties, such as nominal maximum aggregate size (NMAS), binder performance grade (PG), binder content (Pb), and reclaimed asphalt pavement (RAP) %, were found to have statistically significant effects on the mixture rutting and cracking resistance, especially the HWTT and IDEAL-CT results. Based on the proposed criteria for rutting strain index (RSI) and apparent damage capacity (Sapp), the asphalt mixtures tested would provide satisfactory rutting performance under heavy traffic, and satisfactory cracking performance under standard traffic. In addition, based on the IDEAL-CT benchmarking results, mixtures with polymer-modified binder and/or smaller NMAS were found to have higher cracking tolerance index (CTindex) results. The information from the research effort will help MaineDOT to achieve its goal to move beyond sole use of volumetric properties for asphalt mixture design and acceptance with the implementation of balanced mix design (BMD) for improving the field performance of asphalt pavements.

In this study, micro-Deval and Los Angeles (LA) abrasion tests were used to evaluate the durability of 72 coarse aggregates used for hot-mix asphalt (HMA) in Maine. Aggregates used in HMA must be durable and resistant to abrasion and degradation. Material loss in HMA pavements has recently been observed by the Maine Department of Transportation. Aggregate degradation has been hypothesized as a possible contributor. The micro-Deval results showed no correlation with results from the LA abrasion; the range in values was quite large. Two alternative methods of analyzing micro-Deval results were employed to measure the change in gradation of aggregate samples. A relatively large portion of tested aggregate sources was found to degrade significantly in the micro-Deval test while having acceptable AASHTO micro-Deval loss values, presumably as the result of fracturing instead of abrasion. The weighted-average method and the area-between-curves method proved to be effective in measuring the change in particle size distribution not captured with the Method 1 micro-Deval loss value. In addition, a significant influence of initial grading size was found in all the micro-Deval data, with finer initial gradations producing higher loss values. The alternative analysis methods for micro-Deval are recommended for use in detecting degradation not captured by the traditional micro-Deval value.

Performance testing has been recognized by state highway agencies and the asphalt paving industry as an important tool to complement volumetric properties for improving asphalt pavement performance. Thus, the Vermont Agency of Transportation (VTrans) began evaluating the Hamburg wheel-tracking test in 2015, Illinois flexibility index test in 2018, and indirect tensile cracking test in 2020 as steps toward balanced mix design (BMD) implementation. These tests were conducted on reheated common plant-produced asphalt mixtures for multiple shadow projects, and the results were analyzed to determine the typical production variability observed in the performance testing results. Production variability is important to assess potential specifications in field production, especially for statistical quality acceptance procedures, such as percent within limits. Based on the percent within limit analysis, most of the modified Type IVS (i.e., 9.5 mm) mixtures would be acceptable based on the proposed performance requirements, while those found not acceptable were close to meeting the performance requirements and would need minimal adjustments. The lot data was further examined to generate typical lot production variability values for potential use in specification development. The information from this research effort will help VTrans achieve its goal to move beyond solely volumetric properties for asphalt mixture design and acceptance and serve as an example for other agencies exploring the implementation of BMD for improving asphalt pavement performance.