Recognizing the importance of in-place density in building cost-effective asphalt pavements, a Federal Highway Administration (FHWA) Demonstration Project was created for “Enhanced Durability of Asphalt Pavements through Increased In-Place Pavement Density.” The objective of the demonstration project was to determine the benefit of additional compaction and show that additional density could be obtained through improved techniques. This project effort included two major components: (1) a literature search to serve as an educational component regarding the best practices for increasing density, and (2) the construction of 10 field demonstration projects. Eight of the 10 states improved densities by at least 1% compared to a control section on their field demonstration projects. There were at least two pavement sections (a control and at least one test section) constructed within each of the 10 states that participated in this field demonstration project. Many of the states constructed more than two pavement sections for a total of 38 sections. There were many variables, including mixture type, construction equipment, and procedures between states and within states. A summary of the methods that states used to obtain increased density generally fell into one of five categories: (1) improving the agency’s specification by including or increasing incentives and increasing the minimum percentage density requirements; (2) making engineering adjustments to the asphalt mixture design to obtain slightly higher optimum asphalt content (although not part of the original goal of the demonstration project); (3) improving consistency as measured by the standard deviation; (4) following best practices; and (5) using new technologies.

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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.

The Superpave® mix design method has been used by the Colorado Department of Transportation since 1995. Initially, the number of design gyrations used was as recommended by SHRP research. However, the asphalt contents of these preliminary paving mixtures were lower than what had been historically successful for similar dense graded mixtures under similar conditions. This lower asphalt content began leading to premature moisture damage failures. Therefore, a study was initiated to determine how the pavements constructed according to the original Superpave compaction criteria densified under traffic. An experiment was designed to evaluate the in situ voids in 22 full-scale pavements over a 6-year period. Projects were selected on the basis of variation in traffic, environment, temperature, and elevation. Results of this research indicate that the original number of design gyrations recommended by SHRP for field-mixed–laboratory-compacted materials results in pavements that did not compact sufficiently under traffic, resulting in higher air voids than desired after 3 years service. An adjustment to the original SHRP design gyration levels was done in a subsequent study based on NCHRP research. The result of this revision has been a decrease in voids levels after 3 years of service with a corresponding improvement in performance. Further fine-tuning is needed and will be implemented in the future.