Impact on Compaction of Virginia’s Dense-Graded Asphalt Surface Mixtures From Recent Changes to Design and Construction Acceptance Criteria

About the report:

In 2016,the Virginia Department of Transportation implemented new design criteria for most dense-graded surface mixtures with the objective of improving material durability.  The 2016 construction season was also significant for a series of pilot projects that were designed to explore how potential incentives for in-place density might affect constructed quality.  These 2016 pilot projects included a special provision for incentivizing density that was notable in that it required direct measurement of in-place density instead of Virginia’s traditional approach, which was to accept compaction using a thin-lift nuclear gauge (an indirect method).  This report documents the in-place density and permeability characteristics of a series of projects that represented both the newly adopted design criteria and the density-based constructed quality incentive.

The new designs were analyzed and contrasted with earlier criteria using data from 2015 trials.  This analysis determined that the new criteria likely explain as much as a 1.5% average improvement in achieved in-place density. The better density is accompanied by substantially improved(lower) in-place permeability.  Selected data from the 2015 trials, as well as earlier research projects, also facilitated analysis to isolate any affect that could be attributed solely to the density acceptance method.  The average improvement with the new acceptance method (but the same mix design)was non-statistically significant at approximately 0.5%, but the percentage of samples passing the minimum density increased by 12 to 13%.   

Additional support from the Federal Highway Administration’s In-Place Density Initiative (2016/2018) enabled the researchers to follow asphalt compaction trends forward for 3 years past the2016 density pilot program.  This allowed for a review of the densities from field cores from 2017 through 2019, which provided for a far more developed discussion of related implementation and benefit consequences than is usually possible. This follow-up review suggested that improved densities (overall averages and percentage of passing samples) have continued.  A closing comparison to the previous-generation design criteria and the traditional density acceptance method suggested improvement in in-place quality that may offset $70M per year in maintenance costs through extended service lives.

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