VDOT Research Council Publishes Two Research Studies
Evaluation of a Density Profiling System for Asphalt Pavement Density Measurement
Abstract - During asphalt pavement construction, obtaining adequate and uniform density throughout the pavement layer through the compaction process is critical to achieving the desired performance. Optimum pavement density reduces oxidation potential and moisture damage, decreases rutting potential, and improves fatigue life. Numerous national research projects have cited pavement density as one of the most influential parameters defining the service life of asphalt pavement.
The Virginia Department of Transportation’s (VDOT) asphalt pavement program requires the contractor to perform nuclear density readings and density measurements from cores and sawn plugs for acceptance. This process is not only destructive and potentially time-consuming but assesses only the density of a small portion of the pavement mat (about 0.003%). Although VDOT and other agencies use these processes, localized substandard density may not always be identified. Furthermore, this process cannot provide real-time feedback during the paving operation. VDOT needs to use methods that can assess a larger sample more rapidly to reduce the risk of accepting substandard material. One such method is by employing density profiling system (DPS) technology. The benefits to agencies using DPS technology include the ability to conduct measurements over the entire surface area of the new asphalt pavement layer, measure the density along the longitudinal joint where many pavements experience premature failures, and implement a statistical evaluation of the achieved density. The benefits to contractors include the ability to evaluate their compaction processes and make adjustments during the course of the work when improvements can still be implemented before paving completion.
The study concluded that DPS technology can describe relative changes in the density of asphalt mixtures placed in the field based on measurements of the dielectric value and that it offers sufficient data volume for density uniformity estimates. The study also concluded that DPS is a promising technology for process control purposes but is still premature for VDOT to use for density acceptance.
The study recommends that VDOT’s Materials Division not consider using DPS at this time to assess the density of paved asphalt mixtures for acceptance purposes because additional studies are needed to better understand the effects of mixture variables on the dielectric results. The study recommends that VDOT’s Materials Division consider using DPS to assess relative changes in paved asphalt mixture density during construction where desired. The study recommends that VDOT’s Materials Division consider using DPS density testing results to evaluate achievable density uniformity. The study also recommends that the Virginia Transportation Research Council and VDOT’s Materials Division submit a research needs statement to the Pavement Research Advisory Committee to help further understand the remaining unknowns through additional testing during future construction seasons.
Pavement Preservation Demonstration Projects for Chip Seal and Microsurfacing Treatments
Abstract - In 2021, the Virginia Department of Transportation (VDOT) collaborated with the National Cooperative Highway Research Program (NCHRP 20-44(26)) project research team to develop three specifications for chip seals, microsurfacing, and fog seals, incorporating recommendations from guidelines developed under the NCHRP 14-37 project and American Association of State Highway and Transportation Officials specifications. Subsequently, during the 2022 construction season, VDOT initiated two demonstration projects constructed under these new specifications. One project involved a chip seal application in the Richmond district, and the other project involved a microsurfacing application with a fog seal treatment on the shoulders in the Hampton Roads district. Each demonstration project included additional sections treated according to VDOT’s current specifications, serving as reference sections for the corresponding demonstration sections.
The study assessed the effectiveness and performance outcomes of the demonstration sections constructed using the new specifications by comparing them with the reference sections constructed using the existing specifications. The effort included comprehensive documentation of preconstruction surface conditions and construction processes across project sections, incorporating qualitative visual inspections, quantitative measurements, and evaluations of material selection, application methods, and quality control measures. Furthermore, researchers assessed the short-term field performance of each section, monitored for up to 13 months, with performance metrics such as texture, friction, rut depth, and ride quality.
The results indicated that the new chip seal specification did not lead to improved performance outcomes for the chip seal (demonstration) section compared with the chip seal (reference) section constructed using VDOT’s current chip seal specification. Various factors affected the field performance of the chip seal treatment under the new specification, including challenges in achieving the target aggregate application rate and non-uniform spreading of aggregates during construction. In addition, differences in structural application, with one section featuring a modified single-layer chip seal application and the other a single-layer chip seal application, contributed to performance disparities. Further assessment through additional field trials is necessary to comprehensively evaluate the effectiveness of the new specification. Similarly, the new microsurfacing specification did not yield short-term performance differences for the microsurfacing (demonstration) section compared with the microsurfacing (reference) section constructed using VDOT’s current microsurfacing specification.
The study recommends conducting additional field trials that include modified single-layer chip seal applications designed according to the new specification developed in this study to assess its effect on chip seal performance. In addition, the study recommends that VDOT should maintain the use of its current special provision for microsurfacing projects.