A new performance-based chip seal mix design method was developed from the HWY-2008-04 project. This report addresses a few research needs that were identified critical to implement the performance-based mix design method.
The overall objectives of this research were to: (1) validate the developed chip seal mix design using analysis of field test sections constructed in different NCDOT Divisions, (2) construct single and multilayer chip seals using the developed mix design procedure, and (3) identify any adjustments and calibrations to the mix design that are necessary based on material type and/or existing surface conditions.
This report presents the findings from field experimentation and analysis as well as laboratory testing of extracted field samples. The research approach involved the construction of field test sections within multiple NCDOT Bituminous Divisions using granite 78M and lightweight aggregate, both with CRS-2L emulsion. In each Division, test sections were constructed using both typical rates that had been used in the Division and the rates determined from the NCSU's performance-based mix design method for comparative purposes. These sections were monitored by the research team to assess performance of the developed mix design under field traffic loading conditions.
Three dimensional laser scan analysis was used to determine changes in the aggregate embedment depth as a function of time and traffic loading. Additionally, the tack lifter test method under development in the NCDOT HWY-2014-03 project was utilized for measuring the existing surface absorption, which is a key adjustment for the developed mix design method. From the constructed field sections, samples were extracted and taken back to the laboratory to verify that the measured material application rates for each section were reasonably close to the target design material application rates.
It was found that the developed embedment depth algorithm effectively determines the embedment depth as emulsion application rate changes for a single layer of aggregate; and also captures the changes in embedment depth over time due to traffic loading. As expected, embedment depth increases as a function of traffic load until the asymptotic final embedment depth is achieved. Chip seal field validation sections constructed using the performance-based mix design performed well under field traffic and climatic conditions in multiple NCDOT Divisions after over one year in service. The performance-based mix design yields acceptable field performance while designing for less material than the existing NCDOT design approach using those same materials. This reduction in material costs in design could increase the cost-effectiveness of chip seals as a pavement preservation alternative. Construction-related material application rate variability was found to be significant in a majority of the field construction efforts and should be further evaluated in order to minimize the impact on chip seal performance.
