Chip seals are widely used in Washington state as a cost-effective pavement surfacing method. However, they can be subject to variations in performance and premature failures. This project developed a quality acceptance specification for chip seal construction that incorporates the use of a laser texture scanner to provide valuable, data-driven insights for quality control and performance assessment to enhance the quality and effectiveness of chip seals.
Approximately 7,000 miles of Washington State Department of Transportation (WSDOT) roadway are paved with chip seals. Chip seals cost less than hot mix asphalt projects and can extend the life of asphalt pavements. However, chip seal construction acceptance relies on the experience and visual observations of field engineers and inspectors. This process has led to significant variations in chip seal performance, with some projects failing prematurely. One critical factor that contributes to these premature failures is the percentage of the chips embedded in the asphalt. Inadequate embedment leads to chip loss, while excessive embedment causes bleeding due to exposed asphalt on the surface. This not only shortens the chip seal life but also poses safety issues, such as hydroplaning and reduced skid resistance.
Currently, no data-driven measures are available to determine the percentage of chip embedment during construction. However, laser technology has been successfully utilized to measure the macrotexture of chip seal projects. To test its use for determining the percentage of embedded chips, the research team collected data with laser scanners during and after the construction of several WSDOT chip seal projects. They included experimental sections in the field to study modified chip gradation and compaction patterns.
The research team found that the rut depth in existing pavement strongly correlates with the percentage of embedded chips. They also found that emulsion and its asphalt residue migrate to rutted wheel paths even after construction, which increases the chip seal’s bleeding potential. It is critical to address rutting in existing pavement before chip seal placement.
When the chip seal is placed on new hot mix asphalt (HMA) inlay, the emulsion can drain into the new HMA if an inadequate fog seal has been applied on top of the HMA. This may lead to raveling (or loss of chips) of the chip seal. An oscillatory roller crushes chips and is not recommended for chip compaction. A combination roller, which consists of pneumatic tires and a steel drum, may be effective in compacting chips in the wheel paths and between the wheel paths. Sharp curves were also found to increase susceptibility to a loss of chips.
The researchers found that the mean profile depth (MPD) measured by the laser scanner, which is inversely related to the percentage of embedded chips, is a good indicator of chip seal performance. They developed a draft quality acceptance specification in terms of MPD. They also developed a program to determine the lower limit of the MPD, based on a formula developed in this study. They proposed an upper MPD limit of 170 mils (0.170 in.).
Report: WA-RD 942.1
Authors:
Haifang Wen
Juan Pinto
Kevin Littleton
Sponsor: WSDOT
WSDOT Technical Monitors:
Karen Carlie
Riley Bender
WSDOT Project Manager: Jon Peterson