Current procedures for asphalt mixture design in North Carolina requires contractors to conform to volumetric requirements on the air void content, voids in mineral aggregate, and other parameters at a fixed, traffic- and layer-specific compaction effort. The presumption in this case is that the mixtures produced under the same guidelines will have similar properties. However, recently, RP2019-20 has evaluated the mechanical properties of asphalt mixtures from across North Carolina produced under the same mixture design guidelines and found notable differences in the resultant performance. The observed material property differences were substantial enough that pavement performance predicted by AASHTO Pavement ME varied by 80% or more with respect to predicted fatigue cracking and up to 40% with respect to predicted asphalt concrete layer rutting. Differences like these are not currently captured in North Carolina mixture design standards and practices, nor are they captured in quality assurance and quality control procedures because direct assessment of mixture performance quality is not incorporated into either activity. This lack of evaluation can have substantial implications in the design, performance, and management of roadways.
Predicted performance of three different asphalt mixtures of the same class
In light of these issues, the proposed research plan will seek to achieve four objectives.
- Identify the most appropriate durability related testing protocol for incorporation into mix design and quality assurance/control operations.
- Establish initial threshold limits for the test identified.
- Develop a draft balanced mix design (BMD) procedure for North Carolina
- Develop a draft protocol for integrating the identified performance tests into quality assurance and quality control operations.
The primary outcome of the proposed research will be a test method and procedure that the NCDOT can deploy in asphalt mixture design and production to ensure that the mixtures delivered in the state have an acceptable level of performance. This effect would ensure better, longer lasting pavements, but also make refinements and improvements to structural design easier. These outcomes can be used by the NCDOT Materials and Tests Unit to improve their asphalt mix design process and may be used to improve quality assurance practices. This outcome could lead to improved asphalt pavement performance and lower long-term costs for North Carolina roadways.
