Vehicle collisions and increases in collisions rates during wet conditions are one of the major safety concerns for the NCDOT. Collision rates increase when the surface is wet because skid resistance reduces under these conditions. In recent years the NCDOT has conducted different research efforts to characterize the friction and texture characteristics of North Carolina mixes. Most recently, NCDOT RP 2020-11 quantified the impact of new asphalt overlays on friction and texture values and RP 2022-5 has been evaluating preliminary friction and texture performance models. Both projects have evaluated the effect of mixture compositional factors on the short- and long-term performance of both friction and texture.
Low macrotexture may contribute to reduced skid resistance values in the field. Revising the existing asphalt mixture categories to solve these problems may result in many practical issues due to contractor practices, familiarity with mixture designs, and unintended consequences to durability. On the other hand, a preliminary evaluation of the surface mixture guidelines in South Carolina and Virginia shows important differences with the NCDOT current practice. Both state DOTs use coarser gradations for dense-graded surface mixtures and have SMA mixes as an option to use in roads with high traffic volumes and high friction demand. Therefore, a research study is needed to identify alternative structural mixture designs that can be specified to ensure adequate friction and texture in North Carolina.
With respect to this need, the proposed research plan will achieve two objectives; 1) characterize friction and texture performance of alternative surface mixture designs from other states, and 2) identify alternative structural mixture designs that can be specified to ensure adequate friction and texture performance in North Carolina. These objectives will be met with six tasks.
- The relevant literature review that focuses on studies on friction and macrotexture performance curves and mixture composition factors that relate to these characteristics. The use of alternative mixture designs, such as SMA, open-graded friction courses (OGFC), and ultrathin bonded wearing courses (UTBWC) and their implementation in other states will be included as well.
- Identify sites in Virginia and South Carolina with coarse-, dense-graded and SMA surfaces (non-OGFC or UTBWC).
- Collect friction and texture values following the same procedure applied in research projects RP 2020-11 and RP 2022-05. These measurements will be collected in close coordination with the SCDOT and VDOT/VTRC personnel.
- Acquire and conduct performance testing of materials from the surface layers from Virginia and South Carolina. This material samples will be used to benchmark the mixes of both states with respect of North Carolina mixes.
- The data analysis will consist of; 1) spatial analysis, 2) site characterization, and 3) comparison of friction and texture values from SCDOT and VDOT mixes with typical NCDOT approved mixes.
- A final report summarizing the methodology, results, and recommendations will be prepared.
The primary outcome of the proposed research will include new or improved asphalt mix design specifications updated to include new mixture categories that could be selected by NCDOT pavement designers in situations that warrant higher surface texture and/or friction. Given the existence of specifications for such mixes in other states, it is likely that the recommendations can have immediate impact. These outcomes can be used by the Traffic Safety and Materials and Test Units of the North Carolina DOT.