Vehicle collisions and increases in collision 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. The precise amount of loss is dependent on many factors, but the consensus among experts is that pavement friction and macrotexture are important factors that affect the skid resistance and changes in this resistance under wet conditions.
Although the NCDOT actively addresses skid resistance issues as they are identified, recent studies have shown that friction and, more notably macrotexture, are negatively affected when the pavements are newly overlaid. While the current studies have successfully identified the potential for issues in recently overlaid projects, they have not yet provided the DOT with a pavement friction and marotexture performance model. That is a function or set of functions capable of capturing and quantifying the magnitude of the change due to an overlay and the longer-term temporal changes in friction and macrotexture over the lifetime of the pavement surface. Without these performance curves it is difficult to impossible to identify performance thresholds, limits on friction and macrotexture that warrant investigation or intervention, and subsequently manage friction performance across the roadway network.
Likewise, the DOT does not have a robust set of data that will allow them to identify specific mixture related causative effects that can contribute to a higher or lower likelihood of surface issues immediately after an overlay and over time.
With respect to this need, the proposed research plan will achieve three objectives; 1) characterize friction and texture performance models, 2) develop friction and texture performance thresholds, and 3) identify asphalt mixture compositional factors (gradation, asphalt content, etc.) that affect the as-constructed macrotexture and friction.
These objectives will be met with five tasks:
1. The relevant literature on friction and macrotexture performance models, threshold analysis, and mixture compositional effects on friction and macrotexture will be reviewed and summarized.
2. Existing continuous friction tests and macrotexture measurements from North Carolina roadways will be cataloged along with relevant pavement details for the sites.
3. Data will be collected to complement and supplement the existing data catalog so that the research team can meet the above stated objectives.
4. The data collected in Task 3 will be analyzed to first calibrate the performance models, then to identify performance thresholds, and finally to detect the asphalt mixture composition related factors that contribute to good or poor performing pavement surfaces with respect to skid resistance.
5. A final report summarizing the methodology, results, and recommendations will be prepared.
The primary outcome of the proposed research will be an initial set of performance models that can be used to assess immediate and potentially long-term friction/macrotexture issues. The research will also produce a set of threshold limits for friction/macrotexture where investigatory and intervention steps need to be taken to control for safety. Finally, the research will produce information on the mixture design factors that contribute to higher or lower friction/macrotexture.
These outcomes can be used by the Traffic Safety and Materials and Test Units of the North Carolina DOT to predict and manage friction and texture performance on roadways and to understand when measurements represent a potential hazard exists. It will also be used to help identify asphalt mixtures with potential friction and macrotexture issues and develop better guidelines, specifications, and operational controls (if necessary) for recently overlaid pavements. This could lead to reduced collision rates on these pavements. overall improved procedures for flexible pavement overlay operations.