During the entire life cycle of a pavement, highway agencies are expected to monitor and maintain an adequate surface roughness (texture) to facilitate friction between car tires and pavement surface. Skid resistance is a measure of the resistance of pavement surface to sliding or skidding of the travelling vehicles. Maintaining adequate texture and proving resistance to polishing due to the effects of traffic are of prime importance in providing skidding resistance. Polishing of the aggregate is the reduction in microtexture, resulting in the smoothing and rounding of exposed aggregates. This process is caused by particle wear on a microscopic scale. It is a well recognized fact that the lower the skid resistance value, the higher the percentage of the traffic accidents, especially during the wet seasons. Traditionally the repair method for a surface friction-deficient or texture-deficient pavement surface, once the friction falls below certain threshold value, is the application of a new surfacing layer. The practice of monitoring and remedying the low skid resistance pavement sections is important; however, it is a passive approach toward the problem. A more proactive approach would be to test the hot mix asphalt in the laboratory during its initial mix design stage to ensure that aggregate combinations used in the asphalt pavement will provide adequate friction as expected over the life of the pavement.
To achieve the screening task for polishing and friction behavior of the HMA during its mix design stage, a laboratory-scale accelerated polishing device that can mimic the actual abrasion and polishing behavior between the vehicle rubber tire and the HMA surface has been developed. The developed polishing device possesses some practical characteristics. These included the versatility of testing HMA specimens that can be prepared with the conventional compaction methods, the specimens can be prepared as part of mix design procedure, test duration is reasonably short, test procedure including test specimen preparation and friction measurements are relatively simple and repetitive, the developed test procedure requires minimum labor efforts, and finally, the test results can provide realistic indication of the polishing and friction behavior of the hot mix asphalt specimens.
The validation of the developed accelerated polishing device included positive comparisons between the HMA polishing behavior from the new test device with the aggregate polishing behavior from the standard accelerated British Polishing Wheel tests. Furthermore, a total of eight pavement sections in service were identified for long term friction and texture measurements, which will afford data correlation between field performance and laboratory obtained behavior. The laboratory tests on the HMA samples prepared with the job mix formula for the eight sections were completed and analyzed to confirm test repeatability of the new device. The laboratory test results, together with statistical correlation analyses, have resulted in the development of useful empirical predictive equations for the Skid Number (SN) measured by the Locked Skid Trailer from the DFT (dynamic Friction Tester) measured skid number. Finally, the British Pendulum number (BPN) from the British Pendulum Tester (BPT) were found to be strongly correlated with the surface texture (MTD: Mean Texture Depth) measured by the sand patch method.