PERFORMANCE OF 18 BITUMINOUS TEST SECTIONS ON A MAJOR URBAN FREEWAY DURING 11 YEARS OF SERVICE

This paper summarizes the results of a field trial conducted on one of the most heavily trafficked roads in North America, Highway 401, which carried some 250,000 vehicles in average daily traffic (ADT) in 1985. The trial involved 18 test sections representing a comprehensive range of asphalt surface course mixes. Mix designs ranged from sand mix to high-stone-content mixes of various materials and compositions. The conditions of the test sections were monitored periodically for 11 yr, during which samples were obtained for laboratory testing and analysis. Although the primary objective of this study was to find out which mix designs would most improve the frictional characteristics of urban freeways, other parameters relating to material properties and performance of the mix designs, and their various relationships, were also evaluated. The evaluation of these test sections led to formation of policies regarding the use of high-quality asphalt mixes on Ontario highways. All the test sections performed better than expected for single-course thin overlays on concrete pavement under heavy traffic. Open and dense mixes performed equally well. It was found that, for optimum friction characteristics for high-speed roads, a surface must possess sufficient macrotexture for bulk surface water drainage and sufficient microtexture for penetrating the remaining thin-water film in the contact area. To design mixtures with these properties, the coarse aggregate content should be set above 50 and 60 percent for dense and open friction-course mixes, respectively. Good-quality, polish-resistant aggregate should be used for both the coarse and fine aggregates.