Overlay Performance in Canadian Strategic Highway Research Program’s Long-Term Pavement Performance Study

The Canadian Long-Term Pavement Performance (C-LTPP) study, initiated in 1989, involves 65 sections in the 24 provincial sites that received rehabilitation comprising various thicknesses of asphalt overlays. The effects of the various alternative rehabilitation treatments on pavement performance in terms of roughness progression under comparative traffic loading, climate, and subgrade soil conditions are described. Roughness trends are the main subject of the C-LTPP study. Progression of roughness for thin overlays (30 to 60 mm) is significantly higher on a national basis than for medium (60 to 100 mm) and thick (100 to 185 mm) overlays. Factor effects, including climatic zone, subgrade type, and traffic level were also evaluated. Some findings are that ( a) in wet, high-freeze zones, thinner overlays show a higher rate of roughness progression than thicker overlays, regardless of subgrade type; ( b) in dry, high-freeze zones, roughness progression for medium and thick overlays is relatively small; ( c) in wet, low-freeze zones, thinner overlays combined with a fine subgrade show the highest rate of roughness progression; ( d) traffic in terms of equivalent single-axle loads (ESALs) appeared to have a limited effect for all the preceding factors—this was attributed largely to all the traffic essentially falling into one level and to the designation of 200,000 ESALs per year as the boundary between low and high traffic levels. In conclusion, the C-LTPP experiment has provided valuable information on roughness trends after only 8 years of observations. The methodology developed in this study for pavement roughness evaluation can be applied to performance trends analysis of other measured LTPP data.