Effects of Milling and Other Repairs on Smoothness of Overlays on Asphalt Pavements

Abstract

The application of asphalt overlays on existing pavements comprises a significant percentage of the maintenance activities undertaken to restore asphalt pavement condition. The improvement of surface smoothness from overlay construction is gaining increasing attention because smoother overlays are likely to have longer lives as a result of decreased dynamic loading and to provide fuel economy benefits and greater comfort to road users. This paper presents a study in which the California Department of Transportation (Caltrans) Pavement Condition Survey (PCS) data were used to investigate the effects of repairs, preoverlay condition, and surface and binder type on initial overlay (postoverlay) smoothness. Linear mixed-effect models were used in the analysis to take into account the variation across random-effect variables. The smoothness of overlays was measured in terms of the international roughness index (IRI). The analysis indicated that the pavement preoverlay condition (IRI) was the most important variable to affect overlay smoothness: pavements with lower preoverlay IRIs were smoother than those with higher preoverlay IRIs. An increase in overlay thickness also was found to have a significant effect on the smoothness when the preoverlay condition was poor. In terms of preoverlay repairs, analysis of the Caltrans PCS data showed that pavements with digouts (milling and patching in the wheelpaths) performed better than did pavements whose entire surface was milled before the overlay. The effects of surface type (dense- or gap-graded versus open-graded) and binder type (rubberized or not rubberized) depended on the preexisting pavement condition, other factors, or both. Possible reasons for differences between this study and previous ones are offered.