PAVEMENT SMOOTHNESS INDEX RELATIONSHIPS

Nearly all State highway agencies use smoothness specifications to ensure that hot-mix asphalt (HMA) and portland cement concrete (PCC) pavements are built to high levels of smoothness. Not only is an initially smooth pavement generally indicative of quality workmanship, but it has been shown to last longer than a pavement built rougher. About half of all current State smoothness specifications for HMA and more then three-fourths of all current PCC smoothness specifications are centered around the Profile Index (PI), as often measured using a profilograph. The vast majority of these specifications utilize a 5-mm (0.2-in.) blanking band in computing PI (i.e., PI5-mm). Unfortunately, because of the technical limitations of the profilograph equipment and PI computation procedures, the adequacy of PI5-mm in characterizing roughness and having it relate to user response has come into question. The International Roughness Index (IRI) or the Profile Index using a 0.0-mm blanking band (PI0.0) seem to provide better measures of smoothness and better correlation with user response. However, one barrier to more widespread implementation of these new smoothness standards is the lack of objective, verifiable correlation methods for use in establishing specification limits using the IRI or PI0.0. Assistance in selecting appropriate IRI and PI0.0 specification limits is needed to provide a basis for modifying current specifications to these more reproducible and portable smoothness indices. This research effort has developed a series of relationships between IRI and PI that can assist States in transitioning to IRI or PI0.0 smoothness specification for HMA and PCC pavements.