International Roughness Index of Asphalt Concrete Overlays: Analysis of Data from Long-Term Pavement Performance Program SPS-5 Projects

Abstract

The Specific Pavement Studies 5 (SPS-5) experiment in the Long-Term Pavement Performance (LTPP) program was developed to investigate the performance of selected asphalt concrete rehabilitation treatment factors. Results of an analysis are presented that was conducted to compare the international roughness index (IRI) values of the test sections before and after overlay and to compare the IRI values that were obtained on the test sections subjected to different treatments. The data presented will be useful for highway agencies to gain an insight into the typical IRI values that can be expected when overlays are placed on asphalt concrete pavements and to obtain information on the reduction in roughness from an asphalt concrete overlay. The analysis indicated that the IRI of a pavement after overlay was not a function of the IRI before overlay. It was observed that a 50-mm-thick overlay (placed in one lift) was capable of achieving similar IRI values as a 125-mm-thick overlay (placed in two lifts). No differences in IRI values were obtained on overlays that were placed on pavement surfaces milled before the overlay when compared with surfaces not milled before the overlay. A frequency analysis of the IRI values after overlay indicated that in 85 percent of the cases, the IRI of the pavement was less than 1.2 m/km for both 50-mm- and 125-mm-thick asphalt concrete overlays.