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.
The international roughness index (IRI) is widely used throughout the world as a measure of road roughness. A quarter-car simulation at 80 km/h is performed on the longitudinal profile to compute IRI. Questions have been raised regarding the applicability of IRI for roads that are used at speeds above or below this simulation speed. To gain more insight into the effects of simulation speed, an investigation was carried out to determine how the roughness computed from the IRI model changes for different simulation speeds of the quarter car. This investigation was performed on an asphalt concrete data set and a jointed portland cement concrete data set. For simulation speeds between 60 and 110 km/h, the response from the IRI model was within ±0.20 m/km of the IRI for 80% of the asphalt sections and 61 % of the concrete sections used in the study. Although the output from the quarter-car model for the different simulation speeds was different from the IRI (simulation speed of 80 km/h), it is unclear to what extent a user's perception of the roughness of a roadway changes with the speed of travel. If examples of roadways are found where the subjective opinion of roadway users of the roughness seems inconsistent with the IRI, it is recommended that the IRI model be used with the current speed limit of the roadway to examine whether the obtained output provides a better match with the opinion expressed by roadway users.
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