Survival Analysis of Concrete Overlays on Low-Volume Roads in Iowa

Concrete overlays are an important component of Iowa’s rural road network. More than 2,000 centerline miles of concrete overlays have been paved in Iowa over the last few decades, including many overlays on low-volume roads serving 400 vehicles per day or less. In this study, a survival analysis was performed using automated pavement condition data collected on Iowa’s concrete overlays to obtain a probabilistic assessment of concrete overlay service life, paying particular attention to the performance of overlays on low-volume roads. Concrete overlays were found to perform very well, with a 30-year survival probability of 85.3% for all projects to rehabilitation or reconstruction, and 76.6% for overlays only on low-volume roads. Survival life was reduced when the survival condition was changed to performance thresholds based on a pavement condition index (PCI) of 60/100 and an International Roughness Index (IRI) value of 170 in./mi, but was still good overall. Overlays tended to reach the failure condition for PCI before IRI, indicating that cracking is a more common distress than surface roughness. Another important finding was that concrete on asphalt overlays had a longer median survival life and a greater 30-year survival probability than concrete on concrete-unbonded overlays. Overall, concrete overlays have performed well in Iowa and are well suited to low-volume roads and rural county highways.