Thickness and Binder Type Evaluation of a 4.75-mm Asphalt Mixture Using Accelerated Pavement Testing

Abstract

An increasing number of transportation agencies are investing in thin overlays consisting of small aggregate size asphalt mixtures to address routine maintenance needs and pavement preservation initiatives. Reported benefits of thin overlays include a smooth riding surface, reduced tire-pavement noise, and the ability to more easily maintain and control grade and cross-slope. The smallest aggregate size mixture the Florida Department of Transportation (FDOT) currently specifies is a 9.5-mm nominal maximum aggregate size (NMAS) mixture. When used as a dense graded friction course, the 9.5-mm NMAS mixture is placed in 1.0 in. lifts, which allows more flexibility and better optimization of milling depths, as well as structural or overbuild lift thickness to achieve a structurally appropriate and economical pavement. However, the ability to place an even thinner asphalt layer may potentially provide designers with more cost-effective alternatives. FDOT recently investigated the layer thickness of a 4.75-mm NMAS mixture using accelerated pavement testing (APT). During the APT study, overlay thicknesses of 0.5, 0.75, and 1.0 in. were placed with PG 67-22 (unmodified) and PG 76-22 (polymer modified) asphalt binders. Pavement instrumentation, laboratory data, and results from a field test section were also considered. This report documents the findings of the APT study and provides recommendations for the use and appropriate thickness of a 4.75-mm NMAS overlay.