Flexible Slurry–Microsurfacing System for Overlay Preparation

Microsurfacing mixtures are made of high-quality aggregate and asphalt emulsion components. They are produced at the project site by a mobile microsurfacing machine that mixes and places the product. Normal micro-surface designs contain approximately 13.5% emulsion and include a performance grade (PG) 64-22 asphalt cement (AC). This study investigates the field performance of softer-AC flexible slurry surfacing mixtures used for preparation and surface courses. A PG 48-34 AC was chosen for flexible slurry–microsurfacing mixtures constructed on four test cells at the Minnesota Road Research Project low-volume road facility near Albertville, Minnesota, in September 2005. This AC was used to produce mixtures for both surface preparation and wear courses. The mixtures used a reduced emulsion percentage for rut filling and an increased emulsion percentage for normal leveling courses. A single surface course mixture with the increased emulsion level was placed over the entire project. Pre- and postconstruction evaluations of cracking, rutting, and smoothness were performed. These evaluations were repeated at 6 months, following one winter of service. At 6 months an overall 71% of transverse cracking had reflected through the microsurface. Reflected distress from longitudinal cracks and patched areas was negligible. The rutting condition at 7 months was found to be similar to that at postconstruction, in which an overall 20% decrease was attributed to microsurfacing. Smoothness was favorably affected by microsurfacing construction. International roughness index values were correlated to the amount of material placed in the surface preparation course.