Chapter 8: Micro-Surfacing

1.0 Description

This document provides an overview of commonly used types of micro-surfacing, including information on materials and specifications, mix design, project selection, details regarding construction, and a troubleshooting guide to assist the Engineer if problems arise during the placement of these mixtures. In addition, it presents checklists at the end of the chapter with suggested field considerations for placing a micro-surfacing.

1.1 General Description

Micro-surfacing is a mixture of asphalt emulsion, graded aggregates, mineral filler, water and other additives. The mixture is made and placed on a continuous basis by mixing components in a predetermined order into a pug mill. The typical mixing order is aggregate followed by Portland cement, water, the additive and the emulsion. Figure 1 illustrates the process of micro-surfacing.

The resulting slurry material is a free flowing composite material that is spread via a spreader box over the existing road surface. The consistency of the slurry material allows it to spread over the pavement, wetting it, and forming an adhesive bond to the pavement.

The slurry mixture contains asphalt emulsion that breaks onto the pavement surface through heterogeneous or homogenous flocculation. The asphalt particles coalesce into films, creating a cohesive mixture. The mixture then cures, by loss of water, into a hardwearing, dense-graded asphalt/aggregate mixture that is bonded to the existing pavement.

A slurry surfacing does not add any structural capacity to an existing pavement; they are applied as a maintenance treatment to improve the functional characteristics of the pavement surface. The types of slurry surfacing and the pavement characteristics they improve are discussed next.

1.2 Micro-Surfacing Applications

Micro-surfacing is a thin surfacing, and can be laid at two to three times the thickness of the largest stone in the grading. The emulsion in the system is always polymer modified and special additives are used to create a chemical break that is largely independent of weather conditions. In breaking, the emulsion forces water from the aggregate surface. Such systems can often be opened to traffic within 1 hour or less of its application under a range of conditions (1).

Micro-surfacing uses high quality aggregates and fast setting/curing, stiff emulsion allowing thicker layers to be placed. Thicker layers create the following extended performance characteristics and applications:

• Correction of minor surface profile irregularities
• Rut filling
• Higher durability
• Night work (or cooler temperatures)

Micro-surfacing is not intended as a crack treatment and will not prevent cracks in the underlying pavement from reflecting through to the surface.