Science Friction

Thin Open-Graded Surfacings Drive Market

By Tom Kuennen, Contributing Editor

Now, more than ever, thin open-graded friction courses (OGFCs) are driving the market for drainable, high-friction, noise-attenuating asphalt mixes.

A thin crumb, rubber-modified, open-graded wearing course is placed on California Highway 1 near Fort Bragg; the mix incorporated a warm-mix additive to ensure workability, following the two-hour drive to the Pacific Coast from the plant.

This new generation of OGFCs is being bolstered by the advent of high-performance “spray” pavers that place thin, polymer-modified OGFCs quickly and with great consistency, and by new materials that halt the potential “draindown” of liquid asphalt within an open-graded mix.

The use of these new OGFCs also is expanding as a result of the general trend toward pavement preservation, as cash-strapped road agencies realize that it’s a lot cheaper to extend pavement life by spending limited funds on preservation techniques at the right time, rather than allow a road to deteriorate to the point of failure, with costly reconstruction the only option.

A conventional OGFC is a layer of asphalt that incorporates a skeleton of uniform aggregate size with a minimum of fines. It features an open aggregate structure in which larger-sized aggregate is held in place by polymer-modified and fiber-modified Superpave performance-graded liquid asphalts. Most OGFCs are 3/4-inch thick, and never thicker than 2 inches.

The OGFC’s open structure of 15 percent or more voids allows water to drain right through the driving or friction course to an impervious intermediate course below, and out into roadside ditches. The result is the near-complete elimination of tire spray and hydroplaning, making a safer pavement and saving lives. It also results in a quieter pavement as noise is attenuated within the gaps between the aggregate. OGFCs should be elevated above the shoulder, as the water drains onto the shoulder and hence to a roadside ditch.

A Variety of Names

Today’s spray-applied OGFCs are known by a variety of names. Oklahoma DOT has expanded the use of what it calls spray-applied ultra-thin bonded wearing courses. In Nevada, they’re known as ultra-thin asphalt concrete surfacings (UTACS, pronounced “you-tacks”), which are gap-graded wearing courses, bonded to the surface by a warm polymer-modified membrane, followed immediately by the hot, gap-graded, ultra-thin asphalt concrete friction layer.

UTACS are similar to Caltrans’ bonded wearing course, “a gap- or open-graded, ultra-thin hot-mix asphalt mixture applied over a thick polymer-modified asphalt emulsion membrane,” to quote the agency. “The emulsion membrane seals the existing surface and produces high binder content at the interface of the existing roadway surface and the gap- or open-graded mix, all in one pass.” Recently, Caltrans has been increasing its use of crumb rubber-modified emulsion for these OGFCs in lieu of polymer modifier.

Such bonded wearing courses are primarily used in high-traffic areas as a surface treatment over hot-mix asphalt or Portland cement concrete pavements. They are placed over structurally-sound pavements as a maintenance treatment, but may also be used in new construction and rehabilitation projects as the final wearing course.

In a bonded-wearing course such as those specified by Caltrans, a polymer-modified asphalt emulsion membrane seals the existing pavement while bonding the gap-graded or open-graded mix to the surface. The thicker nature of the membrane allows it to wick upwards into the mix, filling voids in the aggregate and creating an interlayer of high cohesion that does not delaminate or bleed, if applied correctly.

A predecessor of today’s thin, open-graded wearing course designs is NovaChip, an ultra-thin, bonded, gap-graded wearing course placed by a specialized paver in one pass. This exclusive pavement process applies an ultra-thin hot-mix wearing course over a polymer-rich asphalt emulsion. The process rapidly secures the lift to the existing surface and allows for minimal traffic delays. Originally developed by a French contractor, the patent for NovaChip is currently owned by Colas S.A. and is licensed for use in the United States by Road Science LLC of Tulsa.

But what’s not happening today is an increase in deeper-lift OGFCs. While most states that use them place OGFCs as thin asphalt lifts at 3/4-inch depth or less, the Oregon DOT has been placing 3/4-inch open-graded mixes in structural layers of 2 inches or more for about 30 years, reports Dr. Steve Muench, et. al., at the University of Washington-Seattle, in a June 2011 report for the Oregon DOT. He adds the Washington State DOT has used similar mixes since the early 1990s, although more sparingly. Due in part to the damage done to OGFCs by studded tires used in Oregon, the report recommends that use of 3/4-inch OGFCs be discontinued in the state (see below).

OGFCs for Cold Weather?

Typically, OGFCs tend to be confined to states without severe winters, as it’s perceived that water trapped within the drainable layer can expand and cause the open-graded layer to ravel, or create dangerous icing, leading to accidents.