Targeted Overlays are "TOPS"

NCAT has been working on FHWA’s Every Day Counts program initiative for Targeted Overlay Pavement Solutions (TOPS). Every Day Counts is a state-based program to encourage the adaptation of proven yet underused innovations to make our transportation system adaptable, sustainable, equitable, and safer for everyone. The TOPS project is included in the program’s sixth round and seeks to improve safety, reduce overall life cycle costs, and increase the performance of our asphalt pavements.

The TOPS initiative highlights seven types of specialty asphalt mixes—

• - asphalt rubber gap-graded
• - crack attenuation mix
• - enhanced friction overlays
• - highly modified asphalt
• - open graded friction course
• - stone matrix asphalt
• - ultra-thin bonded wearing course

NCAT’s role in the project is to produce case studies, how-to documents, webinars, and workshops to assist state DOTs in learning about the specialty mixes and developing specifications for their use. We are continuing the work with our partners ARA and Weris as we move into the webinar and workshop phases.

The TOPS products are not new, but they may have limited use in some states. Below is a brief introduction to each mix.

Asphalt Rubber Gap-Graded Mixtures

Asphalt rubber gap-graded (ARGG) mixtures use an asphalt rubber binder that contains approximately 20% ground tire rubber. They are typically small NMAS mixes with a top aggregate size of 3/8-inch or ½-inch. The gradation of the aggregate is gap-graded to allow for a higher binder content and space for the rubber particles. These mixes are quite durable and have good resistance to rutting and cracking. ARGG mixtures are most often used as a surface layer, are placed 1.25 inches to 2.25 inches thick, and generally have good friction properties. These types of mixes are used in urban areas with considerable stop-and-go traffic, such as intersections.

Crack Attenuating Mix

Crack attenuating mix (CAM) is a fine-graded mixture with a high-binder content that is placed as a half to one-inch-thick interlayer between the existing pavement and a surface asphalt layer to reduce reflective cracking. CAM design relies on the traditional volumetric mix design approach but also typically includes testing to assess rutting and cracking resistance. The mixes tend to be more expensive because of a high polymer binder content and the use of high quality aggregates. This cost is offset by reduced maintenance and longer life. CAM mixes have been successfully used on U.S. routes, interstates, and state highways, as well as farm-to-market roadways, and business highways as an interlayer.

Enhanced Friction Overlay

An enhanced friction overlay (EFO) is a 4.75 mm nominal maximum aggregate size gap-graded mixture that uses calcined bauxite. Calcined bauxite is a hard, angular aggregate that provides excellent friction performance. Calcined bauxite is commonly used in high-friction surface treatments (HFST). EFOs use a polymer-modified asphalt binder and typically have a higher asphalt binder content than conventional mixtures.

EFO mixtures are best used in locations with high crash rates, such as curves, deceleration ramps, and intersection approaches. This mix is more expensive than traditional mixes because of the modified binder and the calcined bauxite, so it is typically placed quite thin with a total compacted thickness of ¾ inch. Studies indicate that EFOs have comparable friction values to HFST with the expectation of longer pavement life and reduced cost.

Highly Modified Asphalt Mixtures

Highly modified asphalt (HiMA) mixtures are produced using asphalt binder containing 7-8% polymer, typically styrene-butadiene-styrene. This polymer rate is more than twice what is used in conventional modified binders. The HiMA acts as an elastic reinforcement in the asphalt binder and improves mixture cracking resistance. In addition, considerable improvement to the rutting performance of HiMA mixtures has been documented.

HiMA mixtures have been used over a wide range of applications ranging from full depth to thin asphalt overlays. Although long-term pavement performance data for HiMA mixtures are not readily available in the field, promising performance in early pavement life has been reported and observed over multiple research cycles on the NCAT Test Track.

Open-Graded Friction Course

Open-graded friction course (OGFC) is a gap-graded asphalt mixture with a high percentage of coarse aggregates almost uniform in size, resulting in a high percentage of air voids (usually 15-25%). Because of its safety and environmental benefits, OGFC has been widely used in the U.S., Europe, and Asia.

OGFC has an open-graded aggregate skeleton with interconnecting voids that allows rainfall to flow through to an impermeable underlying layer, and eventually to the pavement edge. Drainage of water from the pavement surface promotes the tire and aggregate interface contact and substantially reduces the likelihood of hydroplaning. OGFC is placed as a surface layer to maintain good friction in wet weather, reduce splash and spray and nighttime glare during wet conditions, enhance the visibility of pavement markings, and provide a smooth pavement.

Stone Matrix Asphalt

Stone matrix asphalt (SMA) is a tough and rut-resistant gap-graded asphalt mixture that relies on a stable stone-on-stone skeleton offering strength, a rich mixture of asphalt binder, and fibers and/or asphalt modifiers that provide durability. SMA was developed in Germany in the 1960s to provide a durable, rut-resistant wearing course that could withstand damage from studded tires for heavily traveled roads. SMA has been used in the U.S. since the 1990s in more than 40 states.

SMA mixtures are most often placed on pavements with heavy traffic, high-stress pavement areas, thin overlays, airfields, and racetracks due to the expectation of increased service life. SMA is more expensive than conventional mixtures, mainly due to higher asphalt contents, specifications for more durable aggregates, and inclusion of fibers as stabilizers. States have reported that SMA pavements generally have better long-term field performance than traditional mixtures.

Ultra-Thin Bonded Wearing Course

Ultra-thin bonded wearing course (UTBWC) is a thin open-graded asphalt layer placed on a polymer-modified tack coat by a specialized spray paver that places the tack coat and the asphalt mixture in a single pass. UTBWC mixtures are often used as a pavement preservation method because they correct minor surface distresses and restore friction and smoothness. These mixes have been used in the U.S. since the early 1990s in a variety of traffic conditions in urban and rural areas.

More information can be found on the FHWA Every Day Counts webpage.

For more information about this article, please contact Adriana Vargas (left), Travis Walbeck (center), or Jim Musselman (right).