High Friction Roads

High Friction Surfacing (HFS), also known as anti-skid surfaces, are pavement surface treatment systems that are composed of tough polish-resistant, abrasion-resistant aggregates bonded to the pavement surface using a resin. Although both hot-applied and cold-applied HFS processes have been used throughout the world, cold applied high friction surfaces are the most commonly used in the U.S.

Aggregate Properties

Aggregates are critical to skid-resistance of the HFS system. The surface of the aggregate provides the microtexture for the pavement, which directly affects skid resistance. Two of the key properties of aggregates used for HFS are polish resistance and abrasion resistance, generally reported in terms of polished stone value (PSV) and aggregate abrasion value (AAV) or LA Abrasion Value. The polished stone value is a measure of how resistant an aggregate is to polishing under traffic wear. The higher the PSV, the better the aggregate can keep its surface texture (microtexture). Abrasion resistance is a measure of how resistant the aggregate is to material loss under traffic. Aggregates used for HFS are generally 3-4 mm nominal maximum size. Specifications generally require a minimum of 95 percent passing the No. 6 sieve and maximum of 5 percent passing the No. 16 sieve.

Calcined bauxite has traditionally been the most commonly used aggregate material used for HFS. However, other aggregates such as flint, granite, and steel slag have also been used.

Binder Properties

Several varieties of resin binders are used in HFS systems, including epoxy-resin, rosin-ester, polyurethane-resin, polyester-resin, and methyl methacrylate binders. A description of the more common binders is provided below:

Epoxy-resin: This type of binder is a two-component system. One part of the system is composed of the resin and an oil to reduce the viscosity of the resin, and the second part is composed of the curing agent, bitumen, oil and accelerators. Both parts are mixed on-site.
Rosin-ester: This type of binder is “premixed”, meaning the resin and the chippings are bagged as a dry powder. Once on-site, the powder is heated and spread.

Polyurethane-resin: A multi-component binder (two liquid and one powder, respectively) which cures chemically and is generally hand applied. The aggregate is then applied after. These binders help with achieving a quicker curing time during cold weather applications.

Acrylic-resin: Similar to epoxy-resin, however it has a faster curing time.

High friction surfaces are used primarily for restoring or enhancing the skid resistance of a pavement surface, but can also be used for lane delineation, bike paths, traffic calming, or for decorative purposes. Areas where high friction or anti-skidding properties are desired, include:

• Horizontal Curves and Ramps
• Intersections/Intersection Approaches
• Steep Grades
• Roundabouts
• Bus Stop Areas
• Pedestrian Walkways
• Bridge Decks
• Pavement Surfaces Susceptible to Icing

Under the current FHWA HFS demonstration study, emphasis will be placed on evaluating the efficacy of HFS for improving skid resistance and reducing accidents at horizontal curves.

HFS for Curves

HFS for an Intersection Approach

HFS for Lane Delineation and Railroad Crossing Safety (photos courtesy of Traffic Calming USA)

HFS for Lane Delineation and Railroad Crossing Safety (photos courtesy of Traffic Calming USA)

HFS for Lane Delineation and Railroad Crossing Safety (photos courtesy of Traffic Calming USA)

• Provides a solution for restoration of skid resistance on existing pavements or enhancement of skid resistance on new pavements
• Provides a solution for “spot treatment” of a pavement where friction demand is highest
• Low Cost Treatment (when compared to removal and replacement existing pavement)
• Can be installed during short (4-6 hour) closures of the pavement and only one lane at a time
• Only minimal additional thickness or (1/8″ – 1/4″) thickness for areas where overhead clearance is limited
• Can be installed over virtually all pavement types
• Can be colored for delineation purposes

• Pavement should be in relatively good condition, with no alligator or map cracking, ravelling, or scaling.
• Open graded (porous asphalt and concrete) pavement surfaces will no longer function as open graded surfaces after installation. These surfaces may require a double layer application to ensure adequate thickness of the binder.
• Pavement surface must be clean and dry for application. Concrete surfaces should be shotblast prior to application for best adhesion. All surfaces should be free of oils, grease, and other latency.
• Resin curing generally produces an exothermic reaction. Higher temperatures will accelerate curing/hardening, while lower temperatures will slow curing/hardening. In general, surface temperatures above 50F are required for installation of most resins.
• Excess aggregate must be thoroughly removed from the new surface prior to opening to traffic. A power broom or vacuum followed by compressed air is the most effective process. The new surface will naturally shed aggregate over the first several days, requiring additional sweeping to remove loose stone.
• Life expectancy is 5-7 years under heavy traffic, 10-15 years for low traffic.