Gripping turn

May 1, 2015

High-friction surfacing can help with curves

Nontangent roadway sections present a unique challenge for pavements.

The pavement surface must satisfy the necessary friction demand to ensure safety and driver comfort based on the geometry (grade, radius of curvature and superelevation) and anticipated speed of vehicles navigating the curve. Considering that the crash rate for curves is approximately three times that for tangent sections and that there are an estimated 10 million curves on two-lane highways alone, ensuring that friction demand can be met is critical.

Under normal operating conditions, virtually any conventional pavement surface will provide adequate friction. Unfortunately, when these conditions are violated due to excessive speed, improperly maintained tires or adverse weather conditions (rain, snow, ice), the higher friction demand cannot always be achieved by conventional paving materials. High-friction surfacing (HFS) is an unconventional material that provides exceptional skid resistance for conditions such as these.  

While a consensus definition for HFS has not yet been established in the U.S., the British Board of Agrément defines HFS as a material “having a minimum skid resistance value (SRV) of 65 measured using the portable Skid-Resistance Tester” (otherwise known as the British Pendulum Tester).

HFS has been used with great success for decades in the United Kingdom and is now in fact mandated for certain roadways, depending on the traffic and geometric characteristics. HFS is not entirely new to the U.S. as it has been used extensively for bridge decks, helping to both seal the bridge deck, while providing added friction under adverse weather conditions (snow and ice). Its use for pavements, however, has been relatively limited until the past five to seven years.

HFS is a thin overlay for pavements that generally consists of a 2- to 3-mm aggregate broadcast over a thin layer of epoxy or methacrylate binder. The aggregates used for HFS are one of the keys to its effectiveness. HFS aggregates must be highly durable, with high resistance to polishing and abrasion. Calcined bauxite has traditionally been used for HFS, but less costly alternatives are currently being evaluated. HFS treatments are typically a single-layer system, but may consist of two layers for bridge decks or open-graded pavement surfaces. HFS cures quickly, allowing it to be installed during short lane closures, minimizing the disruption to traffic. While HFS has traditionally been applied by hand, tremendous advances have been made in mechanical application devices to greatly increase the efficiency of installation. 

Under evaluation 

Recognizing the need for higher friction at curves and the proven effectiveness of HFS overseas the Federal Highway Administration (FHWA) initiated the Surface Enhancements at Horizontal Curves (SEAHC) program in 2008. This program specifically focuses on the evaluation of HFS for reducing crashes at curves through demonstration installations at high-crash locations throughout the country. To date, 20 HFS demonstrations have been completed in eight states, using four different HFS products under the SEAHC program. Installations have been completed on a variety of pavement surfaces, including concrete, dense-graded asphalt, stone-matrix asphalt and chip seals.    

Each demonstration is evaluated in terms of crash data, surface characteristics and product performance. Crash data for at least three years prior to and three years following installation are compiled in order to evaluate the effectiveness of the HFS in reducing crashes. Texture and friction measurements are conducted on the original pavement surface just prior to HFS installation, on the HFS surface immediately following installation and at one year after installation. Texture measurements are collected using the Circular Track Meter, ASTM E965 “Sand Patch” method and the Transtec RoboTex texture measurement device. Friction measurements are collected with the Dynamic Friction Tester and, at the option of the participating state agency, the ASTM E274 locked wheel skid trailer.

Although final results have not yet been compiled as the initial demonstrations are still within the three-year evaluation period, results to date have been very promising. One state reported an overall crash reduction of 70% after one year for the five sites where HFS was installed. Another state has experienced virtual elimination of crashes on a curve that had become well-known for being closed due to crashes every time it rained.

Friction fixes

In addition to the crash-reduction benefits realized through this study, a number of lessons have been learned that will help agencies in the development of specifications for HFS in their state. Among them:

HFS should not be applied to highly distressed pavements. Any cracking in the existing pavement will reflect through the HFS, allowing the underlying pavement to continue to deteriorate. HFS is not a “healing” treatment for a pavement, but simply a treatment for increasing skid resistance;

HFS has an initial “wear-in” period after opening to traffic. Friction and texture values immediately after installation will measure artificially high and will slowly decrease to a stable level after the first several weeks of traffic wear; and  

The ultimate performance of HFS is highly dependent on the initial installation process. The pavement surface must be clean and dry to ensure proper bonding of the binder material, and the binder must be applied uniformly to the proper film thickness to ensure the aggregates are adequately seated in the binder. Nonuniform thickness may result in uneven wear of the material, particularly on roadways where snowplows are used. Inadequate film thickness will result in premature loss of aggregate, leaving only the binder layer on the pavement surface. 

Alternative rock

One of the drawbacks to conventional calcined bauxite aggregate HFS is cost. While its polish resistance and abrasion characteristics make it an ideal material for maintaining high friction over time, bauxite must be imported from overseas, making it a very costly aggregate. As part of the SEAHC effort, FHWA, in conjunction with private industry, initiated an aggregate durability test at the National Center for Asphalt Technology (NCAT) to evaluate alternative aggregates that are locally available in the U.S.  

Eight different aggregate types were tested, each installed at the same time, using the same binder and the same installation crew (complimentary services provided by Dow POLY-CARB Inc.). Full-scale test sections were installed on a nontangent section of the NCAT test track to test accelerated wear under more than 2 million 18,000-lb truck axle loadings over a six-month period. Additionally, small-scale test pads were fabricated for testing in a laboratory accelerated-loading device, which applied 140,000 cycles to each test pad. Similar to the SEAHC demonstrations, texture and friction measurements were collected on both the full-scale sections and laboratory test pads immediately after installation and after accelerated loading was complete. The aggregate types included three more commonly used aggregates: bauxite, granite and flit, as well as five other aggregates that are not commonly used but readily available: taconite, emery, basalt, steel slag and silica.  

Although results have not been formally published, these tests confirmed that bauxite is a superior aggregate in terms of initial friction and minimal loss of friction after testing. However, several of the other aggregates also showed good performance in terms of loss of friction after testing. While bauxite is still recommended as the premium aggregate for HFS, the results of this study should give agencies a greater comfort level with alternative aggregates that are more cost-effective for projects where the premium solution may not be necessary.

Further education

As the use of HFS has grown, so has the need for establishing best practices within the industry. Agencies are seeking a better understanding of HFS and the associated cost versus benefit as a tool for improving highway safety. In addition to the SEAHC program, other efforts currently under way to provide this information include the AASHTO NTPEP Test Deck and the ATSSA High Friction Surfacing Council.  

The AASHTO NTPEP Test Deck will provide an opportunity for HFS vendors to have their product evaluated side by side with other products during an unbiased test of the materials on concrete and asphalt pavements, as well as a bridge deck. The NTPEP Test Deck is scheduled to be constructed in summer 2012 and evaluated over a two-year period.  

The ATSSA High Friction Surfacing Council was recently established to provide a home for HFS within the highway community. The council consists of representatives from state highway agencies, the FHWA and numerous HFS industry representatives, and seeks to advance the body of knowledge for HFS such that it will be more readily used as a tool for improving highway safety.  

Friction gained

SEAHC has clearly demonstrated that a properly installed HFS surface on a high-crash curve will significantly reduce crashes. In addition, the project has generated a tremendous amount of information with regard to the installation and performance of HFS materials over various pavement types and under varying conditions. While HFS has been demonstrated to be a valuable tool for improving safety on nontangent sections of roadways, we suggest that you add HFS to your arsenal of pitches in working toward a no-hitter on crashes and fatalities. ST

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