1. What is the life expectancy of HFST?
- Life expectancy will vary with climate and roadway characteristics, such as geometry, volume and mix of traffic types, and the nature of traffic movements. Accordingly, it is difficult to generalize.
- Due to its relatively recent adoption in the United States, a limited number of HFST installations are older than 8 years, but there are a number of installations throughout the country that are still intact and performing well in reducing crashes after 5-8 years of usage without failure.
- HFST testing on test tracks with accelerated aging/loading has shown some HFST sections continuing to last and provide friction enhancement capability with over 25 million equivalent single-axle loads (ESALs) of wear.
- International experience indicates at least 7-12 years of service can be expected with correctly applied installations. A recent report by the Road Surface Treatment Association from the United Kingdom reports an average service life of 12 years for cold applied HFST, such as those used in the United States.
- Vendors have reported 5-8 years of service life for traffic volumes around 15,000 vehicles per day, and up to 5 years with traffic volumes closer to 50,000 vehicles per day.
- Just like pavement performance, HFST wear is dependent on many factors, such as initial construction quality, friction demand, and traffic volume as well as the severity of the climate and the weight and number of heavy truck axles using the facility.
- Vehicular traffic with heavy chains and/or studded tires will increase wear within the wheel path of the HFST.
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2. Are these materials resistant to fuels and common de-icing agents?
- HFST installations are resistant to both fuel spills and deicing chemicals in roadway environments. Once cured, the polymer resin will remain stable even when exposed to incidental levels of solvents or diesel fuel, for example.
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3. What is the difference between HFST and other pavement preservation treatments such as microsurfacing and chip seal?
- HFST is a safety treatment made for spot locations with high friction demand (shear forces) such as severe `curves and braking areas. By using calcined bauxite, which is much smaller (1-3 mm nominal size) and possesses very anti-abrasion and polish-resistant material, as an aggregate, high friction numbers can be maintained over time. Installed HFST friction numbers (from locked-wheel skid testing) are over 70 and many times in the high 80s or low 90s. The HFST resin binder material is a thermosetting, polymer-based material and is an 8 to 12 year friction-enhancing treatment.
- Other treatments such as microsurfacing and chip seals are pavement preservation treatments that may also improve skid resistance, but are generally used to extend the life of the pavement. These treatments produce initial friction readings generally lower than HFST, and that friction generally declines with time due to raveling, flushing, or polishing of the aggregate. Please refer to FHWA Turner-Fairbank Highway Research Center (TFHRC) publication titled "Evaluation of Pavement Safety Performance," which included a description of various pavement treatment types. The publication is available at: https://www.fhwa.dot.gov/publications/research/safety/14065/14065.pdf
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4. Is it always worth the investment to use HFST compared to other friction treatments?
- The answer to this question likely depends on: (1) the condition of the existing pavement, since HFST only lasts as long as the pavement it is placed on, and (2) the severity and type of crash the location is experiencing. For a deteriorating pavement with substandard friction characteristics and minimal pavement life, the problem can be dealt with using more conventional pavement treatments to restore standard friction characteristics, at least for the short term. However, HFST may still be a good idea if the location has challenging geometric features and high incidences of speeding, since this tends to escalate pavement polishing and the problem may return even with new pavement installation.
- Many pavement preservation treatments (microsurfacing, chip seal, etc.) can improve available friction, especially on a polished pavement, and FHWA is currently working to establish and quantify the beneficial impacts of improved and enhanced friction as a result of the installation of selected pavement preservation treatments.
- It may not always be necessary to apply the treatment that provides the highest, most sustained friction at a particular site. Consideration should be given to the nature and severity of existing traction problems, life expectancy of the installation, and the friction demand produced by the traffic at the location.
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5. Do these materials crack? If so, how do they resist crack erosion?
- The materials typically do not crack on their own, but any cracks (and joints) in the underlying pavement will generally reflect through.
- If there is concern with erosion (pumping of subbase fines) at reflected cracks, these cracks can be sealed using conventional crack-sealing methods, ensuring that the sealant is recessed slightly below the top surface of the HFST and not overbanded onto the HFST.
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6. Are there any issues with thermoplastic on HFST?
- No. Thermoplastic bonds extremely well and will in fact last longer when applied to the textured surface of HFST. Other marking materials also bond well, but pavement marking tape should be avoided since it does not adhere well due the textured surface of the HFST.
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7. What aggregates are appropriate for HFST and what sources are available?
- Several indigenous aggregates can initially improve the friction of a pavement surface. However, it is important to maintain a distinction between improved initial friction and the long-lasting friction benefit from a true HFST. This benefit comes from the use of calcined bauxite aggregate which is highly abrasion- and polish-resistant.
- Bauxite is a natural resource, mined in many countries, principally for its use in the production of aluminum. Calcined bauxite used for HFST is categorized as ‘non-metallurgical refractory grade’ which comes from high-quality bauxite that is calcined, or heat treated, at 2900-3000°F, to produce a dense, high-purity, stable aggregate. Refractory grade calcined bauxite has a high-alumina (≥
82 percentAl203), low-alkali content (≥0.4 percent) and a bulk density of ≥ 3.0 with very low residual moisture levels.
- After the calcination process, the aggregate is subsequently crushed and sieved to a specific gradation to meet the specification requirements for HFST.
- Calcined bauxite has a resistance to polishing and wear that is superior to other aggregates.
- Generally, vendors that sell HFST systems procure the aggregate from mineral companies that specialize in the mining, calcining, crushing, and grading process for specialized aggregates. A U.S. Geological Survey report states that a world bauxite shortage is not foreseen for the next century. https://minerals.usgs.gov/minerals/
- Other aggregates such as flint, basalt, taconite, and granite have been evaluated but have not performed as well as calcined bauxite in terms of sustained frictional resistance.
- Alternative aggregates can be placed in a similar manner and may perform adequately in a less demanding environment, but in critical locations they have not provided the duration of friction service to be classified as HFST. Other aggregates, such as flint, basalt, taconite and granite have and continue to be evaluated for performance, but at this time only calcined bauxite is recommended to provide the expected safety performance and durability required in an HFST application.
- Please refer to #13 below for several HFST aggregate durability studies related to aggregate testing from the National Center for Asphalt Technology (NCAT).
- Other aggregates other than Calcined Bauxite can be considered as acceptable provided these aggregates met the testing requirement as outlined in Section 7, Method of Testing, the AASHTO Standard Practice for High Friction Surface Treatment for Asphalt and Concrete Pavements AASHTO Designation: PP 79-14 (2016).
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8. Can the leftover/reclaimed aggregate be reused?
- During the HFST installation process, the calcined bauxite aggregate is typically installed to refusal over the wet resin–i.e., excess bauxite material is placed that does not adhere to the resin and is swept up and reclaimed prior to opening the treatment area to traffic.
- Recovery of excess aggregate at a project site and subsequent use of the reclaimed material should be considered very carefully. HFST aggregate is supplied to a specific grading specification and, if not managed carefully, the process of reclamation may introduce uncontrolled particles, leaving a material that does not have an equal distribution of particle sizes as originally supplied.
- If reclamation is considered acceptable within an agency’s specification, then the following should be considered:
- Place only enough excess bauxite material to completely cover all wet resin without undue excess placement.
- Clean roadway areas and adjacent areas thoroughly prior to placement and never attempt to reclaim bauxite off roadway shoulder/gravel edges to prevent the introduction of foreign material into the reclaimed material.
- All recovery sweeping equipment must be thoroughly cleaned and inspected prior to the sweeping operation to prevent the introduction of foreign matter into the reclaimed aggregate.
- Upon completion of the sweeping operation, estimate the accumulation weight and obtain a representative test sample to identify the gradation of the reclaimed material.
- Complete and record gradation tests of the reclaimed aggregate.
- Under no circumstances should reclaimed aggregate be introduced in lower dilution ratios than 1 to 2 bags of prime (new) aggregate. Reintroduction should only be performed using aggregate hoppers that feed aggregate to the final discharge point by screw conveyors. Application equipment with conveyor belt discharge should not be used to blend reclaimed aggregate with prime product.
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9. If a large amount of calcined bauxite comes from international sources, does the Buy America provision apply when using Federal funds for HFST?
- No, Buy America applies only to steel products.
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10. What are some typical polished stone values (PSV) for HFST aggregates using ASTM E 303/AASHTO T 278 British Portable Pendulum and ASTM D 3319/AASHTO T 279, Accelerated Polishing Machine?
- Testing the HFST aggregate for PSV is fairly common practice. Keep in mind that PSV testing gained popularity in in Europe and the British method for testing PSV (EN 1097-8) is different than the ASTM method even though the testing apparatus is the same. Also, the gradation of aggregate specified for testing PSV per ASTM is different than that commonly specified for HFST. This requires a modification of the test method that has not yet been defined within the ASTM. In order to use the PSV requirement for high-friction aggregates within a specification, a value >40 will be required. This value is determined by using the primary "main scale" of the British Pendulum rather than "F-scale" which is used when measuring PSV in accordance with the British (EN 1097-8) test method.
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11. Several HFST aggregate durability studies were published in cooperation with FHWA and the National Center for Asphalt Technology (NCAT):
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12. Can HFST be placed on a bridge deck?
- HFST can be applied to bridge decks. The common method for applying epoxy overlays to bridge decks for bridge preservation is to apply a double layer of resin binder and aggregate for insurance against water penetration of the deck, and to provide added durability to the installation. For installations focused on bridge preservation, the aggregate friction and polish resistance is generally not as important as aggregate hardness, so the aggregates used may not all meet the PSV qualification for HFST.
- If this same bridge was in a curve and was also identified as having a crash problem, a double layer of HFST could be substituted for a bridge deck overlay and accomplish both purposes–protection and crash reduction. This has been done in many locations across the country, especially on roadways or ramps where an adjacent roadway segment is considered for HFST. Many interchanges and ramps, for example, may have non-standard geometry resulting in crash problems on both bridge and roadway segments. An HFST treatment to both would allow for friction enhancement and crash reduction regardless of the underlying roadway type.
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13. There has been some discussion of placing HFST in a double layer. Why is this done?
- Double-layer HFST is generally used to provide a more durable, longer-lasting surface in aggressive environments (i.e., where vehicles are equipped with studded tires or snow chains, or where snowplows are common) or on roadways with very high traffic volumes.
- In addition to the double layer installation on bridge decks (noted above), Double-layer HFST applications have also been successfully installed on open-graded friction course (OGFC) asphalt pavements. Using a double-layer treatment on OGFC seals the voids in the pavement surface with the first layer and maintains the proper binder depth of the top course with the second layer, which is necessary for the aggregate embedment in the final HFST riding surface.
- A double layer should not affect noise any more than a single layer, as the surface texture generally remains the same.
- Care must be taken when considering more than one layer of HFST. Since a double layer is less flexible than a single layer, a sound pavement is most important. Any pavement with signs of pattern cracking should likely be replaced prior to HFST installation. Discuss pavement condition assessment with an expert prior to scoping for an HFST application.
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14. Are HFST systems proprietary? Are there any good examples of generic specification?
- All HFST products are non-proprietary. In general, all the equipment and materials are commercially available. Some agencies will install HFST with their own maintenance crews.
- Many State and local agencies now have a specification or special provision for HFST. Standards and specifications used in States such as Florida, South Carolina, Maryland, California, Illinois, Arkansas, Georgia, Pennsylvania, and Texas are often used as guidance for agencies that are developing their own standards for HFST.
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15. Is there a certification process for contractors who want to perform this type of work?
- Not at this time. It would be beneficial to have a certification process since, during installation, we have experienced some good contractors but also a few who were not quite as knowledgeable. The best way to address the issue of contractor performance is to develop a good specification that requires several quality checks and holds the bidders accountable for meeting the specifications for a proper installation. FHWA is working diligently to try to help State agencies write a better specification to cover some of these concerns. However, certification does not always ensure good installation because it is possible that workers who are not trained or familiar with the process could end up doing the work. A good specification makes worker knowledge and experience less of a factor in the quality of the end product.
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16. Are there specifications for HFST?
- The AASHTO Standard Practice for High Friction Surface Treatment for Asphalt and Concrete Pavements AASHTO Designation: PP 79-14 (2016) is available for agencies to adopt in whole or in parts to their construction specifications.
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