Florida Tackles Premature Failures of HFSTs
Florida has tackled occasional premature failures of high friction surface treatments (HFSTs) with a new specification and guidebook for use in the Sunshine State.
The State of Florida has used high friction surface treatments since 2006 to reduce wet weather crashes on tight curves and intersections and to maintain bridge decks, said Bryan Wilson and Anol Mukhopadhyay, Ph.D., associate research scientists, Texas A&M Transportation Institute, in their 2016 technical guidance, Alternative Aggregates and Materials for High Friction Surface Treatments.
However, Florida DOT has reported issues related to premature failure of the treatment. Produced in cooperation with the Florida DOT and Federal Highway Administration, Alternative Aggregates and Materials included:
- review of the literature and interviews with industry experts on the state of the practice of HFSTs;
- documentation of all existing HFST projects in Florida with analysis of their performance based on distress, skid resistance, and crash reduction;
- performance of field testing on six projects, evaluating their present performance and conducting forensic analyses as needed;
- performance of lab tests on different aspects of HFST materials and construction practices to improve durability and reduce costs; and
- development of a revised HFST specification for Florida and an HFST guidelines brochure.
HFSTs Successful Since 2006
Despite their success since 2006, various sections of HFST have experienced severe cracking and potholing of the pavement, delamination from the existing surface, and raveling of the aggregate from the resin binder, the authors say. Another issue with HFST is that the high material costs limit the number of sections that can be treated annually. “Consequently, there is a need to research the best materials and practices to eliminate premature HFST failures and reduce overall HFST costs, while maintaining or improving treatment life and friction performance,” Wilson and Mukhopadhyay said.
Project documentation and field testing indicated the bid unit cost of HFST in Florida was between $26 and $40 per square yard, and the total unit cost (including traffic control, repairs, striping, etc.) was between $36 and $113 per square yard.
- Crash reduction from HFSTs was most effective on tight curves, where the average reduction in crash rate was 32 and 75 percent for total and wet weather crashes, respectively. Crash rates did not notably change on wide curves/tangent but increased on average for intersections/approach.
- HFSTs are a cost-effective treatment for tight curves with a history of crashes. The average benefit-cost (BC) ratio on tight curves with a history of crashes was between 18 and 26 (depending on calculation method), with some sections greater than 50 and as high as 118.
- HFSTs are not cost effective, from a crash reduction perspective, on wide curves/tangents with no history of crashes. The cost effectiveness for bridge deck preservation was not part of this research.
- The cost effectiveness of HFST at intersections/approaches is still inconclusive. Half the observed sections had good BC ratios, while the other half had increased crashes.
- Different distress types were observed in the field (potholing, aggregate loss, surface cracking, substrate tearing, splotchy texture). Through forensic evaluation, several of the failure mechanisms were identified as discussed in the background section.
Highlighted research findings from the laboratory testing include:
- Aggregate loss is a function of binder thickness, where aggregate loss greatly increased for binder thicknesses less than 50 mils.
- A double application of HFST can mitigate issues with aggregate loss.
- Single applications on concrete appear to wear away faster than single applications on HMA; however, this phenomenon has not been proven or disproven yet in the field environment.
- The concrete tested had a coefficient of thermal expansion (COTE) value of 5 x 10-6/°F and asphalt between 7 and 9 x 10-6/°F. In comparison, the COTE of HFST is drastically higher, between 14 and 30 x 10-6/°F. This would imply that stress builds up between HFST and the substrate during thermal cycling.
- The standard gel time test was not drastically responsive to changes in proportioning until a 25 percent change, but it is much more sensitive to temperature.
- A thin HMA overlay mix was successfully designed using PG76-22 binder and a coarse-size calcined bauxite. The design had good rutting resistance. The friction properties, while better than other HMA designs, are inferior to HFST.
- Two calcined bauxite aggregates with similar aluminum-oxide (Al2O3) contents both performed well in HFST friction testing. Another aggregate of unknown composition had good friction properties, but was not on par with calcined bauxite. (Of particular concern was the high micro-deval mass loss.)
Based on these and other findings, Wilson and Mukhopadhyay recommended the following:
Candidate Projects. The researchers recommend that agencies strongly consider applying HFST on tight crash-prone curves. Agencies should carefully consider if HFST would be effective at reducing crashes at intersection approaches and within intersections (i.e., identify if a significant source of accidents is skid related).
When applied on wide curves and tangent sections (i.e., for maintenance of bridge decks), agencies should not expect to see significant economic benefits from crash reduction; however, benefits from pavement preservation may warrant the cost.
As concerns the existing surface type, the researchers do not recommend placing HFST on open graded friction course (OGFC) pavements. While some projects have successful performance over OGFC, the material often causes more problems like excessive draindown, requirement of double-lift HFST, substrate cracking, and stripping. These surfaces should be milled out and inlaid with dense-graded mix prior to applying HFST.
The department may consider a minimum substrate strength requirement, but stricter requirements on the existing surface type and distress condition should resolve issues with substrate failure. The researchers don’t recommend excessively stringent requirements on surface condition, as it could unnecessarily drive up the costs of HFST application with marginal benefits to long-term performance.
Materials. The two calcined bauxite sources evaluated in this research are acceptable for HFST. The researchers recommend lowering the minimum required Al2O3 content for calcined bauxite to 86 percent.
The minimum required content could likely be lowered further with negligible effect to performance; however, the data are not yet available to support this decision. The unknown aggregate type tested in this research may be suited for bridge-deck preservation.
The current specification for polymer resin binders is adequate. In the future, the department may consider a specification with greater flexibility to mitigate problems with thermal incompatibility. This change would need to be balanced with the binder strength and hardness.
An alternative approach to HFST could be incorporating calcined bauxite into HMA design. While a successful design can be created, it is likely an infeasible option considering the economics of producing and constructing such small quantities. It may only be feasible if paving a winding rural road. Even then, the friction performance of this design is inferior to HFST.
Construction. The researchers recommend a contractor requirement to place an HFST test section before full-scale application. This gives the contractor a chance to identify and fix problems, and of equal importance, the Florida DOT inspector will become familiar with the process and potential issues.
Surface preparation of all projects should include a high-pressure air wash after sweeping to remove remaining dust and debris. Concrete surfaces should require shot blasting to a texture level of concrete surface profile (CSP) 5.
Many problems associated with poor mixing, uneven resin binder thickness, and insufficient aggregate coverage could be solved by using automated application equipment. The researchers recommend Florida DOT adopt this requirement into its specification. This is expected to increase the cost of HFST early on, especially since very few companies have this capability. With time, as more vendors enter to compete, the costs are expected to decrease.
Neither the gel time test nor the Fourier transform infrared spectroscopy (FTIR) test are recommended as quality control methods. The tests are not sensitive enough to misproportioning except at extremes; therefore, the tests are not substitutes for proper maintenance and calibration of the application equipment.
Still, the simple gel time test does have a place to ensure against major problems. The required mil thickness in the current HFST specification is adequate and the researchers recommend that contractors and inspectors check the actual mil thickness from time to time with a thin film thickness gauge.
Currently the contractor is required to sweep after the initial cure and do follow-up sweeping after two weeks. The researchers recommend another follow-up sweep between 24 to 48 hours on high-volume roadways.
HFST Spec, Guidelines
After discussions with FDOT and industry leaders, many of these recommendations have been incorporated into a revised FDOT specification. In addition, a user-ready booklet, titled High Friction Surface Treatment Guidelines: Project Selection, Materials, and Construction, was developed to mirror the FDOT specification and provide additional insight into many requirements and recommendations.
The document can be obtained by contacting Charles Holzschuher at the FDOT State Materials Office (email@example.com), or downloaded at no charge here.
Adapted by Pavement Preservation Journal from the original report and excerpted with permission from Pavement Preservation Journal of FP2 Inc. For more information visit fp2.org.
New guidance may be downloaded here.