Rock on Your High Friction Surface Treatment
In July of this year, Michael Heitzman, PhD, PE, with the assistance of Pamela Turner and Mary Greer of the National Center for Asphalt Technology at Auburn University, Auburn, Alabama, released NCAT Report 15-04 to share the findings of a high friction surface treatment (HFST) study of alternative aggregates to calcined bauxite. The point of the study was to determine if there are good alternatives to calcined bauxite, which is a product that must be imported to the United States, for HFST. The alternative aggregates offered for study were granite, flint, basalt, silica sand, steel slag, emery and taconite. Researchers at NCAT used three studies—two lab-based and one field-based—to determine the long-term friction loss trend—also called the terminal friction—of the seven aggregates suggested for HFST.
In essence, the study found all eight aggregates, which includes the original bauxite, maintained good macro-texture, but the measured friction for alternative HFST aggregates was not equal to bauxite. Let’s take a look at how the researchers came to these conclusions and what other facts they learned about the potential aggregates for HFST.
First, the Federal Highway Administration Roadway Departure Safety Program includes guidance and tools to address crashes on wet pavement. Pavement friction is one component of the program and one of the tools in the design of pavement friction is HFST.
The authors of NCAT Report 15-04 point out, HFST is an important application for critical safety locations such as bridge decks, horizontal curves and high speed deceleration ramps, which asphalt contractors pave on a regular basis. FHWA worked with American Association of State Highway and Transportation Officials and American Traffic Safety Services Association to develop a guide spec PP 79-14 Standard Practice for High Friction Surface Treatment for Asphalt and Concrete Pavements. This guide recognizes calcined bauxite aggregate for use in the HFST. The authors outlined where bauxite comes from, and that’s mainly China.
According to the report: “Bauxite is mined in many countries, but the USA produces less than 1 percent of the product. The large majority of bauxite ore is used for the production of aluminum.
Smaller amounts are used in chemical processes, industrial abrasives, and in the refractory industry. The suppliers of calcined bauxite for the refractory industry are the primary source for the calcined bauxite used for HFST. The amount used for HFST is a small fraction of the calcined bauxite produced for the refractory industry in the USA. In the USA, the majority of calcined bauxite is imported from China. The calcination cost, crushing requirements, and transportation cost make bauxite a more expensive product compared to other aggregates.”
The series of studies discussed in “High Friction Surface Treatment Alternative Aggregates Study” divided the sets of aggregates and tests thus:
“The scope of LAB-1 evaluated HFST test slabs with the bauxite and seven alternative aggregates under accelerated laboratory polishing and testing procedures. The scope of FIELD evaluated friction performance of HFST pavement test sections with the same eight aggregates under heavy truck loading in the west end super-elevated curve at the NCAT Pavement Test Track. LAB-2 evaluated the influence of particle size on HFST friction performance and examined other laboratory aggregate tests as a simpler approach to qualifying friction aggregates in HFST specifications.”
The results of the three studies-within-the-study were thus:
(Kristin, please right&left justify the next 3 long paragraphs and inset the text because they’re a direct, block quote from a document.)
LAB-1 showed that none of the seven alternative HFST aggregates provided friction comparable to bauxite based on the DFT [dynamic friction tester] measurements. Bauxite maintained a terminal friction value—DFT(40)—above 0.80; four aggregates—taconite, basalt, emery and flint—maintained values above 0.60, and two aggregates—silica sand and slag—measured terminal values at or below 0.50. All of the surfaces, except slag, maintained surface macro-texture mean profile depth (MPD) values at or above 1.4 mm. Even though bauxite measured the highest DFT friction values, the bauxite surface texture was lower than most of the aggregates. All of the test surfaces had surface texture values much higher than MPD in the range of 0.30 to 0.50 mm for conventional dense graded asphalt mixtures. The data showed no correlation between the terminal friction and terminal surface texture.
The first step of the FIELD analysis examined the changes in HFST friction and texture to establish terminal values. All of the sections, except for basalt, showed a 0.20 to 0.30 mm drop in MPD texture values after approximately one month of traffic, and in most cases texture continued to gradually decrease an additional 0.10 to 0.20 mm MPD through six months of traffic conditioning. The terminal texture values ranged from 1.10 to 1.50 mm MPD for all sections, except steel slag, which dropped below 0.90 mm. After one month of traffic, the wheel path DFT friction values for all of the HFST test sections had a general surface friction reduction of 0.15. The most probable explanation for the friction reduction within the first month is the traffic abrasion wearing down the sharp edges of the crushed faces of the aggregate particles. Most of the HFST test sections maintained their relative ranking of surface friction throughout the six-month conditioning period. The three sections conditioned for an additional 18 months showed no change in the ranking of friction performance. Locked-wheel skid trailer data for the three longer sections was only reliable for the extended 18 months. The trend lines generated by the skid trailer SN40R data sets showed bauxite friction dropped from 70 to 63, flint dropped from 54 to 43, and granite dropped from 54 to 40. The results clearly show the bauxite HFST test section maintained higher friction levels over the 24 months of accelerated NCAT Pavement Test Track truck traffic conditioning.
LAB-2 had two objectives: (1) evaluate the influence of particle size and (2) examine other laboratory aggregate tests. … The surface macro-texture response from the particle size evaluation was consistent with LAB-1. The CTM [circular texture meter] measured some texture reduction after the first period of TWPD [three wheel polishing device] conditioning but no change after the second period of conditioning. As expected, the HFST surface macro-texture decreased as the size of the aggregate particles decreased. Surfaces with No.6 sieve particles measured 2.2 mm MPD and surfaces with No.16 sieve particles measured 1.1 mm. The friction response showed a similar trend during conditioning. Friction reduced after the first period of TWPD conditioning and did not change after additional conditioning. The terminal DFT(40) friction values for all four aggregates reacted similarly to the differences in particle size. There was marginal change in measured friction surfaces with No.12 and No.8 particles. Friction reduced for surfaces with either No.16 or No.6 particles. The evaluation of macro-texture and friction combined showed friction increased as macro-texture increased up to a MPD of 2.0 mm. Friction decreased on the conditioned slabs with MPD above 2.0 mm. For the second objective, the aggregates showed differences in Micro-Deval mass loss, but none of the aggregates reached a terminal mass loss. Mass loss results ranked bauxite as the best performer and taconite as the lowest performer. The rank order of the mass loss results agreed with the DFT(40) friction results, except flint aggregate ranked second for mass loss and fourth (last) for friction. The AIMS [aggregate image measurement system] measurements quantified the shape of all four aggregates in a narrow range of 6 to 8 on a scale of 0 to 20 and there was very little change in particle shape after Micro-Deval conditioning. Particle shape did not correlate to friction, so shape was not given further consideration. The AIMS angularity test results showed bauxite and taconite are very similar. The flint sample had a higher mean angularity and the slag aggregate had the highest angularity. The anticipated trend would be higher angularity achieves higher friction, but actual LAB-2 results show no correlation between particle angularity and DFT friction. Overall, the use of Micro-Deval and AIMS to condition and measure aggregate characteristics did not correlate to the aggregates’ friction measurements.
As mentioned initially, all eight surfaces maintained good macro-texture, which the authors defined as predominantly MPD>1.0 mm. The series of three studies offered these additional conclusions, according to the report authors:
• The eight surfaces measured terminal DFT(40) values in the range of 0.84 to 0.49 in LAB-1 and 0.79 to 0.43 in FIELD.
• Terminal surface characteristics were achieved after early conditioning (less than one month for FIELD friction) both in the laboratory and in the field. The terminal texture and friction characteristics decreased very slowly during additional conditioning.
• For each aggregate tested in LAB-2, the surface with MPD of 1.50 to 2.00 mm measured the highest DFT(40) friction.
• There was no correlation between HFST surface friction and AIMS particle shape and angularity.
• There is no correlation between HFST surface macro-texture and friction.
• A DFT measures higher friction values than a locked-wheel skid trailer.
Future studies could look into whether common friction aggregates with good micro-texture used in the United States could be combined with HFST pavement surfaces with high macro-texture to reduce crash rates comparable to HFST with bauxite.