Annotated Literature Review for NCHRP Report 640 (2009)

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214 Table 106: Summary of Friction Numbers Measured on the Test Lane and Control Lane Section (Friction Number) Test Experiment H I J K L Conc. Test 15.2 16.5 14.7 15.2 18.7 17.7 I Control 14.9 15.8 15.8 16.5 16.5 13.8 Test 18.6 17.6 18.1 21.6 19.0 21.2 II Control 18.5 19.7 16.7 18.4 18.9 18.5 Test 18.5 14.0 13.5 16.2 13.4 15.6 III Control 17.2 22.3 16.6 11.0 12.2 14.1 Test 12.6 11.1 13.4 16.8 15.7 16.3 IV Control 14.9 25.5 15.2 14.8 14.0 18.2 Test 19.7 19.7 87.9(1) 25.0 21.7 22.6 V Control 39.9(1) 19.9 91.0(1) 20.5 22.0 21.0 Test 11.9 15.4 14.7 18.5 13.7 15.1 VI Control 15.0 15.4 16.4 15.7 16.0 17.5 (1) Sections were not fully covered with snow because of problems with some of the snow towers. 1.53.6 Rehabilitation Practices No specifics on rehabilitations practices were given. 1.53.7 Performance A second experiment conducted by Flintsch was conducted to compare the wearing surfaces during a rain event. The experiment involved producing artificial rain and visually comparing each wearing surface through the use of video. Flintsch states that the OGFC enhanced splash and spray performance when compared to the other hot mix asphalt surfaces. 1.53.8 Structural Design No specifics or inclusions with structural design were given. 1.53.9 Limitations No specific limitations were given. 1.54 Fortes, R.M. and J.V. Merighi. “Open-graded HMAC Considering the Stone-on-Stone Contact.” Proceedings of the International Conference on Design and Construction of Long Lasting Asphalt Pavements. Auburn, Alabama. June 2004. 1.54.1 General This paper presents the results of a research study to evaluate the use of stone matrix asphalt (SMA) and open-graded friction course (OGFC) for an urban expressway in Sao Paulo, Brazil. The authors describe a pavement that is seven or eight lanes in each direction that carries almost 400,000 vehicles per day in each direction. Approximately 22 percent of the vehicles are heavy trucks and the trucks are limited to the two outside lanes at speeds approximately one-third of small vehicular traffic. The primary problems

215 associated with the expressway are rutting, reflective cracking, aggregate polishing and low skid-resistance. Fortes and Meighi also describe a problem with large amounts of splash and spray originating from the heavy trucks during a rain event. 1.54.2 Benefits of Permeable Asphalt Mixtures The authors did not specifically conduct testing to evaluate the benefits of permeable friction courses, however, numerous literature were referenced that indicated that permeable friction courses could reduce splash and spray, improve frictional properties, reduce the potential for hydroplaning and reduce tire-pavement noise. 1.54.3 Materials and Design The paper described a mix design procedure that utilizes 50 blows per face of the Marshall compaction hammer for the standard compactive effort. A single granite aggregate source was used for all testing. The authors reported a Los Angeles Abrasion loss of 25.1 percent for the coarse aggregates and a sand equivalency of 66 percent for the fine aggregates. Two asphalt binders were included within the study: an AC-20 and a polymer modified (SBS) asphalt binder. Six different aggregate stockpiles were used to fabricate ten different open-graded gradations which are illustrated in Figure 23. In order to evaluate the existence of stone-on-stone contact within the OGFC mixtures, the authors used methods developed during NCHRP 9-8 for SMA mixes. The voids in coarse aggregate (VCA) of the aggregate fraction was determined by adding a “low asphalt content” and compacting with 50 blows per face of the Marshall hammer. [Based upon the reference provided, it appears that the authors utilized 2 percent asphalt binder.] Of the ten gradations fabricated within this study, the authors state that only two meet the requirements of having stone-on-stone contact (shown as bold lines in Figure 23). These two gradations were further investigated. [Based upon the experiences of the NCHRP 9-41 researchers, it is very difficult to design an open-graded friction course that does not have stone-on-stone contact. Evaluation of the gradations in Figure 23 suggests that there are at least three gradations that are coarser and more gap-graded than the two highlighted gradations that the authors indicate did not have stone-on-stone contact. The researchers tried to reproduce the values for VCA of the coarse aggregate fraction provided in the paper as well as other volumetric properties without success. It appears that the authors of this paper used an erroneous equation for calculating the VCA of the coarse aggregate fraction. Equation 2 from the paper is provided verbatim along with descriptions of each equation component below.

216 Fortes and Merighi OGFC Gradations 0 10 20 30 40 50 60 70 80 90 100 Sieve Size, mm (Raised to 0.45) Pe rc en t P as si ng 25.019.012.59.54.750.075 2.36 Figure 23: Gradations Used by Fortes and Merighi ( )[ ] 100×× −×= aagr agDaagr LAVCA γγ γγγ where, VCALA = Coarse aggregate voids compacted with low asphalt binder, % γagr = bulk specific gravity of the compacted specimen, kN/m3 γa = unit weight of water, kN/m3 γagD = unit weight of the coarse aggregate fraction, kN/m3. In contrast to the above equation, the following is a correct equation when determining the VCA of the coarse aggregate fraction when using a “low asphalt binder content”: ( ) ⎥⎦ ⎤⎢⎣ ⎡ ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ −×−= b ca mb P G GVCA 1100100 where, Gmb = bulk specific gravity of the compacted mix, Gca = bulk specific gravity of the coarse aggregate, Pb = percent asphalt binder in the mixture (by total mixture mass).

217 Therefore, it appears that the authors were in error when calculating the VCA of the coarse aggregate fraction. Because of these errors, the NCHRP 9-41 researchers tried to recalculate some volumetric properties from the data available within the paper; however, problems were again experienced. Data for the unit weight of the compacted mixtures along with calculated air voids were provided. When the theoretical maximum density was back calculated from these data, the “Rice” was higher than the reported aggregate specific gravities. Therefore, it is uncertain whether other properties are correct within the paper. There remaining part of this review only provides general conclusions provided by the authors.] The two selected gradations were combined with an AC-20 asphalt binder at three asphalt binder contents (not identical) and a polymer modified binder at a single binder content. These eight mixes were then compared to two typical dense-graded mixes. Volumetric properties of all the mixes were evaluated. Specimens containing the polymer-modified binders had higher air void contents than the OGFC mixes containing the AC-20 asphalt binder. This is likely the result of using identical compaction temperatures for each mix with the Marshall hammer. Tests conducted on the ten different mixtures included Marshall stability (60˚C), indirect tensile strength (25˚C) and laboratory permeability testing. Results of all three tests indicated that the addition of the polymer modified binder improved the performance of the OGFC mixtures. Not surprisingly, however, the Marshall stabilities and indirect tensile strengths of the dense-graded mixes were higher than the OGFC mixes. The authors also employed an unconfined static creep test at test temperatures of 25, 40 and 50 C to evaluate the potential for permanent deformation for each of the ten mixtures. This testing was conducted because of the rutting problems described previously. Specimens for this testing were 100 mm (4 in) diameter samples. [No specimen heights were provided; however a figure within the paper suggests that the height of the sample was approximately 50 mm (2 in.) in height]. For the static creep test, the authors capped each end of the sample with a resinous material. A load of 0.55 MPa (approximately 80 psi) was placed onto the sample for 1,000 seconds. The load was then removed and the deformation measured for another 1,000 seconds (rebound). This loading configuration was conducted for a total of four steps. Results from this testing indicated that the OGFC mixtures had less potential for permanent deformation than the dense-graded mixes and that the OGFC mixes containing the polymer-modified binders performed best. 1.54.4 Construction Practices No information is provided on construction practices of friction course. 1.54.5 Maintenance Practices No information is provided on maintenance practices of friction course. 1.54.6 Rehabilitation Practices No information is provided on construction practices of friction course.