Annotated Literature Review for NCHRP Report 640 (2009)

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293 In many cases, OGFC layers are not considered to provide structural capacity. Therefore, the underlying pavement must be constructed to withstand the effects of traffic and the OGFC is solely a sacrificial layer. 1.77 Poulikakos, L.D., S. Takahashi and M.N. Partl. “Evaluation of Improved Asphalt by Various Test Methods.” Report Nr. 113/13 (EMPA No. FE 860076). EMPA. October 2006. 1.77.1 General This report documents a joint Swiss-Japanese research study focusing on the optimization of porous asphalt gradations. Porous asphalts consist mainly of coarse aggregates with small amounts of fine aggregates and fillers resulting in a mix with an open texture and permeable structure. Due to the open texture and permeable structure, porous asphalt improves frictional properties and provides good visibility by reducing splash and spray on wet surfaces. However, porous asphalt mixes can suffer from problems. Because of the permeable structure, asphalt binder films coating aggregates within porous asphalt are exposed to air and water, increasing the risk of binder aging. This report documents a study to evaluate different tests for characterizing porous asphalt for optimizing gradations and improving mix design methods. 1.77.2 Benefits of Permeable Asphalt Mixtures Open-graded asphalt is widely used in Western Europe and Japan. Following are benefits identified by the authors when using OGFCs: • Reduced splash and spray levels from vehicle tires in wet conditions • Prevention of hydroplaning • Improved skid resistance in wet conditions • Reduction in vehicle induced traffic noise generated at the tire/pavement interface • Reduction in headlight glare from oncoming vehicles on wet pavements • Improved nighttime wet weather visibility 1.77.3 Materials and Mix Design The main focus area of this report was a research effort to evaluate the Dry Packing Method of evaluating porous asphalt gradations. This methodology is summarized in Figure 27. The Dry Packing Method (DPM) entails designing a porous asphalt to a design porosity (air void content) based upon the packing characteristics of selected aggregates. The process begins by separating the selected aggregates by size. Figure 27 illustrates the aggregates separated into five sizes. Initially, the largest aggregate size is blended with the finer fraction until the combination of aggregates A and B reach the maximum density (AB). Then AB is combined with aggregate C and the maximum density is determined. The process continues until all but the finest fraction have been combined. Finally, the smallest fraction of aggregate is added (blended) until the target air void content is achieved. [This method appears to have some of the same concepts as the Bailey Method currently being used to design some dense-graded mixes in the US.] A variation of the DPM has also been used to design porous asphalt. This method, called the Wet Packing Method (WPM), includes the same methodology; however, the aggregates are coated with asphalt binder prior to compaction.

294 Figure 27: Dry Packing Method for Porous Asphalt Typical gradation requirements for porous asphalt in Switzerland are provided in Table 139. The wearing courses shown in Table 139 (labeled as DRA) are designed to have 21 to 27 percent air voids. Typical gradation requirements used in Japan are illustrated in Table 140. Table 139: Gradation Requirements in Switzerland Percent Passing Sieve Size, mm DRA 6 DRA 11 DRAT16 DRAT 22 22.4 100 90 – 100 16.0 100 90 – 100 25 – 60 11.2 100 90 – 100 20 – 50 15 – 30 5.6 90 – 100 15 – 40 10 – 25 10 – 20 2.8 15 – 40 8 – 20 7 – 17 --- 2.0 10 – 25 --- --- 6 – 15 0.5 4 – 10 4 – 10 4 – 10 4 – 10 0.09 3 – 5 3 – 5 3 – 5 3 – 5 DRA – Wearing Courses DRAT – Base Courses

295 Table 140: Gradation Requirements in Japan Percent Passing Sieve Size, mm 0/20 mm 0/13 mm 19 95 – 100 13.2 53 – 78 92 – 100 9.5 35 – 62 62 – 81 4.75 10 – 31 10 – 31 2.36 10 – 21 10 – 21 0.60 4 – 17 4 – 17 0.30 3 – 12 3 – 12 0.15 3 – 8 3 – 8 0.075 2 - 7 2 - 7 The authors report conducting a number of different tests on mixes prepared to meet both the Swiss and Japanese gradation requirements. Mixes used during the research were compacted with a Superpave gyratory compactor. Instead of compacting samples to a certain number of gyrations, the samples were compacted to a target air void content of 22 percent. Tests conducted included permeability testing in accordance with a Japanese Standard laboratory test, an interlayer shear test, indirect tensile test, co-axial shear test, wheel tracking, Cantabro Abrasion loss, and particle loss by shearing. The interlayer shear test was used to analyze the interlayer adhesion between porous asphalt and dense-graded HMA. The Layer Parallel Direct Shear test (Figure 28) was conducted on samples prepared in the laboratory. Double layer samples were first made by preparing full height dense-graded samples in the Superpave gyratory compactor. After cooling to room temperature, these samples were cut in half. The half samples were then placed back into the gyratory mold and porous asphalt added and compacted. A typical indirect tensile strength test was conducted. To evaluate different conditions, samples were tested at two different temperatures. In order to evaluate the ability of the different porous asphalt mixes to resist thermal cracking, tests were conducted at 32°F (0°). Samples were also prepared for moisture susceptibility testing. Conditioned samples were placed in a 140°F (60°C) water bath.

296 Figure 28: Schematic of Layer parallel Direct Shear Test The Co-Axial Shear test was conducted to evaluate modulus values for the different porous asphalt mixtures. Tests were conducted at various temperatures and frequencies. To run this test, Superpave gyratory specimens were cored to produce asphalt concrete rings with a 6 in (150 mm) outer diameter and a 55 mm inner diameter. The specimens were then glued to the test molds and the test rod within the center of the sample. The test rod was then torqued to creating the loading. Deformation was measured with LVDTs. Two wheel tracking tests were conducted. First, the French Wheel tracking test was conducted. Tests were conducted at 140°F (60°C). Secondly, the Japan Highway Public Corporation test was run. Within this test, a steel wheel loaded to 686 N is tracked on the sample for 1 hour. This testing was conducted to evaluate the potential for permanent deformation in the various porous asphalt mixes. Cantabro abrasion loss test was used to evaluate the resistance to particle loss by abrasion and impact effects. For this study, specimens were conditioned at -4°F (-20°F) prior to testing. Another test related to particle loss conducted by the authors was the particle loss by shearing force test. Mixture was placed in a mold and exposed to a rotating wheel. Based upon the laboratory testing, the authors state that mixes designed using the packing theory performed better. The most obvious improvements were observed in the Cantabro Abrasion test and Co-Axial Shear test. Specific conclusions by the authors include: • Good repeatability was achieved in all the tests used in the study • Non SBS modified asphalt binders produced specimens with lowest G*.

297 • Aging should be used in order to properly identify long term effects of any packing theory • The Japanese asphalt binder used was stiffer than asphalt binder with NAF used in the Swiss SPA mix • The Japanese binder yielded a smaller phase angle than the Swiss polymer modified binder (Black diagram). Clear differences in Black diagram between neat asphalt binders and polymer modified binders were observed. • The Interlayer shear strength of packed mix increased with aging • Interlayer shear tests indicated no significant differences between dry and wet conditioning • CAST tests show that in the post-compacted stage the packed mixes are stiffer at high frequencies (unaged and aged) • In the French wheel tracking tests, the packed and unaged mixes behave quite similar whereas the unpacked mixes differed considerably • In the aged stage all mixes perform very similar in the French wheel tracking test and the packing theory does not show an improvement. • The packed theory does show an overall increase in the stiffness of the Japanese and Swiss mixes for aged material as seen in CAST. This increase is especially prevalent at lower temperatures and higher frequencies. 1.77.4 Construction Practices No specifics were given on construction practices 1.77.6 Rehabilitation Practices No specifics were given on rehabilitation practices. 1.77.7 Performance No performance measures were given. 1.77.8 Structural Design No specifics on inclusion within mix design were given. 1.77.9 Limitations Typical problems with porous asphalt listed by the authors are as follows: • Hardening of binder (aging) due to oxidation effects of air reduces durability • Draindown in the pavement • Stripping of the binder from the aggregate during service due to high exposure of binder to water • Densification under traffic, reducing permeability to water Loss of permeability to water due to clogging by road debris and detritus