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256 1.68.5 Maintenance Practices McDaniel did not discuss maintenance practices. 1.68.6 Rehabilitation Practices McDaniel did not discuss rehabilitation practices. 1.68.7 Performance McDaniel did not discuss performance of permeable asphalt mixes. 1.68.8 Structural Design McDaniel did not discuss structural design. 1.68.9 Limitations McDaniel did not discuss limitations. 1.69 McDaniel, R. S. and W. Thornton. âField Evaluation of a Porous Friction Course for Noise Control.â TRB 2005 Annual Meeting CD-ROM. Transportation Research Board. National Research Council. Washington, D.C. 2005. 1.69.1 General This paper describes the design, construction and test performance of a porous friction course (PFC) placed on I-74, east of Indianapolis, in 2003. McDaniel and Thornton indicate that this section of PFC was placed adjacent to and compared against a stone matrix asphalt (SMA) and a Superpave HMA section. The authors indicate that the PFC showed significantly better (early life) performance in terms of noise reduction, increasing surface texture and friction, and reducing splash and spray. They conclude that PFC provides an economical way to enhance different performance related properties of HMA. McDaniel and Thornton indicate that the Indiana DOT had experienced clogging problems with OGFC in the past, and that it remains to be seen whether the new PFC, with its higher void content, would exhibit better (long term) performance. Although the initial performance of PFC seems to be promising, they suggest the use of long term performance evaluation for making decisions regarding its use in future. 1.69.2 Benefits of Permeable Asphalt Mixtures McDaniel and Thornton indicate a number of benefits of PFC in their literature review. Based on mostly reports on the European experience, the authors mention that PFCs have a significantly higher void content compared to OGFCs (18-22 percent compared to 10- 15 percent), and that PFCs have proven to be durable, to possess good surface friction and to decrease splash and spray during rain. They indicate that aggregate quality and gradation provide enhanced microtexture and macrotexture in PFCs. They also mention that the microtexture, which is the fine scale texture of the aggregates, influence frictional
257 properties at low speeds, and that the macrotexture influences drainage of water and affects rate of decrease in frictional properties with increasing speeds. 1.69.3 Materials and Design McDaniel and Thornton indicate that the PFC used in this study was made up of primarily steel slag aggregates, and consisted of SBS modified asphalt binder and cellulose fiber. The PFC mix was designed with a target air void content of 18-22 percent, using 20 gyrations of the Superpave gyratory compactor. After design, the PFC samples at optimum asphalt content were subjected to the Cantabro test for evaluation of resistance against abrasion. In this test, unaged and aged samples compacted with the Marshall hammer were subjected to 300 revolutions in a Los Angeles abrasion machine. The mass loss during this process is determined on the basis of percentage of original mass of the specimen. McDaniel and Thornton indicate that the allowable mass loss of unaged and aged samples are taken as 25 percent and 30 percent, respectively, by some European and South African specifications. The specific results of mix design and Cantabro test for the PFC are shown in Table 122. The properties of SMA and HMA are also shown in Table 122. Table 122: Materials, Mix Design and Test Results Mix Materials/Properties PFC SMA HMA Design Traffic --- 10-30 million ESALs 10-30 million ESALs Aggregates 90 % steel slag, 10 % sand 80 % steel slag, 10 % stone sand, 10 % mineral filler Coarse aggregate â 50 % steel slag and 50 % dolomite; dolomitic manufactured sand Sieve Size, mm Percent Passing Sieve Size, mm Percent Passing Sieve Size, mm Percent Passing 12.5 100 12.5 100 12.5 100 9.5 83 9.5 85 9.5 94 4.75 28 4.75 39 4.75 64 2.36 12 2.36 27 2.36 46 1.18 9 1.18 21 1.18 --- 0.6 6 0.6 18 0.6 17 0.3 5 0.3 15 0.3 --- 0.15 3 0.15 13 0.15 --- Gradation 0.075 2.4 0.075 10.1 0.075 5.5 Asphalt Type SBS modified PG 76- 22 SBS modified PG 76- 22 PG 76-22 (source different from those used for SMA and PFC) Asphalt Content 5.7 5.5 5.7 Fiber Cellulose fiber, 0.3 % Cellulose fiber, 0.1 % --- Gyration 20 100 100 Voids, % 23.1 4.0 4.0 VMA, % --- 17.7 15.5 Cantabro Loss, unaged 15 --- --- Cantabro Loss, aged 24.9 --- ---
258 1.69.4 Construction Practices McDaniel and Thornton indicate that the PFC mix was constructed in August 2003, using a material transfer device (MTV), which is commonly used in HMA construction in Indiana. One pass from each of two steel wheel rollers was found to be sufficient for compaction. No problem was reported during construction. The authors caution that over rolling can lead to aggregate breakdown and mention that relatively little compactive effort should be sufficient to bring the coarse aggregate in contact for proper construction of PFC. McDaniel and Thornton mention that similar rollers and roller passes were used for construction of the SMA section. No information is provided on the construction of the HMA section, but the authors indicate that there was no problem reported during the construction of the HMA section. 1.69.5 Maintenance Practices No information has been provided regarding maintenance practices. 1.69.6 Rehabilitation Practices No information has been provided regarding rehabilitation practices 1.69.7 Performance McDaniel and Thornton provide information on performance of the PFC section, in terms of results of different tests conducted to evaluate noise, surface texture and friction and splash and spray. For all the different types of results, the PFC showed lower noise than the SMA and the HMA. The different results are shown in Table 123. McDaniel and Thornton show in a figure that the overall sound pressure level is the lowest for the PFC. They explain the effect of reduction of noise with an example. For a line source of noise (a busy roadway is approximated as a line source), noise is attenuated by 3 dB per doubling of noise from the source. A 3dB reduction in noise source level will lead to a 50% decrease in the distance at which a given noise level is measured relative to the original sound. For
259 Table 123: Results of All Sound Measurements Method Average CPX Sound Pressure Levels (Time averaged level over the length of pavement, LAEQ) Speed PFC SMA HMA 72 kph 89.7 dBA 94.2 dBA 93.0 dBA 97 kph 92.6 dBA 97.6 dBA 96.4 dBA Average 91.2 dBA 95.9 dBA 94.7 dBA CPX Difference from PFC 0.0 dBA 4.7 dBA 3.5 dBA Speed Vehicle Impala 68.1 dBA 74.8 dBA 72.6 dBA Volvo 70.1 dBA 75.5 dBA 75.2 dBA Silverado 71.6 dBA 77.0 dBA 74.5 dBA 80 kph Average 69.9 dBA 75.8 dBA 74.1 dBA Impala 71.7 dBA 78.5 dBA NA* Volvo 74.3 dBA 80.5 dBA NA Silverado 74.4 dBA 79.4 dBA NA 110 kph Average 73.5 dBA 79.5 dBA NA Pass-By Difference from PFC 0.0 dBA 6.0 dBA --- * Could not be tested due to speed limits Results of surface texture measurements are shown in Table 124. McDaniel and Thornton indicate that the PFC and SMA showed significantly more texture than the HMA, with the PFC showing the maximum depth. The variability in measured values was also found to be higher for the PFC and the SMA. The authors indicate that this is expected since these mixes have gap-graded aggregate structures, and a lower texture is expected in the SMA due to the presence of the mastic of asphalt binder and fibers. Table 124: Results of Surface Texture Measurement Mix Mean profile Depth, mm (Standard Deviation) PFC 1.37 (0.13) SMA 1.17 (0.14) HMA 0.30 (0.05) The results of dynamic friction measurement are presented in Table 125. McDaniel and Thornton indicate that for the DFT measurements, the PFC and the HMA show comparable values whereas the SMA showed the lowest values. The authors point out that the SMA is expected to have lower values because of the presence of the mastic of asphalt binder and filler in the gap-graded aggregate structure and that these values should increase the traffic wears off the asphalt binder film and exposes the steel slag aggregate.
