Annotated Literature Review for NCHRP Report 640 (2009)

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9 levels increased as speed increased above 50 to 60 km/hr. This indicated to the authors that a reduction in noise levels is primarily seen and most effective at higher speeds. Results from testing with the noise trailer also showed a relationship between permeability and noise levels. As permeability increased, noise levels generally decreased. Also, porous asphalt mixes having a smaller maximum aggregate size or more continuous grading have a lower noise level than coarser porous asphalt layers. Wayside measurements were conducted on sections of roadway where a comparison between porous asphalt and typical dense-graded layers could be made. For single vehicle cars, a level of reduction between 1 and 5 dB(A) was observed between porous asphalt and the dense-graded layers. Noise levels for a traffic stream (Leq) showed reductions between 0 and 3.5 dB(A). Besides the reduction in noise levels, the authors also found that porous asphalt reduces disturbing noise at higher sound frequencies. By removing the disturbing higher frequency noise, persons living near a roadway having a porous asphalt wearing layer provided positive comments. 1.3.8 Structural Design The only specifics on structural design mentioned by the authors was that typical layer thicknesses ranged from 28 to 50 mm. Porous asphalt mixes having a maximum aggregate size of 10 mm are typically placed 28 to 42 mm thick while porous asphalt mixes having a maximum aggregate size of 16 mm are typically placed from 43 to 50 mm thick. 1.3.9 Limitations Disadvantages of porous asphalt highlighted by the authors included: • Reduction in the advantages listed above over time due to clogging of the layer. • Unknown durability of porous asphalt layers. • Special requirements (e.g., lateral drainage in urban areas). • Unfavorable frictional properties at lower speeds. • Different winter maintenance requirements. • Difficulty in conducting repairs. • Costs 1.4 Ruiz, A., R. Alberola, F. Pérez, and B. Sánchez. “Porous Asphalt Mixtures in Spain.” Transportation Research Record No. 1265. Transportation Research Board. National Research Council. Washington, D.C. pp. 87-94. 1990. 1.4.1 General This paper describes the use of porous asphalt mixtures in Spain up to 1990. Ruiz et al indicate that the first application of porous asphalt in Spain was placed in 1980 on four experimental sections in northern Spain. The purpose of the trial sections was to improve safety in rainy areas. In 1986, the use of porous asphalt began to increase because doubts about the durability of porous asphalt were eliminated. Based upon the success of porous

10 asphalt mixtures, it was not only used to provide a safe wearing course but also to provide a durable surface with a smooth, safe and quiet ride in any type of weather. Ruiz et al indicate that as of 1990, approximately 3 million square meters of porous asphalt mixture had been placed. Porous asphalt was being used for all types of roadways and all types of traffic conditions. The most common practice was to place porous asphalt at a thickness of 40 mm. In addition to being used as the wearing course of new pavements, Ruiz et al indicate that the main application of porous asphalt mixtures had been to repair aged or slippery surfaces of existing pavements that had not shown structural problems. Porous asphalt mixtures are also used in short stretches (300 m) in areas that are difficult to drain (e.g. horizontal and vertical curves. 1.4.2 Benefits of Porous Asphalt Mixtures Ruiz et al indicate that porous asphalt mixtures provide a durable surface with a smooth, safe and quiet ride. 1.4.3 Materials and Design Spain utilizes two porous asphalt gradation bands, P12 and PA12 (Table 4). Ruiz et al indicate that selection of the aggregate gradation influences the water drainage capacity, resistance to particle loss (durability), resistance to rutting and the macrotexture of the pavement surface. The P12 grading band results in porous mixtures with air void contents between about 15 and 22 percent, while the PA12 results in air void contents up to 25 percent. Table 4: Gradation Bands for Porous Asphalt Mixtures in Spain Gradation Bands, Percent Passing Sieve Size, mm P12 PA12 20 mm 100 100 12.5 mm 75-100 70-100 10 mm 60-90 50-80 5 mm 32-50 15-30 2.5 mm 10-18 10-22 0.63 mm 6-12 6-13 0.08 mm 3-6 3-6 The aggregate gradation bands both contain a large percentage of coarse aggregate (defined as larger than 2.5 mm) in order to accommodate the other components of the porous mixture. Ruiz et al indicate that selection of the amount of fine aggregate is important. The fine aggregate content must be low enough to prevent closing up of air voids and must not separate the coarse aggregate particles. Separation of the coarse aggregate particles will increase the potential for rutting. Ruiz et al also indicate that some amount of filler (finer than 0.075 mm material) is needed to give cohesion to the mixture to help prevent particle loss.

11 Ruiz et al provide recommendations for aggregate properties in the form of Los Angeles Abrasion, flakiness index, particles with two or more fractured faces and sand equivalency. [These properties can be generally characterized as toughness, shape, angularity and cleanliness, respectively.] Because aggregate breakdown can lead to particle loss, raveling and the closing up of the surface texture, Spain requires a Los Angeles Abrasion value of 20 percent maximum. For the same reasons, a flakiness index of 25 percent or less is also required. To ensure angular aggregates, Spain requires 75 percent or more coarse aggregate particles with two or more fractured faces. The sand equivalency value for fine aggregates must be higher than 50 percent. Non-polishing aggregates are also required to maintain a good, durable microtexture on the pavement surface. The Spanish specification sets the polished stone value above 0.45 for traffic volumes greater than 800 trucks a day per lane and 0.40 for all other traffic categories. Ruiz et al indicate that stiff asphalt binders are needed for porous asphalt mixtures. Stiff binders are needed to resist against particle loss and to get longer durability through thicker asphalt binder films. In 1990, approximately 80 percent of all porous asphalt mixtures utilize polymer modified binders. The most common types of polymers are EVA and SBS. The philosophy of designing porous asphalt mixtures in Spain is two-fold: provide a minimum binder content to assure against particle loss and a maximum binder content to avoid draindown. The resistance to particle loss is determined through the use of the Cantabro Abrasion test. This test is used to determine the minimum asphalt binder content that will provide a maximum of 25 percent particle loss. No specific test was specified at the time of this paper to evaluate the draindown potential for porous asphalt mixtures. Air void contents are calculated during mix design. The minimum air void content of 20 percent defines the maximum asphalt binder content. Ruiz et al indicate that binder contents are generally 4.5 percent. 1.4.4 Construction Practices Ruiz et al indicate that whenever porous asphalt is placed over an existing pavement, all distressed areas must first be repaired and the surface leveled. The underlying layer must also be impermeable and structurally sound. Ruiz et al recommend a quick setting emulsion be placed over the underlying layer at a residual rate of 500 to 600 gr/m2. Underlying layers that are open or highly polished may require a slurry seal prior to placement of the porous asphalt layer. Porous asphalt mixtures in Spain are produced in typical HMA production facilities. Production rates should correspond to the paving equipment such that stops of the paving train are minimized. Mixing temperatures are generally 140 to 150°C (280 to 300˚F) with the temperature never exceeding 160°C (320˚F). During transportation, trucks should be covered to prevent temperature loss. Porous asphalt should not be placed when the temperature is less than 8°C (4˚F). The mix temperature should not get below 120°C (250˚F) during compaction. Compaction of the mixture on the roadway is conducted with steel-wheel rollers, without vibration, having a

12 total weight of 10 metric tons. Two steel-wheel rollers are generally used. The first roller makes 4 to 5 passes and the second roller makes 2 to 3 passes to smooth out all roller marks. Two methodologies of constructing shoulders with porous asphalt have been used in Spain. First, the porous asphalt has been extended over the entire shoulder while the second method entails extending the porous asphalt 50 cm (1.6ft) onto the shoulder. For quality control of the production process, samples are prepared and the air voids are controlled. The amount of compaction is controlled in the field by means of a permeability test. The permeability test uses water and was developed at the University of Santander. 1.4.5 Maintenance Practices The primary problem with porous asphalt in Spain has been particle loss. Particle loss generally occurs shortly after traffic. Ruiz et al indicate that this problem generally originates from placing the porous asphalt too cold, not enough compaction, or from draindown problems. The maintenance activities generally consist of milling the section in question and filling with new porous asphalt mixture. In one case, the poor performing section was overlaid by a new porous asphalt mixture, with no problems as of publication date. Spain’s experience with winter maintenance has not been extensive. In areas of high snow falls, porous asphalt is generally not used. In warmer areas, when winter maintenance is needed, more salt and an increase in salting are generally used. Ruiz et al indicates that approximately double the amount of salt is needed. 1.4.6 Rehabilitation Practices No specifics of rehabilitation were described in the paper. 1.4.7 Performance The authors indicate that the performance of porous mixtures with more than 20 percent air voids has been much better than porous asphalt mixtures with 15 to 18 percent air voids. Better durability and less clogging has been noted in the mixes having more than 20 percent air voids. 1.4.8 Structural Design A lift thickness of 40 mm has been established in Spain. The possibility of using thicker lifts has not been considered. Ruiz et al indicate that the water absorption capacity with 40 mm is thought sufficient; therefore, the thickness specified in Spain is based upon the volume of potential rainfall. Ruiz et al also indicate that the same structural value is assigned to porous asphalt mixtures as other open or semi-open conventional asphalt mixtures such as road bases. When porous asphalt mixtures are used above pavement structures containing cement-treated road bases, an additional 20 mm of hot mix asphalt is provided to assist in preventing reflective cracking. Reflective cracking that appears in