Annotated Literature Review for NCHRP Report 640 (2009)

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70 17 10 30 33 10 0 5 10 15 20 25 30 35 <6 6-8 8-10 10-12 >12 Estimated Average Service Life Pe rc en ta ge o f S ta te s Figure 10: Reported Service Lives for OGFC Performance of OGFC in terms of durability and surface friction were reported by highway agencies in scales of poor to excellent. In terms of durability, 11 percent of the states (surveyed) reported poor performance, 11 percent reported fair performance, 37 percent reported good performance, and 37 percent reported very good performance, whereas 4 percent indicate that they have observed excellent performance of OGFC. In terms of surface friction, none of the states reported poor performance, 4 percent reported fair performance, 11 percent reported good performance, and 55 percent reported very good performance, whereas 30 percent stated that they have observed excellent performance of OGFC. This indicates that OGFCs have generally provided good surface frictional properties as intended. 1.14.8 Structural Design No information on structural design of porous asphalt mixtures was given. 1.14.9 Limitations No information on limitations of porous asphalt mixtures was given. 1.15 Watson, D., A. Johnson and D. Jared. “Georgia Department of Transportation’s Progress in Open-Graded Friction Course Development.” Transportation Research Record No: 1616. Transportation Research Board. National Research Council. Washington, D.C. 1998. 1.15.1 General In this paper Watson et al provide a comprehensive review of mix design and construction practices of porous asphalt mixes in Georgia. Watson et al describe the

71 conventional Georgia DOT (GDOT) open-graded friction course (OGFC) mix, the porous European mix (PEM) and an overview of changes made to incorporate the good qualities of the PEM into OGFC. With respect to materials and mix design, Watson et al mention that the use of a coarser gradation, polymer modified asphalt binder and fiber as stabilizers have enabled GDOT to produce mixes without draindown problems, and at the same time to achieve mixes with thicker asphalt films and greater durability. The elimination of the draindown problem has also enabled GDOT to produce these mixes at higher temperatures and thus improved workability. Watson et al mentions several modifications that have been made in plants and construction practices to produce better porous mixes. These modifications include use of automatic feeders to introducing fibers into the mix, blending of asphalt binder and polymer at the asphalt terminal as opposed to a HMA plant, and the increase in production temperature. For construction, the rate of spreading has been increased to obtain thicker mats, and hence avoid irregularities on the surface. The use of proper truck insulation procedures, continuity of operation and material transfer devices has also been discussed. Drainage is ensured with an in-place permeability testing procedure. Watson et al provides indications of better results, in terms of smoothness and permeability, with modified mixes. They discuss the higher cost associated with the modified mixes and show results of life cycle cost analyses to prove that the advantages of the modified mix far outweighs its higher initial cost. Watson et al mentions that the improved performance of the modified porous mixes have made it the surface mix of choice for GDOT for all interstates and state routes with high traffic volumes. 1.15.2 Benefits of Permeable Asphalt Mixtures Watson et al mention that the primary benefits of porous asphalt mixes include reduction of hydroplaning and increasing driver safety through rapid removal of surface water from roadways during light to moderate rainstorms. They mention that draining of water through the pores (and not over the surface) results in increased coefficient of friction between tires and the pavement surface. They also mention that porous mixes reduce splash and spray, improve nighttime visibility, and improve the visibility of traffic striping. 1.15.3 Materials and Design Watson et al provide descriptions of the older D type (original OGFC) mixes, the PEM as well as the hybrid OGFC mix that was developed by GDOT since 1992, to accommodate some of the good qualities of the PEM into the conventional OGFC. They mention that as a result of these changes, the permeability of the porous mixes has improved considerably and that the currently used OGFC (referred to as modified OGFC) is much coarser compared to the D mix, but slightly finer than the PEM. Watson et al has presented the different changes that have been made and their basis, which has these have been summarized in Table 40 in this review.

72 Table 40: Materials and Mix Design Material/Design Changes Aggregate properties No changes have been made. Los Angeles abrasion (loss) < 50%, Soundness (loss) < 15% Flat and elongated particles allowed (5:1 ratio) < 10%, Mica schist allowed < 10% (Silica-rich aggregates only shall be used (e.g. granites). Carbonate-rich aggregates (e.g. limestones) are excluded. Soundness loss is measured using magnesium sulfate (MgSO4). Mix Characteristics Percent Passing Tolerance Sieve Size, mm D mix (9.5 mm) PEM Modified OGFC (12.5 mm) 19 mm 100 100 ±0.0 % 12.5 mm 100 90-100 85-100 ±6.1 % 9.5 mm 85-100 35-60 55-75 ±5.6 % 4.75 mm 20-40 10-25 15-25 ±5.7 % 2.36 mm 5-10 5-10 5-10 ±4.6 % 75 μm 2-4 1-4 2-4 ±2.0 % Mix Design Asphalt Content 6.0-7.25 5.5-7.0 5.75-7.25 ±0.4 Hydrated lime is added as an antistripping agent. Asphalt Binder GDOT has primarily used styrene butadiene (SB) and styrene butadiene styrene (SBS), to modify asphalt binders to increase binder stiffness to 8-10 times that of neat asphalt binder. Empirical tests have been replaced with Superpave tests - a phase angle requirement of less than 75° has been added to help ensure that polymer modification is used to meet the binder grade requirements, and Superpave PG76-22 binder is now used. Mineral Fiber Mineral fibers have been added to the materials to stabilize the asphalt film on aggregates, and hence reduce draindown. Draindown Potential The draindown susceptibility of GDOT modified OGFC mixes can be determined using a test developed by the National Center for Asphalt Technology (NCAT). The NCAT procedure specifies that 0.3% is the maximum permissible draindown. Modified OGFC mixes which contain fibers and polymers have met this requirement. In general, Watson et al mentions that the coarse gradation has improved permeability, and the use of a combination of polymer modification of asphalt binder and fibers as stabilizers has enabled GDOT to produce porous mixes with thicker asphalt films, and to use production temperatures higher than that used for conventional mixes. 1.15.4 Construction Practices Watson et al mentions different modifications that have been made for production and paving of porous mixes as well as permeability tests carried out in the field for evaluation of the completed porous mix pavement. These modifications are summarized in Table 41.

73 Table 41: Construction Practices Step Changes Production 1. Fibers are introduced both in batch and drum mix plants via a separate fiber hopper and feeding system (instead of manual feeding). Fibers are fluffed and blown at a specified rate into the drum at drum mix plants and into the mixing chamber at batch plants. 2. Polymer modification of asphalt occurs at centralized terminal. This has resulted in greater quality control, and the capability of producing modified mixes with certified test results available prior to use. 3. The mixing temperature has been increased to 163oC to provide greater workability. This capability also helps in more effective removal of aggregate moisture, and hence improves adhesion of asphalt and aggregates, and improves durability. 4. Dry and wet mixing times are increased to prevent clumping of fibers. Construction The recommended spread rate has been increased to 49 kg/m 2 (90 lb./yd.2 ) from 41 kg/m 2 (75 lbs./yd.2 ) to increase the thickness of the courses and hence avoid streaking and resulting loss of smoothness. In addition to ensuring proper truck insulation and continuity of operation, material transfer devices are to be used to avoid the formation of cold lumps and resulting blemishes on the surface. In-place Testing GDOT performs permeability testing using a falling head permeameter. This device allows the user to determine a permeability coefficient, represented in meters (feet) per day, for the mix being tested. The apparatus consists of a circular base plate, a grease gun, and a cylindrical plastic water container. The base plate is fitted with two rubber O- rings mounted near the outer edge and placed 16 mm (¾”) apart. The base plate is placed in contact with the pavement surface, and grease is pumped between the O-rings, resulting in an impermeable seal. The plastic cylinder is then filled with water, which is allowed to flow through the base plate into the pavement below. By timing the flow of 1600 ml of water, a permeability coefficient can be calculated which is based on the known thickness of the pavement. Watson et al indicates the benefits achieved through the modifications in mix design, production and construction process, by citing examples of improved smoothness and increased permeability. They mention that the smoothness levels on one project, consisting of over 136 lane-km (85 lane-miles) of modified OGFC overlay, averaged 143 mm/km (9 in./mi.). During fiscal year 1996, the average smoothness level statewide for modified OGFC was 158 mm/km (10 in./mi.), whereas dense-graded mixes averaged 397 mm/km (25 in./mi.). With respect to drainage capabilities, they indicate that modified OGFC typically drains 73 m/day (240 ft./day), significantly better than conventional OGFC (39 m/day, 130 ft./day). Watson et al provides some cost comparison between modified and conventional OGFC. They state that the modified OGFC costs 34 percent more, because of additional materials needed in the mix and the equipment needed to introduce these materials in the plant, as well as increased production temperature and slower production rate. Using annualized cost in life cycle cost analysis, Watson et al shows that the modified OGFC would outweigh the conventional OGFC as a more desirable mix if it would last just 19 months longer. The authors have considered a life of 8 years for the conventional mix and 10-12 years for the modified mix. 1.15.5 Maintenance Practices No information on maintenance practices of porous asphalt mixtures has been provided.