Annotated Literature Review for NCHRP Report 640 (2009)

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114 1.26.1 General This paper describes an effort to develop a 2/4 gap-graded 0/6 porous asphalt mixture. This designation means that the aggregate gradation has a maximum aggregate size of 6mm with a gap in the grading between 2 and 4mm. 1.26.2 Benefits of Permeable Asphalt Mixes The authors mention frictional properties and reduced tire/pavement noise as benefits. 1.26.3 Materials and Mix Design Specific recommendations for individual material requirements were not provided. Also, gradation requirements for individual sieves were not provided; however, between 10 and 12 percent of aggregate pass the 2mm size for the mix type. A polymer-modified binder was included. The Cantabro Abrasion test was used during mix design using a test temperature of 20˚C. Other tests used to evaluate the mix included the rotary shearing press test (rutting test) and the LCPC Duriez test (moisture susceptibility). 1.26.4 Construction Practices The authors state that the porous asphalt was produced in a drum mix facility. The tack coat used on the existing layer was a 65 percent emulsion placed at a rate ranging from 350 to 450 g/m2 of residual binder. The mix was placed with pavers over the full width of the roadway (11.5m). Both vibratory and static steel wheel rollers were used to compact the mix, generally using four to six passes. 1.26.5 Maintenance Practices No specifics on maintenance practices were given. 1.26.6 Rehabilitation Practices No specifics on rehabilitation were given. 1.26.7 Performance The authors used permeability tests conducted in the field as a performance measure. 1.26.8 Structural Design No specifics on inclusion within structural design were given. 1.26.9 Limitations No specific limitations were given. 1.27 Bishop, M. C. and M. F. Oliver. “Open Graded Friction Course Pavements In British Columbia.” Proceedings of the 46th Annual Conference of the Canadian Technological Asphalt Association. Toronto, Canada. 2001. 1.27.1 General In this paper Bishop and Oliver describe several OGFC projects constructed in British Columbia (BC), Canada. The authors present their experiences regarding benefits of

115 OGFC, project selection, mix design, construction, structural strength, performance, maintenance, rehabilitation and cost of OGFC. Based on the experience of the OGFC projects and literature review, the authors make several recommendations for designing and constructing good performing OGFC mixes. These recommendations include proper repair or rehabilitation of existing roads to remove any distressed areas before overlaying with OGFC, providing adequate thickness of the OGFC mixes, reducing draindown with the use of fibers and polymer modification, using the right balance of air voids and film thickness and following certain construction practices. These recommended construction practices include use of tack coat, not sealing longitudinal joint faces, producing mixes at a lower temperature compared to conventional mixes, providing unimpeded edge drains and using light rolling for compaction. Regarding maintenance Bishop and Oliver mention that close attention should be paid to OGFC surfaces, and that the greater cost of using more salts in such surfaces is well justified, considering its benefits. They indicate that the best option for repair of slight distresses is to apply a light seal or an open graded asphalt mix as early as possible. Regarding rehabilitation Bishop and Oliver mention that a second OGFC layer could be applied on an existing OGFC mix, as long the latter is sealed properly, and that the entire width of the pavement needs to be rehabilitated at once to ensure adequate drainage. 1.27.2 Benefits of Permeable Asphalt Mixtures Bishop and Oliver mention the following benefits of OGFC mixes: 1) Higher friction and shorter stopping distances in wet conditions; 2) Improved visibility by reduction of glare, and hence reduction in accidents and congestion; 3) Increased rutting resistance due to “rock-to-rock contact” in OGFC mixes; and 4) Reduction of traffic noise. 1.27.3 Materials and Design Bishop and Oliver provide a description of materials and mix designs used in several projects, and also provide recommendations, based on their experience and literature surveys. Table 65 summarizes the materials and mix designs for the projects.

116 Table 65: Materials and Mix Design Used in Projects in BC Project Materials/ Construction Observations/Special Provisions Highway 19 – Nanaimo Projects – Project 1, traffic volume of 54,000 AADT. 100% fractured 12.5 mm maximum size aggregate; spreading rate of 40 kg/m², equivalent to a thickness of 18 mm. 1. Lift thickness insufficient for covering existing irregularities and providing internal drainage. 2. Drainage to edge drain was affected by shoulder gravel and concrete curb. 3. Provision of drainage through butt joints must be made, to allow flow of water particularly when a downward slope towards an abutting dense-graded pavement mix is present. Highway 19 – Nanaimo Projects – Project 2 traffic of 54,000 AADT. application rate to 75 kg./m² (≈43 mm) 1. The edge of the mat was left 100 mm short of the existing concrete curbs; gutters were made for proper drainage. 2. Angled transverse butt joints were specified for tying to existing pavement. 3. The rate of application of tack coat was increased to 0.75 l/m² to ensure sealing of the underlying pavements. Vancouver Island Highway Project - Duke Point Access - Traffic volumes were projected to be 5000 AADT with a high percentage of loaded trucks. A 50 blow Marshall mix design method was used to provide 15% air voids with a 15 micron film thickness. The fines returned from baghouse dust collection system were not put back in the mix, to increase air voids (to approximately 18%) and film thickness (to approximately 20 microns). 1. Design procedures were based on South African process as well as experiences from the Nanaimo projects. 2. All tangents were constructed with a 2.5 % crown to improve the internal drainage of the pavement. This was done to mitigate the internal drainage problems expected because of the width of grade being constructed (4 lanes + shoulders + median). Highway 16 near Terrace in 1993; carries a significant number of heavy logging trucks. Class D seal coat aggregate; aggregates and mix design similar to those used in Nanaimo Projects Pavement is still performing well (reported in 2001) Likely Road - 150 Mile House; placed in 1990. This was a relatively thin lift utilizing a gradation similar to that used in Nanaimo. The lift thickness would put it in the “Carpet Coat” category. This was initially surfaced with a graded aggregate seal. Over time the seal had deteriorated exposing the concrete. In 1995 this section was resurfaced with a thin lift OGFC pavement. Reports to date indicate that it is performing well and providing the desired qualities of enhanced friction and surface drainage.

117 Table 65: Materials and mix design used in projects in BC (continued) Project Materials/Mixes/ Structure/ Construction Observations/Special Provisions Coquihalla Highway - Phase 2 placed in 1997 on Phase 2 of the Coquihalla Highway. Using the same parameters as the Nanaimo projects --- Nakusp This project was completed in the summer of 2000, located in an area with significant freeze thaw cycles and intensive winter maintenance. The mix design used on this project had 13.8% air voids and a film thickness of 17.2 microns. The mix was produced with 85% 12.5 mm maximum size material from a single stockpile and 15% 16.0 mm maximum size rock that was produced for the conventional hot mix used on this project. An anti stripping agent was added to the asphalt cement. Problems encountered in this project include those with gradation, erosion of shoulder material, intersection with side roads and achieving adequate film thickness. The details are as follows. 1) 15 % crushed 16.0 mm aggregate was added to make the gradation coarser. A minimum limit on percentage retained on 9.5 mm (instead of the 100 percent passing), and splitting of stockpile on 4.75 m sieve were suggested; 2) Difficulty in creating a drainage path between the OGFC and the curb; 3) It was determined that putting roadside barriers with asphalts curbs behind them is a good idea for ensuring proper flow of water along the curb; 4) To reduce problems associated with intersecting roads, the limits of the OGFC were extended and also by milling and creating the intersections at an angle to facilitate drainage through OGFC; 5) Lack of finish rolling can cause problem with smoothness; 6) Tenderness of mix can be reduced by lowering compaction temperature and adding silicone to reduce the effect of free moisture; 7) The South African recommendation of considering edge drains in projects with curbs and gutters was considered; 7) Design film thickness values were not obtained, probably due to degradation of aggregates in the plant. The authors point out the following observations: a) A 50 blow Marshall procedure can be used. b) Draindown can be minimized using a rubber or polymer-modified asphalt and by adding cellulose fiber. c) A clear relationship between draindown problems and the degree to which the mix could be described as single sized. Bishop and Oliver point out that a number of agencies that experienced difficulties with draindown used gradations where all the material was essentially retained on one or two sieves. They postulate that agencies that allow gradations with more fines appeared to have less concerns with draindown. Based on the success of the first projects in British Columbia, the targets were set at 15% minimum air voids and 15 micron minimum film thickness after deduction of absorbed asphalt. They suggest the gradation given in Table 66.

118 Table 66: Suggested Gradation Sieve Size, mm Percent passing 16 12.5 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075 100 92-100 75-90 50-70 10-35 0-21 0-10 0-8 0-6 0-4 Regarding materials, the authors mention that aggregates should be clean, sound, polish resistant with a minimum of flat particles, and that asphalt should generally be of a grade equal or harder than that used for dense-graded mixes. 1.27.4 Construction Practices Bishop and Oliver mention that asphalt surfaces to be overlaid are tack coated at an application rate of 0.75 l/m², and that it is essential that the underlying pavement be sealed as thoroughly as possible to prevent delamination of the surface course and degradation of underlying pavement layers. They mention that longitudinal joints are not painted with an asphaltic material to prevent sealing and reducing drainage, and that the film thickness of the OGFC mix should be sufficient to ensure a satisfactory bond. Bishop and Oliver indicate that OGFC mix using conventional asphalt binder should be produced at a temperature approximately 10°C lower than for conventional HMA. This reduces draindown of the asphalt component. This reduction in mixing temperature might not be required with polymer/rubber-modified asphalts because of their inherently higher viscosity. They mention that maximum compaction is achieved very easily because of the high stone content. A single pass with a non-vibrating 10-12 tonne steel roller is usually sufficient to compact the material. They also point out that handwork is quite difficult because of the lack of fines and rapid cooling due to the open graded material. 1.27.5 Maintenance Practices Bishop and Oliver mention that surface temperatures on OGFC mixes could be lower than in conventional mixes and proper attention must be paid to prevent formation of black ice. They indicate that the critical temperature is 0-3 degree Celsius and that the additional attention and amount of salt needed for these mixes are more than offset by the benefits from these mixes. Regarding drainage maintenance, Bishop and Oliver mention that OGFC mixes tend to self-flush the materials in the voids, and clogging is not a problem. However, they indicate problems do occur in maintaining transverse drainage in pavements with adjacent curb. These drainage paths need to be cleaned properly. The authors also mention the problem of water draining longitudinally to dense graded mixes, when the

119 longitudinal slope is greater than the transverse slope. They mention that a solution is to cut a transverse butt joint to make the transverse flow rate greater than the longitudinal flow rate. To maintain the functionality of OGFC mixes, Bishop and Oliver point out that it is necessary to seal slight raveling with light application of asphalt sealant, and that periodic monitoring of surface should be made to detect any signs of major loss of aggregate or potholes. If such a problem is detected, they suggest that an open graded asphalt patching mix be used as soon as possible to avoid further deterioration. 1.27.6 Rehabilitation Practices Bishop and Oliver cite references and experiences to contend that life expectancies for OGFC pavements are about 12 years, compared to 14 to 15 years for conventional pavements. They mention that while riding surfaces will not be obtained by using methods such as hot in-place recycling on existing OGFC mixes, it is possible to use another OGFC layer as a rehabilitation method, provided the existing OGFC is sealed properly. They also mention that it is important to rehabilitate the entire pavement, including the shoulders to make sure that adequate drainage is ensured. 1.27.7 Performance Bishop and Oliver mention results of performance evaluations conducted primarily on the two Nanaimo projects. Their observations are summarized in Table 67. Table 67: Results of Performance Evaluations of Different Projects Property Performance of different projects Overall Pavement Quality The mix, including the longitudinal joints is performing well. Some asphalt stripping in the wheelpath has been noted, along with indications of underlying base problems in some other small areas. Noise Levels A reduction of 5db (30 percent reduction in total noise levels) was achieved in the first Nanaimo project. Skid Resistance An initial and one year skid resistance test with the British Pendulum tester indicated reduction of 10 percent and 20 percent in stopping distance, respectively. Internal Drainage No clogging and reduction in drainage has been noted. Road Splash and Glare Significant reduction (as obtained from visual observation) in spray and glare has been made possible with the use OGFC mixes. Service Life Experience indicates that these pavements will provide a reasonable service life with acceptable levels of noise reduction, enhanced skid resistance and superior drainage.