Annotated Literature Review for NCHRP Report 640 (2009)

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141 The third experiment again included three gradations representing the three maximum aggregate sizes. All three were again open-graded the only difference being that the three gradations became similar at 10 percent passing rather than 7.7 percent. [In essence, there was very little difference between the second and third experiment gradations.] All three gradations were used to prepare mix at 4 percent asphalt binder. Again, Cantabro Abrasion, rutting, permeability and interconnecting voids were evaluated. Similar relationships between maximum aggregate seize and the responses were observed during the third experiment. 1.32.4 Construction Practices No specifics on construction practices were given. 1.32.5 Maintenance Practices No specifics on maintenance practices were given. 1.32.6 Rehabilitation Practices No specifics on rehabilitation practices were given. 1.32.7 Performance No specific performance measures were given. 1.32.8 Structural Design No specifics on inclusion within structural design were given. 1.32.9 Limitations No specific limitations were given. 1.33 Moore, L. M. and R. G Hicks. “Design, Construction, and Maintenance Guidelines for Porous Asphalt Pavements.” Transportation Research Record No: 1778. Transportation Research Board. National Research Council. Washington, D.C. 2001. 1.33.1 General This paper presents an overview of the use of porous pavements – historical perspective, design, construction, quality control and maintenance practices in Oregon. Moore et al mentions that since the 1970s porous asphalt mixes have shown good performance and, hence, this type of mix continues to be a preferred mix in Oregon. They gave a detailed description of performance evaluations, mix design and construction criteria that have been developed for open graded asphalt mixes in Oregon. The authors indicate significant benefits of open-graded asphalt mixes, and special considerations that need to be made for design, construction, maintenance and rehabilitation of pavements with these mixes. Major mix design considerations include specifications on retained strength, compaction air voids and draindown, whereas principal construction considerations include the use of appropriate mixing temperatures and compaction equipment. Moore et al provide a description of quality control

142 considerations, which include putting more emphasis on asphalt content and percent passing the 0.075 mm sieve and use of these considerations for development of pay factors. In terms of structural design and limitations of use Moore et al mention the specification of minimum depths, repair of existing surfaces prior to the placement of open-graded asphalt mixes, and the recommendations on not using the open graded mixes on low volume rods, roads which need much hand work and roads with heavy snowfalls where steel plows are used. The paper indicates that the Oregon Department of Transportation (ODOT) has developed specifications on mix design and construction of open-graded mixes, on the basis of test sections, research projects and surveys. Although adequate recommendations have been developed for addressing maintenance problems, it seems that the issue of adopting a proper rehabilitation methodology is still being researched (at the time of publication of this paper). 1.33.2 Benefits of Permeable Asphalt Mixtures In discussing the reasons for using porous asphalt mixes in Oregon, Moore et al mention the following benefits: safety through improved high speed frictional properties and reduced splash and spray, user comfort during winter driving, quieter roads and similar performance as dense-graded mixes historically used for wearing surfaces. 1.33.3 Materials and Design Moore et al mention that in Oregon, porous asphalt mixes, called open-graded asphalt concrete, is characterized by the use of a large percentage of coarse aggregate in the mix without a significant portion of fines, as found in dense graded mixes. They mention that of the different classes of asphalt concrete (class B through F), E and F are open-graded, whereas, the rest are dense graded. Relevant information provided on these two classes of mix is summarized in Table 77.

143 Table 77: Use and Characteristic of Open-Graded Asphalt Concrete Class of Mix Primary Use Nominal Maximum Aggregate Size Gradation E Nonstructural thin overlays (25 mm) to improve skid and hydroplaning resistance. 19 mm Not provided F Thin overlays (50 mm) and for wearing courses for new pavement construction or structural overlays on all highways up to 100 mm; being recommended for use on many Oregon roadways, including Interstate highways. 25 mm a) 1984 Sieve Size, mm % passing 25 99-100 19 85-96 12.5 60-71 6.3 17-31 2 7-19 0.43 ---- 0.08 1-6 Mineral Filler 0.0-1.5 Asphalt Cement --- ---------------------- a) 1998 Sieve Size, mm % passing 25 99-100 19 85-96 12.5 55-71 6.3 15-30 2 5-15 0.43 ---- 0.08 1-6 Asphalt Cement 4-8 Moore et al mention that early mix designs were carried out following the Hveem mix design procedure, and the mix design procedures for open-graded mixes were adopted from mix design procedures from B/C class mixes, by considering the facts that the open- graded mixes have higher voids and they have lower strength and stability when tested in an unconfined mode. Moore et al state that in 1992, Oregon DOT (ODOT) changed the mix design procedure to include a draindown test and criteria, procedure for determination of bulk specific gravity of compacted specimen from saturated surface dry method to geometric method and specification of modified binder. The new and old ODOT specifications are shown in Table 78.

144 Table 78: ODOT Specifications for Open-Graded Asphalt Mix Criteria Criteria in 1986 Criteria in 1999 1. Asphalt Film Thickness 2. Design Air Voids % (DAV) a) 1st Compaction b) 2nd Compaction (min.) 11-13 8 13.5-16.0 n/a 3. Hveem Stability, minimum a) 1st Compaction b) 2nd Compaction 26 30 n/a n/a 4. IRS @ DAV, minimum 75 80* 5. Draindown n/a 70-80 6. Voids Filled with Asphalt (VFA) n/a Note: These mixes used to be designed with 1% Portland cement as mineral filler to stiffen the hot asphalt during transportation and laydown. The recent switch to the use of PBA-5 and PBA-6 asphalts has eliminated the need for mineral filler. * TSR on surrogate dense-graded mix. 1.33.4 Construction Practices Moore et al indicate that standard drum or batch plants are used for producing the open- graded asphalt mix and that conventional paving equipment, with some minor adjustments in construction practices, are used for laydown and compaction in Oregon. They mention that for these types of mixes, draindown of asphalt binder and cooling of the mix are the primary concerns during transportation and placement. The concerns, performance issues and steps taken to avoid them are summarized in Table 79.

145 Table 79: Construction Concerns and Special Considerations for Open-Graded Asphalt Mixes Concern Performance issue Special considerations Excessive draindown of asphalt binder Results in fat spots in the finished surface which may appear during rolling or within a few weeks of paving Proper selection of mixing and compacting temperatures, the use of modified binders (PBA-6), and use of fibers; mixture deposited in windrows from “belly-dump” trucks and transferred to the paver via a pick-up machine is the most common method of delivering Class F mix. End dump trucks have been successfully used to deliver Class F mix, but experience has been that the use of end dump trucks depositing directly into the paver is most likely to result in fat spots on the finished surface. To date, a material transfer vehicle (MTV) has not been used on a Class F mix project. Cooling of mix Results in “chunks” which, if not removed or broken up prior to depositing into the paver, can result in tears in the mat, differential compaction, and poor ride. The “chunks” can be minimized by increasing the temperature, tarping loads, and using trucks with insulated beds to deliver the mixture; the recommended mixing temperature for the Class F mix is based on an asphalt viscosity of 700 to 900 cst. For these projects, the maximum mix temperature was 129°C (265°F) at the plant. Minimum allowable temperature during laydown was 96°C (205°F). This is comparable to 163°C (325°F) and 116°C (240°F) for dense-graded mixes. The lower temperatures for the Class F mix help promote thick film coatings during hauling and laydown. Use of PBA-6 binders modified with polymers and/or ground tire rubber have also minimized the draindown. Class F mixes are evaluated in the field. Experienced personnel will adjust asphalt content and mixing temperature during production and laydown to optimize film coatings for durability and temperature for workability needed to achieve a smooth ride. Compaction --- A minimum relative density is not specified. The specifications requires a minimum of four coverages with a minimum 7 Mg (8 ton) gross static steel-wheeled roller prior to mix cooling below 80°C. Additional passes may be necessary to eliminate roller marks. Vibratory compaction is not allowed to avoid fracture of aggregate. Moore et al mentions some special considerations for unusual constructions considerations, as summarized in Table 80.

146 Table 80: Special Considerations Special Case Concern Recommendations Long Hauls Excessive draindown of the asphalt binder, cooling of the mix, or both. Jobs far from the asphalt concrete plant are not recommended for Class F mix; although success has been obtained with one-way haul distances of up to 112 km (70 mi), the current policy is to stay below 56 km (35 mi). Weather conditions and asphalt grade may also influence the recommended haul distance. Inlays Adequate drainage; mixes must be allowed to drain Drainage is accomplished by daylighting the mix on the shoulder through a series of outlet trenches. If adequate drainage outlets are provided, Class F mixes may be used for inlays. Although ODOT has used this option on occasion, it is not recommended for standard practice. Hand work The Class F mix, because of its coarse texture, is difficult to rake and is not easily placed where an abundance of hand work is necessary. Usually not specified for tapers, road approaches, or in city streets where there are inlets and manholes to work around. Night paving Projects in high traffic areas require night paving. In these cases, the mix must be heated to higher temperatures to get the necessary temperature for compaction; high temperature can cause more draindown of asphalt binder When F-mix has been placed at night, fibers have been used to minimize draindown related to the higher temperatures. Moore et al mention that in early work, quality control was ensured by controlling aggregate gradation, asphalt content, moisture and compaction and also by enforcing pay adjustment factors that are used for dense-graded mixes. A study started in 1995 has, however, shown that some of these variables are more important than the others and there are some considerations that must be made for the unique properties of open-graded asphalt mixtures. The primary observations are that fat spots and rutting are caused by excess asphalt and/or excess fines, the important factors that need to be controlled are

147 aggregate gradation, asphalt binder content and mix moisture, and that relatively more weightage should be given to asphalt binder content and percent passing the 0.075 mm sieve for pay adjustments. 1.33.5 Maintenance Practices Moore et al has indicated several maintenance related issues unique to open-graded asphalt mixes. These issues and recommended maintenance practices are summarized in Table 81. They mention that a 1997 survey of ODOT maintenance practices indicates that blade patching with dense-graded hot mix was the most widely used technique, and the most successful with a mean success rating of 8.0 out of a possible 10.0. Mill and inlay and screed patch with dense-graded hot mix were also widely used and reasonably successful. Only three respondents had milled and inlayed with Class F mix and their experiences varied widely with a minimum success rating of 3 and maximum success rating of 10. The authors note that Class F mix (open graded asphalt mix) is not readily available to maintenance personnel in small quantities and that traditional maintenance techniques have used dense graded mixes. Table 81: Maintenance Issues and Recommendations Maintenance Work Issues Recommendations Snow plow damage/ type of plow used In areas where steel plow blades (without the rubber cover) are used, damage to the Class F mix is greater because the plow blade can break and/or remove the large surface rock. This damage then results in greater future raveling, due to the loss of surface integrity. Fog Seals are applied periodically to minimize raveling; Chip/Sand Seals are normally used to cover gouges caused by snow plows. The gouges do not generally cause structural damage but do create aesthetic problems. De-icing Class F mix requires more de-icing chemicals because of the higher amount of voids in the mix; the chemicals enter the pavement structure and, thus, higher concentrations are needed to keep the surface from icing. -------- Patching The issue of patching includes both blade/screed and inlay patching. Although successful inlay patches have been made using F-mix, the type of asphalt mix the maintenance forces have available for use is typically a dense-graded commercial mix. Inlay patching with dense- graded hot mix may block the drainage path in the Class F mix causing problems such as black ice and patch deterioration (due to water infiltration). The greatest concern regarding blade patching is getting the 15 mm – 40 mm thick patch to adhere to the existing Class F mix pavement as well as they traditionally do for dense- graded mixes. Maintenance crews report that these thin patches on the Class F mix last about half as long as patches on dense-graded mixes. Even if F-mix were readily available, blade patches with this mix are not feasible because of the inability to “feather” the edge. Patching is normally used to repair isolated distress areas; generally a Class C mix is used. As long as the patched areas are small, the benefits of the open- graded mixes are not lost.

148 1.33.6 Rehabilitation Practices Moore et al indicate two types of rehabilitation practices for open-graded asphalt mixes: 1) overlays and 2) mill and fill. They mention that overlays are done with either open- graded or dense-graded mixes, and that mill and fill operation consists of milling off the open-graded mix and replacing with another open-graded mix. They mention that adequate care should be taken to insure the new material placed as part of the mill and fill operation is able to drain completely. The authors mention that there are several considerations that need to be made for rehabilitation of pavements with open-graded asphalt mixes, and that the appropriate method of rehabilitation of such pavements is being researched. They indicate that a small number of such projects have been rehabilitated or have been marked for rehabilitation and that important considerations include inlay repairs prior to overlaying, changing the wearing surface mix type, and drainage issues with a middle layer of open-graded asphalt. Moore et al mention that the European experience indicates a preference for mill and inlay with recycling and that this approach eliminates the challenges associated with overlays and porous pavements. 1.33.7 Performance Moore et al provide an evaluation of performance of different open-graded asphalt mixes and some comparison of their performance with that of dense-graded mixes (such as classes B and C). These evaluations and comparisons, which have been made with respect to three principal benefits of open-graded asphalt mixes – enhancement of friction, reduction of splash and spray and reduction of noise, are summarized in Table 82.

149 Table 82: Evaluation of Performance of Open-Graded Asphalt Mixes Property Tests, Results and Observation Inferences and Recommendations Friction Friction was measured at a traveling speed of 64 km (40 mph) and expressed as a friction number (FN) (AASHTO T-242). The friction numbers for the pavement management sections used to model the performance were analyzed using standard statistics. Speed gradients were determined for several projects. The speed gradients were determined as the slope of the FN versus speed curve from 64 km (40 mph) to 88 km (55 mph). The tests were performed in a conventional manner on dry pavements and then repeated on the same sections during heavy rainfall The resulting average friction numbers of 50.5 for the Class F mix and 53.7 for the Class B mix are very close. The data suggest that under dry conditions, both mix types provide acceptable frictional properties; Speed gradients indicate that the Class F mix had a slightly improved speed gradient in dry conditions and a much improved gradient during rainy conditions when free water was present on the pavement. Splash and Spray --------- This feature was obvious when driving on the selected projects during wet weather. Although there was no objective test data available related to splash and spray, the observed amount of water splash and spray for the Class F mix projects was much less than for the Class B/C mix projects. Oregon has received numerous comments from motorists noting improved visibility when traveling on Class F mix pavements during rainy weather. For Oregon’s unique climatic condition, with nearly 6 months of rainy weather, this advantage may greatly reduce the number of vehicle accidents. Noise Characteristics A field investigation was conducted to determine the noise level on both mixes. Two types of noise measurements were taken. The first was roadside noise and the second was interior vehicle noise. In order to remove any geometric variables, test sites were chosen where fairly new pavement types existed, and overlays of Class F mix were planned in the near future. Tests were then performed before and after overlay at identical locations. For all sites in this study, the noise measurements were taken 50 feet from the centerline of the closest directional travel lanes. The results indicated porous pavements reduced noise in the higher frequency zones. This conclusion was supported mostly from the roadside measurements, and not from those taken in the interior of the vehicle. A possible explanation for this is that the higher frequencies are dampened by the vehicle shell. As high frequency noises have a shorter wavelength, they would be more apt to be reflected off the vehicle’s thin shell, and would hide some of the data and make Class F mix pavements appear a little noisier inside than outside. 1.33.8 Structural Design Moore et al indicate that the 1993 AASHTO Guide for Design of Pavement Structures and other deflection based procedures are used for structural design of open-graded asphalt mixes in Oregon. Moore et al mention that a minimum thickness of 50 mm has been specified for class F mix (increased from 37.5 mm in the past, to reduce laydown and compaction problems). A maximum thickness of 100 mm (in two 50 mm lifts) has been used. Regarding extensively cracked existing pavement, Moore et al mention that class F mix is allowed