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83 The objective of this project was to recommend design, con- struction, and maintenance guidelines that will maximize the advantages and minimize the disadvantages associated with PFC use. Within this project, PFCs have been defined as spe- cialty type open-graded friction courses that are specifically designed to have high air void contents, above 18 percent, for removing water from the pavement surface. In order to accom- plish the project objectives, a literature review and survey of agencies was conducted. No field or laboratory research was conducted. Following are conclusions based on the findings of this research: ⢠The use of PFC layers has many benefits that can be catego- rized as related to safety, driver comfort, and environmen- tal. Benefits related to improved safety included reduced potential for hydroplaning, improved wet weather friction, reduced splash/spray, reduced glare, and improved visibility of pavement markings. Benefits related to improved driver comfort include smooth pavements, increased confidence of drivers during rain events through the reduced potential for hydroplaning, reduced splash and spray and reduced glare, and reduced potential for permanent deformation. Benefits related to the environment include smoother pavements, thus improved fuel economy, and reduced pavement noise levels. ⢠The design of PFC mixes contains four primary steps: 1) selection of appropriate materials; 2) selection of a design gradation; 3) selection of optimum asphalt binder con- tent; and 4) performance testing. The literature indicated that angular aggregates having the proper shape are desir- able within PFCs. Stiff, modified binders have provided the best performance. Optimum asphalt binder content should be selected based upon balancing durability and draindown potential. The Cantabro Abrasion loss test is the most common test method for evaluating the dura- bility of PFC mixes. Use of the voids in coarse aggregate concept for ensuring stone-on-stone contact should be included within the design of PFC mixes. Fibers are the most efficient method for minimizing the potential for draindown problems. ⢠Inclusion of PFC within the structural capacity of pave- ments varies greatly. Some agencies do not account for any structural capacity for PFC layers while others assign some percentage of structural capacity of dense-graded layer. When characterizing PFC mixes for inclusion of the new Mechanistic-Empirical Pavement Design Guide using the dynamic modulus test, a confinement pressure is needed. Two parameters are important in selecting an appropriate lift thickness for PFCs: rain intensity and PFC permeability. ⢠Construction of PFC mixes is similar to typical dense- graded HMA with some slight modification. The primary modification is the need for addition of fibers to the produc- tion process, if used. Mixing times should be slightly longer than typical mixes to ensure that the stabilizing additives are sufficiently dispersed within the mix. The mix should be protected from cooling during transportation. At a min- imum, tarps should be used to minimize the amount of cooling that takes place during transportation. Material transfer vehicles that remix the PFC before being deposited into the paver are desirable. Conventional steel wheel rollers should be used to compact the PFC. Vibratory rollers should be discouraged as these rollers have the potential to frac- ture aggregates during compaction. Pneumatic tire rollers should not be used on PFC layers. Pneumatic tire rollers tend to pick-up mix during compaction. Typically, two to four passes of a breakdown roller and one to two rolls of a finish roller are sufficient to compact PFC layers. The goal of compaction is not to achieve a certain density; rather, the goal is to seat the aggregates. Roll-down of PFC is about 10 to 20 percent of the lift thickness. ⢠Cleaning of clogged PFC layers is not a common practice within the Untied States. General maintenance activities usually include small patches in localized areas. When C H A P T E R 1 3 Conclusions
dense-graded mix is used for small patches, the patch should be rotated 45 degrees so that water can flow around the patch. No evidence was found that fog seals are an effective maintenance technique. Winter maintenance activities vary greatly around the United States and the world. Sev- eral references were found that indicated that experience was the best method for developing a winter maintenance program. The literature was explicit in stating that winter maintenance of PFC layers is more difficult than for dense- graded layers. PFCs are generally colder than dense-graded layers, indicating that PFC layers reach freezing sooner than dense-graded layers and stay at freezing temperatures longer. As such, more winter maintenance chemicals are needed for PFCs. When snowplows are employed, rubber- tipped blades have decreased the amount of damage to PFC layers. ⢠The most common rehabilitation method for PFC layers is to mill and replace. There is evidence that overlays should not be placed over PFC layers without sufficiently sealing the PFC layer. Overlays of unsealed PFC layers have shown the propensity for moisture damage within the pavement structure. ⢠There are few specific limitations on the use of PFCs. These mixes should not be utilized in areas with large amounts of dirt and debris. This will lead to the PFC lay- ers clogging. PFCs should not be used in areas with high yearly snow fall rates. Winter maintenance can be expen- sive in these areas and snowplows have been shown to damage PFC layers. PFCs should not be used when long haul distances or haul times are needed. This will allow the PFC to cool during transportation and likely cause con- struction problems. 84
