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81 Based upon the literature review and survey of agencies, it is obvious that the use of PFC layers provides a safe riding sur- face for the traveling public. Benefits related to PFCs include: reduced potential for hydroplaning, reduced splash/spray, improved wet weather frictional properties, improved visibil- ity of pavement markings, reduced glare, smooth riding layers, and resistance to permanent deformation. Because of these benefits, PFCs should be considered as a viable option on any high-speed, high-traffic volume roadway. However, there are some areas that need to be further researched. Following are areas deemed needing further research: 1. As discussed within the section on winter maintenance, there are really no consistent guidelines for winter mainte- nance of PFC layers. Research is needed to evaluate the effec- tiveness of various winter maintenance techniques. Included within this research should be comparisons between differ- ent winter maintenance chemicals and application rates on existing PFC layers. These evaluations of chemicals and rates should be conducted for various winter condi- tions. The literature seemed to indicate different prob- lems encountered for different winter conditions. Winter conditions that should be evaluated include hoar frosts, light snowfalls, heavy snowfalls, sleet, freezing rain, slush, etc. Consideration of snowplow operations also could be included. The research should not be limited to northern tier states as hoar frosts also are considered a concern in southern tier states. 2. One of the methods for maintaining the beneficial charac- teristics of PFCs is to conduct general maintenance, specif- ically cleaning dirt and debris from the PFC layer. General maintenance of this type many extend the performance life of PFC layers. Within the United States, these general maintenance activities have not been practiced. Research is needed to identify an effective method(s) for cleaning PFC layers that are not cost prohibitive. The selected method(s) must efficiently clean PFC layers while not damaging the PFC. Also included within this research would be docu- menting how the cleaning activities influence the beneficial characteristics of PFC layers. Test pavements should be identified and several techniques of cleaning evaluated. A measure of permeability should be used as the perfor- mance measure to define the effectiveness of each cleaning technique. Noise levels, wet weather, friction and perme- ability should be conducted during the life of the pavement to determine the effectiveness of this general maintenance activity in maintaining the beneficial attributes. 3. A need for evaluating PFC layers is a standard piece of equipment and test method for estimating permeability. As described in the previous chapter, permeability was identi- fied as the most important performance measure for PFCs. Research is needed to develop/select a piece of equipment and test method to evaluate permeability. 4. As a potential surrogate to a permeameter, equipment that measures the amount and density of splash and spray may be warranted. A device of this nature would likely indicate when PFCs have become clogged and provide a trigger for when general maintenance is needed. A device of this nature should be vehicle mounted with its own source of water. A very important benefit of equipment of this nature would be that testing could be conducted near highway speeds and negate the need for fixed traffic control. To the authorsâ knowledge there is no such piece of equipment. Therefore, research activities would involve development, evaluation and standardization. 5. Within Volume II of this report, a mix design practice was proposed. This mix design method should be field vali- dated. Research should be conducted to validate the proce- dure in the laboratory and field. Laboratory validation would entail identifying a range of materials having a range of physical properties and determining whether the method can successfully proportion the materials to meet the mix design specifications. Field validation would entail deter- mining whether the designed PFC mixes can be successfully C H A P T E R 1 2 Future Research Needs
produced and placed. Performance of the constructed test sections should be monitored over time. 6. A potential need within the mix design method is another method for measuring moisture sensitivity. Past moisture susceptibility tests, namely tensile strength ratios, have been conducted using Marshall compacted samples. It is unclear how the Superpave gyratory compactor will affect tensile strength measurements on PFC mixes. Other tests also have been specified in Europe. Moisture condition- ing of Cantabro Abrasion loss samples may prove suc- cessful. Research should be conducted to determine the best method for evaluating moisture resistance of PFCs. Materials of known moisture performance should be used in this research. 7. Three issues related to the inclusion of PFCs in pavement design need to be researched. First, a method for selecting minimum lift thickness was proposed. This method should be validated. Validation should include field work to docu- ment how water flows within a PFC layer. Findings from this research can be used to refine the selection procedure to prevent water sheets on the PFC surface. Also included in the research should be determination of a representative value for permeability. This research would entail designing various PFC mixtures and determining the level of perme- ability. These values can be validated by permeability test- ing of constructed PFC layers. 8. A practical problem related to minimum lift thickness that needs to be researched is determination of a minimum lift thickness to maximum aggregate size ratio (t/mas). Within this document, a minimum ratio of 2 was approximated; however, no references were found to substantiate this value. A minimum t/mas should be selected based on construction issues. A proper t/mas will allow the PFC to be placed and compacted using typical construction procedures while maintaining an appropriate level of permeability. Another element that should be included within this research is the influence of lift thickness on the ability of the PFC layer to maintain permeability. Some references indicated that thicker layers of PFC had better self-cleaning characteristics. 9. The final research need related to structural design is to determine how to account for PFCs in structural capacity. There are several aspects that need to be investigated. First is the proper method to characterize the properties of PFC for use in the Mechanistic-Empirical pavement Design Guide. Currently, HMA is characterized using the dynamic modulus test. However, research is needed to determine the needed input values for PFCs. One reference showed confined testing was appropriate for PFCs; however, no rec- ommendations were provided for the correct confining pressure. Research should be conducted to determine if the dynamic modulus test is the appropriate test and the proper test conditions if it is the appropriate test. The second issue related to structural design is the reduced temperatures in layers underlying PFCs. Current temperatures with pave- ment depth models do not account for the increased stiff- ness of underlying layers and, hence, increased structural capacity. Accounting for the increased capacity of under- lying layers may improve the cost-benefit of using PFCs, especially combined with all of the other safety benefits real- ized when utilizing PFC layers. 82
