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79 Probably the biggest deterrent for states to use PFC pave- ments is freezing weather. This probably is the primary reason that, with only a few exceptions, southern tier states are cur- rently the predominant users of PFCs in the United States. It stands to reason that one limitation with the use of PFC pave- ments would be snow and ice. PFCs have lower thermal con- ductivity than dense-graded mixes which means that the temperature of the road surface drops below freezing sooner than dense asphalt and will stay below freezing longer than for dense-graded mixtures (24). The primary concern becomes a winter maintenance issue, especially winter icing. Winter maintenance is different for porous pavements because of the . . . different temperature behavior for porous asphalt, and because of difficulty in maintaining a sufficient salt level at the level of contact between tire and pavementâ(49). Other authors have concluded that ice forms quicker on porous asphalt than a dense mix. Porous asphalt will contain moisture in its voids if the humidity level is high for a long period of time, allowing the porous asphalt to be more sus- ceptible to freezing when the road is wet (24). Although not mentioned specifically, Bishop and Oliver caution against the use of OGFC mixes in areas that would not get early or close attention for winter maintenance because of their location or importance (53). In their paper, Ruiz et al. (16) indicate that the use of porous asphalt mixtures should be studied carefully prior to being placed in the following four situations: areas of frequent snow, urban or industrial areas, areas with a high potential for reflec- tive cracking, and bridge pavements. In his paper, Alderson identified intersections near quarries or farms as additional limitations where PFC pavements are not recommended (19). Quarries and farms would increase the potential for rapid clogging from debris. Moore and Hicks (47) mention three conditions under which open-graded mix in Oregon is not recommended for use. These are: 1) low-volume roads with ADT of less than 1,000; 2) curbed areas or areas requiring handwork; and 3) heavily snow plowed areas where steel plow blades are used. For this level of traffic the technical benefits are not as noticeable due to the low volume of traffic and lack of heavy loads. Oregonâs Class F mix is not recommended for use in areas with curbs or where a significant amount of handwork or feathering is required. The mixâs aggregate size and aggregate gradation make handwork difficult around utility appurtenances and at driveways. Also, curbs block the drainage of the Class F mix. As a result of snowplow damage Oregonâs Class F mix is no longer recommended in areas where plowing is frequent (47, 48, 66). The snowplows can cause raveling and gouging resulting in a higher rate of surface deterioration. The determination of frequency of plowing is on an individual project basis, but generally involves the elevation, any existing plow damage, and existing chain-up areas or snow zones (47, 48). Though clogging is a major concern for PFC pavements, it is mentioned here as a limitation, but should be more of a main- tenance issue with states rather than a deterrent or limitation for using PFC. Clogging is a concern mainly in the shoulder areas of the roadway because of the collection of debris. Other authors have suggested placing an impermeable surface dress- ing to mitigate these problems. Rogge and Hunt note that clogging occurs, . . . but clogged pavements still allows drainage through the pavement, whereas dense-graded pavements do not (48). As pavement life proceeds, the clogging of the drainage structure from debris and fines leads to the reduction in the permeability of the surface over a period of time (76, 77). A clogged permeable layer will have reduced drainage and water storage capacity (78). Huber in his NCHRP synthesis paper (7) stated that significant clogging will reduce all of the benefits related to permeability. Huber also states that clogged PFC pavements also may accelerate moisture damage within underlying pavement layers. Huber states that the placement C H A P T E R 1 1 Limitations on the Use of Permeable Friction Courses
of PFC may create a moist microenvironment at the surface of the underlying layer (bottom of the PFC). The increased humidity caused by the moisture may retard the evaporation of moisture from the underlying layer resulting in moisture damage within the existing pavement. In their paper, Van Heystraeten and Moraux (22) stated that roads with a high potential for debris, such as near farming operations, should not include PFC because of the high potential for rapid clog- ging. They also stated that PFCs should not be used on low- volume, low-speed pavements because when traffic volumes or traffic speed is low, the self-cleaning attributes of PFC are negated. Self-cleaning occurs because of the pumping and suc- tion of the tires of numerous fast-moving vehicles. The final location that Van Heystraeten and Moraux (22) recommended not using porous asphalt included areas subjected to very high shearing forces at the tire/pavement interface. Although not labeled as limitations, two papers gave a list of disadvantages when using PFCs. Lefebvre (21) listed some disadvantages, stating that PFCs generally cost more than dense-graded layers as a result of requiring high-quality, polish- resistant aggregates and polymer-modified asphalt binders. Also, pavement markings have to be adapted for PFCs. Special impervious layers specifically placed below PFCs also increase construction costs. Another disadvantage of using PFCs is the relatively shorter economic life. Finally, Lefebvre (21) stated that maintenance is generally more expensive, especially win- ter maintenance. In another paper, Bolzan et al. (26) in their opening paragraph provide several disadvantages of using PFCs. They mention that these disadvantages include increased costs, relatively low structural strength due to its high void content, possibly shorter service life, complications to winter maintenance procedures, maintenance patching difficulties, susceptibility to high stress sites, and requirement of minimiz- ing the drainage path length to allow water passing through the layer to enter the drainage system (26). Kandhal (4) provides a number of situations where PFC should not be used. PFCs should not be used on projects that include long-haul distances. Long-haul distances increase the potential for draindown and/or cooling of the mix. Oregon restricts haul distance to 35 miles (56 km). PFCs should be free draining at the pavement edge and should not be used as an inlay. Handwork is difficult with PFC mixes, so projects that include a lot of handwork should probably not include PFC. Kandhal (4) noted that PFC should not be used in snow zones where extensive snow plowing is required. PFCs may ravel and shove in some critical pavement locations such as intersections, locations with heavy turning movements, ramp terminals, curbed sections, and other adverse geometric locations. The final limitation noted by Kandhal (4) has to do with under- lying layers. PFCs should not be placed on a permeable layer, as water can infiltrate a permeable underlying layer causing moisture damage. The Massachusetts Highway Department recognizes the following limitations of open-grade mixes: 1. They can be prone to premature raveling and weathering due to oxidation and hardening of the binder. 2. Application of thermoplastic paint markings can heat up the pavement surface and cause localized draindown of the binder material from the aggregate. This can lead to delamination and /or raveling of the mix under the thermo- plastic line markings. 3. Snow plows can strike off raised markers and bounce along the surface causing a âchatterâ or plow marks in the surface of the layer. 4. Primary causes for failure were raveling and delamination (62). Though these conclusions were drawn by the pavement man- agement section of the Massachusetts Department of High- ways in February 2001, it is not clear whether the experiences were related to older OGFC pavements rather than the new generation PFC mixtures. Definitely raveling and delamina- tion was an issue with the older OGFC pavements. The open structure of PFC mixtures exposes the surface to the effects of air and water, thus leading to rapid aging of the binder which in turn can lead to particle loss and adhesive failure (77). How- ever, these drawbacks have been vastly improved with the use of polymer-modified binders and fibers. Rogge and Hunt (48) also concluded that the main physical/ mechanical distress in PFC is raveling or particle loss. How- ever, they concluded the problem results from cold mix, low compaction, or segregation from the binder at the time of construction. After construction, PFC contains a lower friction value when braking with locked wheels. When the wheels lock, they begin to melt a thin layer of binder on the pavement surface, which creates a slippery surface. This is only true when the wheels are locked. When the ABS braking system is used, the braking distance on porous asphalt is similar to that of dense- graded hot mix asphalt, maybe even shorter. This layer of binder is worn off after approximately 3-6 months (24). Bolzan et al. (26) mentioned the increased costs as a limita- tion for use of PFC pavements. Until states realize the benefits of PFC pavements over conventional dense-graded pavements, costs will be a deterrent. It is felt that costs should not be con- sidered a limitation. States use costs as a deterrent for using new-generation products as there are never funds available to accomplish the needs before them. As environmental issues (such as noise reduction in pavements) are moved to the fore- front, PFCs will be given greater attention for use. When this occurs, the biggest challenge the states will face is the necessary maintenance required when PFC pavements are used. 80
