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55 Similar to any HMA mixture, construction of PFC pave- ment layers includes four primary phases: production, trans- portation, placement and compaction. Another important aspect of construction is QC/QA. Many of the best practices for constructing PFC pavement layers can be taken from the construction of SMA. Both mix types utilized a large fraction of coarse aggregates and generally require the use of stabiliz- ing additives. Therefore, in addition to the literature, reports, and survey dealing with PFCs (and OGFCs), guidelines devel- oped during NCHRP Project 9-8 (57) also were consulted to develop guidelines on the construction of PFCs. Another valu- able reference utilized during the development of Construc- tion Guidelines was the Hot-Mix Asphalt Paving Handbook (2000) (58). This chapter presents best practices guidelines for the construction of PFC pavement layers as no references were identified that evaluated construction practices. Only experi- ences were found in the literature. Plant Production Production of PFC at a typical HMA plant encompasses those same procedures that would ordinarily be performed at the plant to manufacture any HMA mixture. Any HMA pro- duction facility capable of producing high-quality HMA can produce high-quality PFC (7). This section provides guid- ance found in the literature review and survey for procedures involving aggregate handling, stabilizing additives, liquid asphalt, mixing times, and plant calibration along with other issues that require special attention when compared to conven- tional HMA production. Aggregates As with the construction of any HMA pavement layer, quality begins with proper aggregate stockpile management. Stockpiles should be built on sloped, clean, stable surfaces with the different stockpiles kept separated (58). Every effort should be made to maintain a relatively low moisture content within the aggregate stockpiles. Low moisture contents and low moisture content variability will allow for easier control of mixing temperature (7). A PFC mixture must contain a high percentage of coarse aggregate in order to provide the desired high air void con- tents and, thus, benefits related to permeability. The high per- centage of coarse aggregate within PFC mixtures provides the stone-on-stone contact necessary to provide a stable pave- ment layer for these high air void content mixes. While it is typical to blend two or three different aggregate stockpiles in the mixture (coarse aggregate, immediate aggregate, and fine aggregate), the coarse aggregate (defined as the material retained on the break point sieve) is usually a high percent of the gradation blend (to 85 percent of the blend). Since the coarse aggregate gradation can have a tremendous effect on the quality of the mixture produced, it is necessary that the aggregates be carefully handled and stockpiled. Considera- tion should be given to feeding the coarse aggregate stockpile through more than one cold feed bin to provide better control over the production process. Using more than one cold feed bin for the coarse aggregate will minimize variability in the coarse aggregate gradation (57). Liquid Asphalt The handling and storage of liquid asphalt binder for PFC production is similar to that for any HMA mixture. If not already equipped, the plant facility should have a different, or second, storage tank designated strictly for modified binders. When modified asphalt binders are used, the storage temper- ature may increase slightly from those of neat asphalt binders. Mechanical agitators may be required within storage tanks when modified binders are used (7). Contractors should fol- low the manufacturersâ recommendations for circulation and storage of modified asphalt binders. Metering and introduc- tion of asphalt binder into the mixture may be done by any C H A P T E R 7 Construction of Permeable Friction Courses
of the standard methods using a temperature compensating system. It is very important, however, that the asphalt binder be metered accurately (4). Stabilizing Additives With the high asphalt binder contents and large fraction of coarse aggregate inherent to PFC mixtures, a stabilizing additive of some type must be used to hold the asphalt binder within the coarse aggregate structure during storage, trans- portation, and placement. Draindown will generally occur at typical production temperatures if a stabilizing additive is not used. When draindown occurs during haul and placement, it results in flushed spots in the finished pavement, termed fat spots (4). Eliminating draindown is helped through modify- ing the asphalt binder and/or the use of fibers. Most PFC mixtures will require the use of both a fiber and a modified asphalt binder to minimize draindown potential and improve durability. Draindown testing conducted during mix design should provide an indication of draindown potential and the needed stabilizing additive(s). Fibers Both cellulose and mineral fibers have been used in PFC mixture production. Dosage rates vary, but typically the rates are 0.3 percent for cellulose and 0.4 percent for mineral fiber, by total mixture mass (4). The survey of agencies yielded spec- ified rates between 0.1 and 0.5 percent. Fibers can generally be purchased in two forms, loose and pelletized (4). Decoene (18) indicated that pelletized fibers were specifically developed for the conditions with drum-mix plants. Fibers in a dry, loose state come packaged in plastic bags or in bulk. Fibers also can be pelletized with the addition of some amount of a binding agent. Asphalt binder and waxy substances have been used as binding agents within pelletized fibers. Both fiber types (loose or pelletized) have been added into batch and drum-mix plants with success. For batch plant production, loose fibers are sometimes delivered to the plant site in bags. The bags are usually made from a material that melts easily at typical mixing tempera- tures (18). The bags can be added directly to the pugmill dur- ing each dry mix cycle. When the bags melt, only the fiber remains. Addition of the bags of fibers can be done by workers on the pugmill platform. At the appropriate time in every dry mix cycle, the workers add the correct number of bags to the pugmill. The bags of fiber can be elevated to the pugmill plat- form by the use of a conveyor belt. While this method of man- ual introduction works satisfactorily, it is labor intensive. Another method for addition of fibers into a batch plant is by blowing them into the plant using a machine typically designed and supplied by the fiber manufacturer (Figure 34). The dry, loose fiber is placed in the hopper of the machine where it is fluffed by large paddles. The fluffed fiber next enters an auger system which conditions the material to a known density. The fiber is then metered by the machine and blown into the pugmill or weigh hopper at the appropriate time. These machines can meter in the proper amount of fiber by mass or blow in a known volume (4). This fiber blowing method also can be used in a drum mix plant. The same machine is used and the fibers are simply blown into the drum. When using this method in a drum mix plant the fiber introduction line should be placed in the drum within 1 ft (0.3 m) upstream of the asphalt binder line (Figure 35) (4). At least one agency has reported that introduction of the fibers at the lime injection point (assuming lime is incorporated into the mix) also worked well (10). They indicated that this allowed the fibers to mix with the aggregates prior to the intro- duction of asphalt binder. No matter the method of introduc- 56 Figure 34. Fiber introduction system. Figure 35. Typical location for introduction of fibers into drum-mix plant.
tion, it is imperative that fibers be captured by the asphalt binder before being exposed to the high-velocity gases in the drum. If the fiber gets into the gas stream, it will enter the dust control system of the plant (4). Whenever loose fibers are blown into the production process, whether a drum mix or batch plant is used, the fiber blowing equipment should be tied into the plant control system (57). The fiber delivery system should be calibrated and continually monitored during production. A common practice is to include a clear section on the hose between the fiber blowing equipment and the introduction point within the production process (Figure 36). This clear section can pro- vide a quick, qualitative evaluation of whether the fiber is being blown properly into the drum. Variations in the amount of fibers within the PFC mix can have a detrimental impact on the finished pavement. The pelletized form of fibers can be used in both drum-mix and batch plants. Pellets are shipped to the plant in bulk form and when needed are placed into a hopper (Figure 37). From the hopper they can be metered and conveyed to the drum or pugmill via a calibrated conveyor belt. Addition of the pellets generally occurs at the RAP collar of a drum mix plant or they are added directly into the pugmill of a batch plant. Whether in the drum or the pugmill, the pellets are mixed with the heated aggregate and the heat from the aggregates causes the binding agent in the pellets to become fluid. This allows the fiber to mix with the aggregate (4). Note that some forms of pelletized fibers do contain a given amount of asphalt binder. In most instances, this amount of asphalt binder is very small and is not included within the total asphalt binder content. Check with the fiber manufacturer to determine the asphalt contents of the pellets. It is imperative that the fiber addition, whether it be loose or pelletized, be calibrated to ensure that the mixture contin- ually receives the correct amount of fiber. If the fiber content is not accurately controlled at the proper level, fat spots can result on the surface of the finished pavement. Also, portions of the mixture will be dry and unworkable (7). For assistance with the fiber storage, handling, and introduction into the mixture, the fiber manufacturer should be consulted. Asphalt Cement Modifiers Another method of providing stabilization to PFC is with the use of asphalt binder modifiers. The asphalt binder in PFC can be modified at the refinery or in some cases the modifier is added at the hot mix plant. For the first method, the hot-mix producer takes delivery of the modified asphalt binder and meters it into the PFC mixture in a traditional manner. Special storage techniques and/or temperatures may be required, as discussed previously. With the second method, the contractor must ensure that the proper amount of modifier is added and thoroughly mixed with the asphalt binder (57). When an asphalt binder modifier is added at the hot-mix plant, two different methods are utilized. The modifier is blended into the asphalt binder either before it is injected into the production process or it is added directly to the dry aggre- gates during production (57). Addition of the modifier to the asphalt binder is accomplished by in-line blending or by blend- ing the two in an auxiliary storage tank. If the modifier is added to the aggregates rather than the asphalt binder, it can be added directly into the pugmill or, in a drum-mix plant, it can be de- livered to the drum via the RAP delivery system. Use of the RAP belt weigh bridge is not recommended because of poor accuracy and special metering devices may be necessary if the RAP feeder cannot be calibrated (57). When a modifier is added directly into the plant and not premixed with the asphalt cement, it is impossible to measure the properties of the mod- ified asphalt binder. The properties of the modified asphalt binder can be estimated in the laboratory by mixing the desired proportion of asphalt cement and modifier and testing (21). Regardless of the form of stabilization, advice and assistance should be sought from the stabilizer supplier. It is imperative 57 Figure 36. Clear section of fiber introduction line. Figure 37. Typical fiber hopper for pelletized fibers.
that the system used to add the modifier be calibrated to en- sure the mixture receives the proper dosage. Mixture Production Production of PFC is similar to the production of standard HMA from the standpoint that care should be taken to ensure a quality mixture is produced. Production of PFC is discussed in this section with special emphasis on production areas where PFC quality may be significantly affected. Plant Calibration It is important that all the feed systems of the plant be carefully calibrated prior to production of PFC. Operation of the aggregate cold feeds can have a significant influence on the finished mixture, even in a batch plant where hot bins exist. Calibration of the aggregate cold feed bins should be performed with care. The stabilizing additive delivery system should be calibrated and continually monitored during production. Variations in the amount of additive can have a detrimental impact on the finished pavement. Stabilizing additive manufacturers will usually assist the hot-mix producer in setting up, calibrating, and monitoring the stabilizing additive system. Plant Production Similar to the production of typical HMA mixtures, mixing temperatures during the production of PFC mixes should be based upon the properties of the asphalt binder (57). Mixing temperatures should not be arbitrarily raised or lowered. Ele- vated mixing temperatures increase the potential for damage to the asphalt binder due to rapid oxidation. This damage can lead to premature distress within PFC layers. Additionally, artifi- cially increasing the mixing temperature can increase the poten- tial for draindown problems during storage, transportation, and placement of PFC. Arbitrarily lowering the mixing temper- ature can result in not removing the needed moisture from the aggregates within the drying process. Moisture remaining within the aggregates can increase potential of moisture- induced damage within PFC layers. Additionally, arbitrarily lowering the mixing temperature will likely result in PFC mix- ture delivered to the construction project that is cooler than the desired compaction temperature. If this occurs, the PFC may not bond with the underlying layer (through the tack coat) and result in increased potential for raveling and delamination. Experience seems to indicate that normal HMA production temperatures or slightly higher are adequate. In addition to the properties of the asphalt binder, the mixing temperature should be chosen to ensure a uniform mixture that allows enough time for transporting, placing, and compaction of the mixture. When using a batch plant to produce PFC, the screening capacity of the screen deck will need to be considered. Since PFC gradations are generally a single-sized aggregate, override of the screen deck and hot bins may occur (4). If this occurs, the rate of production should be decreased. Choubane et al. (59) designed some flushing problems that occurred in Florida during construction of open-graded mixes. Based upon an investigation into the problems, the flush- ing problems were traced back to start up problems each day. Therefore, they concluded that both proper start-up and clean- out procedures were needed with open-graded mixes. Mixing Time When adding fibers to the PFC mixture, experience has shown that the mixing time should be increased slightly over that of conventional HMA (4). This additional time allows for the fibers to be sufficiently distributed within the mixture. In a batch plant, this requires that both the dry and wet cycles be in- creased from 5 to 15 seconds each. In a parallel flow drum plant, the asphalt binder injection line may be relocated, usually extended when pelletized fibers are used (57). This allows for more complete mixing of the pellets before the asphalt binder is added. In both cases, the proper mixing times can be esti- mated by visual inspection of the mixture. If clumps of fibers or pellets still exist intact in the mixture at the discharge chute, or if aggregate particles are not sufficiently coated, mixing times should be increased or other changes made. For other plants such as double-barrel drum mixers and plants with coater boxes, the effective mixing time can be adjusted in a number of ways including reduction rate, slope reduction of the drum, etc. Mixture Storage The PFC mixture should not be stored at elevated tem- peratures for extended periods of time as this could facili- tate draindown (4). In general, experience has shown that PFC can be stored for 2 hours or less without detriment. In no instance should the PFC mixture be stored in the silo overnight. During the survey, agencies were equally split on having a maximum silo storage time. Those that did limit storage time had limits from 1 to 12 hours. For those agencies indi- cating that they specified PFC mixes, the maximum storage time was typically 2 hours. Transportation The PFC mixture is transported to the project site using the same equipment used for dense-graded HMA (7). Generally, no additional precautions are required; however, there are some best practices that should be followed. 58
The goal of this phase of construction is to deliver the PFC mix to the project site at the appropriate temperature. There are three methods used to ensure that the PFC will arrive at the appropriate temperature for placement and compaction. The first two limit the amount of time the PFC is in the transport vehicle: limiting haul distance or limiting haul time. These two methods assume that the mix leaves the plant at a tempera- ture near mixing temperature. The third method is to specify a minimum mix temperature upon arrival at the project site. From the survey of agencies, most have requirements for min- imum mix temperature at the project site; however, one agency limited haul distance to 50 miles (80 km) and one limited haul time to no more than 1 hour. Minimum delivery temperatures ranged greatly in both the survey and literature review. A spec- ified minimum delivery temperature should be based upon climate and typical asphalt binders used in PFC by the agency. Hauling One of the keys to successful PFC projects is having adequate transportation to supply mix to the paver so that the paver does not have to stop and wait on materials (4). Since the contrac- tor often does not own the trucks, communication with the trucking operator is essential to avoid delays. Because of the bonding tendency of the modified asphalt binder generally used in PFCs, the truck beds should be cleaned frequently and a heavy and thorough coat of an asphalt release agent applied. Also, truck beds should be raised after spray- ing to drain any puddles of the release agent. Excess release agent, if not removed, will cool the PFC and cause cold lumps in the mix (4). Most agencies have approved lists of release agents. Use of fuel oils in any form should be strictly prohibited. Haul trucks should be covered with a tarp to prevent exces- sive crusting of the mix during transportation (7). Based upon the survey, most agencies require trucks to be tarped. Cold lumps do not break down readily and can cause pulls in the mat. Since long haul distances will compound this problem, the haul distance should be kept under approximately 50 miles (80 km). To combat this problem, some agencies require insu- lated truck beds (7). As an alternative to insulated tuck beds, a âheated dump bodyâ may be used. A heated dump body refers to a transport vehicle capable of diverting engine exhaust (Figure 38) and transmitting the heat evenly throughout the dump body to help keep the PFC from excessively cooling. Haul Time Haul distance is important. however, the haul time should govern over haul length. For PFC mixtures, haul time should be limited to less than 2 hours haul time, but preferably less than one hour. Haul times for PFC should be as short as pos- sible. It is important that the temperature of the PFC mixture not be raised arbitrarily high in order to facilitate a longer haul time (57). The increased temperature in coordination with the vibration provided during haul can amplify the probabil- ity of draindown occurring. The mixture should arrive at the paving site so that it is placed at the appropriate compaction temperature. Placement Placement of PFC is similar to placement of typical dense- graded HMA. Typical asphalt pavers are utilized. Weather Limitations In order to achieve proper placement and compaction, PFC should not be placed in cold or inclement weather. A minimum pavement temperature of at least 50°F (10 °C) for placement of PFC mixture is recommended (19). Ambient air temperature should also be at least 50°F (10°C) and rising though some agencies specify higher temperatures (19). However, the abil- ity to place PFC will also depend on wind conditions, humid- ity, the lift thickness being placed, and the temperature of the existing pavement. Local experience with paving mixtures that include very stiff asphalt binders (polymer-modified) should be considered when specifying weather limitations. Pavement Surface Preparation Prior to placing PFC, preparation of the surface to be cov- ered will depend on the type of surface onto which the PFC will be placed. The preparation method used is generally the same as for conventional HMA mixtures. PFCs should enable rain water to penetrate the surfacing and be laterally drained 59 Figure 38. Exhaust system for heated dump body.
off to the side of the road by flowing on an impermeable inter- face between the PFC and underlying layer. Therefore, the PFC should only be placed on an impermeable HMA layer or on a portland cement concrete pavement (21). Placement on an impermeable layer will help ensure that during rainfall, the water will pass through the PFC and not be trapped in the underlying pavement layer, thus helping to minimize the potential for moisture damage (stripping). PFC should not be placed on rutted asphalt pavement. The rutted surface should be milled first or reshaped to that depth, which allows the water to flow to the side of the road before placement of the PFC mixture. The PFC mat should be daylighted on the shoulder so that rain water percolating through the PFC can drain out freely at its edge (21). A strip at least 4 in. (0.1 m) wide should be left between the PFC and any grass area. If the PFC is not laid over the entire width of an existing pavement, includ- ing the paved shoulder, then it should extend at least 12 to 20 in. (0.3 to 0.5 m) onto the paved shoulder with a tapered profile for safety associated with pavement edge drop-off. Wagner and Kim (60) described a device to construct tapered pavement edges. Two methodologies of constructing shoulders have been used in Spain (16). First, PFC has been extended over the entire shoulder and the second method has been to extend the PFC 1.6 ft (50 cm) onto the shoulder. Lefebvre (21) illustrated three methods for construction of shoulders with PFC (Figure 39). To have an impervious underlying layer, or to make it imper- vious, is one of the imperatives of PFC. When overlaying an existing asphalt road with PFC, the underlying asphalt should be as impervious as possible. When an old pavement surface is to be covered by a PFC then proper repair should first be performed (16). Areas containing large permanent deforma- tions should be milled or filled using a leveling course. If the condition of the in-place mix is sufficiently bad, it may have to be removed to some predetermined depth. Any distressed areas should be properly repaired. A freshly compacted dense-graded HMA course may have as much as 8 percent air voids in the mat and thus may be per- meable to water. Alderson (19) states that pavements can be considered impervious if they have less than 5 percent air voids. If this condition is not available, the existing pavement should be sealed using a heavy tack coat, fog coat or other type seal (19). It is essential to provide a uniform tack coat at an adequate application rate to fill and seal the surface voids of the underlying layer. For old distressed surfaces, the method used to make the surface impermeable will depend on the severity of the pave- ment distress. Lightly and randomly cracked surfaces should have wide cracks cleaned and sealed by bridging. If the entire surface is randomly cracked, a full-width treatment is neces- sary to make it impervious. Types of materials and their appli- cation rates need not vary from that of conventional HMA construction. When sealing the underlying pavement with a tack coat it is recommended that a 50 percent diluted slow- setting emulsion tack coat at a rate of 0.05 to 0.10 gallons per square yard be applied (4). Ruiz et al. (16) recommended 0.11 to 0.13 gallons per square yard. In British Columbia, a rate of 0.17 gallons per square yard is utilized (53). The application rate should be high enough to completely fill the surface voids. A slow-setting emulsion tack coat is likely to penetrate the surface voids more effectively than an asphalt cement tack coat (4). However, others have recommended quick-setting emulsions (16). Most dense-graded HMA surfaces become reasonably impervious after two to three years of traffic. Such surfaces will not need any sealing prior to placing PFC. How- ever, if the existing surface is highly polished, a slurry seal may be required (16). Severely cracked surfaces may require a impervious membrane be used. The survey indicated that a wide range of tack coat materials have been used with PFCs including emulsions and neat asphalt binder. 60 4 cm channel 1 2 1 4 cm 10 cm 2 soft shoulder 4 cm shoulder 1 2 hard shoulder slow lane 30 to 50 cm 2 to 2.5 cm 1 - Impervious Layer 2 - Porous asphalt Figure 39. Examples of daylighting porous asphalt mixtures (21).
Sealing or use of a tack coat should never be used when a PFC is placed on another PFC surface. Paver Operation PFC mixtures are placed using conventional asphalt pavers. However, a hot screed is very important to prevent pulling of the mat. A propane torch or some other means to heat the paver screed before each startup is important. Charging the Paver The PFC mixture is normally delivered to the paver in the traditional manner of backing in trucks. A material transfer vehicle (MTV) also can be used for PFC. Some agencies cur- rently specify MTVs. The use of a remixing material transfer device for transferring the PFC from the trucks to the paver is optional, but highly recommended. It remixes most cold lumps produced during transportation, and also allows con- tinuous operation of the paver for smoother surfaces. If the mixture is dumped directly into the hopper of the paver, the trucks should not back into the paver. With PFCs, the result- ing depression is more difficult to roll smooth than for dense- graded HMA. Placing PFC mixture in a windrow for pick-up is allowed; however, the length of the windrow should be closely con- trolled. Mixture placed within a windrow will lose heat more quickly than mixture placed with a MTV or directly into the hopper. Weather conditions also should be considered before using the windrow technique. During favorable weather conditions, windrow length should not be more than 150 ft (50 m) (7). Paver Calibration Prior to placement of the PFC, the paver should be cor- rectly calibrated. This is no different than when placing con- ventional HMA and involves the flow gates, the slat conveyors, and the augers. The flow gates should be set to allow the slat conveyors to deliver the proper amount of mixture to the augers. When extendable screeds are utilized, auger exten- sions should be used (7). Without the use of auger extensions, the coarse aggregates tend to be pushed to the edge of the mat, leaving the asphalt binder behind. Paver Speed When placing PFC, the paving speed is for the most part dictated by the ability of the rolling operation to compact the mixture. It is critical that the plant production, mixture delivery, and ability to compact be coordinated so that the paver does not have to continually stop and start (58). Paver stops and starts should be held to an absolute minimum because they will likely have a significant negative impact on ride qual- ity (smoothness). In addition to continuous paver movement, the PFC mix- ture delivery and paver speed should be calibrated so that the augers can be kept turning 85-90 percent of the time (57). This helps ensure the slowest possible speed for the augers. Running the augers very fast for short periods of time should be avoided. The high auger speed may have a tendency to shear the mortar from the coarse aggregate thus causing fat spots in the pave- ment. The paver wings should not be lifted except when the material is to be discarded. Lift Thickness The majority of PFC pavements placed in the United States have been placed between 1.25 to 2 in. (32 and 50 mm) in thickness. It is imperative that the construction inspector not try to balance yield by adjusting the thickness of the PFC lift. This can cause unsatisfactory pavements to be built. A tolerance of ± 0.25 in. (6 mm) in the lift thickness should be allowed (57). Though no guidance was found, minimum lift thicknesses should be about two times the maximum aggre- gate size of the gradation. Placement and Finishing Immediately behind the paver, PFC mixtures are known to be harsh and very sticky. For this reason a minimum of rak- ing and hand work should be performed (21). When needed, hand placement of the material can be accomplished with care. Longitudinal joints in the PFC pavement are constructed by placing the mix approximately 1â8 in. (3 mm) above the pre- viously placed and compacted lane. Therefore, it is impor- tant for the edge of the screed or extension to follow the joint exactly to prevent excessive overlap. Longitudinal joints should not be tacked unless they are at the crown of the pavement (4). Tacked longitudinal joints will prevent the flow of water across the joint. Transverse joints placed against a previously laid PFC are constructed by starting with the screed one foot behind the joint and laying the screed flat on the previously laid PFC mat (57). Then hot PFC mix is augered in front of the screed and then drug off the new joint when travel begins. The joint should then be cross rolled with a steel wheel breakdown roller. Compaction Initially, compaction should be as intense as in the case of traditional bituminous mixes, in order to keep subsequent post-compaction as reduced as possible. To achieve correct compaction on PFC, the roller must follow close behind the 61
paver, passing over immediately after the laying, in order for the temperature to be sufficient (57). Conventional steel wheel rollers are used to compact the PFC. No pneumatic tire rolling is required. It is critical to keep the breakdown roller within 50 ft (15 m) of the paver to com- pact while it is still hot and workable. The breakdown roller usually completes one to two complete coverages of the mat in static mode to compact a thin lift (3â4 in. or 20 mm) PFC. Rolling No minimum density is recommended for PFC. Rather than having a density requirement, some agencies control compaction by permeability tests performed on the com- pleted PFC mat (13), though this practice is not common. Densification of PFC mixture should be accomplished as quickly as possible after placement. By its very nature PFC becomes difficult to compact once it begins to cool. For this reason it is imperative that the rollers be kept immediately behind the paver (57). Rolldown of PFC mixtures is slightly less than one-half that for conventional mixtures. While conventional HMA mix- tures rolldown approximately 20-25 percent of the lift thick- ness, PFC will normally rolldown 10 to 15 percent of the lift thickness (7). Breakdown rolling should begin immediately behind the paver and the roller should stay close behind the paver at all times. If the rolling operation gets behind, placement of PFC should slow until the rollers catch up with the paver (57). Steel wheeled rollers weighing 9 Mg (10 tons) should be used when compacting the PFC mixture (7). Roller speed should not exceed 3 miles/hr (5 km/hr) and the drive roll should be kept nearest the paver. Two to four passes of the breakdown rollers should be sufficient. If it becomes necessary for the rollers to sit idle they should be taken off the mat if possible. Idle rollers sitting on the mat can cause unnecessary rough- ness in the finished surface. Static steel wheel finish rollers are used to remove any roller marks from the pavement surface. It is normal practice to mix a minimum amount of release agent with the water in the roller drum to prevent the asphalt binder from sticking to the drum. Excessive amounts of water should not be used (57). Vibratory rollers should not be used on PFC. The breakdown roller may have to be operated in a vibratory mode at transverse joints and occasionally longitudinal joints to help knock down a high joint. Generally, use of vibratory compaction should be discouraged (4). If vibrating is allowed, it must be used with caution. The vibration of the roller may break aggregate and/or force the mortar to the surface of the mat. Pneumatic-tired rollers are not recommended for use on PFC (4). The rubber tires tend to pick up the mortar causing surface deficiencies. One of the main differences between PFC and dense- graded mixtures is that the goal for compaction is quite dif- ferent. With dense-graded mixtures, compaction is necessary to make the mixture impermeable so that water does not in- filtrate the layer through interconnected air voids. With the PFC mixtures, compaction equipment is used only to seat the mixture in the tack coat to provide a good bond at the inter- face of layers. Otherwise, the mixture is intended to be highly permeable to transfer water through the layer onto the shoul- der or edge of the pavement. Where air voids during con- struction are generally reduced to between 5 to 7 percent for dense-graded mixtures, PFC should have above 18 percent air voids immediately after construction. Density Requirements Density of PFC mixtures is seldom checked since there is no attempt to compact the mix to a minimum density level. If density results are desired in order to verify that field air voids are adequately high enough to promote water drainage, the method of determining in-place air voids is critical. Since water freely drains from the mixture, the conventional method of using the saturated surface-dry condition (AASHTO T166) does not apply. One method is to measure the height and diameter of a core specimen and calculate the bulk specific gravity based on a volumetric relationship. Another alternative is to use the vacuum sealing method described in AASHTO TP69-04. In previous research conducted at the National Center for Asphalt Technology (NCAT), the plastic bags used in the vacuum sealing procedure frequently developed punc- tures so that a double-bag procedure was used with the test method (40). Quality Control/Quality Assurance PFC mixture furnished by the contractor should conform to the job-mix formula requirements, within allowable devia- tions from the targets. Testing included within a QC/QA pro- gram should include gradations, asphalt binder content, and draindown. Gradations and asphalt binder content testing is conducted to provide an indication that the mixture is produced according to the job mix formula, while draindown testing is conducted to ensure that the stabilizing additives are being properly added. In Spain, air void contents are con- trolled (16) by dry production. After completion of construction, smoothness testing should be conducted. Smoothness testing should be conducted to ensure that construction practices occurred that would not adversely affect operational control of aircraft. In Belgium, a field permeability device is specified at the time of construction to evaluate whether the PFC layer was properly constructed (18). In Spain, permeability testing is 62
conducted to control the amount of compaction under the roller (16). Similarly, permeability has been used as quality control in Argentina (26) and Japan (20). Another test that has been adopted in quality control prac- tices is the Cantabro Abrasion loss test. This method has been utilized in Argentina during quality control (26). Sholar et al. (61) described the development of percent within limits (PWL) for open-graded mixes in Florida. They indicated that development of the PWL system for open- graded mixes was done in a manner similar to development by PWL specifications for dense-graded Superpave design HMA; however, the material properties used for payment, standard deviations and specification limits recommended are unique to open-graded mixes. Four material properties were selected that were believed to be related to performance: asphalt binder content, percent passing 3â8 in. (9.5 mm) sieve, percent passing No. 4 (4.75 mm), and percent passing No. 8 (2.36 mm) sieve. Quality control data from five different con- struction projects were used to develop variance and standard deviation values for each of these properties. The standard deviations along with calculated specification limits and im- plemented specification are provided in Table 32. For each material property, pay factors are calculated as: Pay Factor (%) = 55 + 0.5 PWL Equation 4 Using the pay factor for each material property, a com- posite pay factor is calculated by multiplying the respective weights for each material property by the individual pay factors. The weightage of each material property was: 1) as- phalt binder content (40 percent); 2) percent passing 3â8 in. sieve (20 percent); 3) percent passing No. 4 sieve (30 per- cent); and 4) percent passing No. 8 sieve (10 percent). Sublots were selected as 500 tons of produced mix with four sublots comprising a lot. Production resulting in less than three sublots was considered âsmall productionâ and treated sep- arately. The pay table for small production is presented in Table 33. Pavement Markings A potential problematic area during the life of PFC pave- ments is pavement markings. The Massachusetts Highway Department has reported performance issues with the use of thermoplastic paint (61). The thermoplastic paint can heat up the asphalt binder at the PFC surface and cause localized draindown. They indicate that this can cause delamination and/or raveling under thermoplastic line markings. No resolu- tions to this problem were provided. Corrigan et al. (63) conducted a study to develop a specifi- cation for longer-lasting and better-performing thermoplastic 63 Property Median Standard Deviation Calculated Specification Limits (standard deviation * 1.645) Implemented Specification Limits Asphalt Binder Content, % 0.24 0.39 ±0.45 Percent passing the 3/8 in. sieve 2.99 4.92 ±6.00 Percent passing the No. 4 sieve 2.10 3.46 ±4.50 Percent passing the No. 8 sieve 1.04 1.72 ±2.50 Property Pay Factor 1-Test Deviation 2-Test Average Deviation Asphalt Binder Content, % 1.00 0.90 0.80 0.00-0.50 0.51-0.60 >0.60 0.00-0.35 0.36-0.42 >0.42 Percent passing the 3/8 in. sieve 1.00 0.90 0.80 0.00-6.50 6.51-7.50 >7.50 0.00-4.60 4.61-5.30 >5.30 Percent passing the No. 4 sieve 1.00 0.90 0.80 0.00-5.00 5.01-6.00 >6.00 0.00-3.54 3.55-4.24 >4.24 Percent passing the No. 8 sieve 1.00 0.90 0.80 0.00-3.00 3.01-3.50 >3.50 0.00-2.12 2.13-2.47 >2.47 Table 33. Pay table for small production (61). Table 32. Specification limits for OGFC (61).
pavement markings. The authors stated durability problems resulting from the use of snowplows as a primary cause for the research. Two parameters were considered: durability and retro-reflectivity of the markings. Both recessed and non- recessed markings were evaluated. Conclusions from the study were that fully recessed thermoplastic traffic markings resulted in the least snowplow change. Permanent inlaid marking tape lacked the needed durability to withstand snowplows. A cost analysis showed that fully recessed thermoplastic traffic mark- ings was cost effective. During the survey, agencies were asked what materials were used for pavement markings on open-graded mixes. Many respondents stated that typical pavement markings were used. The most frequently listed marking type included waterborne paint and thermoplastic. Some northern tier agencies men- tioned epoxy. 64
