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198 A Manual for Design of Hot Mix Asphalt with Commentary Table 11-2. Fine aggregate quality requirements for PFC. Spec. Spec. Test Method Minimum Maximum Soundness (5Cycles), % AASHTO T 104 Sodium Sulfate - 15 Magnesium Sulfate - 20 Uncompacted Voids AASHTO T 304, 45 - Method A Sand Equivalency AASHTO T 176 50 - discussion also applies to PFC mixtures. However, because of the high binder content and open- graded aggregate in PFC mixtures, a stiff asphalt binder is needed to ensure a durable mixture. For pavements with design traffic levels of 10 million ESALs and higher, the high-temperature performance grade should be increased by two grades (12C) over that which would normally be used for the given conditions. For lower design traffic levels, the high-temperature performance grade should be increased at least one grade (6C). The use of polymer modified binders or asphalt rubber binders is strongly indicated for PFC mixtures. Stabilizing Additives Stabilizing additives are needed within PFC to prevent the draindown of asphalt binder from the coarse aggregate during transportation and placement. Stabilizing additives, such as cellulose fiber, mineral fiber, and polymers, have been used successfully to minimize draindown potential. When using polymer or rubber as a stabilizer, the amount of additive added should be that amount necessary to meet the specified performance grade of the asphalt binder. Cellulose fibers are typically added to a PFC mixture at a dosage rate of 0.3% by total mixture mass. Requirements for cellulose fibers are presented in Table 11-3. Mineral fibers are typically added at a dosage rate of 0.4% of total mixture mass. Requirements for mineral fibers are provided in Table 11-4. Experience has shown that fibers are the best draindown inhibitor. Step 2--Trial Gradations As with any HMA, specified aggregate gradations should be based on aggregate volume and not aggregate mass. However, for most PFC mixtures, the specific gravities of the different aggregate stockpiles are close enough to make the gradations based on mass percentages similar to that based on volumetric percentages. The specified PFC gradation bands presented in Table 11-5 are based on % passing by volume. Selection of Trial Gradations The initial trial gradations must be selected to be within the master specification ranges presented in Table 11-5 and illustrated in Figures 11-2 through 11-4. It is recommended that at least three trial gradations be initially evaluated. These three trial gradations should fall along and in the middle of the coarse and fine limits of the gradation range. These trial gradations are obtained by adjusting the amount of fine and coarse aggregates in each blend. Determination of VCA in the Coarse Aggregate Fraction For best performance, the PFC mixture must have a coarse aggregate skeleton with stone- on-stone contact. The coarse aggregate fraction of the blend is that portion of the total aggregate

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Design of Open-Graded Mixtures 199 Table 11-3. Cellulose fiber requirements. Property Requirement Sieve Analysis Method A Alpine Sieve1 Analysis Fiber Length 6-mm (0.25 in.) Maximum Passing 0.150-mm (No. 100 sieve) 70+10% Method B Mesh Screen2 Analysis Fiber Length 6-mm (0.25 in.) Maximum Passing 0.850-mm (No. 20) sieve 85+10% 0.425-mm (No. 40) sieve 65+10% 0.160-mm (No. 140) sieve 30+10% Ash Content3 18+5% non-volatiles pH4 7.5+1.0% Oil Absorption5 5.0+1.0% (times fiber mass) Moisture Content6 Less than 5% (by mass) 1 Method A Alpine Sieve Analysis. This test is performed using an Alpine Air jet Sieve (type 200LS). A representative 5-g sample of fiber is sieved for 14 minutes at a controlled vacuum of 75 kPa (11 psi) of water. The portion remaining on the screen is weighed. 2 Medthod B Mesh Screen Analysis. This test is performed using standard 0.850, 0.425, 0.250, 0.180, 0.150, and 0.106-mm sieves, nylon brushes, and a shaker. A representative 10 gram sample of fiber is sieved, using a shaker and two nylon brushes on each screen. The amount retained on each sieve is weighed and the percentage passing calculated. Repeatability of this method is suspect and needs to be verified. 3 Ash Content. A representative 2-3 gram sample of fiber is placed in a tared crucible and heated between 595 and 650C (1100 and 1200F) for not less than 2 hours. The crucible and ash are cooled in a desiccator and weighed. 4 pH Test. Five grams of fiber are added to 100 ml of distilled water, stirred, and let sit for 30 minutes. The pH is determined with a probe calibrated with pH 7.0 buffer. 5 Oil Absorption Test. Five grams of fiber are accurately weighed and suspended in an excess of mineral spirits for not less than 5 minutes to ensure total saturation. It is then placed in a screen mesh strainer (approximately 0.5 mm2 opening size) and shaken on a wrist action shaker for 10 minutes [approximately 32-mm (1 in) motion at 240 shakes per minute]. The shaken mass is then transferred without touching to a tared container and weighed. Results are reported as the amount (number of times its own weight) the fibers are able to absorb. 6 Moisture Content. Ten grams of fiber are weighed and placed in a 121C (250F) forced air oven for 2 hours. The sample is then re-weighed immediately upon removal from the oven. Table 11-4. Mineral fiber quality requirements. Property Requirement Size Analysis Fiber Length1 6-mm (0.25 in.) Maximum mean test value Thickness2 0.005-mm (0.0002 in.) Maximum mean test value Shot Content3 Passing 0.250-mm (No. 60) sieve 90+5% Passing 0.005-mm (No.230) sieve 70+10% 1 The fiber length is determined according to the Bauer McNett fractionation. 2 The fiber diameter is determined by measuring at least 200 fibers in a phase contrast microscope. 3 Shot content is a measure of non-fibrous material. The shot content is determined on vibrating sieves. Two sieves, 0.250 and 0.063 are typically utilized. For additional information see ASTM C612. Table 11-5. PFC gradation specification bands. Sieve Size, mm 9.5-mm PFC 12.5-mm PFC 19-mm PFC Grading Requirements % Passing 25mm 100 19mm 100 85-100 12.5mm 100 80-100 55-70 9.5mm 85-100 35-60 --- 4.75mm 20-30 10-25 10-25 2.36mm 5-15 5-10 5-10 75m 0-4 0-4 0-4

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200 A Manual for Design of Hot Mix Asphalt with Commentary 19.0 mm OGFC Gradation Requirements 100 90 80 70 Percent Passing 60 50 40 30 20 10 0 0.075 0.6 1.18 2.36 4.75 9.5 12.5 19 25 Sieve Size, mm (raised to 0.45 power) Figure 11-2. 19.0-mm PFC gradation requirements. retained on the breakpoint sieve. The breakpoint sieve is defined as the finest (smallest) sieve to retain 10 percent of the aggregate gradation. The voids in coarse aggregate for the coarse aggregate fraction (VCADRC) are determined using AASHTO T 19. When the dry-rodded density of the coarse aggregate fraction has been determined, the VCADRC for the fraction can be cal- culated using the following equation: Gca w - s VCADRC = 100 (11-1) Gca w 12.5 mm OGFC Gradation Requirements 100 90 80 70 Percent Passing 60 50 40 30 20 10 0 0.075 0.6 1.18 2.36 4.75 9.5 12.5 19 Sieve Size, mm (raised to 0.45 power) Figure 11-3. 12.5-mm PFC gradation requirements.

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Design of Open-Graded Mixtures 201 9.5 mm OGFC Gradation Requirements 100 90 80 70 Percent Passing 60 50 40 30 20 10 0 0.075 0.6 1.18 2.36 4.75 9.5 12.5 19 Sieve Size, mm (raised to 0.45 power) Figure 11-4. 9.5-mm PFC gradation requirements. where VCADRC = voids in coarse aggregate in dry-rodded condition s = unit weight of the coarse aggregate fraction in the dry-rodded condition (kg/m3), w = unit weight of water (998 kg/m3), and Gca = bulk specific gravity of the coarse aggregate The results from this test are compared to the VCA in the compacted PFC mixture (VCAMIX). Similar to GGHMA and SMA, when the VCAMIX is equal to or less than the VCADRC, stone-on-stone contact exists. Selection of Trial Asphalt Binder Content The minimum desired asphalt binder content for PFC mixtures is presented in Table 11-6. Table 11-6 illustrates that the minimum asphalt binder content for PFCs is based on the com- bined bulk specific gravity of the aggregates used in the mix. These minimum asphalt binder contents are provided to ensure a sufficient volume of asphalt binder in the PFC mix. It is rec- ommended that the mixture be designed at some amount over the minimum to allow for adjust- ments during plant production without falling below the minimum requirement. As a starting point for trial asphalt binder contents of PFCs, for aggregates with combined bulk specific grav- ities less than or equal to 2.75, an asphalt binder content between 6 and 6.5% should be selected. If the combined bulk specific gravity of the coarse aggregate exceeds 2.75, the trial asphalt binder content can be reduced slightly. Sample Preparation As with the design of any HMA, the aggregates to be used in the mixture should be dried to a constant mass and separated by dry-sieving into individual size fractions. The following size fractions are recommended: 19.0 to 12.5 mm 12.5 to 9.5 mm

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202 A Manual for Design of Hot Mix Asphalt with Commentary Table 11-6. Minimum asphalt content requirements for aggregates with varying bulk specific gravities--PFCs. Combined Aggregate Minimum Bulk Asphalt Content Specific Gravity Based on Mass, % 2.40 6.8 2.45 6.7 2.50 6.6 2.55 6.5 2.60 6.3 2.65 6.2 2.70 6.1 2.75 6.0 2.80 5.9 2.85 5.8 2.90 5.7 2.95 5.6 3.00 5.5 9.5 to 4.75 mm 4.75 to 2.36 mm Passing 2.36 mm After separating the aggregates into individual size fractions, they should be recombined at the proper percentages based on the gradation blend being used. The mixing and compaction temperatures are determined in accordance with AASHTO T 245, Section 3.3.1. Mixing temperature will be the temperature needed to produce an asphalt binder viscosity of 17020 cSt. Compaction temperature will be the temperature required to provide an asphalt binder viscosity of 28030 cSt. However, although these temperatures work for neat asphalt binders, the selected temperatures may need to be changed for polymer-modified asphalt binders. The asphalt binder supplier's guidelines for mixing and compaction temperatures should be used. When preparing PFC in the laboratory, a mechanical mixing apparatus should be used. Aggregate batches and asphalt binder are heated to a temperature no more than 28C greater than the temperature established for mixing. The heated aggregate batch is placed in the mechan- ical mixing container. Asphalt binder and any stabilizing additive are placed in the container at the required masses. Mix the aggregate, asphalt binder, and stabilizing additives rapidly until thoroughly coated. Mixing times for PFC should be slightly longer than for conventional mix- tures to ensure that the stabilizing additives are thoroughly dispersed within the mixture. After mixing, the PFC mixture should be short-term aged in accordance with AASHTO R 30. For aggregate blends having combined water absorption values less than 2%, the mixture should be aged for 2 hours. If the water absorption of the aggregate blend is 2% or more, the mixture should be aged for 4 hours. Number of Samples Typically, 12 samples are initially required: four samples for each three trial gradations. Each sample is mixed with the trial asphalt binder content and three of the four samples for each trial gradation are compacted. The remaining sample of each trial gradation is used to determine the theoretical maximum density according to AASHTO T 209.