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A Manual for Design of Hot-Mix Asphalt with Commentary (2011)

Chapter: Chapter 11 - Design of Open-Graded Mixtures

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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 11 - Design of Open-Graded Mixtures." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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194 Open-graded friction course (OGFC) is a specialty HMA that uses an extremely open aggregate gradation to improve frictional resistance, reduce splash and spray, improve nighttime visibility, reduce hydroplaning, or reduce pavement noise levels. OGFC is specifically designed to have a large percentage of a single size coarse aggregate with a low percentage of fine aggregates and a very low percentage of materials finer than 0.075 mm (dust or mineral filler). A relatively single size coarse aggregate combined with a low amount of fine aggregate and dust provides for a much more open aggregate gradation compared to other HMA mix types. OGFCs were first developed during the 1940s through experimentation with plant-mix seal coats. Even though these plant-mix seal coats provided excellent frictional properties, their use spread slowly because they required a different mix design method and special construction considerations than typically used HMA. It was not until the 1970s, when the FHWA published a formalized mix design procedure that the use of OGFCs began to increase. This procedure entailed an aggregate gradation requirement, a surface capacity of coarse aggregate, determination of fine aggregate content, determination of optimum mixing temperature, and determination of the resistance of the designed mixture to moisture. During the 1970s and 1980s, some agencies observed performance problems when using OGFCs. Primarily, the problems were raveling and delamination. These distresses were caused by problems associated with mix design, material specification, and construction. The primary issue involved mix temperature during construction. The gradations associated with OGFCs were much coarser than typical dense-graded mixes. Additionally, unmodified asphalt binders were used with these OGFCs at that time. Because of the open nature of the aggregate gradation, there were problems with the asphalt binder draining from the coarse aggregate during transport. To combat these draindown problems, the mix temperature was reduced during production. This reduction in temperature resulted in two problems. First, the internal moisture within the aggregates was not removed and, second, the OGFC did not bond with the existing pavement when placed. These two issues resulted in the raveling and delamination problems frequently encountered with OGFCs during the 1970s and 1980s. During the 1990s, major improvements were made in HMA specifications and design methods. Additionally, new technologies were adopted from Europe for stone matrix asphalt (SMA), gap-graded hot mix asphalt (GGHMA), and OGFC. These improvements and ideas have resulted in new methods for designing and constructing OGFC, notably the inclusion of polymer-modified asphalt binders and stabilizing materials, which have significantly improved the resulting mixtures and the overall performance of OGFCs. Stability additives, such as mineral filler now help prevent draindown and allow higher mixing temperatures. This allows better bonding between the OGFC and the underlying layer through the use of a proper tack coat, reducing the potential for delamination. Additionally, the higher mixing temperatures help to dry the aggregate more C H A P T E R 1 1 Design of Open-Graded Mixtures

completely, which reduces the potential for moisture damage and raveling. The use of polymer- modified binders in OGFCs has increased the durability of these mixes. All of these improvements have led to increased OGFC service life. Within the overall category of OGFC, there are two predominant types used in the United States, which can be generically called permeable friction courses (PFCs) and asphalt concrete friction courses (ACFCs). PFCs are an OGFC that are specifically designed to have high air void contents, typically in the range of 18 to 22%, which helps remove water from the pavement surface during a rain event. PFCs have been referred to as porous European mixes in the United States and are effective in improving frictional resistance, reducing splash and spray, improving nighttime visibility, reducing hydroplaning, and reducing pavement noise levels. The term ACFC is applied to OGFC mixes that are not specifically designed for removing water from the pavement surface. Some agencies within the United States use ACFCs as a wearing surface to simply improve frictional resistance and reduce tire/pavement noise levels. These agencies typically include 8 to 9% of rubber modified asphalt binders within ACFCs. Though ACFCs are designed to have relatively high air void contents (10 to 15%), they are not specifically designed to remove large volumes of water from the pavement surface. Of the two OGFC categories, ACFCs are likely more effective at reducing pavement noise levels. However, some agencies have become concerned about the durability of ACFCs, particularly when their air void contents approach 10%. PFCs have become the more common type of OGFC in the United States, and the remainder of this chapter deals specifically with the design of this mixture type. Overview of PFC Mix Design Procedure The design of PFC mixtures is similar to the design of SMA and GGHMA in that PFC should have stone-on-stone contact and low potential for draindown. However, because of the past problems dealing with durability, there is a laboratory test designed specifically to evaluate the potential durability problems of PFC mixtures. NCHRP Report 640 provides much useful information on the mix design, construction, and maintenance of permeable friction courses. The design of PFCs consists of five primary steps (Figure 11-1). The first step is to select suitable materials. Materials needing selection include coarse aggregates, fine aggregates, asphalt binder, and stabilizing additives. Step 2 includes blending three trial gradations using the selected aggregate stockpiles. For each trial gradation, asphalt binder is added and the mixture compacted. The third step in the mix design procedure entails evaluating the three compacted trial blends in order to select the design gradation. Next, the selected design gradation is fixed and the asphalt binder content is varied. The resulting mixtures are evaluated in order to select optimum asphalt binder. Finally, the design gradation at optimum asphalt binder content is evaluated for moisture susceptibility. This manual does not include provisions for performance testing of PFC mixtures, because there is limited experience at present in performing and interpreting rut resistance tests on this type of material. The information given in this chapter is largely taken from Design Construction and Performance of New-Generation Open-Graded Friction Courses, NCAT Report 00-01, by Mallic, Kandhal, Cooley, and Watson. This report is an excellent reference for technicians and engineers involved in the design of OGFC mixtures. Step 1—OGFC Materials Selection The first step in the PFC mix design procedure is to select suitable materials: coarse aggregates, fine aggregates, asphalt binder, and stabilizing additives. Aggregates used in PFC should be cubical, angular, and roughly textured. The stability and strength of PFCs are derived from the stone Design of Open-Graded Mixtures 195

196 A Manual for Design of Hot Mix Asphalt with Commentary Identify Materials Select MaterialsStabilizer Aggregates Asphalt Cement Select Trial Gradations Determine VCA of coarse aggregate in dry- rodded condition Medium % passing Break Point Sieve Low % passing Break Point Sieve High % passing Break Point Sieve Add asphalt cement and compact Analyze data and select optimum gradation Fix gradation and vary asphalt cement content Determine optimum asphalt cement content Conduct moisture susceptibility Within Specifications Step 1 Step 5 Step 4 Step 3 Step 2 Design of OGFC Mixtures Meet all specifications ? End No Yes Figure 11-1. Flow diagram illustrating PFC mix design methodology.

skeleton and, therefore, the shape and angularity should be such that the aggregates will not slide past each other. Angular, cubical, and textured aggregate particles will lock together providing a stable layer of PFC. Because of the open-graded aggregate structure, the aggregate surface area of PFCs is very low. Like GGHMA and SMA, PFC mixes are required to have a relatively high asphalt binder content. Therefore, the aggregates are coated with a thick film of asphalt binder and the properties of the asphalt binder are important to the performance of PFC. The asphalt binder must be very stiff at high temperatures to resist the abrading action of traffic; however, they should also perform at intermediate and low temperatures. Modified binders are not necessarily required; however, experience indicates better and longer service when modified binders are used. Because of the high asphalt binder content and low aggregate surface area, PFC mixes have a high potential for draindown problems. In order to combat the draindown problems, stabilizing additives are used. The most common stabilizing additive is fiber. Asphalt binder modifiers that stiffen the asphalt binder can also be a considered a stabilizing additive. However, fibers are more effective at reducing draindown potential. The following sections provide requirements for the various materials used in PFC mixes. These requirements are provided for guidance to agencies not having experience with these types of mix- tures. Some agencies have successfully used other test methods and criteria for specifying materials. Coarse Aggregate The success of a PFC pavement depends heavily on the existence of particle-on-particle contact. Therefore, in addition to particle shape, angularity, and texture, the toughness and durability of the coarse aggregates must be such that they will not degrade during production, construction, and service. Table 11-1 presents coarse aggregate requirements for PFC mixtures. Fine Aggregate The role of fine aggregates in a PFC is to assist the coarse aggregate particles in maintaining stability. However, the fine aggregates must also resist the effects of weathering. Therefore, the primary requirements for fine aggregates within a PFC are to ensure a durable and angular material. Requirements for fine aggregates for use in PFCs are provided in Table 11-2. Asphalt Binder Asphalt binders should meet the performance grade requirements of AASHTO M 320-04. Chapter 8 discussed binder selection for dense-graded HMA mixtures in detail; much of this Design of Open-Graded Mixtures 197 Test Method Spec. Minimum Spec. Maximum Los Angeles Abrasion, % Loss AASHTO T 96 - a Flat or Elongated, % ASTM D 4791 2 to 1 - 50 Soundness (5 Cycles), % AASHTO T 104 Sodium Sulfate - 15 Magnesium Sulfate - 20 Uncompacted Voids AASHTO T 326 45 - Method A aAggregates with L.A. Abrasion loss values up to 50 have been successfully used to produce OGFC mixtures. However, when the L.A. Abrasion exceeds approximately 30, excessive breakdown may occur in the laboratory compaction process or during in-place compaction. Table 11-1. Coarse aggregate quality requirements for PFC.

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 (12°C) 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 (6°C). 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 198 A Manual for Design of Hot Mix Asphalt with Commentary Test Method Spec. Minimum Spec. Maximum Soundness (5Cycles), % AASHTO T 104 Sodium Sulfate - 15 Magnesium Sulfate - 20 Uncompacted Voids AASHTO T 304, Method A 45 - Sand Equivalency AASHTO T 176 50 - Table 11-2. Fine aggregate quality requirements for PFC.

Design of Open-Graded Mixtures 199 Property Requirement Sieve Analysis Method A – Alpine Sieve 1 Analysis Fiber Length 6-mm (0.25 in.) Maximum Passing 0.150-mm (No. 100 sieve) 70+ 10% Method B – Mesh Screen 2 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 Content 3 18+ 5% non-volatiles pH 4 7.5+ 1.0% Oil Absorption 5 5.0+ 1.0% (times fiber mass) Moisture Content 6 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 650°C (1100 and 1200°F) 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 (1¼ 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 121°C (250°F) forced air oven for 2 hours. The sample is then re-weighed immediately upon removal from the oven. Property Requirement Size Analysis Fiber Lengt h 1 6-mm (0.25 in.) Maximum mean test value Thickness 2 0.005-mm (0.0002 in.) Maximum mean test value Shot Conten t 3 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. 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 75µm 0-4 0-4 0-4 Table 11-3. Cellulose fiber requirements. Table 11-4. Mineral fiber quality requirements. Table 11-5. PFC gradation specification bands.

200 A Manual for Design of Hot Mix Asphalt with Commentary 19.0 mm OGFC Gradation Requirements 0 10 20 30 40 50 60 70 80 90 100 Sieve Size, mm (raised to 0.45 power) Pe rc en t P as si ng 0.60.075 1.18 2.36 4.75 9.5 12.5 19 25 12.5 mm OGFC Gradation Requirements 0 10 20 30 40 50 60 70 80 90 100 Sieve Size, mm (raised to 0.45 power) Pe rc en t P as si ng 0.60.075 1.18 2.36 4.75 9.5 12.5 19 Figure 11-2. 19.0-mm PFC gradation requirements. Figure 11-3. 12.5-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: VCA G G DRC ca w s ca w = −γ γ γ 100 11 1( )-

where VCADRC = voids in coarse aggregate in dry-rodded condition γs = unit weight of the coarse aggregate fraction in the dry-rodded condition (kg/m3), γw = 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 Design of Open-Graded Mixtures 201 9.5 mm OGFC Gradation Requirements 0 10 20 30 40 50 60 70 80 90 100 Sieve Size, mm (raised to 0.45 power) Pe rc en t P as si ng 0.60.075 1.18 2.36 4.75 9.5 12.5 19 Figure 11-4. 9.5-mm PFC gradation requirements.

• 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 170±20 cSt. Compaction temperature will be the temperature required to provide an asphalt binder viscosity of 280±30 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 28°C 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. 202 A Manual for Design of Hot Mix Asphalt with Commentary Combined Aggregate Bulk Specific Gravity Minimum Asphalt Content 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 Table 11-6. Minimum asphalt content requirements for aggregates with varying bulk specific gravities—PFCs.

Sample Compaction Specimens should be compacted at the established compaction temperature after laboratory short-term aging. Laboratory samples of PFC are compacted using 50 SGC gyrations. More than 50 gyrations should not be used; PFC is relatively easy to compact in the laboratory and exceeding this compactive effort can cause excessive aggregate breakdown. Step 3—Selection of Optimum Gradation After the samples have been compacted, extruded, and allowed to cool, they are tested to determine their bulk specific gravity, Gmb, using dimensional analysis. Dimensional analysis entails calculating the volume of the sample by obtaining four height measurements with a calibrated caliper, with each measurement being 90 degrees apart. The area of the specimen is then multiplied by the average height to obtain the sample volume: where V = specimen volume, in3 h = specimen height, in D = specimen diameter, in Then Gmb is determined by dividing the dry mass of the sample by the sample volume. Uncom- pacted samples are used to determine the theoretical maximum density, Gmm (AASHTO T 209). Using Gmb, Gmm, and Gca, percent air voids, or voids in the total mixture (VTM) and VCAMIX are calculated. The VTM and VCAMIX are calculated by equations 3 and 4 below. where Pca = percent of coarse aggregate in the mixture Gmb= combined bulk specific gravity of the total aggregate Gca = bulk specific gravity of the coarse aggregate Once VTM and VCAMIX are determined, each trial blend mixture is compared to the PFC mixture requirements, which are presented in Table 11-7. The trial blend with the highest air void content that meets the 18% minimum and exhibits stone-on-stone contact is considered the design gradation. The Cantabro Abrasion test or draindown test may be required in order to select the design gradation. Step 4—Selection of Optimum Asphalt Binder Content Once the design gradation has been selected, it is necessary to evaluate various asphalt binder contents in order to select the optimum binder content. Additional samples are prepared using the design gradation and at least three asphalt binder contents. Eighteen samples are needed for this procedure. This provides for three compacted (for Gmb and Cantabro Abrasion Loss) and VCA G P G MIX mb ca ca = − ⎛ ⎝⎜ ⎞ ⎠⎟100 11 4  ( )- VTM G G mb mm = −⎛ ⎝⎜ ⎞ ⎠⎟100 1 11 3 ( )- V h D = × ⎛⎝⎜ ⎞⎠⎟π 2 11 2 2 ( )- Design of Open-Graded Mixtures 203

three uncompacted samples (one for determination of theoretical maximum density and two for draindown testing) at each of the three asphalt binder contents. Optimum asphalt binder content is selected as the binder content that meets all of the requirements of Table 11-7. Cantabro Abrasion Loss Test The Cantabro Abrasion test is used as a durability indicator during the design of PFC mix- tures. In this test, three PFC specimens compacted with 50 SGC gyrations are used to evaluate the durability of a PFC mixture at a given asphalt binder content. To begin the test, the mass of each specimen is weighed to the nearest 0.1 gram. A single test specimen is then placed in the Los Angeles Abrasion drum without the charge of steel spheres. The Los Angeles Abrasion machine is operated for 300 gyrations at a speed of 30 to 33 rpm and a test temperature of 25±5°C. After 300 gyrations, the test specimen is removed from the drum and its mass determined to the near- est 0.1 gram. The percentage of abrasion loss is calculated as follows: where PL = percent loss P1 = mass of specimen prior to test, gram P2 = mass of specimen after 300 gyrations, gram The test is repeated for the remaining two specimens. The average results from the three specimens are reported as the Cantabro Abrasion Loss. Resistance to abrasion generally improves with an increase in asphalt binder content or the use of a stiffer asphalt binder. Figure 11-5 illus- trates a sample after the Cantabro Abrasion Loss test. Draindown Sensitivity The draindown sensitivity of the selected mixture is determined in accordance with AASHTO T 305 except that a 2.36-mm wire mesh basket should be used. Draindown testing is conducted at a temperature of 15°C higher than the anticipated production temperature. Permeability Testing A laboratory permeability test is conducted on the selected PFC mixture. Laboratory permeabil- ity values greater than 100 m/day are recommended. Permeability of asphalt concrete mixtures can be measured using the provisional standard ASTM PS 129, Measurement of Permeability of Bituminous Paving Mixtures Using a Flexible Wall Permeameter. PL P P P = −( )1 2 2 100 11 5( )- 204 A Manual for Design of Hot Mix Asphalt with Commentary Property Requirement Asphalt Binder, % Table 10-6 Air Void Content, % 1 18 to 22 Cantabro Loss % 15 max. VCAMIX% Less than VCADRC Tensile Strength Ratio 0.70 min. Draindown at Production Temperature, % 0.30 max 1 Air void requirements are provided for PFC mixes but not ACFC mixes. Table 11-7. PFC mixture specification for SGC compacted designs.

Step 5—Moisture Susceptibility Moisture susceptibility of the selected mixture is determined using the modified Lottman method in accordance with AASHTO T 283 with one freeze-thaw cycle. The AASHTO T 283 method should be modified as follows: (1) PFC specimens should be compacted with SGC 50 gyrations; (2) no specific air void content level is required; (3) a vacuum of 26 inches Hg is applied for 10 minutes to saturate the compacted specimens, with no specific saturation level required; and (4) the specimens are kept submerged in water during the freeze-thaw cycle. Trouble Shooting PFC Mix Designs If the designer cannot produce a mixture that meets all requirements, remedial action will be necessary. Some suggestions to improve mixture properties are provided below. Air Voids The amount of air voids in the mixture can be controlled by the asphalt binder content. How- ever, lowering the asphalt binder content below the minimum to achieve a proper amount of air voids violates the required minimum asphalt binder content (Table 11-6). Instead, the aggregate gradation must be modified to increase the space for additional asphalt binder without decreas- ing the voids below an acceptable level. Decreasing the percent passing the breakpoint sieve will generally increase the air void content at a given asphalt binder content. Voids in the Coarse Aggregate If the VCAmix is higher than that in the dry-rodded condition (VCADRC), then the mixture gra- dation must be modified. This is typically done by decreasing the % passing the breakpoint sieve. Cantabro Abrasion Loss If the Cantabro Abrasion loss is greater than 15%, then either more asphalt binder or a binder with a greater high-temperature stiffness is needed. Design of Open-Graded Mixtures 205 Figure 11-5. Illustration of sample after Cantabro Abrasion test.

Moisture Susceptibility If the mixture fails to meet the moisture susceptibility requirements, lime or liquid anti-strip additives can be used. If these measures prove ineffective, the aggregate source or asphalt binder source can be changed to obtain better aggregate/asphalt binder compatibility. Draindown Sensitivity Problems with draindown sensitivity can be remedied by increasing the amount of stabilizing additive or by selecting a different stabilizing additive. Fibers have been shown to be very effective in reducing draindown. Bibliography AASHTO Standards AASHTO M 320, Performance-Graded Asphalt Binder AASHTO R 30, Mixture Conditioning of Hot-Mix Asphalt AASHTO T 19, Bulk Density (“Unit Weight”) and Voids in Aggregate AASHTO T 96, Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine AASHTO T 104, Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate AASHTO T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalency Test AASHTO T 209, Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures AASHTO T 245, Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus AASHTO T 283, Resistance of Compacted Asphalt Mixtures to Moisture-Induced Damage AASHTO T 304, Uncompacted Void Content of Fine Aggregate AASHTO T 305, Determination of Draindown Characteristics in Uncompacted Asphalt Mixtures AASHTO T 326, Uncompacted Void Content of Coarse Aggregate (As Influenced by Particle Shape, Surface Texture and Grading) Other Standards ASTM C 612, Mineral Fiber Block and Board Insulation ASTM D 4791, Flat Particles, Elongated Particles or Flat and Elongated Particles in Coarse Aggregate ASTM PS 129, Measurement of Permeability of Bituminous Paving Mixtures Using a Flexible Wall Permeameter Other Publications Cooley, L. A., et al. (2009) NCHRP Report 640: Construction and Maintenance Practices for Permeable Friction Courses, TRB, National Research Council, Washington, DC, 90 pp. Mallick, R. B., et al. (2001) Design Construction and Performance of New-Generation Open-Graded Friction Courses, NCAT Report 00-01. NCAT. Auburn University. Auburn, AL. NAPA (2002) Open-Graded Asphalt Friction Courses: Design, Construction and Maintenance (IS-115), Lanham, MD, 22 pp. 206 A Manual for Design of Hot Mix Asphalt with Commentary

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary incorporates the many advances in materials characterization and hot-mix asphalt (HMA) mix design technology developed since the conclusion of the Strategic Highway Research Program (SHRP).

The final report on the project that developed NCHRP Report 673 and Appendixes C through F to NCHRP Report 673 were published as NCHRP Web-Only Document 159. The titles of the appendixes are as follows:

• Appendix C: Course Manual

• Appendix D: Draft Specification for Volumetric Mix Design of Dense-Graded HMA

• Appendix E: Draft Practice for Volumetric Mix Design of Dense-Graded HMA

• Appendix F: Tutorial

The companion Excel spreadsheet HMA tool and the training course materials described in NCHRP Report 673 are available for download from the Internet.

In January 2012, TRB released NCHRP Report 714: Special Mixture Design Considerations and Methods for Warm Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot Mix Asphalt with Commentary. The report presents special mixture design considerations and methods used with warm mix asphalt.

In January 2012, TRB released an errata to NCHRP Report 673: Page 41, Table 4-7, and page 123, Table 8-10, respectively, should be replaced with a new table.

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