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Design of Gap-Graded HMA Mixtures 179 angularity by measuring uncompacted voids. However, the Rigden voids test is smaller in scale, and the mineral filler is compacted with a small drop hammer prior to determining the void content. Mineral fillers with very high Rigden voids can sometimes cause excessive stiffening in SMA mixtures. The equipment and test method for conducting the Dry Compaction Test can be found in the National Asphalt Pavement Association's Information Series 127, "Evaluation of Baghouse Fines for Hot Mix Asphalt." Other requirements for mineral fillers can be found in AASHTO M-17, "Mineral Fillers for Bituminous Paving Mixtures." However, the gradation requirements stated in AASHTO M-17 should only be used for guidance. The important gradation is that of the designed GGHMA and not the mineral filler. Stabilizing Additives Stabilizing additives are needed in GGHMA to prevent the draining of mortar from the coarse aggregate skeleton during storage, transportation, and placement. Stabilizing additives such as cellulose fiber, mineral fiber, and polymers have been used with success to minimize draindown potential. Other types of fibers have been used with success; however, the most common types are cellulose and mineral fibers. When using a polymer as a stabilizer, the amount of polymer added should be that amount necessary to meet the performance grade of the asphalt binder. Step 2--Trial Gradations As with any HMA, specified aggregate gradations should be based on aggregate volume and not aggregate mass. However, for most conventional HMA mixtures (dense-graded), the specific gravities of the different aggregate stockpiles are close enough to make a gradation based on mass percentages similar to that based on volumetric percentages. With GGHMA, the specific gravities of the different aggregate components are not always similar. This is especially true when commercial fillers are used in GGHMA. Therefore, the gradation bands presented in Table 10-3 are based on % passing by volume. Similar to those for dense-graded mixes, GGHMA gradation bands are described by the nominal maximum aggregate size (NMAS) of the gradation. The following section provides guidance in the form of an example problem on how to blend aggregate components based on volumes to meet the gradation bands in Table 10-3. However, if the bulk specific gravities of the different stockpiles (including mineral filler) used to compose the aggregate blend vary by 0.02 or less, gradations based on mass percentages can be used. Table 10-3. Stone matrix asphalt gradation specification bands (Percent Passing by Volume). Sieve 19-mm NMAS 12.5-mm NMAS 9.5-mm NMAS Size, mm Min. Max. Min. Max. Min. Max. 25.0 100 100 19.0 90 100 100 100 12.5 50 88 90 100 100 100 9.5 25 60 26 88 70 95 4.75 20 28 20 35 30 50 2.36 16 24 16 24 20 30 1.18 -- -- -- -- -- 21 0.6 -- -- -- -- -- 18 0.3 -- -- -- -- -- 15 0.075 8.0 11.0 8.0 11.0 8.0 12.0 Note: NMAS Nominal Maximum Aggregate Size one sieve size larger than the first sieve that retains more than 10%.

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180 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 10-1. Blending Aggregates to Meet GGHMA Gradation Requirements Washed Sieve Analyses As with any blending problem, the first step is to perform washed sieve analyses based on mass for the various stockpiles to be used in the GGHMA mixture, following procedures described in AASHTO T 27. For this example, a 19.0-mm GGHMA mixture is to be blended. Table 10-4 provides the results of washed gradation tests performed on four stockpiles, which are to be blended for this example problem. Also needed to determine aggregate gradations based on volume are the bulk specific gravities (Gsb) of the different stockpiles. Table 10-4 also provides the Gsb values for each stockpile. Notice that the Gsb values differ by more than 0.02 in Table 10-4. Determine Percent Mass Retained After performing the washed sieve analyses, the percent mass retained on each sieve for the different stockpiles is determined. For a given sieve, this is done by subtracting the percent passing the given sieve from the percent passing the next larger sieve. For example, using Aggregate C in Table 10-4, the % mass retained on the 4.75-mm sieve would be calculated as % Retained on 4.75 -mm Sieve = 84.6 - 48.9 = 35.7% Table 10-4. Results of gradation and specific gravity tests for stockpiles to be used in example GGHMA mix design. Stockpile and Percent Passing Based on Mass, % Sieve, mm Aggregate A Aggregate B Aggregate C Mineral Filler 25.0 100.0 100.0 100.0 100.0 19.0 95.0 100.0 100.0 100.0 12.5 66.0 71.0 97.4 100.0 9.5 43.0 46.0 84.6 100.0 4.75 9.0 6.0 48.9 100.0 2.36 5.0 4.0 27.8 100.0 1.18 2.0 4.0 16.6 100.0 0.60 2.0 3.0 10.7 100.0 0.30 2.0 3.0 7.6 100.0 0.075 1.0 1.5 4.6 72.5 Gsb 2.616 2.734 2.736 2.401 Table 10-5. Percent by mass retained on each sieve. Sieve, Percent Mass Retained per Sieve mm Aggregate A Aggregate B Aggregate C Mineral Filler 25.0 0.0 0.0 0.0 0.0 19.0 5.0 0.0 0.0 0.0 12.5 29.0 30.0 2.6 0.0 9.5 23.0 24.0 12.8 0.0 4.75 34.0 40.0 35.7 0.0 2.36 4.0 2.0 21.1 0.0 1.18 3.0 0.0 11.2 0.0 0.60 0.0 1.0 5.9 0.0 0.30 0.0 0.0 3.1 0.0 0.075 1.0 1.5 3.0 27.5 -0.075 1.0 1.5 4.6 27.5 Total, 100 100 100 100

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Design of Gap-Graded HMA Mixtures 181 Example Problem 10-1. (Continued) where 84.6 = percent mass passing 9.5-mm sieve (Table 10-4) 48.9 = percent mass passing 4.75-mm sieve (Table 10-4) 35.7 = percent mass retained on 4.75-mm sieve Table 10-5 presents the values of percent mass retained for all sieves and stockpiles. Note that a row has been added to reflect that material finer than the 0.075-mm (-0.075) sieve is included. Calculate Percent Mass Retained In this calculation a simple assumption is made, "Assume the Mass of Each Aggregate Stockpile is 100 grams." Using this assumption allows for the mass that would be retained of each size fraction for each stockpile to be determined and can be shown to be equal to the numbers shown in Table 10-5. Convert Percent Mass Retained to Volume per Sieve In this step of developing an SMA gradation, the values for percent mass retained determined previously are converted to volumes per sieve. To make this conversion, the bulk specific gravity of the individual stockpiles is needed. The volume of aggregate retained on each sieve for each stockpile can be determined from the following equation: Magg Vagg = Gsb w where Vagg = volume of aggregate retained on a given sieve, cm3 Magg = mass of aggregate retained on a given sieve, g w = unit weight of water (1.0 g/cm3) The following calculation demonstrates how the volume is calculated for the aggregate retained on the 4.75-mm sieve of Aggregate C. 35.7g Volume = 13.05 cm3 2.736 1.0 g cm3 where 35.7 g = mass of Aggregate C retained on 4.75-mm sieve (Table 10-5) 2.736 = bulk specific gravity of Aggregate C (Table 10-4) 1.0 g/cm3 = unit weight of water (w) 13.05 cm3 = volume of Aggregate C retained on 4.75-mm sieve The volumes retained on all sieves for each of the four stockpiles are provided in Table 10-6. (continued on next page)

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182 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 10-1. (Continued) Table 10-6. Volumes of aggregate retained on each sieve. Sieve, Volume of Aggregate Retained per Sieve, cm3 mm Aggregate A Aggregate B Aggregate C Mineral Filler 25.0 0.00 0.00 0.00 0.00 19.0 1.91 0.00 0.00 0.00 12.5 11.09 10.97 0.95 0.00 9.5 8.79 8.78 4.68 0.00 4.75 13.00 14.63 13.05 0.00 2.36 1.53 0.73 7.71 0.00 1.18 1.15 0.00 4.09 0.00 0.60 0.00 0.37 2.16 0.00 0.30 0.00 0.00 1.13 0.00 0.075 0.38 0.55 1.10 11.45 -0.075 0.38 0.55 1.68 30.20 Blend Stockpiles The values provided in Table 10-7 are used to blend the different stockpiles to meet the desired gradation based on volumes. This process is identical to blending stockpiles by mass and is a trial and error process. To perform the blending, select the estimated percentages of the different stockpiles to be used. For this example, the following percentages will be evaluated first: Stockpile % Blend Aggregate A 30 Aggregate B 30 Aggregate C 30 Mineral filler 10 The percent of each stockpile in the blend is multiplied by the volume retained on a given sieve for each stockpile to determine the total volume retained on Table 10-7. Total volumes retained per sieve. Volume Retained Sieve, mm per Sieve, cm3 25.0 0.00 19.0 0.57 12.5 6.90 9.5 6.67 4.75 12.20 2.36 2.99 1.18 1.57 0.60 0.76 0.30 0.34 0.075 1.75 -0.075 3.80 Total Volume, 37.55

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Design of Gap-Graded HMA Mixtures 183 Example Problem 10-1. (Continued) that sieve. Using the 4.75-mm sieve as an example, the total volume retained on the 4.75-mm sieve would be calculated as follows: Total Volume Retained on 4.7 - mm sieve = ( 0.30 13.00 ) + ( 0.30 14.63) + (0.30 13.05) + (0.10 0.00 ) = 12.20 cm3 where 0.30, 0.30, 0.30 and 0.10 are the percentages by mass of each aggregate in blend expressed as decimals; and 13.00, 14.63, 13.05, and 0.00 are the % volume retained on 4.75-mm sieve for each stockpile (Table 10-6). This calculation is performed for each of the sieves in the gradation. Table 10-7 presents the total volume retained for each of the sieves in the gradation. Now, based on the total volume retained per sieve and the summed total volume of the blended aggregates, the percent retained per sieve by volume can be determined for the blend. This is accomplished for a given sieve by dividing the volume retained on that sieve by the total volume of the blend. The following equation illustrates this calculation for the 4.75-mm sieve. %Volume Retained on 4.7-mm Sieve = 12.20 100 37.55 = 32.50% where 12.20 = volume retained on 4.75-mm sieve (Table 10-7) 37.55 = summed total volume of blend (Table 10-7) 32.50 = percent volume of blend retained on 4.75-mm sieve Table 10-8 provides the percents retained based on volumes for each of the sieves and converts this to percent volume passing. Using the % retained per sieve based on volume, the % passing by volume for the gradation can be determined similar to the method used with gradations based on mass. Determine the cumulative % retained for each sieve and then subtract from 100. Now, the blended gradation is compared to the required gradation band Table 10-8. Percent passing based on volumes. Percent Retained Cumulative Percent Passing by Sieve, mm Per Sieve Percent Retained Volume 25.0 0.0 0.0 100.0 19.0 1.5 1.5 98.5 12.5 18.4 19.9 80.1 9.5 17.8 37.7 62.3 4.75 32.5 70.1 29.9 2.36 8.0 78.1 21.9 1.18 4.2 82.3 17.7 0.60 2.0 84.3 15.7 0.30 0.9 85.2 14.8 0.075 4.7 89.9 10.1 -0.075 10.1 100.0 --- (continued on next page)

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184 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 10-1. (Continued) Table 10-9. Comparison of gradation blend based on volume to specified gradation band. Gradation Band Sieve, mm Requirements Blend Percent Passing 25.0 100 100 19.0 90-100 98.5 12.5 50-88 80.1 9.5 25-60 62.3* 4.75 20-28 29.9* 2.36 16-24 21.9 1.18 --- 17.7 0.60 --- 15.7 0.30 --- 14.8 0.075 8-10 10.1 * Does not meet requirements (also based on volume) provided in Table 10-3. Table 10-9 compares the gradation band for a 19.0-mm NMAS GGHMA to the gradation shown in Table 10-8. Based on Table 10-9, the blended gradation did not meet the specified gradation band for a 19.0-mm nominal maximum aggregate size GGHMA. Therefore, different blending percentages for the various stockpiles are needed. Below are the percentages of the four stockpiles used for the second trial. Stockpile % Blend Aggregate A 40 Aggregate B 41 Aggregate C 10 Mineral Filler 9 Table 10-10 presents the blended gradation of the four aggregates for the second trial. The second trial blend percentages were used along with the values of Table 10-6 to determine the percent passing by volume for the blend. Table 10-10. Percents passing based on volumes. Percent Cumulative Percent Sieve, Retained Percent Passing by Percent Passing by Gradation mm Per Sieve Retained Volume Mass Band by by Volume by Volume (For Comparison) Volume 25.0 0.0 0.0 100.0 100.0 100 19.0 2.0 2.0 98.0 98.0 90-100 12.5 24.0 26.0 74.0 74.3 50-88 9.5 20.1 46.1 53.9 53.5 25-60 4.75 33.2 79.3 21.7 20.0 20-28 2.36 4.5 83.7 16.3 15.4 16-24 1.18 2.3 86.0 14.0 13.1 --- 0.60 1.0 87.0 13.0 12.1 --- 0.30 0.3 87.3 12.7 11.8 --- 0.075 4.0 91.3 8.7 8.0 8-11 -0.075 8.7 100.0 --- --- ---

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Design of Gap-Graded HMA Mixtures 185 Example Problem 10-1. (Continued) Based on Table 10-10, the following percentages produce a gradation based on volume, which meets the 19.0-mm nominal maximum aggregate size gradation band for GGHMA. Stockpile % Blend by Mass Aggregate A 40 Aggregate B 41 Aggregate C 10 Mineral Filler 9 Selection of Trial Gradations When designing GGHMA mixtures, the initial trial gradations should be selected to be within the master specification range shown in Table 10-3. To design a GGHMA mixture it is recommended that at least three trial gradations be initially evaluated. It is suggested that the three trial grada- tions 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. The percent passing the 0.075-mm sieve should be approximately 10 percent for each trial gradation. Determination of VCA in the Coarse Aggregate Fraction For best performance, the GGHMA mixtures must have a coarse aggregate skeleton with stone-on-stone contact. The coarse aggregate fraction is not defined by a particular sieve size but rather is that portion of the total aggregate blend retained on the breakpoint sieve. The breakpoint sieve is defined as the finest (smallest) sieve to retain at least 10% of the aggregate gradation (Figure 10-4). 100.0 100.0 100.0 90.0 87.0 80.0 70.0 Percent Passing 60.0 59.2 50.0 40.0 35.5 30.0 Blend 2 Break Point Sieve 20.0 20.6 Finest Sieve to Have 12.3 12.8 13.5 at Least 10 Percent 10.0 10.5 7.5 Retained 0.0 .075 .15 .30 .60 1.18 2.36 4.75 9.5 12.5 19.0 25.0 Sieve Size (mm) Raised to 0.45 Power Figure 10-4. Definition of breakpoint sieve.

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186 A Manual for Design of Hot Mix Asphalt with Commentary Figure 10-5. Method of determining VCA in dry-rodded condition. The method of measuring the existence of stone-on-stone contact is called the voids in coarse aggregate (VCA) method. The concept is quite simple and practical. The first step is to determine the VCA of the coarse aggregate fraction only (material larger than breakpoint sieve) in a dry- rodded condition (VCADRC) (Figure 10-5). AASHTO T 19, "Bulk Density ("Unit Weight") and Voids in Aggregate," is used to compact the aggregate. Then, using Equation 10-1, the VCADRC can be calculated. The VCADRC is nothing more than the volume between the coarse aggregate parti- cles after compaction in accordance with AASHTO T 19. When asphalt binder is added and the GGHMA is compacted, the VCA will again be calculated (VCAMIX). This calculation for VCAMIX also calculates the volume between the coarse aggregate particles. As long as the volume between the coarse aggregate particles is less in the compacted GGHMA (VCAMIX) than the coarse aggregate only (VCADRC), then the GGHMA is deemed to have stone-on-stone contact and the aggregate structure is acceptable. This means that the GGHMA mixture has been compacted more than the dry-rodded condition of the aggregate; therefore, the coarse aggregate particles within the compacted mixture must be compacted closer than the dry-rodded condition and stone-on-stone contact exists. Gca w - s VCADRC = 100 (10-1) Gca w 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 fraction

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Design of Gap-Graded HMA Mixtures 187 Table 10-11. Minimum asphalt binder content requirements for aggregates with varying bulk specific gravities. Combined Aggregate Bulk Minimum 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 Selection of Target Asphalt Content The minimum desired asphalt binder content for GGHMA mixtures is presented in Table 10-11. This table illustrates that the minimum asphalt binder content is based on the combined bulk specific gravity of the aggregates used in the mix. These minimum asphalt binder contents are provided to ensure enough volume of asphalt binder exists in the GGHMA mix to provide a desirable mortar and, thus, a durable mixture. It is recommended that the mixture be designed at 0.3% above the minimum values given in Table 10-11 to allow for adjustments during plant production without falling below the minimum requirement. For example, for a GGHMA mixture to be made with an aggregate blend having a combined bulk specific gravity of 2.75, the minimum asphalt content is 6.0% by mass, and the target asphalt content would be 6.0 + 0.3 = 6.3% by mass. The minimum binder content values given in Table 10-11 have been calculated so that, in most cases, the resulting mixes will meet the suggested minimum VMA of 17.0% at 4.0% air voids for GGHMA mixtures. Sample Preparation As with the laboratory design of any HMA, the aggregates to be used in GGHMA should be dried to a constant mass and separated by dry-sieving into individual size fractions. The following size fractions are recommended: 37.5 mm to 25.0 mm 25.0 mm to 19.0 mm 19.0 mm to 12.5 mm 12.5 mm to 9.5 mm 9.5 mm to 4.75 mm 4.75 mm to 2.36 mm Passing 2.36 mm (for 25.0-, 19.0-, 12.5-, and 9.5-mm NMAS gradations) 2.36 mm to 1.18 mm (for 4.75-mm NMAS gradations) Passing 1.18 mm (for 4.75-mm NMAS gradations) After separating the aggregates into individual size fractions, they should be recombined at the proper percentages based on the gradation blend being evaluated. 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