Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 129


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 128
128 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 8-1. (Continued) Column 6 lists the composition of the mix by weight percentage, calculated using Equations 8-4 through 8-11. The procedure described above is tedious and prone to error if done by hand, so it is normally done with the aid of a spreadsheet designed to perform the calculations needed during the mix design process. It should be emphasized that when developing trial mix designs, calculations such as those given in Table 8-13 are only estimates of the actual mix composition. That is because the actual air void content and the amount of absorbed binder can only be accurately determined by making and testing HMA specimens in the laboratory. The air void content in this example was assumed to be the target value of 4%, even though it would be mostly luck if any of the trial mixtures produced exactly 4% air voids. As described above, the amount of absorbed asphalt is usually only estimated when designing initial trial batches; the actual amount of absorbed asphalt binder might vary significantly from this estimated value. These differences will usually mean that the actual mix composition will differ significantly from the initial estimate as calculated in this example. Step 10. Evaluate and Refine Trial Mixtures As discussed previously, when developing mix designs with new aggregates, three initial trial mixes are prepared, representing dense/fine, dense/dense, and dense/coarse aggregate gradations. When modifying an existing mix design, one or two trial batches might be prepared by making slight adjustments in the existing aggregate blend. In either case, the next step in the mix design process is the same: the trial mixtures must be evaluated to determine if any meet the given specifications or, if none meet all criteria, the mix closest to the specifications must be identified and the way it must be modified to produce an acceptable mix must be determined. Table 8-13 showed the results of an initial estimate of volumetric composition for a trial batch. But as described at the end of Step 9, the values in this table are only estimates--the actual composition for this and any other trial mixtures must be determined in the laboratory, by batching, mix- ing, compacting, and testing laboratory specimens for each of these trial mixtures. The steps involved in this process are described below, using the same trial mix design summarized in Table 8-13. Calculate Batch Weights Weights for trial batches are easily calculated from the mix composition by weight percent, as calculated in the example above and shown in Table 8-14. First, the number and size of gyratory specimens must be determined; for normal volumetric analysis, two specimens 150 mm in diameter by 115 mm high are normally required. For moisture resistance testing, as described later in this chapter, six specimens 150 mm in diameter by 95 mm high are needed. Some performance tests require compacted specimens 150 mm in diameter by 165 mm high. The total weight of material needed for a batch is then calculated using the following equation: Wmix = GmbVspec N spec (8-12)

OCR for page 128
Design of Dense-Graded HMA Mixtures 129 Example Problem 8-2. Calculating Trial Mix Batch Weights For example, for the trial mix described in Table 8-13, the bulk specific gravity is estimated to be 2.536. If two 150-mm-diameter by 115-mm-high cylinders are to be prepared, the amount of mixture needed is calculated as 2.536 2,439 2 = 12,369 grams. The weight of each component is then calculated by multiplying the total required by weight by the weight percentage of the mix component and dividing by 100%. Continuing with the same example, a table is easily constructed showing the weight percentages and the batch weights for each of the mix components. This is shown in Table 8-14; the weight percentages are the same as those listed in Table 8-13, although carried out to an extra decimal place for greater accuracy in calculating batch weights. Table 8-14. Example calculation of batch weights for mixture listed in table 8-13. Percent by Total Mix Batch Weight, Mix Component Weight grams (P) (12,369 P/100) Air --- --- Total Asphalt Binder 4.60 569 Absorbed Asphalt Binder --- --- No. 7 Traprock 22.90 2,832 Traprock screenings 27.67 3,422 Manufactured sand 27.67 3,422 Natural sand 14.31 1,770 Mineral filler 2.86 354 Total 100.00 12,369 Note: Calculations do not agree exactly because of rounding where Wmix = total weight of mix in batch, g Gmb = estimated bulk specific gravity of mix Vspec = Volume of specimen, cm3 = 2,439 for 150-mm diameter by 115-mm high (including 20% extra) = 2,015 for 150-mm diameter by 95-mm high (including 20% extra) = 3,499 for 150-mm diameter by 165-mm high (including 20% extra) Nspec = number of specimens, normally two Batch Aggregates Although the batch weights given in Table 8-14 could be used to weigh out material for the trial batch directly, this is not recommended because many aggregates tend to segregate during stockpiling, sampling, and handling in the laboratory, so that direct batching of aggregates will often produce specimens with aggregate gradations deviating significantly from the desired target gradation. For this reason, when handling and batching aggregates in the laboratory, they are often broken down and weighed in a number of size fractions. This helps ensure that the aggregate gradation actually used in the specimen is close to the target gradation. The way in which an aggregate is broken down is a matter of judgment and experience. Some engineers or

OCR for page 128
130 A Manual for Design of Hot Mix Asphalt with Commentary technicians may choose to completely break down aggregates, while some may break down aggre- gates into only a few size fractions. A typical and fairly conservative approach is to break down aggregates into the following size fractions: 37.5 to 50.0 mm 25.0 to 37.5 mm 19.0 to 25.0 mm 12.5 to 19.0 mm 9.5 to 4.75 mm 2.36 to 4.75 mm Passing 2.36 mm If there is less than 5% within one of these size fractions, it can be combined with an adjacent fraction. Mineral filler is not normally broken down prior to batching. HMA Tools includes the worksheet, "Batch," which calculates batch weights for different levels of aggregate processing. The technician enters which trial batch (out of up to seven) the batching sheet is being prepared for, the number and dimensions of laboratory specimens, and the desired percentage of extra material. The worksheet then provides the appropriate batch weights, including the binder weight and batch weights of each aggregate; coarse aggregates are broken down completely, while fine aggregates are broken down in three different ways--completely, partially (by groups of two sieves), and with no breakdown (that is, a single weight for each fine aggregate). The technician performing the mix design can select among the three different ways of breaking down the fine aggregate in the batching process. Example Problem 8-3. Breaking Down Aggregates and Calculating Aggregate Batch Weights An example of aggregate breakdown and batching is shown in Tables 8-15 and 8-16, using the same example problem described in Tables 8-13 and 8-14. Table 8-15 lists the gradations for the coarse and fine aggregates for the example problem. Table 8-16 describes the breakdown and lists the batch weights for each of the four aggregates. The No. 7 traprock is broken down into five size fractions, ranging from the 12.5- to 19.0-mm fraction to the passing 2.36-mm fraction. The screenings are broken down into three fractions: 4.75 to 9.5 mm, 2.36 to 4.75 mm and passing 2.36 mm. The two sands are both broken down into two Table 8-15. Aggregate gradations for example batching problem. Sieve Weight Percent Passing for Aggregate: Size No. 7 Traprock Manufactured Natural Mineral (mm) Traprock Screenings Sand Sand Filler 19.0 100 100 100 100 100 12.5 93 100 100 100 100 9.5 53 100 100 100 100 4.75 32 90 97 99 100 2.36 9 57 78 91 100 1.18 2 34 51 70 100 0.600 1 25 32 53 100 0.300 1 19 14 29 100 0.150 1 14 11 17 92 0.075 1 8 7 9 79

OCR for page 128
Design of Dense-Graded HMA Mixtures 131 Example Problem 8-3. (Continued) Table 8-16. Aggregate breakdown and batch weight for example 3. Batch Size Fraction Wt. % Weight (g) No. 7 Traprock 12.5 to 19.0 mm 7 198 9.5 to 12. 5 mm 40 1,133 4.75 to 9. 5mm 21 595 2.36 to 4.75 mm 23 651 Passing 2.36 mm 9 255 Total 100 2,832 Traprock Screenings 4.75 to 9. 5 mm 10 342 2.36 to 4.75 mm 33 1,129 Passing 2.36 mm 57 1,950 Total 100 3,421 Manufactured Sand 2.36 to 9.5 mm 22 753 Passing 2.36 mm 78 2,669 Total 100 3,422 Natural Sand 2.36 to 9.5 mm 9 160 Passing 2.36 mm 91 1,611 Total 100 1,771 Mineral Filler Passing 0.075 mm 100 354 fractions: 2.36 to 9.5 mm and passing 2.36 mm. For both sands, the 4.75 to 9.5 fraction contained less than 5%, and this fraction was combined with the 2.36- to 4.75-mm fraction to create a 2.36- to 9.5-mm fraction. The mineral filler does not need to be further broken down, and the batch weight is as given in Table 8-14. Heat Aggregates and Asphalt Binder After batching out the aggregates, they are combined in a suitable metal container and heated in an oven to the desired compaction temperature. Asphalt binder is also heated to the same temperature. The binder will be weighed out into the aggregate after it has been thoroughly heated; the amount placed in the oven should be more than enough to provide for the given batch weight (569 g in this example). It is essential that the aggregates and binders are thoroughly heated to the proper temperature before mixing. For non-modified binders, the mixing and compaction temperatures are calculated on the basis of binder viscosity. The mixing temperature range is that providing a binder viscosity of from 150 to 190 Pa-s, while the compaction temperature range is that providing a binder viscosity of from 250 to 310 Pa-s. For non-modified binders, mixing and compaction temperatures can be estimated using a viscosity-temperature chart, as shown in Figure 8-7. Log viscosity is plotted against temperature, and a curve fit through the data points. The mixing and compaction temperature ranges can then be estimated from the chart as shown. For modified binders, the manufacturer should supply information concerning the mixing and compaction temperature for HMA made with their product. Mixing and compaction temperatures are usually provided by suppliers as part of the specification data provided on the bill of lading for a given binder.

OCR for page 128
132 A Manual for Design of Hot Mix Asphalt with Commentary 10,000 Viscosity, mPa-s 1,000 250 to 310 mPa-s 100 150 to 190 mPa-s compaction mixing temperature temperature 138 to 144 C 150 to 156 C 10 100 110 120 130 140 150 160 170 180 Temperature, OC Figure 8-7. Example viscosity-temperature chart showing determination of mixing and compaction temperature ranges for a non-modified binder. A major research project on mixing and compaction temperatures for HMA was completed as this manual was being completed; the results have been compiled in NCHRP Report 648: Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. Two new promising procedures for determining mixing and compaction temperatures were recommended for further evaluation. The phase angle method involves developing a high-temperature master curve for binder phase angle, determining the frequency where the phase angle is 86 degrees and then applying empirical equations to determine mixing and compaction temperatures. In the steady shear method, viscosity is determined at high shear stresses over a range of temperatures. Viscosity values at a shear stress of 500 MPa are then plotted on a log viscosity versus log temperature chart to determine mixing and compaction temperatures. At the time this manual was being completed, neither method had been accepted as an AASHTO standard, but it is possible than one or both methods could be adopted in the future. Heating materials prior to compaction will typically take from 2 to 4 hours, but the actual time required to heat aggregates and binders to reach the specified mixing temperature will vary considerably depending on the size and type of oven used, the amount of material being heated, and the properties of the aggregate. The oven should be set to a temperature about 15C above the mixing temperature range. The actual temperature of the aggregates and binder should be checked prior to mixing and compaction with a properly calibrated electronic thermometer. Mixing bowls, mixing paddles/stirrers, and gyratory compaction molds must also be heated to the same compaction temperature range prior to compacting specimens. Mix Aggregate and Asphalt Binder Because the use of 150-mm-diameter specimens requires very large batch sizes, laboratory mixing should be done with a large, heavy-duty mechanical mixer. Mixing should be done quickly and efficiently, so that the materials do not cool significantly before mixing is completed. If the mix is the first one to be prepared that day, the mixer should be "buttered" first. This is important because significant amounts of binder and fine aggregate will stick to the bowl and stirrer during mixing. If the mixer is not buttered first, binder and fines will be removed from the batch, and its composition will not be as designed. The mixer can be buttered either by mixing a batch of HMA or sand asphalt that is then discarded. The composition of these materials is

OCR for page 128
Design of Dense-Graded HMA Mixtures 133 Figure 8-8. Typical mixer used in preparing HMA specimens in the laboratory. not important--their only purpose is to coat the mixing bowl and stirrer with binder and fine aggregate. The mixer need only be buttered prior to the first batch of the day. After that, the mixing bowl and stirrer should remain well coated as additional batches are prepared, so that additional buttering is not needed. The procedure for actual mixing of HMA in the laboratory is as follows. Place the heated bowl on an electronic balance, and zero the balance. Form a depression in the center of the aggregate, and weight out the appropriate amount of hot binder into the aggregate. Place the bowl with aggregate and asphalt on the mixer stand, attach the heated stirring attachment, and begin mixing. Mix just until the aggregate is thoroughly coated with binder--too much mixing can cause the aggregate to break down, changing the aggregate gradation in the specimen. The mix is now ready for short-term oven conditioning, as described below. Figure 8-8 shows a typical mixer used in preparing HMA specimens in the laboratory. When HMA is produced in a plant, it is not immediately placed and compacted. Often it is held in a silo, placed in a truck, hauled to the site, and then placed and compacted. During this time period the hot aggregate in the mix may absorb significant amounts of asphalt binder, potentially changing the composition and properties of the mix. Short-term oven conditioning of HMA in the laboratory, as described below, is designed to imitate the absorption of binder that occurs during actual production. Short-Term Oven Conditioning The procedure for performing short-term oven conditioning is described in AASHTO R 30, Mixture Conditioning of Hot-Mix Asphalt. Immediately after mixing the aggregate and asphalt, place it in a shallow metal pan, spreading it out evenly until the depth is between 25 and 50 mm. Place the mix in a forced-draft oven, pre-heated to within 3C of the midpoint of the compaction

OCR for page 128
134 A Manual for Design of Hot Mix Asphalt with Commentary Figure 8-9. Short-term oven conditioning of HMA mixture in the laboratory. temperature range for the mixture. Condition the mix for a total time of 2 hours 5 minutes, stirring after 1 hour 5 minutes. For mixtures having a water absorption value over 2%, the conditioning time should be extended to 4 hours 5 minutes, and the mixture should be stirred every hour. Also, if the mix is to be used to prepare specimens for performance testing, the con- ditioning time should be 4 hours 5 minutes at 135C, regardless of the aggregate absorption. Specimens are compacted immediately after completion of short-term oven conditioning. Figure 8-9 shows HMA mixture spread out in a pan for short-term oven conditioning. Compact Laboratory Specimens In this mix design method, specimens are compacted using the Superpave gyratory compactor (SGC), using the standard angle of gyration of 1.25, and a compaction pressure of 600 kPa, as described in AASHTO T 312, Preparing and Determining the Density of Hot-Mix Asphalt Specimens by Means of the Superpave Gyratory Compactor. It is essential that the SGC is properly maintained and calibrated; engineers and technicians should refer to appropriate specifications and the manufacturer's instructions for information on maintaining and calibrating their device. Prior to compacting specimens, make sure that the mold, top plate, and base plate have been heated to the compaction temperature for the mix. This generally takes about an hour in an oven set to the compaction temperature. The HMA mix must be short-term conditioned, as described above, prior to compaction. Remove the molds and plates from the oven, place the base plate inside the mold and place a paper disk on the base plate. Then, place the hot mixture in the mold, level, and cover with another paper disk. Place the top plate over the paper disk, and place the mold in the compactor. Set the compactor to the appropriate level of Ndesign (see Table 8-2) and compact the specimen. After compaction is complete, remove the mold from the SGC, and then remove the specimen from the mold. SGCs are normally equipped with a sample press for extruding compacted specimens from molds. Removing the specimens should be done slowly to avoid distorting or even breaking the specimen. Waiting a few minutes after completing compaction to allow the specimen to cool can help prevent damage to the specimen during de-molding. Specimens compacted to high air void levels--about 6% or more--can be even more prone to damage during de-molding and may require additional cooling before removal from the SGC mold.

OCR for page 128
Design of Dense-Graded HMA Mixtures 135 Figure 8-10. Mold, top and base plates, and paper disks used in compacting specimens with the superpave gyratory compactor. Remove the paper disks from the top and bottom of the specimen and allow the specimen to cool at room temperature. Handle freshly compacted specimens carefully to avoid damaging them. Specimens must be completely cool prior to performing bulk specific gravity tests, as required for volumetric analysis. Figure 8-10 shows an SGC mold, top and base plates, and the paper disks used in compacting specimens. Calculate Volumetric Composition of Laboratory Specimens Chapter 5 of this manual described in detail volumetric analysis of HMA in the laboratory, including the primary tests involved--bulk and maximum theoretical specific gravity of HMA mixtures. Interested readers or those unsure of the details of these tests and the calculations used in volumetric analysis may wish to review Chapter 5. Some of the calculations are similar to those used in estimating the composition of trial batches as presented previously in Step 9. The information presented here is meant only to be a brief review of the major features of volumetric analysis. Volumetric analysis of compacted HMA mixtures involves two laboratory tests: bulk specific gravity of the compacted HMA mixture and theoretical maximum specific gravity of the loose HMA mixture. As discussed in Chapter 5 of this manual, there are two procedures for determining bulk specific gravity of HMA mixtures: AASHTO T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens, and AASHTO T 275, Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin- Coated Specimens. AASHTO T 166 can be used for most HMA mixtures; however, if the absorption of the specimens during AASHTO T 166 is greater than 2.0%, AASHTO T 275 should be used. The procedure for theoretical maximum specific gravity is given in AASHTO T 209, Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures. Bulk specific gravity gives the specific gravity of the compacted specimen, including air voids within the mixture. The theoretical maximum specific gravity is the specific gravity of the

OCR for page 128
136 A Manual for Design of Hot Mix Asphalt with Commentary mixture at zero air voids; if a laboratory specimen could be compacted to zero air voids, the bulk and theoretical maximum specific gravity values would be equal. One of the most important equations used in volumetric analysis of HMA is Equation 8-13, which relates air void content, bulk specific gravity of the compacted mixture, and maximum theoretical specific gravity: G VA = 100 1 - mb (8-13) Gmm where VA = air void content, volume % Gmb = bulk specific gravity of compacted mixture Gmm = maximum theoretical specific gravity of loose mixture Various other equations given in Chapter 5 are used to calculate other important properties or volumetric factors of the mixture: Total asphalt content by weight (Pb) Effective asphalt content by weight (Pbe) Absorbed asphalt content by weight (Pba) Total asphalt content by volume (VB) Effective asphalt content by volume (VBE) Absorbed asphalt content by volume (VBA) Voids in the mineral aggregate (VMA) Voids filled with asphalt (VFA) Dust/binder ratio (D/B) Apparent film thickness (AFT) The normal practice in HMA mix design is to determine the bulk specific gravity of two com- pacted specimens, and then heat these specimens and break them up and use the resulting loose mixture to determine the maximum theoretical specific gravity of the mixture. Alternately, extra loose mixture can be prepared when the specimens are compacted and used in the determina- tion of maximum specific gravity. Actual calculation of air void content, VMA, and other volu- metric factors is usually done using a spreadsheet such as HMA Tools. Most SGCs also include spreadsheets for performing these calculations. Values for specified volumetric factors are then compared to the requirements for the mixture. In a complete mix design for new materials, this comparison is made for three trial mixtures made at widely different coarse aggregate contents, to determine the aggregate blend that will provide the proper volumetric composition. In many cases, an existing mix design is being slightly modified, and only one or two trial mixtures will be evaluated. Example Problem 8-4. Volumetric Analysis of an HMA Mixture Table 8-17 summarizes a typical volumetric analysis as performed in the evaluation of three trial mixes. The dense/ fine mixture in Table 8-17 is the same trial mix used in the examples given in Tables 8-13 through 8-16. The other two trial mixes--the dense/dense and dense/coarse--have been developed using the same binder and same aggregates, but blended in different proportions, as listed in Table 8-18; this table also includes proportions for a fourth trial mix, discussed below. Table 8-17 shows the specific gravity test data and calculations and the results of volumetric analysis for all three trial mixtures. Table 8-17 includes specification limits, and also lists equations that can be used for calculating the various volumetric factors, such as air void content and VMA.

OCR for page 128
Design of Dense-Graded HMA Mixtures 137 Example Problem 8-4. (Continued) Table 8-17. Summary of volumetric analysis for example HMA mix design. Specification Trial Mix 1: Trial Mix 2: Trial Mix 3: Property Equation Min. Max. Dense/Fine Dense/Dense Dense/Coarse Mix Mix Mix Bulk Specific Gravity of Compacted Mixture Dry mass in air, g --- --- --- 5,221.0 5,190.3 5,135.7 5,392.8 5,321.5 5,175.7 Saturated, surface-dry mass in air --- --- --- 5,241.1 5,211.4 5,153.0 5,414.4 5,343.6 5,212.3 Mass in water --- --- --- 3,160.9 3,140.3 3,172.9 3,324.2 3,228.7 3,137.2 Bulk specific gravity, dry basis 5-1 --- --- 2.510 2.506 2.594 2.580 2.516 2.494 Theoretical Maximum Specific Gravity of Loose Mixture Dry mass in air, g --- --- --- 2,109.7 2,245.5 2,225.8 2,156.9 2,076.4 2,332.7 Mass in water --- --- --- 1,312.3 1,394.3 1,394.0 1,352.9 1,312.0 1,471.5 Theoretical maximum specific gravity 5-2 --- --- 2.646 2.638 2.676 2.683 2.716 2.709 Average --- --- --- 2.642 2.679 2.713 Volumetric Analysis Aggregate bulk specific gravity, dry basis 5-3 --- --- 2.846 2.879 2.915 Air void content, Vol. % 5-4 3.5 4.5 5.1 3.5 7.6 VMA, Vol. % 5-11 14.0 16.0 15.9 14.2 17.9 Asphalt content, Wt. % 5-5 --- --- 4.60 4.55 4.53 Effective asphalt content, Wt. % 5-9 --- --- 4.44 4.27 4.22 Effective asphalt content, Vol. % 5-8 --- --- 10.9 10.8 10.3 VFA, Vol. % 5-12 --- --- 68.2 75.7 57.4 Mineral filler (dust) content, Wt. % 8-10 --- --- 7.9 6.6 5.1 Dust/binder ratio 8-11 0.8 1.6 1.79 1.54 1.20 Table 8-18. Aggregate proportions for trial mixes listed in Table 8-17. Trial Mix Trial Mix Trial Mix Trial Mix Aggregate No. 1: No. 2: No. 3: No. 4: Dense/Fine Dense/Dense Dense/Coarse Dense/Fine No. 7 Traprock 24 45 68 58 Screenings 29 21 12 16 Manufactured sand 29 21 12 16 Natural sand 15 10 5 8 Mineral dust 3 3 3 2 Most often, calculations such as those used in compiling Table 8-17 are done using a spreadsheet developed for this purpose, such as HMA Tools or spreadsheets included with many SGCs. In HMA Tools, specific gravity data for the trial batches are entered in the worksheet "Specific_Gravity"; the necessary calculations are performed and the resulting volumetric composition is summarized in the worksheet "Trial_Blends." Three specified volumetric factors are shown in Table 8-17 and included in most volumetric analyses: Air void content VMA Dust/binder ratio The air void content in this example has an allowable range of 3.5 to 4.5%. This range has been established for practical purposes, since it is very difficult to match the target air void content of 4.0% exactly. Also, it must be realized that laboratory mix designs almost always must be adjusted during field production, so attempting to exactly meet air void requirements in a laboratory mix design is usually pointless. VMA requirements for dense-graded HMA mixtures are given in Table 8-5; the allowable range for VMA for a 12.5-mm NMAS mixture is from 15.0 to 17.0%. The specified range for dust/binder ratio is 0.8 to 1.6, as given in Table 8-12 previously.

OCR for page 128
138 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 8-5. Adjusting an HMA Trial Mixture Looking at the three specified volumetric factors in Table 8-17, the dense/dense trial mixture appears to meet all specification requirements--the average air void content of 3.5% is acceptable, as is the average VMA of 14.2%; and the dust/binder ratio of 1.54% is also within the specified range. As long as the workability of this mix is acceptable, it would be an acceptable final mix design. However, the air void content, VMA, and dust/binder ratio values are all close to various limits. It is therefore desirable to adjust this mixture to obtain values closer to the midpoint for air voids, VMA, and dust/binder ratio. In deciding how to adjust the composition of the fourth trial mix, it is helpful to present the data in Tables 8-17 and 8-18 graphically. Figure 8-11 shows the gradation plot (top) and CMD plot (bottom) for the three initial trial mixes. This figure also includes the fourth trial mix, shown as the dashed line. The plots are 100 Trial, voids/VMA No. 1, 5.1/15.9 80 % Passing by Weight No. 2, 3.5/14.2 No. 3, 7.6/17.9 60 Trial No. 4 Min., 3.5 / 14.0 40 Max., 4.5 /16.0 Max. Dens. 20 0 0.01 0.10 1.00 10.00 100.00 1000.00 Particle Size, mm coarse dense fine Deviation from Maximum Density Gradation 15 0 -15 0.01 0.10 1.00 10.00 100.00 Particle Size, mm Figure 8-11. Top: gradation plot for example mix design, including fourth trial mix; bottom: CMD plot for example mix design.

OCR for page 128
Design of Dense-Graded HMA Mixtures 139 Example Problem 8-5. (Continued) as produced in HMA Tools, though similar plots can be prepared using other spreadsheets or software packages. The legend, given in the top plot, includes the air void content and VMA values for each of the initial three trial mixes, as listed in Table 8-18. It also includes the minimum and maximum for air void content and VMA, given in the legend under "Min." and "Max." As noted above, the second trial mix--the dense/dense mix--meets all requirements for the mix design. However, both the measured air void content (3.5%) and the VMA value of 14.1% are at or very near to the minimum values of 3.5% and 14.0%, respectively. It appears that the air void content and VMA could be increased by either making the gradation slightly finer or slightly coarser. However, because the dust/binder ratio of the dense/fine mix is too high (and only marginal for the dense/dense trial mix), making the fourth trial mix slightly coarser will ensure that the dust/binder ratio remains within allowable limits. The aggregate blend for the fourth trial mix is therefore designed to be intermediate between the dense/dense gradation and the dense/coarse gradation, as is shown in Figure 8-11. It also has slightly less mineral dust (2% rather than 3%) in order to keep the dust/binder ratio near the center portion of the specification. The next step in the mix design process is to calculate mix proportions and batch weights for the fourth trial mix, in the same way as was done for the initial trial mixtures. Two specimens are then prepared and their bulk and theoretical maximum specific gravity values determined. A second volumetric analysis is performed to determine if this fourth trial mix better meets the specified requirements. The results of specific gravity tests and the resulting volumetric analysis are shown in Table 8-19. The specification properties--air void content, VMA and dust/binder ratio--are well within the specified range. The air void content and VMA are slightly high, but this is desirable since both will tend to drop during field production. Therefore, trial mix four is accepted as the final laboratory mix design. Table 8-19. Summary of volumetric analysis trial mix four for example HMA mix design. Specification Dense/Fine Property Min. Max. Mix Dry mass in air, g --- --- 5,221.3 5,182.9 Saturated, surface-dry mass in air --- --- 5,237.5 5,206.6 Mass in water --- --- 3,214.6 3,200.4 Bulk specific gravity, dry basis --- --- 2.581 2.583 Dry mass in air, g --- --- 2,115.7 2,268.2 Mass in water --- --- 1,328.6 1,422.7 Theoretical maximum specific gravity --- --- 2.651 2.658 Average --- --- 2.685 Aggregate bulk specific gravity, dry basis --- --- 2.900 Air void content, Vol. % 3.5 4.5 3.8 VMA, Vol. % 14.0 16.0 15.0 Asphalt content, Wt. % --- --- 4.59 Effective asphalt content, Wt. % --- --- 4.45 Effective asphalt content, Vol. % --- --- 11.2 VFA, Vol. % --- --- 74.5 Mineral filler (dust) content, Wt. % --- --- 5.0 Dust/binder ratio 0.8 1.6 1.13

OCR for page 128
140 A Manual for Design of Hot Mix Asphalt with Commentary Adjusting Aggregate Proportions to Meet VMA and Other Volumetric Requirements The procedure described here for adjusting aggregate proportions to meet the given require- ments for VMA and air void content is straightforward. However, a short discussion may help inexperienced technicians and engineers better understand this important topic. First, it should be again emphasized that the procedure presented here is only one of many possible techniques for preparing aggregate blends for trial mixtures prepared during the mix design process. This manual is in large part intended as an instructional tool for the inexperienced; for this reason, the approach given here is relatively simple and flexible and relies on HMA Tools for performing cumbersome calculations. Technicians and engineers who have successfully used other procedures with consistently satisfactory results should continue to use them. Those who try the procedures given here, but think they need some additional tools should look into other procedures; as discussed earlier in this chapter, the Bailey method has recently become a very popular method for blending aggregates to meet volumetric requirements. The relationship between VMA, air void content, and effective asphalt content must be under- stood to fully appreciate the procedure given in this manual. VMA is composed of air voids and effective asphalt (a small amount of asphalt binder is absorbed into the aggregate surface). There- fore, if the target VMA is fixed, once the target air void content is met, the effective asphalt content is also met. Therefore, there is no need to simultaneously evaluate air void content and VMA--once the proper air void content is obtained, the VMA level will also meet requirements. HMA Tools makes this calculation for the user, so there is no need to calculate the binder content. However, there is some flexibility in selecting target values for VMA and air void content, which indirectly allows for adjustments in asphalt binder content. Lower VMA values will give less binder, higher VMA values will give more binder. Lower air void contents will provide additional binder at a given VMA value, while higher air void contents will provide less binder at a given VMA value. In the first trial batch in a series, HMA Tools assumes that the amount of absorbed binder is one-half the calculated water absorption (calculated from aggregate bulk and apparent specific gravity values). However, after this first trial batch, HMA Tools compares estimated absorption values with those actually measured in the laboratory and adjusts absorption values in subsequent batches accordingly. The general rule given here for adjusting aggregate blends to meet VMA requirements is that the closer an aggregate gradation is to a maximum density gradation, the lower will be its VMA. Technicians and engineers should remember that this is only an approximate rule. The maximum density gradation is only approximated by the 0.45 power law; the actual maximum density gradation for a given set of aggregates may deviate significantly from this. Furthermore, some aggregates have unique properties that will affect mixture VMA in unusual ways. For example, relatively soft aggregates can break down during compaction--especially at high gyration levels-- making it difficult to reach high VMA values. Other aggregates, with unusually good texture or angular shape, will tend to increase VMA, even when their addition would seem likely to make the aggregate blend denser. The specifications given in this manual for dense-graded HMA may require some mix designs to be adjusted by increasing the mineral filler content. Although a certain amount of mineral filler is necessary for good rut resistance and durability, adding mineral filler to a mix design will normally tend to reduce VMA. Thus, adding mineral filler to a mix design will require adjusting the gradation to provide additional VMA to compensate for the effect of increasing mineral filler. Aggregate blending is one of the most critical aspects of the HMA mix design process, and proficiency requires practice, experience, and judgment. Conduct Performance Testing as Required The final stage of laboratory work in an HMA design involves evaluating the performance of the mixture. Chapter 6 of this manual presents a thorough discussion of various factors

OCR for page 128
Design of Dense-Graded HMA Mixtures 141 affecting HMA performance and ways of evaluating this performance through mixture testing and analysis. The following discussion is limited to the practical application of performance test- ing as part of the routine design of dense-graded HMA mixtures. This involves the evaluation of (1) moisture resistance for all mixtures and (2) evaluation of rut resistance for mixtures designed for traffic levels of 3 million ESALs and higher. As discussed in Chapter 6, more advanced types of performance testing, such as the IDT creep and strength test and fatigue testing, are in general not suitable for use in routine mix design, though they may be useful in research and in developing HMA mixtures for critical or special applications. Evaluate Moisture Resistance The moisture resistance of all dense-graded HMA mixtures should be evaluated using AASHTO T 283. Moisture resistance testing is normally performed after a mix design has been developed that meets all requirements for binder grade, mixture composition, and compaction. As also discussed in Chapter 6, in AASHTO T 283, specimens are prepared to an air void content of 7.0 0.5%, then divided into two subsets with approximately equal average air void contents. The tensile strength of one subset is measured dry. The tensile strength of the second subset is measured after conditioning by vacuum saturation followed by a freeze-thaw cycle and a warm-water soak. The ratio of the average tensile strength of the conditioned to unconditioned subsets and a visual assessment of stripping is used to assess moisture sensitivity. A mixture is considered acceptable if the tensile strength ratio is equal to or greater than 80% and there is no visual evidence of stripping in the conditioned test specimens. There are several ways of improving the performance of mixtures initially failing these require- ments for moisture resistance. Small amounts of anti-strip additives can be added to the mixture. Anti-strip suppliers should be contacted concerning recommended products and concentrations for a given HMA mix design. Local hot-mix suppliers may be able to offer suggestions concerning the most effective anti-strip additives for local materials. In many cases, the binder supplier can blend an appropriate anti-strip additive at the necessary concentration directly into the binder. This is a simple but effective approach that requires no special equipment at the plant. One of the most effective and least expensive anti-strip additives is hydrated lime. When used to help prevent moisture damage, hydrated lime should be blended with water to form a slurry and applied directly to the aggregate prior to heating. The typical concentration is 1% hydrated lime by aggregate weight. If used in a laboratory mix design, a slurry composed of 50% hydrated lime and 50% water by weight is prepared and applied to the aggregate prior to heating. Besides anti-strip additives, the other ways of improving moisture damage are to change the binder, the aggregate, or both materials. Different binders can exhibit a wide range in susceptibility to moisture damage. Aggregates that are most susceptible to moisture damage are those that contain significant amounts of quartz, including many igneous rocks. Eliminating these aggregates from a mix design can, in some cases, significantly improve moisture resistance. Evaluate Rut Resistance Before discussing rut resistance testing in detail, it must be noted that the design procedure set forth in this manual has been structured to provide HMA mix designs that will exhibit a high level of rut resistance. The level of reliability against excessive rutting--even without performance testing--ranges from 90 to over 99%, with a typical level of about 95% reliability for design traffic levels of 3 million ESALs and higher. The purpose of rut resistance testing is to increase this level of reliability. For three of the rut resistance tests discussed below--the flow number from the asphalt mixture performance tester (AMPT), the repeated shear at constant height (RSCH) test, and the high-temperature indirect tension (HT-IDT) strength test--the suggested minimum or maximum test values were determined specifically to increase the level of reliability against excessive rutting from about 95 to 98% and higher. It must be emphasized that the reliability

OCR for page 128
142 A Manual for Design of Hot Mix Asphalt with Commentary achieved through the recommended performance tests is a result of applying both the suggested mix design procedure and the selected performance test together. If the given guidelines for performance test results are applied to mixtures designed following some other procedure, the resulting level of reliability will not necessarily be the same. It might be similar, or it might be lower or higher. It should also be noted that the specified test values have in most cases been selected so that if the procedures given in this manual are followed, most of the resulting HMA mixtures will pass the selected performance test. It is estimated that only about 10 to 20% of properly designed mixtures will fail. Thus, the suggested rutting performance tests not only increase reliability against excessive rutting to a very high level, they do so in a relatively efficient way. The suggested maximum rut depths for the asphalt pavement analyzer (APA) and the Hamburg Wheel-Track (HWT) tests were taken from specifications already in place in numerous states. In this case, implementation of these performance tests will certainly increase the reliability against excessive rutting, but the specific amount of improvement is unknown as is the percentage of mixes likely to fail the tests. However, because these tests with the stated maximum rut depths have been implemented in several states, it is likely that the increase in reliability and the rejection rate will both be reasonable. The various rut resistance tests and guidelines are summarized below. The stated minimum or maximum values for each test should be considered guidelines. Although based either on a careful analysis of laboratory and field data or on existing standards, it is quite possible that these values will need to be adjusted by the specifying agency for optimum results in the region. Factors that need to be considered when making such adjustments are climate, the types and grades of binders commonly used in a given locale, aggregates with unusual properties, and typical traffic mixes and traffic levels. For various reasons, some agencies may wish to alter the conditions a test is run under, which will significantly alter the resulting test values and the appropriate specification values. For details on the proper procedures for performing each test, laboratory engineers and technicians should refer to the appropriate standard test method as listed at the end of this chapter. Some additional background on performance testing in general and on these five tests in particular is provided in Chapter 6 of this manual. The Asphalt Mixture Performance Tester. The asphalt mixture performance tester (AMPT) was initially called the simple performance test system or SPT. Details of the latest equipment specification and test procedure are given in NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester. Tests are performed on specimens cored and trimmed from large gyratory specimens to final nominal dimensions of 100 mm diameter by 150 mm high. There are three different tests for rut resistance using the AMPT: the dynamic modulus (sometimes referred to as the E* test), the repeated load test (also called the flow number test) and the flow time test. To use the E* test to evaluate rut resistance the E* Implementation Program software must be used. This software was not yet commercially available at the time this manual was written, but should soon be available from Table 8-20. AASHTO. In the flow number test, a 600-kPa load is applied to the specimen every second, until Recommended the flow point is reached. The flow point represents failure of the specimen, as evidenced by an minimum flow increasing rate of total permanent strain during the test. Flow number tests are run at the average, number requirements. 7-day maximum pavement temperature 20 mm below the surface, at 50% reliability as determined using LTPPBind, Version 3.1. Test specimens should be prepared at the expected in-place air Traffic Minimum void content, typically about 7%. Table 8-20 lists minimum values for flow number determined Level Flow Number Million Cycles using the AMPT. ESALs The flow time test is similar to the flow number test, but a constant load is applied to the <3 --- 3 to < 10 53 specimen and the total deformation is monitored. Test temperature and specimen preparation 10 to < 30 190 are identical to those used in the flow number test described previously. It is simply a static creep 30 740 test and the flow time is the loading time required to initiate tertiary creep, which is the point at

OCR for page 128
Design of Dense-Graded HMA Mixtures 143 which the rate of deformation begins to increase. Recommended minimum flow times are given Table 8-21. in Table 8-21. Recommended minimum flow time The Asphalt Pavement Analyzer. The asphalt pavement analyzer (APA) is growing in requirements. popularity among pavement agencies as a test for evaluating the rut resistance of HMA pavements. Traffic Minimum At the time this manual was written, nine states had specifications for performance testing of Level Flow Time HMA pavements using the APA device. The APA test method is available as AASHTO TP 63, Million s ESALs which was developed from the test procedure found in Appendix B of NCHRP Report 508: <3 --- Accelerated Laboratory Rutting Tests--Evaluation of the Asphalt Pavement Analyzer. In the APA test, 3 to < 10 20 a pressurized hose is placed over a short cylindrical HMA specimen and a wheel is repeatedly 10 to < 30 72 30 280 passed over the hose. The rut depth is measured after several thousand cycles. Details of the APA test are given in Chapter 6 of this manual. Typical conditions for the APA test--and the ones suggested here for using this procedure as a performance test--are as follows: Hose pressure: 100 lb/in2 Wheel load: 100 lbf Seating cycles: 50 Test cycles: 8,000 Specimen size: 75-mm deep by 150-mm diameter Specimen air void content: 4.0 1.0% Rut depth calculated as the average of three tests of two specimens (six specimens total) The APA test is most frequently run at 64C. However, to account for differences in local climate, Table 8-22. it is suggested that the APA test be run at the temperature corresponding to the high-temperature Recommended binder performance grade specified for the project by the agency for traffic levels of 3 million maximum rut ESALs or more. Suggested criteria for the APA in terms of maximum rut depth after 8,000 loading depths for the and 50 seating cycles are given in Table 8-22. These guidelines are based on values used by the APA test. Oklahoma Department of Transportation and are fairly typical for agencies using this test. Traffic Maximum However, suitable limits for the APA test will depend on the test conditions--if the test conditions Level Rut Depth Million mm are varied, the maximum rut depths may also need to be changed. Furthermore, as with the other ESALs performance test guidelines given in this manual, agencies using the APA as a performance test <3 --- should modify the maximum allowable rut depths given in Table 8-22 if, in their judgment, such 3 to < 10 5 10 to < 30 4 modifications are needed to account for unusual conditions or materials. 30 3 The Hamburg Wheel-Track Test. The Hamburg wheel-track test (here termed the "Hamburg test") is, like the APA test discussed above, a "torture" test for evaluating the rut resistance or moisture resistance of HMA mixtures, but the procedure given here is meant only to evaluate rut resistance. In the Hamburg test, a 204-mm (8-in)-diameter, 47-mm-wide steel wheel is passed over an HMA slab immersed in a heated water bath. The Hamburg test is not as widely used as the APA, so it is not possible to provide typical test conditions and guidelines. The following test conditions are used by the Texas Department of Transportation: Specimen dimensions: 150 mm (6 in) in diameter, 62 2 mm- (2.4 in) thick Wheel load: 705 2 N (158 0.5 lb) Air void content: 7 1% Test temperature: 50 1C The requirements for the Texas version of the Hamburg test are given in Table 8-23. As dis- cussed above, developing typical guidelines for the Hamburg test is difficult because the test is not widely used. A detailed procedure for the Hamburg wheel-track test is given in AASHTO T 324. Because this test is not as widely used as some others of this type, agencies wishing to use the Hamburg test as a performance test should consider performing an engineering study to develop appropriate requirements for their local conditions and materials.

OCR for page 128
144 A Manual for Design of Hot Mix Asphalt with Commentary Table 8-23. Texas requirements for Hamburg wheel tracking test. High Temperature Minimum Passes to Binder Grade 0.5-inch Rut Depth PG 64 or lower 10,000 PG 70 15,000 PG 76 or higher 20,000 Superpave Shear Tester/Repeated Shear at Constant Height. The Superpave Shear Tester, or SST, can also be used effectively to evaluate the rut resistance of HMA mixtures using the repeated shear at constant height (RSCH) test. Like the flow number test, the RSCH test is a repeated load test, however, the load is applied in shear rather than in compression as in the flow number test. The primary test result is the maximum permanent shear strain (MPSS), which is the total accumulated shear strain at 5,000 loading cycles. However, the SST is a complicated, Table 8-24. Recommended expensive piece of equipment and the RSCH test can be difficult to run. Therefore, it is not maximum values for recommended that commercial laboratories, hot-mix producers, and similar organizations MPSS determined using purchase SST devices for use in routine mix design work. Either the AMPT or the IDT strength test the SST/RSCH test. are far better suited for routine use in HMA mix design and analysis. However, some laboratories in the United States and Canada have SST devices and use them regularly both for research Traffic Maximum Value Level for MPSS purposes and in the design and analysis of critical or unusual HMA mix designs. As an aid to these Million % laboratories, Table 8-24 gives the recommended maximum allowable values for MPSS. AASHTO ESALs has developed a standard procedure for the test, listed under Standard T 320, Determining the <3 --- 3 to < 10 3.4 Permanent Shear Strain and Stiffness of Asphalt Mixtures Using the Superpave Shear Tester. The 10 to < 30 2.1 maximum values for MPSS given in Table 8-24 are based on specimens prepared at 3.0 0.5% 30 0.8 air void content, as recommended in AASHTO T 320. Indirect Tensile Strength at High Temperatures. Recently some studies have been done supporting the use of high-temperature indirect tensile (HT-IDT) strength to evaluate the rut resistance of HMA mixtures; a good description of this test (and other performance tests) Table 8-25. can be found in "New Simple Performance Tests for Asphalt Mixes" in Transportation Research Recommended Circular E-C068. Although not as widely used as the other methods given here, it is a very simple minimum high- inexpensive test that most engineers and technicians are already familiar with. The HT/IDT temperature indirect strength test is performed as described in AASHTO T 283 for unconditioned (dry) specimens, tensile strength but at a test temperature that is 10C below the average, 7-day maximum pavement temperature, requirements. 20 mm below the pavement surface at 50% reliability, as determined using LTPPBind, Version 3.1. Minimum Unlike AASHTO T 283, in this procedure, specimens should be compacted using the design Traffic HT/IDT gyrations, which should produce an air void content close to 4.0%. As described in AASHTO T 283, Level Strength Million kPa the specimens are conditioned at the test temperature by placing them in a water bath controlled to ESALs within 0.5C of the test temperature for 2 hours 10 minutes. The specimens should be wrapped <3 --- 3 to < 10 270 tightly in plastic or placed in a heavy-duty, leak-proof plastic bag prior to conditioning, to prevent 10 to < 30 380 them from getting wet. Table 8-25 lists recommended minimum values for HT/IDT strength 30 500 determined following this protocol. Design Traffic Speed, Depth within the Pavement and Performance Test Requirements Earlier in this chapter the effect of traffic speed on binder grade was discussed--as design traffic speed decreases, the required high-temperature binder grade increases significantly (see Table 8-1). This is because loading at low speeds will cause much more rutting in a pavement than loading at fast speeds, all else being equal. For this reason, the test requirements given above for the various performance tests should be adjusted if the design traffic speed is slow (25 to < 70 kph or 15 to < 45 mph) or very slow (< 25 kph or < 15 mph). Perhaps the simplest approach to making