A Manual for Design of Hot-Mix Asphalt with Commentary (2011) / Chapter Skim
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Pages 101-147
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From page 101... ...
Engineers and technicians using RAP in their mix designs should make sure they read Chapter 9 carefully and understand the proper procedures for incorporating RAP into HMA mixtures. Chapter 6 presents an in-depth discussion of performance testing of HMA mixtures, including background information important in understanding how and why many of these procedures were developed.
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From page 102... ...
The specific requirements for air void content, VMA, VFA, and stability and flow varied over time and from agency to agency. In the Asphalt Institute's publication Mix Design Methods for Asphalt Concrete and Other Mix Types (MS-2)
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From page 103... ...
However, engineers eventually realized that such mixtures often exhibited durability problems, and minimum VMA values such as those given by the Asphalt Institute were established. Although the current Asphalt Institute version of the Marshall method does not explicitly specify maximum values for VMA, the combination of specifying air void content and VFA in fact indirectly establishes such maximums.
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From page 104... ...
4. Trial mixtures are evaluated on the basis of volumetric composition, with requirements for design air void content, VMA, and VFA.
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From page 105... ...
Ndesign was (and still is) the actual point at which air void content, VMA, and VFA are specified -- the design compaction level.
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From page 106... ...
Calculate Aggregate Content 8. Proportion Aggregates for Trial Mixtures 106 A Manual for Design of Hot Mix Asphalt with Commentary
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From page 107... ...
Evaluate and Refine Trial Mixtures 11. Compile Mix Design Report Most of these steps are straightforward and easily accomplished.
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From page 108... ...
, the required binder grade, the target VMA, and the target air void content. Completion of this worksheet is essential, because much of this information is used in other parts of HMA Tools.
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From page 109... ...
The second approach is to select the performance grade of the new binder and then determine the minimum and maximum amounts of RAP that can be added while still meeting the established performance grade requirements for the final effective binder grade. Either approach can be used with HMA Tools, which gives both the final effective binder grade for a given RAP content and the new binder performance grade, and also the minimum and maximum allowable RAP content for a given new binder performance grade and RAP stockpile or blend of RAP stockpiles.
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From page 110... ...
if a polymermodified asphalt binder is used, and if the mix successfully meets appropriate performance testing requirements, as discussed later in this chapter. A second important high-temperature grade adjustment must be made for temporary construction.
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From page 111... ...
Brown and associates at the National Center for Asphalt Technology (NCAT) in 2004 published the results of research on this topic in NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot Mix Asphalt Pavements.
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From page 112... ...
These target values should be used for the initial development of a mix design -- for determining the composition of trial mixtures to be evaluated and refined in the laboratory during the mix design process. The design VMA value can be adjusted during the later stages of the mix design process or during construction, in order to further refine the mix or to adjust for field production.
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From page 113... ...
It will, however, also tend to decrease rut resistance. Increasing the design air void content by 0.5% will have the opposite effect -- it will slightly decrease the design binder content and produce a mix that is more difficult to compact, while increasing rut resistance.
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From page 114... ...
In the example above, this would result in a total target binder content of 12%. A more accurate estimate would be to calculate the volume of water absorbed by the aggregate, divide this by two, and add it to the target VBE value: where Vb = total asphalt content by volume % VBE = effective asphalt content by volume % VMA = voids in the mineral aggregate = Vbe + air void content Gsb = aggregate bulk specific gravity Pwa = water absorption of the aggregate, weight % The best approach to estimating absorbed binder and the resulting total binder content is to use past experience.
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From page 115... ...
Even though most HMA mixtures do not precisely follow a maximum density gradation, it is often used as a reference when proportioning aggregates. A good estimate of the maximum density aggregate gradation for a given aggregate size can be estimated using the 0.45 power gradation: where % PMD = percent passing for maximum density gradation d = sieve size, mm D = maximum sieve size for gradation, mm % % ( )
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From page 116... ...
= percent passing, continuous maximum density gradation, for sieve size d2 d1 = one sieve size larger than d2 P(d1) = percent passing sieve d1 For example, in a selected aggregate gradation the percent passing the 4.75-mm sieve is 84%.
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From page 117... ...
The top portion of the figure shows a traditional gradation plot, including the maximum density gradation. The bottom chart in Figure 8-5 shows the CMD plots for these Design of Dense-Graded HMA Mixtures 117 0 20 40 60 80 100 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm Pe rc en t P as si ng maximum density gradation aggregategradation -15 0 15 0.010 0.100 1.000 10.000 100.000 Sieve Size, mm % D ev ia tio n fro m M ax .
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From page 118... ...
This is in fact typical for aggregate blends used in HMA. However, even though the deviations from the maximum density gradation for the fine aggregate portion of many aggregate blends 118 A Manual for Design of Hot Mix Asphalt with Commentary 0 20 40 60 80 100 Pe rc en t P as si ng Dense/fine Dense/dense Dense/coarse SMA Max.
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From page 119... ...
It is strongly suggested that during the initial trial batches only the aggregate gradation should be modified, while the asphalt binder content is kept constant. This will make the way changes in the aggregate gradation are affecting VMA and air void content much clearer.
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From page 120... ...
This should become apparent when plotting gradations and VMA values for trial mixtures. It is especially important when modifying existing mix designs to make use of experience with the given aggregates.
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From page 121... ...
After this, the design proceeds as before, determining the air void content and VMA for the trial batches and then making further refinements in the aggregate gradation as needed until the desired mix properties are met. Guidelines for Aggregate Gradations As with previous HMA mix design methods, there are limits for aggregate gradation for each NMAS; suggested control points for aggregate gradations for dense-graded HMA mixtures are listed in Tables 8-6 and 8-7.
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From page 122... ...
The gradation plots included in the worksheet "Trial_Blends" in HMA Tools include boundaries showing the control limits for a given mixture. In order for the proper limits to be included in the plot, the proper value for the aggregate NMAS must be entered in the worksheet "General." Check Aggregate Specification Properties As in the Superpave method, there are four aggregate specification properties: (1)
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From page 123... ...
An important step in the mix design process is to determine specification property values for aggregate blends, to ensure that the blends will likely meet specification property requirements. For the initial trial batches, the specification properties of the aggregate blends are normally estimated mathematically, by calculating a weighted average for each property.
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From page 124... ...
Step 9. Calculate Trial Mix Proportions by Weight and Check Dust/Binder Ratio At this point in the HMA mix design, the amount of air voids, binder, and aggregate has been determined on a volume basis, and up to three different aggregate blends have been developed -- on a proportion-by-weight basis.
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From page 125... ...
Ps = total aggregate content, % by total mix weight Then, calculate the effective asphalt binder content by weight: where Pbe = effective binder content, % by total mix weight Vbe = effective binder content, % by total mix volume Gb = binder specific gravity Vsb = aggregate content, % by total mix volume Gsb = overall bulk specific gravity of aggregate (Equation 8-4) Calculate the percent by weight of each aggregate: where Ps1 = weight percent (by total mix)
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From page 126... ...
of aggregate 2 P0.075/s3 = % passing the 0.075-mm sieve for aggregate 3 Ps3 = weight percent (by total mix) of aggregate 3 Calculate the dust/binder ratio, using the effective asphalt binder content: where D/B = dust/binder ratio, calculated using effective binder content P0.075 = mineral dust content, % by total mix weight (Equation 8-10)
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From page 127... ...
Percent by Total Mix Weight Air 4.00 -- - -- - -- - 0.0 Total Asphalt Binder 11.38 1.025 -- - -- - 4.60 Absorbed Asphalt Binder -0.40 1.025 -- - -- - (0.16) Effective Asphalt Binder 10.98 1.025 -- - -- - 4.44 No.
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From page 128... ...
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.
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From page 129... ...
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.
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From page 130... ...
130 A Manual for Design of Hot Mix Asphalt with Commentary 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.
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From page 131... ...
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.
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From page 132... ...
The composition of these materials is 132 A Manual for Design of Hot Mix Asphalt with Commentary 10 100 1,000 10,000 100 110 120 130 140 150 160 170 180 Temperature, OC Vi sc os ity , m Pa -s 250 to 310 mPa-s 150 to 190 mPa-s compaction temperature 138 to 144 °C mixing temperature 150 to 156 °C Figure 8-7. Example viscosity-temperature chart showing determination of mixing and compaction temperature ranges for a non-modified binder.
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From page 133... ...
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.
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From page 134... ...
134 A Manual for Design of Hot Mix Asphalt with Commentary Figure 8-9. Short-term oven conditioning of HMA mixture in the laboratory.
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From page 135... ...
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 Design of Dense-Graded HMA Mixtures 135 Figure 8-10.
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From page 136... ...
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.
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From page 137... ...
Max. Trial Mix 1: Dense/Fine Mix Trial Mix 2: Dense/Dense Mix Trial Mix 3: Dense/Coarse 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.
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From page 138... ...
Top: gradation plot for example mix design, including fourth trial mix; bottom: CMD plot for example mix design.
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From page 139... ...
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.
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From page 140... ...
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)
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From page 141... ...
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.
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From page 142... ...
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.
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From page 143... ...
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 64°C.
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From page 144... ...
The maximum values for MPSS given in Table 8-24 are based on specimens prepared at 3.0 ± 0.5% air void content, as recommended in AASHTO T 320. Indirect Tensile Strength at High Temperatures.
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From page 145... ...
If the mix still fails to meet the requirements for rut resistance testing, the mix design will have to be modified. Rut resistance of an HMA mix design can be improved as follows: • Increase the binder high-temperature grade.
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From page 146... ...
R 35, Standard Practice for Superpave Volumetric Design for Hot-Mix Asphalt (HMA) T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens T 209, Theoretical Maximum Specification Gravity and Density of Bituminous Paving Mixtures T 269, Percent Air Voids in Compacted Dense and Open Asphalt Mixtures T 275, Bulk Specific Gravity of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens T 283, Resistance of Compacted Asphalt Mixture to Moisture-Induced Damage T 312, Preparing and Determining the Density of Hot-Mix Asphalt Specimens by Means of the Superpave Gyratory Compactor T 320, Determining the Permanent Shear Strain and Stiffness of Asphalt Mixtures Using the Superpave Shear Tester.
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From page 147... ...
(2010) NCHRP Report 648: Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt, TRB, National Research Council, Washington, DC, 77 pp.
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