A Manual for Design of Hot-Mix Asphalt with Commentary (2011) / Chapter Skim
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225 Commentary to the Mix Design Manual for Hot Mix Asphalt
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This chapter serves only as an introduction to the Manual and contains no critical information that requires additional supporting details or justification. 226 C H A P T E R 1 Introduction
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This chapter of the Manual presents general background information on construction materials and flexible pavements. It is intended for engineers and technicians with little background in these subjects.
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Performance grading of asphalt binders is described in some detail, including discussion of the various test methods involved and presentation of some of the pertinent specifications in tabular format. The chapter concludes with a few paragraphs giving practical guidelines for the selection of binders during the mix design process.
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However, the minimum value in the Manual for the highest design traffic level is 98%, with the option of a further reduction to 95% if experience with local conditions and materials warrant such a reduction. This approach represents a compromise between the recommendations of NCHRP Report 539 and the reluctance of many engineers to reduce minimum CAFF values to 95% without further experience with HMA mixtures produced with coarse aggregates exhibiting lower values for fractured faces.
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Sources for critical tables in chapter 4 of the mix design manual.
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For Calculation of Source 5-1 Bulk specific gravity of compacted specimen AASHTO T 166 5-2 Maximum specific gravity of loose mixture AASHTO T 209 5-3 Bulk specific gravity of aggregate blend AASHTO R 35; TAI SP-2, MS-2 5-4 Air void content of compacted specimen,% by mixture volume AASHTO R 35, T 269 5-5 Total asphalt binder content of mixture,% by mixture mass By definition 5-6 Total asphalt binder content of mixture,% by mixture volume By definition 5-7 Absorbed asphalt binder content,% by mixture volume By Definition 5-8 Effective asphalt binder content,% by mixture volume By Definition 5-9 Effective asphalt binder content,% by mixture mass By Definition 5-10 Absorbed asphalt binder content,% by mixture mass By Definition 5-11 Voids in mineral aggregate,% by mixture volume AASHTO R 35 5-12 Voids filled with asphalt,% by volume AASHTO R 35 5-13 Apparent film thickness NCHRP Report 567 (4) 5-14 Aggregate specific surface (method 1)
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232 C H A P T E R 6 Evaluating the Performance of Asphalt Concrete Mixtures Performance Test Used to Evaluate References Asphalt Mixture Performance Test System (AMPT) Rut resistance, dynamic modulus 5, 6 Superpave shear tester, repeated shear at constant height test Rut resistance AASHTO T 320, 7, 8 High-temperature IDT strength test Rut resistance 9, 10, 11 Asphalt pavement analyzer (APA)
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. Commentary to the Mix Design Manual for Hot Mix Asphalt 233 Typical Effects of Increasing Given Factor within Normal Specification Limits While Other Factors Are Held Constant within Normal Specification Limits "↑ " indicates improved performance; "↓ " indicates reduced performance Component Factor Resistance to Rutting and Permanent Deformation Resistance to Fatigue Cracking Resistance to Low Temperature Cracking Durability/ Resistance to Penetration by Water and Air Resistance to Moisture Damage Increasing High Temperature Binder Grade ↑↑↑ Increasing Low Temperature Binder Grade ↓↓↓Asphalt Binder Increasing Intermediate Temperature Binder Stiffness ↑↓ Increasing Aggregate Angularity ↑↑ Increasing Proportion of Flat and Elongated Particles Increasing Nominal Maximum Aggregate Size ↓ ↓ ↓ Increasing Mineral Filler Content and/or Dust/Binder Ratio ↑↑ ↑ Aggregates Increasing Clay Content ↓ Increasing Design Compaction Level ↑↑ ↑↑ Increasing Design Air Voids ↑↑ Increasing Design VMA and/or Design Binder Content ↓↓ ↑ ↓ Volumetric Properties Increasing Field Air Voids ↓↓ ↓↓ ↓ ↓↓↓ ↓↓ Table 4.
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234 A Manual for Design of Hot Mix Asphalt with Commentary Component Factor Resistance to Rutting and Permanent Deformation Resistance to Fatigue Cracking Resistance to Low Temperature Cracking Durability/ Resistance to Penetration by Water and Air Resistance to Moisture Damage Increasing High Temperature Binder Grade M 320, 16, 17, 18 Increasing Low Temperature Binder Grade M 320, 19 Asphalt Binder Increasing Intermediate Temperature Binder Stiffness M 320, Note 1 Increasing Aggregate Angularity M 323, 1, 20, 21 Increasing Proportion of Flat and Elongated Particles Increasing Nominal Maximum Aggregate Size Notes 2, 3 Notes 2, 3 Note 4 Increasing Mineral Filler Content and/or Dust/Binder Ratio 4, 16 4 Aggregates Increasing Clay Content M 323, 20 Increasing Design Compaction Level M 323, 16 M 323, 16 Increasing Design Air Voids 16 Increasing Design VMA and/or Design Binder Content 16, 17, 18 16, 22, 13, 24 20 Volumetric Properties Increasing Field Air Voids 16, 17, 18 17, 23, 24 20 16, 25, 26 Note 5 1Most research suggests that the fatigue resistance of an HMA mixture shows a complex relationship with its stiffness and indirectlywith binder stiffness; for thin pavements, fatigue resistance decreases with increasing modulus, whereas for thick pavements, fatigue resistance increases with increasing modulus (see references 7 and 8)
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Rismantojo, NCHRP Report 557: Aggregate Tests for Hot-Mix Asphalt Mixtures Used in Pavements, Washington, DC: Transportation Research Board, 2006, 38 pp. Commentary to the Mix Design Manual for Hot Mix Asphalt 235 Grade Adjustment for Average Vehicle Speed in kph (mph)
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. The most recent version of the resistivity/rutting equation gives allowable traffic as a function of mixture composition, compaction, and air voids: where TR = million ESALs to an average rut depth of 7.2 mm (50% confidence level)
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(N K V V Meq s QC IP− ) Commentary to the Mix Design Manual for Hot Mix Asphalt 237
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The only other performance testing normally required for any routine mix design is rut-resistance testing. This is because of the high level of complexity and cost for performing tests to characterize resistance to fatigue cracking and thermal cracking.
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Commentary to the Mix Design Manual for Hot Mix Asphalt 239 HMA Property Rutting Thermal Cracking Alligator Cracking HMA ≥ 5 in Alligator Cracking HMA < 3 in Longitudinal Cracking High Temperature Binder Grade Increase to improve Increase to improve Decrease to improve Decrease to improve Low Temperature Binder Grade Decrease to improve Design VMA Decrease to improve Increase to improve Increase to improve Increase to improve Design VFA Increase to improve Filler Content Increase to improve In-Place Air Voids Decrease to improve Decrease to improve Decrease to improve Decrease to improve Decrease to improve Table 8. Summary of effect of mixture composition on performance predictions -- table 6-6 in the mix design manual.
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. 240 C H A P T E R 7 Selection of Asphalt Concrete Mix Type
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However, for some of the performance tests developing meaningful guidelines for interpreting results was a complex task. Chapter 8 presents a brief history of HMA mix design methods, in order to provide inexperienced technicians and engineers with some background.
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. Lift thickness values are based on recommendations given 242 A Manual for Design of Hot Mix Asphalt with Commentary From NCHRP Report 573: 20-Year Design Traffic, Million ESALs Ndesign for Binders < PG 76-XX Ndesign for Binders ≥ PG 76XX or for Binders Used in Mixes Placed > 100 mm from Pavement Surface Ndesign in AASHTO R 35 and in the Mix Design Manual < 0.30 50 NA 50 0.30 to < 3.0 65 50 75 3.0 to < 10 80 65 100 10 to < 30 80 65 100 > 30 100 80 125 Table 9.
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From page 243... ...
, and so the stated minimum VMA values are not ambiguous. However, specifying a minimum VMA and a design air void content also implies a minimum VFA, since VFA = (VMA-4.0)
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There are two reasons for this increase. The first is that research performed during NCHRP Projects 9-25 and 9-31 showed a 244 A Manual for Design of Hot Mix Asphalt with Commentary NMAS, mm Design Traffic Level, Million ESALs Minimum VMA, % Minimum VFA, % Maximum VFA, % Calculated Minimum VFA, % Calculated Maximum VMA, % 4.75 < 3.0 16.0 70 80 75.0 20.0 4.75 ≥ 3.0 16.0 75 78 75.0 18.2 9.5 < 3.0 15.0 65 78 73.3 18.2 9.5 ≥ 3.0 15.0 73 76 73.3 16.7 12.5 All 14.0 65 75 71.4 16.0 19 All 13.0 65 75 69.2 16.0 25 ≥ 0.3 12.0 65 75 66.7 16.0 25 < 0.3 12.0 67 75 66.7 16.0 37.5 All 11.0 64 75 63.6 16.0 Table 11.
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For the other two mixes for each project -- one above and one below the design binder content -- the design air void content was estimated using the following equation (3) : where Vadb = Estimated design air voids at some binder content Pb Vad = Design air voids at design binder content Pbd V V P Padb ad b bd= − −( )
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The flow time values in Table 15 and the flow number values given in Table 13 are intended to be, for all practical purposes, equivalent. 246 A Manual for Design of Hot Mix Asphalt with Commentary Traffic Level Million ESALs Minimum Flow Number Cycles < 3 -- - 3 to < 10 53 10 to < 30 190 ≥ 30 740 Table 13.
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Recommended minimum flow time requirements -- table 8-21 in the mix design manual.
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Recommended maximum rut depths for the APA test -- table 8-22 in the mix design manual.
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Commentary to the Mix Design Manual for Hot Mix Asphalt 249 High-Temperature Binder Grade Minimum Passes to 0.5-inch Rut Depth PG 64 or lower 10,000 PG 70 15,000 PG 76 or higher 20,000 Table 18. Texas requirements for Hamburg wheel tracking test -- table 8-23 in the mix design manual (44)
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Traffic Level Million ESALs Minimum HT/IDT Strength kPa < 3 -- - 3 to < 10 270 10 to < 30 380 ≥ 30 500 Table 20. Recommended minimum hightemperature indirect tensile strength requirements -- table 8-25 in the mix design manual.
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|/sin δ Pa VTM Vol.% VMA Vol.% MPSS @ 3% Voids % HT/IDT Strength lb/in2 Traffic @ 7% Voids MESALs KY limestone, 19 mm, coarse PG 64-22 54 100 2660 4.32 14,500 4.40 15.10 149.6 4.75 5.59 KY limestone, 19 mm, coarse PG 64-22 54 50 2660 4.32 14,500 3.30 17.40 114.1 7.97 0.78 KY limestone, 19 mm, dense PG 64-22 54 100 2648 4.75 14,500 3.60 12.10 226.8 3.92 13.14 KY limestone, 19 mm, dense PG 64-22 54 50 2648 4.75 14,500 4.00 13.60 182.0 4.83 3.68 Crushed gravel, 19 mm, coarse PG 64-22 54 100 2566 3.76 14,500 3.20 14.00 202.0 3.62 2.91 Crushed gravel, 19 mm, coarse PG 64-22 54 75 2566 3.76 14,500 4.20 14.90 192.1 3.71 2.29 Crushed gravel, 19 mm, dense PG 64-22 54 100 2575 4.32 14,500 4.40 12.80 316.9 1.83 10.09 Crushed gravel, 19 mm, dense PG 64-22 54 75 2575 4.32 14,500 3.40 13.00 273.5 2.64 4.31 VA limestone, 9.5 mm, coarse PG 64-22 54 100 2671 4.40 14,500 4.30 15.80 160.0 4.71 4.76 VA limestone, 9.5 mm, coarse PG 64-22 54 50 2671 4.40 14,500 3.50 18.20 6.46 0.75 VA limestone, 9.5 mm, coarse PG 76-16 54 100 2671 4.40 66,200 4.30 15.80 314.8 1.61 38.29 VA limestone, 9.5 mm, coarse PG 58-28 54 100 2671 4.40 4,880 4.30 15.80 108.6 6.21 1.07 VA limestone, 9.5 mm, fine PG 64-22 54 100 2659 6.21 14,500 4.50 18.70 199.6 4.19 6.48 VA limestone, 9.5 mm, fine PG 64-22 54 50 2659 6.21 14,500 3.90 20.30 7.39 1.44 VA limestone, 9.5 mm, fine PG 76-16 54 100 2659 6.21 66,200 4.50 18.70 416.8 2.12 52.14 Granite, 12.5 mm, dense PG 64-22 54 125 2632 4.79 14,500 4.20 13.10 2.33 16.37 Granite, 12.5 mm, dense PG 76-16 54 125 2632 4.79 66,200 4.20 13.10 589.6 0.24 131.71 Granite, 12.5 mm, dense PG 58-28 54 125 2632 4.79 4,880 4.20 13.10 164.8 2.54 3.67 Granite, 12.5 mm, fine PG 64-22 54 125 2631 6.15 14,500 3.50 14.90 2.20 14.49 Granite, 12.5 mm, fine PG 76-16 54 125 2631 6.15 66,200 3.50 14.90 550.6 0.33 116.59 Granite, 12.5 mm, fine PG 58-28 54 125 2631 6.15 4,880 3.50 14.90 171.4 3.00 3.25 VA limestone, 9.5 mm, coarse PG 64-22 60 100 2671 4.40 6,920 4.30 15.80 116.2 4.90 1.72 VA limestone, 9.5 mm, coarse PG 76-16 60 100 2671 4.40 33,100 4.30 15.80 235.5 2.39 14.78 VA limestone, 9.5 mm, fine PG 64-22 60 100 2659 6.21 6,920 4.50 18.70 133.1 5.42 2.35 VA limestone, 9.5 mm, fine PG 76-16 60 100 2659 6.21 33,100 4.50 18.70 300.6 2.73 20.13 Granite, 12.5 mm, dense PG 64-22 60 125 2632 4.79 6,920 4.20 13.10 198.2 2.36 5.93 Granite, 12.5 mm, dense PG 76-16 60 125 2632 4.79 33,100 4.20 13.10 436.1 0.65 50.85 Granite, 12.5 mm, fine PG 64-22 60 125 2631 6.15 6,920 3.50 14.90 247.2 2.51 5.25 Granite, 12.5 mm, fine PG 76-16 60 125 2631 6.15 33,100 3.50 14.90 460.8 1.30 45.01 Table 21. Data used in developing correlations between maximum permanent shear strain, high-temperature IDT strength and estimated allowable traffic (4)
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These adjustments were calculated directly from Equation 6 given above, using a traffic speed of 70 kph for fast traffic, 25 kph for slow traffic and 10 kph for very slow traffic. 252 A Manual for Design of Hot Mix Asphalt with Commentary
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4. Calculation of the required new binder grade needed to achieve a specified binder grade, given a certain RAP content and a binder grade for the RAP binder.
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For gradation analysis, the percentage of each stockpile based on the total weight of aggregate is needed. The percentage of each new aggregate stockpile based on the total weight of aggregate is given by Equation 10: where Pnewk = percentage of new aggregate k, weight% of total aggregate psnewk = percentage of new aggregate stockpile k in the total blend, weight% wbRAPTotal = total weight of RAP binder from all RAP stockpiles, weight% The percentage of each RAP aggregate based on the total weight of aggregate is given by Equation 11: where pRAPi = percentage of RAP aggregate i, weight% of total aggregate psRAPi = percentage of RAP stockpile i in the total blend, weight% wbRAPi = weight of RAP binder from RAP stockpile i, weight% wbRAPTotal = total weight of RAP binder from all RAP stockpiles, weight% For each sieve, the gradation of the blend of the stockpiles is then computed using the percentage of each stockpile based on the total weight of aggregate using Equation 12: tpp p pp p ppnewk k k n RAPi i= × ⎛ ⎝⎜ ⎞⎠⎟ + × ⎛ ⎝⎜ = ∑ 100 1001 ⎞⎠⎟ = ∑ i j 1 12( )
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X X n i i n = = ∑ 1 13( ) Commentary to the Mix Design Manual for Hot Mix Asphalt 255
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σ2w − σ2PM/Max and σPM/Max is the maximum allowable standard deviation for percent passing for the selected sieve. Equations 1 and 2 can be rewritten for asphalt content rather than for aggregate percent passing, calculated using the following equation: where σBM = standard deviation of binder content (weight%)
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From page 257... ...
This is equivalent to saying that the amount of RAP should be limited so that the overall variability in HMA production is not increased above what it would normally be without the addition of any RAP. This approach simplifies some of the calculations, and as discussed below, if typical variability in aggregate% passing and binder contents are assumed, the mix designer can determine the maximum allowable RAP without knowing the existing HMA variability or the maximum allowable variability desired by the producer.
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For example, for the case where σPN = σPM/max = 2.0, and σPR is estimated to be 4.0, if the RAP standard deviation is based on n = 5 samples, the maximum allowable RAP content is 17%. However, if the RAP standard deviation is based on n = 10 samples, the maximum allowable RAP content increases to 27%.
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From page 259... ...
HMA Tools calculates mean, standard deviation, the upper confidence limit for standard deviation, and the maximum allowable RAP content based on variability for up to four different RAP stockpiles. This analysis uses Equations 13 through 20 along with the assumptions described concerning typical variability in HMA production and batching variability.
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From page 260... ...
In using Figure 7 in an actual mix design, all aggregate sizes would be evaluated, and the overall maximum allowable RAP content would be the lowest value calculated for all sieve sizes. The maximum RAP content based on asphalt binder standard deviation should also be evaluated (Figure 8, or Figure 9-4 in the Manual)
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From page 261... ...
for the RAP stockpile blend Using Equations 21 and 22, Figures 11 and 12 were developed, respectively, showing maximum allowable RAP content as a function of average RAP standard deviation values. It should be noted that these charts are conservative and like Figures 7 and 8 are based on standard deviation values calculated using N = 5 independent samples of RAP.
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Maximum RAP content as a function of average standard deviation for asphalt binder content (Figure 9-6 in the Mix Design Manual)
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where Gsb = estimated bulk specific gravity of the RAP aggregate Gse = effective specific gravity of the RAP aggregate from Equation 23 Pba = estimated binder absorption for the RAP, wt% of aggregate Gb = estimated specific gravity of the RAP binder The overall error associated with this analysis is difficult to quantify. It depends on the precision of the maximum specific gravity measurement, the accuracy of the RAP binder content measurement, and the estimated RAP binder absorption and specific gravity.
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264 A Manual for Design of Hot Mix Asphalt with Commentary -0.040 -0.030 -0.020 -0.010 0.000 0.010 0.020 0.030 0.040 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 ERROR IN RAP BINDER CONTENT OR ABSORPTION, % ER R O R IN B UL K SP EC IF IC G RA VI TY O F R A P A G G RE G AT E RAP Binder Content RAP Binder Absorption Figure 13. Potential errors in bulk specific gravity of the RAP aggregate for errors in RAP binder content and binder absorption (Figure 9-7 in the Mix Design Manual)
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Instead, requirements for fine aggregate angularity and sand equivalency are given; these requirements are identical to those given in Chapter 8 for dense-graded HMA designed for traffic levels of 30 million ESALs or more. • Table 10-11 is used explicitly to establish minimum asphalt binder contents; it is identical to Table X2.1 given in R 46, but according to R 46, the requirements in X2.1 are only to be used when the "standard" minimum binder content of 6.0% cannot be met.
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2001. The mix design procedure, tables, equations, and other critical information given in Chapter 11 are taken from this report.
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Typical amounts of mineral dust generated during HMA plant production for different aggregates and gradations (Table 12-1 in the Mix Design Manual)
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Equations 25 and 26 (Equations 12-2 and 12-3, respectively, in the Manual) for calculating the upper and lower control limits for an X-bar control chart are taken from Hot Mix Asphalt Construction, Instructor Manual, Part A: Lecture Notes (49)
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From page 269... ...
are used to calculate lower and upper control limits for range charts (R charts) for quality assurance of HMA production: where LCL = lower control limit UCL = upper control limit D3, D4 = factors for computing control limits for standard deviation control charts; see Table 25 R – = overall range UCL D R= × = × =4 2 12 1 35 2 86 28.
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The rules for interpreting statistical control charts (page 12-21) are based on those in reference 50.
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