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Asphalt Concrete Mix Design

Asphalt Concrete Mix Design. Superpave. Superpave Volumetric Mix Design. Goals Compaction method which simulates field Accommodates large size aggregates Measure of compactibility Able to use in field labs Address durability issues Film thickness Environmental. Specimen Preparation.

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Asphalt Concrete Mix Design

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  1. Asphalt Concrete Mix Design Superpave

  2. Superpave Volumetric Mix Design • Goals • Compaction method which simulates field • Accommodates large size aggregates • Measure of compactibility • Able to use in field labs • Address durability issues • Film thickness • Environmental

  3. Specimen Preparation • Mechanical mixer • 0.170 Pa-s binder viscosity • Short term oven aging • 4 hours at 135°C • 2 hours at 135°C (optional)

  4. Mixing/Compaction Temps Viscosity, Pa s 10 5 1 .5 Compaction Range .3 .2 Mixing Range .1 100 110 120 130 140 150 160 170 180 190 200 Temperature, C

  5. 150 mm Specimen Preparation • Specimen Height • Mix Design - 115 mm (4700 g) • Moisture Sens. - 95 mm (3500 g) • Loose Specimen for Max. Theor. (Rice) • varies with nominal max size • 19 mm (2000 g) • 12.5 mm (1500 g)

  6. Mixing Place pre-heated aggregate in bowl and add hot asphalt

  7. Mixing Place bowl on mixer and mix until aggregate is well-coated

  8. Short Term Aging Empty mix into pan and place in oven to simulate short term aging 2 hours for low absorption aggregates 4 hours for high absorption aggregates

  9. Short Term Aging Important • Allows time for aggregate to absorb asphalt • Helps minimize variability in volumetric calculations • Most terms dependent upon volumes which change with changes in the amount (volume) of absorbed asphalt

  10. Compaction Place funnel on top of mold and place mix in mold. Take care not to allow the mix to segregate.

  11. Compaction Place another paper on top of mix and place mold in compactor.

  12. Compaction Example of typical full-size compactors.

  13. Compaction Key Components of Gyratory Compactor height measurement control and data acquisition panel reaction frame loading ram mold tilt bar rotating base

  14. Compaction • Gyratory compactor • Axial and shearing action • 150 mm diameter molds • Aggregate size up to 37.5 mm • Height measurement during compaction • Allows densification during compaction to be evaluated Ram pressure 600 kPa 1.25o

  15. Compaction After aging, take mix and preheated mold from oven. Place paper in bottom of mold.

  16. Compaction Once compaction is finished, extrude sample from mold.

  17. Compaction Remove the paper and label samples.

  18. SGC Results % Gmm 10 100 1000 Log Gyrations

  19. Three Points on SGC Curve % Gmm Nmax Ndes Nini 10 100 1000 Log Gyrations

  20. Design Compaction • Ndes based on • average design high air temp • traffic level • Log Nmax = 1.10 Log Ndes • Log Nini = 0.45 Log Ndes % Gmm Nmax Ndes Nini 10 100 1000 Log Gyrations

  21. Data Presentation % Gmm 100 98 Nmax = 174 96 Ndes = 109 94 Nini = 8 92 Specimen 1 90 Specimen 2 88 Average 86 84 1 10 100 1000 Number of Gyrations

  22. Superpave Mix Design Analysis

  23. Superpave Testing • Specimen heights • Mixture volumetrics • Air voids • Voids in mineral aggregate (VMA) • Voids filled with asphalt (VFA) • Mixture density characteristics • Dust proportion • Moisture sensitivity

  24. Superpave Mix Design gw Vmx • Gmb (estimated) = Wm Where: Wm = mass of specimen, g Vmx = volume of compaction mold (cm3) gw = density of water, g/cm3 Assumption: specimen is smooth-sided cylinder

  25. Superpave Mix Design • However, surface irregularities cause the volume of the specimen to be slightly less than volume of cylinder • Actual bulk specific gravity measurement of compacted sample used to determine correction factor, C: Gmb (measured) Gmb (estimated) C = % Gmm = Gmb (estimated) C / Gmm (measured)

  26. Superpave Mix Design • Determine mix properties at NDesign and compare to criteria • Air voids 4% (or 96% Gmm) • VMA See table • VFA See table • %Gmm at Nini < 89% • %Gmmat Nmax < 98% • Dust proportion 0.6 to 1.2

  27. SGC Results % Gmm Increasing asphalt cement content 96% (4% Voids) Ndes Nmax Nini 10 100 1000 Log Gyrations Each line = avg. of two samples

  28. Superpave Mix Design • VMA requirements: • Nominal max agg size Min. VMA • 9.5 mm 15 • 12.5 mm 14 • 19 mm 13 • 35 mm 12 • 37.5 mm 11

  29. Superpave Mix Design • VFA requirements: • Traffic (millions of ESALs) Range of VFA < 0.3 70 to 80 1 to 3 65 to 78 > 3.0 65 to 75

  30. % weight of - 0.075 material % weight of effective asphalt 0.6 < < 1.2 Superpave Mix Design Effective asphalt content is asphalt on surface of aggregate (asphalt not absorbed by aggregate)

  31. Superpave Mix Design • Moisture Sensitivity • Prepare set of 6 specimens • 6 to 8% voids • Represents anticipated in-service voids • Determine tensile strength of 3 of specimens • Condition remaining 3 in water bath (60oC, 24 hr.) • Option for freeze cycle • Bring to test temperature (25oC) and determine wet (conditioned) tensile strength

  32. Moisture SensitivityAASHTO T 283 • Measured on proposed aggregate blend and asphalt content • Reduced compactive effort to increase voids 3 Conditioned Specimens 3 Dry Specimens Vacuum saturate specimens Soak at 60oC for 24 hours Soak at 25oC for 2 hours

  33. Avg. wet tensile strength TSR = Avg. dry tensile strength Moisture SensitivityAASHTO T 283 Determine the tensile strengths of both sets of 3 specimens Calculate the Tensile Strength Ratio (TSR) Minimum of 80% needed

  34. Moisture SensitivityAASHTO T 283 Indirect tensile strength apparatus for 100 mm specimens

  35. Example of Superpave Mix Design

  36. Trial Gradations 19.0 mm Nominal Mixture 100.0 90.0 80.0 70.0 Trial Blend 3 Trial Blend 1 60.0 50.0 % PASSING 40.0 Trial Blend 2 30.0 20.0 10.0 0.0 0.075 2.36 19.0 Sieve Size (mm) raised to 0.45 power

  37. Aggregate Consensus Properties • Blended Aggregate properties are determined Property Criteria Blend 1 Blend 2 Blend 3 Coarse Ang. 95%/90% min. 96%/92% 95%/92% 97%/93% Fine Ang. 45% min. 46% 46% 48% FLat/Elong. 10% max. 0% 0% 0% Sand Equiv. 45 min. 59 58 54 Combined Gsb n/a 2.699 2.697 2.701 Combined Gsa n/a 2.768 2.769 2.767

  38. Compaction Characteristics %Gmm • Blend %AC Nini Ndes Nmax • 1 4.3% 86.9% 96.0% 97.4% • 2 4.5% 85.9% 96.0% 97.7% • 3 4.7% 87.1% 96.0% 97.3%

  39. Volumetric Properties Blend %AC %Air %VMA %VFA DP 1 4.3% 4.0% 12.7% 68.5% 0.86 2 4.5% 4.0% 13.0% 69.2% 0.78 3 4.0% 4.0% 13.5% 70.1% 0.88

  40. VFA Blend 3 % binder Va % binder % binder %Gmm at Nini DP %Gmm at Nmax % binder % binder Selection of Design Asphalt Binder Content VMA % binder

  41. Questions - ?

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