5- Superpave. Asphalt Concrete Mix Design

Transcription

1 5- Superpave Asphalt Concrete Mix Design 1

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 2

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

4 Rotational Viscometer (Brookfield) Torque Motor Inner Cylinder Thermosel Environmental Chamber Digital Temperature Controller 4

5 Viscosity, Pa s 10 5 Mixing/Compaction Temps Compaction Range Mixing Range 0.28± ± Temperature, C 5

6 Specimen Preparation Specimen Height Mix Design mm (4700 g) Moisture Sens mm (3500 g) Loose Specimen for Max. Theor. (Rice) varies with nominal max size 19 mm (2000 g) 12.5 mm (1500 g) 150 mm 6

7 Mixing Place pre-heated aggregate in bowl and add hot asphalt 7

8 Mixing Place bowl on mixer and mix until aggregate is well-coated 8

9 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

10 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

11 Compaction Place funnel on top of mold and place mix in mold. Take care not to allow the mix to segregate. 11

12 Compaction Place another paper on top of mix and place mold in compactor. 12

13 Compaction Example of typical full-size compactors. 13

14 Compaction Key Components of Gyratory Compactor height measurement reaction frame tilt bar control and data acquisition panel loading ram mold rotating base 14

15 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 1.25 o 15

16 Compaction After aging, take mix and preheated mold from oven. Place paper in bottom of mold. 16

17 Compaction Once compaction is finished, extrude sample from mold. 17

18 Compaction Remove the paper and label samples. 18

19 SGC Results % G mm Log Gyrations 19

20 Three Points on SGC Curve % G mm N des N max N ini Log Gyrations 20

21 Design Compaction N des based on average design high air temp traffic level Log N max = 1.10 Log N des Log N ini = 0.45 Log N des % G mm N ini N des N max Log Gyrations 21

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23 % G mm N max = 174 N des = 109 N ini = 8 Data Presentation Number of Gyrations Specimen 1 Specimen 2 Average 23

24 Superpave Mix Design Analysis 24

25 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 25

26 Superpave Mix Design G mb (estimated) = W m g w V mx Where: W m = mass of specimen, g V mx = volume of compaction mold (cm 3 ) g w = density of water, g/cm 3 Assumption: specimen is smooth-sided cylinder 26

27 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: C = G mb (measured) G mb (estimated) % G mm = G mb (estimated) C / G mm (measured) 27

28 Superpave Mix Design Determine mix properties at N Design and compare to criteria Air voids 4% (or 96% G mm ) VMA See table VFA See table %G mm at N ini < 89% %G mm at N max < 98% Dust proportion 0.6 to

29 SGC Results % G mm 96% (4% Voids) Increasing asphalt cement content N ini N des N max Log Gyrations Each line = avg. of two samples 29

30 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 30

31 Superpave Mix Design VFA requirements: Traffic (millions of ESALs) Range of VFA < to 80 1 to 3 65 to 78 > to 75 31

32 Superpave Mix Design % weight of material 0.6 < < 1.2 % weight of effective asphalt Effective asphalt content is asphalt on surface of aggregate (asphalt not absorbed by aggregate) If the dust level is too high the mix will be brittle if it is too low the mix will have insufficient stability 32

33 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 (60 o C, 24 hr.) Option for freeze cycle Bring to test temperature (25 o C) and determine wet (conditioned) tensile strength 33

34 Moisture Sensitivity AASHTO 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 60 o C for 24 hours Soak at 25 o C for 2 hours 34

35 Moisture Sensitivity AASHTO T 283 Determine the tensile strengths of both sets of 3 specimens Calculate the Tensile Strength Ratio (TSR) TSR = Avg. wet tensile strength Avg. dry tensile strength Minimum of 80% needed 35

36 Moisture Sensitivity AASHTO T 283 Indirect tensile strength apparatus for 100 mm specimens 36

37 Example of Superpave Mix Design 37

38 % PASSING Trial Gradations 19.0 mm Nominal Mixture Trial Blend Trial Blend 1 Trial Blend Sieve Size (mm) raised to 0.45 power 38

39 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 Combined G sb n/a Combined G sa n/a

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41 % G mm N max = 174 N des = 109 N ini = 8 Data Presentation Number of Gyrations Specimen 1 Specimen 2 Average 41

42 Compaction Characteristics %G mm Blend %AC N ini N des N max 1 4.3% 86.9% 96.0% 97.4% 2 4.5% 85.9% 96.0% 97.7% 3 4.7% 87.1% 95.0% 97.3% 42

43 Va 100 % Gmm@ Ndes % Gmm@ Ndes* Gmm* Ps VMA 100 ( ) Gsb P P (0.4**(4 V b, estimated bi a )) VMA estimated VMA initial C* (4 Va ) C = 0.1 if Va < 4.0% or 0.2 if Va > 4.0% 43

44 44 ) 4.0 ( 100 estimated estimated estimated VMA VMA VFA a ini ini estimated V N Gmm N Gmm max max a estimated V N Gmm N mm bestimated sb se sb se b s estimated be P G G G G G P P ) * )*( * (, estimated P be P DP,.075

45 Volumetric Properties Blend %AC %Air %VMA %VFA DP 1 4.3% 4.0% 12.7% 68.5% % 4.0% 13.0% 69.2% % 4.0% 13.5% 70.1%

46 SGC Results % G mm 96% (4% Voids) Increasing asphalt cement content N ini N des N max Log Gyrations Each line = avg. of two samples 46

47 Selection of Design Asphalt Binder Content V a VMA % binder Blend 3 % binder VFA DP % binder % binder %G mm at N ini %G mm at N max % binder % binder 47

48 Moisture Sensitivity AASHTO T 283 Determine the tensile strengths of both sets of 3 specimens Calculate the Tensile Strength Ratio (TSR) TSR = Avg. wet tensile strength Avg. dry tensile strength Minimum of 80% needed 48

49 Questions -? 49