Eshan Dave Department of Civil Engineering University of Minnesota Duluth NCAUPG Technical Conference, St. Louis MO 01/24/2013

Transcription

1 Eshan Dave Department of Civil Engineering University of Minnesota Duluth NCAUPG Technical Conference, St. Louis MO 01/24/2013

2 Outline 1. Low temperature cracking and fracture energy 2. Fracture energy tests 3. Effectiveness of fracture energy as low temperature cracking performance predictor 4. Low temperature cracking mix specifications 5. Current and future research 2

3 Low Temperature Cracking (LTC) Problem LTC is a major cause of pavement deterioration in regions with severe winter climates 3

4 Mechanism of Thermal Cracking Asphalt pavement contraction during cooling induces tensile thermal stresses Thermal stresses are greatest at critical low temperatures or very fast cooling rates Thermally induced stresses well in excess of material strength result in onset of thermal cracking SUPERPAVE: Binder properties at low-temperatures are used as the main criteria to address thermal cracking 4

5 Why do we need specify LTC performance of asphalt mix? Binder is important, but does not completely control material behavior: Aggregate/mastic effects on mixture creep/fracture properties Effects of RAP, RAS, WMA, and other additives Mixture volumetrics voids, aggregate structure Plant/field aging Structural effects of fracture process Mix testing can provide: Improved comparisons between LTC performance of asphalt mixes Inputs for performance prediction models Input for maintenance decisions Insight for policy decisions 5

6 Cracking of Asphalt Materials Load Quasi-brittle fracture CMOD Load Softening G f Load Crack formation Crack Mouth Opening Displacement (CMOD) Onset of damage δ c σ t Cracking Damage Zone 6

7 Fracture Energy, G f P Quasi-brittle fracture CMOD, u Load, P S f Softening (damage) Fracture Energy, G f : Energy required to create unit fracture surface P Crack Mouth Opening Displacement (CMOD), u Fracture work: Area under Load-Displacement curve G f = Fracture Work Fracture Area 7

8 Tensile Strength-Fracture Energy Comparison Indirect Tensile Strength (MPa) More Ductile (Compliant) Fracture Energy (J/m 2 ) Data from Iowa Project Data from Other Projects Source: Wagoner and Buttlar, 2007 Fracture energy appears to disseminate between mixes better than IDT strength Especially prevalent with modified binders Data supports need for true fracture mechanics approach 8

9 Test Temperature and Fracture Energy 2500 PG58-28 PG64-22 G f =1299.3Exp 0.047*T 2000 r 2 = r 2 = Fracture Energy, G f (J/m 2 ) In terms of fracture energy the temperature spread is consistently greater than 6ºC G f =627.1Exp 0.041*T r 2 = Temperature, T ( o C) 9

10 Outline 1. Low temperature cracking and fracture energy 2. Fracture energy tests 3. Effectiveness of fracture energy as low temperature cracking performance predictor 4. Low temperature cracking mix specifications 5. Current and future research 10

11 Disk-Shaped Compact Tension (DCT ) Test ASTM D Loading Rate: Crack Mouth Opening Displacement CMOD Rate = 1.0 mm/min Measurements: CMOD Load P CMOD, u P Wagoner, Buttlar and Paulino, Experimental Mechanics, 45:270, ASTM D7313: Standard Test Method for Determining Fracture Energy of Asphalt-Aggregate Mixtures Using the Disk-Shaped Compact Tension Geometry (2008) 11

12 Semi-circular Bend, SC(B) Test Test Control: Constant CMOD Measurements: CMOD Load Li and Marasteanu, J. Asphalt Pav. Tech., 73: ,

13 Outline 1. Low temperature cracking and fracture energy 2. Fracture energy tests 3. Effectiveness of fracture energy as low temperature cracking performance predictor 4. Low temperature cracking mix specifications 5. Current and future research 13

14 Low Temperature Cracking Pooled Fund Study Investigation of Low Temperature Cracking in Asphalt Pavements TPF-5(132) Phase II Participating States: Minnesota (Lead State) Connecticut Iowa New York North Dakota Wisconsin LTC performance specifications for asphalt mixes Final Report: 14

15 Results from Phase-I of Pooled Fund Study 15

16 Results for Phase-II of Pooled Fund Study Results for validation sections: 16

17 Results from other Minnesota Mixes Field Cores (TH371) RP6: Good performing section (2005 construction) RP17/21.5: Poor performing section (2004 construction) Plant Produced Mixes (TH9 and TH70) TH9 TH70 Gradation coarse fine Design Traffic Level 1 3 Million ESALs Design Gyrations 60 Binder PG58-34 RAP [%] Total AC [%] New AC Ratio [%]

18 RP Results for TH371 Sections North Bound Crack Count South Bound Crack Count Fracture Energy [J/m 2 ] Wear Fracture Energy [J/m 2 ] RP 6 RP 17 RP

19 TH371 Recovered Binder Grade and Fracture Energy 500 DCT Fracture Energy (J/m2) Low Temperature Grade of Recovered Binder ( º C) 19

20 DCT Testing: TH9 & TH70 Samples TH9 Coarse Gradation Total AC = 4.0% TH70 Fine Gradation Total AC = 5.2% Fracture Energy [J/m 2 ] Tested at -24C Tested at -18C 0 TH9 TH70

21 Outline 1. Low temperature cracking and fracture energy 2. Fracture energy tests 3. Effectiveness of fracture energy as low temperature cracking performance predictor 4. Low temperature cracking mix specifications 5. Current and future research 21

22 LTC Performance Specifications Based on traffic levels Limits based on: Fracture energy 10 º C above 98% reliability Superpave Low Temperature PG (PGLT) Low temperature cracking performance model (IlliTC) Limits DCT Fracture Energy (J/m 2 ) IlliTC Cracking Prediction (m/km) High (> 30M ESALs) Project Criticality / Traffic Level Medium (10 30M ESALs) Low (< 10M ESALs) < 4 < 64 Not required 22

23 IlliTC: Low Temperature Cracking Model Self contained user-friendly software for conducting low temperature cracking simulations Simulations are based on the fracture mechanics principles Accounts for pavement structure, non uniform temperature profiles, and mix viscoelastic and fracture properties Has intuitive graphical user interface 23

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28 Overview 1. Low temperature cracking and mix tests 2. Fracture energy tests 3. Effectiveness of fracture energy as low temperature cracking performance predictor 4. Low temperature cracking mix specifications 5. Current and future research 28

29 MnDOT DCT Low Temperature Fracture Testing Pilot Project Develop DCT specimen fabrication and testing capabilities at MnDOT Between 3 5 pavement projects will be used for pilot implementation/trial of LTC mix specifications Lab testing will be conducted during mix design and production Testing will be conducted during 2013 construction season 29

30 MnDOT Study on Asphalt Performance Test Phase-I: Synthesis of Asphalt Performance Tests Several cracking tests are available Two most promising tests: Disk Shaped Compact Tension Fracture Test (DCT) Dry Indirect Tensile Strength (ITS) from TSR Phase-II: Currently Underway Task-1: Analysis of Laboratory Test and Field Performance Data Task-2: Laboratory Testing and Field Performance Evaluation of Field Sections DCT versus Field Performance Task-3: Analysis of Lab Testing and Field Performance Data Preliminary Performance Specification 30

31 Summary Low temperature cracking performance of pavements show very good correlation to mix fracture energies Fracture energy based low temperature cracking mix specifications have been developed through Pooled Fund Study The fracture energy based specifications are supplemented with thermal cracking prediction model (IlliTC) for high traffic roadways Currently studies are underway to: Conduct pilot implementation of fracture energy based specifications Further validate the fracture energy recommendations and develop performance based specifications 31

32 Thank you for your attention Questions? Contributors: Bill Buttlar Tim Clyne Mihai Marasteanu Luke Johanneck Contact: Eshan V. Dave Acknowledgements: US Department of Transportation (FHWA) Minnesota Department of Transportation Connecticut Department of Transportation Iowa Department of Transportation North Dakota Department of Transportation New York State Department of Transportation Wisconsin Department of Transportation Nextrans Research Center 32