Mechanistic-Empirical Pavement Design Guide: A User s Perspective. Brian D. Prowell, Ph.D., P.E.

Transcription

1 Mechanistic-Empirical Pavement Design Guide: A User s Perspective Brian D. Prowell, Ph.D., P.E.

2 Empirical Approach Based on results of experiments or experience Scientific basis not established AASHTO 86/93 an empirical design method Refer to AASHO Road Test

3 Typical Loop Layout--AASHO Road Test Loop 5 Single Axles 22.4 kips, Tandems 40 kips Loop 6 Single Axles 30.0 kips, Tandems 48 kips

4 AASHTO Flexible Design Method Empirical Method based on AASHO Road Test Design Inputs (Example): Delta psi = 1.2 ( ) Reliability = 90% Standard Deviation = 0.45 Effective Resilient Modulus = 6177 psi Design Traffic = 5.7 million ESALs

5 Empirical Approach AASHTO Flexible Pavement Performance Equation Solve for SN using nomograph or bisection method

6 Empirical Approach AASHTO Flexible Pavement Performance Equation log 10 W 18 = (Z R ) (S 0 ) + (9.36)(log (SN + 1)) {log 10 [ΔPSI/( )]/[ /(SN + 1) 5.19 } + (2.32) (log 10 M R ) Solve for SN using nomograph or bisection method

7 AASHTO 93 Pavement Design Guide Example Design Section SN 7.5 in HMA 6.0 in SP C 12.5 in SP B a 1 = 0.39 a 2 = 0.138, d 2 = 1.2 a 3 = 0.149, d 3 = Required SN = 5.4 Total = 5.4

8 Mechanistic-Empirical Mechanistic: Determine stresses, strains, or deflections at critical locations in a pavement structure. Empirical: Relates stresses, strains, or deflections to pavement performance. Often referred to as Transfer Functions.

9 Important Advantages Relate material properties to design More accurate portrayal of traffic effects and changes in axle loading Effects of construction variability and traffic variability can be accounted for Pavement layers can be engineered for expected distresses

10 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Base Subgrade

11 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Base Subgrade

12 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Base Subgrade

13 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Repeated Bending Base Subgrade

14 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Repeated Bending Leads to Fatigue Cracking Base Subgrade

15 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Repeated Bending Leads to Fatigue Cracking Base Subgrade Repeated Deformation

16 Pavement Design Addresses Two Primary Distresses: Fatigue and Rutting HMA Repeated Bending Leads to Fatigue Cracking Base Subgrade Repeated Deformation Leads to Rutting

17 Mechanistic-Empirical Pavement Design Priest and Timm, 2006

18 M-E design process requires Material properties of each layer (E i, µ i ) Thickness of each layer (h i ) and load (P, a)

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20 Level 1 Hierarchical Input Levels Advanced materials testing (E*, M R ) Level 2 Available test procedures (like CBR) with correlation equations Level 3 Default values

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22 Traffic

23 M-E PDG Traffic Inputs Traffic Volume AADTT Directional and lane distributions Growth factor Speed Vehicle Classification and axle-distribution Level I site specific load spectra Level III default distributions by road class

24 MEASURED PAVEMENT RESPONSE

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26 Climate

27 Enhanced Integrated Climatic Model Weather station or location Heat capacity, thermal conductivity, depth of water table Output Calculates every 6 minutes for design life; up to 7 layers. Temperatures collected into 5 bins for analysis Correction factors to adjust the optimum modulus of unbound layers (moisture and freeze/thaw based)

28 Example Temperature Histogram NCAT Test Track

29 Material Properties

30 Unbound Layers Level I Resilient modulus using stress dependent FEM. Not calibrated at this time! Level II Resilient Modulus based on correlations with empirical tests Can use other M-E program to determine average stress dependent modulus (better approach)

31 Correlations

32 Using Resilient Modulus Data Material k 1 k 2 k 3 R 2 Granite Base Subgrade where: M r = resilient modulus, p a = atmospheric pressure ( psi), θ = bulk stress = σ 1 +σ 2 +σ 3 = σ 1 +2σ x,y, σ 1 = major principal stress = σ z + p o, σ 3 = minor principal stress/confining pressure = σ x,y + k o (p o ) σ z = vertical stress from wheel load(s) calculated using layered-elastic theory, σ x,y = horizontal stress from wheel load(s) calculated using layered-elastic theory, p o = at-rest vertical pressure from overburden of paving layers above unbound layer or subgrade, k o = at-rest earth pressure coefficient, τ oct = octahedral shear stress = 1/3((σ 1- σ 2 ) 2 +(σ 1- σ 3 ) 2 +(σ 2 -σ 3 ) 2 ) 1/2, and k 1, k 2, k 3 = regression coefficients.

33 Asphalt Inputs Binder Properties Volumetric Properties Dynamic Modulus Levels II and III use Witzak Equation to predict from gradation, binder and volumetric properties Level I test or use Hirsch model Low Temperature IDT

34 Test Samples for SPT Tests 150 mm tall by 100 mm diameter, cored from SGC

35 Dynamic Modulus σ o sin(ωt) φ/ω σ,ε σ o ε o ε o sin(ωt-φ) Time, t

36 Faster Traffic or Lower Temperatures Slower Traffic or Higher Temperatures

37 Structural Modeling AC Pavements Multi-layer elastic solution Main engine: JULEA 2-D Finite element analysis For special loading conditions, Non-linear unbound material characterization

38 How well does it work?

39 Evaluated Using Performance of Structural Sections from 2003

40 Observed Cracking in Field

41 Observed Cracking in Field Section Failure Date Cracking, % of Cracking, % of Total Lane Wheel Path N1 6/14/ N2 7/19/ N8 8/15/ Failure defined as 20% cracking in total lane area, For MEPDG, damage (D) = 100% for 50% cracking Of wheel path, For N6 D = 0.7 at end of loading

42 MEPDG Cracking N1

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44 MEPDG N2 and N6

45 Using Endurance Limit for Pavement Design Perpetual Pavements

46 Definition of the Endurance Limit HMA Fatigue Endurance Limit A level of strain below which there is no cumulative damage over an indefinite number of load cycles. From NCHRP 9-44 HMA Endurance Limit Workshop, August 2007

47 Practical Definition of the Endurance Limit Nunn defined long-life pavement as those that last 40 years without structural strengthening

48 Practical Definition of the Endurance Limit 500 million load repetitions is approximate maximum in 40 years Assume shift factor of 10 recommended by SHRP Endurance Limit is that laboratory strain that provides for 50 million cycles to failure

49 Regions of Fatigue Behavior DeBenedetto, 1996

50 Regions of Fatigue Behavior 50% Initial Stiffness DeBenedetto, 1996

51 Beam Fatigue

52 AASHTO T321 - Beam Fatigue Test Results ß 1 = shift factor Between lab and field

53 Transfer Function Coefficients from Beam Fatigue Testing Mix K 1 K 2 R 2 Endurance Limit, ms PG E PG Opt E PG E All 19.0 mm NMAS mix with same aggregate, Opt.+ has additional 0.7% asphalt

54 Fatigue Shift Factor Laboratory fatigue tests underestimate field fatigue life, e.g. number of repetitions to some level of cracking Shift factor accounts for difference Ranges from 10 to 100 SHRP recommended 10 used in NCHRP 9-38 Accounts for factors such as healing,

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57 PG at Optimum

58 Summary of Predicted E-L Mix Beam Fatigue Beam Fatigue Round Robin Predicted 95% Lower Confidence Limit Predicted 95% Lower Confidence Limit PG NA NA PG NA NA PG PG Opt PG PG Opt NA NA

59 Four Analyses: Sensitivity Analysis 1. Comparison of conventional and perpetual pavement thickness for 2003 NCAT Test Track structural sections and loading conditions 2. Evaluate sensitivity of perpetual pavement thickness for NCAT Test Track traffic 3. Repeat 1. using MEPDG s Truck Traffic Classification No. 1 for principal arterials 4. Repeat 2. using MEPDG s Truck Traffic Classification No. 1 for principal arterials

60 10.6% cracking vs. 0% cracking of total lane area Used in original Test Track design Value calculated using the multi-variable, non-linear stress sensitivity model for unbound layers

61 Test Track Traffic

62 Required Thickness MEPDG Traffic

63 Perpetual Pavement Thickness M-E PDG Traffic

64 Summary Mechanistic-empirical design programs require more inputs than 93 AASHTO Some are readily available, some are not Nationally calibrated fatigue models appear to do a good job at predicting cracking; rutting overestimated M-E PDG and PerRoad can both be used for perpetual designs; M-E PDG more conservative

65 Questions? Contact Information: Brian Prowell Advanced Material Services, LLC 2515 E. Glenn Ave., Suite 107 Auburn, AL 36830