Impact of Subgrade Strength on HMA Section Thickness

Transcription

1 Impact of Subgrade Strength on HMA Section Thickness Purdue Road School William R. Vavrik, Ph.D., P.E. 07 Mar 18 Bill Vavrik 1

2 Questions we will attempt to cover History of pavement design Pavement ME Design What is it? A look at typical pavement designs and the move to perpetual design 2

3 Pavement Design Methodologies Experience Empirical Statistical models from road tests Mechanistic-empirical Calculation of pavement responses, i.e., stresses, strains, deformations Empirical pavement performance models Mechanistic Calculation of pavement responses Mechanics-based pavement performance models Common Practice State-of-the-practice State-of-the-art Empirical Mechanistic- Empirical Mechanistic 3

4 AASHTO Pavement Design Guide Empirical methodology based on AASHO Road Test in the late 1950 s Several versions: 1961 (Interim Guide), 1972, 1986, Guide highlights need for mechanistic design 4

5 AASHO Road Test (late 1950 s) 5

6 AASHO Road Test Location Ottawa, IL 6

7 One Rainfall Zone 7

8 One Temperature Zone... 8

9 SOLVING PROBLEMS OF GLOBAL IMPORTANCE One Subgrade Type A-6 / A-7-6 (Clay) Poor Drainage 9

10 Limited Set of Materials One asphalt concrete ¾-inch surface course 1-inch binder course Four base materials Well-graded crushed limestone (main experiment) Well-graded uncrushed gravel (special studies) Bituminous-treated base (special studies) Cement-treated base (special studies) One uniform sand/gravel subbase 10

11 Controlled Construction Methods... 11

12 SOLVING PROBLEMS OF GLOBAL IMPORTANCE 1950s Vehicle Loads... (And on a specific stretch of roadway!) 12

13 Limitations: Huge Extrapolation 13

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15 Field Performance - The LTPP Study AASHO Road Test Site 15

16 New Design Guide An Analysis Method X Design Method The answer is Amount of distress Not layer thickness 16

17 Mechanistic-Empirical Design Climate Traffic Materials Structure Damage Response Time Damage Accumulation Field Distress 17

18 Design Parameters Over Pavement Life Each load application CTB Modulus PCC Modulus Traffic AC Modulus Granular Base Modulus Subgrade Modulus Time, years 18

19 Speed Impact 0.7 HMA Rutting, in Traffic Speed, mph 19

20 Effect of Traffic Wander Total Rutting (in) Wander, inches Wander = 0" Wander = 12" Wander = 24" Lateral Distance from Center Line (in) 20

21 Thickness Impact 35.0% 30.0% Alligator Cracking % 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% AC Thickness (in) SG Mr = 30 ksi SG Mr = 25 ksi SG Mr = 20 ksi SG Mr = 15 ksi SG Mr = 8,000 SG Mr = 3,000 21

22 Asphalt Density Impact Rutting (in) In place air voids (%) 22

23 Bottom-up Cracking Percent Cracked Percent Alligator Cracked at 20 Years Phoenix Atlanta Reno Minneapolis Surface Thickness (in.) 23

24 Rutting 1.8 Total Rutting at 20 Years Rut Depth (in.) Phoenix Atlanta Reno Minneapolis Surface Thickness (in.) 24

25 Effect of HMA Thickness Total Rutting (in) Time (Month) 25

26 Effect of HMA Mixtures 0.12 Low Ac Rutting (in) Intermediate High Time (Month) 26

27 Effect of Truck Operating Speed Ac Rutting (in) Time (Month) 27

28 Effect of Climate on Cracking 700 Damage (%) Phoenix Tennessee Time (Month) Minnesota 28

29 Effect of Ground Water Table Damage (%) Time (Month) 60 29

30 Effect of Bedrock Elevation Ac Rutting (in) Time (Month) 30

31 Effect of Bedrock Elevation Total Rutting (in) Time (Month) 31

32 Effect of Traffic 0.3 Total Rutting (in) High Low Time (Month) 32

33 Let s look at a typical design 33

34 A typical InDOT medium volume interstate design Base Case 2 SMA 2 Intermediate 10 Base 14 Stabilized Subgrade 7,500 psi Natural Subgrade 3,000 psi 34

35 Variations on the theme Base Case Agg Base RBB 2 SMA 2 Intermediate 2 SMA 2 SMA 8.5 Base 8.5 Base 10 Base 2 RBB 2 RBB 14 Stabilized Subgrade 6.0 Agg 30,000 psi 14 1B Modified Natural Subgrade 14 1B Modified 7,500 psi Natural Subgrade Natural Subgrade 3,000 psi 35

36 Measuring field properties 1B Modified Mean Modulus = 89 ksi Minimum Value = 15 ksi Standard Deviation = 50.6 ksi Low Quartile = 50 ksi 36

37 Measuring field properties - Subgrade Mean Modulus = 18 ksi Minimum Value = 5 ksi Standard Deviation = 4.1 ksi Low Quartile = 15 ksi 37

38 What if we use values below the lower quartile in the design Base Case Revised Inputs 2 SMA 2 SMA 2 Intermediate 2 Intermediate 10 Base 8.5 Base 14 Stabilized Subgrade 7,500 psi 14 Stabilized Subgrade 35,000 psi Natural Subgrade 3,000 psi Natural Subgrade 13,000 psi 38

39 The variations also reduce in thickness Base Case 2 SMA 2 Intermediate 10 Base Agg Base 2 SMA 8.5 Base 2 RBB 2 RBB RBB 2 SMA 8.5 Base 2 RBB 6.0 Agg 30,000 psi 14 Stabilized Subgrade Natural Subgrade 14 1B Modified Natural Subgrade 35,000 psi 13,000 psi 14 1B Modified Natural Subgrade 39

40 What if we use the average in-place values and ensure perpetual design? Protect against bottom up fatigue 50-year analysis at 90% reliability under 10% cracking Will allow for mill & fill cycles with occasional added thickness and perpetual life Initial Construction Tied to Typical Values Use SMA for rut prevention Initial IRI = 50 Evaluate roughness & AC rutting at 20 yrs, 90% reliability 40

41 Perpetual design with as-built inputs Base Case 2 SMA 2 Intermediate 6.5 Base Agg Base 2 SMA 4 Base 2 RBB 6.0 Agg RBB 2 SMA 4.75 Base 2 RBB 14 Stabilized Subgrade Natural Subgrade 14 1B Modified Natural Subgrade 14 1B Modified Natural Subgrade 41

42 Find the pavement that gives you the best value and uses the best design strategy Each Layer Designed for its purpose SMA Surface long life Base Mix to provide stability Rich bottom binder protect for fatigue 1B Modified for stability Consider adding durability test for longer life MOP consideration of subgrade soils Design Considerations different by distress IRI 20 yr Initial 50 in/mi Rutting 20 yr Thermal Cracking 20 yr Asphalt fatigue 50 yr 10% or less 2 SMA 4.75 Base 2 RBB 14 1B Modified Natural Subgrade 42

43 MEPDG is not perfect, but Great pavement design tool Design, rehabilitation Analysis of effects of many variables (e.g., construction deficiencies) Prediction of performance: use for LCCA. Pavement management needs & uses. Truck cost allocation of axle weights. Excellent University training tool. 43

44 Questions 44

45 Thank You! William R. Vavrik, Ph.D., P.E. Vice President & Principal Engineer Applied Research Associates, Inc. 505 W. University Avenue Champaign, IL (217) AAE 45