Design Considerations for Perpetual Pavements. Gary L. Fitts, P.E. Sr. Field Engineer Asphalt Institute

Transcription

1 Design Considerations for Perpetual Pavements Gary L. Fitts, P.E. Sr. Field Engineer Asphalt Institute

2 Topics How do perpetual pavements differ from other pavement designs? What procedures/tools are available for evaluating a pavement design? I-49 example, etc

3 Pavement Response to Load HMA Repeated Bending Leads to Fatigue Cracking Base Subgrade Repeated Deformation Leads to Rutting

4 Perpetual Pavements Designed so the pavement structural layers perform without significant damage For light-duty pavements, limited damage per loaded axle application For heavy-duty pavements, minimal damage per loaded axle Pavement thickness is not sensitive to increase in the number of axle loads Surface/wearing course replaced periodically Replacement interval depends on mixture/materials type, traffic conditions, etc.

5 1993 AASHTO Guide Increasing ESAL results in increased SN, i.e., thicker pavement

6 Mechanistic-Empirical Performance Criteria Limit Bending to < 70 (Monismith, Von Quintus, Nunn, Thompson) Thick HMA (> 8 ) Subbase (as required) Limit Vertical Compression to < 200 Subgrade (Monismith, Nunn)

7 Perpetual Pavement Design Strategy-Fatigue Strain below fatigue limit = Indefinite Fatigue Life Reduce tensile strain by increasing pavement thickness and/or increasing stiffness Compressive Strain Thick/Stiff HMA Strain Indefinite Fatigue Life Tensile Strain Fatigue Life

8 Beam Fatigue Testing of HMA Load Load Temperature: 20C Controlled strain various levels Constantly monitor load (force) Failure defined as ½ S initial Reaction Deflection Reaction

9 Stiffness (Mpa) Typical 800 Stiffness Data Cycles vs. Stiffness ,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 Cycles

10 Stiffness (Mpa) Typical 100 Stiffness Data Cycles vs. Stiffness ,000,000 2,000,000 3,000,000 4,000,000 5,000,000 Cycles Actual endurance limit depends on materials and mixture design

11 Damage Accumulation-Fatigue Strain HMA These loads cause damage These loads don t Subgrade Fatigue Life

12 Thicker, stiffer pavement Strain HMA Subgrade Fatigue Life

13 Thinner, deflecting pavement Strain HMA Subgrade Fatigue Life

14 Fatigue Damage N 2.83*10 6 1/ t HMA Subgrade Axle loads resulting in flexural strain levels exceeding -70με cause fatigue damage Strain levels calculated using elastic layer theory Damage is accumulated for an anticipated axle loads and used to predict life Defined as 0.1N, or 10% damage

15 Rutting/Permanent Deformation Structural rutting is accumulated subgrade permanent deformation If the subgrade deforms, the pavement above it conforms to the shape of the underlying foundation Related to the vertical compressive strain at the top of the subgrade Rutting in the asphalt layer is addressed through mixture/materials selection Particularly for upper 4 inches of pavement

16 Mechanistic-Empirical Performance Criteria Thick HMA (> 8 ) Limit Vertical Compression to < 200 Subbase (as required) Subgrade (Monismith, Nunn)

17 Rate of Rutting vs. Total Asphalt Thickness 1000 Subgrade deformation Rate of rutting (mm/msa) TRL Thickness of bituminous layer (mm)

18 Structural Rutting N 6.026*10 8 1/ v 3.87 HMA Subgrade Only axle loads resulting in vertical compressive strain exceeding 200με cause permanent deformation Strains are calculated using elastic layer theory Damage is accumulated for an anticipated axle load spectrum and used to predict life Defined as 0.1N, or 10% damage

19 Stiff, Rut Resistant Upper Layers 0.50 Nom. SMA 1 Nom. Superpave Particularly important in upper 4-6 inches of pavement Use polymer-modified asphalt binders, mixtures that develop aggregate interlock

20 Shear Stress Within an Asphalt Pavement 120 psi 3 6 >35 psi >30 psi >25 psi E HMA = 500 ksi E SG = 10 ksi

21 How to begin? Develop a trial pavement design Using AASHTO, AI, or British method Identify key inputs for elastic layer theory/perroad Modulus, Poisson s ratio for each pavement layer and subgrade, thickness for each pavement layer Layer stiffness values may vary according to season Damage function constants (k-values) for HMA, subgrade Traffic load spectra

22 I-49 Extension, Caddo Parish, Louisiana New Interstate highway construction, Shreveport to Arkansas state line Preliminary designs performed for asphalt and concrete pavements

23 Project Location Denver

24 Input data sources Obtained inputs used for AASHTO design from LaDOTD pavement design office Used FWD data from similar projects to estimate layer stiffnesses to be used as input in the pavement analysis Weather service climatological data for Shreveport

25 Traffic data AADT: 10,000 % trucks: 21.2% Growth rate: 2.9% This critical input is much easier to provide than in the previous version of PerRoad!!!

26 Materials data inputs Season Avg. air temperature, F Estimated pavement temperature, F Duration E*, ksi Winter weeks 1600 Spring/fall weeks 1250 Summer weeks 1100 Other layer stiffnesses: EHMA, psi MnRoad relationship, shifted to meet Louisiana data E base = 45,000 psi E subbase = 25,000 psi E subgrade = 10,000 psi Air Temperature, C

27 Materials data inputs-conservative design values Season Avg. air temperature, F Estimated pavement temperature, F Duration Winter weeks 800 Spring/fall weeks 625 Summer weeks 550 E*, ksi Other layer stiffnesses: E base = 30,000 psi E subbase = 15,000 psi E subgrade = 7,000 psi

28 Summary of results T HMA, in % below limit 2 Fatigue Estimated life, years Probabilistic Permanent Deformation % below limit 3 Estimated life, years Monte Carlo simulation, 5000 cycles 2. Fatigue Threshold = -70 με 3. Deformation Threshold = 200 με What if we raised our requirements for subgrade/subbase/base?

29 Summary of results, revised T HMA, in % below limit 2 Fatigue Estimated life, years Probabilistic Permanent Deformation % below limit 3 Estimated life, years Other layer stiffnesses: E base = 35,000 psi E subbase = 20,000 psi E subgrade = 10,000 psi

30 Time to Damage = 0.1 Fatigue & Deformation Fatigue Deformation Fatigue+ Deformation HMA Thickness

31 Observations Marginal improvement to the foundation can significantly reduce the HMA thickness necessary Proof rolling Consider PDPBT or FWD requirement for pavement foundation Need to continue collecting and analyzing FWD data to identify seasonal stiffness values using different mixture types around the State

32 Considerations for all layers Initial compaction is critically important HMA must be compacted to a nonporous condition for optimal performance Support conditions and lift thicknesses must allow compaction to be achievable Fine-graded mixtures, 3X NMS Coarse-graded mixtures, 4X NMS Consider including loaded wheel test requirements (APA or HWT) for premium mixtures Many agencies are reducing N des levels when using asphalt binders that require polymer modification

33 Summary PerRoad is available and easy to use for evaluating pavement designs with respect to mechanistic perpetual pavement criteria Input data are similar to what are needed when using the ME Design Guide developed in NCHRP 1-37A

34 Perpetual Pavement Resources Check APA website ( for references, software, etc. Keep alert for articles in trade literature, research reports, etc