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Flexible ME Review

Mechanistic-Empirical Design Review: Flexible and Rigid New Design, Partial Reconstruction and Overlays. Flexible ME Review. Summary of Typical Design Process. 1. Determine design inputs traffic, materials properties, construction quality, environment and their interactions costs

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Flexible ME Review

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  1. Mechanistic-Empirical Design Review:Flexible and RigidNew Design, Partial Reconstructionand Overlays

  2. Flexible ME Review

  3. Summary of Typical Design Process 1. Determine design inputs • traffic, materials properties, construction quality, environment and their interactions • costs • other design contraints (bridge heights, utilities) 2. Select some alternative strategies • AC/AB/ASB • AC/AB • AC • AC with Rich Bottom • AC/AB/CTB

  4. 3. For varying thicknesses for each strategy calculate critical strains, stresses for each distress (rutting, fatigue, crushing) for • stiffnesses for environmental conditions, construction • each axle type/load (if axle load spectrum) or an ESAL 4. Sum damage using performance models (n/N for each traffic/stiffness case) across design life for each distress 5. Determine lowest cost structure for each strategy for which S(n/N) < 1 6. Select lowest cost strategy

  5. Example - Minimal input • Traffic • Materials properties • stiffnesses, poisson ratios • design equations • Calculations

  6. Mechanistic-Empirical AC on AC Overlay Thickness Designand Partial Reconstruction

  7. Design Inputs • Traffic, Environment, Reliability as for new pavement • Existing materials properties and thicknesses • Existing structural condition, surface condition, ride quality • Overlay material(s) properties as for new pavement • New materials properties, if reworking any existing layers

  8. Traffic - Past and Future • Can convert to ESALs or use axle load spectrum • Past • if possibility of remaining life in asphalt concrete • Future • as for new pavement design

  9. Existing Materials Stiffnesses: Deflection Testing

  10. Deflection Testing Equipment • Considerations • loads • load duration (frequency) • multiple sensors for back-calculation • cost of operation • reliability • Want loads to be similar to those of traffic • want to measure stiffnesses under traffic conditions due to non-linearities of materials

  11. Falling Weight Deflectometer (FWD)

  12. Layout of sensors Rubber pad 150 mm radius Load Sensor 1 2 3 4 5 6 7 mm 0 200 300 600 900 1200 1500

  13. Typical deflection bowl

  14. Back-Calculation of Stiffnesses • Need multiple sensors at distance from load • Assume (typically) • thicknesses • poisson ratios • Adjust stiffnesses (E, moduli) so that calculated, measured deflections match • Deflections measured are a “snapshot” • Must compensate for AC temperatures at time of testing • Need to apply seasonal factors

  15. Back-Calculation Example • Deflection distance (m) 0 0.2 0.3 0.6 0.9 1.2 1.5 • Deflections measured (microns = 10-6 m) 270 225 192 126 87 63 5 207 mm AC, 280 mm AB, 240 mm ASB Load = 66.7 kN, air temp = 20 C, surface = 18.3 C Calculated deflections 0 0.2 0.3 0.6 0.9 1.2 1.5 m A B C

  16. Pavement Assessment for Overlay or Reconstruction • Non-structural design criteria • Skid resistance • Ride quality • Structural design inputs • Surface condition, to help determine stiffness of surface layers, remaining life • Structural condition, to help determine stiffnesses, thicknesses, seasonal environmental conditions

  17. Pavement Characteristics Affecting Tire/Pavement Noise Non-Structural Properties λ > 0.5 m Roughness 50mm <λ< 500 mm 0.5 mm <λ< 50mm λ< 0.5 mm

  18. Structural Condition Assessment • Condition survey of existing distresses • Destructive testing • Materials sampling • Testing at depth • Lab testing for AC stiffness, fatigue relationSoils stiffness, rutting behaviorCTB stiffness, crushing • Non-destructive testing • Deflections • Wave propagation

  19. Determination of Soils Layer Types • Gradation • Atterberg limits • liquid limit • plastic limit • Granular layers may be contaminated with fines pumped from below or washed in

  20. Determination of Thicknesses • Cores • Dynamic Cone Penetrometer (DCP) • Ground Penetrating Radar (GPR) • resolution issues

  21. Dynamic Cone Penetrometer • Thickness and indirect estimates of stiffness/strength • 2 to 3 person hand operation • for thick AC pavements, core 38 mm hole to drive DCP through

  22. Performance Equations: Subgrade Strain Rutting Criteria • May be conservative for rehabilitation if unbound layers are undisturbed during construction because of effects of past traffic: • compaction • hardening • back-calculated stiffnesses can provide information on stiffness • DCP provides information on hardening

  23. Subgrade Strain Rutting Criteria • May also be conservative for rehabilitation if thick AC, high traffic

  24. Overlay Design Special Considerations • pre-overlay repairs • reflection crack control • recycling • subdrainage • shoulders/widening • lane/curb/bridge height matching

  25. AC Overlay Design Steps if Don’t Expect Reflection Cracking • Divide project into representative structures • deflections, back-calculated moduli • condition survey • Select design sections • Characterize existing structure • linear elastic model inputs (E, m, thickness) • lab testing, back-calculations, coring, as-builts, condition survey

  26. Design Steps (2) • If included in method, determine remaining life • For several overlay thicknesses, calculate critical strains • fatigue • subgrade rutting • Calculate Nf, Nr for each overlay thickness

  27. Design Steps (3) • Plot Nf, Nr vs. overlay thickness • Select thicknesses that provides adequate design life for fatigue cracking, subgrade rutting

  28. Mechanistic-Empirical Overlay Design Review - What Would You Do? (no reflection) • Traffic • Existing structure, condition • Materials properties • existing • new • Calculations

  29. Reflection cracking strategies • AC on AC delaying strategies • engineering textiles • open graded AC • chip seals • AC on AC prevention strategy • Grind AC in place • Use as: aggregate base, or stabilized base with asphalt emulsion or foamed asphalt • AC on PCC delaying strategies • crack and seat/break and seat/rubblize, then overlay, maybe use engineering textile

  30. Mechanistic-Empirical Overlay Design Review - What Would You Do If Recycling • Traffic • Existing structure, condition • Materials properties • existing • reworked • new • Calculations

  31. Rigid ME Design Review

  32. Summary of Typical Design Process 1. Determine design inputs • traffic, materials properties, construction quality, environment and their interactions • costs • other design constraints (bridge heights, utilities) 2. Select some alternative strategies • base layer types (AC, CTB, LCB, granular) • slab lengths, widths • joint designs (dowels, tie bars, aggregate interlock)

  33. 3. For varying thicknesses for each strategy calculate critical stresses for each distress (cracking, faulting) for • slab shapes for environmental conditions • each axle type/load (if axle load spectrum) or an ESAL 4. Sum damage (n/N for each traffic/stiffness case) across design life for each distress 5. Determine lowest cost structure for each strategy for which S(n/N) < 1 6. Select lowest cost strategy

  34. Where/When to Calculate for Fatigue • Transverse • Mid-slab edge • Daytime (maximum curl) • Corner • Near the corner • Night time (maximum curl) • Longitudinal • Somewhere mid-slab, off of edge • Night time (maximum curl)

  35. Top

  36. Top

  37. With -15 C gradient Top

  38. Performance Models for Faulting FaultD = CESAL0.25 * [0.0628 – 0.0628 * Cd + 0.3673 * 10-8 * Bstress2 + 0.4116 * 10-5 * Jtspace2 + 0.7466 * 10-9 * FI2 * Precip0.5 – 0.009503 * Basetype – 0.01917 * Widenlane + 0.0009217 * Age] where: CESAL = Cumulative 18-kip (80-kN) equivalent single axle loads, millions Bstress = Maximum dowel/concrete bearing stress, lb./in.2 Jtspace = Mean transverse joint spacing, ft. Basetype = Base type (0 = nonstabilized base; 1 = stabilized base) Widenlane = Widened lane (0 = not widened, 1 = widened) Cd = Modified AASHTO drainage coefficient, calculated from database information FI = Mean annual freezing index, degree-days Precip = Mean annual precipitation Age = Pavement age, years

  39. Faulting vs Dowel Bearing Stress

  40. Fault Depth at 30 years vs Base Type

  41. Example - Minimal input • Traffic • Materials properties • Calculations

  42. Example - Maximum input • Traffic • Materials • Design Options • Design Equations • Calculations

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