Mechanistic-Empirical Design Review: Flexible and Rigid New Design, Partial Reconstruction and Overlays

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Mechanistic-Empirical Design ReviewFlexible
and RigidNew Design, Partial
Reconstructionand Overlays
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Flexible ME Review
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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

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• 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) lt 1
• 6. Select lowest cost strategy

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Example - Minimal input

• Traffic
• Materials properties
• stiffnesses, poisson ratios
• design equations
• Calculations

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Mechanistic-Empirical AC on AC Overlay Thickness
Designand Partial Reconstruction
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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

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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

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Existing Materials Stiffnesses Deflection Testing
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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

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Falling Weight Deflectometer (FWD)
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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
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Typical deflection bowl
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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

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Back-Calculation Example

• Deflection distance (m) 0 0.2 0.3 0.6 0.9 1.2 1.
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• 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

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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

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Pavement Characteristics Affecting Tire/Pavement
Noise
Non-Structural Properties
? gt 0.5 m
Roughness
50mm lt?lt 500 mm
0.5 mm lt?lt 50mm
?lt 0.5 mm
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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

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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

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Determination of Thicknesses

• Cores
• Dynamic Cone Penetrometer (DCP)
• Ground Penetrating Radar (GPR)
• resolution issues

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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

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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

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Subgrade Strain Rutting Criteria

• May also be conservative for rehabilitation if
  thick AC, high traffic

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Overlay Design Special Considerations

• pre-overlay repairs
• reflection crack control
• recycling
• subdrainage
• shoulders/widening
• lane/curb/bridge height matching

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AC Overlay Design Steps if Dont 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

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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

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Design Steps (3)

• Plot Nf, Nr vs. overlay thickness
• Select thicknesses that provides adequate design
  life for fatigue cracking, subgrade rutting

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Mechanistic-Empirical Overlay Design Review -
What Would You Do? (no reflection)

• Traffic
• Existing structure, condition
• Materials properties
• existing
• new
• Calculations

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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

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Mechanistic-Empirical Overlay Design Review -
What Would You Do If Recycling

• Traffic
• Existing structure, condition
• Materials properties
• existing
• reworked
• new
• Calculations

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Rigid ME Design Review
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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)

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• 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) lt 1
• 6. Select lowest cost strategy

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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)

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With -15 C gradient
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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

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Faulting vs Dowel Bearing Stress
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Fault Depth at 30 years vs Base Type
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Example - Minimal input

• Traffic
• Materials properties
• Calculations

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Example - Maximum input

• Traffic
• Materials
• Design Options
• Design Equations
• Calculations