pavement design overview · 2014. 4. 2. · basic aashto flexible pavement design method •...
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Pavement Design Overview
Rebecca S. McDaniel February 2, 2012
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Plan • Review types of pavements
– Features – Advantages and Disadvantages – Typical Distresses
• Common design techniques/considerations – AASHTO – Mechanistic-Empirical
• Resources
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Basic Pavement Types
• Unbound
• Flexible
• Rigid
• Composite
Primary difference is in how loads are distributed to subgrade.
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Typical Pavement Layers • Wearing course or surface • Base course • Subbase • Subgrade
– Compacted – Stabilized (lime, cement, fly ash) – Natural
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Subbase/Base Courses May be used for: • Drainage • Construction platform • Controlling pumping • Controlling frost action • Controlling shrink and swell of subgrade • Optional for rigid pavements depending
on conditions
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Surface Courses • Safety • Traffic Loads • Environmental Factors
– Temperature extremes – Moisture
• Other Considerations – Economics – Initial and Life Cycle – Traffic Disruptions – Smoothness – Noise
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Flexible Pavements • Multiple, thin layers • Each layer spreads load
to next layer • Pavement deflects • Loads over a smaller
area of subgrade • Easily, routinely recycled • Typical service lives 15-
20 years to first rehab
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Advantages of Flexible Pavement
• Adjusts to limited differential settlement • Easily, quickly constructed and repaired • Additional thickness can be added • Quieter and smoother (generally) • More “forgiving” • Can be used for a wide range of traffic
levels in different applications (mainline, shoulders, ramps, parking, etc.)
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Disadvantages
• Generally shorter service life before first rehabilitation
• May experience moisture problems • Distresses include rutting, cracking
(thermal, reflective, fatigue)
• Properties may change over time as pavement ages
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Repeated Bending
Leads to Fatigue Cracking
Fatigue Cracking
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Perpetual Pavement • Asphalt pavement designed to last over
50 years without major structural rehabilitation needing only periodic surface renewal. – Full-depth pavement – constructed on
subgrade – Deep-strength pavement – constructed on
thin granular base course – AKA extended-life pavement or long-life
pavement
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Perpetual Pavement Concept • Asphalt pavements with high enough
strength will not exhibit structural failures. • Distresses will initiate at the surface,
typically in the form of rutting or cracking. • Surface distresses can be removed/
repaired relatively easily, • Before causing structural damage, • Leaving most of pavement in place,
performing well.
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Perpetual Pavement Features • Each layer designed to resist specific
distresses • Base – designed to resist fatigue and
moisture damage, to be durable • Intermediate/binder – designed for
durability and stability (rut resistance) • Surface – designed to resist surface
initiated distresses (top-down cracking, rutting, other)
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Surface Renewal • Repair surface distresses before they
become structural – Mill and fill – Thin overlay
• Quick • Cost effective
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Rigid Pavements • Generally stiffer – little
bending • Distribute loads over large
area of subgrade • Can be recycled, but less
common • Service lives 20-40 years
(to first major rehab)
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Advantages of Rigid Pavement • Good durability and long service life • Minor variations in subgrade strength have
little effect • Withstand repeated flooding and subsurface
water without deterioration (as long as base and/or subgrade are resistant to moisture damage)
• Can be used in wide range of applications but typically higher traffic volumes.
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Disadvantages
• Distresses may be harder/more expensive to repair – Cracking, faulting, punchouts (CRC),
roughness • May polish (lose frictional properties) over time • Needs even subgrade support (slab to slab) • Generally (but not always) considered more
expensive to construct
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Concrete Slab Temperature and Moisture Gradients
Slab wetter on top
Slab dryer on top
Curling Warping
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Choosing a Pavement Type • Many states have guidelines or policies • Driven by engineering and economic
considerations (preferred) • Sometimes influenced by other
considerations
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Pavement Design Considerations
• Pavement Performance • Traffic • Subgrade Soil Conditions • Availability and Cost of Materials • Environment • Drainage • Reliability • Life Cycle Costs • Constructability • Shoulder Design
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State-of-the-practice
State-of-the-art Actual current practice
Design Methodologies
Empirical Mechanistic- Empirical Mechanistic
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AASHO Road Test
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AASHO Road Test Achievements
• Serviceability concept - PSI • Traffic damage factors – ESALs • Structural number concept – SN • Empirical Process
– Statistical models from road test
• Simplified Pavement Design • Used for over 50 years
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Serviceability
• Ability of a pavement to serve the traffic for which it was designed
• User rating of performance plus measured physical features of the pavement (such as rut depth, cracking, etc.)
• When serviceability reaches a certain level, rehab or maintenance is needed
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Structural Number Concept • Determine SN needed to carry the traffic over
the soil conditions in the region • Empirical layer coefficients (ai) reflect how that
material will contribute to the structural strength of the pavement
• Determine layer thicknesses (Di) to achieve required SN
SN = a1D1 + a2D2 + a3D3 +…
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Basic AASHTO Flexible Pavement Design Method
• Determine acceptable terminal serviceability, pt • Convert traffic volumes to number of
equivalent 18-kip single axle loads (ESAL) • Determine the structural number, SN, needed • Determine the layer coefficients, ai, for materials
you want to use • Solve layer thickness equations for individual
layer thicknesses
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Basic AASHTO Rigid Pavement Design Method
• Select terminal serviceability • Determine number of ESALs • Determine the modulus of subgrade
reaction • Determine the slab thickness needed
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Limitations of AASHO Road Test
• One climate – Ottawa, Illinois • Limited Span – two years • Limited Traffic – generally < 2 million • 1950’s vehicles • 1950’s materials and construction • Only new construction
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• • • • • •
•
Data Limits (AASHO Road Test)
Current Designs >100 million
<2 million
AXLE LOAD REPETITIONS
PAVE
MEN
T TH
ICK
NES
S Current design traffic is far beyond road test limits
What Is Wrong with Present System?
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What Would be Better?
ε t ε
δ
Fundamental Mechanistic - Empirical Principles
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M-E Design • Considers applied stresses and resulting
strains • Uses fundamental engineering properties
that can be measured • Computes reactions to stresses and strains • Predicts distresses using empirical models • Feasible with improved computing
capabilities
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• Mechanistic-Empirical Pavement Design Guide (now marketed as DARWin-ME)
• Allow analysis of designs for: – New construction (flexible and rigid) – Composite pavement designs – Rehabilitation and overlays
• Evaluates effects of design alternatives and specification changes
MEPDG
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M-E Pavement Design Process
Climate
Structure
Response
Materials
Distress Time Damage
Accumulation
Traffic
Iterations
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Basic Concept Behind MEPDG • Determine acceptable levels of distress • Estimate traffic loading • Determine material properties and climatic
effects on those materials • Select trial structure • Calculate distresses in that structure based
on response to traffic and climate • Are distresses acceptable? If not, try again.
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Hierarchical Levels
Level Source Usage
Three Defaults Routine projects
Two Correlations Routine significant projects
One Project specific data
Research, forensics and high level projects
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Numerous Input Parameters
• Materials properties change with time and environment
• Calculates incremental damage for each load
• Damage is dependent upon stress-strain and material properties at time of loading
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Pavement Design Variables
Time, years
Granular Base Modulus
Subgrade Modulus
Traffic PCC Modulus CTB
Modulus
Each load application
2 8 6 4 0
AC Modulus
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Fatigue Cracking
Thermal Cracking
Longitudinal Cracking
IRI
Rutting
Predicted Distresses - Flexible
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Graph Example Output – Rutting
Total
AC
Base
Subgrade
Verify against design criterion specified by agency
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Rigid Pavement Performance
Transverse Cracking Punchout
IRI
Faulting
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Slab Thickness
0.35
1.23
0.80
1.37
0.87
5.00
0.0
1.0
2.0
3.0
4.0
5.0
Slab Thickness 10" Slab Thickness 14"
Design Variables
Dis
tres
s R
atio
(to
Ref
eren
ce) Cracking
FaultingIRI
Reference DesignSlab Thickness = 12"Cracking = 18.1%Faulting = 0.23 in.IRI = 192.1 in/mile
Slab Thickness - JPCP
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Constructability
• Remember the pavement must be constructed.
• Use reasonable layer thicknesses – Too thick or too thin will result in
inadequate compaction – See design manuals and specifications for
details
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Pavement Design Resources • AASHTO 1993 Pavement Design Guide
• http://www.in.gov/indot/design_manual/
• MEPDG not available online – alternates include:
o Perpetual Pavement Design Software – PerRoad http://asphaltroads.org/PerpetualPavement
o http://www.acpa.org/StreetPave/index.asp
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Rebecca S. McDaniel, PE, PhD Technical Director North Central Superpave Center 765/463-2317 ext. 226 [email protected] https://engineering.purdue.edu/NCSC/