Transcript Slide 1

Pavement Design
Overview
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Department Network
Materials
Asphalt Pavement Failure and Distress Modes
Pavement Design
Important Considerations for Prime Consultants
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Context – Department Network
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31,300 2-lane km of roads
8 basic types of roads or pavements:
1. Ice
- 0 km
2. Earth
- 0 km
3. Gravel
~ 3,000 km
4. Thin Surfacing ~ 800 km (includes oiled)
5. Asphalt
~ 27,400 km (includes soil
cement base)
6. Composite
~ 60 km
7. Concrete
~ 60 km
8. Interlocking
- 0 km
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Typical Asphalt Pavement Structure
ASPHALT
GRANULAR BASE COURSE
SUBGRADE
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Materials - Subgrade
• Subgrade
– Is clay, i.e. typically weak
– Obtained from within the highway right of way
or from a borrow source (e.g. a farmer’s field)
– Ideally low plastic in nature
– Roadway embankment constructed in
accordance with specification 2.3 - Grading
– Placed and compacted at or near optimum
moisture content, in 150 mm lifts
– Min. QA req’mnts in Appendix B of ECG vol. 2
– Uniformity
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Materials – Granular Base Course
• Granular Base Course (GBC)
– Is crushed gravel
– Is almost all from glacio-fluvial deposits
– Produced in accordance with spec 3.2
– Constructed in accordance with spec 3.6, QA
req’mnts in Appendix B of ECG vol. 2
– Placed and compacted “quasi optimum
moisture”
– Placed and compacted in lifts between 100
and 200 mm thick
– 98% of control strip density
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Materials – Asphalt Concrete
Pavement (ACP)
• ACP is 95% aggregate and 5% asphalt cement
• Specifications 3.50 (ACP EPS) and 5.7 (Supply
of Asphalt)
• Visco-elastic
– Elastic – resists deformation, returns to it’s original
shape (Winter or faster rates of loading)
– Viscous – gradual deformation with strain (Summer or
slower rates of loading)
• Performance Graded (PG) asphalt cements PG
XX-YY
– Thermal cracking (e.g. PG 58-34 vs PG 58-28)
– Rutting (e.g. PG 70-28 vs PG 58-28)
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Failure Modes of Asphalt
Pavements
Tensile strain at
bottom of ACP
Compressive strain
at top of subgrade
ASPHALT
GRANULAR BASE COURSE
SUBGRADE
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Fatigue Cracking
• Occurs in the wheel paths
• Is in the asphalt layer only
• Starts at the bottom of the asphalt layer and
works up
• Also called alligator cracking
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Fatigue Cracking
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Subgrade Rutting
• This distress also manifests in the wheel paths
• Will show up in the granular and asphalt layers
also
• Typically wider “bowl” rutting
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Other Failure Modes
• Asphalt mix rutting
– Related to heavy and slow moving trucks
– Typically seen at intersections, VIS
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Other Failure Modes
• Shear failure rutting (asphalt layer)
– Related to heavy and slow moving trucks
– Typically seen at intersections, VIS
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Other Distresses
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Ravelling
Segregation
Potholes
Top-down cracking
Other non-load related cracking
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Centre of paver cracking
Longitudinal joint cracking
Block cracking (CSBC related or age/oxidation related)
Thermal cracking in asphalt layer
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PG 52-28: 180 cracks/km
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PG 52-34: 4 cracks/km
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Pavement Design Theory
• “The goal of structural design is to determine the
number, material composition and thickness of
the different layers within a pavement structure
required to accommodate a given loading
regime.” [ref: http://www.pavementinteractive.org/article/structuraldesignmethods/]
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Pavement Design Methods
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Standard sections
Empirical design methods (AASHTO ’93)
Mechanistic design methods
Mechanistic-Empirical design methods (MEPDG)
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AT Pavement Design Methodology
• Based on AASHTO 1993
method
• http://www.transportation.
alberta.ca/Content/docTy
pe233/Production/paved
m2.pdf
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Rehabilitation Designs
• Pavement is triggered
for rehab through our
PMS
• Methodology follows
our guidelines for
assessing pavement
preservations
strategies
• http://www.transportati
on.alberta.ca/Content/
docType233/Productio
n/gappts.pdf
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Related Design Bulletins
• Design Bulletin #13 – mix type and asphalt
cement grade selection
• Design Bulletin #15 – minimum first stage
pavement thicknesses
• Design Bulletin #27 – service class and width
requirements
• Design Bulletin #77 – special design
considerations
• http://transportation.alberta.ca/649.htm
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Pavement Design Inputs –
Design Life
• Typically 20 years for new construction
– Environment (climate) is a challenge
– May go with a 10 year design based on economics for
rehabilitation designs
– 50 year design where there are infrastructure
constraints (e.g. bridge deck approaches, roundabouts,
etc.)
– Longer design lives still require interim surface
rehabilitation because of environment
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Pavement Design Inputs –
Loading Regime
• It’s all about the trucks (and busses)!!
– Rule of thumb is 1 truck = 1,000 cars
• Determine the number of trucks and busses and
their loads
• But trucks vary from single units to B-trains to
triple trailers
• So we simplify various trucks and busses to
Equivalent Single Axle Load (ESAL)
• ESAL been basis for pavement design in North
America for 50 years
• Move is now away from ESAL toward axle load
spectra (but need accurate WIM data)
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Pavement Design Inputs –
Other
• Subgrade strength/existing pavement strength
• Design reliability
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Other Key Considerations in
Asphalt Pavement Design
• Low temperature cracking resistance
– a function of low temperature asphalt cement grade
• Rutting resistance
– a function of the aggregate skeleton and the high
temperature asphalt cement grade
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Pavement Design Process
• Regional consultant is assigned the pavement
design
• Process for design outlined in section 5.3 of ECG
vol. 1
• TSB audits the pavement design
• TSB also can provide in-house designs when
needed (limited capacity)
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Rehabilitation Process
1. Determine ESAL
2. Structural capacity: Falling Weight Deflectometer
(FWD) data analysis
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Evaluate for both 10 and 20 year options
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FWD Data Analysis
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Rehabilitation Example con’t.
3. Ride (IRI): is the pavement above, at, below or
well below trigger?
AADT
IRI TRIGGER (mm/m)
<400
3.0
400 – 1500
2.6
1501 – 6000
2.3
6001 – 8000
2.1
> 8000
1.9
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IRI Data
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Rehabilitation Example con’t.
4. Conduct a field inspection to assess general
condition and distress (rutting, cracking, etc.)
frequency and severity; measure width
5. Talk to the MCI
6. Select feasible treatment options (considering
width):
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No treatment
Mill and inlay
Overlay
FDR, etc.
7. Perform a life cycle cost analysis (LCCA)
8. Select preferred treatment
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Prime Consultant Responsibilities
• Section 9 (Surfacing Design) of ECG vol. 1
• Confirm pavement condition has not changed
from pavement design report
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Important Considerations –
Best Before Date
• All designs now have a “best before date” that is
included in all pavement design reports because:
– Design ESAL may change (rule of thumb: a doubling of
ESAL needs another 30 mm of ACP)
– FWD data may change (only good for 3 to 5 years)
– Width requirements may change (based on AADT)
• Older designs may not have a date so use FWD
test date
• Before (ideally 12-18 months before) a project is
tendered the best before date should be checked
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Example FWD Data Difference
• Design have a “best before date” that is included
in all pavement design reports
• Width
• Design ESAL
• Age of Design
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Important Considerations –
Mix Type and Grade Rationalization
• Individual pavement designs may often be
combined into one construction tender
• Mix types and asphalt cement grades may be
rationalized (simplified) in consultation with TSB
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Important Considerations –
Pre-construction Repairs
• Typical repairs:
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Spray patching
Crack mill and fill
Spot mill and inlay
Failure repairs
• Quantities in B estimate are an
estimate only
• Design may be a few years old
• Expectation is that prime
consultant will confirm quantities and
appropriateness of recommendations
• Significant changes should be vetted through TSB
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Important Considerations –
Width Confirmation
• Occasionally actual widths vary from AT crosssection data
• Can impact future treatment options
• Design report will request prime consultant to
confirm widths post-construction
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Important Considerations –
Longitudinal Joints
• Not in wheel paths
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Future Direction for Pavement
Design
• Move toward more mechanistic based pavement
design through AASHTO’s Mechanistic-Empirical
Pavement Design Guide (MEPDG)
– Much more data intensive
• Axle Load Spectra
• Climate station data
• Materials inputs such as dynamic modulus, etc.
– Requires calibration to Alberta performance
• http://www.darwinme.org/MEDesign/Index.html
• http://www.industrymailout.com/Industry/Landing
Page.aspx?id=1344725&lm=59549778&q=64027
2799&qz=5d0b25e6c46365572698c310ab68cf94
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Future Direction for Pavement
Design
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Questions?
[email protected]
ph: 780-415-0691
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