Pavement Design

Overview

• Department Network • Materials• Asphalt Pavement Failure and Distress Modes• Pavement Design • Important Considerations for Prime Consultants

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Context – Department Network

• 31,300 2-lane km of roads• 8 basic types of roads or pavements:

1. Ice - 0 km2. Earth - 0 km3. Gravel ~ 3,000 km4. Thin Surfacing ~ 800 km (includes

oiled)5. Asphalt ~ 27,400 km

(includes soil cement base)6. Composite ~ 60 km7. Concrete ~ 60 km8. Interlocking - 0 km

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Typical Asphalt Pavement Structure

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ASPHALT

GRANULAR BASE COURSE

SUBGRADE

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

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ASPHALT

GRANULAR BASE COURSE

SUBGRADE

Tensile strain at bottom of ACP

Compressive strain at top of subgrade

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

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

• Ravelling• Segregation• Potholes• Top-down cracking• Other non-load related cracking

– 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

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/structural-designmethods/]

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Pavement Design Methods

• 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/docType233/Production/pavedm2.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.transportation.alberta.ca/Content/docType233/Production/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 ESAL2. Structural capacity: Falling Weight Deflectometer

(FWD) data analysis– 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?

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AADT IRI TRIGGER (mm/m)

<400 3.0

400 – 1500 2.6

1501 – 6000 2.3

6001 – 8000 2.1

> 8000 1.9

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 MCI6. Select feasible treatment options (considering

width):• 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:

– 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

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consultant will confirm quantities and appropriateness of recommendations• Significant changes should be vetted through TSB

Important Considerations – Width Confirmation• Occasionally actual widths vary from AT cross-

section 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=640272799&qz=5d0b25e6c46365572698c310ab68cf94

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Future Direction for Pavement Design

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Questions?

marta.juhasz@gov.ab.caph: 780-415-0691

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