Pavement Design

Overview Department Network Materials Asphalt Pavement Failure and Distress Modes Pavement Design Important Considerations for Prime Consultants 2

Context – Department Network 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 3

Typical Asphalt Pavement Structure 4

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 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 5

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 6

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 vs PG 58-28) –Rutting (e.g. PG vs PG 58-28) 7

Failure Modes of Asphalt Pavements 8 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 9

10 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 11

12

Other Failure Modes Asphalt mix rutting –Related to heavy and slow moving trucks –Typically seen at intersections, VIS 13

Other Failure Modes Shear failure rutting (asphalt layer) –Related to heavy and slow moving trucks –Typically seen at intersections, VIS 14

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 15

16 PG 52-28: 180 cracks/km

17 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: designmethods/] designmethods/ 18

Pavement Design Methods Standard sections Empirical design methods (AASHTO ’93) Mechanistic design methods Mechanistic-Empirical design methods (MEPDG) 19

AT Pavement Design Methodology Based on AASHTO 1993 method alberta.ca/Content/docTy pe233/Production/paved m2.pdfhttp:// alberta.ca/Content/docTy pe233/Production/paved m2.pdf 20

Rehabilitation Designs Pavement is triggered for rehab through our PMS Methodology follows our guidelines for assessing pavement preservations strategies on.alberta.ca/Content/ docType233/Productio n/gappts.pdfhttp:// on.alberta.ca/Content/ docType233/Productio n/gappts.pdf 21

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 22

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 23

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) 24

Pavement Design Inputs – Other Subgrade strength/existing pavement strength Design reliability 25

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 26

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) 27

Rehabilitation Process 1.Determine ESAL 2.Structural capacity: Falling Weight Deflectometer (FWD) data analysis –Evaluate for both 10 and 20 year options 28

FWD Data Analysis 29

Rehabilitation Example con’t. 3.Ride (IRI): is the pavement above, at, below or well below trigger? 30 AADTIRI TRIGGER (mm/m) < – – – >

IRI Data 31

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): No treatment Mill and inlay Overlay FDR, etc. 7.Perform a life cycle cost analysis (LCCA) 8.Select preferred treatment 32

Prime Consultant Responsibilities Section 9 (Surfacing Design) of ECG vol. 1 Confirm pavement condition has not changed from pavement design report 33

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 months before) a project is tendered the best before date should be checked 34

Example FWD Data Difference Design have a “best before date” that is included in all pavement design reports Width Design ESAL Age of Design 35

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 36

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 37 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 38

Important Considerations – Longitudinal Joints Not in wheel paths 39

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 Page.aspx?id= &lm= &q= &qz=5d0b25e6c c310ab68cf94http:// Page.aspx?id= &lm= &q= &qz=5d0b25e6c c310ab68cf94 40

Future Direction for Pavement Design 41

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