Pavement Design Overview Department Network Materials Asphalt Pavement

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 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 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 58 -34 vs PG 58 -28) – Rutting (e. g. PG 70 -28 vs PG 58 -28) 7

Failure Modes of Asphalt Pavements Tensile strain at bottom of ACP Compressive strain at top of subgrade 8

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

Fatigue Cracking 10

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

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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

PG 52 -28: 180 cracks/km 16

PG 52 -34: 4 cracks/km 17

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/] 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 • http: //www. transportation. alberta. ca/Content/doc. Ty pe 233/Production/paved m 2. pdf 20

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/ doc. Type 233/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 • http: //transportation. alberta. ca/649. htm 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? AADT IRI TRIGGER (mm/m) <400 3. 0 400 – 1500 2. 6 1501 – 6000 2. 3 6001 – 8000 2. 1 > 8000 1. 9 30

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 12 -18 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 consultant will confirm quantities and appropriateness of recommendations • Significant changes should be vetted through TSB 37

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 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 • http: //www. darwinme. org/MEDesign/Index. html • http: //www. industrymailout. com/Industry/Landing Page. aspx? id=1344725&lm=59549778&q=64027 2799&qz=5 d 0 b 25 e 6 c 46365572698 c 310 ab 68 cf 94 40

Future Direction for Pavement Design 41

Questions? marta. juhasz@gov. ab. ca ph: 780 -415 -0691 42