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Pavement Design

Pavement Design. CEE 320 Anne Goodchild. Design Parameters. Subgrade Loads Environment. Subgrade. Characterized by strength and/or stiffness California Bearing Ratio (CBR) Measures shearing resistance Units: percent Typical values: 0 to 20 Resilient Modulus (M R )

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Pavement Design

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  1. Pavement Design CEE 320Anne Goodchild

  2. Design Parameters • Subgrade • Loads • Environment

  3. Subgrade • Characterized by strength and/or stiffness • California Bearing Ratio (CBR) • Measures shearing resistance • Units: percent • Typical values: 0 to 20 • Resilient Modulus (MR) • Measures stress-strain relationship • Units: psi or MPa • Typical values: 3,000 to 40,000 psi Picture from University of Tokyo Geotechnical Engineering Lab

  4. Subgrade Some Typical Values

  5. Loads • Load characterization • Tire loads • Axle and tire configurations • Load repetition • Traffic distribution • Vehicle speed

  6. Load Quantification • Equivalent Single Axle Load (ESAL) • Converts wheel loads of various magnitudes and repetitions ("mixed traffic") to an equivalent number of "standard" or "equivalent" loads • Based on the amount of damage they do to the pavement • Commonly used standard load is the 18,000 lb. equivalent single axle load • Load Equivalency • Generalized fourth power approximation

  7. Typical LEFs Notice that cars are insignificant and thus usually ignored in pavement design.

  8. LEF Example The standard axle weights for a standing-room-only loaded Metro articulated bus (60 ft. Flyer) are: AxleEmptyFull Steering 13,000 lb. 17,000 lb. Middle 15,000 lb. 20,000 lb. Rear 9,000 lb. 14,000 lb. Using the 4th power approximation, determine the total equivalent damage caused by this bus in terms of ESALs when it is empty. How about when it is full?

  9. LEF Example • Empty • (13,000/18,000)4 = 0.272 • (15,000/18,000)4 = 0.482 • (9,000/18,000)4 = 0.063 • Total = 0.817 ESALs • Full • (17,000/18,000)4 = 0.795 • (20,000/18,000)4 = 1.524 • (14,000/18,000)4 = 0.366 • Total = 2.685 ESALs • Increase in total weight = 14,000 lb. (about 80 people) or 39% • Increase in ESALs is 1.868 (229%)

  10. Pavement Design • Several typical methods • Design catalog • Empirical • Regression analysis from real-world tests

  11. Design Catalog Example design catalog from the Washington Asphalt Pavement Association (WAPA) for residential streets

  12. Empirical • 1993 AASHTO Flexible Equation • 1993 AASHTO Rigid Equation

  13. Terms – Flexible • W18 (loading) • Predicted number of ESALs over the pavement’s life. • SN (structural number) • Abstract number expressing structural strength requirement • SN = a1D1 + a2D2m2 + a3D3m3 + … • ΔPSI (change in present serviceability index) • Change in serviceability index over the useful pavement life • Typically from 1.5 to 3.0 • MR (subgrade resilient modulus) • Typically from 3,000 to 30,000 psi (10,000 psi is pretty good)

  14. Terms – Rigid • D (slab depth) • Abstract number expressing structural strength • SN = a1D1 + a2D2m2 + a3D3m3 + … • S’c (PCC modulus of rupture) • A measure of PCC flexural strength • Usually between 600 and 850 psi • Cd (drainage coefficient) • Relative loss of strength due to drainage characteristics and the total time it is exposed to near-saturated conditions • Usually taken as 1.0

  15. Terms – Rigid • J (load transfer coefficient) • Accounts for load transfer efficiency • Lower J-factors = better load transfer • Between 3.8 (undoweled JPCP) and 2.3 (CRCP with tied shoulders) • Ec (PCC elastic modulus) • 4,000,000 psi is a good estimate • k (modulus of subgrade reaction) • Estimates the support of the PCC slab by the underlying layers • Usually between 50 and 1000 psi/inch

  16. Reliability • ZR*S0 • ZR – probability that serviceability will be maintained at adequate levels from a user’s point of view throughout the design life of the facility. • Accounts for inherent uncertainty in design. • Want higher reliability on more “important” facilities. Of course this has cost implications • S0 –standard deviation in traffic, variability in materials and construction practices

  17. Reliability Reliability = P [Y > X] X = Probability distribution of stress(e.g., from loading, environment, etc.) Y = Probability distribution of strength(variations in construction, material, etc.) Probability Stress/Strength

  18. WSDOT Flexible Table

  19. ATB – Asphault Treated Base • Dense-graded HMA with a larger nominal maximum aggregate size (1 inch) intended for use as a base course • Poor subgrade in high volume cases • Also called asphalt concrete base (ACB)

  20. WSDOT Rigid Table

  21. Design Utilities From Pavement Interactive http://pavementinteractive.org/index.php?title=Module:Structural_Design (see lower right of page for the “design utilities”)

  22. Design Example – Part 1 A WSDOT traffic count on Interstate 82 in Yakima gives the following numbers: ParameterDataWSDOT Assumptions AADT 18,674 vehicles Singles 971 vehicles 0.40 ESALs/truck Doubles 1,176 vehicles 1.00 ESALs/truck Trains 280 vehicles 1.75 ESALs/truck Assume a 40-year pavement design life with a 1% growth rate compounded annually. How many ESALs do you predict this pavement will by subjected to over its lifetime if its lifetime were to start in the same year as the traffic count?

  23. Part 1 • First year ESALs • ESALs in traffic count year = 0.40(971) + 1.00(1176) + 1.75(280) = 2,054.4 ESALs/day • Total ESALs = 2,054.4 × 365 = 749,856

  24. Part 1 • In 40 years • Total ESALs = 749,856((1+0.01)40-1)/0.01 = 36,657,740 ESALs

  25. Design Example – Part 2 • Design a flexible pavement for this number of ESALs using (1) the WSDOT table. Assume the following: • Reliability = 95% (ZR = -1.645 , S0 = 0.50) • ΔPSI = 1.5 (p0 = 4.5, pt = 3.0) • 2 layers (HMA surface and crushed stone base) HMA coefficient = 0.44, minimum depth = 4 inches Base coefficient = 0.13, minimum depth = 6 inches Base MR = 28,000 psi • Subgrade MR = 9,000 psi

  26. Design Part 2 • Design period ESALs = 25 to 50 million • Subgrade condition = Average • HMA surface course = 0.35 ft. • HMA base course = 0.75 ft. • Crushed stone = 0.45 ft. • Total depth of HMA = 1.1 ft = 13 inches • Total depth of base = 0.45 ft. = 5.4 inches, this is rounded up to the minimum of 6 inches • SN = 0.44(13 inches) + 0.13(6 inches) = 6.50

  27. Design Example – Part 3 • Design a doweled JPCP rigid pavement for this number of ESALs using the WSDOT table. Assume the following: • Reliability = 95% (ZR = -1.645 , S0 = 0.40) • ΔPSI = 1.5 (p0 = 4.5, pt = 3.0) • EPCC = 4,000,000 psi • S’C = 700 psi • Drainage factor (Cd) = 1.0 • Load transfer coefficient (J) = 2.7 • Modulus of subgrade reaction (k) = 400 psi/in • HMA base material

  28. Design Example – Part 3 • Doweled joints, HMA base material • 25 to 50 million ESALs • Reliability = 95% • 0.97 ft. = 12 inches

  29. Primary References • Mannering, F.L.; Kilareski, W.P. and Washburn, S.S. (2005). Principles of Highway Engineering and Traffic Analysis, Third Edition. Chapter 4 • Muench, S.T.; Mahoney, J.P. and Pierce, L.M. (2003) The WSDOT Pavement Guide Interactive. WSDOT, Olympia, WA. http://training.ce.washington.edu/WSDOT • Muench, S.T. (2002) WAPA Asphalt Pavement Guide. WAPA, Seattle, WA. http://www.asphaltwa.com

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