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Atlantic City, NJ / April 25, 2012

Federal Aviation Administration Airport Pavement Working Group Meeting M echanistic- E mpirical PCN Procedure Gabriel Bazi, PhD, PE. Atlantic City, NJ / April 25, 2012. History of Dynatest ME PCN Procedure. Revised ME PCN Procedure.

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Atlantic City, NJ / April 25, 2012

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  1. Federal Aviation Administration Airport Pavement Working Group Meeting Mechanistic-Empirical PCN Procedure Gabriel Bazi, PhD, PE Atlantic City, NJ / April 25, 2012

  2. History of Dynatest ME PCN Procedure

  3. Revised ME PCN Procedure • Existing and revised procedures are similar except for the last step in PCN calculation • Existing Procedure • Permissible response • ESWL • PCN • Revised procedure • Permissible response • Permissible gross weight • PCN Presented today

  4. ME PCN Procedure Advantages • ME PCN procedure has same advantages as ME analysis • Flexible and rigid pavements • Rehabilitation or new design • Layer moduli (stiffnesses) • No need for layer equivalency factors/equivalent pavement… • Considers the failure modes of all layers: • Fatigue cracking for AC and PCC layers • Permanent deformation for unbound layers (base, subbase, subgrade)…

  5. ME PCN Procedure Advantages • Considers various seasons • AC moduli change due to temperature • Unbound layer moduli change due to moisture (& freezing and thawing) • Different aircrafts (weights, repetitions….) • Aircraft lateral wander • Normal distribution • P/C • …

  6. ME PCN Procedure • 100% compatible with ME design • If Remaining life (RL) = Design Life (DL)  PCN = ACN • If RL < DL  PCN < ACN • If RL > DL  PCN > ACN

  7. Layer Moduli • New Design • WES modulus procedure for unbound layer moduli [UFC TM 5-822-13/AFJMAN 32-1018] • Currently used in FAARFIELD • Lab/Assumed • Existing structures (Rehabilitation) • HWD testing and backcalculation • LWD for unbound layers

  8. Flexible Pavement Structural Evaluation • Calculate critical stresses and strains under load • Fatigue cracking: Horizontal tensile strain at bottom of AC • Permanent deformation in unbound layers: Vertical compressive stress or strain on top of layer Vertical compressive stress or strain on top of the base layer AC Horizontal tensile strain at the bottom of the AC layer Vertical compressive stress or strain on top of the subgrade AB SG

  9. Rigid Pavement Design • Calculate critical stresses under load • Fatigue cracking: Horizontal tensile stress at bottom of PCC PCC Horizontal tensile stress at the bottom of the PCC layer Support

  10. ME Structural Evaluation • For each failure criterion, calculate the total damage (dt) using empirical models • If dt 1.0  Structure is adequate • If dt> 1.0  Structure is not adequate • Remaining life • Limit remaining life to 40 years Structural Evaluation is Complete

  11. Permissible Gross Weight (wp) Each aircraft from mix is evaluated separately (or just evaluate aircraft with largest ACN): Total damage (dt) calculated for all aircrafts combined If dt < 1  Increase dt to 1: wp > gross weight  PCN > ACN If dt > 1  Decrease dt to 1: wp < gross weight  PCN < ACN

  12. Permissible Gross Weight (wp) 1) 2) • The permissible gross weight is the weight that would cause a total damage of 1.0 with the traffic mix converted to each aircraft. where w: gross weight of aircraft, wp: permissible gross weight, and dt: total damage B: parameter from empirical model Permissible Weight Multiplier 1) Assuming one model is used per criterion 2) Refer to Appendix A for alternative calculation procedure

  13. Various Forms of Empirical Models Use B in previous equation to determine wp

  14. PCN Calculation • Using the permissible gross weight, calculate the ACN for each aircraft and assign it as its PCN • COMFAA • Power curve fit using ACNs for min. and max. weights • Polynomial curve fit using ACNs for min., max., ½ min. and 2max. weights • Select the largest PCN from the mix as the PCN for the structure

  15. Subgrade Category for ACN/PCN • If subgrade category (CBR or k-value) changes • along a feature (e.g. FWD test points along a runway, or various CBR tests on a taxiway…) • determine average conditions for PCN calculation • over various seasons • determine weighted average conditions for PCN calculation

  16. Flexible Pavement Example • 2 Seasons: Season 2 (9 months) Season 1 (3 months) h = 6 in AC: E1= 250 ksi E1= 500 ksi AB: h = 36 in E2= 37.5 ksi E2= 50 ksi E3= 6 ksi CBR = 4 Code D E3= 24 ksi CBR = 16 Code A SG: h = ∞  = 0.35 for all layers

  17. Aircraft Mix used in the Analyses B727-200 A330-200 Notes: Coverages assumed to be same for AC fatigue and AB/SG PD failure criteria.  Load uniformly distributed throughout the year

  18. AC Fatigue Cracking Failure Model • AC Fatigue Cracking Failure Model (USACE/FAA) log(C) = 2.68 – 5log() – 2.665log(E) where: C = number of coverages to failure E = AC modulus (psi)  = horizontal strain at the bottom of the surface asphalt layer • For this model:

  19. AB/SG Permanent Deformation Model • Unbound Material Permanent Deformation (Kirk) where: 1,p = vertical stress on top of unbound layer, ksi N = number of coverages in millions E = modulus of material, ksi E0 = 23.2 ksi C = 1.16 for E<E0 C = 1 for E E0 • For this model:

  20. Flexible Pavement Example • Analysis period = 20 years 1) Analysis performed using ELMOD (w/ LET) 2) Calculated using a maximum remaining life of 40 years • Lowest dtB = 0.826  PCN analysis is controlled by the AC layer fatigue cracking

  21. PCN • Weighted subgrade modulus • CBR = 13  Subgrade code A • PCN Calculation: 1) ACN for permissible gross weight calculated using COMFAA • PCN: 45/F/A/X/T

  22. ACN-PCN Comparison • Largest ACN: • PCN: 45/F/A/X/T • Remaining life (7.7 years) < Design life (20 years)  PCN < ACN • Analysis controlled by AC fatigue cracking • About 3-inch of AC are needed to restore the structural capacity

  23. Subgrade Category Selection • ACN-PCN Ratios calculated for various subgrade categories • ACN-PCN ratios are almost the same for all subgrade categories • Subgrade code selection is not critical (at this level) • Subgrade properties are important in ME analysis

  24. If Only SG Criterion was Considered!! • Permissible Weight Multiplier (dtB) = 1.237 > 1.0 • PCN >ACN • Knowing that the pavement is failing (RL = 7.7 years) Consider all failure criteria for PCN Calculation

  25. Notes • For PCN calculation, limit calculated remaining life to 40 years • PCN values are associated with the traffic used in the evaluation • An increase in traffic during the evaluation period will reduce the PCN • PCN is highly dependent on aircraft mix • Existing structures: PCN to be calculated for existing conditions and after rehabilitation, if needed • When PCN is evaluated for several points on a feature, report 84th percentile (Average – standard deviation)

  26. Notes • Possibly complement the PCN number with the additional number of allowable coverages of aircraft having ACN = PCN • Instead of having unlimited operations when ACN/PCN ≤ 1.0

  27. Discussion & Questions

  28. Thank You Thank You

  29. Appendix A: Permissible Gross Weight (wp) • For every aircraft, calculate the equivalent number of load repetitions that would cause the same total damage (dt) as the aircraft mix = Allowable number of load reps. (N)  Total damage (dt) • Calculate the allowable stress or strain using the empirical model for the equivalent number of load repetitions calculated in step 1 • Calculate the permissible gross weight (wp) as the MTOW multiplied by the ratio of allowable stress or strain (from step 2) and the actual stress or strain of that aircraft (use of response ratio is valid if contact area is constant) Note: If various seasons are available, the use of any season would provide same permissible gross weight

  30. Appendix B: Rigid Pavement Example • 2 Seasons: Season 2 (6 months) Season 1 (6 months) h = 14 in PCC: E1= 4,000 ksi,  = 0.15 E1= 4,000 ksi,  = 0.15 E3= 7.5 ksi,  = 0.35 k = 82.4 pci 1) Code D E3= 15 ksi,  = 0.35 k = 141.4 pci 1) Code C SG: h = ∞ 1) Equation used to convert E (psi) to k (pci): ESG = 26k1.284 FAA AC 150/5320-6E page 34 Note: Interface assumed to be bonded

  31. Aircraft Mix used in the Analyses A320-100 B737-800 Note: Load uniformly distributed throughout the year

  32. PCC Fatigue Cracking Failure Model • PCC Fatigue Cracking Failure Model (PCA) where: PCC = Tensile stress at bottom of PCC (ksi) N = No. of coverages to failure in millions E = Modulus of PCC (ksi) • For this model

  33. Rigid Pavement Example • Analysis period = 20 years 1) Responses calculated using MnLayer at center of slab • dtB = 1  PCN = ACN • Remaining life = Analysis period (20 years)

  34. PCN • Weighted k-value  Subgrade Code C • PCN Calculation: 1) ACN for permissible gross weight calculated using COMFAA • PCN: 54/R/C/W/T

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