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

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Presentation on theme: "Atlantic City, NJ / April 25, 2012 Federal Aviation Administration Airport Pavement Working Group Meeting Mechanistic-Empirical PCN Procedure Gabriel Bazi,"— Presentation transcript:

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

2 DCIDCI History of Dynatest ME PCN Procedure ELPCN 1987 – 1998 DOS Version WinPCN 1998 – 2003 Windows Version ELMOD PCN 2003 – Present ELMOD sub-module 2

3 DCIDCI 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 3 Presented today

4 DCIDCI 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)… 4

5 DCIDCI 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 … 5

6 DCIDCI 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 6

7 DCIDCI Layer Moduli New Design WES modulus procedure for unbound layer moduli [UFC TM /AFJMAN ] Currently used in FAARFIELD Lab/Assumed Existing structures (Rehabilitation) HWD testing and backcalculation LWD for unbound layers 7

8 DCIDCI 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 Horizontal tensile strain at the bottom of the AC layer Vertical compressive stress or strain on top of the base layer Vertical compressive stress or strain on top of the subgrade AC AB SG

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

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

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

12 DCIDCI Permissible Gross Weight (w p ) 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, w p : permissible gross weight, and d t : total damage B : parameter from empirical model 12 1) Assuming one model is used per criterion 2) Refer to Appendix A for alternative calculation procedure Permissible Weight Multiplier

13 DCIDCI Various Forms of Empirical Models 13 Use B in previous equation to determine w p

14 DCIDCI 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 14

15 DCIDCI 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 15

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

17 DCIDCI 17 Aircraft Mix used in the Analyses Aircraft MTOW (lb) % Gross on One Main Gear Type Wheel Spacing (in) Axle Spacing (in) Tire Pressure (psi) Annual Coverages n Adv. B C Basic 185, Dual ,000 A std 509, Dual Tandem ,000 A B Notes: Coverages assumed to be same for AC fatigue and AB/SG PD failure criteria. Load uniformly distributed throughout the year

18 DCIDCI AC Fatigue Cracking Failure Model AC Fatigue Cracking Failure Model (USACE/FAA) log(C) = 2.68 – 5 log( ) – 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: 18

19 DCIDCI 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 E 0 = 23.2 ksi C = 1.16 for E

20 DCIDCI 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 dt B = PCN analysis is controlled by the AC layer fatigue cracking 20 LayerCriterion Total Damage dt Remaining Life (Years) 1) B Permissible Weight Multiplier (dt B ) 1AC Fatigue Cracking – AB Permanent Deformation – SG Permanent Deformation0.103> 40– ) Lowest Permissible Weight Multiplier (dt B )0.826

21 DCIDCI 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 21 Aircraft Gross Weight w (lb) Permissible Weight Multiplier (dt B ) Permissible Gross Weight (w p ) ACN = PCN 1) Adv. B C Basic 185, , A std509, , Select largest ACN as PCN for the structure45.4

22 DCIDCI 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 22 AircraftACN A std57/F/A

23 DCIDCI 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 23 Subgrade CodeACNPCNACN-PCN Ratio A57.0/F/A45.4/F/A/X/T B61.9/F/B48.3/F/B/X/T 1.28 C71.6/F/C54.8/F/C/X/T 1.31 D96.8/F/D73.1/F/D/X/T 1.32

24 DCIDCI If Only SG Criterion was Considered!! Permissible Weight Multiplier (dt B ) = > 1.0 PCN >ACN Knowing that the pavement is failing (RL = 7.7 years) 24 C ONSIDER ALL FAILURE CRITERIA FOR PCN C ALCULATION

25 DCIDCI 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 84 th percentile (Average – standard deviation) 25

26 DCIDCI 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

27 DCIDCI 27 Discussion & Questions

28 DCIDCI 28 Thank You

29 DCIDCI Appendix A: Permissible Gross Weight (w p ) 1.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 ( d t ) 2.Calculate the allowable stress or strain using the empirical model for the equivalent number of load repetitions calculated in step 1 3.Calculate the permissible gross weight (w p ) 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 29

30 DCIDCI Appendix B: Rigid Pavement Example 2 Seasons: 30 E 1 = 4,000 ksi, = 0.15 E 3 = 7.5 ksi, = 0.35 k = 82.4 pci 1) Code D PCC: SG: Season 1 (6 months) h = 14 in h = Season 2 (6 months) E 3 = 15 ksi, = 0.35 k = pci 1) Code C E 1 = 4,000 ksi, = ) Equation used to convert E (psi) to k (pci): E SG = 26 k FAA AC 150/5320-6E page 34 Note: Interface assumed to be bonded

31 DCIDCI 31 Aircraft Mix used in the Analyses Aircraft MTOW (lb) % Gross on One Main Gear Load (lb) Gear Type Wheel Spacing (in) Tire Pressure (psi) Annual Coverages n B , ,725 Dual342051,200 A , ,874 Dual ,500 A B Note: Load uniformly distributed throughout the year

32 DCIDCI 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 32

33 DCIDCI Rigid Pavement Example Analysis period = 20 years 1) Responses calculated using MnLayer at center of slab dt B = 1 PCN = ACN Remaining life = Analysis period (20 years) 33 LayerCriterion Total Damage dt Remaining Life (Years) 1) B Permissible Weight Multiplier (dt B ) 1PCC Fatigue Cracking –

34 DCIDCI PCN Weighted k-value Subgrade Code C PCN Calculation: 1) ACN for permissible gross weight calculated using COMFAA PCN: 54/R/C/W/T 34 Aircraft Gross Weight w (lb) Permissible Weight Multiplier (dt B ) Permissible Gross Weight (w p ) ACN = PCN 1) B , , A , , Select largest ACN as PCN for the structure54.1


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