1. AAR-410 February 2, 20051 Alpha Factor Determination for 6-Wheel Gears u Gordon Hayhoe, AAR-410, FAA William J. Hughes Technical Center, Atlantic City, New Jersey, U.S.A. u Need for evaluation u Full-scale test structures and results u Procedure for calculating alpha factors u Alpha factor Proposals for consideration by ICAO u Implications for thickness design

2. AAR-410 February 2, 20052 B-777 Six-Wheel ACNs u For flexible pavements, the ACNs initially computed for B-777 6-wheel gears appeared to be unreasonably high. u The FAA had similar concerns about the existing CBR method for 6-wheel gears. u A380 also has 6-wheel body gears.

3. AAR-410 February 2, 20053 Alpha Factors – MWHGL Data

4. AAR-410 February 2, 20054 Interim 6-Wheel Alpha Factor at 10,000 Coverages u 4-Wheel alpha = 0.825 u “Original” 6-wheel alpha = 0.788 (inception to 1995) u “Interim” 6-wheel alpha = 0.72 (1995 to present) u Current 12-wheel alpha = 0.722

5. AAR-410 February 2, 20055 Alpha Factors – MWHGL Data C5-A as one 12-wheel gear C5-A as two 6-wheel gears

6. AAR-410 February 2, 20056 National Airport Pavement Test Facility (NAPTF) for 6-Wheel Tests u Joint FAA and Boeing. u Testing is funded and conducted entirely by the FAA. u Tests run on flexible test items to compare 4-wheel and 6-wheel gears. u Construction cycles CC1 etc.

7. AAR-410 February 2, 20057 NAPTF Construction Cycles u CC1 = original construction. u Conventional and stabilized base flexible on low- strength subgrade (LFC and LFS). u Conventional and stabilized base flexible on medium- strength subgrade (MFC and MFS). u CC2 = rigid pavements, trafficking completed. u CC3 = flexible pavement reconstruction with four conventional test items, trafficking and posttraffic testing completed.

8. AAR-410 February 2, 20058 CC3 Test Pavements - Profile Direction of Traffic

9. AAR-410 February 2, 20059 North, 6-Wheel Track LFC1 LFC2 LFC3 LFC4

10. AAR-410 February 2, 200510 Trench in LFC2 Flexible

11. AAR-410 February 2, 200511 Computation of Alpha Factor u Pass/Coverage ratios calculated from surface coverages in test wander pattern: u 4-Wheel = 2.36 for CC3 and 2.06 for CC1 u 6-Wheel = 1.57 u Subgrade CBR = trench measurements. u Total structure thicknesses are known. u Contact area = 265 square inches. u Compute Alpha using COMFAA.

12. AAR-410 February 2, 200512 CBR Equations Post-MWHGL equation: t =  (A c ) 0.5 [-0.0481 – 1.1562 (log CBR/P) – 0.6414 (log CBR/P) 2 – 0.473 (log CBR/P) 3 ] Pre-MWHGL equation: Solve the Post-MWHGL equation for  OR t = Total Thickness P = ESWL

13. AAR-410 February 2, 200513 Change the Input Alpha until the design thickness is equal to the test structure thickness.

14. AAR-410 February 2, 200514 MWHGL Subgrade CBR Measurements u The CBR of the subgrade for each MWHGL test item was calculated from all available measurements: u After construction, before traffic. u Trench and pit after traffic at surface, 12-inch, and 24-inch depth.

15. AAR-410 February 2, 200515 Summary of NAPTF Flexible Pavement Full-Scale Test Results * Extrapolated from rut depth curve Bold = corrected values

16. AAR-410 February 2, 200516 NAPTF and MWHGL Alpha Factor Results (No conversion of NAPTF to MWHGL structures)

17. AAR-410 February 2, 200517 LEDFAA 1.3 Flexible Failure Model

18. AAR-410 February 2, 200518 NAPTF versus MWHGL Test Results u NAPTF pavements tended to last longer than MWHGL pavements. Possible reasons for this are: u Indoor NAPTF operation means lower asphalt temperatures. u NAPTF asphalt and base layers are thicker. u NAPTF subbase material is of higher quality (strength – screenings versus uncrushed aggregate).

19. AAR-410 February 2, 200519 Procedure for Converting NAPTF Structures to Equivalent MWHGL Structures (Example) (a) real structure, 29.0 in. (b) convert 2 in. AC to 3.2 in. CA (E.F. 1.6) (c) add 3.2 in. CA to exist. 8 in. CA = 11.2 in. CA (d) convert 5.2 in. CA to 8.3 in. SQS (E.F. 1.6) (e) convert 16 in. HQS to 19.2 in. SQS (E.F. 1.2) (f) equivalent MWHGL structure, 36.5 in. Steps:

20. AAR-410 February 2, 200520 NAPTF Flexible Pavement Equivalent Thicknesses and Alpha Factors

21. AAR-410 February 2, 200521 NAPTF and MWHGL Alpha Factor Results (With conversion of NAPTF to MWHGL structures)

22. AAR-410 February 2, 200522 NAPTF and MWHGL Alpha Factor Results No structure conversions and C5-A as two 6-wheel gears NAPTF structures converted to equivalent MWHGL structures (SQS = 1.6 x CA) and C5-A as two 6-wheel gears

23. AAR-410 February 2, 200523 4- and 6-Wheel Alpha Factors for Base-to-Subbase Equivalency = 1.4 Alpha factor quadratic curve fit intercepts at 10,000 coverages: 4-wheel  = 0.806 6-wheel  = 0.7178 From MWHGL report: 4-wheel  = 0.825 6-wheel  = 0.788

24. AAR-410 February 2, 200524 4- and 6-Wheel Alpha Factors for Base-to-Subbase Equivalency = 1.6 Alpha factor quadratic curve fit intercepts at 10,000 coverages: 4-wheel  = 0.832 6-wheel  = 0.7295 From MWHGL report: 4-wheel  = 0.825 6-wheel  = 0.788

25. AAR-410 February 2, 200525 Subbase Equivalency Factors u Burns, C.D., R.H. Ledbetter, and R.W. Grau. u “Study of Behavior of Bituminous-Stabilized Pavement Layers,” Miscellaneous Paper S- 73-4, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississipi, March 1973. u Bituminous stabilized base, asphalt base, bituminous stabilized subbase.

26. AAR-410 February 2, 200526 Subbase Equivalencies for 12-Wheel Traffic BLS stabilized layers replaced by MWHGL equivalent thicknesses

27. AAR-410 February 2, 200527 Subbase Equivalencies for 12-Wheel Traffic BLS stabilized layers replaced by MWHGL equivalent thicknesses

28. AAR-410 February 2, 200528 Alpha Factor Results - Discussion u Conversion of NAPTF structures gives better agreement with MWHGL test results. u This indicates that extra conservatism for subgrade protection has been built into the design procedure by increasing minimum thickness requirements for surface (5 in versus 3 in) and base (8 in versus 6 in) without reducing total thickness. u If 150/5320-6D is used to calibrate LEDFAA then LEDFAA is also conservative.

29. AAR-410 February 2, 200529 MWHGL Designs versus Current FAA CBR Designs u The MWHGL alpha factor curves give design thicknesses for structures with 3-in asphalt and 6-in base, and for material properties the same as the MWHGL test materials. u Thickness designs for other layer thicknesses and properties must be converted to MWHGL compatible structures to give the same level of subgrade protection. 0.87 x 33 in 1.15 x 28.7 in

30. AAR-410 February 2, 200530 Alpha Factor Results - Discussion u But, overconservative thicknesses for subgrade protection may provide other benefits for operation with heavy aircraft loads. u Safety factor for structural failure. u Compaction rutting in base and subbase materials. u Fatigue cracking of stabilized layers. u LEDFAA and FEDFAA are therefore being calibrated against -6D designs (5 and 8+ in), not MWHGL designs (3 and 6 in).

31. AAR-410 February 2, 200531 LEDFAA 1.3 Flexible Failure Model

32. AAR-410 February 2, 200532 North, 6-Wheel Track LFC1 LFC2 LFC3 LFC4 Subgrade CBR = 3.3

33. AAR-410 February 2, 200533 LFC1 Center Line, 6-Wheel Track LFC1 CBR = 4.3

34. February 2, 200534 CC-3 PHASE-2: LFC-1 CL TRAFFIC TESTS Pass No = 0 Pass No = 66 Pass No = 132 Pass No = 198 Pass No = 264 Pass No = 330

35. February 2, 200535 CC-3 PHASE-2 LFC-1 CL TRAFFIC TEST RESULTS

36. AAR-410 February 2, 200536 CC3-LFC1 Traffic Results Summary u A relatively small change in subgrade CBR can produce a very significant change in the magnitude and character of flexible pavement structural performance. u Very large deformations can occur at, say, 5 passes, even when the life to the failure criterion is as large as 100 passes. u This is the basis for the 240 coverage requirement in Engineering Brief No. 65, “Minimum Requirements to Widen Existing 150 ‑ Foot Wide Runways for Airbus A380 Operations.”