AAR-410 February 2, 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 AAR-410 February 2, B-777 Six-Wheel ACNs u For flexible pavements, the ACNs initially computed for B 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. AAR-410 February 2, Alpha Factors MWHGL Data AAR-410 February 2, Interim 6-Wheel Alpha Factor at 10,000 Coverages u 4-Wheel alpha = u Original 6-wheel alpha = (inception to 1995) u Interim 6-wheel alpha = 0.72 (1995 to present) u Current 12-wheel alpha = 0.722 AAR-410 February 2, Alpha Factors MWHGL Data C5-A as one 12-wheel gear C5-A as two 6-wheel gears AAR-410 February 2, 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. AAR-410 February 2, 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. AAR-410 February 2, CC3 Test Pavements - Profile Direction of Traffic AAR-410 February 2, North, 6-Wheel Track LFC1 LFC2 LFC3 LFC4 AAR-410 February 2, Trench in LFC2 Flexible AAR-410 February 2, 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. AAR-410 February 2, CBR Equations Post-MWHGL equation: t = (A c ) 0.5 [ (log CBR/P) (log CBR/P) 2 (log CBR/P) 3 ] Pre-MWHGL equation: Solve the Post-MWHGL equation for OR t = Total Thickness P = ESWL AAR-410 February 2, Change the Input Alpha until the design thickness is equal to the test structure thickness. AAR-410 February 2, 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. AAR-410 February 2, Summary of NAPTF Flexible Pavement Full-Scale Test Results * Extrapolated from rut depth curve Bold = corrected values AAR-410 February 2, NAPTF and MWHGL Alpha Factor Results (No conversion of NAPTF to MWHGL structures) AAR-410 February 2, LEDFAA 1.3 Flexible Failure Model AAR-410 February 2, 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). AAR-410 February 2, 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: AAR-410 February 2, NAPTF Flexible Pavement Equivalent Thicknesses and Alpha Factors AAR-410 February 2, NAPTF and MWHGL Alpha Factor Results (With conversion of NAPTF to MWHGL structures) AAR-410 February 2, 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 AAR-410 February 2, and 6-Wheel Alpha Factors for Base-to-Subbase Equivalency = 1.4 Alpha factor quadratic curve fit intercepts at 10,000 coverages: 4-wheel = wheel = From MWHGL report: 4-wheel = wheel = 0.788 AAR-410 February 2, and 6-Wheel Alpha Factors for Base-to-Subbase Equivalency = 1.6 Alpha factor quadratic curve fit intercepts at 10,000 coverages: 4-wheel = wheel = From MWHGL report: 4-wheel = wheel = 0.788 AAR-410 February 2, 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 u Bituminous stabilized base, asphalt base, bituminous stabilized subbase. AAR-410 February 2, Subbase Equivalencies for 12-Wheel Traffic BLS stabilized layers replaced by MWHGL equivalent thicknesses AAR-410 February 2, Subbase Equivalencies for 12-Wheel Traffic BLS stabilized layers replaced by MWHGL equivalent thicknesses AAR-410 February 2, 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. AAR-410 February 2, 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 x 33 in 1.15 x 28.7 in AAR-410 February 2, 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). AAR-410 February 2, LEDFAA 1.3 Flexible Failure Model AAR-410 February 2, North, 6-Wheel Track LFC1 LFC2 LFC3 LFC4 Subgrade CBR = 3.3 AAR-410 February 2, LFC1 Center Line, 6-Wheel Track LFC1 CBR = 4.3 February 2, 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 February 2, CC-3 PHASE-2 LFC-1 CL TRAFFIC TEST RESULTS AAR-410 February 2, 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.