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Ministère de l'Écologie, de l'Énergie,du Développement durable et de l'Aménagement du territoire

Comparison of US and French rational procedures for the design of flexible airfield pavements. Cécile CARON, Jean-Noël THEILLOUT STAC French Technical Center for Civil Aviation France David BRILL FAA Airport Technology R&D Branch.

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Ministère de l'Écologie, de l'Énergie,du Développement durable et de l'Aménagement du territoire

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  1. Comparison of US and French rational procedures for the design of flexible airfield pavements Cécile CARON, Jean-Noël THEILLOUT STAC French Technical Center for Civil Aviation France David BRILL FAA Airport Technology R&D Branch Direction générale de l’Aviation civile, Service technique de l’aviation civile Ministèrede l'Écologie, de l'Énergie,du Développement durable et de l'Aménagement du territoire

  2. US software: FAARFIELD became the FAA standard design software for airfield pavements in September 2009 (AC 150/5320-6E) The rational method: Status of computer softwares French software: Alizé-Airfield pavement Not yet fully operational for design applications but already useful for research and as a expert analysis tool

  3. Main improvements CBR design Rational design Traffic mix: Need to use conversion procedures (to equivalent departures or passes of a single design aircraft or a reference load) Input of the entire traffic mix US: Pass-to-coverage ratio depends on the structure depth; aircraft wander fixed s=0.775m (30.5 in.) FR: Computation of strain distribution (not just a single maximum) at all points across the pavement for a given depth, and for all wandering positions Aircraft wander: Reduction in damage due to aircraft wander is the same for all pavement thicknesses • Introduction of: • the layered elastic analysis • the CDF (Cumulative Damage Factor ) using Miner’s rule: ESWL concept Boussinesq modelization Cumulative damage=S(ni/Ni) ni number of coverages of airplane i Ni number of allowable coverages

  4. Main improvements CBR design Rational design Pavement failure caused by overstressing the subgrade => maximum vertical deflection at top of subgrade = pavement damage indicator Failure criteria are: - tensile strains at the bottom of asphalt layers - vertical compressive strains at the top of subgrade The superior load spreading characteristics of bound layers are acknowledged by using granular layer equivalency factors, not related to laboratory-determined mechanical properties of materials • Input data are modulus values • Ability to incorporate alternative construction practices and innovative materials (high performance and reclaimed materials) • - Ability to deal with ageing and seasonal effects

  5. Main discrepancies: determination of allowable strains FAILURE MODEL Mechanistic-empirical approach Computed strains < Allowable strains Theoretical analysis of load distribution Experimental data STRUCTURAL MODEL Choice of strain criteria Calibration of strain criteria Mechanical computations Calibration tests Allowable strains Computed strains

  6. Main discrepancies: determination of allowable strains Failure criteria determined from in-door full-scale tests conducted from 1999 until the present at the National Airport Pavement Test Facility (Atlantic City, NJ) • Failure criteria: • Asphalt layers: laboratory fatigue test  bi-logarithmic Wölher laws • UGA and Soils : empirical strain failure criteria

  7. Main discrepancies: determination of allowable strains Alizé subgrade a = 0.2 Faarfield subgrade a = 0.07 At top of subgrade Allowable strains Alizé > Faarfield Calculated thicknesses Alizé < Faarfield

  8. Aim: quantify the change in output data with respect to the change in input parameters. Sensitivity of the CDF computed at top of the subgrade in both US and French rational softwares In both methods, CDF is more sensitive to variations in gross weight and in subgrade CBR than in other input variables (asphalt thickness and moduli, number of aircraft passes). Sensitivity of the method to all the variables considered is in most cases higher in FAARFIELD than in Alizé-Airfield pavement (possibly due to the different empirical determination of subgrade strain-based criteria). Comparative study: 1/ Sensitivity study

  9. Comparative study: 2/Computed mechanical values Faarfield output files Alize profiles • When similar structural and loading conditions are selected, computation of mechanical values et critical levels of the designed structures (vertical strain and stress at top of subgrade and x- and y- components of horizontal strain at the bottom of asphalt layers) is nearly identical in both programs. • This result is quite satisfactory and is in accordance with field data (measurement of strains from embedded gauges on US and French test facilities).

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