The U.S. Military (Army, Air Force and Navy) and Federal Aviation Administration (FAA) thickness design procedures for flexible airport pavements are based on the CBR (California Bearing Ratio) method. This method was originally developed in the 1940s for design of flexible pavements to supportthe then new heavy bombers. The original airfield design curves were an extrapolation, based on shear stress, of the California pavement design curves for highway pavements. The extrapolated curves were modified and verified by extensive full scale field testing. In the late 1960s and early 1970s as a result of extensive prototype testing with full-scale multi-wheel aircraft assemblies, the classic CBR equation was modified with the introduction of a thickness reduction factor (Alpha factor). The thickness reduction factor was to account for traffic volume and for better than expectedperformance from multi-wheel assemblies. Although the CBR design procedure has been in use unchanged for design and evaluation of both military andcivil airfield pavement since 1970, the basic mechanistic nature of the procedure has not been recognized, even by the developers of the procedure.Because of the lack of understanding of the criteria contained within theCBR procedure, the methodology has been unjustly criticized in recent years. By using Frohlich's stress concentration factors, the basic mechanistic foundation for the CBR equation can be explained and the performance criteria clearly identified. The performance criteria contained in the CBR procedure can be computed directly without resorting to the equivalent single-wheel load (ESWL) concept. These criteria were recalibrated to availableprototype test data, resulting in new design criteria without resorting to thickness adjustment factors. Since the new criteria represents a major change in the CBR design methodology, new prototype pavement sections havebeen constructed that directly related to the new design methodology. As developed, the design criteria are very similar and can be directly related to the vertical strain criteria in use by the Corps of Engineers and FAA. Because of the improved predictive capability of the new criteria, the new CBR design methodology is being adopted by the military for design and evaluation of flexible pavements. In the near future, the classic CBR equation will be replaced with the new criteria and the CBR procedure as it isnow known will no longer exist. The new design procedure is considered tobe a major advancement in pavement design technology and will result in asimple and easy to apply design methodology. For the covering abstract see ITRD E139491.
Samenvatting