The current Corps of Engineers design procedures for rigid airfield pavements is based on the Westergaard free edge stress slab theory, and a proposed procedure is based on the multilayer elastic theory. These two design procedures have been expanded to airfield pavement designs expressed in probabilistic and reliability terms. Further developments were required in these procedures to make the analysis more practicable. Two major investigations were conducted: (a) Evaluation and use of the composite modulus of elasticity for layers beneath the rigid pavement, and (b) Evaluation of the maximum tensile stress at the bottom of the slab for different aircraft types. Derivations obtained from the investigation of the composite modulus and maximum tensile stress are reported and are included in computer programs for probabilistic/reliability analysis of rigid pavements. The approximate closed form (Taylor series expansion) is utilized. Example runs of the computer program are presented.

The paper was organized to present the various factors which influence the current design criteria with a brief explanation of how the numerical values of each was derived.

A procedure was developed to analyze layered elastic flexible pavement systems in terms of probability and reliability. A computer program RELIBSIA was prepared to carry out the computations. Rosenblueth method, instead of the conventional Taylor series expansion, us used to estimate the expected value and variance of the strains (dependent parameters) based on the input mean values of independent parameters, i.e., aircraft load, layer thicknesses, and material moduli. The relationships between the reliability level and the allowable strain repetition of the designed system which is established with results computed using RELIBISA provide a decision-making tool for engineers to design pavements at desired reliability level. The design can be optimized by selecting thicknesses of the bituminous concrete and the base layers so that the pavement is failed in fatigue cracking and subgrade failure at nearly the same traffic level and the same reliability level. The reliability-strain curves have steeper slopes with the bituminous concrete strain failure criterion than with the subgrade strain failure criterion, indicating that for flexible pavements designed using the Corps of Engineer's failure criteria, the design has a greater degree of uncertainty in preventing subgrade failure than fatigue cracking of the bituminous concrete surface corps. However, this may not be true in real cases because the bituminous concrete failure criteria are determined based on controlled laboratory test data which do not consider the uncertainties existing in the laboratory-to-field correlations.

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