Effect of untreated aggregates nonlinearity on pavement design

This paper describes the effect of untreated aggregates (UA) nonlinearity on the design of conventional flexible and inverted pavements. A set of 360 structures were analyzed using the French mechanistic-empirical design method with the critical responses calculated by assuming the UA layer to behave as either linear or nonlinear elastic. For most flexible pavement sections, the required minimum thickness of the UA layer was found to be higher by applying a nonlinear analysis in comparison with linear behaviour reaching a difference as high as 135%. This is explained by the fact that in flexible pavements, the UA layer is placed directly above the subgrade under different lifts of hot-mix asphalt (HMA) layers. This configuration reduces the stresses induced by truck loadings on the UA layer, leading to small values for the calculated stress-dependent moduli as compared to the default elastic resilient modulus value used in elastic analysis. As for inverted pavements with relatively thick HMA layers, the stress-dependent moduli were found to be higher than those assumed for linear analysis given the critical placement of the UA layer within this type of pavement structure. The linear analysis did not provide practical solutions when the design cumulative 13t ESALs exceeds 6 million. This finding is further confirmed for inverted pavements with a very thin HMA layer, as no practical solution was achieved using linear analysis, and only nonlinear analysis provided solutions. Therefore, it is recommended that nonlinearity should be taken into account when analyzing and designing flexible and inverted pavement structures.