During the analysis and design of flexible pavements, the nonlinear elastic behavior of unbound aggregate layers and subgrade soils is a crucial component for establishing proper material characterization. This paper presents a validation case study regarding the nonlinear elastic behavior of unbound aggregate specimens in repeated load triaxial testing. An advanced triaxial testing framework, the University of Illinois FastCell (UI-FastCell), was selected for this study considering its capability to conduct cycled independent stress from both axial and radial directions. In total, ten unbound aggregate materials were tested in the laboratory under varying repeated loading conditions. A newly developed 2D axisymmetric finite element method (FEM)-based analytical tool was used to simulate the nonlinear elastic behavior based on a secant modulus iteration scheme. A comparison of the resilient stress–strain curves between the experimental and simulation results showed good agreement. Based on the findings of this study, the advanced characterization of material nonlinearity was shown to be both necessary and practical by utilizing FEM-based analytical solutions for flexible pavements.