Selection of Soil–Aggregate Mixtures Based on Multistage Permanent Deformation

Although mechanistic–empirical approaches have gained increasing prominence in pavement design, the evaluation and selection of soil–aggregate mixtures still frequently rely on gradation criteria and the California Bearing Ratio (CBR), which may not fully capture the behavior of these materials under repeated loading, particularly in the case of tropical lateritic soils. In this context, this study aimed to evaluate the use of permanent deformation obtained from multistage repeated-load triaxial testing as a performance-oriented tool for comparing and ranking soil–aggregate compositions. Three mixtures composed of lateritic soil and basalt aggregate (SA50-50, SA40-60, and SA30-70) were proportioned to comply with the same reference gradation envelope and evaluated through CBR, resilient modulus, and multistage permanent deformation tests. These tests were complemented by a single-stage permanent deformation test conducted under the same stress state for all three mixtures. Increasing the aggregate content increased the CBR from 61% to 144%, whereas the average resilient modulus ranged from 301 to 356 MPa, without showing a consistent increasing trend. SA40-60 exhibited the lowest permanent deformation levels along all three multistage loading paths, with final permanent strains of approximately 0.85%, 0.81%, and 2.12% for TC50, TC70, and TC100, respectively. The single-stage test showed the same overall trend as the multistage tests, with a final permanent deformation of 1.09 mm for SA40-60, compared with 2.17 mm and 3.52 mm for SA50-50 and SA30-70, respectively. The results indicate that conventional parameters and resilient response, when considered individually, do not fully describe the behavior of these mixtures with respect to the accumulation of irreversible deformation. Multistage permanent deformation testing therefore proved to be a useful tool for comparing different soil–aggregate mixtures and supporting the selection of the best-performing composition among those evaluated.