Influence of Subgrade Treatment Type on the Seasonal Performance of Low-Volume Asphalt Pavement Structures

This study investigates the seasonal structural behaviour of flexible pavement structures constructed on subgrades with varying types of treatment. Eight road sections in Lithuania, featuring natural subgrades or soils stabilised with lime, cement, or hydraulic road binder (HRB), were evaluated using Falling Weight Deflectometer (FWD) testing during thawed and recovered states. Structural condition was assessed using deflection-based indices: the Surface Curvature Index (SCI), Base Damage Index (BDI), and Base Curvature Index (BCI). Seasonal changes were quantified, and Wilcoxon signed-rank tests were applied to assess the statistical significance of deflection differences. The results revealed that the untreated subgrades experienced the largest seasonal softening, with BCI increases of up to 45%. Cement stabilization provided the most effective mitigation, limiting the BCI to 14% and preserving the stability of SCI. Lime-treated sections showed a dosage-dependent improvement, while HRB treatment yielded results comparable to high-percentage lime stabilisation. The study confirms that the type and dosage of subgrade treatment significantly influence pavement resistance under freeze-thaw conditions and highlights the importance of evaluating the geometry of the deflection bowl to correctly interpret structural indicators. These findings contribute to improved mechanistic understanding of seasonal load response in flexible pavements and inform best practices for subgrade stabilisation.