An integrated packing-moisture control approach in bitumen-stabilized materials (BSM) design

The mechanical performance of bitumen-stabilized materials (BSM) hinges on two interrelated factors, including aggregate packing and moisture control. However, current design practices offer little guidance on how to systematically balance these variables. This study, therefore, proposes an integrated volumetric framework that couples the Bailey packing principles with a liquid-filled-voids (VFL) criterion to identify the optimum aggregate gradation and total water content for mixtures composed entirely of reclaimed asphalt pavement (RAP). Four gradations were manufactured by blending coarse and fine RAP to represent 60 %, 80 %, 90 %, and 100 % of their loose-unit-weight (LUW) packing states. Each blend was stabilized with a 3 % residual bitumen emulsion and total water dosages ranging from 3 % to 6 %. Compaction behaviour was captured through the dry density, voids filled with liquid (VFL), and voids in the mixture (V_m), which were monitored for up to 100 gyrations. In contrast, water loss due to compaction, long-term evaporation, and 56-day indirect tensile strength (ITS) were also assessed. Results indicate that the 80LUW gradation, combined with a total water dosage of 4 %-4.5 %, produced the densest internal structure (V_m ≈ 8.4 %) and maintained VFL at the 85 % threshold that prevents liquid seepage. In contrast, finer (60 %LUW) and coarser (90–100 %LUW) gradations exhibited excessive voids at comparable moisture levels. The proposed framework thus offers a practical way for selecting gradation–moisture combinations, reducing experimental repetition and advancing the sustainable use of BSMs.