Size effect and compaction method on mechanical strength, rutting resistance, and shear parameters of asphalt mixtures

This study systematically investigates the effects of the specimen size and compaction method on the mechanical strength, dynamic stability, internal friction angle, and cohesive force of asphalt mixtures under elevated temperature conditions (60 °C). Two types of asphalt mixtures (AC-13 and AC-20) with three gradation types (upper specification limit, skeleton-dense gradation, and lower specification limit) were evaluated. Two compaction methods – Marshall compaction method (MCM) and vertical vibration compaction method (VVCM) – and three specimen sizes were employed: cylindrical specimens with diameters of 101.6, 152.4 and 200 mm, and slab specimens with thicknesses of 50, 80 and 160 mm. The unconfined compressive strength (UCS), indirect tensile strength (ITS), and dynamic stability (DS) were measured, and the internal friction angle (φ) and cohesive force (c) were subsequently calculated. The results indicate that at 60 °C, the mechanical strengths (UCS and ITS), DS, and c decrease with increasing specimen size, whereas the φ remains largely unaffected by size changes. VVCM-compacted specimens exhibit higher mechanical strengths, φ, and c than their MCM-compacted counterparts. As the asphalt–aggregate ratio increases, the mechanical strength and DS first increase and then decrease, while the φ shows a monotonic decline. Coarser gradations yield a higher φ, whereas finer gradations enhance c. These findings provide practical guidance for selecting appropriate specimen dimensions and compaction methods in the performance evaluation of asphalt mixtures.