The effect of recycled steel fibres and modified rubber aggregates from waste tires on the performance and molecular interface behaviours of cement-stabilised macadam

As a high-volume pavement base, cement-stabilised macadam (CSM) consumes massive natural aggregates, while incorporating rubber aggregate and recycled steel fibres derived from waste tires could lower life-cycle environmental impacts. The rubber aggregate was pretreated with three different modification solutions. Modified rubber aggregates replaced fine aggregates at 5%, 10% and 15% by volume, and recycled steel fibres were added at 1% by volume. The specimens were assessed for mechanical properties, drying shrinkage, freeze–thaw durability and molecular interfacial behaviour. Among treatments, PVA modification yielded the largest gains, particularly when combined with steel fibres. Compared with CSM, the unconfined compressive strength (UCS) declined slightly by 8.8%, and the splitting tensile strength (STS) remained essentially unchanged. By contrast, the flexural tensile strength (FTS) exhibited a clear advantage, increasing by 10.7%. Shrinkage strain decreased by 44.5% relative to CSM. Alongside a higher post-freeze–thaw strength retention ratio, reaching up to 86.9%. Molecular-dynamics simulations corroborated the experimental trends, indicating stronger interfacial traction and pull-out responses and a wider interfacial interaction zone that explain the macroscopic improvements. Overall, incorporating waste-tire rubber aggregates and recycled steel fibres into cement-stabilised macadam enabled strength-retaining and toughness-enhancing modification, thereby supporting the high-value utilisation of waste-tire components in pavement base applications.