Hybrid Mechanical Behaviour of Cold-Recycled Mixtures with Foamed Bitumen

• A cold-recycled mixture was engineered to exhibit hybrid performance. • Triaxial testing revealed non-negligible cohesion and a high friction angle. • Roller-compacted slabs enabled advanced mechanical characterisation. • Stiffness and fatigue test results indicate an intermediate structural response. • Results support CRM use as a structural layer in pavement rehabilitation. This study presents the formulation and laboratory validation of a cold recycled mixture (CRM) with foamed bitumen, designed to deliberately exhibit a hybrid mechanical behaviour, positioned between lightly stabilised granular materials and conventional bound pavement layers. The mixture incorporates a high reclaimed asphalt content (>90%) and optimised proportions of foamed bitumen (2.6%) and cement (2%), promoting enhanced bonding while preserving the advantages of cold recycling technologies as a sustainable paving solution. A comprehensive laboratory testing programme was carried out, including indirect tensile testing, stiffness characterisation, fatigue assessment using four-point bending, permanent deformation evaluation, and monotonic triaxial compression testing to capture stress-dependent structural behaviour. The results show that the CRM develops non-negligible cohesion (c = 148 kPa) and a high friction angle (φ = 41°), together with deformation modulus values ranging from 1900 to 2600 MPa in triaxial testing and a stiffness modulus between 2200 and 3400 MPa in four-point bending tests. These properties place the material clearly above granular or lightly stabilised mixtures, while remaining below the stiffness levels commonly reported for hot-mix asphalt. Fatigue testing showed that the CRM can sustain cyclic loading at low strain levels, exhibiting a cohesive, bonded response, while diverging from conventional asphalt behaviour at higher deformations. Overall, the results confirm that cold recycled mixtures with foamed bitumen can be engineered to achieve a structurally relevant hybrid behaviour, providing a mechanically robust, structurally relevant, and resource-efficient solution for pavement rehabilitation and maintenance.