Pushing the boundaries for asphalt recycling in surface layers

The increasing emphasis on circular economy principles and sustainability in road construction has driven efforts to maximize the use of reclaimed asphalt (RA) in surface layers. However, incorporating high RA contents raises durability concerns, necessitating robust evaluation frameworks. This study introduces a two-level approach to assess the suitability of RA materials and mixtures for surface applications. At Level 1, RA characterization is performed using the cracking-based brittleness index (CBI), a solvent-free mechanistic test based on indirect tensile principles. CBI effectively captures the global response of RA materials, correlating well with binder ageing severity: higher CBI values indicate increased brittleness and ageing. Results of 6 RA materials showed significant variability among RA sources, with highly aged materials exhibiting the highest CBI values.
At Level 2, mixture performance is evaluated through a balanced nix design (BMD) approach on 6 asphalt concrete (AC 10) mixtures containing 50% RA binder replacement, including one reference AC 10 mixture with no RA. Key performance indicators included compactability (air voids at 60 gyrations), cracking resistance, via semi-circular bending (SCB) and indirect tensile (IDT) tests, rutting resistance, water sensitivity, and raveling. While rutting, water sensitivity, and raveling tests showed no risks and the respective Flemish tender specification requirements were met, risks emerged in compactability and cracking resistance for a number of mixtures incorporating RA. Cracking resistance of the mixtures with RA was consistently lower than the reference mixture based on the SCB tests, for the considered mix designs and RA materials, showing possible cracking resistance risks. In general, SCB tests proved more discriminative than IDT, and a strong correlation was observed between CBI and cracking resistance indices (CRI), reinforcing the predictive value of RA-level screening.
Overall, the proposed framework enables early identification of risky RA materials and mixtures, supporting informed decisions for the inclusion of RA in surface layers without compromising durability. These findings contribute