Shin SATO

Influence of Deterioration and Recycling Processes Associated with Repeated Use of Reclaimed Aggregate on the Performance of Reclaimed Asphalt Mixtures

Osamu TAKAHASHI

In Japan, more than 70% of asphalt mixtures currently produced are reclaimed asphalt mixtures (RAM) incorporating reclaimed aggregate, and the reclaimed aggregate content (RAP content, R-rate) has been increasing annually. Accordingly, the repeated use of reclaimed aggregate is expected to expand further. However, the mechanical performance of RAM containing repeatedly recycled reclaimed aggregate, as well as the characteristics of recycled asphalt binder, have not been sufficiently clarified, and knowledge regarding their actual conditions and associated technical challenges remains limited.
In recent years, polymer-modified asphalt (PMA) has been increasingly adopted to enhance pavement durability and functionality. Consequently, reclaimed aggregates containing aged PMA have also increased. Because PMA exhibits low penetration even in an unaged state, deterioration cannot be appropriately evaluated solely by penetration when the aged binder originates from PMA. Therefore, more advanced quality evaluation methods for reclaimed aggregate are required.
Previous studies have proposed mix design approaches that restrict the R-rate according to reclaimed aggregate quality, demonstrating that even out-of-specification materials may be utilized by adjusting the R-rate based on the degree of aging. However, in practice, the R-rate in Japan is strongly influenced by regional supply–demand conditions, resulting in considerable regional variation. Moreover, the relationship between aged binder penetration and appropriate R-rate has not been sufficiently established.
This study evaluated the influence of reduced penetration of aged binder due to repeated recycling on the mechanical properties of RAM and the rheological characteristics of recycled binder. Mixtures with an R-rate of 60% were prepared using reclaimed aggregates both within and outside specification limits. Indirect tensile tests and high-temperature Cantabro tests were conducted. Recovered binders were further evaluated using the Force Ductility Test (FDT) and Dynamic Shear Rheometer (DSR). Both straight asphalt and PMA were considered as original binders in reclaimed aggregate.
The results indicated that crack resistance decreased with increasing binder aging, regardless of binder type. However, mixtures containing PMA-derived binder exhibited a more gradual reduction in crack resistance, suggesting a mitigating effect of PMA. The FD value corresponding to low crack resistance was generally below 50 N·mm. Compared with DSR, FDT was more sensitive to aging-induced changes in binder properties. These findings suggest that FDT, owing to its simplicity and cost efficiency, is applicable to recycled binder evaluation and RAM mix design.