Evaluation of binder-aggregate adhesion in hot-recycled asphalt mixtures as a function of the production temperature

When recycling reclaimed asphalt (RA) in new hot-mix asphalt (HMA), higher temperatures of the virgin aggregate allow the mobilization of a higher amount of the binder in the RA. However, this implies a more severe short-term aging of the virgin bitumen and poorer properties of the aged-virgin bitumen blend, due to the lower virgin bitumen/RA bitumen ratio. On the contrary, the adoption of lower temperatures has the opposite effect (lower mobilization of the RA binder but higher performance of the bituminous blend). In addition, the reduction of material heating results in a lower bitumen viscosity, which may determine lower compactability and lower adhesiveness. Previous studies showed that a reduction of 30 °C in the mixing temperature of AC containing RA does not imply a significant increase of the air voids content but allows improving the material performance against cracking, fatigue, and rutting. Moreover, the lower mixing temperature also preserves the effectiveness of the rejuvenating agent. To have a deeper understanding of this phenomenon, the objectives of the research were: (i) evaluating how the binder adhesive properties changes when varying the content of aged bitumen; (ii) assessing if the adhesion is higher on virgin aggregate or on RA particles, coated with aged bitumen; (iii) understanding how the blending temperature influence the binder-aggregate adhesion.

To this aim, binder bond strength (BBS) tests were carried out at 25 °C. The experimental program provided 2 types of substrates, simulating virgin limestone aggregate and RA, 3 RA/virgin binder proportions (20/80, 35/65 and 50/50), 2 types of rejuvenator in the binder (coded with the letters A and B), 2 bitumen application temperatures (140 °C and 170 °C), and 5 repetitions.

The results showed that the adhesive properties of the binder decreased when increasing the aged bitumen content from 20% to 50%. The t-test α values obtained when comparing the pull-off tensile strength (POTS) of the blends with 20% and 35% of RA bitumen and the blends with 35% and 50% of RA bitumen were respectively 1.2×10^-6 and 6.1×10^-7, confirming the decreasing trend of POTS with aged bitumen content. Moreover, for high RA bitumen contents (35% and 50%), the adhesion on the RA substrate was higher than on the limestone substrate (α = 0.004). Between the two rejuvenators, the type B allowed obtaining higher POTS values for high RA bitumen contents (35% and 50%), as confirmed by α = 0.008. Differently, the bitumen application temperature (140 °C or 170 °C) did not significantly influence the POTS (α = 0.50). This indicates that the increase of adhesiveness that can be obtained at higher temperature was approximately balanced by the more severe aging underwent by the binder. However, as in site the lower mixing temperature implies the lower mobilization of the RA binder, thus a lower RA/virgin bitumen proportion, from the experimental results it can be stated that the reduction of the mix temperature is beneficial for the adhesion between the binder and both the virgin and the pre-coated RA aggregates.