oa Recycling Roads by Rejuvenating Oxidized Asphalt Binder for Sustainable Transport Infrastructures

The use of Reclaimed Asphalt Pavement (RAP) material is widespread although the overall amount in the mix and the plant technology vary for each country. Reusing RAP allows for potential benefits due to less material consumption and possible energy savings although the benefit greatly varies depending upon the technology adopted for recycling (hot, warm, or cold asphalt recycled mixes) and material performance during the service life. Performance of recycled asphalt, among several other factors, is strongly related to the binder characteristics which presents himself in a oxidized state due to weathering effects (UV radiation, humidity) and load-related distresses accumulated over the service life.

The present research investigates the use of bio-rejuvenators to recover 100% cold RAP mix and evaluates its possible recycle as a construction material for new transport infrastructures. In particular, the rejuvenated asphalt binder was analyzed through several laboratory experimentations including: standard physical tests, rheology tests in the in-service temperature domain, fatigue resistance, and rutting potential as well as chemical characterization through infrared spectroscopy (FTIR). A multi-approach assessment was indeed found to be essential to comprehensively evaluate the “new” performance of the recycled binder. The Pressure Aging Vessel (PAV) was also adopted to simulate the in-service aging of the rejuvenated binder according to the standards and the multi-approach testing was again adopted to analyze the properties of rejuvenated binder after aging. This allowed characterizing the potential and durability of the “second-life” asphalt.

Asphalt binder was extracted from RAP material following the standard centrifuge extraction and solvent recovery procedure. Preliminary analyses were conducted on the extracted oxidized binder (control binder) and the following was shown, as expected: very low penetration (5.4 dmm); very high softening point (72 °C) and dynamic viscosity; great stiffness and low phase angle value at high temperature (Dynamic Shear Rheometer - DSR analysis); brittle behavior at low temperature (Bending Beam Rheometer – BBR analysis); poor fatigue resistance (linear amplitude DSR test) regardless of the testing temperature but good resistance to rutting (multiple stress creep recovery DSR test) at high and very high temperatures; solid gel structure (microscope imaging analysis) and consequently reduced maltene phase (FTIR analysis).

Several quantities of rejuvenator were cold added to 100% RAP mix and the same binder extraction process was performed. Besides the common quantity of rejuvenator-final performance approach, the rejuvenated binder was also evaluated by taking into account the contact time, which represented the time since the rejuvenator was applied to the oxidized binder, and the relative humidity of the RAP before mixing. It was found indeed that rejuvenators commonly have a differential effect depending on the “curing” time, therefore changing the physical, rheological and chemical properties of rejuvenated binder over time. Relative humidity of RAP material, essential on a plant scale where RAP is commonly open-air stored, was proved to affect with lower magnitude the properties of rejuvenated binder.

The amount of rejuvenator greatly affected the final binder performance; in particular, a greater content helps in improving physical and rheological characteristics in the low-temperature domain. Fatigue damage of the binder was increased (compared to oxidized RAP binder without rejuvenator) regardless of the rejuvenator content but resistance to rutting was reduced. However, the correct proportion of rejuvenator amount depending on RAP humidity and contact time allowed for an acceptable balance of performance in the overall temperature domain. Infrared spectroscopy analysis showed the evolution of carbonyl, sulfoxide and other bands in the rejuvenated asphalt binder.

The adoption of rejuvenators can be considered as an optimal way to improve the recycling of RAP and therefore increasing the amount of recycled material to be included into asphalt mixes without lowering performance and durability; this will allow for substantial environmental savings leading the way towards sustainable transport infrastructures. However, rejuvenators should be always carefully calibrated depending upon the in-service climate conditions of the recycled pavement as well as the initial RAP condition to provide the maximum benefit.