Rejuvenating agents are intended to restore aged (oxidized) asphalt binders by restoring the original ratio of asphaltenes to maltenes. As most of these products are proprietary, there is no defined formula for them. However, their primary components are bituminous; a rejuvenating product must contain maltenes in order to improve the asphaltenes to maltenes ratio (Bayer). When used properly, rejuvenating agents will decrease the proportion of new asphalt.
Rejuvenator formulae are proprietary and may change in time. The commons duties of rejuvenators are come in the following:
- Reduce modulus / viscosity (Softening)
- Restore balance of asphalt fractions
- Restore phase/colloidal stability
- Reduce brittleness / improve damage resistance
- Restore “healing” ability
- Equal (or better) aging behavior than original binder
Select Grade of New Asphalt Binder
The grade of virgin and aged bitumen of RAP can be one of the important factors and specifies viscosity of final bitumen of recycled asphalt mixture. For determining the bitumen viscosity of the target mixture selecting grade of virgin bitumen in the recycled asphalt mixture is necessary. In fact, based on the viscosity of the bitumen in the RAP, the viscosity of new bitumen can be determined to achieve the final bitumen viscosity. The final bitumen specifications usually depend on the type of project, weather conditions and the amount and type of traffic. Using the bitumen viscosity mixing table, new grade of bitumen can be determined.
Most laboratory studies have concluded that when the percentage of RAP is low up to 15% or 20%; the properties of recycled asphalt and virgin asphalt mixtures are almost similar (Mcdaniel and Anderson, 2001; Mcdaniel et al., 2002). Indeed, a low rate of RAP will not significantly affect the stiffness and strength of the final asphalt mixture.
Current Super paves guidelines for selecting the virgin binder grade:
- Up to 15% RAP Binder: Use the normal binder grade
- 15%-25% RAP Binder: Use one binder grade lower,
e.g., PG 58-28 instead of PG 64-22 or Pen grade 85/100 instead of 60/70
- Over 25% RAP Binder (High RAP >25%): use additives for modification of bitumen and using blending charts to determine required binder grade
Additives & Antistriping
Parallel to the economic and environmental benefits of using RAP, asphalt industry tends to increase the percentage use of RAP in recycled hot mix asphalt; in order to achieve this target; overcome the limitations and uncertainties, multiple technologies and especially rejuvenators and additives are needed.
One of the important factors effects on the interaction between virgin binder and RAP bitumen in asphalt mixes is percentage of RAP content. The effects of RAP on physical and rheological properties of the final bituminous blend are subjected to wide researches. In these researches bitumen was extracted and recovered from asphalt mixes, and then it was tested in laboratory . Tests results showed that the effects of RAP bitumen on the final blend varied in proportion to RAP content.
The percentage of RAP that can be incorporated into asphalt mixtures depends on many factors in production process. These factors affect the interaction between RAP and virgin bitumen and consequently impact the performance of asphalt mixtures. The maximum amount of reclaimed asphalt is mainly limited by the production technology.
In order to increase the percentage of RAP in the asphalt mixture, the most suitable method is the use of polymer additives and Antistripping. Suitable additives are that have elasto-plastomer capability to increase the efficiency of asphalt, improve performance and increasing the interaction between old and new bitumen. It would be better additives contain anti-stripping materials, in this case, while reducing the cost of using anti-stripping, the addition of this material separately during the production stage is also eliminated. Since mixing time is short, therefore a suitable additive for producing of recycled asphalt must have the above conditions, and also cause new and old bitumen mix be homogenized. In one reported practical case two samples of additives, which had elasto-plastomer and anti-stripping were used in experiments and the results were compared with and without additives.
The applicability of recycled asphalt was investigated in laboratory, and asphalt mixtures containing 20%, 40% and 50% of RAP were designed. Marshall Tests have been performed on various formulations using paving grade bitumen and modified bitumen as binders. Modified bitumen contains of two different sample polymer. This investigation led to the conclusion that the performance of recycled asphalt mixture containing 20% of RAP is significantly closer to results obtained with virgin asphalt mixture without recycled materials. In addition, the use of mentioned polymer improves the properties of recycled asphalt mixtures even with high amounts of RAP.
