The potential sustainable benefits of alternative binder use are positive for the asphalt industry, the environment and society.
Alternative binders (ABs) are available for optimizing higher recycling rates to improve economic and field performance. They also can modify and improve petroleum-based binder quality and be used as recycling agents.
Available ABs are bio-based, recycled oils or tall oil from paper manufacturing, available for use in relatively low doses with conventional asphalt that supports high binder replacement (up to 50%) mix designs when coupled with quality virgin binders and RAP management.
Contractors and DOTs suggest six items when evaluating alternative binders: equal or better performance; cost; ability to specify and accept; availability of supply; environmental health and safety; and recyclability/compatibility. They also suggest that some binders are better than others. Many interviewees expressed concern with the use of recycled engine oil bottoms (REOB).
Using ABs will require more rigorous virgin, alternative, and recycled binder analysis. In the short-term, rheological parameters will likely be added to asphalt binder specifications. This would put responsibility on asphalt suppliers to provide binders with relatively low-aging susceptibility and low REOB levels, as well as help reduce the risk of early cracking.
As recycled materials and AB contents increase, other mixture design parameters will need to be adjusted to meet gradation and volumetric requirements. Mixture design will soon rely on mixture tests conducted per a balanced mix design (BMD) method framework consistent with the outcomes of NCHRP Project 20-07 Task 406.
Producers may need additional asphalt binder tanks and/or blending equipment depending on local specifications, number of mix designs, and recycle levels. There may be a need for odor capture systems on plant tanks, air systems and silos. Source changes will need to be planned and managed to maintain mixture quality and desired recycled materials use. This also may result in additional design or QC lab resources to monitor indicators of binder source change.
RAP and RAS production, handling, and management practices may need to be refined for optimization. As recycled materials use increases, potential plant changes could include additional feed bins, drum length increases or external mixers, burner reconfiguration, and flighting.
Likely even more important are partnerships with virgin binder suppliers to obtain consistent supply. Recent work clearly shows that binders meeting the same PG grade can lead to very different levels of allowable RAP use and field performance. This is because the chemical composition of two binders meeting the same PG grade may be significantly different.
In rural locations there is one potentially significant impact: the use of cold recycling techniques with synthetic binders. Currently available binders can be used with 100% RAP to produce mix properties equal to or better than HMA. So in rural locations where hot plants have to be mobilized, aggregates have to be hauled long distances, or temporary pits have to be opened, cold in-place recycling (CIR) with synthetic binder could well become a cost-effective alternative.
Asphalt industry stakeholders must participate in shaping change associated with alternative binder use, specifically relative to test method selection, material and design specifications, and acceptance specifications.