Effect of Mixing, Transport, and Placement Operations on the Blending and Performance of RAP Mixtures
A major question with respect to central plant produced asphalt mixtures containing reclaimed asphalt pavements (RAP) and recycled asphalt shingles (RAS) is the degree of mobilization and blending that occurs between the asphalt cement in the RAP and any virgin asphalt cement that is included. This degree of blending has a substantial effect on the rheology of the binder in the asphalt mixture and as a result the immediate constructability and long-term performance of the mixture. Most current national guidance on this issue assumes 100% blending of the two components and thus full credit is given to the asphalt cement in RAP. This assumption could be wrong particularly for the southern states with harder PGAC grade. However, much of this guidance was developed decades ago when RAP contents were low and any errors in the conclusions were minimized. In recent years agencies have begun using greater proportions of RAP/RAS in their mixtures and also using newer asphalts and additives. At the same time, agencies are reporting greater instances of durability problems with asphalt mixtures, which while not directly linked to increased RAP/RAS contents in mixtures does correlate to their increased usage. In light of the above issues, there is a need to properly quantify and understand how mixing, transport, and placement of RAP mixtures affects the interaction of asphalt cement from RAP/RAS with newly added virgin asphalt cement and how additives might influence this relationship.
The objective of this research is to investigate the effects of mixing, transport, and placement on the performance of mixtures containing RAP/RAS. Specifically, this study must consider, but not be limited to the aspects given in the tasks.
Agencies have greatly increased the use of RAP in their asphalt mixtures and will likely continue to do so. It is well known that blending of the virgin and RAP asphalt affect performance and that this blending is time and temperature dependent. Yet, there is little guidance on how to best mix and proper times for storage, transport, and placement of these mixtures to ensure ideal performance. Guidelines on these factors that considers real-world constraints and is representative of different field conditions will help improve the quality of the asphalt mixtures that are produced.
Recycling of asphalt materials has existed since the very early days of asphalt paving, with the Warren Brothers experimenting with recycling as early as 1915 (NCHRP 1978). Despite very early trials, asphalt mixture recycling was not common until the 1973 oil crisis when the price of asphalt spiked (Epps et al. 1980). Research on high-recycled content mixtures commenced immediately, with at least half of the states trying some form of recycling by 1976 (NCHRP 1978). At the time only six states were working on central plant experiments with reclaimed asphalt pavement (RAP). These early projects focused on maximizing RAP usage, with only a few projects using less than 50 percent RAP, and several attempting 100 percent RAP. It was found that most plant configurations could not achieve satisfactory air quality when producing 100 percent RAP mixtures and that most drum plants needed 30 to 50 percent new aggregate, additional moisture, reduced production rates, or more control of exit temperatures to achieve satisfactory air quality (NCHRP 1978). When the Strategic Highway Research Program (SHRP) began in the late 1980s, RAP and other recycled materials were not directly considered and thus did not shape the development of the Superpave mixture design, which relies on some implicit relationships between laboratory and field operations in order to work. Consequently, there has been a lack of uniform guidance in how to account for RAP in Superpave volumetric mixture designs and virgin binder selection. Also there has been a lack of studies on how RAP use affects some of the implicit laboratory to field relationships needed to make sure that the mixture design (mixture engineering) and production, transportation and placement (mixture manufacturing) are properly linked to provide sufficient performance. Interim guidelines for the incorporation of RAP into mixture design were provided in the late 1990s, which were followed in the early 2000s with the more thorough guidelines developed under NCHRP 9-12. The NCHRP 9-12 guidelines were eventually incorporated into AASHTO M 323 and R 35 (Copeland 2011). While some states have followed the guidelines from NCHRP 9-12, many others have developed their own procedures for handling and specifying recycled materials (Georgia 2014, North Carolina 2018, Maryland 2014, Massachusetts 2015, New Hampshire 2012, New York 2012, Texas 2014). These guidelines were largely silent on the role of mixing, transport, and placement.
Limited data on the effects of storage, transport, and placement time exist in the literature. Jacques et al. (2016) showed that storage time in the silo changed the mechanical properties of RAP mixtures. Howard et al. (2014) investigated the role of haul time and included a mixture with RAP. They concluded that there was no difference in the mixture when the hauling time was between 1 and 8 hours.
Four main challenges to designing mixtures with RAP in the laboratory were identified by Zhou et al. (2011): evaluating the blending of binders, determining the bulk specific gravity of RAP aggregates, simulating plant handling in the laboratory, and selecting mixing and compaction temperatures. Although these questions were posed specifically for RAP, they also extend to RAS. These questions are not new to recycled materials, with the uncertainty in binder blending noted as far back as 1980 (Epps et al. 1980). While the recent literature contains many studies to identify the blending, definitive answers remain elusive and lie at the heart of inconsistencies in RAP mixtures as well as form the barriers to greater RAP usage in asphalt mixtures. Updates to guidelines through NCHRP 1-52 and 1-54 are evidence of changes in discrepancies between the assumed and implicit relationship between laboratory to field condition specifically when RAP/RAS is involved.
The uncertainties about the effects of RAP has led some agencies to pause the RAP and RAS usage in the premium surface course mixes. There is also a need for methods to determining quantity of RAP actually used in the production mix.
- Review the literature on recent advances in characterizing and quantifying blending in asphalt concrete mixtures containing RAP/RAS.
- Measure the performance properties of various plant produced mixtures containing different levels of RAP/RAS at different times during the production process.
- Establish a relationship between production conditions (as assessed by experiments identified in the literature review such as SCB, DCt, Hamburg, etc.) and performance properties considering factors related to:
a. material type (e.g. polymer and non-polymer modified binders, rejuvenators/additives, etc.),
b. production type (e.g. batch versus drum plants, double versus single drums, etc.),
c. storage and transportation conditions during production (e.g. storage in silos for extensive durations of time or long hauls),
d. mixture type and in service conditions (e.g. dense graded versus open graded mixes, and climatic conditions at the pavement location),
e. aggregate type that is in contact with the binder in the asphalt mixture (e.g. aggregate with typical surface mineralogies and specific surface areas), and
f. RAP source especially differences between northern and southern climates.
- Develop recommendations with respect to production on the use and performance of RAP/RAS mixtures.
|Sponsoring Committee:||AFK50, Structural Requirements of Asphalt Mixtures
|Research Period:||24 - 36 months|
|RNS Developer:||Shane Underwood and Amit Bhasin|
|Source Info:||AFK50 with support from AFK20|
|Index Terms:||Reclaimed asphalt pavements, Admixtures, Mix design, Asphalt mixtures, Recycled materials, Asphalt pavements, Paving, |
|Cosponsoring Committees:||AFK20, Asphalt Binders|