Due to environmental and economic benefits, reclaimed asphalt pavement (RAP) has now been routinely used in pavement construction.
There has been a request from the asphalt paving industry to increase RAP limits allowed in asphalt pavement mixtures to achieve an even greater economic benefit. According to the latest survey conducted by the National Asphalt Pavement Association (NAPA) and the Federal Highway Administration (FHWA), approximately 76.9 million tons of RAP were used in new asphalt pavement mixtures during the 2016 construction season, resulting in a cost savings of more than $2 billion compared to the use of virgin materials. The number of states reporting average RAP percentages of 20% or greater has increased significantly from 10 states in 2009 to 29 states in 2016. The national average percentage of RAP used in asphalt mixtures also has increased from 15.6% in 2009 to 20.5% in 2016, clearly indicating the trend of increasing RAP content used in asphalt mixtures.
However, in spite of the increased use of RAP, there is a still a lingering concern regarding the efficiency of RAP usage in asphalt mixtures, that is, how much aged asphalt binder in RAP is actually mobilized and blended into virgin asphalt binder to form a uniform binder blend and to coat new and RAP aggregate particles? Since RAP has experienced a lifetime of aging, RAP binder is very stiff and is hard to blend and mix with virgin binder. If RAP binder cannot be fully mobilized and made available for reuse as a binding material, there will be less effective asphalt binder than expected or designed to coat new and recycled aggregates and bind them together, resulting in premature distresses such as cracking, raveling and moisture damage. This will lead to a shortened service life of asphalt pavements or increased cost for pavement maintenance and repair, making it unrealistic to save money by using more RAP.
The University of Tennessee researchers have recently finished a research project sponsored by the Tennessee Department of Transportation aiming to solve the issue of RAP binder blending in recycled asphalt mixtures. They developed a laboratory procedure to determine the efficiency of RAP recycling during the plant mixing process. Since virgin and aged asphalt are both black and sticky, one cannot physically or visually differentiate between them once they are blended together. UT researchers used an advanced testing technique called gel permeation chromatography (GPC) to differentiate RAP binder from virgin binder. GPC separates asphalt molecules according to their sizes just like sieves do to aggregate particles in sieve analysis. RAP binders have more large molecules and fewer small molecules than virgin asphalt, which can be used to differentiate between virgin and RAP binders.
As shown in Figure 1, two possible blending scenarios may happen during plant mixing. If a total blending occurs, in which RAP binder is fully mobilized and blended into virgin binder, then virgin and RAP aggregate particles are both coated with a film of homogeneous asphalt blend. Otherwise, RAP binder is partially mobilized, resulting in virgin aggregate coated with uniform asphalt blend, and RAP aggregate coated with residual RAP binder and then further covered by a film of uniform asphalt blend.
To validate the proposed method for determining blending efficiency of RAP binder, a laboratory mixing experiment was performed. Asphalt mixtures containing 10%, 20%, 30%, 40%, 50% and 80% RAP were fabricated in the laboratory at 165°C and two minutes of mixing time. To separate virgin aggregate after mixing to acquire the asphalt blend for GPC testing, some round-shaped aggregate particles were used as special aggregate so that they could be easily identified and separated from the mixture.
Immediately after mixing was finished, the special aggregate particles were separated and the asphalt coating was then extracted and recovered using a chemical method. The recovered asphalt blend was tested using GPC, and the portion of RAP binder in the blend was determined. With other known parameters in asphalt mixture design, such as virgin asphalt content of the asphalt mixture and asphalt content in RAP, the total amount of RAP binder in the asphalt blend could be determined and the mobilization rate, which is the percentage of mobilized asphalt in RAP binder, could be obtained by dividing the total amount of RAP binder in the blend by the total amount of RAP binder in the mixture.
Figure 2 shows the mobilization rates of the asphalt mixtures made with different RAP contents. It can be clearly seen that the mobilization rate decreased with the increase in RAP content in asphalt mixtures. The low RAP mixtures (10% and 20%) showed a mobilization rate close to 100%, whereas the mobilization rate dropped from 73% at 30% RAP content to 24% at 80% RAP content. The researchers further explored the effects of several factors affecting the blending efficiency of RAP binder, such as mixing temperature, mixing duration and warm-mix asphalt technologies. Some general conclusions are given below:
- Mixing at a higher temperature improved the blending between aged and virgin binders. However, caution should be exercised to avoid potential oxidation of virgin asphalt;
- An increase in mixing time enhanced the blending efficiency, but excessive long mixing beyond 2½ minutes showed little effect; and
- The foaming technology and warm-mix asphalt surfactants seem to improve the blending efficiency of the RAP binder.
A preliminary study was conducted to determine the mobilization rates of three plant-prepared asphalt mixtures containing high percentages of RAP. All three mixtures contained 50% RAP and were produced in an asphalt plant in three different ways: hot mix, hot mix with a rejuvenator, and warm mix using a foaming technology. The rejuvenator content was 0.4% by weight of asphalt binder. During the production process, the rejuvenator was first added into the virgin binder. The mixing temperature was 145°C and 155°C for warm mix and hot mix, respectively. After production, the mixtures were stocked in a silo for another two hours. Figure 3 shows the mobilization rates of the three high RAP plant mixtures. It can be seen that all three mixtures showed a mobilization rate of over 80%. Rejuvenator and foaming technology appeared to have a positive impact of blending efficiency of RAP. The fact that plant mixtures had a much higher efficiency than laboratory-prepared mixtures is due to the silo storage during plant production, which significantly improved the diffusion between aged and virgin binder and helped form a homogeneous asphalt blend.
In summary, a laboratory procedure was proposed for determining the blending efficiency of asphalt mixtures containing RAP, which has the potential to solve the problem of blending efficiency of high RAP contents in asphalt mixtures. The preliminary blending results seem promising, indicating that total blending is possible even for high RAP mixtures. However, more comprehensive future studies are still needed to consider various materials and mixing conditions, and to verify the current findings under these circumstances. The detailed laboratory procedure for testing blending efficiency can be found in doi: 10.3141/2506-08.