Asphalt producers are increasingly relying on reclaimed asphalt pavement (RAP) to meet environmental goals and cost-efficiency targets. Using RAP can allow aggregate and aged binder to be reused, reducing the need for virgin materials and lowering overall material costs. However, one key challenge that working with RAP poses to plant operators and mix engineers is variability.  

 

When RAP is inconsistent in attributes such as gradation, binder content, or moisture, the final mix performance can suffer. Producers may experience quality issues, lower production rates, reduced pavement life, and even failures if not processing RAP properly. Additionally, switching between RAP piles without following best practices can lead to variability in quality and performance between different batches of asphalt mix.  

 

The good news is RAP variability can be significantly reduced to produce more consistent, high-performing asphalt mixes when adhering to protocols for proper stockpile management, plant operations, and quality control practices and service lives.  

 

What Causes RAP Mix Variability? 

To work with RAP effectively, producers need to understand that RAP is not a naturally uniform material. A waste byproduct of roadway resurfacing projects, reclaimed asphalt pavement is often sourced from different roadways. Even RAP collected from a single project can exhibit different attributes including aged binder properties, aggregate gradation, and materials used to construct the roadway. A single RAP pile at an asphalt plant could contain material from a combination of residential streets or high-volume highways with different original mixed designs, properties, and contamination levels. 

 

Environmental factors can also impact RAP stockpiles and the variability of the material. Moisture can fluctuate with weather conditions and drainage infrastructure in storage. This can further degrade the integrity of the aged binder in the RAP while also adding to the energy and time needed to preheat and dry the RAP before it can be integrated into a mix. The binder in roadways naturally oxidizes over time (contributing to the original failure of the source roadway), and prolonged RAP storage in open-air, uncovered stockpiles may cause the binder in the material to age even further.  

 

Starting with RAP Stockpile Management Best Practices  

While RAP is a recycled material, historically left in landfills, plant operators who treat RAP as a valuable material may achieve less variability in quality and output in their resulting mixes. Best practices for managing RAP stockpiles can include: 

  • Cover the RAP Pile: Minimize moisture and exposure to the elements with a tarp, bins, or other shelter buildouts. 
  • Segregate the RAP: Keep RAP from different sources separate to simplify testing and formulation. 
  • Fractionate the RAP: Screen the RAP based on particle size to make it easier to incorporate higher levels of RAP into a mix design. 
  • Homogenize the RAP Pile: Blend or mix the RAP pile to promote consistency during long-term storage and settling. 

 

Plants that follow best practices for RAP stockpile management can mitigate some of these issues. Additional operational considerations that should be kept in mind include: 

  • Variations in RAP feed rates that can cause differences in the final mix proportions 
  • Inadequate bin cleaning that can introduce contaminants into the mix 
  • Managing material homogenization to avoid segregation of fine and coarse particles during handling 

 

Improving Control with Fractionation and Screening 

Processing RAP is a multi-step practice. Fractionating, or dividing up RAP based on material sizes from coarse to fine, can help with consistency and quality in the final mix. Smaller and larger RAP components will often behave differently in a mix, making it harder to control variability. Instead, separating these materials through several stages of screening can help engineers tailor their designs to the RAP on hand and better balance the aggregate gradation and binder quantities. For high-RAP mixes, fractionation is essential to meet gradation specifications. 

 

Screening RAP can help with more than just fractionation. Screening can also remove foreign matter and contaminants that can directly impact performance of the final asphalt mix. Plants can consider specialized equipment such as scalping screens to remove large material before it is incorporated into a drum mixer. These screens typically have multiple decks to allow material of different sizes to pass through while removing impurities.  

 

RAP Mix Production Controls and Quality Testing 

Once properly prepared reclaimed asphalt pavement reaches the plant, tight process control becomes critical as with any asphalt mix run. Automation and monitoring tools can help implement practices to detect variability in real time. 

 

Cold feed bins and feeders can be calibrated to regulate the amount of RAP introduced to the mix, avoiding the variability that can come from changes in feed rates. In addition, monitoring drying and mixing temperatures can help avoid impacting the quality of the RAP being processed. RAP with high moisture content may require more energy to dry, increasing the risk of burning the aged binder. Consistent temperatures ensure even coating and reduce the chance of thermal degradation. 

 

The blend ratios established for RAP and virgin materials are typically established through rigorous testing using samples from the existing RAP. Plant operators can help limit mix variability by maintaining these ratios throughout the process and only making adjustments based on test results. Mix engineers can support this process by taking regular samples to confirm consistency across batches while adjusting for current composition.  

 

Working with Asphalt Additives in RAP Mixes 

When and how to introduce asphalt rejuvenators, like ReLIXER®, or other additives to a RAP mix in a production plant also requires attention. Dosing an asphalt mix properly can help reduce variation in the mix quality. For example, depending on how the plant is designed, ReLIXER can be easily incorporated into operations via direct addition to the virgin binder tank, inline injection system, or treatment of RAP prior to entering the mix drum.  

 

Regardless of method, accurate dosing and adequate mixing can be essential to achieving consistent results. Additive suppliers may provide dosage guidelines based on binder content, RAP stiffness, and mix type, and quality control labs should verify performance improvements through both binder testing and mix testing. Employing a balanced mix design (BMD) approach can also help mix designers identify where the mix does, or does not, meet performance specifications.  

 

 

Reducing RAP variability during plant production is a challenge that can be overcome through careful sourcing, storing, processing, and testing methods. By doing so, plant operators may achieve lower costs while conserving natural resources and supporting sustainability goals through reduced need for virgin materials. Additives like asphalt rejuvenators can help improve the functional characteristics of aged bitumen in RAP to produce high-performing roadway surfaces, but the overall quality of the mix depends on a strong operational foundation. 

 

For more information about asphalt rejuvenation, RAP, and ReLIXER, contact info@sripath.com. Â