Features
Business
Pits & Quarries
International experiences with high RAP content mixtures
March 24, 2025 By Mike Aurilio
(iStock / Getty Images) As producing more sustainable pavements becomes a necessity, the use of recycled materials must gain traction.
Efforts must be taken by all industry partners to ensure the successful use of recycled materials such as reclaimed asphalt pavements (RAP). Hot Mix Asphalt including more than 25 per cent RAP are typically considered as high RAP mixtures. Research and production of high RAP mixtures has successfully proven that these pavements will work under different climatic and traffic conditions. Engineering the mixture properly ensures that it is possible. To do so, we must understand the materials used, the way RAP impacts the mixture behaviour, the location of the mixture in the pavement structure and ensure proper quality control procedures are in place.
Balancing contradicting properties of asphalt mixtures
To understand the role RAP plays in impacting pavement performance, we must first consider the general behaviour of asphalt mixtures. Asphalt cement and mixtures are viscoelastic materials in nature. This means that under certain temperatures and loading conditions the material will exhibit either vicious-dominated or elastic-dominated behaviour. Therefore, balancing these contradicting properties is necessary for ensuring the pavement will resist rutting and handle the expected level of strain induced by traffic. Often, there is a misconception that higher stiffness means more brittle, which results in overlooking valuable opportunities to improve the durability of our pavements. If properly designed, increasing the stiffness of a pavement structure can potentially reduce fatigue cracking through reducing the tensile strain in asphalt concrete layer. However, introducing a stiff material with a brittle behaviour can cause an issue. Asphalt mixtures with higher stiffness and yet maintaining the required resilience under loading can be effectively designed nowadays.
Binder-related mix properties can be used to control how the material responds to loading. Increasing the asphalt cement content or using a softer binder both promote a more “viscous-like” behaviour that can help with low temperature properties while SBS polymer modification adds a “rubbery” behaviour which can make the pavement more strain tolerant. Looking at the way a material functions in the pavement structure, it can be realized that a material that is too soft may experience larger strains and become more prone to fatigue cracking.
In other words, on the plus side a stiffer material can resist deflection and reduce the level of maximum strain that the material experiences in the pavement where an equivalent softer material will have the opposite behaviour. This larger strain level can accelerate cracking and premature failure of the material.
This brings us back to RAP. Properties of RAP will vary depending on the source and age of the material, but one major characteristic is hardening due to oxidation. Pavement materials will oxidize at different rates. The nominal maximum aggregate size, binder source, presence of additives and permeability introduced by construction issues can impact the rate of oxidation. RAP is a hardened material and introducing this into new mixtures must be accommodated properly to ensure performance is maintained. Generally, introducing RAP will increase the relative stiffness of the mix, which, when designed for, can improve the pavement performance. One of the biggest issues is ensuring that the mix does not react poorly under repeated traffic loading. Increasing stiffness alone without considering the impact on the material response to loading can prove to be detrimental to performance.
Jurisdictions that have been successful in increasing their RAP usage have focused on three key areas: redesigning the mixtures, characterizing the materials and processing the RAP stockpiles. These ideas are interrelated and although there are many examples of how to produce high RAP mixtures successfully, it is important to note that it is necessary to experiment with these ideas to ensure that they work at the local level. The most important factor is to validate that the performance in the field and adjust as necessary. For example, including fractionation requirements in a quality control plan will improve consistency, but the chosen particle size will also influence the diffusion rate of aged binder. How much aged binder is contributed to the overall mixture will influence how the RAP impacts the mixture properties and may impact the other mixture parameters. Large, multi-source stockpiles may need to be treated differently than smaller, more uniform stockpiles.
Success in Sweden
The Ontario Asphalt Expert Task Group put together a literature review on international experiences with high RAP mixtures and there is a great deal of detail that is applicable to not only high RAP mixtures, but moderate RAP usage as well. One of the jurisdictions highlighted in that document was Sweden, which successfully uses anywhere from 10 to 50 per cent RAP depending on the pavement traffic and location. Sweden does some interesting things that help promote consistency in their mixtures. They limit stockpiles to single sources of RAP and analyze the gradation, asphalt cement content, asphalt cement stiffness and perform petrographic analysis of the aggregates. By characterizing the RAP properties, they can effectively compensate for the change in properties. Understanding the RAP binder stiffness and particle size gives mixture designers a better understanding of how the RAP binder will blend with the virgin binder. In addition to this, it is important to Swedish authorities that the mixture containing RAP has similar properties to the virgin mixtures they produce. Modulus (stiffness) is measured, and the mixtures are subjected to dynamic creep testing as well as indirect tensile strength testing. The modulus is important for ensuring that the pavement meets their structural design requirements. Swedish authorities expect the mixture to reach a minimum stiffness of 5500 MPa at 10°C for heavy traffic. Their biggest concern is over softening the mixture with virgin asphalt cement and making the mixture too soft.
U.S. state of Georgia took steps to address cracking issues
Another great example which is a little closer to home comes from Georgia. They generally had cracking issues with their pavements and took many steps to address this including reducing the number of gyrations (SuperPave mixtures), finer gradations and using mixture performance testing for specialty mixtures. These measures to increase asphalt cement content helped, but additional RAP specific requirements are also used. Georgia DOT is required to approve each stockpile before use and limits the amount of RAP being used based on the asphalt cement content and percent passing the #200 sieve of the RAP. To further ensure sufficient binder is in the mixture, Georgia employs the Corrected Optimum Asphalt Content methodology. This essentially assumes not all the binder in the RAP is available and requires additional virgin asphalt cement to be used to compensate for the unavailable binder.
High RAP mixes perform well when engineered properly
The Ontario Asphalt Expert Task Group covers many other jurisdictions from similar climates to completely different climates, high traffic and low traffic environments. The engineering principles will generally remain valid regardless of traffic and climate, but it is important to determine specifics for local conditions. Georgia employs a 60:40 Corrected Optimum asphalt Content, but will a different ratio be required for more highly aged RAP binders? Questions like these are important to answer, but the data collected by so many different Departments of Transportation and researchers prove that high RAP mixtures are possible and perform very well when engineered properly. The full webinar is available here: https://vimeo.com/1014810357?share=copy
Mike Aurilio is the Terminal Manager for Yellowline Asphalt Products Ltd. Aurilio would like to thank his co-authors/friends Amma Agbedor, Yashar Almandary, Doubra Ambaiowei and Pejoohan Tavassoti for their help putting this literature review together.
Print this page