Effects of production stages on blending and mechanical properties of asphalt mixtures with reclaimed asphalt pavement

Reclaimed asphalt pavement (RAP) has gained popularity as a substitute to virgin aggregates and binders used in asphalt mixtures. The use of RAP has made a good saving in terms of conserved asphalt binder. Unfortunately, aged RAP binder increases the brittleness and stiffness of reclaimed asphalt mixtures and this leads to reduced resistance to thermal and fatigue cracking in pavements.

Researchers have found that using softer virgin binder in the mix design stage could be a solution to alleviating the stiffening effect of RAP binder in high percentage RAP mixtures. Therefore, the performance of RAP mixtures could be affected by the degree of blending between the stiff aged binder and the soft virgin binder used in high percentage RAP mixtures.

Several studies have been done to establish the degree of blending that occurs between reclaimed asphalt and virgin asphalt binders. However, the effects of the production stages of RAP mixtures on their blending as well as mechanical attributes are still unclear, particularly for high percentage RAP mixtures.

The production stages that potentially can affect the blending between the virgin and RAP binders include the RAP binder transfer stage, mechanical blending stage, and diffusion stage. A previous study on a 26% RAP mixture identified the diffusion stage as the most dominant stage that affects mechanical attributes as well as the blending of reclaimed asphalt mixtures. These findings however need verification with higher percentage RAP mixture and other RAP sources.

Kun Zhang and Balasingam Muhunthan at Washington State University conducted a study in order to verify if the diffusion stage is the dominant mechanism for asphalt mixtures with other RAP sources at the RAP binder replacement ratio of 26% as well as 50%. Their research work is published in journal, Construction and Building Materials.

In view of the rheological attributes of the RAP mixtures, for example dynamic modulus and creep compliance, the authors observed that the effects of productions stages or binder blending were more critical at the intermediate temperature than at low temperatures. The diffusion stage was the most dominant stage that influenced the rheological attributes of RAP mixtures. For this reason, sufficient storage time in a silo was necessary to allow for the diffusion process to be complete between the virgin and RAP binders in a RAP mixture.

The diffusions stage was also the dominant stage that affected the fracture attributes at 20 °C for mixtures with 26% RAP binder replacement ratio. For mixtures with 50% RAP binder replacement ratio, mechanical blending could considerably enhance ductility and cracking resistance of high percentage RAP mixtures.

When the percentage of RAP was increased in the mixture, the stiffening effect of the aged RAP binder on the rheological as well as fracture attributes of the mixture could be amplified owing to clusters of RAP material. Therefore, adequate mixing time was necessary to enable the breakup of the RAP clusters to alleviate stiffening effects and enhance the bonding between the blended binder and aggregates.