Ryohei GAKUHARI

Study on the Mix Design for HMA Mixtures Using Low Quality RAP

Osamu TAKAHASHI, Takeshi NAKAMURA

 In the Japanese paving industry, the addition of reclaimed asphalt pavement (RAP) into asphalt mixtures has become a common practice, with agencies developing procedures to successfully increase the content of this material. For instance, in 2016, the ratio of reclaimed asphalt contained in asphalt pavement structures was well over 75%, and this percentage is expected to continue growing. Previous studies have shown that asphalt mixtures composed of RAP that has already been used repeatedly are failing to meet the expected performance. This matter is of concern to the industry, since the use of low quality RAP will only increase with the pass of time. To address this concern, knowledge is needed on how RAP should be handled while still increasing the recycle rate. Only a few reports have addressed this matter before, and further study on the handling of low quality RAP is highly needed. 
 In the present research, a direct tensile strength test was conducted on specimens containing RAP to properly evaluate fracture resistance and provide further knowledge in regards to low quality RAP handling. As expected, results indicate that HMA mixture containing low quality RAP exhibit lower resistance to fracture, in comparison to HMA mixture composed only of virgin materials. Results also showed how HMA mixtures have a tendency to exhibit lower performance to fracture with the increase in RAP content. The experimental work of this study also revealed that mixtures with high contents of RAP would exhibit more deterioration, causing poor compaction of the specimens that were prepared.  
 On the other hand, the present study revealed that HMA mixtures containing RAP that has been used several times before can retain their desired properties if mixtures are designed using higher amounts of virgin materials. Concretely, results obtained from experimental work indicate that HMA mixtures composed of low quality RAP can exhibit the expected performance needed for in-field service.