CAM mix design with local aggregates
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Crack Attenuating Mix (CAM) is relatively a new asphalt mixture type used by the Texas Department of Transportation (TxDOT). These mixtures are fine graded, designed using relatively small stone sized aggregates and screenings with a high asphalt content, specifically for retarding the reflective cracking in thin asphalt overlays without sacrificing rutting resistance. The main goal of this study is to promote the use of local aggregates in CAM design. In order to achieve the goal, the development of a mix design procedure and aggregate quality guidelines for CAM design with local aggregates are essential. Several aggregate sources were identified and characterized in terms of their frictional, strength, and durability properties. A refined mix design procedure is recommended using a Superpave gyratory compactor with the capability to measure shear stress during compaction. The emphasis was on preventing rutting and flushing resulting from overfilling of binder in these mixtures, which was observed during compaction by consistent shear failures. The Hamburg Wheel Tracking Device (HWTD) was used for performance evaluation of rutting, whereas the Overlay Tester (OT) was used for cracking evaluation. It is hypothesized that current performance criteria in terms of HWTD and OT are too severe for being applicable to CAM mixes with local aggregates. A modification in HWTD failure criterion is recommended and is used in developing new aggregate guidelines. A correlation analysis was conducted relating aggregate properties and CAM performance in rutting and cracking. Finally, aggregate quality guidelines were developed for CAM using local aggregates based on the correlation study. The mix design recommendations as well as aggregate guidelines and quality criteria provided in the report are based on laboratory investigations and field validation of these is strongly recommended before implementation.