Influence of asphalt-binder source on CAM mix rutting and cracking performance: A laboratory case study
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Chinese Society of Pavement Engineering. Crack attenuating mixtures, denoted as CAM, are some of the mixes commonly used in the State of Texas (USA) to mitigate reflective cracking in overlays, both in flexiblehot mix asphalt (HMA) and rigid concrete pavements. Typically designed at 98% lab density, with high quality aggregates, these fine-graded HMA mixes are rich in asphalt-binder (minimum 6.5%), predominantly using PG 76-22. In this study, three PG 76-22 asphalt-binders from three different sources (denoted as A, B, and C) were evaluated in the laboratory for their potential to meet the CAM Balanced Mix-Design (BMD) requirements when used in combination with limestone aggregates and 1% hydrated lime. Laboratory tests conducted included the Hamburg wheel tracking test (i.e., for rutting and moisture damage [stripping] evaluation), the Overlay test (i.e., for cracking evaluation), and the asphalt-binder rheology, namely the dynamic shear rheometer (DSR) and the bending beam rheometer (BBR). The corresponding results indicated that not all PG 76-22 asphalt-binders are manufactured equally and that material source has a profound influence on both the asphalt-binder rheological properties and the overall performance of the resulting HMA mix. In fact, one of the PG 76-22 asphalt-binder graded out as a PG 82-22 and could not meet the BMD performance requirements for a CAM mix-design. As part of the quality control/assurance protocols and to ensure that the right materials as designed and specified are utilized, the overall findings of study suggests that it is imperative that all asphalt-binders delivered to a given construction site must be sampled and tested for its rheological properties and graded accordingly. Provided high quality materials are used, the study also indicated that a CAM mix could satisfactorily be designed at a lower lab density than 98% (i.e., 96.5 - 97.5%); which translates into cost savings in terms of the asphalt-binder content.
