Effect of crumb rubber gradation on asphalt binder modification: rheological evaluation, optimization and selection

The objective of this study was to evaluate the effect of various crumb rubber (CR) gradations on asphalt binder performance leading to the development of a rational asphalt-rubber (AR) binder selection procedure. The scope encompassed advanced asphalt binder rheological characterization and statistical analyses in order to formulate a methodical procedure to select an optimum AR material. A total of twenty asphalt binders covering over 5000 data points were utilized, including two virgin binders, and eighteen laboratory prepared AR binders at one CR dosage of 20% but with varying CR gradations. Temperature-frequency oscillation and multiple stress creep and recovery (MSCR) tests were conducted on all the twenty asphalts. G* shear modulus master curves developed from the oscillation test results indicated that AR binders with finer CR particles in the gradation produced flatter master curves that had high G* magnitudes, which are indicative of high strength, better rutting resistance, and reduced viscosity-temperature susceptibility than those with coarser gradation. MSCR test findings complemented the oscillation test results. Analyses of Variance and Dunnett's statistical tests were performed on the binder rheological properties. From statistical analyses, it was concluded that the variation in CR gradations could significantly affect the binders' properties and resistance to distresses. Based on the results of rheological experimentation and statistical analyses, a methodical selection procedure for AR binders was devised by developing Ashby plots for the binder rheological parameters. These Ashby plots are envisioned to assist the pavement engineers and contractors in choosing the best suitable AR binder with the essential properties required to satisfy the performance criteria.