Viability Assessment of the Use of Ground Tire Rubber (GTR) as a Modifier in Asphalt Binders

The total number of generated scrap automobile tires in the U.S. exceeds 300 million per year, which is roughly one scrap tire per person. Improper disposal of these scrap tires is a major concern to environmental protection agencies as they become a nuisance for public health and the environment. The objective of the present study is to assess the viability of the use of scrap tires in the form of ground tire rubber (GTR) in preparing new asphalt mixes. To this end, this study evaluated a commonly used performance graded (PG) asphalt binder (PG 64-22 marketed as PG 67-22) modified with different amounts (0%, 10%, 15%, and 20% by the weight of the binder) of two different graded GTR samples (Mesh #30 and Mesh #40). The blended GTR-modified binder or asphalt rubber (AR) samples were then investigated through a series of laboratory tests that included Fourier Transform Infrared (FTIR), Atomic Force Microscopy (AFM) imaging, Standard Penetration, Rotational Viscometer (RV), Dynamic Shear Rheometer (DSR), and a moisture susceptibility test via the Sessile Drop (SD) method. The FTIR analysis revealed the presence of alkanes, alkyl halides, carboxylic acids, etc. in the PG 64-22 binder, whereas the GTR-modified binders exhibited various amines and nitro compounds. The AFM image analysis showed that there were ‘bee-like’ structures in the PG 64-22 binder, and GTR-modification densified the ‘bee-like’ structures, indicating an additional concentration of Styrene-Butadiene-Styrene (SBS) and/or Styrene-Butadiene-Rubber (SBR) in the GTR-modified binder. Penetration, RV, and DSR test results showed that GTR addition increased stiffness and rutting resistance of the base binder. Moisture susceptibility analysis showed that the addition of GTR increased the surface free energy, cohesive energy, and aggregate compatibility, which are desired for improved moisture resistance. Based on the findings of this study, GTR is found to be a viable modifier for asphalt binders.