One of the major recycling markets of waste tires generated in the United States is represented by Ground Tire Rubber (GTR). Crumb Rubber (CR) is the terminology used for GTR materials reduced to a particle size ranging from 4.75 mm to less than 0.075 mm and is a commonly used additive for asphalt modification. While environmentally friendly and presents a major sustainable alternative to other engineering materials; the use of CR additives in asphalt did not develop as a solution to an environmental problem. CR is used to improve asphalt binder properties, and in turn to improve pavement performance in the field.Despite the environmental benefits and improved pavement performance, the adoption of CR Modified (CRM) materials in the field has been limited. This is partially due to challenges and limitations on characterizing the engineering behavior of CRM binders relating to pavement performance, as compared to standard Styrene-Butadiene-Styrene (SBS) polymer modifiers. This research is an effort to address some of these challenges. The major contributions of this work are outlined below:• Identify a feasible Dynamic Shear Rheometer (DSR) testing geometry for characterizing CRM binder rheology, through establishing a link to asphalt mixture mechanical properties.• Evaluate the interaction mechanisms between CR and asphalt binders and the influence of CR swelling on CRM binder performance.• Propose practical QC/QA test methods for identifying levels CR pre-treatment, as well as for estimating the engineering properties of swollen CR in asphalt binder and its’ relationship to performance.Due to the presence of swollen CR particles as suspensions in asphalt binder, a major challenge for adopting CRM binders is that they cannot be specified using standard rheological characterization methods. This is especially true at high concentrations and/or large CR particle sizes. The first contribution of this dissertation addresses this challenge by identifying a suitable and efficient DSR geometry for CRM binder characterization through a comparative analysis for binder rheology and mixture properties covering a wide range of CR concentrations and sizes. The second contribution addresses the nature of interaction between CR and asphalt binder at elevated temperatures. The changes in base asphalt rheology due to the absorption/diffusion of the light asphalt binder fractions (aromatic oils) into CR were used to identify the influence of interaction conditions on binder stiffening and softening effects of CR. The properties of swollen CR were found to be related to fatigue performance. CRM binder testing and characterization continues to rely on empirical tests that indicate modification, but do not address fundamental engineering behavior relating to pavement performance. The third contribution of this dissertation provides new QC/QA test methods that can be used during construction. A method utilizing CR swelling to estimate the extent of CR pre-treatment and to identify sufficiently pre-treated CR materials in the laboratory before being incorporated into CRM mixtures is proposed. In addition, alternative parameters derived from rheological characteristics of CRM binders were evaluated as indicators of overall binder performance and provided strong correlations to CRM binder fatigue and rutting characteristics. These parameters can be obtained and applied to QC screening of CRM binders without the need for micromechanical modeling or advanced testing/characterization techniques. This dissertation presents a comprehensive comparative analysis evaluating performance characteristics in CRM binders as influenced by different material variables including a wide range of base asphalt binder Performance Grades and CR sizes. The conclusions drawn herein provide a better understanding of CR modification techniques relative to performance-based specifications on asphalt binders and will encourage the overall adoption of CRM materials in paving applications.
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