Modification of asphalt by crumb rubber modifier (CRM) is mostly done through exchange of components between asphalt and CRM. At 160°C and 10 Hz, CRM absorbs aromatics of asphalt and swells; however, at 220°C and 50 Hz, CRM dissolves into the asphalt; this process leads to the release of different components of CRM including carbon black, fillers, and polymeric and oily components. Depending on interaction conditions, CRM dissolution is performed differently, and therefore the state and role of the released components can be different. In this research different mechanisms of dissolution of one size of CRM in asphalt matrix and their effects on property development of modified asphalt were studied by using different analytic techniques, including dissolution test, dynamic shear rheometer (DSR), thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy. Dissolution test results in combination with DSR results indicate the state and the role of CRM particles in the matrix, while TGA and FTIR spectroscopy show the type and amount of released components from CRM. The results reveal that for the materials used in this study interaction temperature is the main factor affecting the mechanism of dissolution of the CRM particles and consequently defines the role of the released components of CRM in asphalt matrix. DSR results indicate that during interactions at 190°C (intermediate interaction temperature), released components are more effective on physical properties of asphalt than are released components during interactions at 220°C (high interaction temperature).

Control of the internal network structure of crumb rubber–modified asphalt (CRMA) was investigated for its effect on property development in the study presented in this paper. The investigation included evaluation by rheological and interrupted shear tests. Rheological testing involved the monitoring of the changes of the produced complex modulus (G*) and phase angle (δ) for CRMA. In the interrupted shear tests, the shear stress profile for the produced CRMA was investigated. It was found that triggering the critical conditions of the combined interaction of speed, temperature, and time was essential to inducing the formation of the three-dimensional network structure within CRMA. Such triggering conditions played an important role in the swelling, dissolution, and release of crumb rubber modifier components into the asphalt liquid phase. Gel permeation chromatography was used to characterize the nature of modifications that led to the occurrence of a three-dimensional network structure within the produced CRMA. The formation of a three-dimensional network structure for CRMA played a major role in the enhancement of its rheological properties in terms of its stiffness and elasticity. Superior improvements in the stiffness and elasticity indicated improved rutting resistance as well as alleviated permanent deformation problems.

Road deterioration inspires researchers to enhance the properties of asphalt binder for better performing mixes. Recycled tire rubber, or crumb rubber modifier (CRM), and used motor oil (UMO) are two modifiers that enhance asphalt binder performance through two different mechanisms. CRM affects high-temperature properties while UMO modifies low-temperature properties. Potential environmental concerns arising from the use of UMO have been raised in the literature. In this paper, the two recycled materials were investigated for their ability to complement each other. Both performance benefits of using both materials and the environmental concerns of using UMO were studied. Four CRM asphalt binders were investigated: two with UMO and two without UMO. Environmental impacts were evaluated using gas chromatography to check air emissions for benzene, toluene, ethyl-benzene, and xylenes (BTEX). The potential for toxic leaching of elements from modified hot mix asphalt (HMA) were checked using the US Environmental Protection Agency (EPA) Toxicity Characteristic Leaching Protocol (TCLP). For asphalt binders modified by CRM-UMO combinations, CRM decreased the amounts of released BTEX components, presumably by absorbing UMO and slowing the release of BTEX. Leaching results concluded that UMO mixtures showed a notable percentage of sulfur (S) as compared to non-UMO mixes. All these leachate components were under EPA limits.

Crumb rubber–modified asphalt (CRMA) extensively suffers from high-temperature storage instability. Because of the higher density of the crumb rubber modifier (CRM) particles, they settle down to the bottom of storage tanks and cause variation between physical properties of the top and bottom samples. The storage stability of the CRMA changes under different interaction parameters and consequently different mechanisms of interaction. In this research the mechanism of separation under different levels of CRM dissolution was investigated by using Stokes’ law. The extent and mechanism of CRM dissolution in asphalt was controlled through regulating interaction temperature and inter action mixing speed. A standard storage stability test was conducted on CRMA samples to measure the corresponding storage stability index. Viscosity of the liquid phase of CRMA was measured at the same temperature as that of the storage stability test (163°C), with the use of a rotational viscometer, to study the effect of viscosity development of the liquid phase on the storage stability of CRMA. Composition analysis was conducted by using thermogravimetric analysis to investigate the effect of changes in CRM composition on the storage stability of CRMA. Results reveal that, in addition to the CRM particle size reduction resulting from CRM dissolution, the viscosity development of the liquid phase affects the storage stability of CRMA with the interaction temperature of 190°C. But increasing the interaction temperature to 220°C changes the mechanism of dissolution and the nature of the residual CRM particles and consequently leads to a different mechanism of separation.

In this work we investigated the performance aspects of addition of used motor oils (UMO) to neat and crumb rubber modified asphalts (CRMA) and related that to the change of molecular size distribution of modified asphalt's fractions; asphaltenes, saturates, naphthene aromatics, and polar aromatics. Based on the results of temperature sweep viscoelastic tests, addition of crumb rubber modifier (CRM) alone or with UMO results in the formation of internal network within the modified asphalt. Based on the results of short and long term aged asphalts, the utilization of combination of UMO and CRM enhanced the aging behavior of asphalt. Bending beam rheometer was utilized to investigate the low temperature behavior of UMO modified asphalts. Based on those tests, the utilization of the UMO and CRM enhanced the low temperature properties of asphalts. Based on the results of the asphalt separation tests and the Gel Permeation Chromatography (GPC) analysis, it was found that saturates and naphthene aromatics are the two asphalt fractions that have similar molecular size fractions as those of UMO. However, UMO only shifts the molecular sizes of saturates after interaction with asphalt. Results also show that polar aromatics pose higher molecular size structures than UMO.

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Asphalt binder is subjected to aging during the construction process and service life of pavement. Aging substantially changes the chemical composition and physical properties of asphalt and leads to premature distresses on pavement. It is speculated that crumb rubber modifier (CRM) reduces the susceptibility of asphalt to aging, but its mechanism is not fully understood. Also, the impact of interaction parameters and exchange of components on the aging behavior of a crumb rubber–modified binder is not fully understood. In this study, the effect of CRM on the short-term aging of asphalt was investigated through an analysis of the distinctions between the impact of different CRM activities (e.g., swelling and dissolution) on the oxidization process of asphalt. The crumb rubber–modified binder and its extracted liquid phase were aged with the use of a rolling thin-film oven. Physical and analytical tests (e.g., dynamic shear rheometer, Fourier transform infrared spectroscopy) were performed before and after aging. The results revealed that CRM affected the short-term aging behavior of asphalt in two ways. First, CRM retarded asphalt oxidization through the release of antioxidants. Second, CRM continued to interact with asphalt during its aging through absorption of aromatics and swelling. The results indicated that, through the accurate control of the interaction conditions, a modified asphalt could be produced with antiaging behavior superior to that of neat asphalt. Moreover, the results showed that an increase in the mixing and compaction temperature of hot-mix asphalt was not a good remediation of the high-temperature workability problem with the crumb rubber–modified binder because it significantly affected the nature of the composite.