Investigation of moisture damage resistance of GTR-modified asphalt binder by static contact angle measurements

• Ground tire rubber (GTR) is a viable modifier of asphalt binder.
• GTR increases viscosity and stiffness of asphalt binder.
• GTR increases the total surface free energy and cohesive energy of binders.
• GTR in binder causes an increase of compatibility between aggregate and binder.
• GTR indicating improved stripping resistance.

More than 300 million scrap tires are being generated every year in the United States and their proper disposal has been a major concern from environmental safety and economical point of views. Using these scrap tires as ground tire rubber (GTR) in asphalt mixes is becoming a common practice now a days. A number of research studies have been conducted to characterize the mechanistic properties of GTR modified asphalt. However, a very limited research has been conducted to evaluate their moisture susceptibility, particularly through fundamental science approaches. Moisture damage or stripping, the loss of strength and stiffness of asphalt mixes due to the existence of moisture, of asphalt pavements is one of the major concerns to the state and federal highway agencies for many years. The objective of this research was to evaluate moisture susceptibility of GTR modified asphalt binders through a surface science approach, in which surface free energy (SFF) components were estimated by following the sessile drop method. In the experimental plan, a commonly used performance grade (PG) binder (PG 64-22) modified with different dosages (0%, 10%, 15%, and 20%) of GTR were tested for static contact angles. Three different probe liquids, namely: water, formamide and ethylene glycol were used to determine contact angle with binder samples using an optical contact angle analyzer (OCA) device. The SFE of five different aggregates, namely: sandstone, gravel, granite, basalt and limestone, were used to evaluate the work of adhesion and compatibility ratio (CR). The SFE results of unmodified and modified binders showed that SFE increased due to the addition of GTR in binder. The work of adhesion of a binder aggregate system also increased with an increase of the GTR content when measured in the absence of water, but it decreased gradually in the presence of water. Granite and basalt showed lowest work of adhesion, whereas sandstone showed the highest work of adhesion. The CR data showed that granite and basalt were the most compatible aggregates with GTR modified binders. The CR value also had an increasing trend with an addition of higher GTR content, indicating an improved moisture resistance. The CR values of all asphalt aggregate systems were found to be greater than 1.5 with 20% GTR, indicating “good” moisture susceptibility.