Tensile strength of asphalt binder and influence of chemical composition on binder rheology and strength

Asphalt mixtures or asphalt concrete are used to pave about 93% of about 2.6 million miles paved roads and highways in the US. Asphalt concrete is a composite of aggregates and asphalt binder; asphalt binder works as a glue to bind the aggregate particles. The mechanical response of the asphalt binder is dependent on the time/rate of loading, temperature and age. An asphalt concrete mixture inherits most of these characteristics from the asphalt binder. Also the asphalt binder plays a critical role in providing the asphalt concrete the ability to resist tensile stresses and relaxing thermally induced stresses that can lead to fatigue and low temperature cracking, respectively. Hence, it is very important (but not sufficient) to ensure that asphalt binders used in the production of asphalt concrete are inherently resistant to cracking, rutting and other distresses that a pavement may undergo. Current binder specification (AASHTO M-320) to evaluate its fatigue cracking is based on the stiffness of the binder and not on its tensile strength. Also, measurements following current specifications are made on test specimens subjected to a uniaxial mode of loading that does not produce the same stress state in the binder as in the case of asphalt concrete.

Another challenge in being able to produce binders with inherently superior performing characteristics is the fact that the asphalt binders produced in a refinery do not have a consistent chemical composition. The chemical composition of asphalt binder depends on the source and refining process of crude oil. There is a need to better quantify the tensile strength of asphalt binder and understand the relationship between the chemical composition of asphalt binders and its mechanical properties. The knowledge from this study can be used to engineer asphalt binders that have superior performance characteristics.

The objective of this research was to quantify the tensile strength of asphalt binder, develop a metric for the tensile strength and identify the relationship between chemical composition and mechanical properties of asphalt binder. Laboratory tests were performed on binders of different grades using a poker chip geometry to simulate confined state by varying the film thickness, rate of loading and modes of loading. The chemical properties of asphalt binder were studied based on SARA fractionation. The findings from this research showed that the modified correspondence principles can unify and explain the rate and mode dependency of asphalt binder. This study also quantified the relationship between chemical composition, and rheological and mechanical properties of asphalt binder. Finally, a composite model was developed based on the individual properties of chemical fractions which could predict the dynamic modulus of the asphaltenes doped and resins doped binder.