Nano-Scale Binder Phase Identification in Asphalt Concrete

Abstract

Asphalt concrete (AC) consists of asphalt binder and aggregate. Aggregate consists of: coarse aggregate and fines. Asphalt binder creates a coating or film around the aggregate, which is defined as the binder phase of AC. Fines are believed to be trapped inside an asphalt film or mixed with asphalt binder, creating a composite material called mastic. Thus, AC has three phases: mastic, asphalt film binder, and coarse aggregate. All these phases play major roles in performance of AC. Researchers have performed various tests on asphalt binder at micro scale to understand the macro scale behavior of AC. However, test methods developed and performed on binders, to this day, are mostly rheological shear and bending beam tests. No studies have been conducted on the compression stiffness or modulus and hardness of and binder, rather than shear and binders stiffness. In addition, the existing tests used in the asphalt area cannot be performed on binder and mastic while they are an integral part of AC. Nanoindentation tests can be performed on aggregate and asphalt binder while they are integral parts of AC. Because, in nanoindentation test, a nanometer size tip, which is smaller than binder film thickness as well as other phases. In the study, Performance Grade (PG) 64–28 was used for the study, same binder had been used afterwards to characterize asphalt and AC. A loading rate of 0.005 mN/sec, a dwell time of 200 sec and a maximum load 0.055 mN were employed in the study. In the current study 20 indentations were done on the asphalt binder sample and 100 indentations were done on AC sample, due to heterogeneity of the sample. However, to identify a specific phase in AC sample, the current study adopts the depth range technique for as same loading protocol. The depth rage of binder phase was acquired by independent indentation on same asphalt binder sample. As, asphalt is known to be a viscoelastic material that exhibits creep behavior, the creep compliance of asphalt binder was used for validation of the depth range assumption. The validation of phase identification was done by comparing the asphalt binder phase creep response while they are integral part of AC with creep response of independent asphalt binder sample under nanoindenter. The comparison shows depth resolution technique can successfully identify the binder phase of AC.