Determining Hardness and Elastic Modulus of Asphalt by Nanoindentation

Nanoindentation is a relatively new technique which has been used to measure nanomechanical properties of surface layers of bulk materials and of thin films. In this study, micromechanical properties such as hardness and Young’s modulus of asphalt binders and asphalt concrete are determined by nanoindentation experiments. Indentation tests are conducted on a base binder and two polymer-modified performance grade (PG) binders such as PG-70-22 and PG76-28. In addition, two Superpave asphalt mixes such as SP-B and SP-III are designed using these PG binders, and the corresponding mixes are compacted to prepare asphalt concrete. Aggregate, matrix (Materials Passing No. 4 sieve) and mastic (Materials Passing No. 200 sieve) phases of each asphalt concrete sample are indented using both Berkovich and Spherical indenters. In nanoindentation, an indenter penetrates into asphalt material and the load (milli-Newton) and the depth (nanometers) of indentation are recorded continuously. Indentation load versus displacement data are analyzed using Oliver and Pharr method to measure hardness and Young’s modulus. The unloading data of base binder is a straight line and therefore could not be analyzed using Oliver and Pharr’s method. However, the indentation data of the PG grade binders are successfully analyzed. Young’s modulus value is less than 3 GPa for mastic, 3 to 12 GPa for matrix, and greater than 12 GPa for aggregate studied herein. Based on the hardness data, mastic is 2 to 15 times softer than matrix materials, and matrix is 10 times softer than aggregate materials. The fact that the properties of the mastic can be measured while in the mixture, this study has great potential for realistic characterization of asphalt mixture components. In this study, spherical indenter is found to be suitable for asphalt binders based on the fact that the spherical indenter produces higher indentation depths than the Berkovich indenter. The study contributes significantly to the use of nanoindentation for transportation material characterization.