# Analysis of Nanoindentation Test Results of Asphalt Mixture with Different Gradations
## 1 Introduction  
Asphalt mixtures are multi-phase composites formed by combining asphalt, filler, coarse and fine aggregates in certain proportions and using specific forming methods. The performance of asphalt mixtures is influenced by a number of factors, such as asphalt, minerals, gradation and the forming process [1, 2, 3]. [PERSON] studied the effect of the interlocking of aggregate on the cracking of mixture, and showed that reaching the interlocking stage during compaction resulted in better rutting resistance [4]. [PERSON] investigated the effect of grading on rutting performance [5]. Numerous domestic scholars have studied the influence of minerals on the performance of mixes at a macro level. The overall performance index of the mixture couples the aggregate and asphalt drastic properties. However, the influence of the interaction between the various parts on the mixture is not well studied. Therefore, it is necessary to find a new way to investigate it. In recent years, the rise of various microscopic test methods for studying asphalt materials has brought new prospects. Among these, nanoindentation technology is gradually becoming a powerful tool for the characterization of microscopic properties of asphalt materials. It can record the load-depth curves during indentation of the indenter onto the sample by touching different phase materials on the surface of the specimen under a high-definition microscope and subsequently fit the test curves with a mechanical model to analyze the test curves. The micromechanicalindices of each phase material can be accurately determined, and the mechanical properties can be tested at the micro- and nano-scale [6; 7; 8]. Many researchers have carried out detailed investigations on the application of nanoindentation technology in asphalt materials and asphalt mixtures, and useful findings have been put forth. In terms of viscoelastic characterization of asphalt, [PERSON] was the first to use the nanoindentation technique to test B50 and B70 asphalt considering the indenter geometry, who found that the viscoelastic parameters correspond well with three deviation creep model parameters [9]. [PERSON] used linear viscoelastic analysis to extract the viscoelastic properties of aged asphalt and asphalt mass samples and calculated the shear relaxation modulus and dynamic modulus. It was shown that the nanoindentation results were consistent with those measured by the dynamic shear rheological (DSR) test tests in terms of magnitude and trend, thereby verifying the feasibility and accuracy of nanoindentation for studying viscoelastic properties of asphalt mastic [10; 11]. For the viscoelastic characterization of asphalt mixture, [PERSON] and [PERSON] et al. used the elastic-viscoelastic correspondence principle to derive the viscoelastic solutions of indentation curves for different loading phased to characterize the creep response of materials during loading [12; 13; 14]. Moreover, [PERSON] et al. quantified the strength loss of asphalt mixture before and after freeze-thawing and the fracture properties of asphalt concrete. Their research demonstrated that nanoindentation is more detailed and accurate in the evaluation of asphalt mixture properties [15; 16]. [PERSON] investigated the effect of warm additives on the nano-mechanical properties of asphalt mixture in three phases (aggregate, interfacial transition zone and mastic), suggested that nanoindentation can reflect the real behavior of asphalt mixture [15; 17]. In terms of the interfacial transition zone (ITZ) of asphalt mixtures, some scholars are devoted to the study of the interface characteristics of asphalt mixture and the mechanical behavior of ITZ [18; 19]. The accuracy of this was verified by energy dispersive spectroscopy (EDS) [20]. The above results show that the nanoindentation test has good applicability and prospects for the characterization of asphalt mixture properties. It may provide a new way to explore the properties of asphalt mixtures.