Experimental investigation and rheological modelling of the fine aggregate matrix (FAM) phase in the multiscale transition from binder to asphalt mixture

In the last decades, the multi-scale approach has gained wide attention for studying and understanding the mechanisms affecting the performance of asphalt mixtures. According to such approach, the asphalt mixture could be seen as an assemblage of components of different length scales, each with its own mechanical properties, and thanks to the investigation on these lower scales a scale-wise insight can be gained and used to capture phenomena not considered in continuum approaches. This Thesis is focused on the interrelation between the Fine Aggregate Matrix (FAM) the asphalt mixtures. Fine aggregates, filler, binder and air voids compose the FAM, which represents the intermediate scale between mastic and asphalt mixture. This phase has a critical role in the overall performance evaluation of asphalt mixture and the simplicity, efficiency and the lower costs/times required to study the FAM make it a very attractive specification-type approach. However, despite the growing interest on FAM testing, there are some concerns about proper FAM mix design. Moreover, there are many lacks in predicting the performances of asphalt mixtures from the FAM phase. The primary objective of this Thesis was the identification of a design method for FAM, which allows recreating the FAM phase, as it exists within the asphalt mixture. The selected design method has shown promising results and seems quite accurate in reproducing the FAM within the asphalt mixture. The second issue which is pursued in this Thesis, is a multi-scale approach based on the rheological 2S2P1D model, allowing to interrelate the four material scales (from binder to asphalt mixture). Firstly, it was verified that the 2S2P1D model remains valid for FAM in the Linear ViscoElastic (LVE) range and that it could adequately fit experimental data of FAM. Then, it was possible to relate the different phases thanks to the definition of interrelationships between one model parameter. The interrelation between the asphalt mixture and the corresponding FAM could be used to predict the rheological properties of the asphalt mixtures starting from FAM tests. This methodology allows making reliable forecasts of the LVE behaviour of the asphalt mixtures, as demonstrated by the results of validation tests. Future developments will investigate other volumetric compositions of mixes, to study the influence of microstructural and volumetric characteristics on the model parameters.