Effects of nano-sized additives on bituminous binders: rheological and morphological characterization

The use of nano-sized particles as reinforcing additives for bituminous binders has become, in recent times, increasingly popular, due to the positive effects that nanoparticles induce on a wide range of materials. A considerable literature has grown up around the theme, and the experimental work presented here will contribute to a deeper understanding of nano-additives effects on rheological and morphological properties, with the main purpose of improving the performance of bituminous materials. Much less is known about the connection between rheological parameters and microstructure morphology, therefore this research has thrown up many questions in need of further investigation. In our specific study-case, Nanoclays (Cloisite 15A) and Carbon nanotubes (CNT7000) were added into single base bitumen samples (AZALT 70/100), through several dosages, expressed in percentages by weight, assessed on the base of data derived from previous studies. Four different operating protocols were adopted in mixing phase, varying the shear mixing equipment and the disintegrating heads. A two-phase blends preparation was adopted: the shear-mixing procedure (performed at low and high speed) leads to a more homogeneous dispersion of the nanoparticles into the bituminous matrix; while the sonication-mixing procedure allows reducing particles’ agglomeration, in order to reach dispersion as much uniform as possible and nanoparticles’ fine separation. Several rheological tests were led, gathering data that let us perform a rheological characterization of the samples. Developing the δ-method provided in literature, the bitumen’s molecular weight distribution (MWD) was determined, by mean of the phase angle of the complex modulus measured in the linear viscoelastic domain. In order to investigate the materials’ morphology, ESEM technology was used, with the main intention to estimate the mixing procedure influence on nanoparticles dispersion degree and to examine the relationship between the microstructure and the rheological properties. Focusing on the approach and the methods we chose to adopt, we based our analysis on the following conceptual framework sequence. At first, Frequency Sweep Tests were carried out on the twenty blends prepared with the abovementioned protocols. Collected data were employed to draw Black Diagrams, which are particularly useful to identify inconsistencies in rheological data and to highlight and compare the revealing differences between the mixtures, and Master Curves, built according to Christensen-Anderson Model and Williams-Landel-Ferry equation for the shift factors determination. All the information about MWD can be found in the rubbery zone of the linear viscoelastic behaviour and a plot of MW and phase angle was pictured. After the simulation of short-term aging of asphalt binders with the Rolling Thin-Film Over Test (RTFOT), Linear Amplitude Tests were performed and Damage curves were drawn. The results showed that asphalts modified with carbon nanotubes have averagely a better behaviour in terms of fatigue resistance. Recent developments in microscope technology broaden research horizons to morphological investigation of both uncoated and “wet” specimens, with a non-destructive procedure. The ESEM study gave us pictures and videos as output, for that reason we weren’t able to easily lead a quantitative analysis, since previous researches have not treated