The Structure of Bitumen: Conceptual Models and Experimental Evidences

Abstract

Bitumen, one of the by-products of petroleum industry processes, is the most common binder used in road pavements and in the construction industry in general. It is a complex organic mixture of a broad range of hydrocarbons classified into four chemical families, collectively known with the acronym SARA fractions, which include saturates, aromatics, resins and asphaltenes. Since the 1940s, researchers working on bitumen and the science behind its existence, nature and application have investigated the spatial organization and arrangement of several molecular species present in the binder. Therefore, several models have been proposed in the literature, and they are more or less corroborated by experimental studies, although most of them are model-dependent; for example, the structural investigations based on scattering techniques. One of the most popular models that has met with a wide consensus (both experimentally and of the modeling/computational type) is the one aiming at the colloidal description of bitumen’s microstructure. Other types of models have appeared in the literature that propose alternative views to the colloidal scheme, equally valid and capable of providing results that comply with experimental and theoretical evidence. Spurred by the constant advancement of research in the field of bitumen science, this literature review is aimed at providing a thorough, continuous and adept state of knowledge on the modeling efforts herein elaborated, in order to more precisely describe the intricacy of the bituminous microstructure. In this body of work, experimental evidence, along with details of bitumen’s microstructure (depicting the colloidal state of bitumen), is particularly emphasized. We will also try to shed light on the evolution of the experimental and theoretical results that have focused on the aspect of the association and aggregation properties of asphaltenes in various models and real systems.

Adsorbing Volatile Organic Compounds within Bitumen Improves Colloidal Stability and Air Quality

“…The first theory, which originates from colloid science, ascribes a colloidal nature to the bitumen structure. In the colloid picture, bitumen contains an asphaltenic core with the highest molecular weight and the most aromaticity dispersing in a surrounding medium of lightweight aromatics, resins, and saturates. − The second theory questions the colloidal dispersion of bitumen and suggests that bitumen is a single-phase homogeneous fluid containing a variety of polar and nonpolar molecules with mutual interactions. − In this theory, all the molecules (including asphaltenes) are kept in solution due to their mutual solubility. Despite the differences in the two theories, the general idea of a suspension of asphaltenes in organic solvents, crude oil, and bitumen has been widely accepted to describe the behavior of asphaltenes in the maltene medium.…”

Using Plastic Waste in a Circular Economy Approach to Improve the Properties of Bituminous Binders

“…Further directions may also be taken in future research, namely evaluating the influence of mixing these waxes with nanoclays [30,66], aged bitumen from reclaimed asphalt pavement (RAP) [67,68], or polymeric capsules containing rejuvenators to promote selfhealing [69]. Likewise, it will be helpful to study the long-term performance of different bituminous mixtures with a binder such as the PMB + wax presented in this study and evaluate their recyclability potential [70], including the advanced characterization after an accelerated ageing procedure [71,72].…”

Optimised Sunflower Oil Content for Encapsulation by Vibrating Technology as a Rejuvenating Solution for Asphalt Self-Healing

“…This modifies the chemical balance of the carbonyl and sulfoxide functional groups [ 8 ], promoting the increase in molecular polarity and agglomeration potential between bitumen molecules [ 9 ]. Such changes have an impact on the physical and rheological properties of the bitumen, increasing its softening point, viscosity, and complex shear modulus |G*|, while decreasing its penetration and phase angle (δ) [ 10 , 11 , 12 ]. This results in a more brittle bitumen, so microcracks appear and propagate on the pavement [ 13 , 14 ].…”