The colloidal structure of bitumen: Consequences on the rheology and on the mechanisms of bitumen modification
Introduction
The earliest record of human use of bitumen to date, is as a hafting material 180,000 years ago in the El Kowm Basin in Syria, where it was applied to stick flint implements to the handles of various tools in a way that persisted until Neolithic time [1]. At first, its adhesive and waterproofing properties were generally emphasized. Even the Bible cites examples such as the waterproofing of Noah's arch, of the Babel tower or of the cradle of Moses [1], [2]. Medical uses were also reported, with bitumen acting as a remedy for various illnesses (trachoma, leprosy, gout, eczema, asthma ...), as a disinfectant or as an insecticide [1], [2], [3]. Another well studied historical application was for the embalming of mummies by the Egyptians [2], [3], [4].
The first mention of the use of bitumen in road construction dates back to Nabopolassar, King of Babylon (625–604 BC): a bitumen-containing mortar cemented both the foundation made of three or more courses of burnt bricks and the stone slabs put on top [2]. However, bitumen essentially disappeared from the pavements until the early 19th century, when the recently rediscovered European sources of natural bitumen led to the development of the modern applications for this material [5]. The use of natural bitumen in road construction started to decay in the 1910s with the advent of vacuum distillation which made it possible to obtain artificial bitumen from crude oil [6] and nowadays, paving grade bitumen is almost exclusively obtained as the vacuum residue of petroleum distillation.
At the present time, 95% of the almost 100 Mt of bitumen that are produced worldwide each year are applied in the paving industry where they essentially act as a binder for mineral aggregates to form asphalt mixes, also called bituminous mixes, asphalt concrete or bituminous concrete. Asphalt mixes are generally fabricated by first heating typically 5 wt.% bitumen up to around 160 °C in order to decrease its viscosity and then blend it with 95 wt.% aggregates. This process is referred to as hot mixes, and the specifications on bitumen for pavements are mostly based on this application. Other techniques to manufacture asphalt mixes using bitumen emulsions or foamed bitumen are also available, but they amount to less than 5% of the total asphalt mix production.
In order that the mixes resist climate and traffic, specifications on paving grade bitumens have become quite severe. The properties that are needed to obtain suitable bitumen are mostly rheological. First, the bitumen has to be fluid enough at high temperature (around 160 °C) to be pumpable and workable to allow for a homogeneous coating of the aggregates upon mixing. Second, it has to become stiff enough at the highest pavement temperature to resist rutting (around 60 °C, depending on local climate). Third, it must remain soft enough at the lowest pavement temperature to resist cracking (down to around − 20 °C, depending on local climate). All these properties are quite opposite, and it is therefore difficult to obtain bitumen that would work under all possible climates. As a consequence, different paving grades exist, the softer being generally suitable for cold climates and the harder, for hotter regions. In order to widen the temperature range of bitumen, additives such as polymers and/or acids are increasingly used.
The purpose of this article is therefore to explain the current status of bitumen science, with special emphasis on the relationships between the structure and the rheological properties. It intends to give a simple physical picture of bitumen that helps understand known results on the effect of different modifiers. Clearly, the emphasis is on paving grade bitumens but many of the features should equally apply to heavy oil or to industrial grades of bitumen.
In a first section, the structure of bitumen is presented using an updated colloidal model and its consequences on the mechanical properties are described. The limits to our current understanding are also highlighted.
Next, three kinds of modifications are discussed. The first one is the modification of bitumen by an acid. This process has become of increasing importance in recent years, especially for the production of harder grades of bitumen. The effect of the acid can directly be understood in the light of the colloidal model. Then, the modification of bitumen by mineral fillers is addressed. Bitumen and mineral filler blends are usually referred to as mastics. Mineral fillers are generally not commercial modifiers as such. Since they are always present as part of the mineral aggregates, a mastic is indeed formed in-situ when manufacturing a hot mix. Therefore, it is in this mastic form that the bitumen really acts within the final mix, hence its critical importance. Finally, the last modification presented is that by a polymer. It is the most important commercial modifier at the moment, with an estimate of 4 Mt of Polymer-Modified Bitumen (PMB) produced in the United States [7]. This represents almost 10% of the total road binder market.
The effects of fillers and polymers are explained in the light of known results on model viscoelastic systems, i.e., suspensions and emulsions, using the Palierne model. This clarifies how and why these additives modify the properties of the bitumens, highlighting the relevant physico-chemical parameters governing the process.
Section snippets
Definitions
Many definitions were proposed for bitumen, asphalt and related substances, some of them quite opposite and sometimes scientifically incorrect [2], [8]. Incorrect definitions are those presenting bitumen as a pasty or semi-solid material [3], when the correct description would be that of a viscous viscoelastic liquid (at room temperature), as will be described in more details in Section 3.
In 1728, Chambers, in the first modern dictionary, defined bitumen as a generic term encompassing naphtha,
Early evaluation of bitumen rheology
The use of bitumen in paving applications has generated a lot of interests in its rheological properties, because of their importance in the manufacture and quality of bituminous pavements. As a matter of fact, the development of the early colloidal model was based on rheological observations (§ 2.7). Long before that, ancient users of bitumen observed the strong effect of temperature on its consistency [2], [3]. But due to its highly viscous character at room temperature, giving rise to a
Acid modification
The modification of bitumen by chemical reactions is not a new subject. As described before (§ 2.2), air-blowing of soft bitumens is an industrial application of the chemical reactivity of bitumen, in use for more than a century. However, the process was quite complicated and could only be done in specific production units only available to refiners.
Still, it was early observed that bitumen could be reacted with other compounds such as sulphur, chlorine or various acids (sulphuric, nitric, acid
Conclusions
Bitumen is a complex mixture of mostly hydrocarbons whose structure is well described by the colloidal model: solid particles (the asphaltenes) with a radius of a few nanometres dispersed in an oily liquid matrix (the maltenes). The most important parameter describing the structure of a bitumen are thus the glass transition temperature of its maltenes and its solid fraction content (or effective asphaltenes content), proportional to its asphaltenes content.
This structure generates two main
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