For good product quality, the main focus is on the binder attributes of bitumen, which can be measured with a rheometer. The various processing and usage steps require different consistencies of bitumen. In road engineering, the hot asphalt-concrete mixtures are produced and homogenized at various temperatures. The asphalt plants used for continuously processing these mixtures can be stationary or mobile, so the mixture must be pumpable and pourable. During the subsequent cooling phase after application, the mixture is continuously compacted on the road. The properties of bitumen can be optimized by testing its viscosity and other rheological parameters that describe its stiffness and compliance. Various measuring geometries and accessories, such as heating devices specialized for bitumen (or asphalt) testing ensure highly accurate results.
Rheological investigation of asphalt and bitumen
Bitumen is a refined residue from crude oil distillation. It is a complex mixture containing many diverse petrochemical components. Asphalt is a mixture of bitumen and mineral components, with bitumen serving as a binder. In some countries (like the USA), however, the terms asphalt and bitumen are used synonymously.
Road paving or asphalt concrete consists of a mixture of mineral materials such as stones, gravel, crushed stones, sand, etc. with bitumen as the binder (or asphalt, respectively). Paving is generally exposed to diverse weather conditions and heavy traffic load. Consequently, road repairs represent a considerable cost factor for road construction authorities. Typical damage is the formation of lane grooves or cracks due to material fatigue or thermal stress.
Lane grooves arise from permanent traffic load. As a result, the structural forces of the mixture become weaker, which may lead to the pavement’s increasing and permanent deformation. Finally, this can give rise to effects like aquaplaning.
Fatigue and thermal cracking are often caused by seasonal temperature changes. Micro-cracks may occur which are no longer able to "heal" by themselves. As a further consequence, the cracks can become larger, and fragments of stone or asphalt can be ripped from the pavement surface due to the frictional forces of tires – in a chain reaction, the penetrating water and ice in winter will further accelerate this damage process, which is especially the case in low-temperature climates.
The goal for public administration, as well as for road-builders, building material manufacturers, and research institutes, is to improve the performance and lifetime of asphalt concrete. This goal can be achieved by using high-quality raw materials and by finding the right texture and mixing proportion of bitumen and mineral materials. Such improvements result in enhanced processability, dimensional stability, resistance towards seasonal weather changes, less lingering of liquids and dust, as well as better grip behavior.
Rheological behavior of asphalt and bitumen
The term "bitumen" is of Celtic origin, and means mineral pitch or "earth resin". This flammable liquid of brown-yellow to black color shows tough, viscoelastic properties at room temperature. It is composed of thousands of high- and low-molecular-weight compounds, including hydrocarbons, resins, paraffins, waxes, fats, heavy oils, lignins, proteins, and asphaltenes. In contrast to asphalt, bitumen is free of solids (e.g. minerals). Natural bitumen consists of natural organic materials.
Bitumen serves foremost as an asphalt binder for road pavement mixtures, in which it binds solid mineral additives such as sand or small stones. For this purpose, it needs to remain stable against demixing at high temperatures. On the other hand, it must not tend towards brittle fracture at low temperatures.
Rheological tests on bitumen
The rheological properties of bitumen can be determined, for example, by oscillatory tests at variable temperature (in the range of T = 0 °C to +70 °C). Synthetic polymers are often used to optimize the viscoelastic behavior of road pavement. Usually, the components of pure bitumen have less influence on its rheological behavior than the added synthetic polymers. Some longstanding tests in the asphalt industry are performed at a single temperature only, which is not sufficient for such a rheologically complex thermoplastic material like bitumen. Traditional specific values such as the softening point (SP), determined with the ring-and-ball (R&B) method, or pen values, determined by needle-penetration tests, allow for rough classification but provide insufficient information about the quality of a delivered product or its suitability for most specific applications.
Polymer-modified bitumen (PMB)
To render bitumen or asphalt binder suitable for special demands, such as road pavement, it is often mixed with polymers to form a so-called polymer-modified bitumen (PMB). Compared to common bitumen, PMB is more cohesive to mineral particles, and the temperature range between the softening point and the breaking point is wider. It shows better structural recovery after load removal and is more resistant to material fatigue. Furthermore, PMB shows better water resistance as well as higher rigidity and durability. PMB is commonly used in areas with an extraordinarily high traffic load; for example on bridges and in open-cell asphalt concrete, which is used to reduce driving noise.
Rheological tests on polymer-modified bitumen
The main purpose of mixing bitumen with polymers is to expand its ductility range, making it stiffer at high temperatures and softer at low temperatures. Additives can comprise up to seven percent of the bitumen. Common additives are blends of thermoplastic polymers such as styrene–butadiene–styrene (SBS), ethylene–vinyl acetate (EVA) or ground tire rubber (GTR). Rheological tests can be employed to determine the temperature-dependent behavior of PMB when heated or melted, for example in termperature tests by using an oscillatory rheometer. Although the percentage of added polymer is quite small in relation to the bitumen, a temperature curve clearly depicts the strong influence of the polymer molecules on the mixture's viscoelastic behavior.
The increasing number of vehicles on the roads of industrialized and developing nations generates millions of used tires every year. The use of tire rubber in asphalt production for road construction not only helps reduce the environmental impact of used tires, but also beneficially modifies the properties of the asphalt for highway construction. The benefits include longer-lasting road surfaces, reduced road maintenance, lower road noise, and shorter breaking distances. Perhaps unsurprisingly, asphalt rubber has become the largest single market for used tires, consuming approximately 12 million tires each year. California and Arizona use the most asphalt rubber in highway construction (over 80 % of asphalt used there is asphalt rubber), although asphalt rubbers can be engineered to perform in any type of climate.
Rheological tests on rubber-modified asphalt
Asphalt manufacturers need to be able to measure the temperature-dependent viscoelastic behavior of asphalt in order to classify the material and predict its performance. Such measurements can be performed with a dynamic shear rheometer, which can be used for rubber-modified asphalt with particles of up to 2 mm in diameter. In general, the addition of tire rubber significantly improves the rheological properties of asphalt, which can be seen as an increase in the material’s stiffness and elasticity, compared with neat paving-grade asphalt.
Other materials and applications
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