Viscosity of Bituminous Materials
Viscosity and Rheology of Binders
The viscosity of a liquid is the property that retards flow, so when a force is applied to a liquid, the higher the viscosity, the slower will be its movement. The viscosity of bitumen is dependent upon both its chemical make-up and its structure. In sol-type bitumens, the asphaltene micelles are well dispersed within the maltenes continuum.
The viscosity depends on the relative amounts of asphaltenes and maltenes, decreasing as the asphaltene content reduces. In gel-type bitumens, where the asphaltene micelles have aggregated, the viscosity is higher and dependent upon the extent of aggregation. The degree of dispersion of the asphaltenes is controlled by the relative amounts of resins, aromatics and saturated oils. If there are sufficient aromatics they form a stabilising layer around the asphaltene micelles, promoting the dispersion. However, if they are not present in sufficient quantity the micelles will tend to join together.
A schematic representation of the two states is shown in Fig. 1. In practice most bitumens are somewhere between these two states. The maltenes continuum is influenced by the saturated oils, which have low molecular weight and consequently a low viscosity. These saturates have little solvent power in relation to the asphaltenes, so that as the saturate fraction increases, there is a greater tendency for the asphaltenes to aggregate to form a gel structure.
Thus a high proportion of saturates on the one hand tends to reduce viscosity because of their low molecular weight, but on the other hand encourages aggregation of the asphaltene micelles, which increases viscosity. The relative importance of these two opposing effects depends on the stabilising influence on the asphaltenes of the aromatics. The asphaltenes exert a strong influence on viscosity in three ways.
Firstly, the viscosity increases as the asphaltene content increases. Secondly, the shape of the asphaltene particles governs the extent of the change in viscosity. The asphaltene particles are thought to be formed from stacks of plate-like sheets of aromatic/naphthenic ring structures. These sheets are held together by hydrogen bonds.
However, the asphaltenes can also form into extended sheets and combine with aromatics and resins so that the particle shape varies. Thirdly, the asphaltenes may tend to aggregate, and the greater the degree of aggregation the higher is the viscosity.
Empirical Measurement of Viscosity
The physical behaviour of bitumen is complex and to describe its properties over a wide range of operating conditions (temperature, loading rate, stress and strain) would require a large number of tests. To avoid this and simplify the situation, the mechanical behaviour and rheological properties of bitumen have traditionally been described using empirical tests and equations.
The two consistency tests required in the European Standard BS EN 12591 to characterise different bitumen paving grades are the needle penetration test (BS EN 1426) and the ring and ball softening point test (BS EN 1427). These tests provide an indication of the consistency (hardness) of the bitumen without completely characterising the viscoelastic response, and form the basis of the bitumen specification. The softening point is the temperature at which a bitumen reaches a specified level of viscosity. This viscosity is defined by the ring and ball test apparatus as the consistency at which a thin disc of bitumen flows under the weight of a 10 mm diameter steel ball by a distance of 25 mm.
Figure 2 shows a diagrammatic representation of the test. The more viscous the bitumen, the higher the temperature at which this level of viscosity is reached. Another test that is commonly applied to bitumens, and is the basis for their characterisation, is the penetration test.
The test measures hardness, but this is related to viscosity. The test consists of measuring the depth to which a needle penetrates a sample of bitumen under a load of 100 g over a period of 5 seconds at a temperature of 25°C. Thus the test differs from the softening point test in that, rather than determining an equiviscous temperature, the viscosity is determined at a particular temperature.
However, because bitumen is viscoelastic, the penetration will depend on the elastic deformation as well as the viscosity. Therefore, since viscosity changes with temperature, different bitumens may have the same hardness at 25°C but different hardnesses at other temperatures. It is the varying elasticity of bitumens which prevents correlation between these empirical tests.
Measurement of Viscosity
Viscosity is the measure of the resistance to flow of a liquid and is defined as the ratio between the applied shear stress and the rate of shear strain measured in units of Pascal seconds (Pa.s).
In addition to this absolute or dynamic viscosity, viscosity can also be measured as kinematic viscosity in units of m2/s or, more commonly, mm2/s with 1 mm2/s being equivalent to 1 centistoke (cSt).
The viscosity of bitumen can be measured with a variety of devices in terms of its absolute and kinematic viscosities. Specifications are generally based on a measure of absolute viscosity at 60°C and a minimum kinematic viscosity at 135°C, using vacuum and atmospheric capillary tube viscometers, respectively.
Absolute viscosity can also be measured using a fundamental method known as the sliding plate viscometer. The sliding plate test monitors force and displacement on a thin layer of bitumen contained between parallel metal plates at varying combinations of temperature and loading time.
The force of resistance, F, depends on the area of the surfaces, A, the distance between them, d, and the speed of movement of one plate relative to the other, V, such that:
The factor ɳ is the coefficient of viscosity (absolute viscosity), and is given by:
The relationship between dynamic viscosity (absolute viscosity) and kinematic viscosity is expressed as:
Kinematic viscosity = Dynamic viscosity / Mass density …….(3)
The rotational viscometer test (ASTM D4402-02) is currently considered to be the most practical means of determining the viscosity of bitumen. The Brookfield rotational viscometer and thermocel system allow the testing of bitumen over a wide range of temperatures (more so than most other viscosity measurement systems).
The rotational viscometer consists of one cylinder rotating coaxially inside a second (static) cylinder containing the bitumen sample, all contained in a thermostatically controlled environment. The material between the inner cylinder and the outer cylinder (chamber) is therefore analogous to the thin bitumen film found in the sliding plate viscometer.
The torque on the rotating cylinder or spindle is used to measure the relative resistance to rotation of the bitumen at a particular temperature and shear rate. The torque value is then altered by means of calibration factors to yield the viscosity of the bitumen.
Influence of Temperature on Viscosity
Bitumens are thermoplastic materials so that they soften as the temperature rises but become hard again when the temperature falls. The extent of the change in viscosity with temperature varies between different bitumens. It is clearly important, in terms of the performance of a bitumen in service, to know the extent of the change in viscosity with temperature.
This is referred to as temperature susceptibility and, for bitumens, is determined from the penetration value, P, and softening point temperature, T. These are related empirically by the expression:
logP = AT + k ……..(4)
where A is the temperature susceptibility of the logarithm of penetration and k is a constant. From this relationship, an expression has been developed (Pfeiffer and Van Doormaal, 1936) that relates A to an index, known as the penetration index, PI, such that for road bitumens the value of PI is about zero.
It has been determined that, for most bitumens, the penetration at their softening point (SP) temperature is about 800. Thus if the penetration at 25°C and the softening point temperature are known, the PI can be evaluated from:
giving PI = -1.7.
Pfeiffer and Van Dormaal produced a nomograph (Fig. 3) to evaluate the above expression, and it can be seen that for the above example a similar result is obtained. Bitumens for road use normally have a PI in the range -2 to +2.
If the PI is low, bitumens are more Newtonian in their behaviour and become very brittle at low temperatures. High-PI bitumens have marked time-dependent elastic properties and give improved resistance to permanent deformation. The influence of chemical composition on temperature susceptibility is illustrated in Fig. 4.
In general the PI increases as the asphaltene content increases at the expense of the aromatics. This change can be achieved by controlled air blowing.
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