Introduction to Viscosity and Rheology of Bituminous Materials
The resistance of a liquid in flowing is defined as the viscosity of that fluid. Under the action of force, the fluid reflects higher resistance when the viscosity is high. The two factors that define the viscosity of bitumen are:
- Chemical Composition of Bitumen
- Structure of Bitumen
The bitumen is a composition of two factors (Structure of Bitumen) asphaltenes miscelles and maltenes. In bitumen sol, the asphaltenes miscelles are dispersed in maltenes continuum. As the asphaltenes content reduces, the viscosity of bitumen also decreases. Aggregation is the factor that governs the viscosity of gel type bitumen. Gel type bitumen have a higher viscosity.
Bitumen Structure Influence on Viscosity
Bitumen consist of resins, aromatics and some form of saturated oil after their manufacture. The asphaltenes dispersion is controlled by the above mentioned elements in bitumen.
The aromatics in sufficient quantity provide a layer around the Asphaltenes. This layer is stable and promotes dispersion. If it’s not sufficient the micelles will join as shown in figure-1.
Fig.1. Sol Type Bitumen Schematic Diagram
Fig.2.Schematic Diagram of Gel type bitumen
Now the saturated oil quantity will affect the maltenes continuum. This has a low viscosity and low molecular weight. The increase in saturated oil quantity would bring the viscosity down, but it increases the aggregation of asphaltenes with aggregates, making the structure gel type.
This on other hand makes the aggregation of asphaltenes miscelles, causing viscosity decrease. So, two opposing factors are said to exist. These effects are depended on the aromatic stabilization.
The influence of asphaltenes on viscosity of bitumen can be summarized as:
- The viscosity increases with increase in asphaltenes quantity
- The change in viscosity is governed by the asphaltenes particles
- The asphaltenes have aggregation tendency. Greater the aggregation, greater is the viscosity.
Measurement of Viscosity of Bitumen
The viscosity can be defined as the ratio of shear stress applied to the rate of shear strain. It is measured in Pascal Seconds. The dynamic viscosity can also be calculated in terms of kinematic viscosity in units m2/s or mm2/s.
1 mm2/s = 1 cSt (Centistoke)
Based on absolute or kinematic viscosity requirement, the bitumen viscosity can be measured by a variety of devices. The specifications kept for measurement is the absolute viscosity at 60 degree Celsius and a Kinematic viscosity at 135 degree Celsius.
The vacuum capillary tube viscometer is used to find absolute viscosity. Atmospheric capillary tube viscometer is employed to measure the kinematic viscosity.
The displacement happening on a thin layer of bitumen can be determined with the help of a sliding plate test. The force in the layer is also calculated. The arrangement involves keeping the bitumen layer between metal plates for different combinations of loading time and temperature.
The resistance of force is represented by ‘F’, which is dependent on the:
- Area of the surfaces – A
- Distance between the surfaces – d
- Relative speed of movement of one plate with respect to other -V
The coefficient of viscosity is represented by ?. This is also called the absolute viscosity. And it is given by,
Now, the most practical means to measure the viscosity of bitumen is with the help of rotational viscometer test (ASTM D4402 -02). The Brookfield and Thermocel rotational viscometer let the measurement of bitumen viscosity at varying temperatures.
Influence of Temperature on The Viscosity of Bitumen
Bitumen tend to soften, when subjected to a temperature rise and will harden when the temperature falls. Bases on different varieties of bitumen, the viscosity changes with temperature also differs.
It is very essential to understand the viscosity variations of the bitumen used in the construction process, with the temperature. This factor is also a governing factor that assures the exposure of bitumen to open environment. The bitumen used for an area having high temperature is not the same that is used in lower temperature areas (cold regions). The serviceability, durability and maintenance factor is dependent primarily on viscosity property of bitumen type used.
The change in viscosity of bitumen with temperature is represented by the value of temperature susceptibility of bitumen. It is determined by the penetration value, P and softening Point temperature, T.
The empirical relation can be given as:
log P = AT +k
‘A’ is designated for temperature susceptibility of the logarithm of penetration value. The value of A varies from 0.0015 to 0.06. This variation itself shows, there would be variation in the response of bitumen with temperature. ‘k’ is a constant.
From the above expression, a relation has been developed by Pfeiffer and Van Doormaal,1936. This new relation relates A to an index named as penetration index, PI.
The relation tells that for road bitumen, the penetration index value PI =0;
It has been observed that most of the bitumen have a penetration of about 800, at their softening point temperature. Thus, the PI can be evaluated, if the penetration at 25 degree Celsius and softening point temperature are known.
Hence, Penetration Index of bitumen can be calculated
The figure below shows the Nomograph to prove the validity of above stated expression by Pfeiffer and Van Doormaal,1936.
Fig.3. Nomograph to evaluated the penetration Index of bitumen, with the known values of softening point temperature and penetration value at 25 degrees.
Penetration Index for Bitumen
A quantitative measure of the response of the bitumen material to a given temperature is represented by Penetration Index Value (PI). The PI index for bitumen that is used for roads have a value ranging from -2 to +2.
When the PI index is low, the bitumen behaves like Newtonian fluids, that they become very brittle at lower temperatures.
When the bitumen has a higher value of Penetration Index, resistance to permanent deformation is improved.
PI increases with the increase of asphaltenes (at the expense of aromatics). So, the penetration index is affected by the bitumen chemical composition to a larger extent. The figure below shows the variation of penetration index with each chemical fraction of bitumen.
Fig.4: Graph showing variation of penetration index with respect to chemical composition as per Lubbers, 1985
The behavior of bitumen after application in the construction can be predicted with the help of PI. High penetration value asphalt binders are called as “soft”. These are used in areas of cold climate. Low penetration numbers are called “hard”, which are employed in hot climates.