Deformation of Bituminous Materials
Resistance of Bitumen to Deformation
Since bitumen is a viscoelastic material, the response to an applied load depends on the size of the load, the temperature, and the duration of its application. In other words there is no simple relationship between stress and strain and it is therefore difficult to predict the elastic modulus (or equivalent Young’s modulus) of bitumen.
To take account of the viscoelastic nature of bitumen, Van der Poel (1954) introduced the concept of stiffness modulus. This modulus is dependent on both temperature and time of loading, and is given by:
St,T = = σ /εt,T
where σ is the tensile stress and εt,T is the resultant strain after loading for time t at temperature T.
Figures 1 and 2 illustrate the effect of loading time and temperature for bitumens of different PI.
For low-PI bitumens (Fig. 1) the stiffness is constant for very short loading times and virtually independent of temperature. This represents elastic behaviour.
For longer loading times the curves have a consistent slope of 45° and have a significant variation with temperature, indicating viscous behaviour. The effect of increasing PI can be seen by comparing Figs 1 and 2.
High-PI bitumens are much stiffer at high temperatures and longer loading times. Thus under conditions that are more likely to give rise to deformation, namely slow moving or stationary traffic and high temperatures, a high-PI bitumen offers greater resistance to deformation by virtue of its higher stiffness and more elastic response.
When considering a bituminous mixture consisting of a graded aggregate bound with bitumen, the stiffness of the mixture is dependent on the stiffness of the bitumen and the quantity and packing of aggregate in the mixture (Van der Poel, 1955).
The quantity and packing of aggregate particles depend on grading, particle shape and texture, and method of compaction.
Determination of Permanent Deformation
Rutting of bituminous pavements is the most common type of failure in the UK. It is therefore important to be able to predict the permanent deformation for a bituminous mixture, and this depends on the low stiffness response, that is the stiffness at long loading times and high temperatures, as well as the balance between the viscous (non-recoverable) and elastic (recoverable) components of the mixture’s deformation.
Two tests that have been commonly used to determine the permanent deformation properties of bituminous mixtures are the creep test (usually under repeated loading) and the wheel tracking test. In the creep test (known as the repeated load axial test (RLAT) in the UK), a repeating uniaxial load of 0.1 MPa, with a loading time of 1 second and a rest period of 1 second, is applied to a cylindrical specimen for 2 hours at 40°C.
During the test, deformation is measured as a function of time. Although simple, the repeated load axial test is extremely convenient and allows the relative performance of different bituminous mixtures to be easily determined. It is often criticised as being too severe as the test does not employ a confining stress.In-situ materials will clearly be confined and the effect of the confining stress on the vertical strain may be important.
However, the severe nature of the test does mean that intrinsically poor materials can easily be identified, and there is good correlation between creep tests and permanent deformation performance in the road. The wheel tracking test can be considered to be a simulative test.
Figure 3 shows a diagrammatic representation of a laboratory-scale wheel tracking test. In the UK, the wheel tracking test is usually carried out at either 45°C or 60°C with an applied wheel load of 520 N. The performance of the bituminous mixture is assessed by measuring the resultant rut depth after a given number of passes or the rate of tracking in millimetres per hour.
Factors Affecting Permanent Deformation
When a stress is applied to a bituminous material, both the aggregate particles and the bitumen will be subjected to the stress. But the aggregate particles, being hard and stiff, will undergo negligible strain, whereas the bitumen, being soft, will undergo considerable strain. Thus deformation is associated with movement in the bitumen, and the extent of the movement will depend on its viscosity.
Bituminous mixtures that utilise a continuously graded aggregate, such as asphalt concretes, rely mainly on aggregate particle interlock for their resistance to deformation. Thus the grading and particle shape of the aggregate are major factors governing deformation.
The characteristics of the fine aggregate are particularly important in gap-graded materials, which rely on a dense bitumen and fine mortar for their strength. These are the hot rolled asphalt and stone mastic asphalt mixtures. Sand particles can vary considerably from spherical glassy grains in dune sands, to angular and relatively rough grains from some pits.
Mixtures made with a range of sands all at the same bitumen content have been shown to give deformations, when tested in the laboratory wheel tracking test, that varied by a factor of 4 from the best to the worst sand (Knight et al., 1979).
Figure 4 shows permanent strain against number of test cycles in a repeated load axial test. It can be seen that permanent strain increases with temperature. This is due to the reduction in viscosity of bitumen, the consequent reduction in bitumen stiffness and the accumulation of repeated, non-recoverable viscous deformations. The figure also indicates the effect of the aggregate grading.
At low temperatures, the permanent strain in continuously graded asphalt concretes and gap graded hot rolled asphalts will be very similar. Here the high degree of aggregate particle interlock in the asphalt concrete and the high viscosity bitumen in the hot rolled asphalt provide a similar resistance to deformation.
However, at higher temperatures, the hot rolled asphalt deforms more due to the reduced bitumen viscosity, which is not compensated by the aggregate interlock effect. In the asphalt concrete, although the bitumen will also be less stiff and viscous, the aggregate grading continues to provide a compensating resistance to deformation.
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