LIST OF EQUATIONS
3. Chapter 3 Theoretical Methods for Modelling Pavement Deterioration Deterioration
3.4. Modelling Pavement Indicators
3.4.6. Development Change in Rutting (ΔRDS) Model based on Climate Change Climate Change
3.4.6.5. Asphalt Material Properties
Morosiuk, Riley and Odoki (2004) stated that various elements affect the mixing properties of asphalt, which eventually affects the performance of asphaltic layers. NDLI (1995) highlighted the criteria used for evaluation of these elements, such as the ability to measure changes in performance, easily obtained without sophisticated tools or equipment, and availability in a typical application. The authors shed light on the most significant mix properties for the plastic deformation model, which are asphalt binder viscosity and voids in mix (VIM). Anyala (2011) agreed with their statement and applied such properties for an improved deterioration model. A similar approach is adopted in this research.
95 Asphalt Binder Viscosity
Basically, at high pavement temperatures, binder viscosity has a significant effect on the stability of an asphalt mix. For such cases, the application softening point (SP) is introduced to measure the viscosity. Morosiuk, Riley and Odoki (2004) defined the softening point (SP) as “the temperature at which bitumen attains a certain level of consistency”. Mixing and placement, voids in mix and pavement temperature are the three main factors that increase the softening point. For example, Daines (1992) highlighted that a high asphalt mix with high voids content is most likely to have age hardening because the softening point is increased. Morosiuk, Riley and Odoki (2004) added that pavement temperature also affects the rate of age hardening. At high pavement temperatures, binder viscosity has a significant effect on the stability of an asphalt mix.
Prediction of Softening Point Value
Rohde (1995) investigated the relationship between pavement age and softening point. He highlighted that the prediction of softening point can be modelled as shown in Figure 3-4.
Figure 3-4: Expected increase in softening point over time by Rohde (1995)
According to Figure 4-4, the softening point rises at the start of the asphalt mix’s life. Rohde (1995) stated that such an increase is due to the asphalt mix having high voids (VIM) content. However, as the pavement structure ages, then the voids in the mix (VIM) start to reduce. Mainly, such reduction is due to traffic load generated
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from heavy vehicles. Thus, the softening point will continue rising at a constant rate.
Rohde (1995) named this phase the hardening stage as voids decrease in the asphalt mix. He concluded that voids designed in the mix range from 2.4% to 9%. Asphalt ageing is most likely to increase the softening point per year in the range from 0.1 °C to 2.9°C. And after 10 years it remains constant with no change. As per the conclusion delivered by Daines (1992), a high voids content is most likely to produce age hardening because the softening point is increased. Morosiuk, Riley and Odoki (2004) and Anyala (2011) agreed that pavement temperature affects the rate of age hardening.
Anyala, Odoki and Baker (2014) considered softening point in their research model in conjunction with age (AGE) of the asphalt layer using the following equation, 3-8.
𝑆𝑃= 𝑎0 × ln(𝐴𝐺𝐸 + 0.0001) + 𝑎1
Equation 3-8:Softening point equation developed by Anyala, Odoki and Baker (2014)
The author follows their proposed equation. The reason behind this is that there are no recorded data that represent the softening point with respect to pavement age in either the Ministry of Public Works or Al Ain City Municipality (Valor 2013).
Therefore, a similar coefficient will be used in this research, which is a0= 2.52 and a1= 70.5. In conclusion, softening point equation and coefficients used by Anyala, Odoki and Baker (2014) as per Equation 3-8 are adopted in this research to determine the change in rutting.
Voids in Mix (VIM)
There is no doubt that Percentage Voids in Mix (VIM) is crucial for asphalt mix property. VIM are related to the stability of the asphalt mix and contribute to the resistance to rutting. Generally, once a new road has been constructed, the level of VIM is considered to be in the high range while, with time, due to continuous traffic compaction, the level of VIM is known to be decreased (NDLI 1995; Nicholls et al.
2007). Rohde (1995) emphasised the importance of voids in the mix; their impact can be the reason for rutting occurrence, especially if they drop below 3%. Such a drop leads to unstable and plastic flow conditions. Axle loads and average speed of heavy vehicles are the leading causes for VIM drops. Basically, loads and duration of such loads will have an impact on the plastic deformation.
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Rohde (1995) highlighted that the rate of decreasing the VIM variable for the first year is not the same for the rest of the mix’s life. The reason for such behaviour occurs in the initial year when the mix receives loads from compaction effort applied during the construction period, then the VIM will receive much lower impact for the same conditions. Anyala, Odoki and Baker (2014) determined the VIM model in conjunction with age (AGE) of asphalt layer, as per Equation 4-9.
𝑉𝐼𝑀 = 𝑎0 × ln(𝐴𝐺𝐸 + 0.0001) + 𝑎1
Equation 3-9: VIM equation developed by Anyala, Odoki and Baker (2014)
The author follows this proposed equation. The reason behind this is that no recorded data represent the VIM concerning pavement age. Moreover, a similar coefficient will be used in this research, which is a0= -0.07 and a1= 1.39. In conclusion, the VIM equation and coefficients used by Anyala, Odoki and Baker (2014) as per Equation 3-9 are adopted in this research to determine the change in rutting.