Asphaltic Binder Viscosity Aging
Hardening of the asphaltic materials may be caused by a reduction in air void contents caused by post-construction compaction and/or by aging (oxidation) of the binder. In CalME the following model is used to describe the binder aging through increase in viscosity:
where DA is the increase in the viscosity constant,
time is age of the material in months,
T is the pavement temperature in degree Celsius at 1/3 depth of the pavement layer,
A =0.7 and B = 0.007 are constants, and
All logs are base 10.
This model is derived from the work by Houston et al. (2007) with some additional assumptions. The model parameter A depends on the A-VTS relationship of the binder, but the variations are very small and an average value of 0.7 may be used for most binders. The model parameter B can be adjusted to account for different types of material. The default values shown above were determined based on preliminary field data collected in California.
Just increasing the viscosity will make the master curve shift to the right, which implies that there will be no hardening effect at high or low temperatures. To allow the hardening effect at both high and low temperatures, an aging factor has been introduced and it is defined as the ratio of the modulus of hardened material to the modulus of the original material. The aging factor is determined by evaluating the effect of a viscosity increase due to aging for the temperature corresponding to a modulus of the original material of 10δ+α/2 under 10 Hz loading frequency. The aging factor is then used to increase the modulus at all temperatures. In essence, applying a uniform aging factor for all temperatures is equivalent to increasing δ. An example of how the aging factor is determined along with the unaged and aged stiffness master curves are shown below. Note that the aging factors are different for different materials even if DA is the same because the aging factor depends on the parameters of the stiffness master curve model itself.
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Demonstration of the effect of increase in A (i.e., DA) and the corresponding unaged and aged stiffness master curves, .png){kind=small}
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, loading frequency = 10Hz.
As an example to demonstrate how they change with time and climate, aging factors have been calculated for an HMA layer in a flexible pavement in the North Coast and Desert climate zones respectively over a thirty-year period. The structure has 150 mm HMA over 300 mm AB-Class 2 followed by subgrade with CL soil. The changes of aging factors over time are shown below, which indicates that the HMA layer stiffness will increase by approximately 50 to 70 percent over 30 years due to aging depending on the climate zone the pavement is in.
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Aging factor calculated for a typical flexible pavement in the North Coast and Desert climate zones respectively.
Aging may be limited by a maximum age in days, beyond which no more aging takes place. This is set to 10 years or 3650 days in CalME.
Houston, W.N., M. W. Mirza, C. E. Zapata, and S. Raghavendra. National Cooperative Highway Research Program. Research Results Digest 324: Simulating the Effects of Hot Mix Asphalt Aging for Performance Testing and Pavement Structural Design. 2007. Available from: onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rrd_324.pdf. (Accessed Jan. 17, 2017)