Phase Angle Master Curves

Phase angle master curves for the three FAM groups were produced by using the shift factors derived from the │G*│ master curves, and they are presented in Figure 4.8 for the three FAM groups.

However, unlike the │G*│ master curves where the rheological data produced a smooth continuous curve in all the FAM groups at all production temperatures of the mixtures, the resulting phase angle master curves for all the FAM groups after the shifting procedure resulted in non-continuous, branching and spread waves at all production temperatures of the mixtures. This fact does not allow the TTSP to be applied for the FAM mixtures. However, the experimental rheological data at all the tested temperatures can still be shifted to the reference temperature with respect to time, applying the Partial Time-Temperature Superposition Principle (PTTSP) to produce the δ master curves, and these curves are presented in Figure 4.8.

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Figure 4.8 δ master curve for: a) Standard-FAM mixtures, b) Mechanical foamed-FAM mixtures, and c) Zeolite-FAM mixtures, at a reference temperature of 25^oC

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0.0001 0.001 0.01 0.1 1 10 100 1000

Phase angle, δ[o]

0.0001 0.001 0.01 0.1 1 10 100 1000

Phase angle, δ(o)

0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000

Phase angle, δ[o]

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From these figures, it is possible to observe that the production temperature and foaming technology have an effect on the phase angle of the materials, and as in the │G*│, this effect is a function of the loading frequency. For instance, the δ master curves for the Standard-FAM mixtures in Figure 4.8.a, show that at high reduced frequencies (low temperatures) these mixtures exhibit lower values of δ (more elastic behaviour) with production temperature. This means that the production temperature, affects the viscous component of the bitumen present in the FAM mixtures, leading to stiffer mixtures, as was seen by an increase in the │G*│ master curves, and less viscous material (lower δ values) with production temperature. This behaviour implies that low production temperatures of 90 and 120^oC could be potentially good for a better fatigue resistance of the mixtures. At low reduced frequencies (high temperatures) instead, the Standard-FAM mixtures manufactured at 90 and 120^oC displayed similar values, and the Standard FAM mixtures manufactured at 160^oC exhibited higher δ values. This behaviour (i.e. more viscous response with production temperature) could be an effect of the frequency on the material.

The phase angle master curves for the Mechanical foamed FAM mixtures (Figure 4.8.b), on the contrary, show slightly smaller phase angle values at lower production temperatures over the reduced frequency range. This move towards a more elastic response at lower production temperatures, is an effect of the increased stiffness observed in the │G*│ master curves of these mixtures at production temperatures of 90 and 120^oC. The phase angle master curves of the Zeolite FAM mixtures in Figure 4.8.c exhibit similar phase angle values within the mixtures across the reduced frequencies range, meaning that for these mixtures, the production temperature did not have an effect on the rheological properties of the mixtures, as was observed also in their │G*│master curves. These results imply that the zeolite containing mixtures are not very sensitive to changes in the mixing temperature of the materials.

Furthermore, comparing the phase angle master curves for the two foaming FAM groups with that of the reference Standard-FAM 160C mixture, both foaming FAM groups exhibit a more elastic response at all production temperatures (lower δ values), at the low frequencies, which corresponds to high temperatures. At the high-reduced frequency end (lower temperatures) the phase angle values are similar.

Due to the complex behaviour of these FAM materials, the effects of production temperature and foaming technology with regards to their phase angle were also evaluated by means of the phase angle isochrones. In these plots, the phase angle data obtained from the DMA test, versus temperature at a constant frequency is used, without the need to perform TTSP manipulations of the raw data. Figure 4.9 presents the isochronal plots for the three FAM groups at 10Hz, which is a frequency commonly used in the design of pavements.

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Figure 4.9 Isochronal plots at 10Hz for: a) Standard-FAM mixtures, b) Mechanical foamed-FAM mixtures, and c) Zeolite-foamed-FAM mixtures, at a reference temperature of 25^oC

10

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The isochronal plots for all three FAM groups illustrate that similar to what has been observed in the δ master curves, the mixtures exhibit a maximum δ value which occurs at 35 or 45^oC, after which there is a decrease in the δ values towards a more elastic behaviour as the temperature increases, and the behaviour of the bitumen is not rheologically dominant.

In terms of the production temperature, the isochronal plots for the Standard FAM mixtures in Figure 4.9a, illustrate that there is a decrease in δ with production temperature from 15 to 45^oC, after which there is a sharp increase in the slope of the δ isochrones for the Standard FAM mixture manufactured at 160^oC. The isochronal plots for the Mechanical foamed FAM mixtures (in Figure 4.9b) on the contrary, show a decrease in δ with lower production temperatures, which is maintained throughout the temperature range of 15 and 65^oC. This reduction in δ is associated with the increased stiffness observed in the │G*│ master curves described previously. The isochronal plots for the Zeolite FAM mixtures in Figure 4.9c, show similar δ values with production temperature of the mixtures, across the temperature range. Thus confirming the little effect that the incorporation of zeolites at different production temperatures had on the overall rheological performance of these mixtures.

Furthermore, the effect of foaming technology, with regards to phase angle, can be observed by comparing the isochronal plots of the phase angle for the Mechanical foamed FAM mixtures and the Zeolite FAM mixtures in in Figure 4.9b and in Figure 4.9c, respectively, to that of the Standard FAM mixture at 160^oC, which is also included in these plots. For instance, Figure 4.9b shows that from 15 to 35^oC, there is no significant difference in the phase angles of the Mechanical foamed-FAM mixtures manufactured at 90 and 120^oC and the Standard FAM-160^oC, while the Mechanical foamed-FAM mixture manufactured at 160^oC, exhibits a sharper increase in the δ values at these testing temperatures. Further increase in the testing temperature for the Mechanical foamed-FAM mixtures results in a move towards a more elastic response, as depicted by the lower δ values compared to those of the Standard-FAM mixture manufactured at 160^oC. Similar to what was observed in the δ master curves, at high temperatures (i.e. above 45^oC) the Standard-FAM 160^oC exhibits higher δ values compared to those of the two foaming FAM groups, reflecting an effect on the frequency and temperature on this behaviour in the FAM materials.

For the Zeolite FAM mixtures, Figure 4.9c shows that this group of mixtures (at all production temperatures) exhibit a higher increase in δ with respect to the Standard-FAM mixture manufactured at 160^oC, from 15 to 45^oC, and they reach the maximum δ value at 45^oC as the Standard-FAM-160^oC mixture, with the same magnitude. Similar to the Mechanical foamed-FAM mixtures at all production temperatures, the effect of further increase in the testing temperature for the Zeolite-FAM mixtures is a move towards a more elastic response, as depicted by the lower δ values compared to those of the Standard-FAM mixture manufactured at 160^oC.

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