Complex Modulus Master Curves

The │G*│ master curves at a reference temperature of 25^oC for the two foaming-RAP groups of mixtures are presented in Figure 5.1. In these curves, the Virgin HMA mixture - no RAP material, no foaming technology –, the HMA-RAP mixture– no foaming technology –, and the 100%RAP mixture are included for comparison as reference mixtures, and also to evaluate blending between the RAP and virgin binders.

Figure 5.1 │G*│ master curve for: a) Mechanical foamed-FAM mixtures, and b) Zeolite-FAM mixtures

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These figures reveal the effect of production temperature on │G*│. In both foaming-RAP FAM groups, the mixtures exhibited a better mechanical capacity as the production temperature increased, as depicted by a shift towards higher │G*│ values with production temperature, over the range of reduced frequencies. However, each mixture exhibits a different shifting of │G*│towards higher or lower values with respect to the Virgin HMA, with production temperature, and also over the range of reduced frequencies.

The different shifting of the │G*│ with reference to the Virgin HMA, could be partially explained by the actual level of blending occurring between the hard bitumen from the RAP and the soft virgin bitumen, which can also be reflected in Figure 5.1. In this figure, the behaviour (i.e. │G*│ values) of the virgin HMA and the RAP material (i.e. 100% RAP mixture) are exhibited, and are significantly different. If complete blending would have occurred between the virgin bitumen and the bitumen from RAP in the HMA-RAP mixture (manufactured at 160^oC), the resultant mixture should present similar properties to that of the Virgin-HMA since, in theory, the combination of the soft virgin bitumen used for the preparation of these materials, and the bitumen from RAP would produce a mixture with similar properties to that of the Virgin HMA. However, the │G*│ values of the HMA-RAP mixture are higher, ending up with a mixture with similar properties to that of the 100% RAP mixture, suggesting that complete blending between the old and new binders did not occur. This means that the behaviour of this mixture is primarily dominated by the hardened bitumen from RAP, and little interaction with the virgin bitumen was obtained. The Mechanical foamed-RAP mixture manufactured at 160^oC exhibits also high │G*│ values, even higher than those reported for the HMA-RAP mixture (Figure 5.1a). This behaviour breaks the analogy of complete blending since the properties of the Mechanical foamed-RAP mixture at 160^oC is out of the range of the properties of its constitutive components. However, these results imply that as observed in the HMA-RAP mixture, full blending was also not obtained in this mixture, and indicates that at this elevated temperature of 160^oC a different mechanism is occurring within the FAM mixtures.

One hypothesis that may explain this behaviour is the internal structure of the FAM-RAP materials.

Unlike conventional full asphalt mixtures, in which aggregates of different sizes coexist (i.e. coarse and fine particles) in these FAM materials, only fine particles (i.e. below 1 mm) are present.

Therefore, in full asphalt mixtures when RAP is incorporated and it behaves as a “black rock”, where incomplete blending occurs, it acts as an intrusion of solid particles that influences the mix volumetrics, properties, and performance through its aggregate gradation properties (McDaniel and Anderson, 2001); However, within FAM materials, when incomplete blending occurs, it is speculated that the FAM-RAP particles (having similar size) bond together with the bitumen forming clusters that influence the behaviour of the final mixture through their hardened bitumen properties, which

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could explain the increased stiffness observed in the Mechanical foamed-RAP manufactured at 160^oC and the HMA-RAP mixtures.

The Zeolite-RAP mixtures manufactured at 90, 120 and 160^oC and the Mechanical foamed-RAP mixtures manufactured at 90 and 120^oC, exhibited a softening effect with respect to the HMA-RAP, as depicted by lower |G*| values. Furthermore, although these mixtures exhibited a different shift of│G*│values with respect to the Virgin-HMA, these mixtures could still be comparable to the Virgin HMA. These results suggest that the level of blending between old-new materials occurred to a different extent in each mixture, depending on the final mixing temperature and production process (i.e. foaming technology).

Following these results, it is theorised that three different component systems are present in these FAM mixtures consisting of: 1) the soft virgin bitumen, 2) part of the hard bitumen from RAP and, 3) the actual bitumen blend composed of the previous two, where the amount of the two last components depends on the mixing temperature of the mixtures, and the foaming technology (i.e. particularly mixing process). Taking into account that all the mixtures contain the same amount of virgin and RAP materials, with the same target bitumen content, and that the level of blending depends on the mixing temperature and mixing mechanism as described previously, these results also imply that the effective binder content in each mixture manufactured at the evaluated temperatures is also different. This observation is important since it suggest that the properties of the final mixtures are affected not only by the rheological properties of the bitumen blend but by their effective binder content, which could also explain the different shifting of the master curves. Figure 5.2. shows a schematic representation to illustrate this concept.

Figure 5.2 Schematic representation of blending between RAP-virgin binders with temperature The final properties of the two foaming FAM groups in combination with RAP material with production temperature are hypothesised to be influenced by the mixing process and the preparation of the materials for each foaming technology, as mentioned previously. It is speculated that manual mixing contributes to a better distribution of the RAP-virgin bitumen blend within the mixture

Complete blending Blending at 90^oC

RAP aggregate Virgin aggregate Virgin bitumen RAP bitumen Bitumen blend

Blending at 120^oC Incomplete blending

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compared to mechanical mixing. In fact, the Zeolite-RAP mixtures exhibited closer │G*│values to those of the Virgin-HMA, than the Mechanical foamed-RAP mixtures, where mechanical mixing was performed.

5.2.2 Phase Angle Master Curves

Figure 5.3 shows the δ master curves for the two groups of mixtures. These curves have been produced by using the same shift factors as the │G*│ master curves. As in the │G*│ master curves, these curves include the Virgin HMA mixture, the HMA-RAP mixture, and the 100%RAP material for comparison purposes.

The δ master curves for the two foaming-RAP mixture groups show a clear decrease in δ values with production temperature (more elastic response), at high reduced frequencies (or low testing temperatures), thus indicating that production temperature affects the viscous component of the bitumen present in the mixtures, as could be expected. For the same amount of RAP material, the production temperature in the two foaming-RAP mixture groups increased │G*│, as observed previously in the master curves, and decreased δ leading to a stiffer and more brittle material at high reduced frequencies (low testing temperatures). Low production temperatures as 90 and 120^oC for the Mechanical-foamed technology and all production temperatures 90-160^oC for the zeolite-based technologies resulted in RAP mixtures with almost similar rheological behaviour (master curves overlapped) with the virgin mixture, meaning a better or similar thermal cracking or fatigue resistance for these mixtures. However at low reduced frequencies (high temperatures), the δ master curve for the two foaming-RAP mixtures groups exhibit an increase in δ values with production temperature, but lower δ values with respect to the Virgin HMA, thus indicating a more viscous material. However, the response of mixtures at these low frequencies could be an effect not only of the loading frequency, but also of the complex behaviour of the materials as a result of the discontinuous curves after the shifting process of the master curves.

The HMA-RAP, the Mechanical foamed-RAP mixture manufactured at 160^oC and the 100%RAP mixtures exhibit an increased elastic response at high reduced frequencies (or low testing temperatures), meaning that producing a 100% RAP mixture and incorporating RAP at 160^oC could negatively affect the fatigue and thermal cracking resistance of the mixtures. Similarly, at lower reduced frequencies, the δ values tend to overlap or present a more viscous behaviour with production temperature, which could be in part a result of the shifting of the isothermal data.

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Figure 5.3 δ master curve for: a) Mechanical foamed-FAM mixtures, and b) Zeolite-FAM mixtures, at a reference temperature of 25^oC

5.2.3 Black Diagrams

The complete rheological behaviour for the two FAM groups is presented in the black space in Figure 5.4. In black diagrams, the data that is in the high-end of the plot is at low temperatures and the data that is at the lower end of the plot is at high temperatures. At the high-end of the plot, the curves for the Mechanical foamed-RAP mixtures in Figure 5.4a, show lower δ values with production temperature, and also with reference to the Virgin HMA. This reduction in the δ values at similar

│G*│ values indicate an increase in the elastic response of the materials, and consequently an increase in their brittle behaviour. After the inflection point, the shift towards a more elastic response becomes more evident. This shift of the black diagram curves is a result of the dual effect of increase

10

0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000

δ[o]

0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000

δ[o]

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in │G*│ and decrease in δ with production temperature, as was also observed in the corresponding master curves. The Zeolite-RAP mixtures, on the other hand, exhibit similar δ values across the

│G*│ values for all with less change in the viscoelastic properties with production temperature.

Furthermore, the two foaming FAM groups with RAP material, exhibit similar phase angle values to those of the Virgin HMA mixture at high │G*│values. The similarity of results at high │G*│ values relates to the upper limiting elastic stiffness of all the FAM mixtures which will tend to be the same (or at least similar). After the inflection point, the foaming FAM groups with RAP material show lower δ values compared to those of the Virgin-HMA, indicating the impact of the inclusion of RAP in the mixture. The same trend is observed for the HMA-RAP mixture which is similar in composition but without the use of a foaming technology. This behaviour shows that part of the viscous component of the mixtures is decreased by the incorporation of RAP material. Due to the increased elastic response of the bitumen present in the RAP material, the behaviour of mixtures containing RAP will naturally differ from those of the virgin mixture.

a)

1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10

10 20 30 40 50 60

│G*│ [Pa]

Phase angle, δ [^o] Virgin-HMA

HMA-RAP

Mechanical foamed-RAP 90C Mechanical foamed-RAP 120C Mechanical foamed-RAP 160C 100% RAP

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Figure 5.4 Black diagram for: a) Mechanical foamed-FAM mixtures, and b) Zeolite-FAM mixtures