Dynamic Shear Modulus Master Curves

Figure 4.6 shows the average │G*│ master curves for the three FAM mixtures groups at a reference temperature of 25^oC. In the FBMs figures, the Standard-FAM mixture produced at 160^oC is also presented as a reference for the control mixture for comparison purposes. These curves correspond to the average of the tested specimens for each type of mixture.

90 91 92 93 94 95 96 97 98 99 100

0 50 100 150 200 250 300 350 400

Remaining mass [%]

Time (min) Zeolite at 90C

Zeolite at 120C Zeolite at 160C

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Figure 4.6 │G*│ master curve for: a) Standard-FAM mixtures, b) Mechanical foamed-FAM mixtures, and c) Zeolite-FAM mixtures, at a reference temperature of 25^oC

1.E+06 1.E+07 1.E+08 1.E+09

0.0001 0.001 0.01 0.1 1 10 100 1000

G* [Pa]

Reduced frequency [Hz]

a) Standard FAM-90C

Standard FAM-120C Standard FAM-160C

1.E+06 1.E+07 1.E+08 1.E+09

0.0001 0.001 0.01 0.1 1 10 100

Complex modulus, G* [Pa]

Reduced frequency [Hz]

b) Mechanical foamed FAM-90C Mechanical foamed FAM-120C Mechanical foamed FAM-160C Standard FAM-160C

1.E+06 1.E+07 1.E+08 1.E+09

0.0001 0.001 0.01 0.1 1 10 100 1000

│G*│ [Pa]

Reduced frequency [Hz]

c) Zeolite FAM-90C

Zeolite FAM-120C Zeolite FAM-160C Standard FAM-160C

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From these master curves, two initial observations can be made: 1) the upper part of the master curves for all three types of mixtures approaches asymptotically to a unique maximum stiffness of the mixes, which is higher than 1x10⁹ Pa for the considered mix design. 2) At lower frequencies (or higher testing temperatures), the modulus approaches a minimum value, which is different for the two foaming FAM groups. For instance, for the Zeolite-FAM mixtures this lower limiting value is very similar at all production temperatures, and also to that of the Standard-FAM mixtures. However, for the Mechanical foamed-FAM mixtures, the minimum stiffness value is different for all production temperatures, and also different to the value reported for the Standard-FAM-160^oC. These differences in the lower limiting values for the Mechanical foamed-FAM mixtures are potentially a consequence of the effects of the foaming technology in the final properties of these mixtures.

With the objective of better quantifying the actual effect of the production temperature and foaming technology on the │G*│ of the FAM mixtures, Figure 4.7 presents the values of │G*│for all the mixtures at two loading frequencies (0.001Hz and 10Hz), at the reference temperature (25^oC).

Data in Figure 4.7 reflects the impact of the production temperature on the changes in the │G*│ of the FAM mixtures manufactured with different technologies, and these changes depend on the loading frequency (Figure 4.7a vs Figure 4.7b). At 10Hz, the Standard-FAM mixtures show an increase in

│G*│ values with increasing production temperature. The│G*│of the Standard FAM mixture manufactured at 120 and 160^oC increased by 11 and 51% respectively, with respect to the Standard FAM mixture manufactured at 90^oC. This increase in the │G*│ values with production temperature can be attributed to better coating of the aggregates with the bitumen at higher production temperatures, which improved the overall adhesive properties of the mixtures and therefore resulted in better mechanical capacity. For the Mechanical foamed-FAM mixtures, data in Figure 4.7a shows that these mixtures do not follow a constant increasing trend in │G*│with production temperature. The changes of the │G*│values in these mixtures are less than 12%, suggesting that there is no effect of temperature for these mixtures. The Zeolite-FAM mixtures, show similar │G*│values at production temperatures of 90 and 120^oC (i.e. changes in │G*│ less than 5%), and a slightly higher increase in

│G*│ at 160^oC (i.e. │G*│of the Zeolite-FAM 160C increased by 13% with respect to that of the Zeolite-FAM 90C). These changes in │G*│ are small, suggesting that the effect of production temperature on these mixtures is not very significant.

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Figure 4.7 │G*│for all mixtures at (a) 10Hz, and (b) 0.001Hz

Figure 4.7b confirms that the│G*│values are smaller at lower reduced frequencies (or higher temperatures). At 0.001Hz, the values of the dynamic modulus are within one or two orders of magnitude lower than those obtained at 10Hz. Furthermore, the changes in│G*│for each type of mixture with production temperature are different compared to those observed previously at higher frequencies. At 0.001Hz, the Standard-FAM mixtures exhibit similar │G*│ values, with slight increase with production temperature (i.e. increases less than 5%). The Mechanical-foamed FAM mixtures on the contrary, show a clear increase in│G*│with lower production temperatures. For

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respectively 4.2 and 3.2 times higher than the│G*│of the Mechanical foamed FAM mixture manufactured at 160^oC. This increased stiffness at low production temperatures for the Mechanical foamed-FAM mixtures appears to be unusual as it was expected that the incorporation of a certain amount of heat to the aggregates for further mixing with the foamed bitumen (which was always heated at 160^oC for foaming) would result in FBMs with better mechanical capacity, reflected in an increase in the │G*│ values. However, this unusual increase in stiffness at lower production temperatures could be attributed to the mixing process employed for the production of these mixtures.

For instance, it is possible that when the foamed bitumen is squirted, it only reaches the portion of aggregates that are on the top, and at low mixing temperatures (i.e. 90 and 120^oC) complete dispersion of the bitumen into the aggregates is not fully achieved. In addition, a portion of the added bitumen remains in the mixer and on the vertical agitator after mixing inducing bitumen loss. These two conditions could result in mixtures with different effective binder contents (which explains the difference in the lower limit stiffness values with respect to the Mechanical foamed FAM-160C and also to the Standard FAM-160C). In addition, at low frequencies, where the increase in │G*│ is more evident with lower production temperatures, the lower effective binder content along with the reduction of the viscosity of the bitumen, allow the fine aggregate skeleton to control the rheological properties of the Mechanical foamed-FAM mixtures, providing an increase in │G*│.

Furthermore, with regards to the Mechanical foamed mixture manufactured at 160^oC, results from both frequencies indicate that producing foamed bitumen mixtures by means of a Mechanical process with a mixing temperature of 160^oC (i.e. aggregates at 160^oC), results in a mixture with similar properties to that of a standard HMA, as could be expected. Considering that the main purpose of the foamed bitumen is to provide adequate workability and coatability conditions to produce mixtures for cold mix processes (i.e. non-heated aggregates) or half-warm mix processes (i.e. partially heated-aggregates) which otherwise could not be achieved with the same bitumen in liquid state, obtaining also environmental benefits. These results imply that extending the production of FBMs for hot processes is not practical, since a mixture with adequate coating and workability conditions promising good mechanical performance at these high temperatures (i.e. HMA) can already be obtained without the need to include special equipment to produce the foamed bitumen, and in addition the benefits for reduced energy consumption within the sustainability framework are no longer effective.

Furthermore, at 0.001Hz, the Zeolite-FAM mixtures exhibited changes in the│G*│values of less than 20%, for all production temperatures. Although the changes in │G*│ for the zeolite containing mixtures are higher than those exhibited at 10Hz (Figure 4.7a), the production temperature still did not result in major differences in the rheological properties of these mixtures. In addition, another important observation for these type of mixtures which is also reflected in Figure 4.7, is that these

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mixtures exhibit comparable│G*│ values to those of the Standard FAM mixtures at the same corresponding production temperature. On average the Zeolite FAM mixtures exhibited changes in

│G*│between 3 to 20%, compared to those of the corresponding Standard-FAM mixtures. These results suggest that the addition of zeolites in the FAM mixtures did not present significant changes in the rheological properties of the final mixtures. It could be possible that during the process of pre-blending Advera® with the bitumen prior to its incorporation into the hot aggregates, the influence of water and foaming process will have disappeared before mixing started making this procedure very similar to just mixing and compacting conventional HMA at lower temperatures. The Mechanical foamed FAM mixtures, on the contrary, exhibit higher changes in the │G*│ compared to those of the Standard FAM mixture at the same corresponding production temperature. On average, the Mechanical foamed FAM mixtures manufactured at 90 and 120^oC exhibited changes in

│G*│between 44 to 191%, compared to those of the Standard-FAM mixtures at the same corresponding production temperatures of 90 and 120^oC. At a production temperature of 160^oC, the│G*│ of the Mechanical foamed FAM mixtures exhibit changes up to 30%, compared to those of the Standard FAM-160^oC. These results suggest that for these types of mixtures, the mixing processes are critical in the rheological response of the materials. In addition, the wide range of changes in│G*│as a function of the foaming technology suggest that the changes in the rheological properties of the mixtures are highly dependent on the production process.