Chapter 4 : Rheological properties of FBMs produced with different foaming technologies 57
4.4 Effect of binder content on the dynamic shear modulus master curves
For this purpose, a new set of mixtures with different penetration grade binders and various bitumen contents were produced using the same aggregate gradation described in Section 3.1. The properties of these mixtures were compared to those of the Mechanical foamed-FAM mixtures manufactured at all the evaluated temperatures to get a better understanding of the rheological properties of these materials with production temperature.
Three additional FAM mixtures were manufactured, and the Standard FAM mixture produced at 160oC, which is the same mixture presented in the first set of FAM mixtures in Section 4.1.3, was used as a reference mixture for comparison purposes. The three additional mixtures comprise a range of asphalt mixture types in terms of binder grades (i.e. using the same bitumen used to manufacture the Standard FAM mixture, labelled here as “reference bitumen” (RB), using a softer penetration grade bitumen (SB) than the RB bitumen and, a harder penetration grade bitumen (HB) than the RB bitumen), and binder contents of 7 and 10.7%.
All mixtures were manufactured using limestone aggregates and were produced at 160oC. The characteristics of these mixtures are shown in Table 4-2. This table includes the Standard FAM mixture manufactured at 160oC, which contains 10.7% binder content. The naming convention for the new set of mixtures is as follows: FAM which denotes the fine aggregate asphalt mixture type, RB, SB and HB denotes the bitumen type in terms of its consistency characteristics, followed by a number that indicates the binder content. For example, FAM-RB-7 corresponds to the FAM mixture, produced with the same bitumen (RB) as the Standard-FAM-160C and containing 7% of bitumen by weight of the total mixture.
Table 4-2 Characteristics of the FAM mixtures with different penetration grade bitumen and binder contents
Mixture
Bitumen Penetration at 25oC
(dmm)
Bitumen content
(%) Aggregates
Air void content
(%)
Standard-FAM-160C 70 10.7 Limestone 9.7
FAM-RB-7 70 7.0 Limestone 8.7
FAM-SB-10 145 10.7 Limestone 8.6
FAM-HB-10 38 10.7 Limestone 8.9
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The average │G*│ master curves at a reference temperature of 25oC for these mixtures are presented in Figure 4.14. Each master curve corresponds to the average of two replicate specimens tested for each type of mixture, after observing low variability.
Figure 4.14 │G*│ master curve for FAM mixtures with various binder contents and penetration grade binders
In this figure, the effect of bitumen grade and bitumen content on the │G*│ of the FAM mixtures can be clearly seen. Each FAM mixture shows a different shifting of the │G*│ master curves, compared to the FAM-160C mixture. For instance, for the same bitumen content used in the Standard-FAM-160C (i.e. 10.7%), the use of a softer bitumen grade resulted in a FAM mixture (FAM-SB-10) with lower │G*│ values, as expected. Similarly, for the same bitumen content (i.e. 10.7%), the FAM mixture manufactured with a harder penetration grade bitumen (FAM-HB-10) exhibited higher │G*│
values as a result of the hardened consistency of the bitumen. Moreover, the FAM-RB-7 mixture, manufactured with the same penetration grade bitumen as the Standard-FAM-160C (i.e. 70/100) but with a lower content (i.e. 7%), exhibited higher │G*│ values to those obtained for the reference Standard-FAM-160C mixture (i.e. which contains 10.7% of the same bitumen). Thus, the stiffness of the FAM mixture increased with lower bitumen content. These results are in good agreement with some results published by other researchers conducted on full asphalt mixtures to evaluate the effect of bitumen and air void content on mix fatigue and stiffness (Harvey and Tsai, 1994).
It is now important to compare the response of the set of FAM mixtures presented in Figure 4.14 to those observed in Figure 4.6b for the Mechanical foamed FAM mixtures, to get a better understanding of the behaviour of the Mechanical foamed FAM mixtures produced at different temperatures. Figure 4.15 combines the │G*│ master curves of the above mentioned mixtures.
1.0E+06 1.0E+07 1.0E+08 1.0E+09
0.0001 0.001 0.01 0.1 1 10 100 1000
│G*│ [Pa]
Reduced frequency [Hz]
FAM-RB-7 FAM-SB-10 FAM-HB-10 Standard FAM-160C
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Figure 4.15 Effect of binder content on the G* of the Mechanical foamed-FAM mixtures
From an initial observation of these master curves, it can be seen that at low reduced frequencies, the
│G*│ values of the Mechanical foamed mixtures manufactured at 90 and 120oC, approach those of the FAM-RB-7 and FAM-HB-10 FAM mixtures. Figure 4.16, studies this aspect in more detail by presenting the values of │G*│for all the mixtures at a frequency of 0.001Hz at the reference temperature (25oC).
Figure 4.16 Effect of binder content on the │G*│ values of the Mechanical foamed-FAM mixtures
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Data in Figure 4.16 corroborates that the │G*│ values of the Mechanical foamed FAM mixtures manufactured at 90 and 120oC, approach more to those of the FAM-RB-7 and FAM-HB-10 FAM mixtures, than to those of the Standard FAM mixture manufactured at 160oC, and even less to the FAM-SB-10. For instance the difference between the │G*│ of the Mechanical foamed mixtures manufactured at 90 and 120oC with the FAM-RB-7 is 19.8 and 27.7% respectively, and with the FAM-HB-10 is 23.8 and 31.8% respectively, while the differences with respect to the Standard HMA are 94.3 and 87.7% for the Mechanical foamed mixtures manufactured at 90 and 120oC, respectively, and with the FAM-SB-10 it is 111.4 and 105.8% respectively.
These results suggest that the behaviour of the Mechanical foamed FAM mixtures manufactured at 90 and 120oC, (which were manufactured with a target bitumen content of 10.7%) is similar to that of a FAM mixture manufactured with lower bitumen content (FAM-RB-7), and/or a stiffer bitumen (FAM-HB-10 FAM), which produced a FAM mixture with high │G*│ values. Therefore, it is clear that the mixing process herein used to manufacture the mechanical foamed FAM mixtures, resulted in mixtures with lower effective binder content at low mixing temperatures. Poor bitumen dispersion within the mixtures at these low mixing temperatures (i.e. 90 and 120oC) and high bitumen loss by means of the mechanical foaming process, were the main factors strongly influencing the final rheological response of these mixtures.
To support this hypothesis, the binder content for the remaining SGC FAM specimens (after coring the DMA FAM testing samples) for both foaming FAM mixtures groups was determined following the BS EN 12697-4:2005, and the results are presented in Table 4-3.
Table 4-3 Bitumen content for both foaming FAM groups of mixtures
Mixture Design bitumen
content (%)
Estimated bitumen content (%)
Difference (%)
Mechanical foamed-FAM-90C 10.7 8.2 (-) 26.5
Mechanical foamed-FAM-120C 10.7 8.4 (-) 24.1
Mechanical foamed-FAM-160C 10.7 9.9 (-) 7.8
Zeolite-FAM-90C 10.7 11.6 (+) 8.1
Zeolite-FAM-120C 10.7 11.6 (+) 8.1
Zeolite-FAM-160C 10.7 - -
Results in Table 4-3 show that the Mechanical foamed FAM mixtures manufactured at 90 and 120oC have lower binder contents than the Mechanical foamed FAM mixture manufactured at 160oC, thus confirming that lower production temperatures resulted in Mechanical foamed FAM mixtures with
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less effective binder content. Indeed, the Mechanical foamed FAM mixtures have lower binder contents compared to the Zeolite FAM mixtures as well, which implies that the mixing process is crucial in the rheological performance of the final foaming FAM mixtures and, manual mixing contributes to the even distribution of the bitumen within the mixture, and less bitumen loss.