Finite Element Simulations Results

The asphalt binder creep stiffness obtained from the backcalculation performed with Hirsch model, Huet-ENTPE formulation and the experimental determined creep stiffness of the extracted binder at -6°C was used as input in the finite element simulations. To further reduce the computational time the simulations were run for selected point on the creep stiffness curve and specifically for creep stiffness at 8, 15, 30, 60 120, 240 480, 960s. Each simulation was performed for each side of the two small sides (~115 × 6.25 mm) of the specific asphalt mixture BBR beam specimen and the results averaged. The results of the FEM simulations are graphically compared to those obtained experimentally form on the asphalt mixtures beams (Figures 6.17 to 6.20).

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Hirsch Huet - ENTPE

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

Figure 6.17. Finite element simulation comparison for mixture 1 and 2, T=-6ºC

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

Figure 6.18. Finite element simulation comparison for mixture 3 and 4, T=-6ºC

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

Figure 6.19. Finite element simulation comparison for mixture 5 and 6, T=-6ºC

Mix 1 Mix 2

Mix 3 Mix 4

Mix 5 Mix 6

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

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Asphalt Mixture Creep Stiffness (GPa)

Time (s)

Experimental Extracted Binder Hirsch Huet - ENTPE

Figure 6.20. Finite element simulation comparison for mixture 7 and 8, T=-6ºC

From the plots it is evident that the finite element simulations that use the back calculated asphalt binder stiffness obtained from the Huet model, coupled with the ENTPE transformation, [6.7] as input, are more accurate than those performed using the Hirsch model predictions and for sure much closer to the asphalt mixture creep stiffness experimental data than what are the simulations that use the creep stiffness of the extracted asphalt binder. However, as in the case of the back calculation of the asphalt binder creep stiffness, it seems that the simulations based on Huet-ENTPE approach and extracted binder data present parallel curves in log scale.

Overall, based on the finite element simulations, the Huet-ENTPE formulation seems to provide the best asphalt binder creep stiffness prediction. Anyway the strong assumption made at the beginning of this Chapter of assuming a single identical value for the aggregate modulus, may be not realistic. Further information is required on the characteristics of RAP and shingles (TOSS and MWSS) included in the different mixtures to elaborate more precise finite element simulation since it may be that such an assumption is masking some effects of the aggregates and the good results obtained may be biased by this.

As in the case of asphalt binder creep stiffness, the simulation results obtained from the extracted asphalt binder data and those obtained from backcalculation are significantly different with higher values for the modeling input. This was expected since the asphalt binder creep stiffness of the extracted binder was much smaller than that predicted from both Huet-ENTPE expression [6.7] and Hirsch model. However, this cannot be related to the finite element simulations itself, even though a more sophisticated 3D geometry reconstruction of the asphalt mixtures beam through X-Ray CT tomography may provide more accurate results. On

Mix 7 Mix 8

the other hand, the price to pay for such an accuracy improvement would be an increased computational time of simulations.

It turns out that also the FE simulations seem to put a spot light on the actual interaction between the aged binder (contained in RAP, TOSS and MWSS) and the virgin binder when mixed together both in the case of mixture preparation (laboratory or plant) and chemical extraction. As mention in the previous section it may be that the very stiff asphalt binder present in the recycled material cannot melt and/or partially blend with the virgin one after heating and mixing. However other explanations can be given for significant difference between the results.

In the case of the mixtures containing only RAP, it was hypnotized that the higher stiffness showed from the backcalculated binder prediction, compared to that of the extracted binder, may be due to an erroneous mixture preparation, where a smaller amount of virgin binder was used, resulting in dryer asphalt mixtures. This can lead to much stiffer asphalt mixtures. On the other hand, when extraction is performed, the asphalt binder blend obtained from virgin and oxidized asphalt binders, presents much different characteristics with smaller creep stiffness.

When analyzing the results for the four asphalt binders - asphalt mixtures containing both RAP and RAS (mixtures # 5, 6, 7 and 8) particular attention should be carried since one of the basic components of shingles is paper backing. This material has fibrous characteristics and thus it significantly contribute to the stiffness of the asphalt mixtures. However when binder extraction is performed fiber are washed away during the extraction process and thus their contribution to the asphalt binder creep stiffness is then removed. This may be an explanation to the significantly higher results obtained from asphalt binder creep stiffness backcalculation compared to the extracted asphalt binder creep stiffness. Analogous conclusions were drawn by Cascione et al. (2010) when investigating the dynamic modulus of asphalt mixtures containing both RAP and RAS and the PG grade of the corresponding extracted asphalt binder. It must be also mentioned that the detection of fibers is not possible during the scanning process and thus they cannot be represented in the finite element simulations.

As for backcalculation models and for Hirsch model in particular, it is known that micromechanical models present some limitations since their formulations relate asphalt binders and asphalt mixtures properties in a fundamental way. On the other hand, Huet -ENTPE expression [6.7] can be categorized as phenomenological model. The advantage of this type of

model is that they are directly fitted to the experimental data. However, little is known about the physical meaning of the parameters they are built on. For this reason a more in dept study should be carried on to investigate how δ, k, h, E∞, and τ affect the predicted properties.