Conclusion

Several conclusions can be drawn from the different Chapters and sections of this thesis.

For asphalt mixture creep stiffness and m-value it can be concluded that:

 Significant increase in asphalt mixture creep stiffness is experienced for a 25% RAP content when RAP is the only recycled material included in the mixtures. On the other hand a significant decrease of the m-value and thus of the relaxation properties of the mix are shown for all RAP percentage investigated.

 For a fixed RAP content of 15% mixtures containing up to 3% of Tear-off Scrap Shingles (TOSS) presents a decrease in creep stiffness. On the other hand when 5%

Manufacturer Waste Scarp Shingles (MWSS) are added to the mix creep stiffness increases significantly. Neither TOSS nor MWSS statistically affect the m-value.

 Tear-off Scrap Shingles and Manufacturer Waste Scrap Shingles do not affect the creep stiffness of mixtures designed with 25% of RAP. However an increase in TOSS is negatively correlated to the m-value resulting in mixture with poorer relaxation properties.

 When using softer binder there is a decrease in creep stiffness for asphalt mixtures containing 5% of Tear-off Scrap Shingles, while there is no binder type effect when Manufacturer Waste Scrap Shingles are present. An increase in m-values is however experienced for softer binder meaning that using a PG 52-34 is beneficial to the relaxation characteristics of the mixtures.

The main findings for thermal stress and critical temperature can be summarized as follow

 When only RAP is present in the mixtures both cooling rate and RAP content are statistically significant. They are positively correlated with thermal stress and critical temperature meaning that, for an increase in RAP content and cooling rate, there is an increase in thermal stress and critical temperature resulting in a more brittle and temperature susceptible mixture.

 For mixture containing 15% of RAP cooling rate has a positive correlation with thermal stress and critical temperature. However the contribution of Recycled Asphalt Shingles to thermal stress is highly significant only for TOSS at 3% resulting in an increase of the stress in the pavement. A significant critical temperature increase is experienced only for a TOSS content of 3%.

 When 25% of RAP is present in the mixtures there is a significant increase in thermal stress when TOSS or MWSS are present up to a 3%. Critical temperature is affected

by all the factors levels showing an increase as the recycled material content increases.

Cooling rate is positively correlated both with thermal stress and critical temperature.

 A softer binder type shows to be statistically significant and negatively correlated with thermal stress. As a result softer binder helps reducing the thermal stress in pavement.

However depending on the type of recycled material included in the mixtures (TOSS or MWSSS) it may not be helpful in decreasing the critical temperature of the specific mixture.

From the digital image analysis it was found that:

 The volumetric fraction and the average distribution of aggregates for the different asphalt mixtures are very similar suggesting that the mixtures were designed for the same amount of aggregate in volume even though they include various type of recycled asphalt material.

 Based on the average values of the particle size distribution obtained from two-dimensional images, the mixtures show very similar gradation curves. This impression is also confirmed by a visual comparison of the mixtures with similar characteristics in terms of RAP, Tear-off Scrap Shingles (TOSS) and Manufacturer Waste Scrap Shingles (MWSS) content.

 The 2- and 3-point correlation functions calculated for asphalt mixtures behave similarly to what would be expected from a theoretical solution using the penetrable spheres model. No large variations are observed between the 2- and 3-point correlation functions. The results contained in S2 and S3 suggest that there is no unexpected pattern in the asphalt mixtures microstructure and thus that the recycled materials added to the mixtures do not deviate the distribution of the mixtures constituents, and in particulars of the aggregates, from that of a typical random heterogeneous material.

From the modeling section it can be concluded that:

 The Hirsch model and the Huet model, coupled with the ENTPE transformation, predict higher asphalt binder creep stiffness than those obtained from the Bending Beam Rheometer testing on the extracted asphalt binder.

 In general the asphalt binder creep stiffness obtained using Huet-ENTPE formulation are parallel to the creep stiffness of the extracted asphalt binder up to 240s in log scale.

 The two dimensional finite element simulations performed using as input the asphalt binder creep stiffness obtained from Hirsch model, Huet-ENTPE approach and from

the extracted binder suggest that, under the assumed hypothesis on the aggregates properties, the ENTPE transformation, with the embedded Huet model, provides a good estimation of the asphalt binder creep stiffness of the mixtures investigated. On the other hand Hirsch model doesn’t seem to give good asphalt binder creep stiffness predictions.

 The two dimensional finite element simulations seem also to indicate that the creep stiffness of the extracted asphalt binder is not representative of the real properties of the binder when in the mix. This is something expected since extraction results in a complete blending of the different binder present in the mixtures and coming from different sources (virgin, RAP, TOSS and MWSS).

 The fibrous materials contained in RAS may contribute to the global stiffness of the recycled asphalt mixtures, however they may be just one of the reason for the difference between the backcalculated and extracted asphalt binder creep stiffness.