Hot mix asphalt needs to be developed to resist particular permanent deformations like rutting and thermal cracking due to increased traffic volumes and vehicle loads. Additives such as carbon black, graphite, carbon fibers are used in the mixture or the binder for improving the mechanical features of asphalt. In this article, the effects of graphite used for developing the mechanical properties of asphalt have been investigated in mixtures tests. Therefore, Marshall stability, indirect tensile stiffness modulus and indirect tensile fatigue tests were performed to bituminous mixtures modified with three different proportions of graphite by weight of bitumen. Stone mastic asphalt mixtures which were manufactured with pure and modified bitumen were aged in different time intervals in the oven. In conclusion, it has defined that Marshall stability values have declined. It has been determined that there is no significant difference in the time-dependent deformation behavior of the original and aged samples in pure and different graphite content although the stiffness modulus and load repeat number of the samples increased with the rise of the aging time. These results shown that graphite generally used for improving the thermal properties in literature were also determined to contribute to mechanical properties of mixtures.

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The durability, fatigue resistance, and low-temperature behavior of asphalt layers are greatly affected by the properties of bitumen. Therefore, the composition of bitumen is frequently modified to improve the performance of asphalt mixtures. Sty-rene-butadiene-styrene (SBS) has been the most often used additive recently. Researchers are trying to improve the cohesive and adhesive properties of binders by such polymer-based additives. In this study, 160/220 penetration grade bitumen and Kraton D 1101 SBS were used. The present study contains a new evaluation for determining the cohesive behavior of SBS-modified binders at -1°C, -3°C, and -5°C. The results of this evaluation were compared to conventional and rheological test results. Penetration, softening point, viscosity, dynamic shear rheometer, and bending beam rheometer tests were therefore conducted. Finally, the results of a low-temperature tensile test were found to be consistent with the results of the other tests; hence, they also confirm the cohesive behavior of SBS-modified binders at low temperatures.

The increasing rate of traffic every day requires the roads to be made more stable. In order to improve the low and high temperature properties of the bituminous mixtures forming the coating layer of the flexible pavements, the binder in the mixture is often modified with various polymers. The most commonly used polymer in the modification is styrenebutadiene-styrene (SBS). However, SBS, which is very successful in improving the mechanical properties of the mixtures, is expensive and has brought the search for cheaper alternative additives. In this context, natural asphalts (NA) which are not subjected to any processes are used as additive. In this study, natural asphalt obtained from Zaho region of Iraq was used in 20%, 35% and 50% of bitumen modification by weight. The mechanical properties of mixtures prepared with different amount of NA modified bitumen were compared with 4% SBS modified bituminous mixtures. As a result, NA modification significantly improved the properties of bituminous mixtures. In particular, above 35% NA, the mixtures exhibited a superior performance by significantly resisting the effect of repeated loads. It was found that an average of 17.3% NA modification exhibited similar behavior with 4% SBS modification and provided significant economic savings.