# Special Issue \"Nanomaterials and Other Additives to Enhance Asphalt Payement Performance\"
## 3 Pavement Layer Mixtures' Modification  
This second group of papers [9; 10; 11; 12; 13] includes several different methods of improving flexible pavement layers, namely, asphalt concrete and granular layers.  
The study [9] uses wax obtained directly or as a by-product from plastic waste to modify binders, specifically by blending it with conventional or modified bitumen and other additives applied in manufacturing asphalt mixtures. Different tests were used: thermogravimetric and spectroscopic analysis, consistency tests, testing of dynamic viscosity at various temperatures, and assessment of the rheologic properties of the binders. As a result, several crucial findings were reached: this sustainable wax promotes lower handling temperatures and improved consistency and rheological behaviour, providing, for example, lower temperature susceptibility and higher permanent deformation resistance.  
The use of plastic waste as an enhancer of bitumen qualities and an asphalt mixture binder also contributes to the value of this type of waste. The study [10] analysed four types of plastic waste using different plastic contents added by the dry process in asphalt concrete manufacturing and compared the results with a conventional mixture without plastic. A set of laboratory tests were used, and the results show that although the addition of plastic waste reduces workability, satisfactory handling conditions are guaranteed, the asphalt concrete becomes more elastic, and its stiffness values are adequate to apply the material in a pavement surface layer. In addition, the resistance to fatigue cracking was observed at a suitable level. Moreover, the resistance to permanent deformation generally improved, although the performance was dependent on plastic type and use context.  
A material that can be used to increase the stiffness of asphalt binders is gilsonite. This was the object of the study [11]. A modified asphalt binder with gilsonite was introduced by the wet-process, aiming to improve rutting resistance and to replace (by mass and volume) part of the coarse fraction of the aggregate with recycled concrete aggregate; this study experimented with these aims for two hot-mix asphalts with different gradations. In conclusion, both materials (gilsonite and recycled concrete aggregate formulated by volume) could be used in hot-mix asphalts for thick-asphalt layers in high-temperature climates and any level of traffic. However, they are not recommended for thin-asphalt layers in low-temperature climates.  
The study [12] examines the effect of Styrene-Butadiene-Styrene and crumb rubber on the rutting, moisture-induced damage, and workability properties of hot-mix asphalt (HMA) mixtures. In this study, three types of mixtures/binders --namely, control (CB), crumb rubber-modified (CRMB), and polymer-modified (PMB)--were evaluated. The results indicate that CRMB mixtures were less workable and exhibited better resistance to rutting than the PMB and CB mixtures. Furthermore, the PMB mixtures had increased resistance to moisture-induced damage, while the effect of the CRMB mixtures was negligible compared to the CB mixtures.  
The study [13] dealt with construction and demolition waste (CDW) and coconut husk materials to be used in the construction of cement-bound granular mixtures (CBGM) to be applied as base and sub-base layers of road pavements. The CDW can be used to obtain good-quality recycled aggregates (RA), and the coconut husk can be processed into coconut fibre (CF). The mechanical performance of CBGM with RA and CF reinforcement was compared with a conventional CBGM with natural aggregate (NA). The results concluded that the CBGM with RA performs similarly to the CBGM with NA. Furthermore, using CF effectively reinforced the integrity of the CBGM specimens at post-test, indicating potential gains in durability after cracking.