## 2 State of the Art
While the controlled mixture with a mineral filler has better resistance to rutting, those evaluated with the modified mixture meet the prescribed rutting depth values. The authors of [15] analyzed the suitability of recycling wood ash (WA) as an alternative filler in bituminous mixtures. The WA used in this study was taken from a landfill located near a food industry plant. Bituminous mortars containing four different amounts of WA were compared to conventional variants with added hydrated lime (HL) in similar amounts. It was found that WA has a lower specific gravity, coarser particles, and a low content of active clay, which affects hydrophilicity. Bituminous mortar with WA was found to have a slightly lower resistance to rutting at high temperatures due to lower stiffness. WA mortars were, however, found to exhibit better fatigue resistance than conventional HL mortars. Bituminous mixtures containing WA had satisfactory Marshall and volumetric properties. The HL and WA mixtures also showed satisfactory resistance to the action of water, which, in both mixtures, decreases with an increase in the amount of filler due to a simultaneous decrease in the proportion of bitumen. According to the proposed methodology, the optimal filler content for HL is 5.5% and 5.6% for WA. WA mixes are more cost-effective as well as more environmentally friendly than HL mixes. By using WA in road construction, a large amount of conventional HL is saved, which reduces the exploitation of natural resources. In [16], biochar was used as a modifier of bitumen obtained from oil processing. Three amounts of biochar: 2%, 4%, and 8%, and two particle sizes: 75-150 \(\upmu\)m and <75 \(\upmu\)m were tested. Viscoelastic properties, rutting resistance, and fatigue resistance were determined via testing (rolling thin-film oven--RTFO, pressure aging vessel--PAV, and dynamic shear rheometer--DSR). The viscous properties were confirmed with increasing temperature in biochar-modified bitumen. The addition of biochar increased the resistance to rutting. The resistance of bitumen to fatigue cracking was reduced by the addition of biochar, but the size of the biochar particles significantly affected the fatigue resistance. The small-particle biochar-modified binder had better resistance to fatigue cracking than the coarse-particle biochar-modified bitumen. In conclusion, at the end of the tests, an optimal amount of biochar of 2-4% and a particle size of less than 75 \(\upmu\)m was determined. In the search for ways to improve the quality and load capacity of asphalt pavements, taking sustainable development into account, the researchers of [17] used biowaste as a modifier in asphalt pavements. As part of this study, laboratory tests were conducted on the use of peanut or groundnut shell ash (GSA) as a bituminous binder modifier, and its effects on pavement performance were tested. The bituminous binder test results showed that GSA increases viscosity, shear strength, stiffness, and elastic recovery and improves rutting resistance. It was found that up to 10% GSA can be used to modify the bituminous binder without the risk of segregation during storage and transportation. The addition of GSA to the bitumen binder increases the bond between the bitumen and the aggregate and improves the stiffness modulus. GSA-modified bituminous mixtures indicate lower thermal sensitivity and, thus, good resistance to rutting. By applying GSA (up to 15%), the resistance of bituminous mixtures to fatigue and the development of microcracks is improved. Based on tests conducted on bituminous binder and bituminous mixtures containing GSA, an optimal dose of 10% (by the weight of the binder) was determined. GSA landfills cause environmental problems, and by using them in asphalt pavements, they reduce the cost of road construction and maintenance, as well as reducing the environmental problems caused by the GSA landfills. Based on the conducted research [18], the optimal share of sugarcane bagasse ash (SCBA) was determined to be 0.34%.