## 7 Conclusions  
The research has examined the application of wood bio-ash (BA) as a substitute for 50% of the conventional mineral filler made from stone material with a carbonate composition. Figure 8: Rut depth RDair. Figure 6: Rutting speed WTSair. Figure 7: The relative rut depth PRDair. A bituminous mixture of asphalt concrete with 50% bio-ash as a filler was tested for its impact on environmental pollution, its water sensitivity, and its resistance to the occurrence of rutting. This research confirmed the possibility of using BA in the asphalt layers of the pavement for the following reasons:  
1. Due to the hazardous substances (sulfates, content of total dissolved solids (TDS), and total chromium) contained in BA, it cannot be deposited in landfills, and the same applies if it is combined with other similar waste [23]. The tests conducted on a crushed asphalt sample and a compacted asphalt roller with BA as a filler confirm that there are no illegal amounts of discharge; therefore, there is no harmful impact on the environment (Tables 6 and 7). Bio-ash, covered with bitumen in the asphalt pavement, is protected from the release of harmful substances into the environment;
2. A ratio of the indirect tensile strength (ITSR = ITSw/ITSd) of 98.9% confirms the good resistance of asphalt with BA to the action of water, which has a direct impact on the durability of the pavement and, therefore, road maintenance costs;
3. By testing the resistance to rutting of the asphalt samples with and without BA, good rutting parameters (WTSair = 0.03 mm/1000 cycles; PRDair = 3.88%; RDair = 1.5 mm) were confirmed. It is important that the addition of bio-ash does not worsen the resistance of the asphalt mixture to the occurrence of plastic deformations, and multiple is used for environmental protection, which also results in positive economic indicators. Laboratory tests have confirmed the justification of using wood ash as a filler in bituminous mixtures, achieving the following advantages:  
* A reduction in the amount of BA waste in landfills and its harmful impact on the environment;
* Enables the use of less industrial stone filler, which is extracted from quarries, the opening and extending of which harm the natural environment;
* Good resistance of the asphalt to the action of water and good resistance to rutting, which directly implies better management of the asphalt pavement. In the end, it should be mentioned that the variability of the WA composition should always be considered, and the possibility of application should be confirmed with full laboratory tests. For the type of asphalt from which a recipe using bio-ash was created in the laboratory at the asphalt plant, the bio-ash from the same power plant/heating plant or from more than one plant should be used if the same biomass is used as the energy source. The knowledge gained through this research has confirmed that the goal of achieving sustainability and innovation in road construction can be achieved by using new component materials in asphalt mixes. Conceptualization, M.S. and S.D. ; methodology M.S., S.D. and K.V. ; investigation, M.S. ; writing--original draft preparation, M.S., S.D. and G.G. ; writing--review and editing, M.S., S.D., G.G. and K.V. All authors have read and agreed to the published version of the manuscript. This research was funded by the Faculty of Civil Engineering and Architecture Osijek (scientific-research project IZIP-GrAFOS-2018), entitled \"The application of ash from wood biomass in layers of pavement structure--Bio PAV\". Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The authors agree to submit test results as appropriate. All tests of hazardous substan ces released from a bituminous mixture using biomass (from the rutting asphalt mixture and the sensitivity of the asphalt samples to water) were conducted in an accredited test laboratory at the Institute IGH d.d. in Zagreb. The authors declare there is no conflict of interest.