## 2 State of the Art  
Asphalt mixtures are used in the construction of the surface, binding, and base layers of an asphalt pavement structure. They are made of crushed coarse and fine aggregates, filler, bituminous binder, and, in some cases, other additives. Asphalt mixtures are quality mixtures with a complex, highly elastic behavior. Many researchers have investigated the potential of bio-ash for usage in asphalt mixtures [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. According to the results of [8], the presence of wood ash can significantly improve the resilient modulus of the asphalt mixture, while stability decreases with increasing wood ash content. However, these values of stability are within the prescribed limits of applicable technical conditions. As the proportion of wood ash increases, permanent deformations decrease, and fatigue resistance improves. Therefore, the authors found that wood ash, as an environmentally sustainable material, is suitable as a replacement material for conventional fillers for the production of hot mix asphalt (HMA) using asphalt concrete AC 10 and bituminous binder PEN 60/70. Wood ash used as a material to replace conventional fillers can not only improve the properties of the asphalt mixture but also reduce environmental pollution. In addition, wood ash can also be used as a bitumen modifier to improve the quality of the asphalt mix. In [9], samples of bituminous mixtures with 25, 50, 75, and 100% rice husk ash (RHA) as a stone filler substitute for granulometric compositions 4 and 5 (in accordance with Iranian standards) and with 4, 4.5, 5, 5.5, 6, and 6.5% binder were made. The Marshall test was performed, and the Marshall parameters were determined. Subsequently, according to the results for specific gravity and Marshall stability and deformation, a certain optimal bitumen content was obtained for each percentage of RHA. Ten types of bitumen mixture samples were prepared: five samples with granulometric composition number 4 and five samples with granulometric composition number 5. These samples were tested for active water (ITSR), rutting, stiffness modulus, and dynamic creep. An increase in RHA content increases the indirect tensile strength of bituminous mixes when compared to control samples for either saturated or dry samples. For samples made with granulometric composition number 4, with an increase in RHA of 25, 50, 75, and 100%, the amount of TSR increased from 7.7, 13, and 18.1% to 22.8%. The TSR values were 4.1, 7.2, 10.9, and 12.6% for granulometric composition number 5. Increasing the proportion of RHA in the bituminous mixture samples significantly increased the stiffness modulus of the samples. When compared to the control samples, this increase was 54% for granulometry 4 and 117% for granulometry 5. The results clearly show that the samples made from RHA have a higher resistance to rutting, and with increasing RHA, the rutting depth decreases. The results of the dynamic creep test confirm the rutting results. According to these results, the change in flow rate increases with the increase in RHA content, which shows a higher resistance to the occurrence of rutting in the samples made with RHA. The optimal RHA content, considering the requirements and limitations of the prescribed conditions, was determined at 26% (number 4) and 48% for the granulometric composition number 5. As part of the research by the authors of [10], the properties of SBS (styrene-butadiene-styrene) with a bituminous binder modified with rice husk ash (RHA) and a conventional binder were tested using a rotational viscosity test and a temperature shear test. The properties of the SBS/RHA-modified bitumen mixture were tested for rutting, water penetration, and cracking at low temperatures. The addition of SBS/RHA reduced the penetration value and ductility of the pure bituminous binder and improved the softening point (RB method) of the base bituminous binder.