## 2 Materials and Methods  
To evaluate the effectiveness of the two biobased recycling agents and the jute fibers for improving the properties of asphalt mixture with 30% RA, 14 mixtures were made for this study. The experimental design of all mixtures was carried out in accordance with a typical asphalt concrete used in base courses (AC\({}_{\text{base}}\) 16) with a nominal maximum aggregate size (NMAS) of 16 mm (according to the Czech technical standard CSN 73 6121:2019 [30], denoted as ACP 16+). The same aggregate size distribution curve (Figure 3), RA content (30% by weight), and bituminous binder content and type (paving grade bitumen 50/70, the standard bitumen in the Czech Republic) were used for all mixes. Bitumen content of RA was found out to be 6%. The targeted binder content in the mixture with 30% RA was 4.5% by weight divided between added amount of virgin binder of 2.7%; the rest was assumed to be reactivated from RA. The amount of added recycling agent is dosed with 6% of the amount of the RA binder content and can be expressed as 0.108% by weight of the asphalt mixture. Fundamental layout of prepared and tested mixtures is shown in Figure 4.  
Figure 3: Granulometric curve of the reference asphalt mixture. In this study, two commercialized biobased recycling agents--RAPFix, noted in this paper as \"RF\" (oil-based additive), and SylvaRoad, denoted as \"SR\" (crude tall oil and crude turpentine oil-derived additive), were selected and dosed with 6% by mass of the aged binder contained in the reclaimed asphalt. This dosage was a result of a comparison of selected tests, such as penetration at 25 \({}^{\circ}\)C, softening point test, frequency sweep test, and MSCR (multiple stress creep recovery test), which will not be presented in this paper. To date, only a few published research articles [31; 32] about the performance of these types of products are available, although their features can be found on company websites. Additionally, this study used natural-based jute fibers to enable the physical enhancement of recycled asphalt mixtures. In the present paper, jute fibers of 20 mm in length were added at a rate of 0.2% by weight of the total mixture. Jute fibers were provided by JUTEKO s.r.o company in form of a yarn with fineness 300 g/km. Yarn was cut to the length of 10 and 20 mm on a precise desktop manual paper cutter and placed in a water bath for at least 4 h at room temperature. This step led to disintegration of the yarn into separate fibers. After the water bath, the fibers were hand-squeezed, placed on a stainless-steel plate, and placed in an oven with fan-induced air circulation. They were dried for at least 12 h at 50 \({}^{\circ}\)C.  
A mild NaOH treatment was performed by immersing dry separated jute fibers into a solution of 0.5% NaOH for 24 h. Then, fibers were washed with copious amount of deionized and tap water and were dried with the same procedure. For preparing the specimens, reclaimed asphalt and virgin aggregates were kept at 160 \({}^{\circ}\)C. Next, the recycling agent was added by two different methods: (i) directly to the RA (oil-based) or (ii) onto the fibers (both types). Recycling agents were hand-mixed with the fibers at room temperature. Their dosage was slightly higher than required amount to compensate for leftovers adsorbed on gloves and the bowl surface. Then, the fibers were moved to another bowl and placed in an oven at 150 \({}^{\circ}\)C for 5 min. After this step, part of the bitumen binder was applied onto fibers, the specimen was quickly hand-mixed, and the fibers partially coated by bitumen were gradually added to the mix with the rest of bitumen, as can be seen in Figure 5. The final mixture was then compacted at 150 \({}^{\circ}\)C, according to EN 12697-30 [33] (by Marshall impact compactor). The asphalt mixes were produced and tested in the lab according to the Czech national specifications provided in the national standard CSN 73 6121:2019 [30]. List and description of all mixtures is displayed in Table 1.  
Figure 4: Test plan scheme.