## 4 Discussion  
As anticipated, the addition of recycling agents led to a decrease in asphalt mix stiffness (IT-CY). The combination of recycling agents and jute fibers resolved the decrease in excess stiffness (IT-CY) when a recycling agent is used. However, data obtained from complex modulus testing did not show this trend. The recycling agent softens aged bitumen in RA and, thus, should enhance miscibility and compactability of the mix. However, there should be more liquid binder available, and the air voids content in mixtures with recycling agents and fibers was higher than that in equivalent mixtures containing only fibers. This indicates a reaction between recycling agents and jute fibers under high mixing and compaction temperature. The method of application of the recycling agent did not make any difference in the case of air voids. However, slight improvements in other characteristics were observed for the mixture with recycling agent added to the RA. Therefore, it is better to apply the recycling additive to the RA.  
The traditional and main issue with the use of cellulose-based vegetable fibers in asphalt mixtures is the lower ITSR. Hydrophilic materials bind water molecules and contribute to the swelling and at least partial debonding of the fiber from the matrix. The second problem is the capillary effect of vegetable fibers, which is good for a plant's growth but not very positive for the bitumen mixtures and their moisture and freeze-thaw susceptibility [41]. Shorter, 10 mm fibers tend to lower the ITSR of the tested mixture even more and increase air voids [42]. It was observed that shorter fibers are prone to absorb more fresh binder and thus decrease compactability and bulk density. On the other hand, they tend to reduce the thermal susceptibility of tested mixtures. The mild alkali treatment of jute fibers showed a moderate ITSR increase, which indicates better fiber-bitumen  
Figure 16: Dynamic (complex) modulus of mixtures with the recycling agent “RF”; reference temperature, T = 20 \({}^{\circ}\)C.  
bonding. In future work, other surface modifications should be researched. Furthermore, better insight into the rheology of vegetable fibers in asphalt mixtures is needed.  
Additives naturally increase the cost of any commercialized mixture. It is important to justify higher costs with positive effects of the additive. Using more RA in asphalt mixes requires either the use of soft binder or a recycling agent, which is a common additive in pavements, with a cost of around 2-3 EUR/t of asphalt mix. In contrast, jute fibers are not a standard additive, but they are commercially available and are one of the most affordable options among vegetable fibers. Their price range is between 0.50 and 1 EUR/kg for raw fibers at the lower end of the range. Producing the yarn raises the cost, and this can be close to 1 EUR/kg and higher. With additional processing, treating to make an additive, and a margin, we can estimate the final cost somewhere around 4-5 EUR/kg of fiber additive. This means that for use in HMA and 0.2% fiber content, the final cost could be not more than EUR 10 per metric ton of mix. Waste jute fibers, e.g., from sugar or coffee bags, can be potentially used, reducing the cost of additive. Together with reduced waste and carbon footprint, this may constitute a good ecofriendly alternative to synthetic counterparts. In addition, vegetable fibers have a combined--stabilizing and reinforcing--effect on asphalt mixtures, which makes them unique and separates them from fine cellulose fibers currently used as stabilizing additives in asphalt mixtures with higher binder content, such as SMA. However, such fibers are expected to be used in the binder and base layer where they can provide additional stiffness and tensile strength. Finally, the use of jute or other vegetable fibers in the construction industry is not limited to asphalt concrete and has shown interesting potential, e.g., in regular [43] and high-strength concrete [44].