Laboratory Study of Asphalt Concrete for Base Course with Reclaimed Asphalt, Recycling Agents, and Jute Fibres

Abstract

The way we treat materials after their lifespan is changing. We are finding a new, more effective way to deal with waste: using it, rather than depositing it in landfills. Since bitumen mixtures are the most popular paving materials by far, and their lifespan is limited, there is a constant availability of old asphalt pavement or reclaimed asphalt (RA). To restore the aged binder properties, we can use recycling agents. In this study, two commercialized biobased recycling agents were used. Furthermore, jute fibers were used as a reinforcement. The influence of the different fiber content and fiber length was investigated in mixtures without the recycling agents. In addition, alkali-treated fibers were used in some mixes for better fiber compatibility with the bitumen matrix. Air voids content, moisture, freeze–thaw susceptibility, stiffness modulus (IT-CY), resistance to crack propagation, and complex modulus tests were conducted. The addition of recycling agents led to a decrease in stiffness. A lower indirect tensile strength ratio (ITSR), increased stiffness, and best crack propagation results were recorded in some mixtures with fibers and recycling agents.

1. Introduction

During recent decades, the world witnessed a change in the manner of using and dealing with materials. From an excessive use of raw materials and a high amount of dumped waste, we have shifted to more sustainable and circular practices, namely, sustainable waste management practices whose main aims are a reduction in the environmental impact of waste, promotion of recycling and resource recovery, and minimizing the use of landfills. Thus, to reduce, reuse, and recycle waste materials became the pillars for a better environment.

The road industry is no exception. Asphalt is one of the most widely used materials for roads construction globally due to its ability to withstand heavy traffic loads and diverse weather conditions and its cost-effectiveness. It is estimated that, as of 2021, 94% of U.S. roads [1] and 90% of European roads [2] are surfaced with asphalt, making it one of the most abundant waste materials. Therefore, repurposing asphalt materials from old road surfaces (reclaimed asphalt) and using it in new construction or maintenance projects is essential to operate in a more environmentally friendly and efficient manner.

According to the latest data from the European Asphalt Pavement Association (EAPA) (Figure 1), in 2021, a total of 41.3 Mt of reclaimed asphalt were available in the asphalt industry within the Europe. Amounts of reclaimed asphalt, however, vary from one country to another: France, Germany, Great Britain, and Italy are the leading countries and have the most reclaimed asphalt (RA). These are followed by the Czech Republic, with 2.4 Mt. Of this amount, 50% (Figure 2) was reused in hot mix asphalt (HMA), warm mix asphalt (WMA), half-WMA, and cold mix asphalt (CMA) mixtures, and 15% is either landfilled or used in other applications [2]. In the previous year, the data from the EAPA showed that 28% of the available RA was dumped in landfills. Thus, the use of these materials is growing gradually year by year [3].

Reclaimed asphalt is a material generated from removed old road surfaces. It is mainly composed of nonrenewable resources, i.e., aggregates and aged bitumen binder (approximately 95% wt. and 5% wt., respectively). Hence, their use and reuse in new asphalt mixtures tends to reduce the demand for virgin materials, the carbon footprint associated with the production of asphalt, and the dumping of asphalt in landfills, leading to better preservation of the environment and cost-effectiveness. The use of RA has also shown some mechanical advantages and properties such as workability [4,5], toughness [5,6], and freeze and thaw resistance [7].

Despite the above-mentioned benefits, the aged binder within RA has arisen as a concern of many researchers since the reclaimed mixtures tend to be stiffer and susceptible to fatigue and thermal cracking if the average content of RA is more than 30% wt. in HMA [8,9]. As a result, it is essential to improve the performance of asphalt mixtures containing RA. Several methods and strategies have been adopted, such as the use of various additives, including polymers and recycling agents in combination with RA [10]. In fact, the incorporation of polymers such as polyethylene terephthalate (PET) in asphalt mixtures have demonstrated their ability to improve long-term performance and mitigate moisture damage [11]. Thus, the usage of PET with RA has become an effective practice to overcome RA deficiencies. As for the incorporation of recycling agents, this has been proven to decrease reclaimed mixtures’ stiffness and restore the aged bituminous binder properties [8,12]. There are several types of recycling agents available on the market, including modified vegetable oils and biobased oils. Although the aim of these additives is the same, i.e., to improve binder properties in the reclaimed asphalt, they have different capabilities in terms of addressing these deficiencies. This paper will be dedicated to studying two types of organic recycling agents and their regeneration mechanisms and effects.

In addition, the use of natural fibers as a stabilizing additive or reinforcement has presented significant environmental advantages as a renewable, sustainable, ecological, biodegradable, and high-quality product. Natural fibers are considered to be not only a more environmentally friendly alternative to manmade fibers, such as synthetic fibers, carbon fibers, glass fibers, and many others, but also more economically friendly since the cost of the production and the processing of natural fibers is very low. From a mechanical point of view, natural fibers have shown good performance when added to asphalt mixtures as they enhance the asphalt mixture performance, greatly extend the lifespan of AC mixtures, increase stiffness, and reduce crack propagation and permanent deformations [13,14,15,16].

Among the natural fiber alternatives, jute fibers have been investigated by many researchers as an appealing additive to reinforce asphalt mixture thanks to their environmental and economical compatibility, availability, annual renewability, biodegradability, and physicochemical properties. However, the research investigating their effects when combined with RA is still limited [17,18]. An extensive review of the use of fibers in asphalt mixtures was carried out by Guo et al. [19]. One subchapter is devoted to the combination of glass, metal, synthetic polymer, lignin, basalt fibers, and recycling agent in mixtures with RA. Similar research was recently conducted by Buritatum et al. [20]. Hemp fibers were used as reinforcement in asphalt concrete with up to 100% RA. This mix was meant to be used for low-traffic roads, and they found that fibers’ length had a more pronounced effect on mechanistic performance improvement than fiber content. Mansourian et al. [21] investigated the fracture resistance of WMA containing jute fibers in the form of 20 mm-long yarn elements and observed a significant improvement in fracture resistance under fracture mode I. This indicated that the tensile strength of the WMA mixtures had increased. It was also found that there was no significant effect on fracture resistance when more than 0.3% of fibers were added. A study conducted by Balreddy et al. [22] investigated the application of four natural biofibers, including jute fibers, in open-graded bituminous concrete (OGBC). They found that fibers can boost OGBC’s performance, in terms of strength and resistance to moisture susceptibility, and can reduce binder draindown. Finally, the use of jute fibers in stone mastic asphalt (SMA) was investigated. In a recent study, Ali Alshehri et al. [23] used jute fibers obtained from rice and sugar sacks and found that jute fibers clearly enhanced the moisture resistance of SMA mixes but not as much as the addition of plastic waste. Kumar et al. [24] compared jute fibers with synthetic fibers and concluded that jute may be suitable for gap-graded asphalt mixes supported by lower permanent deformation of the mix and marginally lower creep modulus. The stiffness of mixtures with jute fibers was lowered by approximately 20%, but the ITSR was the same. Furthermore, an estimated construction cost was calculated. It was found that the cost of one metric ton of SMA with jute fibers was approximately 18% lower than the cost of the mix using a synthetic counterpart.

Aside from all the denoted advantages that natural vegetable fibers can provide, their composition—cellulose, hemicellulose, pectin, lignin, and wax [25]—make them hydrophilic in nature due to the large number of hydrophilic hydroxyl groups. This increases their moisture absorption and weakens their adhesion to hydrophobic matrices. To enhance interface bonding between the fibers and matrix, various chemical treatments can be applied to the jute fibers, such as alkali treatments, mercerization, an acetylation, among others [25,26]. In general, the use of chemical treatments, such as alkali treatment, has been reported to produce a decrease in the weight and diameter of fibers and an increase in tensile properties [27]. The effect of water and the potential faster deterioration of vegetable fibers and, consequently, deterioration of the entire pavement are legitimate concerns. Banerjee and Ghosh [28] conducted a study regarding the mechanical behavior of jute in bituminous binder. After enzymatic treatment simulating microbial attack, 6 months of hygral treatment was shown to be insufficient to break the jute–bitumen interface and degrade the coated fiber. This strong interface is created due to the coexistence of chemical and physical bonding between jute and bitumen. Therefore, they concluded that bitumen acts as a protection for jute against microbial attack. They also found that the thermal condition of bitumen overlaying deteriorates the strength of jute by about 10%. This study revealed that jute is very compatible with bitumen and reinforces the system to a considerable extent either in the form of fiber or fabric.

Nevertheless, material aging affects every organic material. Cellulose-based fibers are especially prone to changes in physical properties after being exposed to a hot environment and, clearly, after exposure to a real environment. Li et al. [29] attempted to study the effects of short-term and long-term aging on pavement that incorporated fine cellulose fibers (as stabilizing additives in SMA). They found that aging of cellulose fiber inevitably decreases the high-temperature performance of asphalt mortar. During the process of aging, the cellulose fiber is oxidized and degraded, resulting in an increase in the amount of ash in it. Under long-term aging, the performance of cellulose fiber is weakened.

In this study, reclaimed asphalt mixtures containing 30% RA with different additives will be investigated. This investigation will include the influence of two types of biobased recycling agents, alkali-treated and non-treated jute fibers, and varying lengths of jute fibers.

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_base 16) with a nominal maximum aggregate size (NMAS) of 16 mm (according to the Czech technical standard ČSN 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.

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 °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 °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 °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 °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 °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 ČSN 73 6121:2019 [30]. List and description of all mixtures is displayed in

Table 1. Description of tested mixtures.

Mixture’s Label	Description
Ref	Reference mixture containing 30% RA but no recycling agents or fibers.
0.1% J300	+0.1% (by weight of the total mixture) of jute fiber with 20 mm-long fibers.
0.2% J300	+0.2% (by weight of the total mixture) of jute fiber with 20 mm-long fibers.
0.2% J300 10 mm	+0.2% (by weight of the total mixture) of jute fiber with 10 mm-long fibers.
0.3% J300	+0.3% (by weight of the total mixture) of jute fiber with 20 mm-long fibers.
0.2% J300 NaOH	+0.2% (by weight of the total mixture) of NaOH-treated jute fiber.
6%SR	+6% (by mass of aged binder) of SR recycling agent.
J300 6%SR	+0.2% (by weight of the total mixture) of jute fiber and 6% (by mass of aged binder) of SR recycling agent added to fibers (ii).
NaOH 6%SR	+0.2% (by weight of the total mixture) of NaOH-treated jute fiber and 6% (by mass of aged binder) of SR recycling agent added to fibers (ii).
NaOH 9%SR	+0.2% (by weight of the total mixture) of NaOH-treated jute fiber and 9% (by mass of aged binder) of SR recycling agent added to fibers (ii).
6%RF	+6% (by mass of aged binder) of RF recycling agent.
6%RF_RA	+0.2% (by weight of the total mixture) of jute fiber and 6% (by mass of aged binder) of RF recycling agent. This recycling agent was added directly to the RA (i).
6%RF_fibers	+0.2% (by weight of the total mixture) of jute fiber and 6% (by mass of aged binder) of RF recycling agent. This recycling agent was added to the fibers (ii).
NaOH 6%RF	+0.2% (by weight of the total mixture) of NaOH-treated jute fiber and 6% (by mass of aged binder) of RF recycling agent. This recycling agent was added to the fibers (ii).

.

To evaluate the performance of asphalt mixtures, a selection of laboratory tests were performed and the results were examined. These tests included the following:

• Air voids content according to EN 12697-8 [34];
• Moisture susceptibility (ITSR) according to EN 12697-12 [35] and the water + one freezing cycle according to AASHTO T283 (modified procedure used at CTU in Prague [36]);
• Determination of stiffness modulus according to EN 12697-26 [37], Annex C—IT-CY test method;
• Resistance to crack propagation according to EN 12697-44 [38] with a modified procedure: loading rate of 2.5 mm/min, diameter of test specimens of 100 mm, and compaction of test specimens by Marshall compactor—like that used for IT-CY stiffness test method (Figure 6);
• Complex modulus according to EN 12697-26 [37], Annex B—4PB-PR method.

3. Results

3.1. Air Voids

Of all the tested mixtures, three mixtures exceeded the limits for the type test (according to ČSN 736121:2019): the reference mixture, the mixture with 10 mm fibers, and the mixture NaOH 6% RF. Surprisingly, the mixture with an increased content of recycling agent (NaOH 9%SR) reached the limit of 7%. It can be observed (Figure 7) that the mixtures with alkali-treated fibers, except for the NaOH 6%SR mixture, always achieved higher air voids. An increase in surface roughness, the surface area of 10 mm, and alkali-treated fibers may be the likely cause of this increase. It was also shown that a higher fiber content leads to an increase in air voids. However, the air voids content of the reference mixture was unusually high, which was also confirmed by the production of verification test specimens (7.2% air voids content). Nevertheless, for mixtures with recycling agents, there was an expected trend of increasing voids after the addition of fibers, except for the mixture J300 NaOH 6%SR. All mixtures met the limits for control tests. The change in fiber content led to the expected increase in air voids with increasing fiber content. The method of adding the RF recycling agent had no effect on the air voids content of the mixtures.

3.2. Moisture and Freeze–Thaw Susceptibility

The results confirmed the trend of a decrease in the indirect tensile strength ratio (ITSR) value after the addition of fibers. When testing the freeze–thaw susceptibility, the decrease was even more significant in most cases. Figure 8 displays the mixtures without recycling agent. The reference mixture showed the best ITSR values, followed by the mixture with alkali-treated fibers (0.2% J300 NaOH). Both showed a slight increase in ITSR after the freeze cycle.

Furthermore, all mixtures with 20 mm fibers showed an increase in dry indirect tensile strength. After the freeze–thaw and water immersion, the strength of mixtures with 20 mm fibers did not drop below the level of the reference mixture, which compensates for the lower value of the ITSR. The lowest indirect tensile strength was achieved by the mixture with 10 mm fibers, followed by the 0.3% J300 mixture with the highest fiber content.

The mixture J300 6% SR achieved the lowest ITSR values of all the tested mixtures, as shown in Figure 9. Treatment of fibers with NaOH led to a significant increase in dry indirect tensile strength, in the case of the “SR” recycling agent, and basic content of 6%. However, an increase in the recycling agent content, up to 9%, led to the opposite effect. On the other hand, ITSR values rose above the 70% mark. Surprisingly, the mixture with the recycling agent “SR” without fibers did not demonstrate any increase in any monitored parameter compared to the reference mixture. In contrast, the application of recycling agent “RF” without fibers resulted in the highest ITSR values and a decrease in indirect tensile strength (ITS). This dry value was again increased by the addition of fibers. Likewise, there was a drop in ITS values after the freeze cycle and water immersion. The difference between applying the recycling agent to the fibers and to the RA was small, but the 6%RF_RA mixture achieved slightly better results. The mixture with alkali-treated fibers achieved high ITS after water immersion. However, subjecting the mixture to a freeze cycle resulted in a drop to the lowest level of the “RF” mixes. A more pronounced reduction in ITSR values can be attributed to the lower content of fresh binder. However, the addition of recycling agent to fiber blends did not lead to the expected reduction in susceptibility. This phenomenon should be further investigated in future research. However, according to ČSN 736121:2019, AC 16_base mix types do not have set ITSR limits. However, for use in practice, it is good for each mixture to have the highest possible values. The asphalt base layer is not often replaced during roadway reconstructions [39]. Once damage/cracks develop in the layer, degradation and the need to replace all bound layers could potentially occur more rapidly.

3.3. Stiffness

Stiffness modulus testing found that the addition of fibers increases the stiffness of the mix. Among the mixtures without recycling agents (Figure 10), almost all mixtures with fibers achieved a higher stiffness than the reference one at all tested temperatures. Only the mixture 0.1% J300 and the mixture with 10 mm-long fibers achieved lower stiffness at 0 °C. Furthermore, there was also a decrease in thermal susceptibility, calculated as the ratio between the stiffness at 0 °C and 27 °C. The mixture with NaOH-treated fibers had a higher stiffness than the equivalent mixture of 0.2% J300. An increase in stiffness was observed with increasing fiber content. Reduction in the fiber’s length led to a slight stiffness increase at 27 °C and a decrease for the remaining two temperatures, resulting in one of the lowest thermal susceptibility values, together with mixture 0.3% J300.

Observing Figure 11, the application of the recycling agent brought about a logical reduction in the stiffness of the mixtures. After the addition of fibers, the value increased, which did not only apply to the mixture with a higher content of recycling agent (NaOH 9%SR). In addition to the previous observation, there was also an obligatory decrease in thermal susceptibility, except in the mixture NaOH 9%SR. With the “SR” recycling agent, there was no general increase in stiffness in the case of alkali-treated fibers compared to the equivalent mixture of J300 6%SR. The application of the “RF” recycling agent resulted in the highest value of thermal susceptibility in all the tested mixtures. After the addition of the fibers, there was a significant reduction in this parameter and an up to twofold increase in stiffness at 15 °C for the mixture with alkali-treated fibers. This mixture also surpassed the stiffness of the reference mixture at 27 and 40 °C.

3.4. Resistance to Crack Propagation

Testing the crack propagation resistance of the mixtures resulted in the following, in some cases very surprising, findings. Among the most surprising findings was that the crack fracture energy at 0 °C in mixes with fibers was generally lower than that of the reference mixture. Crack fracture energy is calculated as the difference between total fracture energy and energy of the maximum force reached in the test. This was improved in mixtures with 0.3% jute fibers, alkali-treated fibers, and 10 mm fibers. However, a relatively significant standard deviation in the latter was also observed. Nevertheless, the fibers did not help increase this value as observed with previous types of mixtures with jute fibers [40]. The trend was notable in fiber-reinforced mixtures regardless of the application of recycling agents, which could potentially cause degradation chemical reactions at higher temperatures. So, the root of the problem lies elsewhere. A partial explanation can be provided by the higher stiffness of mixtures with fibers and the absorbency of vegetable fibers, which absorbed part of the free binder. This theory is slightly supported by the results of the best performing mixtures with recycling agents, where there was a decrease in the fracture energy difference if compared to the reference mix.

The mixtures with fibers and without recycling agents performed well at 25 °C, as can be seen in Figure 12. It should be noted that the 0.3% J300 mixture was inadvertently tested at 15°. The fracture toughness of this mixture at 25 °C is provided for interest only. Unfortunately, the data for the mix with 10 mm fibers were not recorded because of technical issues. An increase in the fracture toughness at 25 °C was observed for mixtures with fibers.

For the mixtures with recycling agents displayed in Figure 13, there was also a decrease in the crack fracture energy at 0 °C. The most significant decrease was for the 6%SR mix. In the case of the other mixtures, the decrease was mitigated, and, in two cases, there was a significant increase in the total fracture energy (J300 6%SR and NaOH 9%SR). The latter reached the highest values of fracture energy up to the maximum force and total fracture energy, together with the minimum difference in crack energy. The other tested mixes generally reached higher critical fracture toughness values. Unfortunately, the fracture energy parameters at 25 °C were not recorded in the case of mixtures without fibers. At this temperature, there was generally a slight increase in the fracture energies and, except for the NaOH 9%SR mixture, an increase in the critical values of the fracture toughness. The mixtures of J300 6%SR and J300 6%RF_RA can be considered as the best at 25 °C. Thus, a slight improvement of the fracture parameters was achieved by applying a recycling agent to the reclaimed asphalt.

3.5. Complex Modulus

The complex modulus determination was conducted on a prismatic beam specimen by the 4PB-PR test method according to EN 12697-26 [37]. The test was performed at four temperatures (0 °C, 10 °C, 20 °C, and 30 °C) and 11 selected test frequencies (50, 30, 20, 15, 10, 8, 5, 3, 2, 1, and 0.1 Hz) for each temperature. This test is conducted by applying a haversine load to achieve a target strain amplitude of 50 macrostrains. The testing temperatures should be decreased from the highest to lowest level. However, based on previous experience with this test, at the temperature of 30 °C, the specimens show damage from time to time. Therefore, the measurement at 30 °C was carried out at the end of the test process, i.e., as the last measurement after the other temperatures. As for the frequencies of loading, the test starts with the highest frequency and proceeds to the lowest one.

The master curves of all mixtures were constructed for a reference temperature of 20 °C using the principle of time–temperature superposition. The norm of the complex modulus |E*| represents the material’s stiffness, which is a tool for achieving a better understanding of the load’s distribution on the pavement’s structure. The area of low temperatures (high frequencies) is defined on the horizontal axis as higher than 1000 Hz, the area of medium temperatures is in the range of 1–1000 Hz, and the area of high temperatures (low frequencies) starts below 1 Hz. Observing Figure 14, a clear increase in the stiffness of the 0.2% J300 mixture in the high- and medium-temperature region can be noted compared to the reference mixture. At low temperatures (high frequencies), the stiffness levelled off. These findings correspond to the stiffness testing by the IT-CY method, and this is desirable behavior for the mixture. However, the other mixtures without recycling agent did not confirm the correlation, and when testing the complex modulus, there was an equalization or decrease in stiffness. The mixture of 0.1% J300 can be rated as the worst; in this mixture, a decrease in stiffness occurred from a balanced area of low temperatures with a decreasing load frequency (increasing temperature). The mixture with 0.3% fibers, on the other hand, showed a desirable decrease at high frequencies and an effort to match the reference mixture in the range of high temperatures. The stiffness curve of the mixture with alkali-treated fibers followed the curve of the reference mixture.

In the mixtures with the recycling agent “SR” (Figure 15), a general decrease in stiffness was noted. The exception was the 6%SR mixture without fibers, where a decrease was recorded only in the high temperature area and, conversely, an increase in the low temperature area. This undesirable behavior is also supported by the high value of temperature sensitivity achieved in stiffness testing by the IT-CY method. The mixture J300 6%SR achieved the lowest stiffness in the entire range of frequencies (temperatures). The mixture with NaOH-treated fibers had a similar stiffness to the reference mixture. The master curves of the two latter mixtures demonstrated the same curve as that of the reference mixture.

Figure 16 shows the mixtures with the “RF” recycling agent, which had the greatest decrease in stiffness. The desirable behavior of lower stiffness around high frequencies and an increase in stiffness at low frequencies was noted in the 6%RF_RA mixture. The opposite trend was then noted for the mixture with NaOH-treated fibers. The master curves of the remaining two mixtures followed the shape of the reference mixture’s curve.

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 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].

5. Conclusions

Based on the research results presented in this study, the following conclusions can be drawn:

• The addition of recycling agent and/or jute fibers (virgin and NaOH-treated fibers) has a positive influence on the compactability of asphalt mixture with 30% RA content.
• A directly proportional relationship between jute fiber content and air voids was observed. The shorter, 10 mm fibers caused higher air voids content as well.
• The use of jute fibers in reclaimed asphalt mixtures has been shown to produce a significant decrease in ITSR. Mixture with short fibers achieved the lowest ITSR in the mixtures without recycling agents. The NaOH-treated fibers showed better ITSR values. The addition of the recycling agent “SR” slightly lowered the ITSR. However, the addition of a higher amount of agent “SR” had an opposite effect, although the effect was caused by the lower dry ITS value. On the other hand, the wet/freeze ITS of most mixtures with fibers stayed above the reference level, which can compensate for the low ITSR. The second recycling agent, “RF”, showed the highest ITSR. Interestingly, mixtures with fibers and without recycling agents achieved higher ITSRs than equivalent mixtures with the recycling agents.
• As predicted, the addition of recycling agents tends to decrease the stiffness modulus, while the addition of jute fibers tends to increase it. Furthermore, fibers that underwent NaOH treatment showed an additional increase in their stiffness modulus. Incorporation of jute fibers led to increased stiffness and to a decline in their thermal susceptibility.
• The addition of virgin jute fibers exhibited the same results in fracture toughness and fracture energy to the maximum force at 0 °C as the reference mixtures, with a decrease in total fracture energy. On the other hand, only a slight increase in total fracture energy was demonstrated in mixtures that had NaOH-treated fibers added. The addition of recycling agent led to an increase in selected parameters. Mixtures with fibers and recycling agents exhibited the best fracture resistance results. At higher temperature (25 °C), the surface treatment did not improve the fracture properties. A combination of fibers and recycling agents showed the best results.
• Similarly to the stiffness modulus, the addition of virgin jute fibers showed higher complex values. Yet, treating the fibers with NaOH resulted in a decrease in the complex modulus across the entire range of frequencies and temperatures.
• Although the effects of adding recycling agent to asphalt mixtures are well known, the addition of recycling agents only showed a similar complex modulus along the low-frequency/high-temperature ranges and higher values along the range of high frequencies/low temperatures. Also, the addition of the recycling agent “SR” to treated fibers showed similar values along the entire range. Nevertheless, a decrease in values was achieved in the mixture with virgin fibers.
• Better results were achieved with recycling agents added to RA.