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Article

Development and Evaluation of Vegetable Resin Bio-Binders as Technological Alternatives to Bitumen

by
Manuel De Rose
1,
Rosolino Vaiana
1,*,
Cesare Oliviero Rossi
2 and
Paolino Caputo
2
1
Department of Civil Engineering, University of Calabria, 87036 Rende, Italy
2
Department of Chemistry and Chemical Technologies, University of Calabria, Via P. Bucci, Cubo 14/D, 87036 Rende, Italy
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(6), 2437; https://doi.org/10.3390/su16062437
Submission received: 27 January 2024 / Revised: 12 March 2024 / Accepted: 13 March 2024 / Published: 15 March 2024
(This article belongs to the Special Issue Sustainable Road Survey and Maintenance: New Tools, Technologies and Practices)
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Abstract

:
Recently, the feasibility of using bio-materials to reduce or completely replace the use of bitumen in asphalt mixture has gained increasing attention. Amongst others, an interesting solution is represented by the use of wood co-products with mineral or vegetable oils. This research focuses on the development of bio-binders using vegetable resin (VR) in unmodified form and waste olive oil (WOO) as the main components; in order to optimize the rheological properties of the blends, crumb rubber from end-of-life tyres (CR), Styrene-Butadiene-Styrene (SBS) and polyethylene waxes (PEW) are used as additives. In particular, this investigation focuses on studying different oil/rosin ratios and polymer contents to provide a clear framework on this bio-binder solution; conventional bituminous binders are taken as a reference. The alternative binders are characterized in terms of conventional properties such as penetration depth and softening point, as well as rheological response. Finally, two of the bio-binders studied are selected with the aim of assessing the mechanical properties of the resulting sustainable asphalt mixture using the Marshall Stability test and the Indirect Tensile Strength test, comparing the results with the threshold values set by an Italian road agency. Thus, this research represents a preliminary analysis of the potential application of bio-binder mixtures within the specification limits imposed by road agencies. Although this research represents a first attempt, the results are promising and prove to be worthy of further investigations.

1. Introduction

In the last decades, the European Union (EU) has been adopting strategies to contribute to the reduction of environmental impacts, energy and water consumption, the exploitation of raw materials, and greenhouse gas (GHG) emissions [1]. In 2019, with the European Green Deal [2], the European Union established wanting to achieve carbon neutrality by 2050, and each economic sector was called to give their contribution, including the road pavement industry. To this end, the EU has advocated for the application of technologies and the use of products with a lower environmental impact [3,4]. Consequently, the road construction sector has been moving towards greener and more sustainable production, promoting the use of alternative materials such as wastes or secondary raw materials in asphalt mixtures [5,6], along with the use of warm/cold mix technologies to reduce energy consumption [7,8].
Among the different solutions, one research area that has gained increasing attention consists of the feasibility of using bio-materials with the aim of reducing or completely replacing the use of bituminous binder [9,10], obtaining what is generally termed a bio-binder.
The dependence from bituminous binder can be reduced or totally overcome depending on the bio-binder blending proportion, obtaining three different products: bitumen modifier, when the replacement of bitumen is less than 10%; bitumen extender (25–75% of replacement); and direct alternative, when bitumen is replaced by 75–100% [11]. A fourth approach involves using bio-binders as recycling agents to maximize the use of Reclaimed Asphalt Pavement (RAP) in the mixtures [12,13,14].
The total replacement of asphalt binder remains the most attractive option for researchers. Among the different materials, one of the most feasible solutions is the use of vegetable resin, or wood resin, blended with mineral or vegetable oils [15,16].
Vegetable resins (VRs) are compounds secreted by plants, either spontaneously or through bark incisions [17]. Resins are a natural and renewable feedstock considered to be the oldest natural polymer [18]. Historically, resin has been extensively used as a component of adhesives and sealants, combined with various polymers, showing comparable properties to petroleum products [19,20]. Indeed, VRs have shown potential feasibility for the development of direct alternatives, with the final goal of substituting bituminous binders in the production of asphalt pavements.
The first studies on the possible use of resin for the development of an alternative binder date back to the 2000s. Martínez-Boza et al. [21] investigated the development of a bio-binder made with wood resin, process aromatic oil, and SBS triblock copolymer, demonstrating that the alternative binders exhibit similar physical characteristics to pure bitumen, although they differ from a chemical point of view [20]. Later, Navarro et al. [22] examined the influence of processing variables (temperature and mixing speed) on the mechanical behaviour of these alternative binders, indicating that the binders underwent oxidative processes under atmospheric conditions. Indeed, in their chemical analysis, Castro-Alonso et al. [23] noted that the presence of saturated fatty acids in these types of binders makes them more prone to crystallization, resulting in increased stiffness [24]. However, it was demonstrated that resin-based bio-binders begin to decompose at temperatures higher than 165 °C, whereas, thermal degradation begins around 190 °C [25]. These findings prove that bio-binders remain thermally stable during the mixing and compaction phases.
In addition, the combination of bio-binders and Reclaimed Asphalt (RA) in new asphalt mixtures could represent an ambitious field of research. Indeed, the nature of bio-binder compounds makes them soft or oily materials, which could rejuvenate the old and stiff binder present in RA [24,26].
Jiménez del Barco-Carrión et al. [27] focused on rejuvenating RA in combination with two different bio-binders: one produced by mixing 20% linseed oil and 80% modified pine-rosin; another one, called ‘BioPhalt®’, provided and patented by the EIFFAGE company, made from vegetable oil, residues of the paper industry, and SBS polymers. Both bio-binders showed promising rejuvenating capacities compared to traditional bituminous binders; however, when subjected to short- and long-term aging investigations, these bio-binders experienced an increase in the storage modulus. According to Espinosa et al. [28], for long-term aged bio-binders, the increase in the complex modulus G* is about 3.5 times higher than that of bituminous binder at lower frequencies, and about 2.5 times higher at higher frequencies.
Regarding the implementation of these bio-binders in asphalt mixture, recent researchers investigated the cohesive and adhesive properties of resin-based bio-binders with different aggregate sources, demonstrating better results in comparison to traditional binders and RA binders [29,30].
Despite the potential of resin bio-binders for substituting neat bitumen in asphalt mixtures, few studies have focused on the mixture scale. A recent study examined the mechanical performance of mixtures composed of 50% RAP and ‘BioPhalt®’, confirming that this bio-binder is suitable for performance at a 20-year design ESAL level greater than 10 million but less than 30 million [31]. Porto et al. [32] investigated the feasibility of using re-refined engine oil bottoms blended with several additives, including resin, to produce direct alternative binders. These blends, when used in 100% RAP mixture, exhibited promising Indirect Tensile Strength (ITS) values and lower moisture susceptibility consistent with the limits of the Italian technical specifications.
In a laboratory investigation conducted by Espinosa et al. [33], a bio-binder produced from by-products of pine resin treatment was used as a total binder replacement and compared with a Hot Mix Asphalt (HMA) made with traditional bitumen. The bio-binder mixture showed good values of Marshall Stability, although lower than the control binder, but still higher than the standard requirement, and with similar ITS values. Additionally, complex modulus tests indicated that the bio-binder mixture exhibited a more elastic behaviour at lower temperatures and, therefore, was more prone to cracking, while the laboratory traffic simulation results at a test temperature of 60 °C showed no difference on the permanent deformation of both mixtures.
Finally, it is better to highlight that some of the abovementioned researchers used resin in modified forms through chemical processes such as esterification or hydrogenation [34,35]. Although these modifications result in a more thermally stable product, they require high reaction temperatures and long residence times [34,35,36], which may increase costs and reduce the potential environmental benefits associated with the use of renewable materials, thereby increasing emissions during the product’s manufacturing [37].

2. Objective of the Study

In light of the above analysis of previous studies on the use of VRs, which highlighted the main benefits and drawbacks of using such materials to replace neat bitumen, the main objectives of this research are confined to the following tasks:
  • Task 1: The development of bio-binders composed of VR in unmodified form, such as waste olive oil (no longer suitable for human consumption), and various additives such as crumb rubber from end-of-life tyres, Styrene-Butadiene-Styrene, and polyethylene waxes to investigate different oil/rosin ratios and different polymer contents;
  • Task 2: Assessment of the conventional properties (penetration depth and softening point) and the rheological responses of the developed bio-binders;
  • Task 3: Manufacturing a bio-asphalt mixture with two of the bio-binders studied and investigating the mechanical properties using the Marshall Stability test and the Indirect Tensile Strength test.
  • Task 4: Comparison of the results to the threshold values required for road construction materials set by an Italian road agency technical specification.
Therefore, this research can be identified as a preliminary analysis of the potential application of bio-binder mixtures within the specification limits imposed by road agencies. For completeness, in order to have a holistic view on the topic, an approach related to the environmental sustainability aspects of bio-binders is presented in a prior study [37].

3. Materials and Methods

The rationale behind the manufacturing of alternative binders primarily stems from the inherent composition of bitumen. Specifically, asphaltene, constituting the high molecular weight component, is substituted with natural resin, while viscous oils, derived from mineral or vegetable sources, serve as substitutes for the saturated and aromatic constituents [38,39]. Usually, synthetic or biopolymers are used to optimize the rheological properties of the binders [40].
Following this approach, the formulation used in this research to prepare the bio-binders relies on the use of five main components:
  • Vegetable resin (VR);
  • Styrene-Butadiene-Styrene (SBS) polymer;
  • Waste olive oil (WOO);
  • Polyethylene wax (PEW);
  • Crumb rubber from end-of-life tyres (CR).
The VR is a biosurfactant component, which is used in a higher percentage compared to the others; it imparts the adhesion/binding properties to the blend [41]. Since vegetable resin is solid at intermediate temperatures and exhibits a brittle behavior (high elastic modulus), it is unsuitable for the binder manufacturing on its own. Therefore, WOO is introduced into the blend to provide energy dissipation and a viscous behavior (the loss modulus, G″, increases with increasing WOO content in the system) [26].
However, the resulting system lacks a consistency similar to that of bitumen; thus, SBS polymer and PEW are added to optimize the rheological properties. Specifically, SBS is widely used in modifying traditional bitumen, providing elastic properties to the blend; an increase in SBS content leads to an increase in the storage modulus, G′ [42]. On the other hand, PEW provides consistency to the binder at intermediate temperatures and imparts fluidity at high temperatures (warm-mix effect) during the mixing and compaction phases of the mixture [8]. Finally, the use of CR imparts elastic properties to the system; its use is similar to that of SBS, but being a secondary raw material, it has a lower cost compared to virgin polymer. CR is used extensively in modifying traditional bitumen (rubberized asphalt) and also improves the acoustic properties of the mixtures [43,44].
For comparative purposes, four bitumen were chosen as references, namely a 70/100 and a 50/70 penetration grade bitumen, a soft-polymer modified bitumen, MOD SF (2.5 ÷ 3.5% of polymers), and a hard-polymer modified bitumen, MOD HD (4.0 ÷ 6.0% of polymers) [45].

3.1. Preparation of Bio-Binders

The manufacturing process of the bio-binders involved a preliminary heating phase in which the materials were placed together in a vessel (less the CR) according to the desired formulation, and then placed in the oven at a temperature of 180 ± 5 °C for 40 min. Subsequently, the preheated batch was placed inside a cylindrical heating element in mica, and all the materials were mixed by means of a high-shear mixer (IKA T25 model) at a speed of 5000 rpm, maintaining the temperature at 180 ± 5 °C (see Figure 1). To ensure the proper development of the polymers and homogeneity of the samples, each bio-binder was mixed for 60 min; in particular, during the first 15 min, the CR was gradually added to ensure adequate digestion into the blend.
The investigated formulations are based on previous trial attempts aimed at achieving a bio-binder with a consistency similar to that of traditional bitumen [38], which led to the development of a base formulation. Starting from the latter, different oil/resin ratios (O/R) and different polymer contents were investigated. The recipe for each blend is shown in Table 1, where each component is expressed as a percentage out of 100. It is worth highlighting that three samples of each formulated binder were manufactured to confirm the reliability of the tests and ensure statistical repeatability.
To investigate the effects of CR content, the blend ‘0.35 O/R-4%SBS’ was selected as the base blend for conducting a gradual decrease in the CR content. In particular, the amount of CR removed from the mixture was replaced by the addition of VR, WOO, and PEW according to their weight ratio, whereas the SBS content remained unchanged; the percentages of the materials are shown in Table 2.

3.2. Conventional Measurements

The conventional properties of the binders, such as penetration grade and softening point, provide valuable information about the physical behaviour of the bio-binders analysed. A penetration test (consistency of the binder at 25 °C) and a ring & ball test (softening temperature of the binder) were performed according to the reference standards EN 1426 [46] and EN 1427 [47], respectively.
The penetration and softening point tests are useful to calculate the penetration index (PI), which gives a good approximation on the behaviour of the binders in terms of stiffness and viscosity. Usually, the value of the PI ranges from around −3 to +7, respectively, for bitumen with high- and low-temperature susceptibility [38]. The PI can be calculated from the following Equation (1):
P I = 1952 − 500 · log P E N 25 − 20 · S P 50 · log P E N 25 − S P − 120
in which P E N 25 represents the value of the penetration depth at 25 °C, expressed in 1/10 mm, and S P is the value of the softening temperature of the binder under consideration.

3.3. Rheological Analysis

Rheological tests were conducted on the manufactured alternative binders using a dynamic shear rheometer (SR5000, Rheometric Scientific, Piscataway, NY, USA) in a plate-to-plate configuration with a gap set at 2 mm (see Figure 2). The test temperature was controlled using a Peltier system with an accuracy of ±0.1 °C. Plate tools with a diameter of 25 mm were used for testing in the temperature range of 25–100 °C. To ensure tests were conducted in the linear viscoelastic (LVE) region at all the testing temperatures, an appropriate stress value was determined through stress sweep tests. For all the binders tested, a stress of 1000 kPa was chosen. Dynamic Temperature Ramp (DTR) tests were performed at a frequency of 1 Hz and at a temperature rate of 1 °C/min, from 25 °C to 100 °C, in order to investigate the material phase transition of the blend.
Rheological analyses were conducted on three samples for each blend, obtained from various parts (upper, median, and bottom) of the batch where mixing took place. Comparing the rheological curves obtained from different samples ensures control over the homogenization of the sample.

3.4. Bio-Binder Mixtures and Mechanical Analysis

Two of the investigated bio-binders were selected to assess their potential use in asphalt mixtures, namely ‘0.55O/R-4% SBS’ and ‘0.35 O/R-4% SBS’.
The aggregates and binders were designed in order to produce a traditional wearing course asphalt mixture for urban roads, with 8 mm of nominal maximum aggregate size, according to one of the main Italian road agency technical specifications [45]. For the mixtures, limestone aggregates were selected, and their particle size distribution is presented in Figure 3. The optimum binder content was determined through a Marshall study. Moreover, for comparative purposes, the same mixture was manufactured by using a traditional 50/70 penetration grade bitumen (B50/70).
The characteristics of the aggregates and the binders are reported in Table 3. Specifically, the density of the bio-binders was estimated according to the standard EN 15326 [48].
For each pair of samples, 2600 g of dry aggregates were heated in an oven at a temperature of 160 °C for 3 h, while the alternative binders were heated at 160 °C for a minimum of 1 h. Subsequently, the aggregates and the binders were mixed at a temperature of 160 °C until complete homogenization. The specimens were manufactured using the Marshall Compaction method [8,49]. For each type of mixture and each type of test, four samples were manufactured.
Due to stability issues observed in binders incorporating crumb rubber (wet process) during storage, the blending of the blends was carried out immediately before their utilization within the bio-binder mixture. Indeed, Lo Presti indicates these types of binders as “Field blends” (cf. [44]), precisely because, in light of the abovementioned problems, they are produced directly on site.
Mechanical performance was evaluated through the Marshall Stability test and the Indirect Tensile Strength test according to the EN standards EN 12697-34 and EN 12697-23, respectively [50,51]; the air void content of the specimens was also recorded [52].

4. Results and Discussion

4.1. Conventional Measurement

The results of the penetration test, ring & ball test, and the associated PIs of the bio-binder blends and the traditional bitumen are summarized in Table 4. Since the tests were carried out on three different samples, the results are presented as average values, with the standard deviation (SD) also reported; note that the PIs are calculated on the average values.
By looking at the penetration test results, it is possible to note that the higher the O/R ratio, the more the penetration depth. In addition, a higher percentage of SBS contributes to a decrease in the penetration depth. Overall, the values range between 204 dmm, corresponding to an O/R ratio equal to 0.55 and the lower percentage of SBS, and 78 dmm, for an O/R ratio of 0.35 and 6% of SBS.
Regarding the ring & ball test, it is necessary to highlight that the behavior of the alternative blends differs from that of the bituminous binders. Indeed, in the case of bio-binders, the ball touches the base plate more quickly upon reaching the softening temperature, whereas the bituminous binders slowly deform under the weight of the ball. This fact could be attributable to the difference in ductility between the binders.
Actually, the higher the O/R ratio, the lower the softening point temperature; however, although the trend in the penetration depth values is well defined, it is not possible to define a noticeable relationship between the O/R ratio, the percentage of SBS in the blend, and the softening point values. Softening temperatures for the blends range between 88.3 and 95.5 °C. This fact could probably be attributable to the intrinsic softening temperatures of the blend’s components. Overall, penetration depth values are always higher than the reference bitumen and less than the 70/100 penetration grade bitumen; similarly, softening temperatures are consistently higher than with bitumen.
The results of the penetration tests conducted on the blends align with the findings in the literature. Martinez-Boza et al. [21] observed a decrease in the penetration value with an increase in the O/R ratio. Jiménez del Barco-Carríòn et al. [24] reported a penetration value of 235 dmm for a bio-binder composed of pine resin and linseed oil, comparable to the values obtained for 0.55 O/R; ‘BioPhalt®’ exhibited a penetration of 147 dmm, although its composition is protected by patent, making a direct comparison challenging. Conversely, bio-binders produced by the Brazilian chemical company Quimigel showed penetration values of 21 and 29 dmm [28].
In comparison to previous studies, the softening point values obtained in this experiment were higher. This difference can be attributed to the presence of crumb rubber or PEW; indeed, the latter exhibits a softening point of approximately 120 °C.
As a consequence, the PIs are significantly higher than those of neat bitumen, namely 70/100 and 50/70 penetration grade bitumen; the same can be said for the polymer modified bitumen (MOD SF and MOD HD), although the difference is slightly lower. According to Munera and Ossa [53], blends of bitumen including polyethylene wax, SBS and CR showed PI values ranging between about 3.5 and 7.5 based on the percentages of the polymers; moreover, high percentages of PEW resulted in a PI value ranging between about 4.0 and 6.5.
Additionally, the conventional properties, obtained by an investigation of the different CR content, are reported in Table 5. In this case, a clear relationship between conventional properties and the blend’s formulations is not evident. It could be possible that a change in the structure of the blends occurred when passing from 11% to 5.5% CR. Indeed, although a reduction in the CR content is expected to result in a softer consistency of the binder at 25 °C [54,55], the penetration depth value decreased from 91 to 69. Thus, further investigation into the chemical structures of the proposed blends is warranted. Indeed, this behavior was found when passing from 16.6% to 11%, and from 5.5% to 0% CR. No defined trends were observed for the softening temperatures. Considering the PIs, it is possible to state that the presence of PEW in the blend has a greater influence on the PI in comparison to CR and SBS, since the values are still high.

4.2. Rheological Analysis

The results of the DTR tests are reported in Figure 4, Figure 5 and Figure 6. In particular, the storage modulus, G′, the loss modulus, G″, and the phase angle, tan δ, are displayed separately, and graphs are grouped according to the O/R ratio, showing the differences between different SBS content.
First of all, by considering G′, it is possible to notice that at intermediate temperatures the values are one or even two orders of magnitude higher in comparison to traditional bitumen. This suggest that mixtures including bio-binders are more prone to cracking during their service life. Additionally, G″ is higher than that of neat bitumen, although not as much as for the storage modulus. In particular, bio-binders exhibit a more elastic behavior in the temperature range between 25 °C and 60 °C, while the loss modulus predominates over the storage modulus for temperatures higher than 60 °C; conversely, for traditional bitumen, G″ is always greater than G′. As a consequence of this predominance, the phase angle tan δ is lower than that of the reference bitumen at service temperatures. The phase transition of bio-binders is positioned between the 50/70 penetration grade bitumen and the soft-polymer modified bitumen, MOD SF. In particular, the transition is shifted to higher temperatures with higher SBS content. By looking at the phase angle of all the blends, it is possible to note that just before the transition phase occurs, the curves display a shoulder, possibly attributable to the presence of CR in the blend. By considering both the softening temperature results and the phase transition of each blend, it is evident that bio-binders show a different behaviour in comparison to traditional bitumen. In fact, the softening point of the bio-binders are always higher than the temperature at which the transition phase occurs, whereas the opposite is observed for bitumen.
The observed trends in the rheological curves are consistent with the studies gathered from the literature. Espinosa et al. [28] noted an increase in the value of the complex modulus, G*, at high frequencies (low temperatures) compared to a traditional bitumen with a penetration grade of 30/45. Similarly, in DTR tests conducted in the same range of temperatures (25–100 °C) by Porto et al. [32], blends containing VR exhibited a prevalence of G′ over G″ at intermediate temperatures and up to 50–60 °C.
For what concerns the analysis of the blend with different CR contents, the rheological analysis is plotted in Figure 7. Generally, a reduction in the CR percentages determined a decrease in the storage modulus, G′, and therefore of the blend’s elastic behaviour. Furthermore, it is noted that lower CR concentrations in the blend correspond to a predominance of the loss modulus G″ over the storage modulus G′ at lower service temperatures compared to observations from Figure 4, Figure 5 and Figure 6. Even in this case, the transition phase temperatures are lower than the softening temperature of the bio-binders.

4.3. Bio-Binder Mixtures and Mechanical Performance

In this section, the preliminary assessment of the mechanical properties of the bio-binders’ mixture is discussed. Specifically, the results obtained as average values from four samples are presented in Table 6 and Table 7, along with the standard deviation (SD) of the loads. The air voids of the mixtures are also reported. Furthermore, the tables provide the technical specification limits set by the Italian road agency, along with a commentary regarding exceeding these threshold values [45].
First of all, it is important to emphasize that both the mixtures comply with the specifications in terms of air voids in the specimens. This indicates that despite the high presence of polymers (SBS and CR) the mixtures achieved a good level of compaction at a temperature of 160 °C. In particular, the mixture with the blend ‘0.55 O/R-4% SBS’ achieved a lower value of air voids, albeit still above the specification limits, probably due to the higher presence of WOO.
From Table 6, which shows the results of the Marshall Stability test, it is possible to note that both the mixtures, albeit slightly, meet the specification limits required by the road agency in terms of stability. Furthermore, they do not deviate significantly from the values obtained from the reference mixture B50/70. However, since the flows are low in comparison to traditional bitumen [56,57], the Marshall quotient results in higher values and does not comply with the specifications. Nonetheless, it is noteworthy that the traditional mixture with a 50/70 pen grade bitumen demonstrated high Marshall quotient results in respect to the specification limit.
Table 7 presents the indirect tensile strength obtained by the innovative asphalt mixtures produced with alternative binders. The Italian technical specifications require a minimum ITS value equal to 0.75 or 0.95 MPa for asphalt concrete with unmodified and polymer-modified bitumen, respectively. It is important to clarify that despite the presence of SBS inside the bio-binders’ blend, it should not be intended as a polymer-modified binder, as this component is fundamentally important for the constitution of the blend itself. Thus, the minimum ITS value for asphalt concrete with unmodified bitumen was chosen for comparison.
As it can be seen, the difference with the traditional mixture B50/70 is significant: the mixture ‘0.55 O/R-4% SBS’ did not meet the ITS requirement, whereas the mixture ‘0.35 O/R-4% SBS’ achieved a result slightly above the specifications. For the latter, the higher amount of VR, which imparts the adhesion properties to the blend, results in a favorable ITS value. On the contrary, an increase in the amount of WOO in the blend softens the resulting mixture. However, the “0.55O/R-4%SBS” blend should not be entirely discarded, as previous studies have demonstrated that blends with similar conventional and rheological properties can maximize the utilization of RAP in mixtures up to 50% [31].
By drawing a comparison with a previous study, Espinosa et al. [33] found a Marshall stability value of 8.84 kN for a mixture including VR-based bio-binders (developed by Quimigel), lower than the control mixture; thus, in general, bio-binder mixtures exhibit lower values compared to reference bitumen while still meeting the specifications imposed by road agencies. In contrast, the ITS values were very high (2.28 MPa) in comparison to the ones obtained in this investigation. Porto et al. [32] obtained ITS values ranging between 0.8 and 1.1 MPa when considering VR-based bio-binders mixed with 100% RAP, which are comparable to those achieved by the mixture “0.35O/R-4%SBS”; moreover, it must be considered that the presence of RAP usually tends to stiffen the mixture, supporting the findings of the present investigation. Overall, the results may be considered promising and warrant further investigations.

5. Conclusions

During this study, conventional properties, rheological analysis, and mechanical tests were carried out to preliminarily assess the main properties of innovative binders made from vegetable resins. The following conclusions can be drawn:
  • The alternative binders exhibited higher values of penetration depth and softening point in comparison to traditional bitumen.
  • From a rheological perspective, the bio-binders studied exhibited a comparable behaviour to polymer-modified bitumen to some extent, as they have a transition phase shifted to higher temperatures and high moduli. However, bio-binders showed higher storage modulus at service temperature, which may affect the in-situ properties of the resultant mixtures.
  • Regarding the mechanical characteristics, the results suggest that higher oil/rosin ratios (0.55 O/R) may be better suited for use as a rejuvenator for high percentages of RAP, whereas lower ratios (0.35 O/R) allow for use in traditional mixtures. Thus, the findings encourage further investigation of these alternative binders.
In light of the results obtained, the authors want to remark that despite their potential adhesive and cohesive properties for the manufacturing of sustainable pavements with good mechanical properties, bio-binders should be studied and validated as a product exhibiting a behavior that is inherently similar to bitumen but may not perfectly match with it.
Nevertheless, this research represents a preliminary step for the development of these bio-binders. Thus, additional investigations are needed to fully characterize the rheological response of the binders, such as low-temperature DTRT and master curves, as well as to assess the durability of the binders by investigating short- and long-term aging. The evaluations of the corresponding mixtures from a mechanical standpoint, through modulus and fatigue tests, will be beneficial for completing the investigation.

Author Contributions

Conceptualization, M.D.R. and R.V.; methodology, R.V., P.C. and C.O.R.; software, M.D.R.; investigation, M.D.R. and P.C.; data curation, M.D.R.; writing—original draft preparation, M.D.R.; writing—review and editing, M.D.R., R.V., P.C. and C.O.R.; supervision, R.V. and C.O.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Acknowledgments

The activities of this study are part of the PRIN2017 project “Urban safety, sustainability, and resilience: three paving solutions, four sets of modules, two platforms”, acronym: USR342 (prot. 2017XYM8KC), which is gratefully acknowledged. The authors would like to acknowledge Esther Elisabetta Tridico for her valuable contribution during the preliminary phase of the experimentation. The authors would like to express their gratitude to the company Perretti Petroli S.p.a. for having kindly supplied the bitumen used for comparative purposes in this experimental investigation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. High-shear mixer and heating elements (under a flow hood) of the MaSt-Lab (Road Pavement, Railways and Airfield Materials Laboratory) of the University of Calabria. (1) High-shear mixer; (2) heating element; (3) temperature controller.
Figure 1. High-shear mixer and heating elements (under a flow hood) of the MaSt-Lab (Road Pavement, Railways and Airfield Materials Laboratory) of the University of Calabria. (1) High-shear mixer; (2) heating element; (3) temperature controller.
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Figure 2. Dynamic Shear Rheometer from the PCAM-Laboratory (Physical Chemistry and Applied Materials Laboratory) of the University of Calabria. (1) Main power controller; (2) rheometer; (3) plate-to-plate geometry; (4) Peltier temperature controller system; (5) bio-binder sample.
Figure 2. Dynamic Shear Rheometer from the PCAM-Laboratory (Physical Chemistry and Applied Materials Laboratory) of the University of Calabria. (1) Main power controller; (2) rheometer; (3) plate-to-plate geometry; (4) Peltier temperature controller system; (5) bio-binder sample.
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Figure 3. Particle size distribution of the aggregates composing the bio-binder mixtures.
Figure 3. Particle size distribution of the aggregates composing the bio-binder mixtures.
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Figure 4. DRT test results of the bio-binders studied with a 0.55 O/R ratio. (a) Storage modulus, G′; (b) loss modulus, G″; (c) phase angle, tan δ.
Figure 4. DRT test results of the bio-binders studied with a 0.55 O/R ratio. (a) Storage modulus, G′; (b) loss modulus, G″; (c) phase angle, tan δ.
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Figure 5. DRT test results of the bio-binders studied with a 0.45 O/R ratio. (a) Storage modulus, G′; (b) loss modulus, G″; (c) phase angle, tan δ.
Figure 5. DRT test results of the bio-binders studied with a 0.45 O/R ratio. (a) Storage modulus, G′; (b) loss modulus, G″; (c) phase angle, tan δ.
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Figure 6. DRT test results of the bio-binders studied with a 0.35 O/R ratio. (a) Storage modulus, G′; (b) loss modulus, G″; (c) phase angle, tan δ.
Figure 6. DRT test results of the bio-binders studied with a 0.35 O/R ratio. (a) Storage modulus, G′; (b) loss modulus, G″; (c) phase angle, tan δ.
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Figure 7. DRT test results of the bio-binders studied with different CR contents. (a) Storage modulus, G′; (b) Loss modulus, G″; (c) Phase angle, tan δ.
Figure 7. DRT test results of the bio-binders studied with different CR contents. (a) Storage modulus, G′; (b) Loss modulus, G″; (c) Phase angle, tan δ.
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Table 1. Type and percentage of the materials used for each of the investigated bio-binder.
Table 1. Type and percentage of the materials used for each of the investigated bio-binder.
Blend NameVR [%]WOO [%]PEW [%]SBS [%]CR [%]
Base Formulation4025101015
0.55 O/R-6% SBS43.223.810.86.016.2
0.55 O/R-4% SBS44.124.311.04.016.6
0.55 O/R-2% SBS45.024.811.32.016.9
0.45 O/R-6% SBS46.220.810.86.016.2
0.45 O/R-4% SBS47.221.211.04.016.6
0.45 O/R-2% SBS48.221.611.32.016.9
0.35 O/R-6% SBS49.617.410.86.016.2
0.35 O/R-4% SBS50.717.711.04.016.6
0.35 O/R-2% SBS51.718.111.32.016.9
Table 2. Type and percentage of the materials used for each bio-binder: investigation of the amount of CR in the blend.
Table 2. Type and percentage of the materials used for each bio-binder: investigation of the amount of CR in the blend.
Blend NameVR [%]WOO [%]PEW [%]SBS [%]CR [%]
0.35 O/R-4% SBS-16.6%CR50.717.711.04.016.6
0.35 O/R-4% SBS-11.0%CR54.219.011.84.011.0
0.35 O/R-4% SBS-5.5%CR57.720.212.64.05.5
0.35 O/R-4% SBS-0.0%CR61.321.413.34.00.0
Table 3. Main characteristics of the bio-binder mixtures.
Table 3. Main characteristics of the bio-binder mixtures.
ID MixtureAggregate Bulk Specific Gravity [g/cm3]Bio-Binder Density
[g/cm3]		Percentage of Bio-Binder
on the Aggregate Weight
[%]		Mixture Theoretical
Maximum Density
[g/cm3]
B50/702.8001.0024.802.554
0.55 O/R-4% SBS2.8001.0265.502.568
0.35 O/R-4% SBS2.8001.0325.502.570
Table 4. Summary of the conventional properties achieved by the bio-binders and the traditional bitumen chosen as comparison.
Table 4. Summary of the conventional properties achieved by the bio-binders and the traditional bitumen chosen as comparison.
Blend NamePenetration (dmm)SDPEN (dmm)Softening Point (°C)SDSP
(°C)		IP
0.55 O/R-6% SBS1755.888.30.59.7
0.55 O/R-4% SBS1917.689.10.310.2
0.55 O/R-2% SBS2045.690.00.410.6
0.45 O/R-6% SBS1205.392.30.58.6
0.45 O/R-4% SBS1426.693.20.49.4
0.45 O/R-2% SBS1808.591.20.310.2
0.35 O/R-6% SBS786.594.90.47.4
0.35 O/R-4% SBS843.493.00.57.5
0.35 O/R-2% SBS943.895.50.38.1
BIT 70/100843.348.00.2−0.4
BIT 50/70541.451.20.4−0.7
MOD SF601.570.80.53.5
MOD HD501.678.10.34.1
Table 5. Summary of the conventional properties, obtained from the investigation of the amount of CR in the bio-binders.
Table 5. Summary of the conventional properties, obtained from the investigation of the amount of CR in the bio-binders.
Blend NamePenetration (dmm)SDPEN
(dmm)		Softening Point (°C)SDSP
(°C)		IP
0.35 O/R-4% SBS-16.6%CR843.493.00.57.5
0.35 O/R-4% SBS-11.0%CR913.090.50.37.4
0.35 O/R-4% SBS-5.5%CR691.892.70.36.8
0.35 O/R-4% SBS-0.0%CR732.699.00.37.6
Table 6. Results of the Marshall Stability test for the two bio-binders’ mixtures.
Table 6. Results of the Marshall Stability test for the two bio-binders’ mixtures.
ID MixtureStability
[kN]		SDS
[kN]		Flow
[mm]		Marshall Quotient [kN/mm]Air Voids
[%]
Technical Specification
Thresholds		>10.000--3.0 ÷ 4.53.0 ÷ 6.0
50/7011.8460.5751.8776.3103.94
0.55 O/R-4% SBS10.1200.5091.4607.0103.46
0.35 O/R-4% SBS10.9400.5071.3508.1004.25
Table 7. Results of the Indirect Tensile Strength test for the two bio binders’ mixtures.
Table 7. Results of the Indirect Tensile Strength test for the two bio binders’ mixtures.
ID MixtureLoad
[N]		SDLoad
[N]		ITS
[MPa]		Air Voids
[%]
Technical Specification Thresholds-->0.7203.0 ÷ 6.0
50/7012,570272.21.2653.83
0.55 O/R-4% SBS4780126.80.4753.33
0.35 O/R-4% SBS7906323.00.7854.16
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De Rose, M.; Vaiana, R.; Rossi, C.O.; Caputo, P. Development and Evaluation of Vegetable Resin Bio-Binders as Technological Alternatives to Bitumen. Sustainability 2024, 16, 2437. https://doi.org/10.3390/su16062437

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De Rose M, Vaiana R, Rossi CO, Caputo P. Development and Evaluation of Vegetable Resin Bio-Binders as Technological Alternatives to Bitumen. Sustainability. 2024; 16(6):2437. https://doi.org/10.3390/su16062437

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De Rose, Manuel, Rosolino Vaiana, Cesare Oliviero Rossi, and Paolino Caputo. 2024. "Development and Evaluation of Vegetable Resin Bio-Binders as Technological Alternatives to Bitumen" Sustainability 16, no. 6: 2437. https://doi.org/10.3390/su16062437

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