Physicochemical and rheological properties of a transparent asphalt binder modified with Nano-TiO2

nanomaterials Article Physicochemical and Rheological Properties of a Transparent Asphalt Binder Modified with Nano-TiO2 Iran Rocha Segundo 1,* , Salmon Landi, Jr. 2, Alexandros Margaritis 3, Georgios Pipintakos 3, Elisabete Freitas 1, Cedric Vuye 3, Johan Blom 3, Tom Tytgat 4, Siegfried Denys 4and Joaquim Carneiro 5 1Department of Civil Engineering, University of Minho, 4800-058 Guimarães, Portugal; [email protected] 2Federal Institute Goiano, Rio Verde 75901-970, Brazil; [email protected] 3Energy and Materials in Infrastructure and Buildings (EMIB) Research Group, University of Antwerp, 2020 Antwerp, Belgium; alexandros.mar[email protected] (A.M.); [email protected] (G.P.); [email protected] (C.V.); [email protected] (J.B.) 4Research Group Sustainable Energy, Air and Water Technology, University of Antwerp, 2020 Antwerp, Belgium; [email protected] (T.T.); [email protected] (S.D.) 5Centre of Physics, University of Minho, 4800-058 Guimarães, Portugal; carneir[email protected] *Correspondence: [email protected] Received: 16 September 2020; Accepted: 23 October 2020; Published: 28 October 2020   Abstract: Transparent binder is used to substitute conventional black asphalt binder and to provide light-colored pavements, whereas nano-TiO 2 has the potential to promote photocatalytic and self-cleaning properties. Together, these materials provide multifunction effects and benefits when the pavement is submitted to high solar irradiation. This paper analyzes the physicochemical and rheological properties of a transparent binder modified with 0.5%, 3.0%, 6.0%, and 10.0% nano-TiO 2 and compares it to the transparent base binder and conventional and polymer modified binders (PMB) without nano-TiO 2 . Their penetration, softening point, dynamic viscosity, master curve, black diagram, Linear Amplitude Sweep (LAS), Multiple Stress Creep Recovery (MSCR), and Fourier Transform Infrared Spectroscopy (FTIR) were obtained. The transparent binders (base and modified) seem to be workable considering their viscosity, and exhibited values between the conventional binder and PMB with respect to rutting resistance, penetration, and softening point. They showed similar behavior to the PMB, demonstrating signs of polymer modification. The addition of TiO 2 seemed to reduce fatigue life, except for the 0.5% content. Nevertheless, its addition in high contents increased the rutting resistance. The TiO 2 modification seems to have little effect on the chemical functional indices. The best percentage of TiO 2 was 0.5%, with respect to fatigue, and 10.0% with respect to permanent deformation. Keywords: asphalt binder; transparent binder; nanomaterials; TiO 2 ; viscoelastic properties; FTIR; photocatalytic asphalt; light-colored asphalt; self-cleaning 1. Introduction For specific applications, it is important to control the light absorption and thermal energy storage in asphalt pavements, which can be carried out by the application of light-colored pavements [ 1 ]. Light and heat are essential influencing factors for asphalt pavements. Firstly, it is well known that they are essential keys for the asphalt binder aging, causing damage to asphalt roads [ 2 , 3 ]. The absence of light profoundly affects the visibility conditions, decreasing safety [ 1 ]. In contrast, a large amount of Nanomaterials 2020,10, 2152; doi:10.3390/nano10112152 www.mdpi.com/journal/nanomaterials Nanomaterials 2020,10, 2152 2 of 19 heating can increase the Urban Heat Island (UHI) effect in urban areas [ 4 ]. The conventional black color of asphalt pavements absorbs light and stores a large amount of thermal energy. According to the World Health Organization (WHO), more than 90% of the global population lives in places where the concentrations of pollutants exceed their limits, presenting, as consequences, intensification of the greenhouse effect, acid rain, and public health problems, for example. The indispensable urgency for the reduction of air pollutants is clear from different scales and needs. As such, the introduction of semiconductor nanoparticles into asphalt mixtures can make part of the solutions available to mitigate air-quality problems [5–11]. It is estimated that 40% of urban areas are covered by pavements due to the rapid human development, affecting the local ecosystems and the subjacent surface conditions. Nowadays, another urban problem is the UHI phenomenon, which is the increase of temperatures in cities in comparison to the colder conditions of suburban zones and rural areas, due to the massive development of urbanization [ 4 , 12 ]. Traditional (asphalt) pavements and roofs absorb and store most of the solar energy during the day, which is released in the form of heat during the night. The dark surfaces of, for example, asphalt pavements are characterized by a sunlight reflection up to only 20%. Therefore, light-colored road pavements can be considered a viable technology to tackle this phenomenon. Additionally, these surfaces reduce the heat convection from pavement to air with a consequent decrease of ambient air temperature. Its high reflectivity reduces the overheating during the summer period, resulting in less distress and increased pavement durability [1,3]. A common practice for separating asphalt binder fractions is to fractionate it based on polarity using different solvents. By this method, the following fractions are obtained: asphaltenes, resins, aromatics, and saturates with decreasing polarity order [ 13 ]. To obtain light-colored asphalt pavements, transparent binders can be used. Thus, they are produced through three different processes: (i) bitumen modification based on the extraction of asphaltenes that are responsible for the black color of bitumen; (ii) synthetic binder production by transparent polymer materials; and (iii) blending specific resins with bio-oils or organic vegetal origin materials [ 1 , 3 ]. Even though they are not bituminous materials, their rheological properties are similar to asphalt bitumens [ 1 ]. They can even contribute to electricity cost savings (and reduced pollutant emissions) due to the increased visibility in dark areas, for example, in tunnels, and, consequently, a reduced need for lighting [1]. Hitherto, little research has been focused on the physicochemical and rheological properties of light-colored binders. Additionally, the use of TiO 2 can bring two benefits in this sense besides environmental effects: (i) the functionalization by providing photocatalytic capability can contribute to the environmental remediation; and (ii) development of lighter asphalt mixture, e.g., from dark brown to light yellow, depending on the used granulates, which can mitigate the UHI. The literature presents a small number of studies addressing the use of the transparent binder. For example, Bocci et al. (2012) produced a light-colored asphalt mixture with conventional aggregates, lime filler, light-colored binder, and TiO 2 powder (1% by aggregate weight). The coefficient of reflection related to night visibility and the luminance of this technology were much higher than those from the conventional asphalt mixture [ 1 ]. Bocci E. and Bocci M. (2014) continued their research on this subject, showing that light-colored dense-graded mixtures have similar mechanical properties when compared to the conventional asphalt mixture. They concluded that the light-colored asphalt pavement presented very high photometric properties even after five months from the traffic opening [ 14 ]. Sengoz et al. (2017) investigated the rheological properties of transparent binder in comparison to a traditional black bitumen. They concluded that the transparent and the traditional black bitumen had a similar performance [3]. Nanomaterials 2020,10, 2152 3 of 19 On the basis of a review of the previous literature, there is a gap considering the rheological behavior of transparent binders modified with nano-TiO 2 . The use of these two materials together would combine their benefits into one single product: an asphalt pavement capable to photodegrade pollutants and avoid the UHI. The main objective of this research is to analyze the physicochemical and rheological properties of a transparent binder modified with nano-TiO 2 (0.5%, 3.0%, 6.0%, and 10.0%) for the understanding of its limitations and definition of suitable destinations. For this, its physical (conventional), rheological, and chemical properties were assessed and compared to those of a conventional asphalt binder and a commercial PMB. 2. Materials 2.1. Binders In this research, the transparent binder Kromatis 50/70 from Total (Rives-en-Seine, France) was used. According to the supplier, this light-colored synthetic binder presents properties similar to other bitumens. It is produced with hydrocarbon resins and low content of asphaltenes, which are removed and replaced with new elastomeric polymers [15]. A conventional 50/70 bitumen and a polymer-modified binder (PMB) (SBS-modified bitumen) were also used in this study. These reference binders were named as N50/70 and PMBTS, respectively. 2.2. TiO2Nanoparticles The semiconductor selected to provide multifunctional properties was the nano-TiO 2 by Quimidroga (Aeroxide TiO 2 P25) (Barcelona, Spain). Its main properties are 80% anatase and 20% rutile crystalline phases, purity >99.5%, and particle size about 23 to 28 nm. 2.3. Sample Preparation The nanoparticles were incorporated into the binder (at 150 ◦ C for 30 min in a low shear mixer with a rotational speed of 1500 RPM) with four different contents: 0.5, 3.0, 6.0 and 10.0% (in the mass of the binder) with a similar modification procedure adopted in the literature review [ 10 , 16 – 18 ]. The particles were placed when the low shear mixer was working, homogenizing the binder already heated to 150 ◦ C. All safety precautions were taken, considering personal protective equipment, engineering control (ventilated enclosures), and hygiene, among other things. The samples were named by the modification content: 0.5%, 3.0%, 6.0% and 10.0%. With the introduction of nano-TiO 2 , the color of the binder changes from dark brown (0%) to light yellow (10.0%), see Figure 1. One blend was prepared for each content. Regarding the performed tests, the number of replicates respected the requirements mentioned in the relevant European standard. Nanomaterials 2020,10, 2152 4 of 19 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 19 Figure 1. Schematic representation of this research. 3. Methods Different tests were carried out, such as penetration, softening point, Dynamic Shear Rheometer (DSR) tests (complex modulus, Linear Amplitude Sweep—LAS, Multiple Stress Creep Recovery— MSCR), Dynamic viscosity and Fourier Transform Infrared (FTIR) spectroscopy, in order to determine conventional, rheological and chemical properties. Figure 1 presents the schematic summary of the preparation and characterization methodology adopted in this paper. 3.1. Penetration and Softening Point Penetration and softening point were tested according to EN 1426/2015 and EN 1427/2015, respectively. They indicate the basic properties of asphalt binders. 3.2. Dynamic Viscosity A dynamic viscosity test was carried out following the EN 13302/2010 standard, but only for the transparent binders (with and without nano-TiO2). The objective was to evaluate the workability of the binders according to Superpave specifications. The highest allowed viscosity to respect the workability is 3 × 103 cP (or 3 Pa·s) at 135 °C [19,20]. Figure 1. Schematic representation of this research. 3. Methods Different tests were carried out, such as penetration, softening point, Dynamic Shear Rheometer (DSR) tests (complex modulus, Linear Amplitude Sweep—LAS, Multiple Stress Creep Recovery—MSCR), Dynamic viscosity and Fourier Transform Infrared (FTIR) spectroscopy, in order to determine conventional, rheological and chemical properties. Figure 1presents the schematic summary of the preparation and characterization methodology adopted in this paper. 3.1. Penetration and Softening Point Penetration and softening point were tested according to EN 1426/2015 and EN 1427/2015, respectively. They indicate the basic properties of asphalt binders. 3.2. Dynamic Viscosity A dynamic viscosity test was carried out following the EN 13302/2010 standard, but only for the transparent binders (with and without nano-TiO 2 ). The objective was to evaluate the workability of the binders according to Superpave specifications. The highest allowed viscosity to respect the workability is 3 ×103cP (or 3 Pa·s) at 135 ◦C [19,20]. Nanomaterials 2020,10, 2152 5 of 19 3.3. Viscoelastic Behavior The viscoelastic behavior of the binders was assessed using the Dynamic Shear Rheometer (DSR). The DSR used in this study is an Anton Paar MCR 500 (Graz, Austria). For temperature ranges from 0 ◦ C to +40 ◦ C and from +40 ◦ C to +80 ◦ C, the 8 mm and 25 mm plate geometries were used, accordingly, as described in EN 14770:2012. For each temperature step (increments of 10 ◦ C), frequency sweep tests were performed (0.1–10 Hz) on two replicates per binder sample, within the linear viscoelastic region (LVER) of the binders. The data were further analyzed using the RHEA ™ software (v2.0, Abatech, Blooming Glen, PA, USA) [ 21 ]. The shifting of the data was performed using the Gordon and Shaw procedure [ 22 ]. The phase angle and complex modulus master curves are presented in their original format, without fitting any mathematical or mechanical models. Black diagrams (complex modulus versus phase angle) aim to identify discrepancies of the rheological data, breakdown of time-temperature equivalence, and thermo-rheological simplicity [ 23 ]. A smooth curve indicates time–temperature equivalence, a typical response of unmodified binders. On the other hand, discontinuities indicate the presence of high wax content bitumen, highly polymer modified bitumen, or a highly asphaltene structured binder [ 23 ]. Additionally, it is possible to notice whether there are different dominances when the binder is a composite [ 23 ]. This phenomenon happens, for example, when the complex modulus trend changes with the increase of the phase angle, a phenomenon known as curling. 3.4. Fatigue Resistance (LAS Test) To evaluate the fatigue resistance of bituminous binders, the Linear Amplitude Sweep (LAS) test was performed. This test is an accelerated method that uses the DSR (8 mm parallel plate geometry at 15 ◦ C), which consists of two steps: (i) firstly, a frequency sweep test, and (ii) secondly, an amplitude sweep test, as described in AASHTO TP 101-14. The frequency sweep test (0.2–30 Hz) is used to define the undamaged properties and fatigue law parameters, at a strain level of 0.1%. A linear amplitude sweep test is performed at 10 Hz, and the strain amplitude is linearly increased over 3000 cycles, from 1% to 30%. The Viscoelastic Continuum Damage (VECD) theory is used to determine the parameters Aand Bof the fatigue law (Equation (1)) [ 24 ], in order to determine the fatigue life (Nf). The failure point is determined as the point when the product of the complex modulus (G*) and phase angle (δ) sine is reduced by 35% from its initial value. N f =AγB(1) Both the fatigue curves and the Nf for strain levels ( γ ) equal to 2.5% and 5%, related to a strong and weak pavement [25], respectively, will be presented. 3.5. Rutting Resistance Indicator (MSCR Test) The Multiple Stress Creep Recovery (MSCR) was performed using the DSR together with the 25 mm plate and 1 mm gap, as described in EN 16659:2015. The test was performed at 50 ◦ C, at two different stress levels (0.1 and 3.2 kPa) over ten load cycles. Each cycle consists of 1 s loading followed by 9 s of a recovery period, from which two parameters are obtained: (i) the non-recoverable creep compliance Jnr (Pa −1 ), which is the ratio between the residual strain and the stress applied; and (ii) the recovery R(%), showing proportionally how much strain the sample recovers at the end of the cycle. R(%) can be used to identify the presence of polymer modifications in the asphalt binders. 3.6. FTIR Chemical characterization of binders has received attention in the literature as it can present functional groups related to the crude oil origin, the polymer modification, and the degree of oxidation [ 26 – 31 ]. Since this paper aims to analyze the chemical characteristics of the transparent binder modified with nano-TiO 2 , three approaches were carried out: (i) identification of FTIR peaks; Nanomaterials 2020,10, 2152 6 of 19 (ii) establishment of a possible relationship between the TiO 2 modification level and related chemical groups; and (iii) comparison of standard indices with reference binders used in this study. The Thermo Scientific Nicolet iS10 Fourier Transform Infrared (FTIR) spectrometer (Waltham, MA, USA) is equipped with an Attenuated Total Reflectance (ATR) fixture and a Smart Orbit Sampling Accessory. The average spectra were obtained after the acquisition of the spectra, 32 repetitive scans in the range 400 cm −1 to 4000 cm −1 with a resolution of 4 cm −1 were performed to deliver an average spectrum. A hot droplet of each binder was placed on the crystal, and its respective spectrum was measured. The chemical structure of the binders was analyzed using indices, I, from the obtained FTIR spectrum [ 32 ]. Each I(Equation (2)) is calculated by the ratio of the peak area of the identified band by the total area (Equation (3)) of the spectrum. IFunctional Group =Ai ΣA(2) ΣA=A1700 +A1600 +A1460 +A1376 +A1030 +A864 +A818 +A743 +A724 +A(2953,2923,2862) (3) where Aiis the peak area of the specific functional group. The areas defined by the introduced baselines and the part of the spectrum were calculated using a specific software Origin. Each peak is attributed to a functional group remaining unaffected during service life, but also to groups responsible for aging or polymer presence [ 26 , 28 ]. When it comes to the groups responsible for TiO 2 , echoing [ 33 – 35 ] in the region below 1000 cm −1 , several peaks are ascribed to TiO 2 presence. Previous researchers have demonstrated that the peak around 657 cm −1 is attributed to Ti-O-Ti stretching vibration, whereas the peak around 590 cm −1 is due to the vibration of Ti-O-O. A broader band of wavenumbers was calculated around these peaks in order to capture their increase by elevating the TiO 2 modification level. It should be noted that a horizontal baseline coinciding with the X-axis was used for the calculation of this area. RI (Equation (4)) was calculated in order to check the relative increase of TiO 2 modification. ITI−O+Ti−O−O is the index of each binder, and I TI-O+Ti-O-O0% is the index of the transparent base binder (0%). RI =ITI−O+Ti−O−O−ITI−O+Ti−O−O0% ITI−O+Ti−O−O0% % (4) More specifically, for asphalt binders, the sulfoxide, carbonyl, aromatic, aliphatic, branched aliphatic, long chains, polybutadiene, and polystyrene indices were calculated. Sulfoxide and carbonyl indices are both related to aging. They were calculated considering the following method: (i) aromatic index: A 1600 / Σ A; (ii) aliphatic index: (A 1460 +A 1376 )/ Σ A; (iii) branched index: A 1360 /(A 1460 +A 1376 ); (iv) long chain index: A724/(A1460 +A1376 ); (v) carbonyl index: A 1700 / Σ A; (vi) sulphoxide index: A 1030 / Σ A; (vii) polybutadiene index: A966/ΣA; and (viii) polystyrene index: A699/ΣA. Aromatic, aliphatic, branched aliphatic, and long chains are the structural groups of asphalt binders. Polybutadiene and polystyrene are associated with SBS modified binders. For details concerning the determination of standard indices related to oxidative aging of the binder, the reader is referred to the protocol described in [ 32 ]. Briefly, a common practice in this processing method is to introduce tangential baselines defined by limits around certain peaks [36]. 4. Results and Discussion 4.1. Penetration and Softening Point Figures 2and 3show the resulting penetration and softening point of the studied binders. When compared to the transparent base binder, the inclusion of 0.5%, 3.0% and 6.0% TiO 2 Nanomaterials 2020,10, 2152 7 of 19 nano-modification decreased the penetration from 49 to 47, 45, and 47 × 10 −1 mm, respectively. For 10.0% TiO 2 , it increased to 53 × 10 −1 mm, which is similar to the N50/70 results. When compared to the PMBTS, all the penetration results were higher. Therefore, the penetration values of the Kromatis binder were between the conventional and the PMB binders. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 19 TiO2, it increased to 53 × 10−1 mm, which is similar to the N50/70 results. When compared to the PMBTS, all the penetration results were higher. Therefore, the penetration values of the Kromatis binder were between the conventional and the PMB binders. Figure 2. Penetration results of the binders of this study. Figure 3. Softening point results of the binders of this study. For the softening point, the results of the transparent binders were around 59 °C. The increase of the TiO2 content gradually increased the softening point by about 4 °C. The transparent binders had a softening point again between those of the conventional and the PMB binders. Figure 2. Penetration results of the binders of this study. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 19 TiO2, it increased to 53 × 10−1 mm, which is similar to the N50/70 results. When compared to the PMBTS, all the penetration results were higher. Therefore, the penetration values of the Kromatis binder were between the conventional and the PMB binders. Figure 2. Penetration results of the binders of this study. Figure 3. Softening point results of the binders of this study. For the softening point, the results of the transparent binders were around 59 °C. The increase of the TiO2 content gradually increased the softening point by about 4 °C. The transparent binders had a softening point again between those of the conventional and the PMB binders. Figure 3. Softening point results of the binders of this study. For the softening point, the results of the transparent binders were around 59 ◦ C. The increase of the TiO 2 content gradually increased the softening point by about 4 ◦ C. The transparent binders had a softening point again between those of the conventional and the PMB binders. Comparing these results to those obtained by Sengoz et al. (2017) for the same binder from the same supplier, they showed that the penetration and softening points were 55 × 10 −1 mm and 56 ◦ C [ 3 ]. Thus, in this research, the transparent base binder had a lower penetration but a higher softening point [3]. Nanomaterials 2020,10, 2152 8 of 19 It can be concluded that the transparent binders had results between those of the conventional and the PMB binders, but closer to those of the conventional one. The incorporation of nano-TiO 2 gradually increased the softening point. For the penetration, the results were lower until 6.0%, but higher for 10.0%. 4.2. Dynamic Viscosity The dynamic viscosity, determined only for the transparent binders, is shown in Figure 4. The introduction of nano-TiO 2 increased dynamic viscosity. At 135 ◦ C, the viscosity increased from 1×103 cP (for the base binder) to 2.3 × 10 3 cP (for the 10.0%). Additionally, all the binders had a viscosity lower than 3 × 10 3 cP, considered as the recommended maximum viscosity criterion under Superpave to guarantee proper binder pumping in the asphalt plant during production [ 37 ]. If this value is higher than 3 × 10 3 cP, excessive energy is needed for the mixing and compaction of asphalt mixtures [ 19 ]. It can be concluded that all the modified transparent binders using the contents of nano-TiO 2 studied (from 0 to 10.0%) are feasible with respect to their workability. It is also interesting that the 10.0% TiO 2 appears to have the highest energy requirements as its viscosity is close to the Superpave threshold. Thus, from an economic point of view, it would be favorable to target lower TiO2levels. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 19 Comparing these results to those obtained by Sengoz et al. (2017) for the same binder from the same supplier, they showed that the penetration and softening points were 55 × 10−1 mm and 56 °C [3]. Thus, in this research, the transparent base binder had a lower penetration but a higher softening point [3]. It can be concluded that the transparent binders had results between those of the conventional and the PMB binders, but closer to those of the conventional one. The incorporation of nano-TiO2 gradually increased the softening point. For the penetration, the results were lower until 6.0%, but higher for 10.0%. 4.2. Dynamic Viscosity The dynamic viscosity, determined only for the transparent binders, is shown in Figure 4. The introduction of nano-TiO2 increased dynamic viscosity. At 135 °C, the viscosity increased from 1 × 103 cP (for the base binder) to 2.3 × 103 cP (for the 10.0%). Additionally, all the binders had a viscosity lower than 3 × 103 cP, considered as the recommended maximum viscosity criterion under Superpave to guarantee proper binder pumping in the asphalt plant during production [37]. If this value is higher than 3 × 103 cP, excessive energy is needed for the mixing and compaction of asphalt mixtures [19]. It can be concluded that all the modified transparent binders using the contents of nano-TiO2 studied (from 0 to 10.0%) are feasible with respect to their workability. It is also interesting that the 10.0% TiO2 appears to have the highest energy requirements as its viscosity is close to the Superpave threshold. Thus, from an economic point of view, it would be favorable to target lower TiO2 levels. Figure 4. Dynamic viscosity results of the binders of this study. In addition, the comparison of the results to those from the literature reveals that a higher viscosity was reached (1 × 103 cP). For example, Sengoz et al. (2017) presented 788 cP in dynamic viscosity for the same transparent binder [3]. 4.3. Viscoelastic Behavior The complex modulus and phase angle master curves are presented in Figure 5a,b. The addition of TiO2 slightly alters the viscoelastic behavior of the transparent binder, leading to a simultaneous small increase of modulus and a decrease of the phase angle. The N50/70 shows a simple viscoelastic Figure 4. Dynamic viscosity results of the binders of this study. In addition, the comparison of the results to those from the literature reveals that a higher viscosity was reached (1 × 10 3 cP). For example, Sengoz et al. (2017) presented 788 cP in dynamic viscosity for the same transparent binder [3]. 4.3. Viscoelastic Behavior The complex modulus and phase angle master curves are presented in Figure 5a,b. The addition of TiO 2 slightly alters the viscoelastic behavior of the transparent binder, leading to a simultaneous small increase of modulus and a decrease of the phase angle. The N50/70 shows a simple viscoelastic behavior with the phase angle gradually approaching the viscous asymptote of 90 ◦ at elevated temperatures, a typical response of an unmodified binder. Concerning the complex modulus, the N50/70 shows similar values to the PMBTS at frequencies above 10 Hz. At low frequencies (related to high temperature), Nanomaterials 2020,10, 2152 9 of 19 N50/70 shows the lowest complex modulus compared to other binders, which was an expected observation, since the modulus of those binders is greatly influenced by the polymer modification. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 19 behavior with the phase angle gradually approaching the viscous asymptote of 90° at elevated temperatures, a typical response of an unmodified binder. Concerning the complex modulus, the N50/70 shows similar values to the PMBTS at frequencies above 10 Hz. At low frequencies (related to high temperature), N50/70 shows the lowest complex modulus compared to other binders, which was an expected observation, since the modulus of those binders is greatly influenced by the polymer modification. Figure 5. (a) Complex modulus and (b) phase angle master curves of the binders of this study. Comparing the transparent binder with the PMBTS, it can be seen that the complex modulus is similar at lower frequencies, while the PMBTS demonstrates a lower modulus at frequencies above a ) b) Figure 5. (a) Complex modulus and (b) phase angle master curves of the binders of this study. Comparing the transparent binder with the PMBTS, it can be seen that the complex modulus is similar at lower frequencies, while the PMBTS demonstrates a lower modulus at frequencies above 0.01 Hz. Looking at the phase angle, both reveal the presence of elastomeric modification, which is visible by the drop of the phase angle at a low reduced frequency. For the PMBTS, the dominance of the polymeric phase starts below 1 Hz and shows at a steady plateau stage of 60 ◦ . On the other hand, the dominance of the polymeric phase for the transparent binders starts after 0.01 Hz (which can Nanomaterials 2020,10, 2152 16 of 19 aliphatic indices, of either of the transparent binders. Additionally, the addition of TiO 2 would reduce the fatigue life of asphalt pavements using the transparent binder. On the contrary, it would increase the rutting resistance for high contents. Transparent binders with TiO 2 give promising results, based on their conventional, rheological, and chemical performance. On the one hand, the best percentage for the addition of TiO 2 , based on the results, without compromising the performance of the transparent binder, was 0.5% with respect to fatigue resistance. On the other hand, 10.0% nano-TiO2was the best with respect to permanent deformation. Although the objectives of this research were achieved, it is essential to carry out this analysis on more samples, as limited numbers were used. The next steps of this research are to evaluate the light-colored and photocatalytic pavements considering the properties of color and photocatalysis and analyze the aging performance of the transparent binders with nano-TiO 2 . Another topic that must be assessed is the analysis of the total life cycle cost (including environmental impact/benefits). Author Contributions: Conceptualization, I.R.S., E.F., and C.V.; methodology, I.R.S., S.L.J., E.F., and C.V.; validation, J.B., T.T., S.D., and J.C.; formal analysis, I.R.S., S.L.J., A.M., and G.P.; investigation, I.R.S., E.F., C.V., T.T., S.D., and J.C.; resources, E.F., C.V., J.B., T.T., S.D., and J.C.; data curation, I.R.S., S.L.J., A.M., and G.P.; writing—original draft preparation, I.R.S., A.M., and G.P.; writing—review and editing, S.L.J., E.F., C.V., J.B., T.T., S.D., and J.C.; visualization, I.R.S, E.F., and C.V.; supervision, E.F., C.V., and J.C.; project administration, E.F., C.V., J.C.; funding acquisition, E.F., C.V., J.B., T.T., S.D., and J.C. All authors have read and agreed to the published version of the manuscript. Funding: FCT partially financed this work—Fundaç ã o para a Ci ê ncia e a Tecnologia—under the projects for Strategic Funding UIDB/04650/2020 and UIDB/04029/2020, and Nanobased concepts for Innovative and Eco-sustainable constructive material surfaces PTDC/FIS/120412/2010. Furthermore, we would like to thank the Industrial Research Fund (IOF) of the University of Antwerp for funding the PAPPoA project (IOF/SBO/41859/2020). Lastly, the first author would like to acknowledge FCT for the PhD scholarship (SFRH/BD/137421/2018). Conflicts of Interest: The authors declare no conflict of interest. References 1. Bocci, M.; Grilli, A.; Cardone,F.; Virgili, A. Clear Asphalt Mixture for Wearing Course in Tunnels: Experimental Application in the Province of Bolzano. In Proceedings of the SIIV—5th International Congress—Sustainability of Road Infrastructures, Rome, Italy, 29–31 October 2012; Elsevier B.V.: Rome, Italy, 2012; Volume 53, pp. 115–124. 2. Chen, Z.; Zhang, H.; Zhu, C.; Zhao, B. 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