Introduction

Growing concerns regarding the climate change effect and other environmental troubles have led the world toward developing sustainable infrastructure [1, 2]. One of the promising development methods is incorporating nanomaterials into road pavement, which enhances durability and reduces the maintenance needs of this infrastructure [3]. Since road pavements are crucial for sustainability implications, researchers are exploring the application of nanomaterials to improve the mechanical performance and other properties of pavement materials like asphalt mixtures [4]. Among these nanomaterials, nanotitanium dioxide (TiO2) has shown positive outcomes for enhancing the properties and performance of asphalt mixtures, which make it an eligible nanomaterial for asphalt pavement applications and construction [4, 5].

Nano-TiO2 has also been commonly used in asphalt pavement for degrading vehicle exhaust pollutants [6]. The combustion of fossil fuels in vehicles releases a range of pollutants, which are significant contributors to air pollution and raise serious concerns for global health [7, 8]. Nano-TiO2 unique photocatalytic properties can efficiently degrade the pollutants onto asphalt pavement and improve air quality [9, 10]. Also, the lighter color of nano-TiO2 may increase the UV reflection and lower the pavement temperature, resulting in a cooler pavement surface [11]. This lower temperature of asphalt pavement can lead to mitigating urban heat island effects [12]. These environmental outcomes of nano-TiO2 incorporation into asphalt pavement and its effects on performance enhancement have drawn the attention of the research and construction field [13].

It has been previously discussed in recent related studies that nano-TiO2 can be obtained through environmentally harmful sulfate or chloride processes, and nano-TiO2 has a larger surface, smaller diameter, and lower opacity compared to normal TiO2, making it potentially advantageous for improving the rheological and mechanical performance of modified asphalt binders and mixtures [14,15,16]. Also, the photocatalytic performance of TiO2-modified asphalt pavement through different incorporation methods has been discussed in previous studies [17]. However, several gaps exist in this field, including studying the effects of different nano-TiO2 polymorphs on the properties of the pavement, rheological and mechanical properties of the asphalt binder and mixture, aging resistance, the relationship between chemical modification and asphalt performance after incorporating nano-TiO2, factors affecting nano-TiO2-modified asphalt pavement photocatalytic performance, and cool pavements for urban heat island mitigation. Also, the comparison between nano-TiO2 and typical TiO2 is missing to justify their applications in pavement construction.

To focus on the mentioned gaps and show a better insight, this study aims to give a critical review of the nano-TiO2 application in asphalt pavement and its effect on performance and environmental impacts. Accordingly, an introduction to nano-TiO2 has been given to understand better its chemical characteristics and relationship with the modified asphalt performance. Then, incorporation methods and probable chemical interactions between asphalt binder and nano-TiO2 are discussed. This section is followed by the rheological and mechanical performance review of nano-TiO2-modified asphalt binder and mixture. Lastly, the environmental performance of using nano-TiO2 in asphalt pavement, including photocatalytic performance and urban heat island mitigation, has been evaluated to develop better sustainability. Figure 1 provides a visual overview of the topics presented in this article.

Fig. 1
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Schematic view of the subjects reviewed in this study

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Introduction to TiO2 and nano-TiO2

Titanium is a light metal with a white-metallic color. Pure titanium is not soluble in water, but it can be dissolved in concentrated acids [18]. TiO2 is the most stable oxide of titanium [19]. Moreover, TiO2 elemental composition is 59.95% titanium and 40.05% oxygen [20]. The distinctive properties of TiO2 can be directly attributed to its polymorphic form, which, in turn, is mainly dependent on the preparation method and post-fabrication heat treatment [21]. Titanium dioxide naturally exists in four polymorphic forms, including anatase, rutile, brookite, and the most uncommon one, TiO2 –B [22].

Figure 2 shows the crystal structure of TiO2 polymorphs and field emission scanning electron microscopy (FE-SEM) image of anatase and brookite. Both rutile and anatase have tetragonal crystal structures, while brookite has an orthorhombic one [23]. Rutile is the most thermally stable phase, and other phases will transform to rutile by heating, while brookite is the least stable polymorph of TiO2 and is difficult to synthesize [24]. Anatase has a higher surface area compared to rutile due to its more open crystal structure and higher number of exposed surface sites [25]. The FE-SEM images are used to assess the distribution and dispersal of different compounds and changes in the surface morphology of the samples [26]. For nano-TiO2 samples, they show the presence of nanocrystalline domains in rutile and dense nanocrystalline at the surface of anatase, but the surface of rutile is relatively smoother than that of anatase [27].

Fig. 2
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a Crystal structures of TiO2 polymorphs [25] b FE-SEM images of anatase (A–C) and rutile TiO2 (D–F) at different magnifications [27]

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Thus, it can be predicted from FE-SEM images that anatase can have better photocatalytic performance than rutile. Additionally, due to the higher surface area and reactivity of anatase, which can allow it to form stronger bonds with asphalt molecules, it may be better for improving asphalt rheological and mechanical properties. However, due to the higher thermal and chemical stability of rutile, the optimum proportions of nano-TiO2 polymorphs for optimum rheological, mechanical, and photocatalytic performance should be evaluated.

Nano-TiO2 is a form of TiO2 with nanometer-ranged particle size (typically less than 100 nm) [28]. This size reduction leads to increased surface area and enhanced reactivity [29, 30]. These features result in an improvement in asphalt pavement properties, including mechanical and photocatalytic performance [31, 32]. However, the smaller particles raise concerns about the environmental impacts of utilizing and disposing of this material [33]. The focus of this study is on nano-TiO2 incorporation in asphalt pavement and its comparison with typical TiO2.

Incorporating TiO2 particles into asphalt materials

Application methods

The methods of nano-TiO2 application in asphalt pavement are critical in determining the effectiveness of the modified pavement [34]. Generally, these incorporation methods can be divided into direct mixing method (i.e., binder modification) and pavement surface applications (coating method and spraying method) [34, 35]. Figure 3 shows the different incorporation methods of TiO2 and nano-TiO2 into asphalt pavement in both laboratory and field areas. It should be noted that the figures for field incorporation methods are related to typical TiO2, and nano-TiO2 field incorporations need more advanced techniques.

Fig. 3
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Methods of incorporating TiO2 and nano-TiO2 into asphalt samples and pavement [35,36,37]

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For the surface spraying method, pretreatment of the pavement surface is important for preparing the surface for better spraying efficiency [38, 39]. One of the most important parts of the spraying method is the solvent of TiO2, which makes it possible to spray. Liquid solvents can directly solve the TiO2 particles and be used as spraying emulsions, while solid solvents should be used with water for spraying [40,41,42]. Post-caring and treatment of the surface is mainly optional and is mostly related to the case.

For the coating method, the procedures are more complicated and require strong solutions to ensure the fluidity and dispersion of nano-TiO2 coating [43, 44]. It has been previously studied that the coating photocatalytic performance significantly improves with the increase of nano-TiO2 content and spraying amount up to 8% and 400 g/m2, respectively, and the recommended maximum spraying amount is 550 g/m2 to maintain skid resistance because excessive dosage thickens the oil membrane, reduces skid resistance, and is unsafe and uneconomical [35].

However, based on the literature review, the surface spraying and coating methods are not well and clearly distinguished in the studies. Although in this study these two methods are separated, some may find these methods similar, especially in field application. However, the focus of this study is to evaluate the effect of nano-TiO2 addition to asphalt binder on the chemical, rheological, and mechanical properties, as well as its environmental comparison with surface application of nano-TiO2 in asphalt pavement.

Chemical or physical interactions of asphalt binder with TiO2 and nano-TiO2

The addition of TiO2 and nano-TiO2 to asphalt binders as modifiers may change the functional group percentages in the asphalt matrix. Thus, studying the chemical behavior and modification mechanism of incorporating these materials into asphalt pavements is necessary. Fourier transform infrared spectrum (FTIR) is a method to evaluate chemical changes and performance evaluation in materials, especially asphalt binders [45,46,47]. Also, it can be used to examine the aging phenomenon of asphalt materials, which is due to the chemical changes in the asphalt matrix [48].

By the addition of 1–5% TiO2 (80% anatase, 20% rutile) and conducting FTIR on modified asphalt samples, it was indicated that increasing TiO2 content results in increasing absorbance in the wavelengths of below 700 1/cm [49]. Another study, in which FTIR was conducted on the UV-aged samples of the addition of 5% TiO2 (80% anatase, 20% rutile) to asphalt binder, showed more ester carboxyl functional groups, which showed more progress in aging, and it was shown that there were some significant peaks at below 700 1/cm wavelengths in the spectrum [39]. In another study, FTIR was conducted on the addition of 0.5–10% nano-TiO2 (80% anatase, 20% rutile) to transparent asphalt samples, and it was shown that the peaks in the spectrum were about 1300 1/cm and below 1000 1/cm wavelength, which shows an increase in aliphatic groups, and no long chains were observed [50].

It should be noted that the peaks beneath 1000 1/cm wavelength can correspond to the polyaromatic groups [47]. Besides, asphaltenes are polyaromatic and heavy compounds in oil reservoirs and asphalt binders [51]. Thus, it can be concluded that the peaks in the spectrums may correspond to more asphaltene content by the addition of TiO2 to asphalt binder.

Figure 4 shows spectrums of nano-TiO2-modified binders (binder 50/70) with different nano-TiO2 content, which indicate no chemical alteration but more absorbance in the wavelengths of between 2800 and 3000, 1200 and 1600, and beneath 1000 1/cm. It has been indicated that differences at wavenumbers of 3000–4000 and 700–900 1/cm can be referred to asphaltenes [52]. More asphaltene content leads to more stiffness and elasticity of the asphalt binder and decreases the high-temperature susceptibility [53]. Also, saturate–aromatic–resin–asphaltene (SARA) analysis on the addition of 5% TiO2 to asphalt binder showed an increase in asphaltene and resin and a reduction in aromatic content [39].

Fig. 4
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FTIR spectrum of nano-TiO2-modified asphalt binder [54]

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Findings may indicate that the TiO2 and nano-TiO2 modification of asphalt binder is related to physical changes, and no new functional groups are created. However, the exact effect of nano-TiO2 on the rheological and mechanical properties of asphalt materials has to be examined.

Mechanical and rheological performance of nano-TiO2-modified asphalt binder and mixture

Fatigue resistance

Repeated vehicular loads contribute to the most common pavement cracking type, fatigue cracking [55]. Nano-TiO2 can form strong interfacial bonding with the asphalt binder molecules, enhancing adhesion between the binder and aggregates [56]. This effect leads to a more durable asphalt pavement capable of withstanding repetitive stress and strain from traffic loading [57]. Table 1 reviews the fatigue resistance of asphalt binders and mixtures after the addition of nano-TiO2. It has been indicated that in almost all cases, the addition of nano-TiO2 can have an enhancing influence on the fatigue lives of conventional binders and mixtures. Also, the addition of other materials like nano-SiO2, multiwalled carbon nanotube (MWCNT), and CaCO3, along with nano-TiO2, can heighten the enhancement of fatigue life caused by this nanomaterial [58,59,60].

Table 1 Fatigue resistance of nano-TiO2-modified asphalt binders and mixtures
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High-temperature performance

Asphalt pavements are constantly exposed to high temperatures, which have recently increased due to the effects of climate change [68, 69]. Thus, it is necessary for binders and mixtures to have great performance against rutting [70]. Incorporated in asphalt pavement, nano-TiO2 can act as a thermal barrier, reflecting a significant amount of solar radiation, reducing the absorption of heat by the pavement surface, and improving rutting resistance [11]. Also, nano-TiO2 high surface area and reactivity lead to increased viscosity and stiffness of the binder at high temperatures, reducing the potential for rutting and permanent deformation [67]. Table 2 reviews the studies related to the high-temperature performance and rutting resistance of nano-TiO2-modified asphalt binders and mixtures. It can be concluded that regardless of binder type, the addition of nano-TiO2 as a modifier leads to improving rutting resistance. These results are compatible with the results taken from the chemical interaction of binder and nano-TiO2, which more stiffness and asphaltene content, signs of better rutting resistance, were concluded after examining chemical experiments.

Table 2 Rutting resistance of nano-TiO2-modified asphalt binders and mixtures
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It should also be noted that using nano-TiO2 along with other materials as binder modifiers may change the performance of the pavement. By incorporating 1–7% nano-TiO2 and 0.4–2.8% nano-SiO2 in a 60/70 binder, it was shown that the incorporation of these nanomaterials results in a higher complex modulus, lower phase angle, and lower permanent deformation [60]. In another study, a combination of 1% organic expanded vermiculite (OEVMT) and organic montmorillonite (OMMT) with 2% nano-TiO2 resulted in better rutting resistance for the binders [71]. Also, using nano-TiO2/CaCO3 resulted in more rutting factor and higher rutting resistance [59]. The incorporation of TiO2 and thermochromic (TC) powder met the maximum rutting depth by conducting Asphalt Pavement Analyzer (APA) rutting test [72]. Also, the addition of 1, 2, and 3% nano-TiO2 along with 2, 4, and 6% thermoplastic polyurethane (TPU) to asphalt binder 80/100 showed better high-temperature performance than the base binder, and TiO2 had a leading effect [73]. The combination of TiO2, ZnO, and basalt fibers as modifiers in asphalt binder led to better resistance to permanent deformation, and nanoparticles also could compensate for the poor cracking resistance of the binder [74].

Low-temperature performance

Low-temperature cracking of asphalt pavement is the main distress of these pavements in cold regions [81]. Table 3 reviews the outcome of nano-TiO2 addition on low-temperature cracking resistance of asphalt binders and mixtures. As it is shown, nano-TiO2 addition increases asphalt viscosity and decreases its cracking resistance. However, all the samples met the standard requirements for cracking resistance, which shows that this nanomodification can provide acceptable low-temperature performance for asphalt samples.

Table 3 Low-temperature cracking resistance of nano-TiO2-modified asphalt binders and mixtures
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Aging resistance

Asphalt pavements are exposed to oxidation and environmental impacts like ultraviolet (UV) rays, which result in asphalt aging. Decrease in durability and increasing potential of cracking due to more stiffness and brittleness are the results of asphalt aging [85]. Evaluating the aging resistance and behavior of these nanomodified binders is necessary for improving pavement performance. Table 4 reviews the related studies and shows that incorporating nano-TiO2 in asphalt binders and mixtures leads to better aging resistance. Studies have shown that the lower aging resistance has an adverse effect on the fatigue cracking of the asphalt mixtures [86]. Thus, better aging resistance to nano-TiO2-modified mixtures can lead to better fatigue resistance, which is in line with the results in the fatigue performance evaluation section.

Table 4 Aging resistance of nano-TiO2-modified asphalt binders and mixtures
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It should be mentioned that the increase in the amount of TiO2 nanoparticles, when used alone, has a better effect on aging resistance than using nanoparticles along with microparticles [87]. Nano-TiO2 particles have a higher surface area and result in better adhesion and mechanical performance in the mixtures, which leads to better aging resistance [60]. Also, the higher surface area may lead to better and more evenly dispersion of the particles in the binder, but the potential agglomeration probability should also be considered [88]. In addition, using nano-TiO2 can lead to more efficient photocatalytic and cool pavement performance, which is discussed in the latter sections. However, the chemical mechanism and exact difference of nano-TiO2 and micro-TiO2 effects on asphalt aging are areas for further investigation.

Rather than directly mixing with asphalt binder, TiO2 can be used as a coating agent or surface spraying on the asphalt pavement. By adding 10, 25, and 50% TiO2 as coating agents, atomic force microscopy (AFM) results showed that the modified binder had a smoother surface and less aging process [89]. Also, using 4 g/L of nano-TiO2 as surface spraying showed better aging resistance for asphalt samples [87]. It has also been indicated that nano-TiO2 can fill the microscopic defects in asphalt binders and lead to a more erosion and aging-resistant asphalt pavement surface [90].

Moisture susceptibility

Water penetration into the pavement layers has negative impacts on the pavement performance [96]. As mentioned before, nano-TiO2 leads to better adhesion between the asphalt binder and aggregate particles [56]. By increasing this adhesion, the asphalt mixture can tolerate more freeze and thaw cycles and shows better moisture susceptibility [97, 98]. Also, due to the higher solubility of nano-TiO2 in water than in asphalt binder, the moisture susceptibility of nano-TiO2-modified asphalt mixtures can be ameliorated [99]. It has also been mentioned before that using nano-TiO2 can reduce the effects of aging factors, which water penetration may be mentioned as one, on the asphalt pavement. It should also be mentioned that although nano-TiO2 has a hydrophilic nature, it can be modified to hydrophobic, which may be used to enhance the moisture susceptibility of the modified or coated pavement [100, 101].

Table 5 reviews the moisture susceptibility of nano-TiO2-modified asphalt mixtures. It is shown that by incorporating nano-TiO2 into asphalt pavement, the Tensile Strength Ratio (TSR) of the mixtures is increased, which shows more durability and better water stability. In another study, it was shown that the addition of 1, 3, and 5% nano-TiO2 to semi-warm asphalt mixtures can lead to more viscosity and, therefore, more adhesion, which leads to better moisture susceptibility (about 16% enhancement). It was also found that this modified asphalt mixture shows a minimum of 75% in terms of Resilient Modulus Ratio (RMR) [102].

Table 5 Moisture susceptibility of nano-TiO2-modified asphalt mixtures
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Optimum dosage

Based on the literature review, the evaluation approach of nano-TiO2 in asphalt pavement in studies can be divided into two forms: evaluation of modified binder with binder tests and evaluation of mixture made by the modified binder with mixture tests. According to the results of fatigue performance, the suggested optimum dosage of nano-TiO2 addition is 5% of the weight of binders for binder performance and 3% of the weight of binder for mixture performance, which is congruous with the results of some other studies [66, 67]. It can be seen that the results for binders and mixtures do not match exactly, which shows that the binder tests are not the definitive predictive parameters for asphalt mixture performance. For high-temperature performance, the optimum dosage of nano-TiO2 is suggested to be 5% for both binder and mixture performance, which is consistent with other studies [77, 80]. For low-temperature performance, aging resistance, and moisture susceptibility, there has not been a clear result for the optimum dosage, but the suggested optimum dosage for other performance criteria (fatigue and high-temperature performance) can meet the minimum and improvement for these parameters, but more research is needed to justify the application. Thus, the literature review shows that the optimum dosage of nano-TiO2 for binder modification to improve the rheological and mechanical performance of asphalt binder and mixture may be around 5% of the weight of the binder. However, this dosage can be vary in different projects due to the type of binder, aggregates, field conditions, and more importantly, the environmental impacts.

Environmental impacts of incorporating nano-TiO2 into asphalt pavement

Nano-TiO2-modified asphalt pavements are credited with many environmental benefits, such as purifying exhaust emission, mitigating the heat island effect, and reducing haze as well as noise [104, 105]. Therefore, the promising environmental benefits of nano-TiO2, as well as the performance of the asphalt pavement, should be considered to accentuate its application. Photocatalytic performance and mitigating urban heat island effect, as two important benefits of using TiO2 and nano-TiO2 in asphalt pavement, are evaluated in the following sections.

Photocatalytic performance

Asphalt pavements are constantly exposed to vehicle exhaust pollutants. A suitable method to degrade these pollutants, which has become popular recently, is using TiO2 in asphalt pavements. TiO2 is a semiconductor material. Accordingly, in terms of solid-state physics, there is a large band gap equal to 3.2 eV for the anatase phase of TiO2 and 3.02 eV for the rutile phase of TiO2 between the conduction band (vacant band) and the valence band (filled with electrons). Due to this band gap, electrons in the valence band cannot move to the conduction band; however, in photocatalysts, light is the decisive factor in helping electrons to be excited to the conduction band (photoexcitation). TiO2 particles applied to surfaces such as roads are highly exposed to UV rays, which contain photons. When TiO2 absorbs a photon that encompasses the energy greater than or equal to its band gap, 3.2 eV or 3.02 eV, it will result in a process in which electrons from the valence band can be excited to the conduction band (e−). In other words, this process will bring about electron–hole pairs (h+) in the valence band [106]. Thereupon, these electrons (e−) and holes (h+) react with oxygen (O2) and water (H2O), respectively, to produce active superoxide anion (O2−) and hydroxyl radical (OH*); this is because holes and electrons are powerful oxidizing and reducing agents in the process. Furthermore, the superoxide anion will react with H+ separated from the water to generate the HO2* radical. Finally, hydroxyl radical (OH*) and HO2* react with organic air pollutants such as NO and NO2, resulting in water-soluble nitric acid (HNO3), which can be later washed away by rainwater or street sprinkling [42, 107]. Nano-TiO2 also shows promising photocatalytic performance, which can be used for degrading pollutants in the pavements. Figure 5 shows an illustration of the photocatalytic performance of nano-TiO2 addition in asphalt pavement.

Fig. 5
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A schematic view of the photocatalytic performance of incorporating nano-TiO2 into asphalt pavement

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Also, studies show that the TiO2 particles can lead to CO2 reduction photocatalytic performance, as UV light on TiO2 catalysts produces separated electrons and holes, reacting with water and CO2 to form oxygen, H radicals, CO, methane, methanol, and hydrogen, which lead to better efficiency when smaller particles are used [108]. Thus, the application of TiO2 nanoparticles in asphalt pavement can show promising outcomes in reducing air pollution.

Several studies have scrutinized the photocatalytic performance of applying TiO2 and nano-TiO2 in asphalt pavements, which leads to pollutant degradation. Accordingly, a brief review of the studies related to photocatalytic pavements regarding the TiO2 and nano-TiO2 incorporation method is presented in Tables 6, 7, and 8. It is shown that different incorporation methods have alternative impacts on photocatalytic performance. This may create a challenge that whether binder modification or surface application can meet the optimum performance and environmental criteria. Also, it is shown that using nano-TiO2 shows better photocatalytic efficiency comparing typical TiO2. Moreover, materials like carbon, cerium, nitrogen, lanthanum, and Fe3+ are used as doping agents and CeO2, steel slag, montmorillonite, AL2O3, rubber, rejuvenators, cationic surfactant, g-C3N4, polystyrene, activated carbon, pyrite, specularite, glass beads, polystyrene, Fe2O3, CeO2, and WO3 are used as additives to enhance photocatalytic performance.

Table 6 Photocatalytic performance of direct mixing incorporation of TiO2 and nano-TiO2 into asphalt pavement
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Table 7 Photocatalytic performance of surface spraying incorporation of TiO2 and nano-TiO2 into asphalt pavement
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Table 8 Photocatalytic performance of coating incorporation of TiO2 and nano-TiO2 into asphalt pavement
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However, the optimum proportion and dosage of nano-TiO2 for the photocatalytic performance of asphalt pavement needs more research. For micro-surfacing applications, research indicated that the optimum dosage could be 11% of the weight of the binder, showing 40% NOx absorption, as well as acceptable performance [109].

There are several factors that affect the degradation efficiency of the pavements, which are shown in Table 9. It should be noted that the reasons for better photocatalytic performance of anatase compared to other TiO2 polymorphs can be as follows: larger band gap, differences in direct and indirect band gap, and better excitons mobility [134].

Table 9 Factors affecting the photocatalytic performance of asphalt pavements
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Also, studies have shown that the smaller TiO2 particles can lead to better photocatalytic efficiency. In nano-TiO2 epoxy emulsified asphalt mixture, the NO-degradation rate is increased from nearly 40% to 70% when particle size is reduced from 10–15 to 5 nm, but the change in CO2 degradation is not significant [116]. For CO2 reduction, laboratory studies have shown the optimum nano-TiO2 particle size can be 14 nm, but using larger or smaller particle sizes than 14 nm can decrease the CO2 reduction [108]. The controversial results may be due to the difference in nano-TiO2 samples in the laboratory. It should be noted that although particle size reduction can be effective on photocatalytic performance, other parameters like surface area, voids, band gap energy, and other physical characteristics should also be considered because they can have more influence on photocatalytic efficiency [135]. Thus, the optimum particle size of nano-TiO2 is an area of further investigation.

All in all, by considering these factors and using an appropriate dosage of materials, the highest efficiency and better photocatalytic performance can be achieved.

Urban heat island mitigation (cool pavement)

There has been a drastic increase in the world population in recent years, resulting in new megacities and existing ones becoming more populated. This has led to the emergence of the urban heat island (UHI) phenomenon, where anthropogenic heat, the blockage effect against urban ventilation, and the implementation of artificial materials result in warmer climatic conditions [139, 140]. Figure 6 shows the schematic view of UHI.

Fig. 6
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Urban heat island profile

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Cool pavements have been introduced for their capability of reducing the pavement surface temperature and mitigating UHI, and they have been categorized into three types: reflective, evaporative, and heat-storage-modified pavements [141, 142]. Reflective technologies such as reflective coating, light-colored pavements, and thermochromic materials are considered suitable strategies to reduce the negative effects of UHI on roads [143, 144]. Also, nanomodifications have been proposed for lowering the pavement temperature and mitigating UHI effects [145, 146]. Accordingly, TiO2 particles can help reduce the UHI effect in urban areas, as they increase the reflectivity and reduce the temperature of materials, including asphalt coatings [147, 148]. Figure 7 shows the beneficial effects of using nano-TiO2 in asphalt pavement regarding pavement cooling.

Fig. 7
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Using nano-TiO2 in asphalt pavement for cool pavements

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The mechanism behind pavement cooling by nano-TiO2 particles can be attributed to higher reflectivity and better thermal conductivity. On the one hand, TiO2 particles absorb light at wavelengths of 275–405 nm and reflect light due to their high refractive index (n = 2.6142 at wavelength = 587.6 nm), which allows them to be used in sunscreens and photography applications, as well as pavement coatings for cool pavements [149, 150]. On the other hand, TiO2 particles have, on average, a higher thermal conductivity than the typical asphalt mixtures [151, 152]. This may lead to better thermal conductivity of nano-TiO2-modified asphalt pavement. However, the majority of studies have focused on the reflective properties, and the thermal conductivity may need further investigation.

A study showed 4–5 °C temperature reduction for asphalt samples and 8–10 °C for binders at the top surface of pigment-modified samples. Also, pigmented mixtures take 25–30% less time to cool down, proving their greater efficiency in heat dissipation. Moreover, red and white pigment-modified asphalt mixtures exhibit decreased rut depth of 35% and 15%, respectively, as compared to typical asphalt mixtures [151].

Furthermore, a study found that the improved thermal behavior of the nanomodified asphalt material could be ascribed to physical modifications that resulted in smoother and lighter-colored surfaces, leading to lower daily surface temperatures and a reduction of the UHI impact of the asphalt [153].

Also, a study found that the green coating with 15% titanium dioxide and 10% floating beads had the best cooling performance, and higher dosages of the coating resulted in better cooling effects [154]. Additionally, by adding 1% TC powder and 3% nano-TiO2 as the fundaments of the thermochromic asphalt mixture, it was found that this addition leads to a reduction in surface temperature of up to 15 °C [72].

In another study, it was concluded that the use of nano-TiO2 in asphalt can potentially increase its albedo and reduce pavement temperatures to address the UHI effect. However, micro-TiO2 may have better reflectance than nano-TiO2 and nano-ZnO [155]. Also, it was found that using 5–30% TiO2 quantum dots in asphalt coatings can lead to a 12–17% increase in solar reflectivity compared to 3% for conventional samples [156].

It should be noted that although nano-TiO2 can be effective in mitigating UHI effects, more research is needed to clarify and justify its application. The effect on TiO2 particle size and incorporation method in pavement cooling efficiency, the improvement of reflectivity and thermal conductivity of nano-TiO2-modified asphalt pavement by the addition of other reflective and conductive materials, and the optimum proportions on nano-TiO2 particles need to be examined to reach better pavement cooling and in result, better sustainability.

Critical discussion and potential downsides

This literature review has focused on the application of nano-TiO2 in asphalt pavement and its effects on the pavement performance. Regarding the application methods, direct mixing, surface spraying, and coating can be used to apply the nano-TiO2 particles in asphalt pavement. The literature review has shown that by using nano-TiO2 as an asphalt binder modifier, the high-temperature performance, fatigue resistance, aging resistance, and moisture susceptibility of the modified binder and mixture are improved. Also, nano-TiO2 can lead to pollutant degradation by photocatalytic performance and urban heat island mitigation by cool pavement performance. However, there are challenges that need to be critically examined to investigate the viability and potential downsides of using nano-TiO2 in asphalt pavement.

Although studies have shown improvements in rheological and mechanical properties of asphalt pavement by using nano-TiO2, the usage of nano-TiO2 is expected to also improve the photocatalytic and cool pavement performance in order to reach better sustainability. Thus, the selection of nano-TiO2 optimum incorporation method (binder modification, surface application, or a combination of both) is a challenge, leading to future investigation. Also, due to the lack of related studies, there need to be research regarding the optimization in mixing (mixing speed, temperature, and time) and spraying (dosage and procedures) and the optimum dosage of nano-TiO2 for maintaining performance criteria, environmental impacts, and long-term performance of the modified asphalt mixture.

Also, the fracture mechanics of the nano-TiO2-modified asphalt mixtures should be considered. As mentioned before, nano-TiO2 can lead to better rutting and fatigue resistance but has a low impact on the low-temperature cracking resistance. Thus, in order to clarify the fracture mechanics of the modified pavement, studying mode I, mode II, and mixed mode I/II is recommended. Related studies have shown that by 0.9% addition of nano-TiO2, the fracture mechanics of the asphalt mixture in both vertical and angular cracks are improved, and the toughness of the mixture is increased [84].

Additionally, the effect on skid resistance of asphalt pavement after nano-TiO2 addition is an area of concern. Accordingly, it has been indicated that with the increase of spraying and coating amount of nano-TiO2, there is a significant reduction in skid resistance (in terms of reduction in the textural depth and friction coefficient), which can lead to lower driving safety and higher accident rate (halving the skid resistance leads to doubling the accident rate) [35, 44, 157]. For controlling the skid resistance, different amounts of nano-TiO2 have been proposed, from 350 to 550 g/m2, to control the textural depth from 0.55 to 1.4 mm, respectively [35, 44]. Therefore, there needs to be more research on the skid resistance of asphalt pavements modified with nano-TiO2, mainly due to the different standards for textural depth and the effect of characteristics of nano-TiO2 particles and the mixture properties on the skid resistance.

The field applications of nano-TiO2 in asphalt pavement construction introduce practical challenges. The photocatalytic efficiency and properties of nano-TiO2 field applications have been discussed before, but some challenges still remain. Using N-doped nano-TiO2 on a selected field road to evaluate its durability when used as photocatalytic coating has shown that the photocatalytic coating can maintain its performance for approximately 13 months, which is caused by affecting and removal of the coating due to the traffic and rain [122]. In another field study in Germany, the TiO2 particles were applied on an epoxy resin layer, which was coated on a selected test road, and the sample was cored and extracted from the pavement and then tested, which showed remarkable photocatalytic performance [131].

Although the field results demonstrate appropriate photocatalytic performance, the field conditions, including severe traffic loading repetitions and runoff due to rain, can affect the photocatalytic efficiency and pavement life span. The abrasion caused by traffic loadings can remove the modified layer form; the pavement surface can impair its performance. Also, the rehabilitation and surface treatment of the pavement are affected by the nano-TiO2 coating, which may be removed from the surface and lose its photocatalytic efficiency due to these proceedings. Another concern regarding nano-TiO2 particles is their probable aggregation chance due to their smaller size, which can cause larger particles and lower surface area. Achieving uniform dispersion and ensuring the stability of nano-TiO2 particles throughout the asphalt mixture can be significantly important for better performance. These challenges can affect both the laboratory and field application, especially the field application due to the lower possible controls on the variables.

In addition, due to its smaller particle size, nano-TiO2 can cause oxidative stress, DNA damage, and genotoxicity in living organisms, ultimately leading to a decrease in growth and reproduction, as well as affecting the microbial communities in soil and water, adversely affecting the overall health of the ecosystem [158]. The recommended exposure limits for fine TiO2 (including pigmentary TiO2) are 2.4 mg/m3 by the US National Institute for Occupational Safety and Health (NIOSH) and 0.3 mg/m3 for ultrafine TiO2 (including nano-TiO2) for up to 10 h per day during a 40-h work week, as a time-weighted average (TWA) concentration [159]. Due to these limits, care should be taken when using nano-TiO2 in both laboratory and field experiments to avoid its harmful effects, especially when workers use it in asphalt pavement construction.

The reflective properties of nano-TiO2-modified asphalt pavement are an area of both benefits and challenges. Studies have shown that by doubling the luminance of the pavement, the night to day accidents will decrease by 19% [160]. Although more luminance of asphalt pavement, especially during the night and in the tunnels, can lead to more safety and fewer accidents, the extra luminance and sun glare can result in more crashes [161]. Also, by reducing the texture depth of the asphalt pavement, the accident rate increases [162]. Because nano-TiO2 addition can affect the micro- and macro-texture as well as the pavement surface color and reflectance, the side effects of the nano-TiO2 reflective pavement need to be considered.

Contaminants leaching from asphalt pavements are an environmentally important concern [163]. Studies have indicated that nanoparticles can leach out and potentially contaminate water bodies or soil, raising concerns about the long-term environmental impacts [164]. For nano-TiO2-modified asphalt pavement, there have not been adequate studies regarding the leaching characteristics. These pavements have different and unique leaching characteristics as they have nanomodified asphalt binder and its interactions with water infiltrating (the solubility of nano-TiO2 in water) and other additives, as well as nanosurface coatings which is affected by runoff water. Therefore, the leaching potential of nano-TiO2 particles from the asphalt pavement into the environment is an important area for further investigation.

Another important point is that although this study has focused on the application of TiO2 particles in asphalt pavement, the usage of nanomaterials in concrete pavements has also been regarded. It has been shown that by using different nanomaterials, the performance and properties of the concrete mixture are improved [165, 166]. This can draw attention to the usage of nano-TiO2-modified composite pavements and lead to further investigations for construction.

Also, for better sustainability approach, the environmental and economical assessment of nano-TiO2-modified asphalt pavement should be considered. Two main tools for this approach can be defined as life cycle assessment (LCA) and life cycle cost analysis (LCCA). Although nano-TiO2 has shown photocatalytic performance, which can lead to lower NOx and CO2 pollution (lower acidification and global warming potential in LCA), the initial process of nano-TiO2 production can produce too much pollutants. Thus, the life cycle emission and pollution degradation of these pavements should be considered. For economic analysis, the initial cost of the nanomaterials can affect the life cycle cost of the pavement, which makes it very crucial to be examined. Also, the maintenance of these modified pavements is a challenge in performance, environment, and economic perspectives.

All in all, the application of nano-TiO2 in asphalt pavement can show improvement in both performance and environmental aspects. But considering both benefits and challenges can lead to a better understanding of the potential advantages and disadvantages of nano-TiO2-modified asphalt pavement. This approach requires a multicriteria decision making for researchers and pavement constructors. Thus, a comprehensive understanding of these critical aspects is necessary to assess the viability and sustainability of implementing nano-TiO2 in asphalt pavement construction.

Conclusions and future research directions

This article presents an overview of the research in the field of incorporating nano-TiO2 in asphalt pavement, emphasizing the chemical, rheological, mechanical, and environmental properties and effects. This article also aims to investigate and determine the possible chemical interactions, optimum dosage for nano-TiO2, and factors affecting its performance by giving in-depth explanations. Below is a summary of some of the most important conclusions deducted from this review study:

  1. 1.

    Chemical analysis shows that nano-TiO2 mainly consists of anatase and rutile polymorphs. FE-SEM images show that the rutile surface is smoother than anatase, which, along with differences in band gaps and better excitons mobility, may lead to better photocatalytic performance of anatase. Also, higher surface area and reactivity of anatase, which can lead to stronger bonds with asphalt molecules, may result in better rheological and mechanical properties.

  2. 2.

    FTIR spectrum shows that modification of asphalt binder with nano-TiO2 may belong to physical reactions, and no chemical alteration is observed. FTIR and SARA analysis may show more stiffness and viscosity of the asphalt binder.

  3. 3.

    Rheological and mechanical assessment of nano-TiO2-modified asphalt binder and mixture may show higher rutting and fatigue resistance due to more viscosity and stiffness. However, low-temperature cracking resistance may be weakened after modification but still meets the minimum criteria. Also, the long- and short-term aging resistance of nano-TiO2-modified asphalt can be enhanced. The optimum dosage for nano-TiO2 in rheological and mechanical performance is variable due to different conditions, but it can be suggested to be 5% of the weight of the binder to improve the characteristics of binders and mixtures.

  4. 4.

    The photocatalytic performance of nano-TiO2 in asphalt pavement shows improvements, but it depends on many factors, including nano-TiO2 phase, size and number of TiO2 particles, a doping method for preparing nano-TiO2, incorporation method, additives, UV light, weather conditions, and experiment duration. By optimizing these conditions, better efficiency for pollution degradation can be obtained.

  5. 5.

    Cool pavements have been introduced to mitigate the urban heat island effects on the environment by different mechanisms. Incorporating nano-TiO2 into asphalt pavement can lead to higher reflectivity, lower surface temperature, and less time to cool down, which could mitigate the urban heat island effects.

Moreover, it is recommended that researchers focus on the points listed below for their future works.

  1. 1.

    Evaluating the different performance and effects of anatase and rutile nano-TiO2, as well as their particle size, for better rheological, mechanical, and photocatalytic performance of nano-TiO2-modified pavement.

  2. 2.

    Conducting more chemical tests on the modification of asphalt binder with nano-TiO2 to acquire a better understanding of the possible reactions and predict the binder and mixture performance and aging resistance, as well as the possible dispersion conditions.

  3. 3.

    Optimum methods, proportions, and dosage of nano-TiO2 incorporation in asphalt pavement for maintaining both the performance and environmental benefits.

  4. 4.

    Better evaluation of nano-TiO2-modified asphalt mixture to highlight the possibility and performance of its utilization in future roads and, especially the long-term performance, skid resistance, and abrasion possibility.

  5. 5.

    Exploring the potential for using nano-TiO2 to enhance the sustainability of asphalt pavement by conducting LCA and LCCA. However, efforts have been made to clarify this part, but no related results were found [167]. Key parameters for LCA may include raw material extraction, manufacturing, application, and disposal, focusing on environmental impacts. For LCCA, parameters encompass initial costs, maintenance, energy use, and long-term performance, evaluating economic aspects. A holistic view of the environmental and economic implications can be obtained by integrating these two methods.

  6. 6.

    Leaching characteristics of nano-TiO2-modified asphalt pavements for clarifying the potential contaminants and their effects on the environment and human health.