Asphalt Road Pavements to Address Climate Change Challenges—An Overview

Abstract

Climate change is already happening. It is one of the significant challenges that the planet has to face. Asphalt road pavements cover a large area of the Earth’s surface; consequently, climate change challenges can significantly affect their performance and serviceability. Thus, pavement solutions have been developed to address the problem. This paper aims to present an overview of those topics to increase awareness among transportation engineers and practitioners. First, the most significant aspects of road asphalt pavements’ materials, design and condition are presented. Second, the most relevant climate change challenges for asphalt pavements are described. Then, different pavement solutions are presented. This overview concludes that there are pavement solutions able to address climate change. These depend on local climate conditions and should be incorporated into the decision-making process in planning, design and maintenance.

1. Introduction

Asphalt road pavements are exposed to the natural environment. The behaviour of bitumen drives the performance of asphalt road pavements. Bitumen is a complex material, exhibiting different behaviours depending on the temperature range. At high temperatures, its behaviour is similar to a Newtonian fluid; at intermediate temperatures, it is similar to a viscoelastic liquid; and at low temperatures, it is similar to a viscoelastic solid [1].

Therefore, climate change challenges will inevitably affect pavement service life. Rutting (permanent deformation) performance is the criterion failure most affected [2]. In a study by Gudipudi et al. about climate change for USA conditions, fatigue increased by 2–9% and rutting by 9–40% at the end of 20 years [3].

Current pavement design methodologies assume that climate conditions are stable over the design period [4], which is not valid. Climate change makes weather patterns less predictable [5]. Petteri Taalas, the Secretary-General of the World Meteorological Organization, said, “the negative trend in climate will continue at least until the 2060s, independent of our success in climate mitigation” [5]. Besides global average surface temperature increase, climate change challenges include droughts, heavy rain and flooding, sea-level rise, coastal floods, and forest fires. The impact of those challenges on asphalt pavement depends on the nature and scale of the climate change challenge. They can range from minor effects, such as travel speed reduction, to infrastructure closure or even infrastructure destruction.

A transport network is an essential infrastructure that supports social and economic activities and, thus, must be operational at all times. The United Nations Sustainable Development Goals explicitly included the accessibility and availability of transport networks (SDG 9.1) [6]. The transport system’s weather-related disruption entails high direct and indirect costs [7,8]. Climate change significantly impacts the pavement’s life cycle cost (LCC) analysis. Qiao, Guo, Stoner and Santos [8] estimated the pavement LCC for 24 climate locations across the United States, concluding that climate change (per °C) increases agency costs by approximately 650–700 million USD/year.

Several studies and projects have been developed to recognise, adapt, and mitigate climate change [9,10]. However, uncertainty linked to climate change challenges is difficult to anticipate. Mahpour and El-Diraby [9] developed flood risk curves, which can assess the expected loss, manage risk, and design risk mitigation strategies. Other researchers use models to predict climate change [2,10]. The most used are based on the Representative Concentration Pathways (RCPs) developed for the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) [11].

It is a matter of great importance that will likely be exacerbated by continuing urbanisation. In 2018, 55% of the world’s population lived in cities, and the United Nations estimates that this percentage will rise to 68% by 2050 [12]. These circumstances can replace permeable lands with impervious surfaces, which causes complex drainage and difficult heat dissipation. It could raise pavement temperature and increase the effects of the floods. Transportation engineers and practitioners should be aware of the impact of climate change challenges on asphalt road pavements and know the main playing variables and available solutions. Although several studies address this topic, most address an issue or develop a pavement solution. This paper aims to present an overview of the climate change challenges with a more significant impact on the performance and serviceability of asphalt mixtures, namely the air temperature and the factors that alter the moisture balance of the asphalt mixture and which, consequently, could influence pavement deterioration (rainfall, flooding, sea-level rise). Moreover, it intends to present pavement solutions to address climate change challenges. However, as it is not possible to pave all areas with pavements that mitigate climate change challenges; predicting climate change and selecting appropriate solutions and locations is essential. It aims to increase awareness of the different stakeholders to incorporate these issues into the decision-making process in planning, design and maintenance.

To pursue it, the rest of the paper is structured as follows:

(a)

Section 2 presents the most significant aspects of road asphalt pavements related to climate change issues, namely the materials that constitute the pavement, the design of the pavement structure, and the pavement condition.

(b)

Section 3 describes the three most relevant climate change challenges. They are the air temperature, the precipitation (floods), and the sea-level rise.

(c)

Section 4 presents solutions for mitigating climate change challenges, namely models to predict climate change and pavement solutions/developments to address the climate change challenges.

(d)

Conclusions and recommendations are suggested in Section 5.

2. Asphalt Pavements

2.1. Materials

A flexible road pavement presents a multi-layer structure, with bound layers (upper layers) and unbound layers (lower layers) over a subgrade (Figure 1). Asphalt mixtures constitute the upper layers (surface, binder, regulation, and base) and are mainly formed by mineral aggregates (≈95 wt%) bound with a bitumen binder (≈5 wt%).

Bitumen is a complex material. It is a residue obtained from the petroleum-refining process [13]. Its behaviour depends heavily on the temperature. It goes from a brittle solid (at low temperatures) to a viscous fluid (at high temperatures) [14]. Therefore, its rheological behaviour is essential for the asphalt mixture’s performance. It should be flexible to accommodate cracking at low-temperature and stiff enough to resist permanent deformation under high-temperatures. In addition, asphalt pavement durability, mainly in the presence of water, depends on the adhesion of the bitumen to the aggregates [15].

Climate change leads to a rise in temperatures and changes in precipitation trends. It is thus essential to consider materials suited to future climates, such as upgraded asphalt binder grades and mixture gradation [10].

2.2. Design

The objective of pavement design is to determine the thicknesses of the pavement layers to be constructed over the subgrade. It depends on paving materials, traffic loads and climate; those thicknesses are defined to control failure. The design has evolved from an empirical to a mechanistic-empirical approach [16,17].

In-service/design temperature greatly influences asphalt mixture stiffness. It varies over time, influenced by weather (such as air temperature, solar radiation, wind, and humidity) and pavement materials parameters (such as thermal and physical properties) [18]. Several models have been developed to predict in-service/design temperature [18,19,20] from the knowledge of air temperature. Adwan, Milad, Memon, Widyatmoko, Ahmat Zanuri, Memon and Yusoff [18] present a review in which 38 prediction models were analysed and discussed. Some models are complex, while others are regionally specific.

Usually, pavement design predicts design temperature by considering monthly temperature fluctuations within a given annual temperature and assumes that it is constant over the pavement design period [16]. However, as temperature depends on complex/erratic natural weather, some authors [21] developed a methodology to incorporate temperature variations into pavement design.

2.3. Condition

Pavement condition tends to degrade over the pavement design mainly due to traffic and climate actions combined. As a result, pavement distresses (such as cracking, surface defects and deformations) begin to appear and increase (if nothing is done), influencing users’ comfort, vehicle operating costs and traffic safety [22,23].

A condition index usually measures pavement performance. The Pavement Condition Index (PCI) is among the most popular at the network-level. It considers the extent and severity of the distresses found in the understudy section. Piryonesi and El-Diraby analysed the effect of climate change (temperature, precipitation and freeze–thaw cycles) on the PCI at 2, 3, 5 and 6 years, using machine learning algorithms [24]. They considered two locations (Ontario and Texas), and the results were location-dependent, i.e., climate change affects regions differently. The international roughness index (IRI) also indicates pavement performance [25]. Lu, et al. [26] show that extreme flooding increases IRI. They also present pavement failure patterns.

At a project-level, some authors [3,27] use the AASHTOWare software [28] to calculate pavement responses due to traffic and climate, predicting thus the progression of distresses (fatigue, cracking, and roughness). For instance, Gudipudi et al. (2017) [2] evaluated the isolated effect of temperature and the effects of both temperature and precipitation on pavement performance. They considered climate projection (five locations and two scenarios) data with the AASHTOWare software. Fatigue increased 2–9% and rutting 9–40% at the end of 20 years. The effect of the precipitation was not noteworthy in the pavement performance.

3. Climate Change Challenges

The addressed challenges are those that affect asphalt mixture performance and road serviceability. Figure 2 depicts the three overviewed climate change challenges. They are the air temperature and the factors that alter the moisture balance of the asphalt mixture (rainfall and floods; sea-level rise). Besides the direct impact on pavement damage, climate change also causes indirect effects, such as traffic delays or interruptions, and increases road traffic accidents [29]. Moreover, they could lead to higher maintenance and operations costs [8,30].

3.1. Air Temperature

Air temperature is not constant. It varies night to day and between seasonal extremes in the hemispheres. The difference between the highest and lowest temperatures can be greater than 55 °C [31].

Knott et al. [32] simulated the pavement response to temperature increases and recommended increasing the thickness of the asphalt layers by 7% to 32% to overcome that rise. Other authors suggest solutions to reduce permanent deformation, such as upgraded binder grades and mixture gradation [6,33,34]. It is essential in urban areas, since the air temperatures are frequently higher than in rural areas. Mainly after sunset, the heat stored in urban structures (e.g., buildings and roads) is slowly released into the urban atmosphere, raising the air temperature. This problem is known as urban heat island (UHI) and is likely to worsen with climate change [35]. Paved surfaces occupy 30–45% of the city area [36], contributing thus significantly to UHI.

In contrast, Sun et al. [37] investigated the impacts of extreme winter temperatures on pavement temperature performance. The results depend on the pavement structure’s temperature distribution and cooling rate.

The same pavement structure can be subject to high and low temperatures yearly, or even for short periods, and the asphalt mixture should perform well in the temperature range it faces. Asphalt mixtures should be modified to perform well at all temperatures [38]. Bayat, et al. [39] measured thermal- and load-induced strains for one year. The air temperature ranged from −0.5 to 28 °C, and the longitudinal strains from 504 to 1600 μm/m. They concluded that thermal-induced strains were higher than load-induced strains. It indicates the impact of temperature on pavement performance.

Another critical issue is the freeze–thaw cycles, mainly in open-graded mixtures [40]. Kwiatkowski Kyle, et al. [41] evaluated the effect of freeze–thaw cycles on a porous asphalt mixture, concluding that pavement damage varies with location and increases with the number of freeze–thaw cycles. They also compared the long-term annual costs (2100-year) of using a proactive (design changes) or a reactive approach (maintenance actions). In 2060, the reactive approach was more profitable than the proactive one, since climate change is expected to warm winters and reduce freeze-thaw cycles.

3.2. Rainfall and Floods

Besides leading to temperature rise, climate change shifts precipitation patterns, making rainfall larger and more frequent. Due to climate change and urbanisation, rainfall severity will further increase [42]. Heavy rain causes floods, the most frequent type of natural disaster [42]. Floods can be categorised into three types [42]:

(a)

Flash floods—caused by heavy rainfall, resulting in a rapid rise of water height.

(b)

River floods—caused when water (from rain or snow melt) rises above the river’s banks.

(c)

Coastal floods—caused by storms that surge, from a tropical cyclone to a tsunami.

Asphalt pavement structures are vulnerable to water; as moisture content increases, it weakens most pavement structures [43,44]. In addition, water on asphalt pavement surfaces could reduce friction between vehicles’ tyres and pavement surfaces, which can increase the number of traffic accidents [29]. Therefore, besides adequate moisture resistance, it is also crucial to have adequate asphalt pavement surface characteristics, such as texture and friction. Researchers have extensively studied mixtures’ water sensitivities using laboratory and computational methods [15,45,46] as well as pavement surface characteristics [47,48]. Sultana, et al. [49] critically reviewed assessing and modelling the deterioration of flood-affected pavements, confirming that floods accelerate pavement degradation. Sultana et al. [40] also developed two mechanistic-empirical deterministic models to quantify rutting and roughness caused by flooding events. According to Mateos, Harvey, Millan, Wu, Paniagua and Paniagua [44], the moisture effect on pavement materials is reversible. However, the pavement damage caused by the traffic loading on the weakened pavement is not. Fairweather and Yeaman [50] monitored the strain (surface and subgrade layers), moisture, and temperature of a pavement that was subjected to an intense rainfall event. The moisture content increased 20% in the surface layer and 10% in the subgrade layer. Lu, Tighe and Xie [26] studied the impact of 33 extreme rainfall events (varying in magnitude, duration, and number of cycles) on pavement damage.

Besides the direct impacts on pavement structures (pavement damage), floods also cause indirect effects such as traffic delays or interruptions. To consider it in transport models, Pregnolato, Ford, Wilkinson and Dawson [43] developed a relationship between road flood water depth and vehicle speed. Texture and friction varies with asphalt pavement type and evolves over time [48,51]. Afonso, et al. [52] analysed successfully the texture and friction of permeable pavements.

3.3. Sea-Level Rise

Another climate change challenge is the sea-level rise. Since 1808, it has risen around 21–24 centimetres (global average), with a higher rate in the last two and a half decades. It is mainly caused by glaciers melting, sea ice melting, and ocean expansion (due to water warms) [53]. It also increases the moisture content of the pavement structure. However, a salty and humid environment affects pavement performance differently [54,55,56].

Wang et al. examined the influence of fresh water and seawater on pavement performance, concluding that seawater had the greatest impact on the water sensitivity of the mixture (cited in [55]). Chu et al. conclude that dry–wet cycles, when compared to a constant salty wet cycle, speed up the erosion mechanism of the sea salt solution on the performance of the asphalt mixture (cited in [55]). The sea spray environment is a complex situation that is challenging to replicate with a salt solution immersion. Therefore, Meng et al. [57] used a salt spray test chamber to study the sea spray erosion effect on the mixture performance at high temperature. They concluded that by increasing the salt concentration and spray time, the mixture’s high-temperature stability decreased, and salt concentration was more damaging than the erosion time (cited in [55]). In turn, Zhao et al. concluded that salt spray erosion greatly influences low-temperature cracking resistance, moisture resistance, and the mixture’s durability. The mixture performance could decrease by more than 40% (also cited in [55]). The greater the salt concentration, the worse the mixture performance at high and low temperatures; the relationship is linear [56].

For more information, Feng et al. review [55] present the damage mechanism and different testing simulations, not only for the salt erosion effects but also for the aggregated effects of other natural factors, such as temperature, moisture, and ultraviolet radiation.

4. Solutions for Mitigating Climate Change Challenges

There are several asphalt pavement solutions to mitigate climate change challenges. Until now, the term asphalt mixture has been used generically. It includes many types of asphalt–aggregate mixtures (dense-graded, open-graded and gap-graded mixtures) and some additives. The mixture and constituent materials should be chosen to satisfy the different pavement performance demands that depend on climate change challenges. However, it is not possible to pave all areas with pavements that mitigate climate change challenges. Therefore, predicting climate change and selecting appropriate solutions and locations is essential.

4.1. Climate Change Prediction

Using conventional asphalt pavement designs (stable climate conditions over the service life) can result in an incorrect pavement solution, since it does not consider the influence of future climate change challenges. To consider this, climate prediction models are crucial. The climate system is a complex system where five significant components interact: the atmosphere, the hydrosphere, the cryosphere, the land surface, and the biosphere [58], and a complete model should comprise all components. Not all predictions require a complete model; consequently, simplifications can be made. Several models have been developed [59]. They can be global (cover the whole Earth) or regional (cover a specific region). Climate change is not equal in all areas of the Earth; it impacts regions differently. Global models do not have enough resolution. Regional models and downscaling methodologies capture the finer-scale change resolution of a particular region. In the context of the Coordinated Regional Climate Downscaling Experiment (CORDEX), regional climate change predictions were improved [60].

Future climate data (predictions) are usually made using projections under different Intergovernmental Panel on Climate Change (IPCC)’s Representative Concentration Pathway (RCP) scenarios. In the Fifth Assessment Report, the IPCC used four pathways: a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0), and one scenario with very high GHG emissions (RCP8.5) [11]. RCP4.5 is an optimistic scenario, while RCP8.5 is the worst scenario.

Regarding temperature, one thing is the air temperature, and another is the pavement temperature. Pavement temperature depends, among other things, on air temperature, solar radiation and wind. Chen, et al. [61] present a review of models to predict pavement temperature. They can be numerical, analytical or empirical. The former two are based on heat conduction’s partial differential equation, while the last is based on statistical analysis. To predict time-dependent pavement temperature using empirical models, neural network-based models have been developed. Currently, the models are developed in dry conditions. Rainfall challenges the model development, and the effect of precipitation is still unclear [59]. Adwan, Milad, Memon, Widyatmoko, Ahmat Zanuri, Memon, and Yusoff [18] also present a comprehensive review of 38 models to predict asphalt pavement temperature and categorize them into numerical, analytical, and empirical. In turn, Tabrizi, et al. [62] developed a model to predict pavement temperatures, using machine learning to optimise deicer applications in cold climates.

Once climate change is predicted, several authors have been using the AASHTOWare Pavement Mechanistic-Empirical design tool [28] to calculate distresses over the lifetime and consequently to predict the impact of climate change challenges on pavement performance [63,64]. It includes the Enhanced Integrated Climatic Model (EICM), which is a one-dimensional coupled heat and moisture flow model that predicts the changes in the behaviour of bound (asphalt mixture) and unbound materials over service time due to climate conditions [65].

4.2. Pavement Solutions

4.2.1. Permeable Pavements

Permeable pavements are a pavement solution that can help flood management. The water can percolate into the pavement layers, reducing thus surface water height. It is also a generic term; permeable pavements include but are not limited to porous asphalt, pervious concrete, and paver blocks. Porous asphalt is a mixture with a high air voids content, so the water drains through the asphalt surface layer [66] (Figure 3). Therefore, it increases safety by decreasing hydroplaning and splashing. In addition, as porous asphalt presents an open-grade surface, it increases skid resistance and reduces traffic noise. In fact, asphalt pavement surface characteristics, such as macrotexture, influence road safety [47].

Permeable pavements also benefits the ecological environment (promotes groundwater resources recharging) [66,67,68].

Despite the advantages, serious issues affect its performance and pavement life. Void structure clogging is one of them [69]. The water drainage through the mixture voids and traffic makes some particles (dust, tyre wear particles, etc.) accumulate in the mixture voids, changing the pavement permeability and durability. Several researchers have been studying this issue. Li, Xu, Chen, Liu, Tan, and Leng [69] analysed, at the microscopic level, the water drainage through the porous asphalt with the seepage depth, velocity, and path. They concluded that the results depend on clogging size particles and mixture voids content. Meng et al. [57] explored the permeability variation in the clogging process, considering the water head height, the mixture gradation and the clogging particles. They concluded that the effect of clogging on the permeability varied with direction. The clogging effect on the transverse permeability was noticeably more remarkable than on the vertical permeability. Again, it depends on the water head height, mixture gradation, and clogging particles. Another issue is the mixture’s durability [66,68,70]; porous asphalt is more exposed to climatic actions and, consequently, is more prone to ageing. Wang, et al. [71] studied the effect of air voids content (16.2, 20.7 and 23.8%), modifier amount (9, 12 and 15%), ageing (short- and long-term). and test temperature (0, −10 and −20 °C) on porous asphalt performance, concluding that the most critical factor was the modifier amount. Next was the test temperature. The effect of air voids content and ageing were not as significant. To improve porous mixture durability, modified bitumen is currently used. Zhang, Sha, Liu, Luan, Gao, Jiang, and Ma [66] present a comprehensive review of this kind of mixture, helping to understand the context better.

It is also essential to assess the effectiveness of permeable pavements in flood management. Liu, et al. [72] compared four permeable pavement structures in terms of infiltration using artificial rainfalls and full-size pavements. They concluded that thicker structures with reasonable combined air voids perform better. Ciriminna, et al. [73] used numerical simulations to assess the effectiveness of four permeable solutions against different rainfall events. All solutions perform better than a conventional impervious pavement. The worst-performing one was the porous asphalt over an impermeable layer. Cheng et al. [74] measured the monthly infiltration rates of two permeable solutions (porous asphalt and porous concrete blocks) and monitored 36 rainfall events. They found a runoff peak reduction from 16% to 55%, depending on rainfall intensity; an average reduction in the rainfall volume of 37.6%; and an infiltration rate decrease over the 15 measured months. The infiltration rate decrease depends on the location. In the porous asphalt solution, the rate remains high at one location and decreases inconsistently in other locations. Those differences show that porous asphalt’s field infiltration depends on several factors, such as construction and environmental conditions. Routine maintenance is required to ensure adequate performance over time.

Researchers have also carried out some life cycle assessment analyses. For instance, Liu, et al. [75] compared a permeable and dense-graded asphalt pavement’s life cycle assessment (cost and environmental perspective). The results show that permeable pavement is costly (high investment in raw materials and maintenance), but the environmental benefits are considerable.

4.2.2. Cool Pavements

Black asphalt pavements absorb and store more heat than other surfaces [76]. Therefore, materials that constitute them present very low reflectivity or albedo. It is worrying mainly in urban areas since it contributes to UHI. UHI is a complex phenomenon where multiple influencing factors (such as albedo, pavement type, pavement thickness, and thermal properties of constituent materials) interact [77]. To counterbalance UHI, some pavement solutions have been developed. They involve using cool pavements, i.e., pavements that contribute to UHI mitigation (Figure 4). It includes cool/reflective/permeable materials and phase change materials (PCMs).

Cool pavement surfaces present a high albedo (proportion of the incident radiation that is reflected by a horizontal surface) and high thermal emissivity, thus absorbing less heat. New asphalt pavement has a typical albedo value of 0.05 [78] and it increases with ageing up to 0.15. With a 0.6-increase in pavement albedo, daytime pavement surface temperature could be reduced by 20 °C [79]. Non-black asphalt pavements are also a solution [80].

Water retention pavements, such as porous asphalt pavement, also mitigate UHI [77,79,81]. That UHI mitigation is not due to a higher albedo, but to the release of the absorbed solar heat in the form of latent heat (due to the evaporation of liquid water) [78]. Their behaviour depends on their air voids content; the cooling effect increases as the air voids content increases [82]. Researchers have enhanced their evaporation characteristics by developing a structure with capillary columns in aggregate. The resulting pavement was 9.4 °C-cooler than a conventional permeable pavement [83].

Other cool pavement technologies are based on high-inertia pavements that add phase change materials (PCMs) to the asphalt mixture [84,85,86]. PCMs can change their phase states, store heat, and control temperature. They absorb heat during the day more slowly, and release the absorbed heat more slowly during the night. According to the composition, PCMs can be inorganic, organic, or composite [84]. Even though inorganic ones have some advantages over organic ones, they are inadequate for asphalt pavements. For asphalt pavements, organic PCMs should be the best choice [84]. One of the most known is polyethene glycol [86]. Road pavement solar collectors are also a solution to absorb heat from the pavement surface [87].

Kappou, et al. [88] presents a comprehensive review on this pavement solution with maintenance solutions. They recommend to use reflective pavements in the maintenance of existing ones and permeable pavements in the construction of new ones. They also outline the main advantages and disadvantages of each solution. Permeable pavements are costly.

4.2.3. Hydrophobic-Deicing Pavements

Climate change causes extreme weather patterns. Although temperatures are rising, causing more precipitation to fall as rain instead of snow, several regions have experienced extreme winter temperatures [89,90,91]. Traffic safety can be severely affected by extreme winter temperatures (snowfall and freezing temperatures) [92]. It is, therefore, essential to control pavement surface snow and ice for safety and mobility purposes. Various deicing technologies have been developed, such as pavement deicing, road heating systems and self-icing melting mixtures.

Chloride-based deicers (e.g., salt) are the most common deicers. Before or after snow, deicers are spread on the pavement surface, forming a liquid anti-icing solution that melts snow and prevents ice formation. Although effective, they are labour and equipment-intensive, cause pollution, and damage pavement and vehicles [93,94]. Therefore, alternative deicers have emerged, such as agro-based deicers, acetates, formats, glycols, and succinates [95]. Despite the advantages of more natural deicers, they are costly and not entirely harmless to the environment [93,96]. Alternative approaches based on heating systems have been developed, such as electric heat and hot water trading systems, hydronic-, microwave-, and infrared-heating systems, and electrically conductive pavement mixtures [93,97]. They are effective, but energy and cost-consuming. Self-icing melting mixtures could also be used. They are prepared by adding anti-icing additives during the mixing process [98]. However, this method cannot remove heavy snow.

4.2.4. Less Temperature-Sensitive Pavements

Bitumen type dramatically influences the thermal properties of the asphalt pavement. It is crucial to select the right one for the local climate since it behaves as a viscous-plastic material in summer and a brittleelastic one in winter. Road pavements experience a wide temperature range. In summer, the pavement surface temperature can be 70 °C; in winter, it can be −40 °C or even less [99].

Several additives, such as polymers and nanomaterials, can make asphalt road pavements less temperature-sensitive [99,100,101,102,103,104]. The selection depends on the local climate, influencing the failure criteria to design asphalt pavement. In cold weather, low-temperature cracking is predominant, while in hot climates, it is rutting. Additives can improve low-temperature and high-temperature performance. In addition, additives can also improve the moisture resistance of asphalt pavements [105,106,107].

The most used polymers in bitumen modification are styrene–butadiene–styrene (SBS), styrene–butadiene rubber (SBR), ethylene vinyl acetate (EVA), and polyethene A [101]. For instance, Sarroukh, et al. [108] compared the rutting resistance of a mixture produced with conventional bitumen to a mixture produced with polymer-modified bitumen. In order to strengthen the temperature effect, wheel tracking tests were carried out at 80 °C instead of the usual 60 °C. The rut depth of the modified mixture was 17% lower. The incorporation of waste plastics also improves rutting resistance [109,110,111]. For more information on polymer-modified asphalt’s advances and challenges, see Zhu, Birgisson and Kringos [105].

Researchers have also been using nanotechnology to improve asphalt pavement performance [112]. Caputo, Porto, Angelico, Loise, Calandra and Oliviero Rossi [103] present a comprehensive review regarding the use of nanomaterials. The following nanomaterials are addressed: silica, ceramic, clay, other oxides, inorganic nanoparticles, organic nanostructures (organically expanded vermiculite and carbon nanostructures), and functionalised nanoparticles (increase affinity with bitumen). Nanotechnology is not a cost-effective solution, despite the small amount used. The authors also present an evaluation of the costs. The costs vary considerably between materials and even for the same nanomaterial. According to the authors, the high costs may be justified by it being a recent technology. They believe that technological development will decrease prices, leading to a demand increase. A recent study [113] used a nano-Al₂O₃ composite modification to improve rutting, cracking and moisture resistance.

Another way to improve the rutting resistance, i.e., the mixture performance at high-temperature, is by mixture grading [114,115]. Aggregate skeletons and air voids influence rut development. Stone Mastic Asphalt (SMA) is a gap-graded skeleton dense form (coarse aggregates) with voids filled with mastic asphalt (fine aggregate, bitumen and additives/stabilisers). Consequently, it presents good resistance to rutting deformation [115,116].

4.2.5. Salt-Resistant Pavements

Sea-level rise exposes the asphalt road pavements to a salty environment. Researchers have studied the effect of that environment on asphalt mixtures and proposed solutions [54,55,117]. One way to improve the adhesion between asphalt and aggregate, and thereby the water sensitivity of the mixture, is by using an anti-stripping agent. By testing four anti-striping agents, Zhang et al. [90] determined the most appropriate, and recommended dosage. The chemical composition of two include amine substances, while the other includes non-amine substances. The best performance was achieved with a non-amine anti-stripping agent with a dosage of 0.5–0.6% by weight of bitumen. Baldino, Angelico, Caputo, Gabriele, and Rossi [54] also tested anti-stripping agents (three, in this case; a cationic, a nonionic, and an amine surfactant) to improve asphalt mixture adhesion in salty and humid environments. Better performance was achieved with the nonionic surfactant.

Zhang, Liu, and Shi [98] studied the corrosive effect of salty and humid environments on asphalt pavement performance (rutting, fatigue, and moisture resistance). For that, the following environments (by immersion) were simulated to speed up the corrosive effect: dry–wet and freeze–thaw cycle in 0%, 5%, and 10% sodium chloride solution. They also studied the effects of six anti-stripping agents. The authors concluded that the best environment to simulate the corrosive effect is the dry–wet cycle in 10% NaCl solution (twelve times). Regarding the anti-stripping agent, the basalt fibre is recommended to improve asphalt pavement performance in coastal areas.

4.2.6. Final Remarks

Future asphalt pavement choices, mainly in urban and coastal areas, is a very complex issue where several factors play a role. The performance of the presented solutions should be simulated in the laboratory, subjected to trial projects, and/or monitored in the field before being applied on a large scale. However, test protocols for extreme weather climates, salty environments, and long-term field applications are not always available, and extreme weather prediction is difficult. In any case, design methodologies should be updated to accommodate climate change fluctuations. Another critical aspect is the life cycle assessment of the solutions and how they compare with traditional solutions.

5. Conclusions

This paper presents the most significant aspects of road asphalt pavements related to climate change issues, the most relevant climate change challenges for asphalt pavements, and shows pavement solutions to address climate change. Different climate change challenges have different pavement solutions. The following table presents pavement solutions recommendations for corresponding climate change challenges (

Table 1. Asphalt pavement’ solutions recommendations to mitigate different climate change challenges.

High Temperature	Low Temperature	Rainfall and Floods	Sea-Level Rise
Decrease bitumen grade	Increase bitumen grade	Permeable pavements	Anti-stripping agents
Cool pavements	Hydrophobic-deicing pavements
Increase aggregate skeletons (e.g., use SMA)	Bitumen/Mixture modification
Bitumen/Mixture modification	Increase asphalt layer thickness
Increase asphalt layer thickness

).

The solutions depend on local climate and should be incorporated into the decision-making process in planning, design, and maintenance.