Recycled Pavement Materials and Urban Microclimate: Albedo and Thermal Capacity Effects on Heat Island Mitigation

Abstract

In Mediterranean cities, high solar radiation combined with limited shading and vegetation intensifies the urban heat island (UHI) phenomenon. As the road network often covers a large portion of the cities’ surfaces and is mostly constructed using asphalt pavements, it can significantly affect the urban microclimate, leading to low thermal comfort and increased energy consumption. Recycled and waste materials are increasingly used in the construction of pavements in accordance with the principle of sustainability for minimizing waste and energy to produce new materials based on a circular economy. The scope of this study is to evaluate the effect of recycled or waste materials used in road pavements on the urban microclimate. The surface and ambient temperature of urban pavements constructed with conventional asphalt and recycled/waste-based mixtures are assessed through simulation. Two study areas comprising large street junctions near metro stations in the city of Thessaloniki, in Greece, are examined under three scenarios: a conventional hot mix asphalt, an asphalt mixture containing steel slag, and a high-albedo mixture. The results of the research suggest that the use of steel slag could reduce the air temperature by 0.9 °C at 15:00, east European summer time (EEST), while the high-albedo scenario could reduce the ambient temperature by 1.6 °C at 16:00. The research results are useful for promoting the use of recycled materials, not only as a means of sustainably using resources but also for the improvement of thermal comfort in urban areas, the mitigation of the UHI effect, and the reduction of heat stress for human health.

1. Introduction

1.1. Problem Statement

The need for the more sustainable use of materials includes the notion of minimizing the quantities of raw materials (i.e., minimization of extracted quantities and reduced production of raw materials) and the waste produced by their replacement at the end of their lifespan. Within the framework of the contemporary structural design of buildings, roads, bridges, etc., apart from the mechanical strength of the materials utilized, their lifecycle behavior, together with their environmental impact, needs to be jointly assessed.

In the meantime, urban street networks constitute an important proportion of the impervious surfaces in densely populated cities. Most conventional materials used in the pavement infrastructures of European urban streets mainly comprise asphaltic products in the surface layer, which contribute to the increase in streets’ surface temperatures during summer periods due to their low reflectance and their high thermal storage capacity, resulting in the release of heat even during the nighttime. Furthermore, the fact that roads and streets often remain exposed to solar radiation during the daytime leads to the development of intensified and extreme temperatures. To counterbalance this, studies suggest strategies to either reduce solar exposure (e.g., by increasing vegetation or other shading strategies) or alter properties such as the reflectivity or the thermal storage of the asphalt pavements (e.g., high-albedo materials, low-thermal-storage materials, and cool materials) to mitigate the urban heat island (UHI) effect.

Τhere is an increased interest in investigating planting streets and sidewalks with either low grass or trees, depending on the functional limitations of the areas. These vegetation strategies can have multiple advantages in the mitigation of the UHI effect and in improving the quality of life in cities. Green areas can reduce heat accumulation, improve thermal comfort through evaporative cooling, and decrease the amount of solar radiation reaching hard urban surfaces, thereby reducing heat storage in areas with high thermal mass [1]. However, despite the indisputable advantages of increasing urban vegetation, there are some parts of the streets where it is not easy to replace the existing high thermal mass and hard surfaces with greener ones. There are multiple reasons that some parts of the streets must maintain their current properties in terms of the surface materials, such as pressing traffic demand in urban areas or underground network infrastructures.

It is therefore very important to consider alternative materials for covering these areas to reduce heat accumulation due to the high absorptivity and high thermal storage of the existing construction of the streets. These alternative materials might include recycled or waste materials so that landfilling issues can be effectively treated at the same time. Waste plastics, waste glass, and other industrial by-products, such as steel slag or crumb rubber from tire processing, are some of the most investigated alternative materials in the engineering of pavement structures [2,3]. Provided that the use of alternative materials does not compromise the pavement’s strength and durability, other advantages could also result from the consideration of alternative materials in terms of their thermal properties, including the reduction of urban noise and a lower environmental footprint during the material’s lifespan (i.e., production, construction, use phase, etc.) [4,5]. The surface layers of asphalt pavements represent a valuable application area for the use of solid waste materials (SWMs) following an effective and resilient planning perspective. However, it is very important to figure out the mechanism of optimum use of these materials in terms of the urban form, climate, and microclimate.

In Greece, urban streets under existing planning strategies remain mostly car-oriented. The street network of the city of Thessaloniki covers 19% of the urban surface [6], while the city suffers from traffic congestion, despite the recent completion of the metro line and the forthcoming extension, which together encompass a total length of 14.4 km [7]. The extended area of street surfaces covered with asphalt pavements and the climate conditions prevailing in the Mediterranean, characterized by high periods of solar availability and high ambient temperatures, result in the deterioration of urban microclimatic conditions, with increased heat stress during the summer period. Therefore, an effective and resilient planning perspective is necessary to mitigate the UHI effect.

To reduce the high temperatures of asphalt pavements in urban areas, the use of alternative materials such as the ones mentioned earlier is investigated in this study in terms of the microclimatic improvement they can provide under specific circumstances and modeling assumptions. Towards this, the performance of asphalt pavements constructed using such recycled materials is first reviewed to provide optimum material compositions that could be further explored in terms of the environmental contribution of asphalt pavements in urban areas.

1.2. Aspects of Material Engineering for Pavement Surfaces

As pavements typically serve longer than their intended design life, various additives or unconventional materials are incorporated into the mix design to produce stable roads, which implies a durable and long-lasting structure with skid- and water-resistant performance [8]. Considering that pavements are most often maintained by replacing the surface layer, it appears that a pavement’s operational service life depends heavily on the mixture design of the surface layer. The performance of pavement surfaces is intended to be distress-resistant, indicating that both structural and functional elements are sufficient. A pavement surface may experience common distresses, such as rutting, stripping, moisture damage, raveling behavior, loss of roughness or texture, top–down fatigue cracking (linked to stiffness modulus and fatigue resistance [9,10]), and inadequate skid resistance, which is directly related to road safety.

Whatever the kind of pavement distress, vehicle movements exhibit social, economic, and environmental implications. The primary factors linked to pavement surface performance are traffic safety, noise level, solar reflectance, stormwater runoff, and greenhouse gas emissions [11]. Consequently, any alteration chosen for traditional asphalt surface mixtures must first satisfy most of the criteria for distress-resistant behavior. Provided that an equivalent performance is guaranteed, the next step is to evaluate their environmental contribution. Following this rationale, a quick overview of the commonly utilized alternative materials in surface layers is presented, including waste plastic, tires, glass, and steel slag.

Plastic waste is a major environmental issue due to its high production and dispersed recycling rates globally. Every minute, one million plastic bottles are manufactured [12], although not all of them are produced equally worldwide. Nevertheless, because plastics are not biodegradable, disposing of their waste is a social and environmental issue in many nations [13,14]. In recent years, the philosophy of recycling and its development have advanced extremely quickly. In fact, in 2022, the EU recycled almost 40% of its waste from plastic packaging [15]. By the end of 2025, Australia wants to achieve a plastic recycling rate of 70% [16]. A promising approach for reducing the effects of waste plastic is to use it in asphalt pavements.

Plastics such as polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), and polyethylene terephthalate (PET) can be added using either a wet or dry process, depending on their melting point [10,17]. Research shows that adding plastics improves the rutting resistance, stiffness, and fatigue life of asphalt, especially under high temperatures [18,19,20]. However, the type and quantity of plastic used must be carefully managed to avoid reduced moisture resistance or durability. Overall, plastic-modified asphalt can offer significant performance benefits, but mix designs must be optimized to balance strength, flexibility, and environmental benefits. The rate at which plastic waste is used to substitute the binder material is also critical; rates lower than approximately 10%, as measured by the mass of the binder, or lower than 0.5–1% as measured by the mass of the mixture [20,21,22].

Another popular material is the crumb rubber from the processing of waste tires. This is mostly used as an asphalt binder additive at optimal rates of 5–20% by mass of the binder [23,24,25]. Again, the disposal rate of waste tires varies throughout the world depending on the recycling rate of old vehicles. This means that where new cars are replacing older ones more frequently, there is a greater number of available waste tires. For example, in China, it was anticipated that the number of new passenger cars would double between 2016 and 2024, thereby leading to increased tire landfilling [26].

In terms of the contribution of crumb rubber on pavement performance, it improves asphalt’s softening point, elasticity, and thus the mixture’s rutting resistance [27,28,29]. Both wet and dry processing methods are used. While crumb rubber generally enhances fatigue performance and stiffness [30,31,32], its effect on moisture sensitivity is mixed and depends on the rubber dosage and processing conditions. Challenges include high energy consumption during mixing and poor storage stability at high temperatures due to binder and crumb rubber separation [33,34]. Advances such as warm mix asphalt (WMA) and finer rubber particles help improve performance and reduce production issues. Finally, research about the impact of crumb rubber on the pavement’s functional characteristics has revealed improvements in macrotexture and skid resistance performance [35,36], aspects that are crucial for urban road safety [37].

Moving forward with glass waste, its use in asphalt mixtures can substitute either the aggregate or the binder. Being non-biodegradable and difficult to recycle fully, glass waste is a viable candidate for reuse in asphalt pavement construction [38,39]. When properly incorporated, it enhances binder properties and surface visibility (due to its high reflectivity), which improves safety in low-light conditions or at night [40]. However, the excess glass content can reduce binder adhesion and increase the risks of bleeding and surface wear. Studies suggest limiting the glass content to 10–15% to ensure good performance. While glass-modified asphalt can offer improved fatigue resistance and adequate skid resistance [40,41], it may also be prone to breakage under traffic [38]. This can raise safety issues for pedestrians and moving vehicles because of the potential of glass dislodging, thereby making it more suitable for low-speed roads or parking areas [11].

As an industrial by-product of steel manufacturing, steel slag is widely used as a coarse aggregate substitute in asphalt mixtures, mainly due to the availability of large waste quantities annually. Indeed, the EU generates steel slag at a rate of 21.8 million tons on an annual basis [10,42], with 24% of this quantity being temporarily stored and landfilled; as such, the potential for its alternative reuses increases. When used in asphalt mixtures, it offers strong mechanical properties, such as high rutting resistance [43,44], fatigue life, and stiffness [44,45], due to its rough texture and interlocking ability [46]. It also improves skid resistance and is suitable for high-traffic roads [47,48,49]. However, challenges include its high specific gravity (affecting mix weight and transport costs), potential expansion due to residual chemicals, and increased binder demand due to high absorption [10]. To manage these, an optimal slag content (around 30% as measured by the mass of the mixture [50,51,52]) and pre-treatment for chemical stability are recommended. Overall, slag is an effective and sustainable material for road construction when used appropriately. In other words, steel slag is best suited as a coarse aggregate substitute because using it to replace fine particles or filler may cause issues with the mixture’s water sensitivity [49].

Given the above remarks, it should be acknowledged that limited information exists in the literature about the mixture’s characteristics of pavement layers at urban roads. Nevertheless, most of them are distressed, thereby necessitating their maintenance and overall management during their lifespan. In the absence of relevant specifications and technical requirements, which were developed ad hoc for the recycled mixtures, it becomes important that any material component intended for either binder or aggregate use in asphalt mixtures is subject to the same requirements for property classification and testing as that for virgin materials. Benchmarking based on a reference condition status is an important challenge when deciding on the use of alternative materials [53].

In addition, newer technologies such as micro-surfacing and chip seal techniques have emerged during recent decades as alternative and cost-effective solutions to meet surface restoration needs. Micro-surfacing consists of a thin, protective layer made of polymer-modified asphalt emulsion, crushed aggregate, water, and additives, and is applied to improve surface texture, seal minor cracks, and slow down deterioration [54,55]. Chip sealing, with or without colored pigments that directly impact the surface albedo, involves spraying a layer of asphalt binder followed by a layer of aggregate chips, which are then rolled into place [56,57]. It can seal cracks, restore skid resistance, and protect the surface from oxidation and moisture. Both preventative techniques are ideal for low-to-moderate-traffic roads with moderate damage or minor surface issues and are more cost-effective than repaving [58]. Although both methods can delay the appearance of major distress, their suitability for low-to-moderate traffic volumes might question their applicability in major urban arterials that serve high volumes of both light- and heavy-duty vehicles daily. As such, apart from the mechanical issues of pavement performance, the environmental assessment of the individual alternative materials and other innovative techniques (e.g., low-carbon photovoltaic coatings [59]) should follow a prioritization and categorization approach.

1.3. Aim and Objectives

The main aim of this research is to investigate solutions for reducing the high temperatures of asphalt pavements in urban areas, where the amount and duration of their exposure to solar radiation are high, and the alternatives for shading through vegetation or other greening interventions are not feasible. To meet the research aim, typical climatic characteristics of a north Mediterranean coastal city were assumed. In particular, the case of a densely populated Greek city, i.e., Thessaloniki, was chosen for the analysis, and two central street junctions were selected to enable a comparative assessment and demonstrate the prioritization concept.

While most of the above-mentioned alternative materials and techniques are more likely to be used on heavy-duty suburban motorways subject to high traffic demand, the issue of the microclimate makes much more sense when the focus of the investigation is on urban roadways. Therefore, taking as a fact the satisfactory or improved pavement performance of the alternative materials, this study goes a step further by figuring out the optimum use of these materials for the improvement of the microclimate in urban areas. In this context, the following objectives were set:

• To define typical pavement structures met in urban streets and assume typical material composition for the mixture of the surface layer;
• To quantify the effect of replacing conventional pavement materials on the microclimatic conditions of the city. To this end, the temperature change caused by the replacement of conventional mixtures with recycled or waste materials in different layouts of the cities’ street network configurations has been calculated.

The rest of this paper is organized as follows: Section 2 includes the description of the simulation parameters, with the assumptions made and the utilized analysis methods and tools. Section 3 and Section 4 present the results of the analysis together with critical discussion points, respectively, and Section 5 summarizes the main findings of the research and indicates future research directions.

2. Materials and Methods

With respect to the objectives of this research, the methodology is structured as follows (Figure 1):

First, the thermal properties of the materials are defined according to the literature.

Second, a base case and a recycled scenario are defined. The base case scenario represents the conventional asphalt mixture, and the recycled scenario represents the steel slag asphalt mixture. Thermal conductivity, heat capacity, and albedo values are retrieved from literature sources for each scenario.

Third, two case study areas (street junctions) are defined in the city of Thessaloniki.

Fourth, the areas are analyzed in terms of surface and ambient temperatures using the Envi-Met simulation software.

The results are analyzed further to assess the potential improvement of the urban microclimate using recycled asphalt pavements.

2.1. Case Study Area

The areas examined belong to the urban street network of the city of Thessaloniki, Greece. The selection of Thessaloniki is justified based on its representativeness for the north Mediterranean coastal climate, dense urban morphology, and pronounced summer heat conditions. During July, the city experiences average high temperatures of 28–31 °C, a range that closely approximates the temperature of other coastal cities in the north Mediterranean, e.g., Venice, Nice, Barcelona, etc. [60]. In addition, limited wind circulation during July, which typically represents the peak of the warm season in Greece, can raise the UHI implications and the thermal stress conditions for urban residents. Therefore, Thessaloniki can be considered a representative case study for urban climate research in the Mediterranean region, although generalization should be avoided.

The areas of the investigation are situated at central positions of the urban tissue and are considered important crossroads in Thessaloniki. They consist of large avenues with dense traffic, mostly used for crossing the city and/or traffic distribution at neighboring districts and municipalities. A metro station of the recently constructed line is also located in both areas. The two study areas are situated at the west and east edges of the historic center of the city of Thessaloniki on the same road axis (Egnatia avenue), denoted as areas A and B in Figure 2. Finally, the fact that these junctions were distanced some meters away from the coastal line could further diminish the potentially favorable impact of the sea on the urban microclimate, thereby making the analysis somewhat suitable even for non-coastal cities.

The east area (A) is named Sintrivani, which is also the name of the nearby metro station. The west area (Β), named the Lagada area after the name of the main street (Lagada Avenue), also has a nearby metro station, named the Democratias station (after the neighboring square). Both areas are characterized by the extensive coverage of hard impermeable surfaces and especially of asphalt pavements. The layout plans of the two areas are presented in Figure 3. The boundaries of the area of the simulation are marked with a grey-toned square.

The buildings around the streets are tall, but most of them cannot contribute to the shadowing of the central area of the junction, where the asphalt surfaces remain exposed to solar radiation for long periods (Figure 4). There are some large trees, especially in the Lagada area, but they still cannot provide shadowing in the junction’s area as they are mostly situated at a significant distance from the center of the street junction. The asphalt surface of the two areas is significantly extensive, covering 44% of the total area (Figure 5). This means that the asphalt is expected to make an important contribution to the local air temperature.

2.2. Material Data Collection

To change the temperature field of a pavement, the thermophysical properties of the pavement should be modified. The main thermal characteristics of a pavement (thermophysical properties and surface properties) depend on its albedo, thermal capacity, and thermal conductivity, as well as the rate of each material in the mixture:

• The surface albedo is a measure of the amount of solar radiation reflected by an object compared to the total solar radiation it receives on its surface, with values ranging from 0 to 1, where 1 indicates total reflection;
• Thermal conductivity is the rate of heat transfer through a unit area of a material per unit temperature gradient under steady-state conditions. It represents the amount of heat conducted per unit time across a unit area for a unit of temperature difference across a unit of thickness and is expressed in W·m⁻¹·K⁻¹ [61];
• The volumetric heat capacity describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase change. The volumetric heat capacity (ρ × Cp) is considered an important parameter with which to assess the energy storage material. The volumetric heat capacity is the product of density ρ and specific heat capacity Cp [62].

2.2.1. Assessment of the Asphalt Mixture’s Thermal Properties

To determine the material properties affecting surface temperatures, typical values synthesized from relevant experience and literature reviews have been proposed for a typical composition of conventional and alternative asphalt mixtures [11,19,20,23,25,50,52]. The results of our research on the typical composition of conventional and alternative asphalt mixtures are summarized in

Table 1. Typical composition of the conventional and alternative asphalt mixtures examined.

Composition (% by Mass of the Mix)	Conventional Hot Mix Asphalt (HMA)	RAP Mix (Aggregate Substitute)	CR Mix (Binder Substitute)	Plastic-Based Mix (Binder Substitute)	Steel Slag Mix (Aggregate Substitute)	RAP-CR Mix
Virgin binder (%)	5	4	4	4	5	3
Virgin aggregate (%)	90	61	90	90	60	61
Waste substitute for binder (%)	-	-	1	1	-	1
Waste substitute for aggregate (%)	-	30	-	-	30	30
Air voids (%)	5	5	5	5	5	5

. For the sake of completeness, apart from those materials described in the introduction, the reclaimed asphalt pavement (RAP) is also given in Table 1, since the practice of milling existing pavement surfaces and reutilizing the reclaimed materials is a common and sustainable practice in pavement engineering. A pertinent report from the Federal Highway Administration (FHWA) [63] indicates that employing RAP at levels of up to 30%, as measured by mixture weight, in the intermediate and surface layers does not undermine pavement performance when compared to control mixtures using virgin aggregates. The binder content in RAP-mixtures appears to be a bit lower because RAP inherently includes a quantity of bitumen. However, for the scope of the present research, RAP and RAP-CR mixtures were excluded from further analysis since a recycled asphalt aggregate is not expected to significantly alter the thermal properties of the mixtures. In addition, Kim and Lee [61] examined a dense-grade asphalt mixture containing 30% recycled aggregate and concluded that the incorporation of recycled materials had no impact on thermal conductivity.

The definitions of the thermal properties of the materials used in the asphalt mixtures have been retrieved from various literature sources and are summarized in Table 2, while the albedo values of the asphalt mixtures, also retrieved from the relevant literature, are presented in Table 3. As can be seen, the values for thermal conductivity, heat capacity, and density vary, depending on several reasons, including differences in the properties of the materials, the method of calculation, experimental conditions, etc. However, all values are considered reliable, as they have been retrieved from scientific sources. Among the collected values, the ones presented in italics have been used in this study as they are the most frequently seen in the literature.

Table 4. Thermal properties of conventional and waste asphalt mixtures.

Properties of Each Mix	Conventional Hot Mix Asphalt (HMA)	CR Mix (Binder Substitute)	Plastic-Based Mix (Binder Substitute)	Steel Slag Mix (30% Aggregate Substitute)
Thermal conductivity (W/m·K)	1.82 1 [61], 1.487 ± 0.0008 [66], 1.16 [81]	1.441 ± 0.0008 [66]	0.68, 1.15 1 [61]	1.98 [82], 2.21 [83], 1.67 [84], 1.3821
Volumetric heat capacity (MJ/m3·K)	2.156, 1.98 [61], 2.214 [81]	2.157, 2.201 ± 0.0006 2 [67]	2.16, 1.67 [61]	2.432

¹ Dry sample. ² 50% content of waste tire powder.

presents the thermal properties of the mixtures retrieved from relevant literature sources or calculated for the scope of the analysis. The values presented in italics are the ones considered for the analysis of this study, while the values in bold italics are values that have been calculated on the basis of the properties of the materials presented in Table 2. For the definition of the volumetric heat capacity of the mixture, the thermal properties of the materials it consists of were considered. The volumetric heat capacity of each mixture was determined by calculating a linear function based on the capacities and volume ratios. This method can provide a representative estimate of the effective volumetric heat capacity [62,64].

Table 2. Thermal properties of the materials used for the examined mixtures.

Material	Asphalt Binder (Bitumen)	CR Binder Substitute	Plastic-Based Binder Substitute 1	Steel Slag Aggregate Substitute	Limestone Aggregate
Thermal conductivity, λ (W/m·K),	0.15–0.17 [65], 0.17, 0.362 ± 0.0003 [66]	0.243 2 [67], 0.275 ± 0.0005 3 [66], 0.418 4 [67]	0.331, 0.414, 0.502 [67]	0.90–1.70 [68]	1.30–3.00 [3] 1.4, 1.7, 2.3 [69], 2.92 [65], 2.8 [70]
Heat capacity (J/Kg·K)	1158 [65], 2093 [61], 1670 ± 0.0007 [66]	1757 [67], 1591 ± 0.0003 [66], 1443 [67]	2092–2301 [67]	732 [68], 810 [71]	790–930 [3], 921 [67], 908,
Density (Kg/m3)	1020–1060 [72]	1340 [67], 322.6 [73]	920, 935, 950 [67]	2230, 3100 [61], 3980 [71]	1650–2500 [67] 2000, 2200, 2600 [69]

¹ Polyethylene. ² Styrene butadiene + rubber carbon black. ³ 50% content of waste tire powder. ⁴ Butadiene-Acrylonitrile Rubber + C.

Table 2. Thermal properties of the materials used for the examined mixtures.

Material	Asphalt Binder (Bitumen)	CR Binder Substitute	Plastic-Based Binder Substitute 1	Steel Slag Aggregate Substitute	Limestone Aggregate
Thermal conductivity, λ (W/m·K),	0.15–0.17 [65], 0.17, 0.362 ± 0.0003 [66]	0.243 2 [67], 0.275 ± 0.0005 3 [66], 0.418 4 [67]	0.331, 0.414, 0.502 [67]	0.90–1.70 [68]	1.30–3.00 [3] 1.4, 1.7, 2.3 [69], 2.92 [65], 2.8 [70]
Heat capacity (J/Kg·K)	1158 [65], 2093 [61], 1670 ± 0.0007 [66]	1757 [67], 1591 ± 0.0003 [66], 1443 [67]	2092–2301 [67]	732 [68], 810 [71]	790–930 [3], 921 [67], 908,
Density (Kg/m3)	1020–1060 [72]	1340 [67], 322.6 [73]	920, 935, 950 [67]	2230, 3100 [61], 3980 [71]	1650–2500 [67] 2000, 2200, 2600 [69]

¹ Polyethylene. ² Styrene butadiene + rubber carbon black. ³ 50% content of waste tire powder. ⁴ Butadiene-Acrylonitrile Rubber + C.

Table 3. Albedo of the conventional and alternative asphalt mixtures examined.

Properties of Each Mix	Conventional Hot Mix Asphalt (HMA) New	Conventional Hot Mix Asphalt (HMA) Aged	CR Mix (Binder Substitute)	Polymer Modified Asphalt (PMA)	Steel Slag Mix (30% Aggregate Substitute)	Pigment Coating
Surface albedo	0.04–0.06 [74], 0.05 [75]	0.09–0.18 [74], 0.14 [76], 0.15 [75], 0.120 [61]	0.08 [76]	0.12 [76]	0.0674 [77], 0.089 [78],	Beige: 0.45 [79], Off-white: 0.55 [80], yellow: 0.62 [79], 0.44 [80], Green: 0.43 [79], 0.27 [80]

Table 3. Albedo of the conventional and alternative asphalt mixtures examined.

Properties of Each Mix	Conventional Hot Mix Asphalt (HMA) New	Conventional Hot Mix Asphalt (HMA) Aged	CR Mix (Binder Substitute)	Polymer Modified Asphalt (PMA)	Steel Slag Mix (30% Aggregate Substitute)	Pigment Coating
Surface albedo	0.04–0.06 [74], 0.05 [75]	0.09–0.18 [74], 0.14 [76], 0.15 [75], 0.120 [61]	0.08 [76]	0.12 [76]	0.0674 [77], 0.089 [78],	Beige: 0.45 [79], Off-white: 0.55 [80], yellow: 0.62 [79], 0.44 [80], Green: 0.43 [79], 0.27 [80]

2.2.2. Definition of Material Use

The steel slag mixture stands out among the presented asphalt mixtures, exhibiting the greatest disparity in thermal conductivity and volumetric heat capacity values when compared to the conventional asphalt mixture. This can be easily perceived because the steel slag mixture is the only one among the analyzed recycled or waste asphalt mixtures in which the recycled material serves as a substitute for the aggregate. In the crumb rubber and plastic-based asphalt mixtures, the recycled material replaces part of the binder, resulting in a very small proportion of the material in the mixture, which cannot significantly alter the properties of the initial mixture (e.g., conventional asphalt). The CR mixture has a thermal conductivity that is marginally lower than the conventional asphalt mixture, along with a slightly higher volumetric heat capacity. The same is also true for the plastic binder substitute asphalt mixture, which has a more significant difference (0.34 W/m·K) in its value of thermal conductivity when compared to the conventional asphalt mixture, but its volumetric heat capacity is still very close to the value of the conventional asphalt mixture (0.16 MJ/m³·K).

Therefore, at this stage, the steel slag asphalt mixture (aggregate substitute) was chosen for further analysis. However, it should be mentioned that although the analysis of the CR and plastic mixture is not included at this stage of the research due to the smaller difference between their thermal characteristics and those of conventional asphalt, it will be worth examining their impact on urban microclimates in a future analysis, especially since these mixtures have higher albedo values (0.08 and 0.12) compared to conventional asphalt (especially the newly constructed asphalt) and the steel slag mixture (Table 3).

Table 5. Selected values for the analysis.

Properties of the Composition of the Mix	Conventional Hot Mix Asphalt (HMA)	Steel Slag Mix (30% Aggregate Substitute)
Thermal conductivity (W/m·K)	1.487	1.980
Volumetric heat capacity (MJ/m3·K)	2.156	2.432
Surface albedo	0.040	0.067, 0.45, 0.62

summarizes the values for the analysis of the comparison of the conventional hot mix asphalt and the steel slag mix.

Moreover, for the other urban materials, the material and tree library of Envi-Met 5.7.1 was used. For the vegetated areas, grass with an average density of 25 cm was used, while for the trees, two sizes of deciduous trees were considered: a tree with a large, dense 25 m canopy and one with a small, dense 5 m canopy. For sidewalks and tiled surfaces, light-grey concrete has been considered, with an albedo value of 0.5, while for the building surfaces, typical exterior insulated plastered walls (with an albedo of 0.4) have been considered.

2.3. Simulation Data and Processing

This research analyzes two central street junctions in the city of Thessaloniki, as presented in Section 2.1. Both examined areas are 100 m × 100 m in size, and their square plan is oriented 34° west from the north axis.

For the scope of this analysis, surface and ambient temperatures are calculated to define the difference due to recycled materials, including aggregates and steel slag in the mixture. For the scope of this analysis, Envi-Met 5.7.1 software was used. The grid size used for the areas was 50 × 50 × 40, corresponding to 2.0 m × 2.0 m × 2.0 m.

 i.

Base case: Conventional asphalt pavements (existing situation).

ii.

Sustainable street scenario: replacement of asphalt pavements with recycled solid waste materials (steel slag).

Climate data for the city of Thessaloniki was retrieved from the Climate.OneBuilding.Org database [60]. The selected EPW file represents the typical meteorological conditions experienced in Thessaloniki over the last 15 years (2009–2023). The simulation was performed for the month of July, between 15:00–21:00 east European summer time (EEST) for all cases.

The choice of this specific period was based on the local climate data, according to which July is the hottest month of the year, with the highest peak temperatures. Furthermore, the highest temperatures in July appear between 16:00 and 18:00 (Figure 6), and the maximum incident solar radiation is at 15:00 EEST (Figure 7). Therefore, to reduce the simulation time, only the period after 15:00 EEST has been simulated.

3. Results

Firstly, the spatial and temporal distributions of the surface temperatures are presented for the two case study areas to illustrate the general condition of the case study area surfaces and the surface temperature variation according to the material and time of day.

Secondly, mean, max, and median temperatures for the asphalt surfaces of each area are presented to quantify the contribution of the examined asphalt mixtures to the pavement surface temperatures.

Thirdly, the surface temperature values of a specific point in the center of both areas are analyzed, including a higher albedo scenario with a value of 0.62.

Finally, the effect of the examined materials on the potential air temperature of the two areas is presented to quantify the improvement that can be achieved by using the waste or recycled asphalt pavement (steel slag aggregate asphalt mixture).

3.1. Surface Temperature

The spatial distribution of the surface temperature for the three different asphalt mixtures (conventional asphalt, the steel slag mixture, and the high-albedo steel slag mixture) for the time range of 15:00–21:00 EEST in the Sintrivani and Lagada areas is presented in Figure 8 and Figure 9, respectively. For the base case scenario, it is observed that the highest temperatures are on the street surface area, while the rest of the surfaces (sidewalks and paved and green areas) present lower surface temperatures. Among the examined asphalt mixtures, the steel slag mixture, and especially the high-albedo steel slag mixture, leads to obvious improvements in the street surface temperatures, especially at 16:00 EEST, where the conventional asphalt presents the highest values. Furthermore, it was also noted that the highest temperatures on the street surface are observed at 16:00, while after 20:00 (both refer to EEST), when the solar radiation is reduced—and especially after sunset—the temperature of the street surface is significantly lower. The distribution of surface temperatures follows a similar pattern for both examined areas. The differences that are observed can be explained by the geometry of the urban space around the street junction, which changes the shadows cast on the ground. In particular, the existence of tall buildings in the western part of the area alters the distribution and values of surface temperatures in the nearby parts of the street. In addition, vegetation, especially large trees situated at the southeast of the Lagada area, also provides shading in some parts of the area, reducing the neighboring surface temperatures.

Table 6. Mean and median temperature of the pavement surface (in °C) for the Sintrivani area.

Sintrivani	Base Case 16:00	Base Case 17:00	Steel Slag 16:00	Steel Slag 17:00	High Albedo 16:00	High Albedo 17:00
mean	40.60	39.18	31.58	33.04	26.34	27.10
median	41.57	40.56	34.96	35.27	26.25	27.28
st. deviation	3.32	3.64	5.52	3.45	0.96	1.18

and

Table 7. Mean and median temperature of the pavement surface (in °C) for the Lagada area.

Lagada	Base Case 16:00	Base Case 17:00	Steel Slag 16:00	Steel Slag 17:00	High Albedo 16:00	High Albedo 17:00
mean	36.77	36.50	31.95	33.82	25.78	26.72
median	41.04	39.00	35.12	35.54	26.34	27.39
st. deviation	8.00	4.22	6.97	3.38	5.64	4.92

present the results of the average and median values of the surface temperatures of the street area for the two examined areas. The base case mean surface value for the Sitrivani area is 40.6 °C at 16:00 and 39.18 °C at 17:00 (both refer to EEST), while for the Lagada area, it was 36.77 °C and 36.5 °C, respectively. It is observed that the mean values for the Lagada area are lower, which can be explained by the fact that there are large trees on both sides of the street in the southeast, which shade part of the street, lowering the surface temperatures. The mean surface value for the steel slag aggregate mixture scenario for the Sintrivani area is 31.58 °C at 16:00 and 33.04 °C at 17:00, and 31.95 °C and 33.82 °C, respectively, for the Lagada area. The surface temperature values are reduced due to the steel slag aggregate. For the Sintrivani area, the reduction ranges between 9.03 °C and 6.14 °C depending on the hour of the day (16:00 or 17:00, EEST).

For the Lagada area, the decrease in the mean surface temperature at 16:00 is 4.82 °C, while at 17:00, it is 2.68 °C. For the high-albedo scenario, the mean surface temperature of the pavement at 16:00 is decreased by 5.24 °C and 6.18 °C, and at 17:00, the decrease is 5.94 °C and 7.10 °C for each area, respectively. The rate of improvement is higher for the Sintrivani area, indicating that urban form and vegetation play important roles when it comes to the thermal performance of the asphalt surfaces in relation to the incident solar radiation they receive. Specifically, for the Sintrivani area, the reduction in the mean value due to the change in the aggregate is 22.2% at 16:00 and 15.7% at 17:00. The reduction observed for the maximum values is 13.3% at 16:00 and 11.1% at 17:00, corresponding to reductions of 5.6 °C and 4.58 °C in the surface temperature at the respective times. For the Lagada area, the reduction of the mean value due to the change in the aggregate is 13.1% at 16:00 and 7.3% at 17:00. The reduction of the maximum value is 14.0% at 16:00 and 7.8% at 17:00, corresponding to reductions of 6.02 °C and 3.13 °C in the surface temperature at the respective times.

Furthermore, it is observed that this reduction is even more important when the albedo of the steel slag asphalt mixture is increased to 0.45. For the Sintrivani area, the mean surface temperature is reduced by 35.1% at 16:00 and 30.8% at 17:00 (compared to the base case). The reduction of the maximum value is 30.3% at 16:00 and 26.0% at 17:00, corresponding to a total reduction of 12.85 °C and 10.76 °C in the surface temperature at the respective times (compared to the base case). For the Lagada area, the mean values are reduced by 29.9% at 16:00 and by 26.8% at 17:00. The reduction of the maximum values is 14.4% at 16:00 and 9.5% at 17:00.

Figure 10a,b shows the mean values for the two examined areas at 16:00 and 17:00.

The addition of the steel slag aggregate to the asphalt mixture and the increase in albedo both reduce the surface temperature of the pavement. However, for the steel slag and the high-albedo pavements, the mean temperature is lower at 16:00 and increases at 17:00, while for the base case (conventional asphalt), the temperature reduces from 16:00 to 17:00.

To verify this result, the surface temperature of a single point in the center of both areas was recorded for all asphalt mixture scenarios (the position of the point in the center of each is marked with a red cross in Figure 5). The specific points were selected in terms of their central location and for not being shadowed by buildings or vegetation.

The temporal distributions of the surface temperature (15:00–21:00) for the base case scenario, the steel slag scenario, and the high-albedo scenario are presented in Figure 11. In addition, for the same point, a higher albedo (0.62) has also been considered to examine its effect on the surface temperature of the examined point. The surface temperature is further reduced by the higher albedo of the pavement. For the Sintrivani area, the reduction ranges between 1 °C and 3.7 °C, depending on the examined hour (highest reduction at 16:00; lowest reduction at 21:00). For the Lagada area, the reduction ranges between 1.1 °C and 3.8 °C, also depending on the examined hour (highest reduction at 16:00; lowest reduction at 21:00).

The examination of point (single value) temperatures at the two examined areas confirms that the peak temperature for the conventional mixture is at 16:00, while for the steel slag mixture, it is at 17:00 (Figure 12). This can be explained by the higher volumetric heat capacity of the steel slag, which stores heat during the day, and its high thermal conductivity, which helps release this heat to the environment through its surface.

Assessment of Model Variability and Uncertainty in the Interpretation of Results

Across both locations, the high-albedo scenario consistently shows the lowest mean and median surface temperatures, followed by the steel slag scenario, with the base case scenario exhibiting the highest values. This indicates a clear cooling effect associated with the alternative pavement materials, particularly high-albedo surfaces that reflect more solar radiation.

The standard deviation (σ) values (Table 6 and Table 7) provide insight into spatial variability and model stability. In the Sintrivani area, variability is relatively low, especially for the high-albedo case (σ ≈ 1 °C), suggesting a consistent surface response. In the Lagada area, the base case and steel slag scenarios show higher σ values (up to 8 °C), indicating greater heterogeneity in surface conditions or local microclimatic effects. Such variability implies that local surface characteristics and shading may significantly influence thermal responses.

The computational model likely includes uncertainties related to:

• Boundary conditions and input data, such as meteorological parameters (solar radiation, wind speed, and humidity);
• Material property assumptions, e.g., emissivity, albedo, and thermal conductivity, which may not perfectly reflect real-world conditions.
• Spatial resolution, where coarse grid cells can smooth out local variations, underestimating temperature extremes.

These uncertainties propagate through the simulation, potentially affecting both the mean temperature estimates (systematic bias) and the standard deviations (random variability). For example, underestimated emissivity or surface roughness could lead to systematically lower modeled temperatures.

While relative differences between materials are large enough to remain statistically meaningful despite modeling uncertainties, the absolute temperature values should be interpreted with caution. The close alignment between mean and median values across most cases suggests a relatively symmetric distribution of temperatures, supporting the reliability of the comparative trends.

3.2. Air Temperature

The examination of pavement surface temperatures showed that the recycled asphalt mixture with the steel slag aggregate can greatly help reduce street overheating in summer. The effect of this reduction on ambient temperatures could improve microclimatic conditions. Figure 13 presents the median values of air temperatures for the two areas between 15:00 and 21:00 for the three asphalt mixtures. It is obvious that the median values of the air temperature are reduced. For the Sintrivani area, the median air temperature is reduced at 16:00 by 0.8 °C for the steel slag scenario when compared to the base case, and by 1.4 °C when comparing the base case and the high-albedo scenarios. This reduction corresponds to a reduction of 2.2% and 3.9%, respectively. For the Lagada area, the median air temperature is reduced by 0.9 °C at 16:00 when comparing the conventional and steel slag scenarios, and by 1.6 °C when comparing the conventional and high-albedo scenarios, which corresponds to a reduction of 2.5% and 4.4%, respectively. It is also observed that, for the steel slag scenario at 19:00, the median air temperature is not reduced but slightly increased in both areas by 0.2 and 0.5 °C for the Sintrivani and Lagada areas, respectively. This might also be explained by the thermal storage ability of the steel slag mixture and its thermal conductivity, resulting in increased heat release into the air after the progressive reduction of solar radiation due to the sunset. However, it is important to note that the increase that is observed in this case can be reversed using a higher surface albedo, as in both examined areas, the high-albedo scenario presents lower temperatures than the conventional scenario for all the time series examined.

The maximum values of the air temperature are also reduced. For the Sintrivani area, the maximum air temperature is reduced at 16:00 by 0.1 °C for the steel slag scenario when compared to the base case, and by 0.3 °C when comparing the base case and the high-albedo scenarios. This reduction corresponds to a reduction of 0.4% and 0.7%, respectively. For the Lagada area, the maximum air temperature is reduced by 0.9 °C at 16:00 when comparing the conventional and steel slag scenarios, and by 1.1 °C when comparing the conventional and high-albedo scenarios, which corresponds to a reduction of 2.4% and 2.9%, respectively. The air temperature distribution for the three examined scenarios at 16:00 (Figure 14) shows that the higher air temperatures are located in areas with asphalt pavement, while it is obvious that, in these areas, the temperature decrease is higher, while the cooling effect is less intense in areas covered with other materials (e.g., tiled sidewalks or grass).

4. Discussion

This research examines surface and ambient temperature changes under the solar radiation effect caused by the waste and/or recycled materials in asphalt mixtures used for the construction of urban pavements. For the scope of this research, the thermal properties of waste and recycled asphalt mixtures were examined by using the existing literature, including experimental and calculation studies. Then, two case study areas in the urban tissue of Thessaloniki, Greece were used to calculate the possible effects of waste asphalt mixtures on the microclimatic conditions of the case study areas. Apart from the base case scenario, which examined the conventional asphalt mixture, two steel slag mixtures were examined: one with a 30% steel slag aggregate and a similar one with a pigment coating to increase surface albedo.

The results of this research show that the area covered with asphalt presents higher surface temperatures in the urban areas examined, contributing to an increase in local urban air temperatures. While conventional asphalt reaches its highest maximum surface temperature (43 °C) at 16:00 EEST, the steel slag asphalt mixtures reach their highest temperature one hour later. This means that there is a shift, which is due to the thermal lag caused by the material’s higher thermal storage properties.

The maximum improvement (compared to the base case scenario) on the mean surface temperature of urban pavements for the steel asphalt mixture can reach up to 9 °C at 16:00 EEST. For the high-albedo (0.45) steel slag asphalt mixture, this improvement is around 14 °C.

The impact of cooler pavements on the ambient temperature is also calculated, proving that there can be an important improvement in reducing air temperatures, especially during the hottest hours of the day. The improvement of median values is calculated to be between 2.2% and 4.4%, with the lower percentage corresponding to the steel slag asphalt mixture for the Sitrivani area and the higher percentage corresponding to the high-albedo pavement for the Lagada area. This reduction could make a significant change in the urban microclimatic conditions of cities, reducing ambient temperatures and enhancing thermal comfort. It is also observed that while the high thermal conductivity of the steel slag might facilitate internal heat transfer within the material, its high heat storage capacity governs the temporal dynamics of heat release, which can cause sustained warmth in urban settings. However, this effect can be diminished by using a higher albedo. The demonstrated temperature reductions, even when accounting for modeling uncertainties, suggest that material-based interventions can play a significant role in urban heat island mitigation strategies.

Other studies on cool materials also corroborate these research findings. Research conducted in another urban area of Athens indicated that substituting conventional pavements with cool pavements could lead to a reduction in the maximum air temperature in that region by 1.2 to 2.0 °C [85,86]. Moreover, research in the suburbs of Athens proves that the difference in average values of air temperature between conventional black asphalt and high-albedo asphalt with a thin white layer at a height of 1.5 m is 7 °C. According to the findings of that research, there was also an average reduction of 5 °C in air temperatures in the simulated region under low-wind-speed conditions [80]. In addition, a study in the Thessaloniki area also examined the possibility of lowering ambient air temperatures under both conservative and radical scenarios, concluding that it is possible to achieve an average reduction of 1 °C. The same study found that urban surface temperatures reached a median of 27.4 °C (a 47% reduction from the initial 40.4 °C), demonstrating that they absorb significantly less heat to be re-emitted after sunset [87].

The examination of higher-albedo pavements proves that there can be an even greater improvement in the pavement’s surface temperature, as the examination of a specific point at the center of the two case study areas showed that the asphalt surface temperature is further reduced with the use of a higher-albedo material (0.62). The average reduction for the whole examined period (15:00–21:00) is 2.5 °C, while the maximum is 3.8 °C at 16:00. However, very high albedo values in urban surfaces should be further examined as there are other parameters that should be considered, such as glaring or the excessive reflection of incident solar radiation, which can cause an increase in air temperatures and damage other materials or objects in the urban equipment and infrastructure [88].

5. Conclusions

Creating climate-neutral urban environments demands a comprehensive approach, since no single solution alone can effectively enhance urban climate resilience [89].

The results of this research are useful, proving that waste materials used in asphalt mixtures, together with high-albedo surfaces, can create important changes in urban microclimates. According to the literature, around one-quarter of fossil fuel consumption and one-third of global air pollution are linked to pavement applications. Thus, sustainable asphalt mixtures could improve environmental quality [81].

There are plenty of studies approaching the sustainability issues of materials by examining the properties of recycled or reused materials (including thermal properties) to prove their compliance with standards. For example, Jahanbakhsh et al., 2020, have demonstrated that the mechanical properties of mixtures can be enhanced with different amounts of reclaimed asphalt pavements (30%, 60%, and 100%), waste engine oil, and a supplementary binder modified with crumb rubber, which are better than or equivalent to those of the conventional mixture [90]. Other studies have analyzed the performance of steel slag asphalt mixtures in order to define the optimum content of steel slag in the mixture that meets all standards for road performance. It is suggested that the content of steel slag should not exceed 60% [84]. It is therefore important for further research to consider different contents of the recycled aggregate to optimize its use, not only in terms of thermal performance but also with regards to other properties, such as strength and durability, which should also be explored from the integrated perspective of the sustainable use of materials.

This research investigated the possible contribution of asphalt mixtures with recycled materials on the urban microclimate in order to quantify the temperature decrease that can be achieved during the summer period and to add one more argument for cleaner technologies in the production of asphalt mixtures with recycled or waste aggregates. Recent research mainly focuses on the mitigation of the UHI phenomenon through different materials, emphasizing the use of cool pavements in terms of high-albedo surfaces or the thermal properties of the materials [91]. Focusing on the performance of waste materials in terms of their effect on urban temperatures has not been extensively analyzed, as there is still little research examining the effect of recycled or reused materials on the UHI phenomenon. The extension of research in this field could provide further motivation for the sustainable use of resources, as it will provide one more reason to promote the use of recycled materials that can reduce urban air temperatures.

The present study concentrated on investigating the effect of steel slag on urban temperatures, given that its thermal properties showed greater differences from those of conventional asphalt mixtures. Moreover, recycling steel slag offers the benefit of being collectable from a limited number of steel plants, which enhances collection efficiency compared to most other solid waste materials. Additionally, it is comparatively easy to manage and ensure consistency in the quality of this waste material [2]. However, future research should also examine the effect of other recycled or waste asphalt mixtures at urban temperatures, as the complexity of the parameters affecting the urban microclimate is not easy to predict in terms of the thermal properties of the materials.

There are some limitations to this research. As for the systematic literature review defining material thermal properties, the use of experimental data would be useful to improve the quality of the input data of the simulation. This would allow for coupling the results on the potential air temperatures and the possible improvement of the microclimate, and provide a more accurate prediction of the improvement of the urban microclimate. In this direction, further research could consider different contents of the steel slag in the asphalt mixture, which is also a parameter that has been proven to affect its albedo and thermal properties. Cao et al. [84] have determined that the thermal conductivity of steel slag asphalt mixtures first rises and then falls as the steel slag content increases, explaining that when the slag content is below 60%, the positive effect of iron-containing oxides on thermal conductivity may prevail. On the contrary, when it exceeds 60%, the increased voids in the mixture may be the primary factor affecting thermal conductivity [84]. Jointly considering both the mechanical and the thermal requirements, a maximum rate of 30% could be proposed based on the literature findings [50,51,52,84].

It is also important to mention that similar studies using Envi-Met software for simulating the microclimate of Thessaloniki have confirmed the reliability of the software by comparing the simulation data with in situ data [87].

Another limitation of this research is that it considers the specific climatic conditions of Thessaloniki, while the climatic data of other cities could provide further results on the possible effect of recycled or waste materials on the challenge of reducing overheating in contemporary cities. Similarly, the expansion of the results on a larger part of the urban street network could verify the additive effect that could be achieved if using recycled or reused materials in pavement asphalt mixtures on a larger scale. A study of urban geometry could provide guidelines for prioritizing the replacement of asphalt pavements in specific typologies of the street network, coupled with vegetation strategies to optimize urban temperature conditions and quality of life in cities.

Despite these positive remarks, implementing large-scale steel slag or high-albedo pavements in densely populated cities such as Thessaloniki faces several practical and economic challenges. Material availability and quality control are primary concerns, as steel slag must be sourced reliably from steel plants and meet strict mechanical and thermal specifications. The transportation costs of slag-based mixtures might be high. Furthermore, the initial construction costs of high-albedo mixtures are higher than conventional pavements, which may deter municipal investment despite long-term environmental benefits. Urban integration adds further complexity, requiring careful planning to minimize traffic disruptions and coordinate multiple stakeholders across a dense cityscape. Finally, limited economic incentives and the current lack of policy support can slow adoption, despite the clear potential for mitigating UHI and improving thermal comfort.

Overall, public policies are necessary to promote research on the sustainable use of resources by encouraging the reuse and recycling of waste, aiming to reduce the environmental impact of extracting raw materials and enhancing circular economy perspectives and climate adaptation frameworks. Large-scale experiments and pilot case studies are essential for evaluating the feasibility, performance, and practical challenges of implementing steel slag or high-albedo pavements in urban environments.