Cool Pavements Research Articles

Micrometeorological effects and thermal-environmental benefits of cool pavements: findings from a detailed observational field study in Pacoima, California

Cool pavements represent one of several strategies that can mitigate the effects of urban overheating by increasing albedo. By definition, this means increasing reflected and potentially re-absorbed short-wave radiation but also decreased surface and air temperatures and longwave upwelling, thus reducing radiant temperatures. So far, real-world studies have been inconclusive as to net effects from cool pavements. A project by GAF installed reflective pavements in Pacoima, California, in summer of 2022. This study set out to perform detailed, high spatiotemporal resolution, multi-platform observations to quantify micrometeorological benefits of the cool pavements and address concerns regarding glare, chemistry/air quality, and pedestrian thermal comfort. Results indicated large variability, as expected, but that the dominant effects were beneficial both in direct side-by-side, real-time comparisons (RT) between test and reference areas, as well as in difference-of-difference (DofD) to quantify local changes in test areas. During a heatwave in September 2022, maximum air-temperature differences (averaged over individual street segments) reached up to −1.9 °C RT in the afternoon. During non-heatwave, hot summer days, the largest street-segment-averaged afternoon air-temperature differences reached up to −1.4 °C RT or −2.8 °C DofD, and surface temperature up to −9.2 °C RT or −12.2 °C DofD. Whereas above values represent maximum effects, more typical street-segment averages also showed statistically significant benefits. In the afternoon, the mean of air-temperature differences was −0.2 °C RT and −1.2 °C DofD. The mean of surface-temperature differences was −2.6 °C RT and −4.9 °C DofD. Indicators of pedestrian thermal comfort also showed variability but predominantly a cooling effect. The mean of differences in mean radiant temperature was between −0.9 and −1.3 °C RT, and for physiological equivalent temperature, between −0.2 °C and −0.6 °C RT and −1.7 °C DofD. In terms of predicted mean vote, the mean of differences was −0.09 RT and −0.32 DofD.

The Concurrent Effect of Building Height Diversity and Cool Pavement Materials on Air Temperature Near the Surface of an Urban Facade: A Case Study of Shahriar Street In Esfahan, Iran

The rise in urban temperatures is a significant threat to the urban heat island effect, increasing building energy demand. This increase is worrisome because the supply of renewable energy is a big challenge. The approach to improving the urban microclimate offers a promising solution. This research investigates the concurrent effect of urban morphological parameters and physical characteristics of urban surfaces, such as cool materials, on the urban microclimate near the building's facades in an urban street. This evaluation was conducted by ENVI-met(v4). The results show that the concurrent effects of increasing building height diversity and by using cool pavement materials are more helpful in reducing the average air temperature of an urban street. Because the amount of shade and wind speed increased as building height variety increased, the absorption of solar radiation decreased as pavement material albedo increased. As a result, these two parameters reduced air temperature by 0.8 °C. Also in the combined scenario of increasing building height diversity and by using cool pavement materials, the air temperature near the building's facades was reduced by 1°C on the first and second floors and by approximately 0.5°C on the upper floors.

The Evaluation of the Temperature Reduction Effects of Cool Roofs and Cool Pavements as Urban Heatwave Mitigation Strategies

Various types of damage, including health risks, caused by abnormal climate conditions have gradually increased in recent years, particularly in cities with a high degree of complexity. In particular, it is important to address heatwave damage risk from both technical and policy perspectives because it poses risks to older adults and/or disadvantaged individuals. This study was performed to assess the quantitative effects of two heatwave mitigation measures, cool roofs and cool pavements, in the Jangyumugye district of Gimhae, Republic of Korea. Thermal imaging cameras were used to measure surface temperatures after the installation of these measures; after the installation of cool roofs, indoor temperatures were also measured using temperature measurement data loggers. Then, a survey was conducted to obtain feedback from users of the associated facilities. From August to October 2019, surface temperatures were significantly reduced following the installation of cool roofs and cool pavements. Cool roofs constructed using slabs and panels provided average temperature reductions of 15.5 °C and 11.6 °C, respectively. Cool pavements showed a temperature reduction of ≥4 °C in both parking lot and alley sites. More than 30% of users noticed a reduction in temperature as a result of heatwave measure installation. These results may be used by the city to make informed policy decisions regarding the implementation of heatwave mitigation measures.

Innovative light-coloured porous asphalt for low-impact pavements: A laboratory investigation

In recent years, research is pushing toward more sustainable solutions in the road sector such as cool pavement and eco-friendly paving components. Cool pavements are advanced pavement technologies characterized by lower surface temperature thanks to the use of light-coloured binders and an open-graded structure. In this framework, the present study intends to provide a laboratory characterization of two different open-graded wearing courses produced with Construction and Demolition waste (CDW) aggregates and virgin aggregates and with a transparent resin-based binder. The final aim of the research is to provide scientific evidence on the possible application of the adopted resin-based transparent binder with light-coloured aggregates for open-graded wearing courses in order to minimize the Urban Heat Island phenomenon. The results show that the addition of 50% of CDW aggregates to the porous asphalt enhances the abrasion resistance and the mechanical performance of the mixture.

Effectiveness of Road Cool Pavements, Greenery, and Canopies to Reduce the Urban Heat Island Effects

The ongoing climate change is manifesting itself through the increasing expansion of Urban Heat Island (UHI) effects. This paper evaluates the microclimate benefits due to cool road pavements, greenery, and photovoltaic canopies in a parking lot in Fondi (Italy), identifying the best strategy to counteract the negative effects of UHIs. The ENVI-met software allowed a microclimatic analysis of the examined area in July 2022 through the comparison of the thermal performances between the current asphalt pavement and ten alternative scenarios. The proposed layouts were investigated in terms of air temperature (AT), surface temperature (ST), mean radiant temperature (MRT), and predicted mean vote (PMV). The results showed that the existing asphalt pavement is the worst one, while the cool pavement integrated with vegetation provides appreciable benefits. Compared to the current layout, a new scenario characterized by light porous concrete for carriageable pavements and sidewalks, concrete grass grid pavers for parking stalls, a 2-m-high border hedge, and 15-m-high trees implies reductions of AT above 3 °C, ST above 30 °C, MRT above 20 °C, and a maximum PMV value equal to 2.2.

Experimentation of Mitigation Strategies to Contrast the Urban Heat Island Effect: A Case Study of an Industrial District in Italy to Implement Environmental Codes

The European goals to reduce CO2 emissions by up to 40% by 2030 and reach carbon neutrality by 2050 cannot ignore the building sector, that accounts for 27% of global greenhouse gas emissions. In the context of the sustainable development goals, it is a key point to consider the reduction of the heat island effect in the urban environment. Considering this background and the proven absence of the clear promotion of urban mitigation measures, the research aims at investigating the influence on several micro-climate parameters of different retrofitting strategies at the building level (green façades) and the cooling strategies at the urban scale (e.g., cool pavements, trees). As a case study, the application of these measures in an industrial district located in Italy is evaluated. ENVI-met software was adopted to perform the outdoor environmental simulations, in order to assess the effectiveness of the mitigation strategies proposed, considering both the whole district and a portion, focusing on urban canyons. Cool pavements proved to be the most promising strategy to both reduce the air temperature and increase the relative humidity. Slighter effects on environmental conditions can be achieved by planting trees and installing green walls that, by contrast, significantly affect the mean radiant temperature and buildings’ surface temperatures, respectively.

Numerical Investigation of the Temperature Field Effect on the Mechanical Responses of Conventional and Cool Pavements.

Conventional asphalt pavement has a deep surface color and large thermal inertia, which leads to the continuous absorption of solar thermal radiation and the sharp rise of surface temperature. This can easily lead to the permanent deformation of pavement, as well as aggravate the urban heat island (UHI) effect. Cool pavement with a reflective coating plays an important role in reducing pavement temperature and alleviating the UHI effect. It is of great significance to study the influence of temperature on the mechanical response of different types of pavement under vehicle loading. Therefore, this study examined the heat exchange theory between pavement and the external environment and utilized the representative climate data of a 24 h period in the summer. Two kinds of three-dimensional finite element models were established for the analysis of temperature distribution and the mechanical responses of conventional pavement and cool pavement. The results show that in this environmental condition, conventional pavement temperatures can exceed 50 °C under high temperatures in summer, which allows for the permanent deformation of pavement and further affects the service life of asphalt pavement. The temperature difference in a conventional pavement surface between 6 h (24.7 °C) and 22 h (30.2 °C) is much less than that between 22 h (30.2 °C) and 13 h (50.1 °C) in the summer. However, the difference in the vertical displacements of the pavement surface between 6 h and 22 h is much larger than that between 22 h and 13 h. One reason is that the difference in temperature distribution between the morning and night leads to changes in pavement structure stiffness, resulting in significant differences in vertical displacement. Cool pavement has a significant cooling effect, which can reduce the surface temperature of a road by more than 15 °C and reduce the vertical displacement of the pavement by approximately 11.3%, which improves the rutting resistance of the pavement. However, the use of cool pavement will not change the horizontal strain at the bottom of the asphalt base and will not improve the fatigue resistance of asphalt pavement. This research will lay the foundation for further clarifying the difference in the mechanical properties between the two types of pavements in the management and maintenance stage.

Evaluation of Microclimate Benefits Due to Cool Pavements and Green Infrastructures on Urban Heat Islands

Ongoing climate change is manifesting in the expansion of the urban heat island (UHI) effect. This paper evaluates the microclimate benefits of cool pavements and green infrastructures in a historical square in Rome, Italy. The ENVI-Met software enabled us to conduct a microclimatic analysis of the examined square in July 2021, through a comparison of the thermal performances of the current asphalt and sampietrini pavement (AS) with three alternatives. The proposed options are to change the existing layout to include: light concrete slabs and green furniture consisting of 5 m high trees (LC+G1), a perimeter hedge with 5 m high trees (LC+G2), and a perimeter hedge with 15 m high trees (LC+G3). The effects of the road pavements as passive countermeasures to the UHI effect are investigated in terms of air temperature (AT), mean radiant temperature (MRT), and predicted mean vote (PMV). The results show that the existing asphalt pavement is the worst option, while the cool pavement integrated with vegetation is greatly beneficial to human thermal comfort. Compared to the current layout, LC+G3 is the best scenario, because it implies an AT reduction higher than 3 °C, a MRT reduction equal to 50%, and a maximum PMV value equal to 2.2.

Materials to Mitigate the Urban Heat Island Effect for Cool Pavement: A Brief Review

The urban heat island (UHI) effect has a significantly negative impact on the living environment in urban areas. Asphalt pavement is one of the most widely used infrastructures that absorbs solar energy, which leads to the UHI effect and premature failure. As a result, cool pavement technology has been rapidly developed in recent years to mitigate the UHI effect originating from asphalt pavement. Although several outstanding review articles have analyzed previous studies on cool pavement technologies, very few review articles have focused on how to design and expand cool pavement technology from a materials perspective. In this mini-review article, the theoretical and practical factors of the solar reflective coatings and phase-change materials, which are significantly dependent on the design of new materials, have been summarized. The main challenges and potential problem-solving ideas have been presented. In a cool pavement, the solar reflective coatings are composed of epoxy resin or acrylic polymer matrix filled with solar reflective nanoparticles, such as TiO2, SiO2, ZnO, Al2O3, or Fe2O3. The main challenges of the solar reflective coatings are the spalling of the coating polymers from the asphalt pavement surface and the dispersion of the solar reflective nanoparticle in the polymer matrix. Most importantly, it is critical to harmonize the balance between the bonding strength, aging rate, solar reflectance, curing requirements, mechanical properties, and durability of the solar reflective coating. For the nanofillers, the cost of the filler materials, the balance between UV, visible light, and near-infrared reflectance and the dispersion status of the nanofillers in the polymer matrix are the primary factors that must be concerned. For the phase-change materials (PCMs), the interaction between the asphalt and the PCMs, the decomposition of the PCMs, the toxicity of the PCMs, the distribution status of the PCMs in the asphalt matrix, and the cost are the main factors that have to be considered in constructions. This review article can not only provide basic knowledge for the development of new solar reflective pavement materials but also serve as a guide for practical applications of cool pavement in the field.

A Literature Review of Cooling Center, Misting Station, Cool Pavement, and Cool Roof Intervention Evaluations

Heat islands and warming temperatures are a growing global public health concern. Although cities are implementing cooling interventions, little is known about their efficacy. We conducted a literature review of field studies measuring the impact of urban cooling interventions, focusing on cooling centers, misting stations, cool pavements, and cool or green roofs. A total of 23 articles met the inclusion criteria. Studies of cooling centers measured the potential impact, based on evaluations of population proximity and heat-vulnerable populations. Reductions in temperature were reported for misting stations and cool pavements across a range of metrics. Misting station use was evaluated with temperature changes and user questionnaires. The benefits and disadvantages of each intervention are presented, and metrics for evaluating cooling interventions are compared. Gaps in the literature include a lack of measured impacts on personal thermal comfort, limited documentation on intervention costs, the need to standardize temperature metrics, and evaluation criteria.

Cool Pavements: State of the Art and New Technologies

With growing urban populations, methods of reducing the urban heat island effect have become increasingly important. Cool pavements altering the heat storage of materials used in pavements can lead to lower surface temperatures and reduce the thermal radiation emitted to the atmosphere. Cool pavement technologies utilize various strategies to reduce the temperature of new and existing pavements, including increased albedo, evaporative cooling, and reduced heat conduction. This process of negative radiation forces helps offset the impacts of increasing atmospheric temperatures. This paper presents an extensive analysis of the state of the art of cool pavements. The properties and principles of cool pavements are reviewed, including reflectivity, thermal emittance, heat transfer, thermal capacity, and permeability. The different types, research directions, and applications of reflective pavements are outlined and discussed. Maintenance and restoration technologies of cool pavements are reviewed, including permeable pavements. Results show that cool pavements have significant temperature reduction potential in the urban environment. This research is important for policy actions of the European Union, noting that European and international business stakeholders have recently expressed their interest in new ways of reducing energy consumption through technologically advanced pavements.

Could a bio-resin and transparent pavement improve the urban environment? An in field thermo-optical investigation and life-cycle assessment

Pavements affect the urban microclimate and the overall quality of the outdoor environment. Cool pavements, in particular, can guarantee lower surface temperature than conventional ones, e.g. asphalt and concrete. These solutions often comprise reflective coatings placed above existing pavements or more complex configurations, i.e. evaporative and heat harvesting solutions. This study investigates in-field the thermo-optical profiles of cool paving systems made with a novel modified epoxy resin binder. Unlike classic binders, the presented binder is transparent and, when combined with the proper aggregate minimizes its contribution to local surface overheating phenomena. The two novel fields presented herein were found to maintain up to 9.2 °C and 6.4 °C lower surface temperature during daytime and up to 8.4 °C and 4.6 °C lower surface temperature during nighttime as compared to the asphalt and concrete fields, respectively. A cradle-to-gate life cycle assessment showed that the novel binders may also decrease pavements’ footprint on human health and ecosystems, while the application of a CO2 off-set model showed their good potentiality to decrease the corresponding emissions. The novel binders, that unlike typical cool paving applications are an integral part of the pavement, are ready to be integrated in real life applications.

Evaluating urban heat island mitigation strategies for a subtropical city centre (a case study in Osaka, Japan)

Urban heat island (UHI) effects were first observed in London in the 19th century. The urban heat island is identified by developing higher temperatures in urban areas than the surrounding rural areas that directly surround them. However, there are some main mitigation strategies to deal with subtropical UHI, such as increasing the albedo of the urban environment (reflective pavement) and developing the green infrastructure (green roof) in urban areas. This study would analyse the microclimate in a subtropical city by evaluating meteorological parameters with a three-dimensional model simulation software of computational fluid dynamics (CFD) named ENVI-Met.To evaluate Urban Heat Island mitigation strategies for a Subtropical City Centre, subtropical city Osaka, Japan, is selected to investigate UHI with modelling. The simulation has been used to applicate the five scenarios (base scenario, cool pavement scenario, cool roof scenario, increasing vegetation coverage scenario, and integrated scenario) with different albedo and vegetation coverage ratios. In this study, outdoor air temperature, sky view factor, net radiation, mean radiant temperature and thermal radiative power are considered as five criteria for evaluating the efficiency of UHI mitigation strategies. The simulation results of the base model (scenario A) with Osaka's current condition are considered the reference value. The relative percentage differences between each scenario with a base model are determined. The increased albedo of urban fabric material (scenario B Cool pavement model) showed the most efficient to mitigate UHI. The relative percentage differences of the five criteria in the Cool pavement model are more significant than other models. Therefore, the results of this study can provide valuable guidance, both for keeping subtropical residents cooler and informing subtropical climate cities that would be sustainable in the future.

Lab-Controlled Experimental Evaluation of Heat-Reflective Coatings by Increasing Surface Albedo for Cool Pavements in Urban Areas

Many studies were conducted to find possible strategies for reducing the urban heat island (UHI) effect during the hot summer months. One of the largest contributors to UHI is the role that paved surfaces play in the warming of urban areas. Solar-reflective cool pavements stay cooler in the sun than traditional pavements. Pavement reflectance can be enhanced by using a reflective surface coating. The use of heat-reflective coatings to combat the effects of pavements on UHI was pre-viously studied but no consistent conclusions were drawn. To find a conclusive solution, this work focuses on the abilities of heat-reflective pavement coatings to reduce UHI in varying weather conditions. Within this context, both concrete and asphalt samples were subject to a series of per-formance tests when applied to a heat-reflective coating, under the influence of normal, windy, and humid conditions. During these tests, the samples were heated with a halogen lamp and the surface temperature profile was measured using an infrared thermal camera. The air temperature was recorded with a thermometer, and the body temperature at multiple depths of the samples was measured using thermocouples. The results from these tests show that the effectiveness of the heat-reflective coating varies under different weather conditions. For instance, the coated samples were about 1 °C cooler for concrete and nearly 5 °C cooler for asphalt, on average. However, this temperature difference was reduced significantly under windy conditions. As such, the findings from this work conclude that the heat-reflective coatings can effectively cool down the pavement by increasing the surface albedo, and thus might be a viable solution to mitigate UHI impacts in the city/urban areas.

Improving outdoor human-thermal environment by optimizing the reflectance of water-retaining pavement through subjective field-based measurements

The role of urban surfaces in climate is recognized worldwide. Cool pavements such as reflective and water-retaining pavements contribute to the mitigation of urban heat. To optimize the cooling performance of a combination of reflective and evaporative characteristics, water-retaining (WR) pavements, with reflectance values of 13% (WR13) and 25% (WR25), were used and compared to a conventional asphalt pavement (AS). In addition to measuring surface and atmospheric temperatures, human-energy balance known as “human-thermal load” was assessed using perceptive responses obtained from participants (n = 78). Experimental insolation conditions were classified as low-, medium-, and high-based on radiation effects. Water-retaining pavements demonstrated significant surface temperature reductions compared to the overall AS; however, air temperature and humidity above the pavement were less sensitive, and they were not significantly different between the two distinct pavements. Under high insolation, local thermal comfort improved and skin temperatures of the lower body reduced. These consequently reduced the temperature perceived in the whole-body and improved the thermal environment on the surface of WR13. These results can be reasonably explained using the human-thermal load. The increase in thermal load from reflected solar radiation is limited below a certain level of insolation. The overall radiation received by WR13 was moderate from both ground-reflected and -sourced infrared radiation sources. The human-thermal load of WR13 was significantly different from that of AS under high insolation. A modest reflective pavement, such as WR13, is anticipated to improve human thermal comfort.

Effect of Sampietrini Pavers on Urban Heat Islands.

Cool pavements are reflective and/or permeable pavements that improve microclimate of urban areas where heat islands cause discomfort to citizens. Stone pavements lower surface temperatures and reduce the amount of heat absorbed. This study assessed, using ENVI-met 4.3 LITE software, how air temperature and predicted mean vote depend on physical properties of the road pavement. A comparative microclimatic analysis was implemented on a rectangular square in Rome (Italy) in the summer, paved in three different ways: asphalt, traditional sampietrini, and permeable sampietrini. The model considered local weather parameters, surrounding fabric, and vegetation to give reliable results in terms of numerical and graphical output using the application tool Leonardo. The tested pavement types affected air temperature during the day, but did not influence this variable in the early morning. Permeable sampietrini pavement was more effective than traditional sampietrini pavement in reducing air temperature compared to the current asphalt surface. The road pavement did not, however, affect human comfort in terms of predicted mean vote. The obtained results are useful for further investigation of parameters that could modify the microclimatic conditions of urban areas.

Urban-Scale Evaluation of Cool Pavement Impacts on the Urban Heat Island Effect and Climate Change.

We implemented a context-sensitive and prospective framework to assess the global warming potential (GWP) impacts of cool pavement strategies on specific roads for different cities. The approach incorporates several interconnections among different elements of the built environment, such as buildings and urban road segments, as well as the transportation fleet, using specific building and pavement information from an urban area. We show that increasing pavement albedo lowers urban air temperatures but can adversely affect the building energy demand in the areas with high incident radiation exposure. The heating energy savings and the radiative forcing effect improve the GWP savings in cold and humid climate conditions. The total GWP savings intensity is sensitive to the city morphology and road traffic. The probabilistic results show that cool pavement strategies can offset 1.0-3.0% and 0.7-6.0% of the total GHG emissions of the U.S. cities Boston and Phoenix, respectively, for a 50-year analysis period. The worldwide range of savings can be as large as 5.0-44.7 Gt of CO2 eq. A paradigm shift in pavement strategy selection is required in most neighborhoods.

The Use of Cool Pavements for the Regeneration of Industrial Districts

Industrial districts are characterized by the presence of low and extensive building volumes and by predominantly sealed, impermeable surfaces, which contribute to several environmental problems and to the deterioration of outdoor human thermal comfort conditions, especially during summer hot days. To tackle these issues, this study proposes an approach for the regeneration of industrial districts based on the application of cool materials. Reflective and evaporative pavements were selected as suitable solutions to reduce summer overheating, while ensuring the functionality required by the industrial production, and contributing to stormwater management. The effectiveness of the approach was tested in a portion of the industrial district of Padua (Italy). In summer conditions, the replacement of conventional pavements with cool materials results in a reduction of the ground surface temperatures up to 14.0 °C and a consequent decrease of the air temperature at pedestrian level between 0.6 and 1.2 °C. The effects of human thermal comfort conditions highly depend on the selected cool material and on the morphology of the urban canyon. Finally, the reduction of external surface and air temperatures also contributes in cooling indoor spaces (average decrease from 1.0 to 2.5 °C), with impacts on the energy efficiency of the industrial buildings.

Temperature of Paved Streets in Urban Mockups and Its Implication of Reflective Cool Pavements

In summer, urban heat islands increase building cooling demands, aggravate air pollution, and cause heat-related illnesses. As a mitigation strategy, reflective cool pavements have been deemed an effective measure to decrease the temperature in urban areas. However, the reflection of paved streets in an urban area will be different from that in an open area. It remains unknown which fraction of paved streets needs to be cooled upmost, and if increasing the albedo of paved streets can effectively reduce their temperature. This study measured the skin temperature of two urban mockups, of which one contained white streets and the other, gray streets. The streets were orientated at different strikes. It was found that in summer the East-West street was hotter than both the cross street and the South-North street. At nighttime, the heat released from building blocks kept the paved street about 0.2 °C hotter than paved areas in open spaces. It was also found that street orientation controlled the skin temperature of an urban street while the sky view factor (or building height and street width) acted in a secondary role only. Increasing the albedo of the paved street in an urban canyon effectively reduced the skin temperature of the street. Reflective pavements should be built preferentially on East-West streets and the cross streets.

How Cool Pavements and Green Roof Affect Building Energy Performances

The urban heat island effect amplifies the global warming phenomenon with sensible aftermaths on the building energy consumptions. The absence of green areas, open water channels as well as the lack of free unsurfaced spaces creates a sort of urban cap and a heat reflection effect, from the bottom upwards, which persists in the early hours of the night. This heating effect exacerbates the use of air conditioners which, if on one hand helps to cool houses, on the other hand rejects outside hot air hindering the cooling effect of nighttime air. The aim of this study is to analyze the effects of urban heat island mitigation techniques on the building energy performances, such as cool pavements and green roofs. Measured temperatures were used to calibrate a model of a densely populated district in Rome by means of ENVI-met software. It was subsequently carried out an energy analysis through the use of TRNSYS in order to study the energy behavior of a reference building in its urban context in function of the variation of the boundary conditions.