Cool Pavements Research Articles

Cool pavements for climate change adaptation

Whitin the framework of road infrastructures, CHM has led a study that investigates construction materials in order to reduce accumulated and released heat on urban surfaces, such as asphalt, especially in summer, seeking to minimize the "urban heat island”.This project is the result of having developed reflective pavements that has passed through laboratory level, manufacture on an industrial scale and their testing on a small scale to ensure proper implementation, as well as their implementation by means of a large-scale demonstrator, on 24,000m2 of pavement in the city of Murcia (LIFE HEATLAND).This asphalt has several benefits with respect to conventional asphalt, it improves the urban environment and life quality of the citizens around the implantation, and which are mainly:• Energy savings in lighting system: air conditioning system and public light• Energy savings and materials savings• Natural improvement: air quality and lower noise• Improvement quality of life: comfort and pedestrian• Economics and ambiental benefitsUntil today, the demonstrator has been monitored and the following conclusions have been obtained, validating this type of pavement as a mechanism to reducing the urban heat island effect:• Initial luminance of reflective pavement 2.5cd / m2 under the lamppost, 150% higher than the conventional asphalt street• Solar reflectance of 30%, almost four times higher than that of conventional asphalt• Average surface temperature with reflective pavement 7-11ºC lower than that of the conventional pavement surface• Areas of the asphalt where rubber has been deposited from the tires as a result of the rolling of vehicles are heated 1-3ºC more than the clean areas• Lower environmental lower noise level of the area: 3 dB (A)

Cool Pavement Strategies for Urban Heat Island Mitigation in Suburban Phoenix, Arizona

Urban areas are characterized by a large proportion of artificial surfaces, such as concrete and asphalt, which absorb and store more heat than natural vegetation, leading to the Urban Heat Island (UHI) effect. Cool pavements, walls, and roofs have been suggested as a solution to mitigate UHI, but their effectiveness depends on local land-use patterns and surrounding urban forms. Meteorological data was collected using a mobile platform in the Power Ranch community of Gilbert, Arizona in the Phoenix Metropolitan Area, a region that experiences harsh summer temperatures. The warmest hour recorded during data collection was 13 August 2015 at 5:00 p.m., with a far-field air temperature of about 42 ∘ C and a low wind speed of 0.45 m/s from East-Southeast (ESE). An uncoupled pavement-urban canyon Computational Fluid Dynamics (CFD) model was developed and validated to study the microclimate of the area. Five scenarios were studied to investigate the effects of different pavements on UHI, replacing all pavements with surfaces of progressively higher albedo: New asphalt concrete, typical concrete, reflective concrete, making only roofs and walls reflective, and finally replacing all artificial surfaces with a reflective coating. While new asphalt surfaces increased the surrounding 2 m air temperatures by up to 0.5 ∘ C, replacing aged asphalt with typical concrete with higher albedo did not significantly decrease it. Reflective concrete pavements decreased air temperature by 0.2–0.4 ∘ C and reflective roofs and walls by 0.4–0.7 ∘ C, while replacing all roofs, walls, and pavements with a reflective coating led to a more significant decrease, of up to 0.8–1.0 ∘ C. Residences downstream of major collector roads experienced a decreased air temperature at the higher end of these ranges. However, large areas of natural surfaces for this community had a significant effect on downstream air temperatures, which limits the UHI mitigation potential of these strategies.

Coupling a Building Energy Simulation Tool with a Microclimate Model to Assess the Impact of Cool Pavements on the Building’s Energy Performance Application in a Dense Residential Area

Replacing conventional pavements with the corresponding high albedo ones constitutes a well-known technique to improve outdoor thermal environment of modern cites. Since most of the existing studies assess the impact of the high albedo pavements at the pedestrian’s height and with respect to thermal comfort, this study aims to examine the effect of the application of highly reflective pavements on the heating and cooling energy needs of a building unit, located inside a dense urban area. Aiming at a higher accuracy of the energy performance simulations, an integrated computational method between ENVI-met model, Meteonorm weather data generator and Energy Plus software is established, to consider the site-specific microclimatic characteristics of the urban areas. The analysis is performed both for the design and the aged albedo values as significant changes may occur due to aging process. The analysis revealed that the application of cool materials on the ground surfaces only marginally affects the energy performance of the examined building unit, both for the design and the aged albedo value; changes on the annual heating and cooling energy demand, for both albedo scenarios did not exceed 1.5% revealing the limited potential of cool pavements regarding the improvement of the energy performance of urban building units.

Lighting Implications of Urban Mitigation Strategies through Cool Pavements: Energy Savings and Visual Comfort

Cool materials with higher solar reflectance compared with conventional materials of the same color are widely used to maintain cooler urban fabrics when exposed to solar irradiation and to mitigate the Urban Heat Island (UHI). Photo-catalytic coatings are also useful to reduce air pollutants. Many studies related to these topics have been carried out during the past few years, although the lighting implication of reflective coatings have hardly been explored. To investigate these aspects, reflective coatings were applied on portions of a road and intensely analyzed in a laboratory and on the field. The applied cool coatings were found to have much higher solar and lighting reflectance than the existing road, which lead to lower surface temperatures up to 9 °C. Non-significant variations of chromaticity coordinates were measured under different lighting conditions. However, these materials showed a relevant variation of directional properties depending on the lighting and observation conditions with respect to conventional pavements. The optical behavior of these materials affects the uniformity of visions for drivers and requires ad-hoc installation of light sources. On the other hand, potential energy savings of up to 75% were calculated for the artificial lighting of a reference road.

Using Cool Pavements to Mitigate Urban Temperatures in a Case Study of Rome (Italy)

The urban density and the design of built and natural environments of cities play a crucial role in defining sustainable patterns. Urban heat islands (UHI) are phenomena tightly associated with the development of cities and urban expansion. Its effect is defined as the increase of the urban air temperature compared to surrounding rural areas. One of the main technology aimed at reducing the urban air temperature is the adoption of cool materials. As a matter of fact, the increasing of solar reflectance of urban materials can lead to reduce the built surface temperatures and mitigate the urban heat island intensity. Its features have vast impacts and implications on energy efficiency, environment and at last on human comfort and health. Measured temperatures were used to calibrate a model of a densely populated neighborhood in Rome inputted in ENVI-met software. The actual temperature field was evaluated in comparison with proposed areas consisting in the adoption of high albedo pavements application. Simulation results showed a significant reduction of air temperature closely correlated with the road solar reflectance.

Human Thermal Comfort: Modeling the Impact of Different Cool Pavement Strategies

Cool pavement strategies could help mitigate the heat island effect and improve the outdoor thermal environment in hot climates. This study investigated the impact of cool pavement strategies on outdoor human thermal comfort. On the basis of the human body energy balance model and the thermal comfort model, the physiological equivalent temperature (PET) was selected to evaluate and compare the effects of different pavement strategies on the outdoor thermal environment for summer and winter in three climates (Sacramento and Los Angeles in California and Phoenix in Arizona). The pavement strategies evaluated included high reflectance, evaporation, high reflectance plus evaporation, and shading along with a baseline for control. The use of high-reflectance pavement reduced pavement surface temperature; however, increasing pavement reflectance increased mean radiant temperature, because of the increased reflected radiation hitting the human body, and consequently increased the risk of reducing human thermal comfort during hot periods by 3% to 8% in the three locations. Enhancing pavement evaporation and shading pavement with trees or other canopies were effective strategies for reducing pavement surface temperature and PET, helping improve human thermal comfort in hot periods by 3% to 28%. For some areas, such as Sacramento, where it is hot in summer and cold in winter, some cool pavement strategies (e.g., pavement evaporation or shading) used to improve the summer thermal environment might make the cold winter slightly colder. Therefore, strategies that can reduce summer hot temperatures but not reduce winter cold temperatures, such as enhancing evaporation or tree shading only in the hot summer, are desirable.

Urban heat island in Padua, Italy: Simulation analysis and mitigation strategies

The paper reports on the study of the UHI phenomenon in Padua, a medium size city of the North-East of Italy. During the 2012–2014 period experimental measurements and simulations were carried out by the Authors in different zones of the city area: urban, sub-urban and rural. The analysis of the data highlights the presence of UHI effect with different magnitudes in function of the zone of the city, up to 7°C.In this study a very famous square of the city (Prato della Valle) was analyzed: it can be considered representative of the phenomenon because of the size and the very different characteristics from UHI effect point of view. The ENVImet simulation model was used to evaluate the effect of some mitigation strategies. Two scenarios besides the actual one (“AsIs” scenario) were analyzed: “Green ground” (halving the asphalt surface and doubling the green and plants surface) and “Cool Pavements” (increasing the albedo of impervious horizontal surfaces). The simulations results are presented both in terms of UHI intensity and mean radiant temperature and thermal comfort sensation. The results are presented both in spatial and temporal terms for a typical summer day. The “Green ground” scenario allows till 1.4°C and 3°C decrease in air temperature, respectively during the night and the day. The same items for the “Cool Pavements” scenario are, respectively, 1.8 and 4°C.

Experimental Analysis of Natural Gravel Covering as Cool Roofing and Cool Pavement

Passive solutions for building energy efficiency represent an interesting research focus nowadays. In particular, natural materials are widely investigated for their potential intrinsic high thermal energy and environmental performance. In this view, natural stones represent a promising solution as building envelope covering and urban pavement. This paper concerns the experimental characterization of several low-cost and local gravel coverings for roofs and urban paving, properly selected for their natural high albedo characteristics. To this aim, the in-field albedo of gravel samples is measured with varying grain size. These in-field measurements are compared to in-lab measurements of solar reflectance and thermal emissivity. The analysis shows a significant variation of the albedo with varying grain size. Both in-lab and in-field measurements agree that the stones with the finest grain size, i.e., fine sand, have the best optic-thermal performance in terms of solar reflectance (62%). This feature results in the reduction of the surface temperature when exposed to solar radiation. Moreover, a natural mixed stone is compared to the high reflectance stone, demonstrating that the chosen stone presents an intrinsic “cool” behavior. Therefore, this natural, low-cost, durable and sustainable material could be successfully considered as a natural cool roof or cool paving solution.

Development and Preliminary Validation of Integrated Local Microclimate Model for Numerical Evaluation of Cool Pavement Strategies

Cool pavement strategies can help mitigate the heat island effect and improve the outdoor thermal environment in hot climates. This study developed a framework for a general integrated local microclimate model that included the pavement structure and near-surface air and considers coupled processes of radiation, convection, conduction, shading, and evaporation. A simplified numerical model based on heat transfer was developed to simulate the thermal behavior of pavements with a focus on the pavement surface. The model was used to simulate the temporal and spatial distribution and variation of pavement temperature during the summer in a hot region (Sacramento, California). Preliminary validation of the model was accomplished with field measurements from experimental test sections on asphalt and concrete pavements (of various materials and structures) and under various weather conditions during summer. The simulated results agreed well with the measurements recorded on the test pavements, but further validation will be required before the use on any pavement under any climatic condition can be considered. The results indicated that the model could be used after further validation to simulate temperature changes on asphalt and concrete pavement surfaces under various climatic conditions. The model could also be used for simulation-based sensitivity analysis of key factors when identifying potential effective strategies for cool pavements, evaluating the thermal behavior of cool pavement strategies, and examining the impacts of cool pavements on near-surface heat island effects (such as human thermal comfort and air quality issues associated with ground-level ozone) over time and climates.

Thermal Performance of Developed Coating Material as Cool Pavement Material for Tropical Regions

The amount of solar energy emitted back from conventional asphalt pavement contributes to the phenomenon of urban heat island (UHI), and the current work was focused on studying the thermal behavior of asphalt pavement developed as a coating material from wasted tile. The surface temperature of asphalt surface with and without coating material for ambient temperature and underground soil temperature was studied. Results showed that the application of developed coating materials could reduce the surface temperature of asphalt pavement up to 4.4°C. Rainfall also played a significant role as a natural coolant during the experimental run by maintaining the surface temperature continuously for 2 days after the rainfall event, with an average surface temperature of 41.9°C. The underground soil temperature of the coated surface showed reduction in the range of 0.8-1.2°C throughout the day. The coating materials achieved their efficiency in reducing the emitted radiation only during noontime and the results show that the developed coating materials have true potential to serve as cool pavement to combat UHI effects.

On Cold Materials of Pavement and High-Temperature Performance of Asphalt Concrete

With global climate becoming warmer more and more attention is being paid to cold materials. Lower surface temperature contributes to decrease the temperature of the ambient air as heat convection intensity from a cooler surface is lower. Such temperature reductions can have significant impacts on cooling energy consumption in urban areas, a fact of particular importance in hot climate cities. The black surface of asphalt pavement absorbs more heat from the sun, and higher temperature of pavement surface contributes to increase the effect of the urban heat island, but affects the performance and life span of a pavement. Asphalt pavements form an integral part of any transportation system and are typically engineered to last 15 years or more, but many have been failing early due to potholes, cracks, raveling and other problems. Cool pavement are mainly aimed to decrease the effect of asphalt pavement on the urban heat island, but the influence of cold materials on the high-temperature performance of asphalt concrete pavement is paid little attention relatively. In this paper, it’s discussed that the effect of asphalt-pavement high temperature and its improving measures. And the mechanism of cool pavements is introduced, and possible technologies applied to asphalt pavements are reviewed. The idea of asphalt concrete pavement with automatic temperature-control is put forward.