Developing Conductive Asphalt Pavement to Mitigate Urban Heat Islands

Abstract

Asphalt pavement is reported as one of the main causes of temperature increase in urban areas. The dark surface of pavements absorbs and stores solar energy during the day and releases it at night, exacerbating a phenomenon called urban heat island (UHI). Although increasing surface reflectivity can reduce heat absorption, the asphalt mixture absorbs some parts of solar energy, causing a higher thermal gradient between the pavement and surrounding air. As a result, more heat is released, increasing the surrounding air temperature. In this study, steel slag aggregates were used to transfer the accumulated heat in the asphalt mixture to the below layers of pavements, reducing the temperature difference between the pavement surface and its surroundings and mitigating the UHI effects.

Keywords: Urban heat islands; Conductive by-products; Heat transfer; cool asphalt pavements

Different Urban Heat Island Contributing Factors

The UHI is an urban area with higher temperatures than its suburbs. UHI effects are attributed to the reduction of green places, trees, vegetation, and the transformation of these natural areas into buildings and road pavements in cities. In general, asphalt pavements account for a considerable part of cities (29 to 39%). Some human activities and inventions in cities, such as asphalt pavement can increase urban temperature from 5 to 15 ℃. Hence, practical methods should be taken into consideration to alleviate the negative effects of asphalt pavements on the UHI effects.

Current Urban Heat Island Mitigation Methods

Researchers have investigated different parameters associated with asphalt pavements to find solutions for this environmental issue. For instance, some studies have proposed different ways to increase the reflectivity of asphalt pavements using high visible and near-infrared reflectance coatings, reflective epoxy resin coatings, and light-color materials for chip sealing of the asphalt pavements' surfaces. Moreover, many studies have also focused on changing the mix designs of asphalt mixture using different conductive and insulation materials, including graphene powder as a binder additive, fly ash cenosphere as asphalt mastic filler, and glass and steel slag as asphalt mixture aggregates. Although an array of studies has been conducted to alter the thermal conductivity of asphalt mixture, the significant role of base layer thermal modification for UHI mitigation has been neglected. Asphalt pavement is comprised of different layers. The top layer is called the asphalt mixture, and the second layer is the base layer. Unlike the asphalt mixture, in which materials are glued with bitumen, the base layer is made of unbounded granular materials.

The Proposed Method for Transferring the Absorbed Heat

In this study, steel slag aggregates, which are industrial by-products, were used to increase the heat transfer of the base layer of asphalt pavement. Compared to conventional base layer materials, steel slag has higher thermal conductance due to its chemical properties such as iron contents. So, some base layer specimens were developed using different percentages of steel slag aggregates (0, 33, 66, 100%). The thermal properties of these specimens were then evaluated using a thermal conductivity instrument (C-Therm), demonstrated in Figure 1. The results showed that the thermal conductivity of the developed base layer was increased by 96%.

Thermal conductivity test; (a) specimens, (b) sensor, (c) C-Therm device

Experimental and Numerical Evaluation Methods

Firstly, asphalt mixture and base layer specimens were developed in the laboratory. They were then placed in the solar simulation setup to investigate the heat transfer rate and temperature variation in asphalt mixture and base layer specimens. This experimental setup was employed to simulate the nighttime heat release, playing a crucial part in UHI exacerbation. The results revealed that the pavement surface temperature and nighttime outgoing heat were reduced by 13% and 38% respectively.

After measuring the thermal properties and temperature variation of pavement layers, numerical models were developed using Ansys Fluent to estimate the nighttime air temperature reduction. Vertical and horizontal planes were used for recording temperatures in pavement layers and the air domain, illustrated in Figure 2. The result showed that using conductive base layers decreased the air temperature above the pavement by 15%.

Figure 2. The vertical plane used for data extraction

Conclusion

The UHI effects stem from asphalt pavements can increase the urban temperature in summer, causing some issues such as heat-related illnesses, higher energy consumption, higher air pollution, and contribution to global warming. In this study, a novel conductive pavement structure was developed which releases less heat at night and has a cooler surface in hot months. This approach can reduce the urban air temperature, mitigating the UHI effects significantly.