ReviewThe urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete
Introduction
The aim of this literature review is to present and define the various developments and studies that have occurred with regard to the thermal properties of asphalt concrete (AC), and their associated effect on the environment. This study summarises the surface temperature, albedo and other limiting factors of AC, and links these closely to the urban heat island (UHI) effect. The UHI effect is defined and analysed, and the mitigating methods for this phenomenon ailing urban environments, as described in the literature, is discussed. This review also identifies areas in which there has been minimal study, and provides scope for future research and development accordingly.
Section snippets
Background to the urban heat island
The UHI is a global issue that threatens the operation and habitability of our cities and urban environments. According to Oke (1982), the concept of the UHI has been well researched and documented; however, the understanding of the topic is quite limited. This has changed in recent years as a result of a greater focus on global warming and climate effects, the greater prevalence of hotter cities, and due to changes in technology for measurement and analysis. The heat island effect is
Timeline of contributions to study
The first studies on the UHI were conducted in the early 1800s, making studies on the topic more than a century old (Yang et al., 2015). The timeline in Table 1 below illustrates some of the most critical contributions to the study of thermal conductivity of asphalt pavements and the associated UHI, and is derived from the various references listed at the end of this paper.
Key causes of the urban heat island
Fig. 1 below illustrates the relationship between atmospheric heat, increased use of manufactured materials, and various other causes that can be attributed to the UHI, which will be discussed further.
Pavement structure
Pavements form the arterial transport connections within our cities, and research shows that they are a powerful contributor to the UHI. Many paths in our cities are referred to as pavements. From pedestrian footpaths and garden paths to highway roads, the term ‘pavement’ is applied to a diverse array of structures. Although there are many studies researching the effect of pavements on UHI, they often do not specify what sort of pavement structure they are researching. Roads are perhaps the
Key mitigation measures
Mitigation measures to combat the UHI effect have been well studied and well documented. Fig. 3 below illustrates some of the common mitigation measures. Many measures have been developed over time, and some of the key measures are outlined in this review. These include designing cool pavements by increasing the albedo of surfaces and making them more reflective, permeable, porous and water retentive; the increased utilisation of green spaces within our urban landscape (Gorsevski et al., 1998,
Discussion
The most common theme within the literature is the need to conduct a greater number of studies within the field. The issue with studies to date is that demonstration projects are not at a large enough scale to quantify the effects of a given mitigation method on an urban environment or city (Santamouris, 2013b). Researchers suggest that a city of the world needs to implement solutions on a large scale to allow researchers to better quantify the effects of the UHI. The range of studies on UHI is
Conclusion
In conclusion, it has been found that the thermal properties of asphalt concrete are a strong contributory factor to the UHI effect in cities. There is a constant need to reduce the effects of the UHI, due to the adverse effect it has on liveability, wellbeing and health in urban environments. The constantly expanding nature of cities and the increased use of hard, heat absorbing substances make a significant contribution to the UHI. Various mitigation measures have been proposed within the
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