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The urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete

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The Urban Heat Island (UHI) is a phenomenon that affects many millions of people worldwide. The higher temperatures experienced in urban areas compared to the surrounding countryside has enormous consequences for the health and wellbeing of people living in cities. The increased use of manmade materials and increased anthropogenic heat production are the main causes of the UHI. This has led to the understanding that increased urbanisation is the primary cause of the urban heat island. The UHI effect also leads to increased energy needs that further contribute to the heating of our urban landscape, and the associated environmental and public health consequences. Pavements and roofs dominate the urban surface exposed to solar irradiation. This review article outlines the contribution that pavements make to the UHI effect and analyses localized and citywide mitigation strategies against the UHI. Asphalt Concrete (AC) is one of the most common pavement surfacing materials and is a significant contributor to the UHI. Densely graded AC has low albedo and high volumetric heat capacity, which results in surface temperatures reaching upwards of 60 °C on hot summer days. Cooling the surface of a pavement by utilizing cool pavements has been a consistent theme in recent literature. Cool pavements can be reflective or evaporative. However, the urban geometry and local atmospheric conditions should dictate whether or not these mitigation strategies should be used. Otherwise both of these pavements can actually increase the UHI effect. Increasing the prevalence of green spaces through the installation of street trees, city parks and rooftop gardens has consistently demonstrated a reduction in the UHI effect. Green spaces also increase the cooling effect derived from water and wind sources. This literature review demonstrates that UHI mitigation techniques are best used in combination with each other. As a result of the study, it was concluded that the current mitigation measures need development to make them relevant to various climates and throughout the year. There are also many possible sources of future study, and alternative measures for mitigation have been described, thereby providing scope for future research and development following this review.
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The Urban Heat Island Effect, its Causes, and Mitigation, with Reference to the
Thermal Properties of Asphalt Concrete
Abbas Mohajerani, Jason Bakaric, Tristan Jeffrey-Bailey
Civil and Infrastructure Engineering, School of Engineering, RMIT University, Melbourne, Victoria,
Australia
Corresponding author: E-mail address: abbas.mohajerani@rmit.edu.au
Citation:
Mohajerani, A. Bakaric, J. and Jeffrey-Bailey, T. 2017, 'The urban heat island effect, its causes, and
mitigation, with reference to the thermal properties of asphalt concrete', Journal of Environmental
Management, Elsevier, United Kingdom, vol. 197, pp. 522-538 ISSN: 0301-4797
https://researchbank.rmit.edu.au/view/rmit:43518
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The Urban Heat Island Effect, its Causes, and Mitigation, with Reference to the
Thermal Properties of Asphalt Concrete
Abbas Mohajerani, Jason Bakaric, Tristan Jeffrey-Bailey
Civil and Infrastructure Engineering, School of Engineering, RMIT University, Melbourne, Victoria,
Australia
ABSTRACT
The Urban Heat Island (UHI) is a phenomenon that affects many millions of people worldwide. The higher
temperatures experienced in urban areas compared to the surrounding countryside has enormous
consequences for the health and wellbeing of people living in cities. The increased use of manmade
materials and increased anthropogenic heat production are the main causes of the UHI. This has led to
the understanding that increased urbanisation is the primary cause of the urban heat island. The UHI effect
also leads to increased energy needs that further contribute to the heating of our urban landscape, and
the associated environmental and public health consequences. Pavements and roofs dominate the urban
surface exposed to solar irradiation. This review article outlines the contribution that pavements make to
the UHI effect and analyses localized and citywide mitigation strategies against the UHI. Asphalt Concrete
(AC) is one of the most common pavement surfacing materials and is a significant contributor to the UHI.
Densely graded AC has low albedo and high volumetric heat capacity, which results in surface
temperatures reaching upwards of 60°C on hot summer days. Cooling the surface of a pavement by
utilizing cool pavements has been a consistent theme in recent literature. Cool pavements can be reflective
or evaporative. However, the urban geometry and local atmospheric conditions should dictate whether or
not these mitigation strategies should be used. Otherwise both of these pavements can actually increase
the UHI effect. Increasing the prevalence of green spaces through the installation of street trees, city parks
and rooftop gardens has consistently demonstrated a reduction in the UHI effect. Green spaces also
increase the cooling effect derived from water and wind sources. This literature review demonstrates that
UHI mitigation techniques are best used in combination with each other. As a result of the study, it was
concluded that the current mitigation measures need development to make them relevant to various
climates and throughout the year. There are also many possible sources of future study, and alternative
measures for mitigation have been described, thereby providing scope for future research and
development following this review.
KEYWORDS: Urban heat island; asphalt concrete; urban heat island causes; urban heat island
consequences; urban heat island mitigation measures
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1. 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.
2. 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 characterised by the development
of noticeably higher temperatures in our cities compared with the countryside that directly surrounds them
(Nakayama and Fujita 2010; Stone, Hess and Frumkin 2010; Synnefa, Karlessi, Gaitani, Santamouris,
Assimakopoulos and Papakatsikas 2011; Santamouris 2013b, 2015a). Initial studies conducted by the
World Meteorological Organisation (1984) and Oke (1987), cited in Gorsevski, Taha, Quattrochi and Luvall
(1998), revealed that the UHI effect can increase air temperature in an urban city by between 2 and 8°C.
Recent studies illustrate that a more accurate range is between 5 and 15°C (Santamouris 2013a). The
heat island effect arises due to the changing nature of our cities, and is the result of a reduction in
vegetation and evapotranspiration, a higher prevalence of dark surfaces with low albedo, and increased
anthropogenic heat production (Stone, Hess and Frumkin 2010). Therefore, the existing surface conditions
of an urban area will directly impact on the chosen UHI mitigation strategies. Akbari and Rose (2008) found
that the average urban surface of four different metropolitan areas in the USA were characterised by 29-
41% vegetation, 19-25% roofs and 29-39% paved surfaces. This demonstrates that over 60% of an urban
surface can be covered by hard, man-made, heat-absorbent surfaces. The authors concluded that
knowledge of the urban fabric and surface conditions of a city is important in order to explore the effects
of possible UHI mitigation measures.
The UHI can be illustrated by drawing a curve from one side of a city to the other, mapping graphically the
temperature change from the rural to the urban environment and back to the rural environment. The ‘island’
would be represented by the large spike in the centre of the graph, which generally mimics the outline of
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the structures within the urban area, and is bound by the cliffs either side that mark the urban and rural
boundary (Oke 1982). By illustrating the UHI in this way, it is easy to recognise the profound increased
temperature difference to which our cities are exposed. Yang, Wang and Kaloush (2015), in their studies
on UHI, illustrate the temperature intensities of UHI for various cities around the world. This effect is only
getting greater, and, hence, it is important that strategies are developed for adapting to, and mitigating the
adverse environmental effects of UHIs (Yang, Wang and Kaloush 2015). In most research to date, the
primary solution to the UHI has been replacing dark materials with high albedo light-coloured materials for
greater solar reflectivity. This, however, is not the only solution (Stempihar, Pourshams-Manzouri, Kaloush
and Rodezno 2012), and further methodologies are illustrated in this review.
2.1. Regional Atmospheric and Geographic Conditions
The regional atmospheric and geographic conditions are a key determinant of the UHI effect and its
intensity in cities. The UHI effect is significantly affected by the geographic features, climatic conditions,
and seasonal variations of a city’s particular location. Even the time of day will dramatically affect the
intensity of the UHI. Take for example night-time, during which the emissivity of a pavement and the heat
radiated into the atmosphere are the most critical contributors to the temperature of the surface and lower
atmosphere within an urban canyon (Santamouris 2013b). Debbage and Shepherd (2015) conducted a
study of the fifty most populous cities in the USA using PRISM data (a climate data collection network that
incorporates various monitoring methods and stringent quality control measures). The authors found that
geography and climate heavily affected the manifestation of the UHI across the US. For example, Salt
Lake City exhibited the highest UHI effect due to the high prevalence of calm, clear and stable conditions
that are ideal for UHI formation. This localised atmospheric condition resulted in the UHI being most
intense during the winter months. In addition, the authors identified mitigation techniques that worked
during summer but increased the UHI in winter. This research demonstrates the importance of tailoring
mitigation techniques to regional conditions.
The UHI intensity is also affected by the built environment of an urban area. The configuration of urban
zones is a topic of some ambiguity in UHI research. The UHI effect has often been linked to urban sprawl,
and, therefore, urban densification has been espoused as a potential mitigation method (Stone 2012 as
cited in Debbage and Shepherd 2015). However, Debbage and Shepherd (2015) suggest that UHI
intensity is more strongly linked to other urbanisation factors, such as urban contiguity, and that both
sprawled and dense cities exhibit the UHI effect. This again demonstrates the irregularity of the UHI effect
and that it is an issue best tackled with localised research and mitigation efforts.
As cited in Oke (1982), and in accordance with Taesler (1981) and Oke (1976), the atmosphere in our
cities can be categorised into two interrelated layers; the urban canopy layer (UCL) and the urban
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boundary layer (UBL). The urban canopy layer, quite similar to a vegetation layer, extends only up to the
roof level of the buildings, whilst the urban boundary layer extends from the mean roof level to a few
kilometres above this. Each layer has a significant effect on the other, with the UCL contributing heating,
cooling and evaporation to the UBL, and the UBL contributing greater mesoscale weather conditions (Oke
1982). Yavuzturk, Ksaibati and Chiasson (2005) conducted an assessment of the temperature fluctuations
in asphalt pavements due to the environmental conditions. In this assessment, they deduced that a city’s
orientation to the wind significantly affects airflow, and the associated cooling that this brings for a
pavement structure. Whilst the critical effect of wind is considered when designing urban structures both
large and small, the effect that airflow has on the UHI and on the thermal properties of pavement materials
appears to not have been comprehensively considered within studies to date. The work of Oke (1982) has
long since deduced that in terms of the meteorological variables, wind speed is the most significant
characteristic affecting UHI intensity, followed by cloud cover. In addition to this, Yavuzturk, Ksaibati and
Chiasson (2005) illustrate that a longer segment of pavement with uninterrupted wind flow results in
increased heat transfer from the pavement, and effectively cools the pavement surface. Various other
factors within the local atmosphere directly contribute to the heat island intensity and its existence within
a city, including the moisture and rainfall availability, geometric properties of a site, thermal properties of
surface and building materials, and local anthropogenic heat (Oke 1982).
3. 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, Wang and Kaloush 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.
Table 1 - Timeline of Contributions
Year
Reference
Findings/Description
1833
Howard, L cited in (Oke
1982)
First to illustrate higher air temperature in cities relative to
surrounding countryside.
1957
Barber, E.
Pioneered work in asphalt pavement temperatures. Sought to
match surface temperature with standard weather report
information.
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1968
Straub, Schenck and
Przbycien
Studied asphalt pavements in New York. Developed computer
model of temperature against air temperature and solar radiation.
Showed solar radiation was critical to surface temperature.
1970
Dempsey and
Thompson
Evaluated frost action in multilayered pavements.
1971
Rumney and Jimenez
Studied pavement temperatures in Tucson, Arizona, southwest
USA; a hot desert-like climate.
1972
Williamson
Used finite difference models to predict temperatures in pavement
at various depths over a short time period. Found albedo
significantly influenced pavement surface temperature. Did not
consider humidity or precipitation.
Christison and
Anderson
Used finite difference model to predict thermal behaviour of
pavements in low temperature environment. Model and practice
agreement was found.
1973
Noss
Analysed pavement temperatures in subgrades as a result of frost
penetration in cold winters.
1974
Berg, R.
Analysed surface energy balance of Portland cement concrete
pavements. Found this approach not perfectly valid for frost and
thaw depth interpretation.
1975
Southgate and Deen
Correlated with a linear relationship pavement temperature at
depth, and surface temperature, over a five-day mean air
temperature history.
1976
Wilson
Attempted use of heat flow equations for the prediction of heat
gradients in asphalt pavements. Results were inaccurate.
1982
Spall
Calculated thermal conductivity and diffusivity of asphalt concrete
by designing a modified calorimeter.
1984
Highter and Wall
For various asphalt mixtures, determined thermal properties.
Found variance in thermal conductivity from changing asphalt
content.
1987
Wolfe, Heath and
Colony
Created heat-transfer equations for the determination of the
cooling rate of fresh asphalt in defined environments.
1989
Huber, Heiman and
Chursinoff
Developed models for short-term and long-term assessment of
pavement and subsoil temperatures in thawed and frozen
pavements
Hsieh, Qin and Ryder
Created 3D estimation models for concrete pavement temperature
and infiltration of rainwater into soil/subgrades.
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1992
Choubane and Tia
Determination of the temperature of rigid pavements in Florida
using a developed quadratic equation.
1993
Solaimanian and Bolzan
Related latitude to maximum and minimum pavement surface
temperature with a parabolic equation.
1995
Inge and Kim
Improved AASHTO method for temperature correction for asphalt
concrete deflections by creating a database method.
1996
Asaeda, Ca and Wake
Analysis of the effect that heat storage of pavement has on the
atmosphere.
1997
Gorsevski, Taha,
Quattrochi and Luvall
Environmental Protection Agency (EPA) launches Urban Heat
Island Mitigation Initiative (UHIMI).
1998
Voller, Newcomb,
Chadbourn, De Sombre,
Timm and Luoma
Predicted construction phase thermal profile of asphalt concrete
pavement with a computer tool.
Lukanen, Han and Skok
Selection of asphalt binder dependent on surface temperature
using a probabilistic method.
Mohseni
Refined work done by Lukanen et al. (1998).
2004
Doulos, Santamouris
and Livada
Analysed 93 commonly used pavement materials for their thermal
characteristics and effect on the urban heat island.
2005
Yavuzturk, Ksaibati and
Chiasson
Assessment of changes in temperature due to thermal
environmental conditions.
2006
Golden and Kaloush
Interpreted the use of mesoscale and microscale techniques for
measurement of surface temperature and albedo of asphalt
pavement.
2007
Scholz and Grabiowecki
Permeable pavement system review.
2012
Stempihar, Pourshams-
Manzouri, Kaloush and
Rodezno
Analysis of thermal performance of porous pavements.
Takebayashi and
Moriyama
Study and comparison of surface heat for various commonly used
pavements.
2013
Santamouris
Analysis of cool pavements and their effect on urban heat island.
2015
Li, H
Evaluation and comparison of the thermal performance of different
pavement materials.
4. Consequences of the Urban Heat Island
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The UHI effect has significant consequences for the liveability in our cities, and is the source of a significant
number of environmental problems in urban areas (Yang, Wang and Kaloush 2015). The warming effect
of urbanisation has critical impacts on health and wellbeing, as well as human comfort and the local
atmosphere (Grimmond 2007). Ichinose, Matsumoto and Kataoka (2008), Synnefa, Karlessi, Gaitani,
Santamouris, Assimakopoulos and Papakatsikas (2011), and Yang, Wang and Kaloush (2015), illustrate
the various consequences associated with the UHI effect; these include:
Increased cooling energy usage and associated costs;
Significant increases in peak energy demand;
The formation of large amounts of smog and air pollutants, and a resulting degradation in the
quality of air;
Increased thermal stress on residents and the public;
Strong impact on urban ecosystems;
A living environment that is significantly degraded; and
A significantly increased level and risk of morbidity or illness due to heat.
Heat stress is a direct consequence of the UHI effect. The negative impact of elevated temperatures on
urban infrastructure, ecosystems, and human health and comfort provides the motivation for finding UHI
mitigation measures. Heatwaves are responsible for a range of serious health issues, and, at their
extreme, can account for a large number of fatalities. Heatwaves also cause an increase in electricity and
water usage (Hatvani-Kovacs, Belusko, Skinner, Pockett, and Boland 2016). Santamouris (2013b) also
illustrates that various studies have been able to measure significant increases in urban temperatures
resulting in double the cooling energy usage of buildings. In addition to this, Clarke (1972), cited in Stone,
Hess and Frumkin (2010), show that most deaths as a result of heatwaves are associated with urban
environments and the UHI effect. Hatvani-Kovacs, Belusko, Skinner, Pockett, and Boland (2016) argue
that an emphasis on the public health risks of heatwaves outweighs discussion on the water and electricity
usage. The authors suggest that these three consequences of heatwaves are intertwined, and that, to
increase a city’s heat resilience, an integrated framework assessing all three of these consequences is
required. Such a policy framework would aim to reduce the dependency on air-conditioning, and increase
the knowledge and awareness of heat stress resilience in relation to the built environment. Lemonsu,
Viguie, Daniel, and Masson (2015) also argue that urban planning and transport policies affect the
magnitude of the UHI effect and suggest that compact cities have a higher risk of heatwaves than areas
of urban sprawl. The authors, however, make the point that measuring heatwave vulnerability depends
largely on the choice of heat stress indicators. Therefore, one must be cautious in the design of mitigation
measures for the UHI to ensure that solutions such as increasing albedo or thermal conductivity do not
have an added adverse effect. It is a common understanding that an increase in reflected UV radiation,
glare and temperature affects the health and wellbeing in our urban environments (Yang, Wang and
Kaloush 2015).
5. Key causes of the Urban Heat Island
Figure 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.
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Fig. 1. How the Urban Heat Island occurs - image sourced from Yamamoto (2006).
5.1. Urbanisation, urban sprawl and increasing population
The most significant cause of the UHI is urbanisation. The constant increase in hard and heat absorbing
surfaces, the density of our cities, and the reduction in natural vegetation are the main contributors to the
heat island effect (Akbari, Pomerantz and Taha 2001; Doulos, Santamouris and Livada 2004; Rossi,
Pisello, Nicolini, Filipponi and Palombo 2014). Takebayashi and Moriyama (2012), in their study of surface
heat budget of various pavement materials in Japan, show that the daytime temperature of normal asphalt
can be up to 20°C hotter than grass. The changes in our cities mean that the amount of vegetation is
decreasing, and pavements now cover an increasingly high proportion of our cities, contributing
significantly to the heat island phenomenon (Santamouris 2013b). The network of paved surfaces and
roads that our cities are increasingly developing, have been shown to account for a larger percentage of
elevated surface temperatures per unit volume compared to any other man made material or structure
(Golden and Kaloush 2006).
The cities of today continue to increase in size and breadth; a phenomenon known as urban sprawl. Stone,
Hess and Frumkin (2010) illustrate that urban sprawl contributes to an increasing number of heatwaves,
and sprawling cities result more frequently in extreme heat events when compared with compact
metropolitan cities. According to the United Nations Population Fund, cited in Stempihar, Pourshams-
Manzouri, Kaloush and Rodezno (2012), by 2030, 61% of the world’s population will live in cities. Rapid
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increases in the world’s population is also a key driver of change in the urban and global environment
through global warming, loss of biodiversity and deforestation (Grimmond 2007). This will result in a
significant contribution to the UHI and to the environment as our urban areas continue to get larger and
denser, vegetation reduces, and energy and cooling costs escalate. Changes due to urbanisation will also
influence the climactic, hydrologic and biophysical cycles in the environment, and adversely affect not only
humans, but natural ecosystems (Gorsevski, Taha, Quattrochi and Luvall 1998).
5.2. More manufactured materials
The increase in the use of engineered materials in our cities, which have greater heat storage capacity as
well as lower albedo, and the associated reduction in native vegetation, have a major influence on the UHI
effect (Golden and Kaloush 2006). The hydraulic, radiative and thermal characteristics of materials used
in modern construction are completely different in comparison to natural soil and rock, vegetation, and
water (Grimmond 2007). The changing materials result in new surface and atmospheric conditions,
thereby altering the exchange of energy and water, as well as the airflow. In addition, combining these
changes in the conditions with the anthropogenic heat from vehicles and people, increased pollution, and
the increased density of our cities, results in a completely different and distinct urban climate today
(Landsberg 1981; Oke 1997; cited in Grimmond 2007).
5.3. Increase in heating and cooling energy needs
Technological developments within society contribute to the UHI effect through increased heating and
cooling. The most significant are air conditioning systems, which, although effective for increasing human
comfort, inherently generate a greater level of heat (Grimmond 2007). Given the density of today’s cities,
there are a larger number of air conditioners that are essential for the functioning of buildings and
structures. Our increasingly hotter climate sees the use of more air conditioners, which significantly
increase the heat of a city, creating thermal discomfort, more greenhouse emissions, and affecting human
wellbeing (Rossi, Pisello, Nicolini, Filipponi and Palombo 2014). In addition to this, electricity demand for
heating and cooling as a result of UHI has a significant effect, with Akbari, Pomerantz and Taha (2001)
finding that for each 1°C increase in temperature, electricity demand increases by between 2 and 4%.
6. 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
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specify what sort of pavement structure they are researching. Roads are perhaps the most significant
pavement type affecting the UHI, and asphalt is widely used as the surfacing material for Australia’s roads.
The overlay thickness is designed to provide additional structural strength as required and densely graded
AC is often used (figure 2) (Austroads 2007). The properties of asphalt vary widely, depending on such
factors as void capacity and mixture content. However, it has been widely confirmed that asphalt
pavements contribute to the UHI.
Fig. 2. Typical mix components for asphalt pavement - image sourced from Austroads 2007
Most studies focus on the albedo, density, permeability and water retention of the top pavement layer
because these properties have been shown to have a strong influence on the pavement’s surface
temperature. However, the structure and purpose of a pavement will impact the mitigation strategies
utilised. Therefore, an understanding of the whole pavement structure is important to adequately assess
the viability of UHI mitigation strategies.
Pavements are categorized as either flexible or rigid. Flexible and rigid pavements are composed of
several layers. A flexible pavement has a wearing surface, typically asphalt, laid on top of a base and then
a subbase. Together, these three layers, and sublayers, form the pavement structure which sit upon the
subgrade. A rigid pavement generally has a thick concrete layer that sits upon a subbase and then the
subgrade. A rigid pavement can leave the concrete exposed as the wearing surface or it can be surfaced
with a thin asphalt layer. Flexible pavements are designed to deflect under loads as horizontal tensile and
compressive strains are produced in the pavement’s layers. Rigid pavements have a high flexural strength
and distribute the load over a wider area of the pavement. Therefore, rigid pavements do not transfer
deformation throughout the layers.
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7. Asphalt Concrete and the Urban Heat Island
According to much of the literature, pavements make a significant contribution to the UHI effect
(Santamouris 2013b). This is due to the significant geographical area that pavements cover in our cities,
and the relatively low albedo of a dark pavement surface (Synnefa, Karlessi, Gaitani, Santamouris,
Assimakopoulos and Papakatsikas 2011). According to Akbari, Menon and Rosenfeld (2009), pavements
cover about forty per cent of the urban environments of cities today. In various studies, mesoscale imagery
from satellites of infrared and thermal activity has shown that pavements are strong sources of heat
radiation (Gorsevski, Taha, Quattrochi and Luvall 1998). The albedo of a surface can be defined as the
fraction of light incident that it reflects (Golden and Kaloush 2006; Bobes-Jesus, Pascual-Muñoz, Castro-
Fresno and Rodriguez-Hernandez 2013). According to Pomerantz, Akbari, Chang, Levinson and Pon
(2003), fresh AC absorbs approximately 95% of sunlight, or, in other words, it has an albedo of 5%. It is
important to note that solar radiation typically comprises 43% solar energy, 52% near infrared light and
5% ultraviolet light (Golden and Kaloush 2006), and a significant percentage of this is absorbed by the
asphalt surface. Consideration must therefore be given to the implementation of appropriate measures
and materials to improve the thermal characteristics of our cities, a topic which has garnered greater
support in recent years (Doulos, Santamouris and Livada 2004).
7.1. Thermal properties of asphalt concrete
An understanding of the thermal conductivity of various forms of AC, and its related contribution to the UHI
effect, is critical to this study. The thermal properties of pavement materials and their albedo are quite
significant determining factors in their performance and behaviour (Li 2015). Dense graded asphalt,
commonly known as Asphalt concrete in Australia, is usually composed of coarse aggregate, fine
aggregate, mineral filler and a bituminous binder (Underwood 1995). It is mixed such that the aggregates
are distributed evenly between coarse and fine, and compacted such that the asphalt pavement is mostly
impervious (Underwood 1995).
In most of the studies undertaken, ordinary black AC pavements in a summer climate have a temperature
range upwards of 60°C (Doulos, Santamouris and Livada 2004; Bobes-Jesus, Pascual-Muñoz, Castro-
Fresno and Rodriguez-Hernandez 2013; Higashiyama, Sano, Nakanishi, Takahashi and Tsukuma 2016;
Santamouris 2013b). This is due to the relatively low albedo of AC compared with other pavement
materials, and its affinity for thermal absorption and conductivity. According to Stempihar, Pourshams-
Manzouri, Kaloush and Rodezno (2012), the thermal properties of pavements fall into two categories:
transfer of energy properties, and thermodynamic characteristics. The transfer of energy properties relates
to conduction and radiation, the characteristics that determine the movement of energy between the
pavement and its surrounding environment. On the other hand, the thermodynamic characteristics refer
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to the energetic equilibrium in the pavement, and concern the volumetric heat capacity and thermal
diffusivity (Stempihar, Pourshams-Manzouri, Kaloush and Rodezno 2012). In most cases, AC with a higher
volumetric heat capacity can store more heat, and hence, generally, has a lower temperature (Yavuzturk,
Ksaibati and Chiasson 2005).
Achieving thermal balance in a pavement structure is quite a complex procedure as it is affected by various
energy transfer processes and thermodynamic factors. Primarily, heat transfer is achieved through the
absorption of solar and infrared radiation, through convection between the pavement surface and the
surrounding air, and through conduction between and within the pavement surface and the underlying
ground (Yavuzturk, Ksaibati and Chiasson 2005; Santamouris 2013b). The degree to which a pavement
is responsive to thermal inputs can be referred to as thermal admittance, or thermal inertia (Oke 1982).
This thermal inertia, and the thermal properties of a pavement are highly affected by the constituents of
the pavement, which is why many solutions to the UHI involve changing the properties of the AC. The
constantly changing properties of each solution makes the thermal conductivity of pavements quite difficult
to obtain, as, according to Stempihar, Pourshams-Manzouri, Kaloush and Rodezno (2012), conductivity is
determined by factors such as the type of AC mix and aggregates used, size of aggregates, moisture
content and mineral content. Table 2 below illustrates some sample thermal conductivities collected from
the literature. It has been shown, however, that the heat flux (heat energy transfer rate through a surface,
per unit area) of asphalt is quite important. An increase in thermal conductivity (heat flux) serves to
decrease the surface temperature of an asphalt pavement, as there is more dissipation of heat off the
surface, as well as more conduction to the ground (Yavuzturk, Ksaibati and Chiasson 2005).
Table 2 - Summary of Thermal Conducti vit ies
Reference
Year
Asphalt Type
Thermal Conductivity
(W/mK)
Conditions
Luca and Mrawira
2005
Dense Graded
Superpave Asphalt
Concrete
1.4-1.8
2,295-2,450kg/m3 bulk
density, 3-7% air voids,
edge heat losses
accounted for
Kavianipour and
Beck
1977
Dry Asphaltic Pavement
2.28-2.88
Garcia,
Norambuena-
Contreras and Partl
2013
Dense Asphalt
Concrete
1.2-1.6
Takebayashi and
Moriyama
2012
Asphalt
1.03
Page 14 of 40
The major variables for the thermal performance of AC are albedo, conductivity, and volumetric heat
capacity, and these parameters influence the balance of transient heat throughout the pavement layers.
This includes the heating of the pavement via solar absorption, the transfer of this heat via conduction,
loss of heat via wind cooling from convection, and, finally, irradiation from the pavement.
Changing the thermal conductivity of the surface affects both the minimum and maximum temperatures.
Lowering the conductivity of a pavement decreases the speed at which it can transfer heat to the soil, and,
in return, gain heat from the soil.
The volumetric heat capacity is the combination of two terms: the specific heat constant and the material’s
density. The volumetric heat capacity gives the amount of energy a material can hold per cubic metre.
8. Key mitigation measures
Mitigation measures to combat the UHI effect have been well studied and well documented. Figure 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, Taha,
Quattrochi and Luvall 1998; Santamouris 2013b); and harnessing the cooling effects of wind and water.
Thermal Conductivity
(W/ m°C)
Carlson, Bhardwaj,
Phelan, Kaloush
and Golden
2010
Hot Mix Asphalt (HMA)
0.896
2,281kg/m3 bulk density
Li
2015
Dense Graded Asphalt
(DG1)
DG1 = 1.73
DG1 = 2399 kg/m3 bulk
density (impermeable)
Open-Graded Asphalt
(OG1)
OG1 = 1.24
OG1 = 2269 kg/m3 bulk
density (permeable)
Page 15 of 40
Fig. 3. Mitigation Measures for the Urban Heat Island – image sourced from Ichinose, Matsumoto and Kataoka (2008).
8.1. Cool Pavements
Cool pavements do not have a standard definition. The United States Environmental Protection Authority
(USEPA) defines cool pavements as “paving materials that...have been otherwise modified to remain
cooler than conventional pavements” (Qin 2015a). Many studies interpret an element of this definition to
mean that a cool pavement must be able to suppress its daily maximum surface temperature relative to
AC (Qin 2015a; Zheng, Han, Wang, Mi, Li and He 2015; Toraldo, Mariani, Alberti and Crispino 2015;
Jiang, Sha, Xiao, Wang and Apeagyei 2016; Higashiyama, Sano, Nakanishi, Takahashi and Tsukuma
2016).
Qin (2015a) derives the following mathematical equation for the daily maximum surface temperature
(Tsmax) of a pavement:
!"#$% & '( )*+,-./
0
1
23'!4
(1)
Page 16 of 40
Γ is the percentage of the thermal absorption to the thermal conduction
R is the albedo
I0 (W/m2) is the daily peak solar irradiation
P is the thermal inertia of the pavement
ω (1/s) is the angular frequency
T0 (°C) is a regressed constant
Based on Equation 1, suppressing the daily maximum surface temperature of a pavement can be achieved
by increasing the albedo, reducing the percentage of thermal absorption to thermal conduction or by
increasing the thermal inertia of the pavement.
Defining a cool pavement by its ability to suppress its daily maximum surface temperature is limited
because it does not account for temperature fluctuations across the time of day, season or other
environmental conditions. For example, increasing the thermal inertia of a pavement may result in a lower
daily maximum surface temperature; however, more heat will be released in the early evening and night-
time (Qin and Hiller 2014). Another limitation of this definition is the assumption that lowering the surface
temperature of a structure will contribute to the mitigation of UHI effects, which is not always the case.
Nevertheless, reducing the daily maximum surface temperature of a pavement remains a key element
within the literature of cool pavements.
8.2. Reflective pavements
The use of reflective pavements is one of the most well studied and most cost effective mitigation
measures for combatting the UHI effect by reducing the surface temperature of the pavement (Synnefa,
Karlessi, Gaitani, Santamouris, Assimakopoulos and Papakatsikas 2011; Rossi, Pisello, Nicolini, Filipponi
and Palombo 2014). It is easier to vary the albedo of a pavement than its thermal inertia. Therefore,
increasing albedo is the simplest method for lowering the maximum daily surface temperature, according
to Equation 1, and a reflective coating can even be applied to existing pavements.
Essentially, to make a pavement more reflective, two parameters can be changed, the colour of the
pavement, and its surface roughness (Santamouris 2013b). According to most studies, the most efficient
and practical means for mitigation of the UHI effect is to make pavement surfaces whiter, or as light-
coloured as possible (Pomerantz, Akbari, Chang, Levinson and Pon 2003; Santamouris 2013b). In its
essence, making a pavement surface a lighter colour decreases the amount of solar radiation that it
absorbs, and increases the amount of light and heat radiation that it reflects back into the atmosphere.
The reflectivity of a pavement structure and the percentage of associated solar radiation absorbed are
Page 17 of 40
also known as the albedo (Golden and Kaloush 2006; Santamouris 2013b; Li 2015; Sen and Roesler
2016). In general, lighter coloured materials, or smoother surfaced pavements have a higher albedo, and,
hence, reflect, rather than absorb, more solar radiation (Doulos, Santamouris and Livada 2004;
Santamouris 2013b). A reflective pavement with an increased albedo can be developed by applying a
reflective paint or a sealant of thin bitumen with exposed light coloured aggregates to the surface of the
pavement.
Reflective pavements can also be referred to as cool pavements, because of their inherent capability to
significantly reduce surface temperature as a result of the higher albedo and increased level of thermal
emissivity. According to Santamouris (2013b), a global expert and key researcher on the UHI effect,
creating cool pavements is a critical heat island mitigation and management method. This is simply
because pavements make up such a high percentage of the heat absorption and re-radiation in our
sprawling cities. Hence albedo is a significant factor in pavement technology and in reducing the UHI.
Ideally, cool pavements would be used on every road construction site; however, their application is often
limited for economic or aesthetic reasons, and, instead, warm or hot pavements are used (Doulos,
Santamouris and Livada 2004). The global implementation of reflective roofing, pavements and other
structures would reduce the air and pavement temperature, offset billions of tons of carbon dioxide
emissions, reduce smog and aid the ailing environment (Pomerantz, Akbari, Chang, Levinson and Pon
2003; Yang, Wang and Kaloush 2015). Hence, making cool pavements more available, durable and
affordable may be an effective area of future study.
Studies have shown that although higher albedo undoubtedly reduces the surface temperature of
pavements, to date, studies considering the detrimental environmental effect of more reflective
pavements, and their contribution to glare, thermal comfort and human health for residents are limited
(Yang, Wang and Kaloush 2015). Solar reflective coatings change the original surface texture of AC, and,
hence, the possible detrimental effects of reflective coatings need to be studied in order to ensure that UHI
mitigation techniques do not have negative and unintended consequences. Kinouchi, Yoshinaka, Fukae
and Kanda (2004) developed darker paint pigments with high solar reflectivity in near-infrared light but low
visual brightness in an effort to reduce the negative consequences of increased glare; a particularly
important consideration for roads. Another important consideration for road design is the skid performance
of the road surface. Increasing albedo by increasing the smoothness of the road surface will negatively
impact skid performance. Zheng, Han, Wang, Mi, Li and He (2015) developed and compared solar
reflective coatings that had sufficient anti-skid performance and low impact on driving glare whilst retaining
their cooling properties (figure 4). Hard machine-made sands and ceramic particles were used as additives
to the reflective coating to improve the anti-skid performance. They found that without these additives the
skid resistance of the coating was half that of regular AC pavement.
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Fig. 4. Outdoor test of various solar reflective coatings - image sourced from Zheng, Han, Wang, Mi, Li and He (2015).
Developing and supplying paints and particle additives for pavement surfaces will increase consumer
demand for manufactured materials, which, in turn, will produce more waste and anthropogenic heat, and
perhaps offset the UHI benefits of the materials. Utilising recycled materials as construction materials will
help to alleviate the environmental stresses associated with increased landfill and will contribute to a
reduced demand for newly manufactured materials. Sano, Tobi, Takahashi and Sakai (2009) developed
a method for producing ceramic waste aggregates from recycling ceramic porcelain insulators, and
Higashiyama, Sano, Nakanishi, Takahashi and Tsukuma (2016) used light coloured ceramic waste
powder as an aggregate in their cement-based grouting materials (CBGMs) for cool pavements.
Combining the properties of cement and AC has become an important feature of current research.
Pomerantz, Akbari, Chang, Levinson and Pon (2003), in their study of cooler and more reflective streets
for heat island mitigation, analyse the difference between normal Portland cement concrete and AC. In
addition, they consider the use of chip seal pavements as a cooler and more efficient surface treatment. It
was found that Portland cement pavement, given its smooth texture and light colour, presents a much
higher albedo and lower surface temperature than AC pavement (Pomerantz, Akbari, Chang, Levinson
and Pon 2003). In addition to this, Portland cement concrete pavement was found to be cooler due to the
fact that it is a mineral that attracts rather than repels water, unlike the oil based asphalt binder. Water has
a passive cooling effect on the pavement, which is mainly due to evaporation. This is consistent with
studies done to date, illustrating that more reflective pavements are positive for the environment.
Page 19 of 40
By resurfacing that achieves a lighter colour, chip seals have also been shown to work quite well at
increasing albedo (Akbari, Pomerantz and Taha 2001; Santamouris 2013b). This is because chip seals
are generally an overlay type structure, in which the aggregate remains exposed, meaning that the albedo
is almost the same as the exposed aggregate (Pomerantz, Akbari, Chang, Levinson and Pon 2003),
thereby resulting in greater heat and light reflection than thermal conduction by the pavement. Whilst the
albedo tends to decrease for a chip sealed pavement due to age and use, it remains higher than that of
AC for about five years, resulting in a reduction in the environmental impact over its life cycle (Qin 2015a).
Chip seals are often only used on rural roads of low traffic volume, and, therefore, whitetopping is a more
appropriate sealing method for medium to high density roads. Whitetopping is the process by which an
aged pavement is resurfaced with around 10cm of light coloured concrete. Low-traffic volume pavements
can be resurfaced with an even thinner surfacing of concrete (Qin 2015a).
Age and weathering can degrade or even enhance the reflectance of cool pavement coatings. The current
practise of assuming a constant albedo over the lifetime of AC pavement overestimates the warming
potential of the pavement. As an AC pavement ages its albedo can increase from .05 to 0.15; this occurs
because the binder oxidises and its aggregates become exposed (Qin 2015a). Sen and Roesler (figure
5)(2016) developed an aging albedo model to determine the significance of the change in albedo of AC
pavement over time, and found that assuming a static albedo over the life of a pavement overestimates
the global warming potential of the pavement by 25%. According to their model the albedo of a pavement
increases rapidly in its first year.
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Fig. 5. Sen and Roesler's aging albedo model (2016).
In contrast to the increasing albedo of AC, the albedo of concrete decreases over time, as dirt, tyre wear
and exposure darken the pavement. Research in this field is important to determine the economy of using
reflective coatings on AC pavement. Future research needs to consider the durability of reflective coatings
to adequately assess whether altering the albedo of AC pavement is actually the most economically viable
way of addressing the UHI effect of AC pavement in the long term.
8.3. Evaporative and water retentive pavements
In addition to reflective pavements, evaporative and water retentive pavements have been topics of much
study, due to their in-built ability to cool a pavement through water evaporation. Evaporation of water is
critical to the creation of urban cool islands within cities and urban environments (Nakayama and
Hashimoto 2011). Porous, water retentive and permeable pavements essentially eliminate the finer
particles from their structure, or replace the finer particles with more porous materials, such as pozzolans,
slag, fly ash, or silica fumes (Santamouris 2013b). The amount of air voids in asphalt depends on the
mixture and can vary between 5 and 30% of the total volume (Hassn, Chiarelli, Dawson and Garcia 2016).
The increased degree of air voids within the pavement means that water can pass into the voids either by
seepage from the soil below, or through surface rainfall. Solar absorption causes heat convection from the
pavement to the air and a conductive heat flux with the lower pavement layers. A saturated pavement
Page 21 of 40
emits less heat because part of the absorbed energy is diverted to evaporate the water into the atmosphere
(Hassn, Chiarelli, Dawson and Garcia 2016). Once thermal exchanges heat the pavement by solar
radiation to a certain temperature, evaporation of the liquid within the voids requires energy usage, thereby
cooling the pavement (Santamouris 2013b). This evaporative cooling technique suppresses the parameter
Γ (Equation 1): the percentage of the absorption to the thermal conduction of solar radiation.
A higher air void content in asphalt decreases the thermal conductivity of the pavement as well as lowers
its specific heat capacity. Hassn, Chiarelli, Dawson and Garcia (2016) determined experimentally in a
laboratory that the surface temperature increase rate was higher for dry asphalt slab samples with a high
air void content than for dry samples with a low void content. This suggests that low thermal conductivity
detrimentally affects a dry pavement’s ability to dissipate heat. However, the surface temperature increase
rate was lower for wet asphalt slab samples with a higher air void content. In wet conditions, a pavement
that holds more water can divert more energy to evaporate the water than a pavement with a higher
conductivity and heat capacity but lower void content. This indicates a key concept in porous pavement
design: the ability to hold water for longer periods of time will increase the cooling ability of an evaporative
pavement.
The easing of stormwater runoff is critical, as it is rainfall that determines the effectiveness of a permeable,
porous and water retentive pavement. The cooling properties of an evaporative pavement depend on
whether the evaporative cooling effects outweigh the low thermal inertia of the pavement. It is critical to
note here that permeable or porous pavements should not be used in areas of the world that do not
experience tropical fluctuations in weather. This is because in hot arid environments the temperature of
these pavements can actually be hotter than ordinary AC, and have a more detrimental effect on the
surrounding environment than their counterparts (Santamouris 2013b).
Evaporative pavements can be designated as porous, permeable, or pervious. Although the terms
permeable, porous and pervious are often used interchangeably when discussing evaporative pavements,
Qin (2015a) offers separate definitions as all three exhibit unique properties. In addition, when discussing
evaporative pavements, it is important to note the entire structure of the pavement. For high traffic load
pavements permeability or porosity often only refers to the top layer of the pavement. If water is able to
permeate throughout the pavement structure it would undermine the base and sub-base. For this reason,
when studies refer to permeable or porous pavements, it is important to qualify the structure of the entire
pavement and to refer to its utility.
8.3.1. Porous Pavers
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Porous asphalt pavement allows water to pass through voids in the pavement structure. Porous pavers
are used in non-road pavement design. These pavers comprise a cellular grid system with holes filled with
materials designed to hold moisture. Grass is the most effective infill for these pavers as grass has a higher
albedo than other fillers (such as dirt), and the transpiration from the grass aids evaporative cooling (Qin
2015a).
8.3.2. Permeable Pavement
Permeable pavements operate on a different principle than porous or pervious pavements. Rather than
allowing rainwater to pass directly through the pavement to the base, permeable pavements specifically
refers to pavements that have a permeable top layer that diverts the water flow to channels so that water
passes around the pavement (Qin 2015a). Permeable pavements have been shown to effectively mitigate
urban issues, such as the heat island effect, and also slow the stormwater runoff of pavements (Li 2015).
They are also gaining wider use within the paving industry, not only due to this ability to reduce stormwater
runoff, but also because of the increased friction properties from the rough texture and the associated
improved safety for vehicles (Stempihar, Pourshams-Manzouri, Kaloush and Rodezno 2012).
According to Takebayashi and Moriyama (2012), in their study of the surface heat budget for various
pavement materials, the albedo and thermal properties of a porous material surface are highly dependent
on the distribution of aggregate within the pavement. The reflectance of solar radiation for porous
pavements during daytime periods is generally considered to be lower than for smooth surfaced
pavements due to the open and coarse structure of the pavement (Golden and Kaloush 2006; Stempihar,
Pourshams-Manzouri, Kaloush and Rodezno 2012; Santamouris 2013b; Qin 2015a). In addition to this,
the less dense structure results in less heat storage and transfer to the surface and subsurface layers,
often resulting in a hotter daytime surface temperature (Stempihar, Pourshams-Manzouri, Kaloush and
Rodezno 2012; Qin and Hiller 2014; Coseo and Larsen 2015). Porous [permeable] pavements, however,
are significantly more effective during night periods due to their high void ratio, giving them increased
thermal insulation and storage properties (Stempihar, Pourshams-Manzouri, Kaloush and Rodezno 2012).
In addition, porous and permeable pavements also effectively mitigate the UHI effect through their
evaporative efficiency, and because they do not store as much heat during the day for re-radiation into the
atmosphere (Golden and Kaloush 2006). According to Equation 1, low solar reflectance combined with a
lower thermal inertia means that a permeable pavement will be hotter during the day and cooler during the
night than a densely graded AC. Porous pavements have been shown through studies (Stempihar,
Pourshams-Manzouri, Kaloush and Rodezno 2012) to have the lowest night-time temperature compared
to other solutions, and should be considered as an effective tool for UHI mitigation in tropical environments.
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As with reflective pavements, using recycled materials in evaporative pavement design can decrease
dependence on newly manufactured materials to further reduce UHI effects. Composites of recycled
rubber, and chitosan, a by-product from the seafood industry, can be used for permeable pavements and
can exhibit stormwater filtration performance comparable to typical stormwater treatment practices
(Murray, Kayla and Brooke 2014). However, permeable pavements are often less durable than AC, and,
due to clogging of the pores, require more maintenance. Both the lower durability and the associated
maintenance increase the costs of the pavement over its lifetime.
8.3.3. Pervious Pavement
According to Qin (2015a) a pervious paver refers to a special type of concrete that has a high level of
porosity that allows water to pass directly through the pavement. The large internal cavities in the concrete
allow water to drain quickly and therefore the cooling effects of this concrete vary greatly. The high solar
absorption of pervious pavers combined with a low thermal inertia causes the pavement to be hotter during
the day.
Qin (2015a) includes pervious asphalt concrete under his designation of pervious pavement. However, in
much of the literature, this is referred to as porous asphalt concrete. Porous asphalt concrete is achieved
by eliminating the finer aggregate in AC, which causes air voids between the larger aggregates. Water
can then filter through the voids and into the sub-base of the pavement. A modified approach can even be
used for highway road engineering by laying a thin surfacing of porous pavement over the impermeable
AC layer. This allows the road to absorb rainwater and divert it to the side of the road as stormwater runoff
while contributing to road safety and cooler pavement design. This porous asphalt behaves similar to most
evaporative pavements with a cooling effect during wet periods and a heating effect during dry periods.
An increasing amount of literature highlights the importance of the evaporation rate on the efficacy of
permeable and porous pavements to reduce the pavement’s temperature. Studies show that keeping
water near the surface of the pavement is critical for improving the thermal capabilities of the pavement.
This can be achieved by improving the capillary action of a pavement or by sprinkling water over the
pavement surface (Li, Harvey and Ge 2014). The capillary effect depends on the air void content, pore
size and permeability of the pavement structure. It is important to note that the positive effect of evaporation
on UHI is dependent on a high air temperature (Li, Harvey and Ge 2014). An evaporative pavement will
only decrease the sensible heat released if the evaporative flux remains high enough to counter the
negative effects of the lower thermal inertia of a porous pavement (Qin and Hiller 2014). More research
on the amount of water required to be held in a pavement’s structure to counter the effects of other heating
sources is required. The dual uses of porous pavement (to promote quick stormwater runoff and combat
the UHI) may not be mutually attainable as the water needs to be held in the pavement structure to cool
Page 24 of 40
the pavement from evaporation. In a compact urban environment there is no quick fix solution and UHI
mitigation measures must consider all the urban geometry and environmental factors to be successful.
8.3.4. Water-retaining pavements
Water-retaining pavements were developed in response to the fast draining of pervious and permeable
pavements, which caused negligible cooling effects in the pavement (Qin 2015a). The most effective
cooling of pavements occurs when water held within the top 25mm of the pavement evaporates
(Nemirosky, Welker and Lee 2013). Water-retaining pavements hold water at the top of the pavement by
having a similar porosity to a permeable pavement but with a considerably lower permeability. Water-
retentive fillers are utilised as an aggregate in the concrete or asphalt. The high absorption of water-
retentive pavements also allows the water held in the sub-base of the pavement to be absorbed. Therefore,
water-retentive pavements are expected to remain cooler longer than permeable or pervious pavements.
Developing water-retentive pavements has become an important aspect of current research. Designing
different fillers and methods for keeping the pavement cooler longer are being researched including using
recycled waste water as a watering agent on water-retentive pavements (Qin 2015a; Jiang, Sha, Xiao,
Wang and Apeagyei 2016).
Water-retentive asphalt concrete has been developed by incorporating water-retentive slurry (WRS) in the
air voids of porous asphalt concrete. The slurry hardens and is able to absorb water and hold it for a period
of time, which significantly decreases the surface temperature of asphalt concrete and is therefore an
effective heat island mitigation measure. Most studies regarding water-retentive asphalt concrete (WRAC)
have focussed on the cooling effect of these pavements. Due to the slurry, WRAC has a lower air void
content than porous asphalt concrete (PAC), which increases the overall strength and stability of the
pavement. The analysis of the microstructure of hardened WRS combined with laboratory testing has
demonstrated that WRAC develops good moisture resistance and rutting resistance compared to PAC.
WRAC has a higher resistance to permanent deformation under high temperatures than PAC and performs
better in low temperature fracture resistance tests. However, WRAC has a lower resistance to low
temperature fracture and lower deformation resistance then densely graded asphalt concrete (Jiang, Sha,
Xiao, Wang and Apeagyei 2016).
Higashiyama, Sano, Nakanishi, Takahashi and Tsukuma (2016) developed water retaining open graded
asphalt pavements by pouring cement-based grouting into the pavement voids. The CBGMs developed
by Higashiyama, Sano, Nakanishi, Takahashi and Tsukuma (2016), to produce a surface temperature rise
reduction function (STRRF), consist of cement (C), ceramic waste powder (CWP), and fly ash (FA) or
natural zeolite (NZ). CBGMs utilising recycled CWP were found to reduce the surface temperature of open
Page 25 of 40
graded asphalt by 10 to 20°C when the open graded asphalt reached daytime temperatures of 60°C or
more.
Jiang, Sha, Xiao, Wang and Apeagyei (2016) used laboratory testing and scanning electron microscopy
to investigate the effect of varying the proportions of ingredients of water retentive slurry (WRS) in porous
asphalt concrete (PAC). The workability, water absorbing capacity, compressive strength and flexural
strength of cured WRS are affected by the mix proportion of ground granulated blast furnace slag
(GGBFS), fly ash (of a porous cellular nature), an alkali activator (calcium hydroxide), and water. The
cementitious and hydraulic properties of GGBFS and fly ash contribute to the long-term strength increase
and resistance to the drying shrinkage of WRAC. The study demonstrates that the optimal mass ratio of
GGBSF to fly ash for compressive and flexural strength is 90% to 10%. Calcium hydroxide has a significant
effect on the water absorbing capacity, compressive strength and flexural strength of hardened WRS. A
calcium hydroxide content of 15% provides adequate hydration with improved mechanical and water
absorbing properties.
Jiang, Sha, Xiao, Wang and Apeagyei (2016) also showed that WRAC has a cooler surface temperature
than PAC. This is due to WRAC’s moisture retentiveness and evaporative cooling, and the off-white colour
that provides high reflectivity of the WRS. Jiang, Sha, Xiao, Wang and Apeagyei (2016) found that the
cooling effects of WRAC decreased as the water retained evaporated, thereby demonstrating that the
effect of WRAC is highly dependent on rainfall or watering; however, due to its higher reflectivity, WRAC
still continually shows lower surface temperatures than PAC both in the daytime and night.
A limitation to much of the current literature is that research regarding ‘cool pavements’ rely on laboratory
testing or field testing that was only conducted over a short period of time and often far from true urban
conditions. Toraldo, Mariani, Alberti and Crispino (2015) studied the UHI effects on five different road
surfaces for a period of a year. Untreated dense and porous asphalt concretes were compared with dense
and porous asphalt concretes that had been sprayed with a photocatalytic coating and with an open graded
asphalt concrete filled with a cement mortar. The authors confirmed that solar irradiance has a great effect
on pavement temperature and that relative humidity influences open graded pavements. Lighter coloured
pavement reduces the surface temperature and open graded asphalt filled with a cement mortar lowers
the surface temperature by about 14°C compared to dense asphalt concrete. Toraldo, Mariani, Alberti and
Crispino (2015) found that open graded asphalt with cement mortar has the lowest temperatures and the
least amount of heat transferred compared to the other mixtures.
8.4. The Urban Canyon
Page 26 of 40
Urban geometry can greatly affect the efficacy of UHI mitigation methods, particularly reflective
pavements. The urban canyon (UC) is a descriptive term for roads and pathways surrounded by the
buildings, walls and roofs of an urban environment. This combination of roads and buildings essentially
creates canyon-like structures that are subjected to many reflections. The geometry of an urban canyon
will affect the magnitude of radiation reaching street level and escaping back into the atmosphere
(Santamouris 2015a). The urban canyon of tall buildings and narrow streets trap heat, and prevent air
circulation (Rajgopalan, Lim and Jamei 2014). The urban geometry is also the greatest man made
influence on wind flow characteristics through an urban zone (Memon, Leung and Liu 2010). Therefore,
understanding the holistic effect of UHI mitigation measures on the entire urban street environment is an
important point of the current research.
An increasing number of studies are exploring the actual effectiveness of UHI mitigation techniques on the
air temperature of an urban area. Many cool pavement studies in the past have focussed on surface
temperature and have often assumed that reducing pavement surface temperature will have a positive
effect on UHI mitigation on a city-scale level. Qin and Hiller (2014) found that the sensible heat released
to the air from the pavement typically follows the pattern of surface temperature and can be reduced by
cool pavements. However, this is not always the case, as environmental factors, such as urban canyons,
complicate the issue.
The height/width aspect ratio of a canyon has an enormous effect on the urban canyon albedo (UCA) and
can cause an increase in air temperature (Memon, Leung and Liu 2010). Urban canyons have lower
albedo than roads and pavements, and absorb more solar radiation (Qin 2015b); an urban canyon will
also act as a reflector, preventing radiation from escaping into the urban boundary layer. An increase in
pavement albedo will only raise the UCA if the h/w aspect ratio 1.0 (Qin 2015b). This has immediate
ramifications for urban planning as it constricts the context in which reflective pavements will reduce the
UHI effect. In fact, raising the pavement’s albedo in an urban canyon can have negative effects as it
increases the amount of solar radiation reflected into the surrounding walls of the buildings and can
increase air temperature (Coseo and Larsen 2015). This additional energy will increase the sensible heat
of the urban zone and may increase night-time heat emission as it is difficult for heat to advect out of an
urban canyon. Increasing the albedo of pavements in an urban canyon will also increase the energy usage
of the surrounding buildings (Yaghoobian and Kleissl 2012). This occurs because the buildings absorb a
greater amount of heat, and, therefore, need to expend more energy for cooling. The exact effect of
increasing pavement albedo needs site specific research because it can be affected by variables such as
wind, sky view factor, building insulation, mirrored windows, building aspect ratios and shapes. More
research is required to determine the adequacy of high albedo pavements in urban zones.
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8.5. Making the environment green
One of the easiest solutions to mitigate the UHI effect is to revert back to the environment of the past. This
is because the reduction in vegetation, and the greater prevalence of hard materials and structures, as
outlined earlier, is a significant cause of the UHI effect (Oke 1982; Akbari, Pomerantz and Taha 2001;
Stone and Norman 2006; Stone, Hess and Frumkin 2010). More trees not only provide shade for buildings
and people, but also reduce the wind speed under the canopies, and cool the air through
evapotranspiration (Akbari, Pomerantz and Taha 2001; Santamouris 2015b). The surface temperature
and the air temperature of urban green areas are lower than those of the surrounding cityscape and even
a relatively small green area can contribute a cooling effect (Doick, Peace and Hutchings 2014; Tan,
Wong, Tan, Jusuf and Chiam 2015). This cooling effect extends its benefits to the surrounding cityscape.
Golden and Kaloush (2006) have been able to illustrate in their subject site that increased canopy
coverage is an effective mitigation measure, as temperatures below the canopy are up to twenty degrees
cooler in the mid-afternoon and at most times during the day. Takebayashi and Moriyama (2012), in their
study of the surface heat of pavements, made a comparison between AC pavements and grass, as many
other studies have done. They illustrated that the average daytime temperature of AC pavements is up to
20°C higher than the grass surface. This is a significant increase, and causes a detrimental effect on the
environment. Urbanisation has undoubtedly created this hazard to the environment and general wellbeing.
Not only does the higher amount of vegetation reduce heat within a city, it also contributes to environmental
impact management. More vegetation prevents smog formation (Gorsevski, Taha, Quattrochi and Luvall
1998), results in less reflection and glare, and greater thermal comfort for an area’s inhabitants (Mullaney,
Lucke and Trueman 2015; Tan, Lau and Ng 2016). The heat related health impacts are also minimised as
a result of the greater prevalence of vegetation in the environment (Stone, Hess and Frumkin 2010; Tan,
Wong, Tan, Jusuf and Chiam 2015). There are many ways to increase the prevalence of vegetation in an
urban zone. For example, urban greenery can be enhanced through the increased installation of city parks,
private gardens, street trees and rooftop gardens.
Research suggests that localised tree planting in small parks with high sky view factors (SVF) significantly
cools down urban air temperatures and that planting trees in the wind path will increase the cooling ability
of each tree (Tan, Lau and Ng 2016). In a five-month case study of a park in central London, Doick, Peace
and Hutchings (2014) found that the cooling effect of the urban greenery was greatest on warm nights with
low wind speed. However, the boundary of the cooling effect was largely variable and they suggest that
urban planners should seek to develop a higher number of small, wooded green spaces for more efficient
and stable cooling effects. The sky view factor (SVF) is an important parameter for the cooling effect of
trees, because a low SVF causes heat to dissipate more slowly at night (Oke 1982; Rafiee, Dias and
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Koomen 2016). Rafiee, Dias and Koomen (2016) also found that a 40m radius around greenery will
experience the greatest reduction in UHI effects.
Research into the role of cool roofs is increasingly placing an emphasis on green roofs. Cool roofs and
green roofs are both effective strategies against the UHI effect. Roofs represent, on average, 20-25% of
urban surfaces in Italy and 60-70% of the building envelope, and, therefore, are an important urban
component to consider (Costanzo, Evola and Marletta 2016). A cool roof increases the albedo of the roof
surface and therefore decreases the amount of solar radiation that is absorbed by the building
(Razzaghmanesh, Beecham and Salemi 2016). However, this will only provide a benefit in summer.
Reducing the solar heat gains from the roof in winter will actually increase the energy needs of the building
(Costanzo, Evola and Marletta 2016). In contrast, a green roof replaces the roof surface exposed to solar
radiation with soil and plants, and creates a more natural environment with many benefits.
Green roofs reduce both the surface temperature and the air temperature of an urban area during a hot
summer day (Razzaghmanesh, Beecham and Salemi 2016). This direct benefit demonstrates that the UHI
effect is resisted by increasing evapotranspiration and by converting solar radiation into latent heat.
Costanzo, Evola and Marletta (2016) found that both reflective and green roofs decrease the roof’s surface
temperature. Reflective roofs perform better in summer; however, green roofs also improve stormwater
runoff mitigation, reduce noise and improve a building’s insulation (Tan, Wong, Tan, Jusuf and Chiam
2015; Razzaghmanesh, Beecham and Salemi 2016). This can have positive effects on energy costs in
both summer and winter, and can reduce carbon emissions. Green roofs lower the annual energy needs
by providing shade and insulation in summer and insulation in winter (Tan, Wong, Tan, Jusuf and Chiam
2015; Costanzo, Evola and Marletta 2016). Reflective roofs, on the other hand, may even require an
increase in energy use during winter. If a city wide green roof strategy was implemented, the air quality
would also likely improve (Gorsevski, Taha, Quattrochi and Luvall 1998).
A deeper green roof lowers the daytime temperature more than a shallow green roof, but increases the
night-time temperature (Razzaghmanesh, Beecham and Salemi 2016). The heat flux during the day moves
from the top of the garden layer to the bottom, and at night moves from the bottom layer to the surface.
This follows the same pattern as dense AC pavements. An increase in the volumetric heat capacity of a
material results in less energy being released immediately during the day with the absorbed energy being
released over a longer period of time. This typically results in higher night-time temperatures.
More research is required to determine the optimum green roof depth and possible ways to mitigate energy
release at night. Despite this, green roofs and green walls are valuable UHI mitigation strategies and would
work well when implemented with reflective pavements. Instead of excess energy being reflected from the
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pavement into the buildings it would be absorbed by the green walls and roofs (Razzaghmanesh, Beecham
and Salemi 2016). This highlights the fact the UHI mitigation relies on a multifaceted approach.
Stone, Hess and Frumkin (2010), in their studies on urban form and sprawling cities, indicate that it is the
responsibility of urban planners to make a difference in the use of vegetation and greenery. By predicting
the effects of heat, they can promote and enforce the installation of trees, green roofs, and greater water
sensitive urban design. Anthropogenic heat production can also be reduced by promoting mass transport,
bicycling and walking. Therefore, it is evident from the literature that increasing tree planting and vegetation
within an urban environment is positive, resulting in less need to consider the effect of the thermal
properties of AC pavements within cities.
8.6. Wind, water and atmosphere
The surrounding atmospheric conditions of an urban zone will affect the UHI. The rapid development of
urban areas disrupts the natural wind patterns and water bodies of the world. Wind and water play an
important role in the UHI phenomenon. Wind speed is one of the most important natural variables on the
UHI effect (Morris, Simmonds and Plummer 2001; Memon, Leung and Liu 2010). Higher wind speeds can
decrease urban temperatures, improve air circulation, improve cooling systems, and dissipate pollutants
(Morris, Simmonds and Plummer 2001; Memon, Leung and Liu 2010, Rajagopalan, Lim and Jamei 2014;
Santamouris 2015a). The greatest night-time UHI intensity is often recorded in clear and calm conditions.
The UHI intensity will decrease with higher wind speeds, higher relative humidity and increased cloud
cover (Morris, Simmonds and Plummer 2001; Santamouris 2015a). The UHI of specific urban zones is
influenced by the urban geometry as well as local atmospheric conditions. The causes of the urban heat
island effect are not the same for different cities or different climates (Mirzaei and Haghighat 2010). Region
specific mitigation strategies can effectively model the urban environment of particular cities. This confirms
that climate and region specific models are important for clearly defined UHI mitigation strategies.
The tall buildings of modern cities prevent air circulation and disrupt the convection of heat away from the
city, which also prevents the dissipation of pollutants. In addition, many UHI mitigation techniques would
be improved by circulating the cooler temperatures throughout the urban zone. Wind moves along paths
of low surface roughness and it is therefore possible to model and predict wind paths. Computational fluid
dynamics (CFD) provide a valuable tool for evaluating urban wind paths and the effect of water bodies on
the urban environment (Memon, Leung and Liu 2010; Tominaga, Sato and Sadohara 2015). Hsieh and
Huang (2016) developed a methodology for analysing urban wind corridors at a scale that would be useful
for urban planners. Detailed quantitative analyses of wind corridors can be used to effectively regulate
urban design in order to utilise wind to ventilate areas and advect heat away from its source to prevent
local heat accumulation. In their case study of Muar, Malaysia, Rajagopalan, Lim and Jamei (2014)
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determined that a step up configuration, figure 6, can distribute the wind efficiently and allow ventilation to
reach the leeward side of the building. The step up configuration places towers towards the windward side
of the block rather than in the middle or leeward.
Fig. 6. Step up configuration of towers showing a) most ventilated, b) less ventilated and c) least ventilated
option (Rajagopalan, Lim and Jamei 2014, pp. 167).
The effect of water bodies on the UHI is becoming an increasingly relevant area of study. The concept of
a water cooling island (WCI) to mitigate the UHI is based on the principle that water evaporation uses
energy that would otherwise be converted into sensible heat. Several studies have found that proximity to
a water body decreases the air temperature. The air temperature above a river can drop by over 5°C
compared to a surrounding urban zone during warmer seasons, and this cooling effect can be propagated
a few hundred metres horizontally and about 80m vertically when a sea breeze blows along the river
(Makamura, Sekine and Narita 1991). A WCI was also observed from field data in Sheffield, UK, as a
mean temperature reduction of 1.5°C was found above a river in spring; this temperature reduction was
again related to wind speed and direction. However, the cooling effect of the river was reduced in summer
as the water temperature of the river increased (Hathway and Sharples 2012).
The reduction in temperature from a water body can increase the human thermal comfort in the WCI zone.
Even relatively small water bodies can increase the human thermal comfort, as Xu, Wei, Huang, Zhu and
Li (2010) found that a small water body (0.087km2) in an urban park in Shanghai increased human thermal
comfort during the hottest part of a summer day. This study observed that the WCI effect was greatest for
an area 10 - 20 metres from the water’s edge. Tominaga, Sato and Sadohara (2015) used CFD simulations
to reproduce the WCI effect of small water bodies and found that a water body could decrease the
pedestrian level temperature by 2°C and that a wind velocity of 3m/s at a height of 10m could propagate
a cooling effect downwind up to a distance of 100m.
Du, Song, Jiang, Kan, Wang and Cai (2016) also confirmed the WCI effect in Shanghai and based their
research on an analysis of eighteen lakes and three rivers. The authors also correlated the cooling effect
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of a water body to several variables. They found that the cooling effect of water bodies is related to their
geometry and that simple shapes, such as squares, rectangles and circles, have a greater cooling effect
than complex shapes. Improving the microclimate around a water body, by surrounding the water body
with a green zone, vastly improves the water body’s cooling ability (Doick, Peace and Hutchings 2014; Du,
Song, Jiang, Kan, Wang and Cai 2016). Surrounded by impervious urban surfaces on hot days the water
body is heated by these materials and the WCI decreases. Green spaces will also improve local air
circulation which was again found to have a large influence on the cooling ability of the water body.
Most of these studies compare the temperatures of and around the water bodies with the surrounding
urban zone. Steeneveld, Koopmans, Heusinkveld, Theeuwes (2014), however, found that water bodies in
Rotterdam actually have higher temperatures than the surrounding rural areas. The harbour in Rotterdam
in summer was cooler than the surrounding urban zone in the hotter daytime periods, but increased the
UHI effect in the evening and at night. This is due to water’s relatively high volumetric heat capacity and
confirms the significance of combining a water body with a green zone and air circulation to effectively
mitigate the UHI effect.
8.7. Combined effects
Mitigation methods should not be considered in isolation. Whilst many studies have shown that it is
possible to produce energy gains and lower temperatures through a pavement with a higher albedo (
Pomerantz, Akbari, Chang, Levinson and Pon 2003; Golden and Kaloush 2006; Santamouris 2013b;
Yang, Wang and Kaloush 2015), this must be combined with other heat island mitigation initiatives
(Doulos, Santamouris and Livada 2004). Adding vegetation and planting at all levels of the urban canyon
make a significant contribution to evaporative cooling and temperature reduction when in combination with
more reflective pavement (Pomerantz, Akbari, Chang, Levinson and Pon 2003; Doulos, Santamouris and
Livada 2004). Green roofs and walls would prevent the increased energy consumption of buildings in an
urban canyon due to reflective pavements (Razzaghmanesh, Beecham and Salemi 2016). Doick, Peace
and Hutchings (2014) found that green spaces should be combined with other cooling strategies and Du,
Song, Jiang, Kan, Wang and Cai (2016) emphasised the importance of water bodies being surrounded by
green zones with good air circulation.
Trees planted in an urban environment typically have a shorter lifespan and require more maintenance
than trees in natural settings. Mullaney, Lucke and Trueman (2015) studied the effect of permeable
pavements on the growth and nutrient status of urban trees and found that tree growth is improved when
a pavement base layer of 20mm diameter drainage aggregate is installed beneath permeable pavements
over wet clayey, and, therefore, poorly-draining soil. This again demonstrates the importance of combining
UHI mitigation strategies.
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8.8. Other methods and future developments
There are many other mitigation measures for the UHI effect that can be used that will not be discussed
in this review, but are listed here for completeness. First and foremost, as suggested by Takebayashi and
Moriyama (2012), and Doulos, Santamouris and Livada (2004), substituting pavement materials and
surfaces for an alternative surface or construction material is an effective and established mitigation
measure. Alternative materials can have desirable thermal properties, of which the benefits cannot be
realised with current solutions. Recycled products and substitutes may also be used to enable the greater
thermal efficiency of pavement structures.
In addition to this, more research should be done on the thermal conductivity and capacitance of AC
pavements to better transmit the heat from the pavement and the subsurface to the air. Whilst a higher
thermal capacity pavement will significantly increase the minimum temperature of pavements, as more
heat is stored, the maximum temperature will be reduced when compared with current methods
(Santamouris 2013b).
There are various future developments for which investigation and analysis is beginning to materialise.
These involve either the substitution of materials or an improvement in current systems; as follows:
Improvements in Reflective Pavements and permeable/porous/water retentive pavements Infrared
reflective paints would allow for a greater reduction in the heat that pavements absorb as compared
with only the reflection of solar radiation. In addition, changing the material constituents of permeable
pavements would have a significant effect on the cooling of the environment and reducing the UHI.
Santamouris (2013b) provides an effective summary concerning the potential for the betterment of
reflective and permeable/porous/water retentive pavements in his published literature.
Thermochromic materials can change their albedo in response to surface temperature by undergoing
a molecular structure transformation. This is a thermally reversible process that changes the visible
colour of the material (Qin 2015a). Thermochromic materials are gradually broken down when exposed
to an outdoor environment.
Harnessing the Heat – The circulation of water beneath pavements in pipework, otherwise known as
asphalt solar collectors, is a key avenue of research. Bobes-Jesus, Pascual-Muñoz, Castro-Fresno
and Rodriguez-Hernandez (2013) have conducted a literature review into the topic, and analysed the
effect of various thermal parameters of pavements for this process. The conclusions state that
comprehensive reviews on the topic are lacking, with almost all studies to date being conducted within
a laboratory, and only analysing individual parameters in each individual research paper. There is
scope for large-scale future development in this field in order to determine its effectiveness as a
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mitigation method. In addition to this, Pascual-Muñoz, Castro-Fresno, Serrano-Bravo and Alonso-
Estébanez (2013) undertook a study on multilayered asphalt pavements that contain a highly porous
middle layer through which water can circulate. This eliminates the need for pipe systems, and uses
the inherent properties of the pavement material to advantage. The authors suggest that this is a
promising source of future research. Thirdly, García and Partl (2014) have conducted a study into the
use of air circulation conduits within pavements to harness heat. Whilst some aspects of the
experimental study proved inefficient, the study removed the inherent issues related to the circulation
of water, and warrants future research.
Photovoltaic pavements A fairly new system and solution that involves completely substituting the
AC pavement in the road for solar cells that convert solar energy into electrical power. At present this
is an expensive solution, and whilst small-scale systems have been implemented in some locations
around the world, further research needs to be done within this field to make the solution feasible and
cost effective.
Studies into green roofs on buildings – Buildings cover a large percentage of the urban environments
we live in; creating a green canopy on the top of existing buildings, and potentially between buildings,
could significantly reduce the effects of the UHI. Studies should be undertaken to estimate and outline
the benefits of covering the buildings of our cities with natural vegetation, and to quantify the potential
costs. Santamouris (2015b) has provided a brief outline of studies conducted and the benefits of green
roofs in the associated literature.
Harnessing the heat with small thermocouples – This does not appear to have been studied within the
literature. Given the current technological advancements, there may be a way to convert heat to
electricity using thermocouples. Making such devices small enough and efficient enough to be housed
within raised reflective pavement markers (RRPMs) or similar could be an effective means of
generating electrical energy from natural heat exchange processes.
Increasing the use of phase-change materials in pavements, and alternate colour pavements for the
reflection of solar and infrared heat, or for improved thermal properties during certain times of the day
or night.
Determining which plants are most effective at UHI mitigation in an urban zone is an important area of
current research. Tan, Wong, Tan, Jusuf and Chiam (2015) suggest a methodology to determine the
selection of plants that are most suitable to enhance the positive effects of rooftop greenery.
Affordable methods using recycled water to absorb the excess energy in urban gardens of high
volumetric heat capacity should be developed.
9. Discussion
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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 well developed but
also suffers from inconsistencies in the experimental and analytical methods. This questions the accuracy
of results and conclusions and makes comparison between studies difficult. Diverse monitoring
techniques, measuring equipment, duration of study, variables considered and rural reference sites all
contribute to this problem. In addition, the methodology to measure UHI intensity has not been agreed
upon (Santamouris 2015a).
It has also been suggested that many of the studies conducted with regard to the UHI effect have been
conducted during the summer and in hot environments. This is because summer is the time of the year
when temperatures are expected to be the hottest, and when it is anticipated that the most detrimental
and negative results will be measured. However, this cannot be proven as there have not been enough
studies in seasons other than summer, and in cooler climates, and researchers insist that this should be
considered in the future (Stempihar, Pourshams-Manzouri, Kaloush and Rodezno 2012; Yang, Wang and
Kaloush 2015). Observational research on the urban heat island is also limited by time and cost restraints.
Current research techniques focus heavily on model simulation to quantify the urban heat island (Mirzaei
and Haghighat 2010). However, the complexities of urban environments make it extremely difficult to
model all possible variables and generalisations are often made. More research is required to enable
integration of the various simulation methods and to make climate and regional specific models.
The implementation of reflective materials for mitigation of the UHI is well documented and researched,
and recognised as an innovative solution. However, to date, studies have not adequately explored the
effect that these materials and the increased reflected heat and light have on the local environment (Yang,
Wang and Kaloush 2015). Future studies need to consider the effect that reflective materials also have on
the surrounding building envelope and the air temperature. Reflective pavements may also undesirably
reduce the pavement temperature in winter, further reinforcing the importance of studies considering
longer time frames. Santamouris (2014) has begun his research to quantify the effect on buildings of
reflective materials; however, more studies need to be undertaken.
Buchin, Hoelscher, Meier, Nehls and Ziegler (2016) argue that the effects of UHI countermeasures are
often only considered on the outdoor environment. The effect of these countermeasures on an indoor
environment is also important to evaluate the health risks of the UHI. The highest potential for reducing
the health related risks of UHI lies in reducing the heat of the indoor environment. Classic UHI
countermeasures, such as cool pavements and urban greenery, often only have a marginal direct impact
Page 35 of 40
on the indoor environment (Buchin, Hoelscher, Meier, Nehls and Ziegler 2016). Passive or active indoor
cooling measures are a far more important variable for combating the heat related health effects of the
UHI.
The UHI countermeasures will have a negligible effect on indoor heat risk in cities with a high prevalence
of air-conditioning. In cities and areas without many air-conditioned buildings the effect of UHI counter
measures and passive cooling on heat related health risks will be higher. Reducing the net heat flux of an
urban environment will have a positive effect on the indoor environment; however, passive cooling
potential is limited by existing building design. Therefore, when considering the health implications of the
UHI a balance needs to be struck between continued air-conditioning and urban planning. Buchin,
Hoelscher, Meier, Nehls and Ziegler (2016) suggest that air-conditioning should be powered by renewable
sources and that only by rigorous passive cooling building design and UHI countermeasures at the urban
scale can the heat related health risks be possibly mitigated without air-conditioning.
The potential negative consequences of UHI countermeasures need to be fully understood before
implementing large scale urban projects. As was already discussed, reflective pavements can cause an
increase in glare for pedestrians and drivers, and may even increase the UHI effects in urban canyons.
There is also some evidence to suggest that water bodies may increase the UHI effects in the early evening
(Steeneveld, Koopmans, Heusinkveld and Theeuwes 2014). Although urban greenery and reflective
surfaces can reduce the temperature and near surface levels of ozone in an urban environment, a
decrease in temperature may actually cause an increase in the concentration of primary pollutants
(Fallman, Forkel and Emeis 2016). This occurs due to a lowering of the turbulent kinetic energy that is
responsible for diffusing these pollutants into the atmosphere (Fallman, Forkel and Emeis 2016). All UHI
mitigation strategies need to be critically evaluated to ensure successful implementation and combination.
10. 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 literature, which include reflective
pavements, evaporative pavements, making the environment greener, and harnessing the cooling effects
of wind and water. It is often found that using several methods in combination with each other is the most
effective strategy for reducing the UHI effect. There are many methods being investigated, and further
studies are required for these measures across all seasons of the year, to ensure that the best possible
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solution for mitigation of the UHI is found. The greatest issue concerning the UHI is that conditions are not
identical in every urban environment. Cities around the world are vastly different, and solutions must be
found that meet and exceed the needs of each individual city around the globe.
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One of the most significant factors affecting pavement-induced Urban Heat Islands is the albedo. Several studies have shown albedo’s effect on air temperature and building energy usage. Albedo depends on the optical properties of the material constituents of the surface layer of the pavement, which can change over time or with additives. The albedo of a series of full-scale asphalt and concrete pavement test sections of varying ages in Rantoul, Illinois (USA) was measured and a non-linear aging albedo model for asphalt pavements was developed. For a hypothetical pavement section, the current practice of assuming a constant albedo over time was found to overestimate the Global Warming Potential of the asphalt pavement by about 25%. In addition, the albedo of translucent polymer fibers was determined to be 0.07 using a spectrophotometer. This does not have a thermal impact on the pavement as long as the fibers remain coated with cement paste. However, if the fibers become exposed because of abrasion, they will have an impact on albedo depending on the pavement area containing exposed fibers.