A Review: Permeability and Thermal Conductivity of Road Surface Layers
as Regulators of Stormwater-Mediated Urban Temperature Buffering
Emad H. Salih1*,
Baydaa Hamdi Salih1
1 Department of Civil Engineering,
Anbar University, Iraq.
Corresponding Author: emadhamde48@uoanbar.edu.iq
Abstract
The rapid
urbanisation has significantly intensified the urban heat island (UHI) effects
and disrupted the natural hydrological processes due to the large-scale
transformation of vegetated land surfaces into impervious traffic
infrastructure. Urban pavements are of utmost importance to microclimate
regulation, as they directly influence heat absorption, thermal stormwater
infiltration, evaporation and surface energy exchange. In recent years, the
development of permeable and thermally adapted pavement surfaces as
multifunctional solutions to mitigate urban overheating and to improve
stormwater management in the context of changing climatic conditions has
attracted increasing attention. This research rigorously analyses the recent
developments (2016-2026) on permeability and thermal conductivity of road
surface layers and its impact on the urban temperature regulation via
stormwater. This study consolidates the current knowledge on porous asphalt,
pervious concrete, permeable interlocking concrete pavements (PICP), cool
pavements and water-retentive pavement technologies from thermal, hydrological,
environmental and sustainability perspectives. Coupled hydrothermal processes
controlling the variation of pavement temperature receive much attention. These
processes include moisture infiltration, latent heat transfer, evaporation,
thermal conductivity, albedo, heat storage and conductive heat transfer in
multilayer pavement systems. The literature review results indicate that
permeable and water-retentive pavements have the potential to significantly
reduce pavement surface temperatures. Cooling effects vary from about 5 °C to
20 °C under ideal ambient and moisture conditions. The higher permeability of
the pavement enhances evaporative cooling and latent heat flux, whereas the
thermal conductivity influences the underground dissipation of heat, the heat
storage properties and the emission of heat at night. Pore structure
characteristics, material composition, moisture availability, meteorological
conditions, traffic loading, ageing behaviour and maintenance procedures have a
great influence on the thermal and hydrological performance of pavement. The
review identifies key research gaps in long-term field performance, integrated
hydrothermal modelling, standardisation of testing procedures, climate-specific
pavement design standards, and the integration of permeable pavements in urban
climate adaptation and stormwater management systems. Ongoing research involves
intelligent permeable pavement systems, phase-change materials (PCMs),
nanotechnology-based binders, AI-enabled monitoring and maintenance prediction,
and multi-functional climate-adaptive urban infrastructure design. The study
emphasises the growing importance of hydrothermally optimised pavement systems
as sustainable solutions to improve urban climate resilience and reduce UHI
intensity and stormwater management in rapidly urbanising areas.
Keywords: permeable pavement, thermal conductivity,
stormwater management, urban heat island, porous asphalt, pervious concrete,
hydrothermal behavior, urban climate adaptation.
1.
Introduction
Modern
cities have been dramatically transformed by accelerated urbanisation and large
land-use changes that have affected the thermal and hydrological dynamics. The
replacement of natural vegetated surfaces with impermeable structures such as
asphalt roads, parking lots, concrete pavements, and urban transportation
systems has significantly altered the surface energy balance and the natural
hydrological cycles. Urban pavements are absorbing large amounts of solar
radiation during the daytime and slowly releasing the stored thermal energy at
nighttime that aggravates the urban heat island (UHI) effect and increases the
ambient urban temperature relative to the surrounding rural areas [1], [2].
Traditional impervious pavements not only reduce the infiltration of rainwater
but also increase stormwater runoff and reduce groundwater recharge, increasing
the risk of urban flooding, overloading drainage systems and limiting
evaporative cooling [3]. Consequently, the road pavement layers have been
rendered a critical interface controlling the interaction of urban hydrology,
atmospheric heat exchange and microclimatic control.
The rising
occurrence of extreme heat events, urban flooding, and climate-induced stress
on infrastructure has caused a global interest in sustainable pavement
technology that can simultaneously mitigate thermal and hydrological issues.
Pavement permeability and thermal conductivity have become two key parameters
controlling the hydrothermal dynamics of urban road systems. Permeable
pavements allow stormwater infiltration and temporary storage of subsurface
water, thus increasing evaporative cooling and decreasing sensible heat
transfer to the urban environment [4]. Thermal conductivity is a measure of the
speed of heat transfer in pavement layers and has a significant effect on
surface temperature variations, heat storage beneath the surface, and the heat
dissipation process at night [5]. The coupling of hydraulic transport systems
and thermal energy exchange constitutes a complex hydrothermal system, which
has a profound influence on urban temperature regulation and local climate
resilience.
Over the
last decade many new pavement technologies have been developed to better manage
storm water and control urban temperatures. These include porous asphalt
pavements, pervious concrete systems, permeable interlocking concrete pavements
(PICP), cool pavements, and water-retentive pavement technologies. These
technologies are increasingly being integrated into sustainable urban drainage
systems (SUDS), low-impact development (LID) policies, green infrastructure
frameworks and sponge city projects in Asia, Europe, North America and
Australia [6], [7]. Field and laboratory studies have demonstrated that
permeable and thermally optimised pavements can reduce stormwater runoff
volumes by 50-95% and also reduce pavement surface temperatures by 5-20 °C
under favourable environmental conditions [8]. However, their performance is
highly influenced by the climatic conditions, the pore structure
characteristics, the moisture retention capacity, the composition of the
materials, the traffic loading, the ageing characteristics and the maintenance
processes.
Although
much progress has been made in the research of pavement hydrology and urban
thermal mitigation, most of previous studies have analysed hydrological
performance and thermal behaviour independently. There has been rather little
attention paid to the interconnected interaction of permeability, thermal
conductivity, moisture transport and stormwater mediated urban temperature
regulation. Still there are significant knowledge gaps on coupled hydrothermal
mechanisms, long-term climatic efficacy, numerical modelling approaches,
thermal deterioration due to maintenance and climate-adaptive pavement
optimisation strategies. In addition, variations in experimental methodology
and absence of standardised assessment frameworks have hindered direct
comparison between different pavement technologies and climatic zones.
The aim of
this review is to give a holistic and critical overview of the recent
developments in hydrothermal pavement research in the period 2016-2026. The
review explicitly investigates the effects of pavement permeability and thermal
conductivity on stormwater infiltration, evaporative cooling, heat transfer,
and urban microclimate. This article reviews the state of the art for permeable
pavement technologies, and critically assesses coupled hydrothermal processes,
numerical simulation approaches, sustainability implications and new
climate-adaptive paving materials. The identification of limitations of current
research and the definition of future research directions for multifunctional
pavement systems that can improve urban climate resilience in face of
increasing environmental and climatic challenges is given much emphasis. The
main objectives of this review are to accomplish the following:
- To investigate the influence of pavement
permeability on stormwater infiltration, moisture retention, and
evaporative cooling processes.
- To examine the effects of thermal conductivity
and related thermal properties on pavement heat storage, heat transfer,
and temperature dynamics.
- To analyze coupled hydrothermal mechanisms
governing stormwater-mediated urban temperature buffering.
- To compare the hydrological, thermal, and
environmental performance of major permeable pavement technologies.
- To evaluate current numerical modeling,
simulation, and machine learning approaches used in hydrothermal pavement
research.
- To identify major research gaps, technical
limitations, and future priorities for climate-responsive pavement
engineering and sustainable urban infrastructure development.
2. Urban
Heat Islands and Pavement Thermal Dynamics
2.1 Urban
Heat Island Mechanisms
The urban
heat island (UHI) effect is one of the major environmental effects of rapid
urbanisation and large-scale land use change. Urban infrastructure is being
developed by replacing natural, vegetated and moisture-retaining landscapes
with impermeable materials such as asphalt pavements, concrete surfaces,
buildings and transportation systems. This transition substantially alters the
thermal and hydrological balance of the urban environment by changing the
mechanisms of surface energy transfer, reducing evapotranspiration and
increasing the retention of heat in urban materials [9]. This means that cities
often have much higher air and surface temperatures than surrounding rural
areas, particularly at night.
The
formation of urban heat islands (UHIs) is driven by a number of physical and
human-induced processes. Factors contributing include anthropogenic heat
emissions, decreased vegetation cover, altered aerodynamic conditions,
increased thermal retention in urban materials, and disturbed radiative energy
balances. Anthropogenic heat released by cars, industrial plants, buildings,
air-conditioning systems and energy production directly contributes to the
atmospheric warming in densely populated urban regions. The simultaneous loss
of green spaces and natural soil surfaces reduces evapotranspiration and limits
the natural cooling capacity of urban ecosystems [10].
The shape of
cities intensifies heat buildup and creates urban canyons as buildings are
tightly clustered together. These buildings hinder air flow, trap long wave
radiation, and restrict heat loss during the night. Consequently, the thermal
energy accumulated during the day is prolonged in metropolitan areas, raising
the nighttime temperatures and thermal discomfort [11].
Urban pavements are a vital part of infrastructure and a significant
contributor to the urban heat island (UHI) effect, representing about 30-45% of
the total urban land area [12]. Traditional asphalt pavements have low albedo
values, typically in the range of 0.05 to 0.20, which indicates that a large
fraction of incoming shortwave solar energy is absorbed rather than being
reflected back to the atmosphere. Their dark colour, high heat capacity and
very low moisture content enable considerable thermal energy storage during the
day and gradual release at night. High solar radiation results in asphalt
pavement surface temperature reaching more than 60-70 °C and greatly
intensifies the sensible heat transfer towards the neighbouring metropolitan
air masses. The thermal response of pavement systems is governed by a surface
energy balance, which results from a combination of interconnected activities
of heat exchange including:
- Absorption of incoming solar radiation
- Reflection of shortwave radiation
- Longwave thermal radiation emission
- Conductive heat transfer within pavement layers
- Convective sensible heat exchange with the
atmosphere
- Latent heat exchange associated with evaporation
- Subsurface heat storage and delayed nocturnal
release
The relative magnitude of these heat fluxes is greatly affected by the
properties of pavement materials, such as albedo, emissivity, thermal
conductivity, permeability, porosity, moisture retention capacity, and surface
roughness. Conventional impervious pavements typically do not allow for latent
heat transfer as little moisture is available for evaporation. Thus, more of
the absorbed solar energy is converted into sensible heat, increasing the
warming of the air near the surface and the storage of heat in urban areas
[14].
Permeable
and water-retentive pavements, on the contrary, alter the dynamics of urban
energy exchange through the infiltration of rainwater and the increase of
evaporative cooling. These systems temporarily store rainfall in interconnected
pore structures, permitting slow evaporation and latent heat uptake from the
pavement surfaces and surrounding atmosphere. Therefore, permeable pavements
can reduce surface temperatures and reduce the intensity of the local urban
heat island under favourable climatic conditions [15].
Climatic and
climatic circumstances greatly influence the thermal behaviour of pavement and
intensity of urban heat island. High solar radiation and low humidity in
hot-arid climates accelerate pavements heating and increase surface
temperatures during daytime. Evaporative cooling, however, may be less
effective in humid climates because of the lower vapour pressure difference
between the pavement surface and the surrounding atmosphere. Additional
environmental factors such as wind speed, cloud cover, frequency of
precipitation and seasonal variations also influence the urban thermal regime
and heat exchange mechanisms of pavements [16].
Modern urban
climate studies are increasingly aware that pavement systems cannot be judged
simply on structural or transportation performance criteria. Modern sustainable
pavement engineering is focused on the multifunctionality of solutions that
simultaneously address stormwater management, mitigation of thermal buildup,
urban climate resilience, and environmental sustainability [17]. This shift has
sparked global interest in cool pavements, permeable pavements and
climate-responsive pavement technology to reduce UHI intensity while promoting
sustainable urban development.
2.2 Thermal Conductivity of Pavement Materials
Thermal conductivity is one of the important physical
properties which govern heat transfer dynamics in pavement systems. It is a
description of the ability of the material to conduct thermal energy from a
region of higher temperature to a region of lower temperature when there is a
temperature gradient. Thermal conductivity regulates the rate of solar heat
absorbed into the subsurface pavement layers and the effectiveness of thermal
energy redistribution and dissipation in pavement engineering. The thermal
conductivity of pavement materials varies greatly depending on the composition
of the material, properties of the aggregates, moisture content, porosity,
density and temperature conditions. The typical thermal conductivity of dense
asphalt mixtures is in the range of 0.7-2.5 W/m·K [18]. Conductive mineral
aggregates in thick pavements They are more conductive and conduct heat better
through the pavement structure. Porous pavements are generally less suitable,
as the pores are filled with air (thermal insulator) [19]. There are many
important factors affecting the thermal conductivity of pavement.
- Aggregate mineralogy and particle distribution
- Asphalt binder composition and rheological
properties
- Air void content and pore connectivity
- Moisture saturation conditions
- Degree of compaction
- Temperature gradients within pavement layers
- Surface ageing and oxidation processes
The mineral composition of the aggregate is important
because the thermal conductivities of the different minerals are quite
different. Generally, quartz aggregates have better heat conductivity than
limestone or lightweight aggregates. Similarly, the thermal storage properties
and the heat transfer efficiency are affected by the properties of asphalt
binder, especially at high temperature where the viscoelastic properties vary
significantly [20]. Another important
regulator is the water content. The thermal conductivity of water is much
higher than that of air. Therefore, thermal conductivity is generally increased
with pavement saturation level [21]. The infiltration of rainwater in permeable
pavements pushes out the air in the pore spaces and therefore increases the
conductive heat transfer in the pavement structure.
As a result,
saturated surfaces can transfer heat more efficiently to the deeper layers,
thus reducing the excessive surface temperatures at midday. However, the
increased conductivity could potentially enhance nocturnal heat dissipation and
modify the daily thermal cycles [22].
The thermal
conductivity and thermal performance of pavement are extremely complicated.
Pavements with high thermal conductivity can distribute absorbed heat more
uniformly within the pavement structure and reduce surface heat accumulation
under daytime conditions. However, these materials are also able to store a lot
of thermal energy and slowly release it during the night, which helps to
prolong the nocturnal warming [23].
Pavements
with low thermal conductivity reduce the heat penetration downwards and
decrease the deep thermal storage. This may reduce heat loss at night but at
the same time increase surface temperatures during the day because of the
storage of absorbed solar energy near the pavement surface. Therefore,
enhancing the thermal conductivity of pavements requires a compromise between
the effectiveness of daytime cooling and the heat dissipation properties at
night [24]. Thermal diffusivity and volumetric heat capacity are also important for
the dynamics of pavement temperature, in addition to thermal conductivity.
Thermal diffusivity governs the rate of temperature change in pavement
materials, while volumetric heat capacity governs the amount of thermal energy
that can be stored in the pavement structure. Dense asphalt pavements with high
heat capacity absorb large amounts of heat during the day and retain it, thus
aggravating the urban warming during the night. Conversely, low-density
permeable pavements tend to show less heat retention and faster thermal
response cycles [25]. Recent research progresses on the pavement materials have
been motivated to modify the thermal conductivity and heat transfer behaviour
by novel technologies, such as:
- Nanomaterial-enhanced asphalt binders
- Recycled and lightweight aggregates
- Reflective mineral additives
- Porous ceramic materials
- Phase-change materials (PCMs)
- Moisture-retentive additives
Among these technologies, phase-change materials are
especially promising as they absorb and release latent heat during the phase
transition and thus stabilise the pavement temperature and mitigate the thermal
fluctuation under harsh climatic circumstances [26], [27]. In addition, the
thermal conductivity of pavement shows spatial heterogeneity in pavement
structures. The top layers receive the most solar exposure and thermal stress,
and the base and subbase layers serve largely to retain heat long-term and
manage moisture. Therefore, the knowledge of vertical temperature gradients and
thermal behaviour of layers is important for the development of climate
responsive pavements, which could be a tool for mitigation of UHI intensity and
improvement of urban thermal resilience.
2.3
Hydrothermal Coupling in Pavements
Hydrothermal
coupling is the simultaneous interaction of thermal transfer processes and
moisture movement in pavement systems. In permeable and water-retentive
pavements the thermal and hydrological processes are tightly coupled as the
pavement temperature dynamics and the thermal conductivity characteristics are
directly influenced by infiltration, evaporation, drainage and moisture
retention [28]. Rainwater enters the interconnected pore networks during
precipitation events and is stored for a period of time in the pavement surface
layers, aggregate bases, and subbase reservoirs. The thermal properties of the
pavement are significantly affected by moisture due to the fact that water has
much higher thermal conductivity and heat capacity than air [21]. With
increasing saturation of the pavement, the conductive heat transport in the
pavement structure becomes more efficient and the surface and subsurface
temperature distributions change.
Evaporative cooling is an important hydrothermal process
influencing the thermal performance of pavements. After precipitation or
artificial irrigation, liquid kept in pavement pores gradually evaporates due
to sun radiation and ambient warmth. Evaporation drains latent heat from both
the surfaces of pavements and the surrounding air, which lowers surface
temperatures and decreases the sensible heat flux into the urban atmosphere
[29]. This evaporative cooling allows permeable pavements to be significantly cooler
than conventional impermeable pavements under hot weather conditions.
Experimental results revealed that the surface temperature of permeable
pavements can be reduced by about 10-15 °C during rainstorms, compared to dry
pavements [30]. For water-retentive pavement systems, reductions of 20 °C have
been shown under high sun radiation and favorable air conditions [31]. The
effectiveness of hydrothermal cooling processes depends on several interacting
parameters, including:
- Moisture storage capacity
- Pore size distribution and connectivity
- Pavement permeability
- Climatic and meteorological conditions
- Relative humidity and wind speed
- Solar radiation intensity
- Pavement thickness and material composition
- Drainage characteristics and evaporation rates
Moisture Availability is critical as evaporative cooling is
reduced substantially when stored water runs out. The results show that the
pavement temperature increases faster under extended dry conditions and the
latent heat exchange decreases [32]. Thus, the thermal performance of permeable
pavements is highly variable and continuously changing with precipitation
regimes, seasonal climatic conditions, and atmospheric moisture demands.
Hydrothermal interactions impact the durability and long-term structural integrity
of pavement. Repeated wetting-drying cycles can alter the material stiffness,
the pore structure stability, thermal expansion properties and mechanical
strength. In cold regions, the freeze-thaw cycles of residual moisture can
accelerate the cracking and deterioration of pavement [33]. Therefore, the
hydrothermal performance should be assessed not only in terms of thermal
mitigation, but also in terms of long-term structural integrity and maintenance
requirements.
Recently, more sophisticated coupled hydrothermal
simulation models are gradually developed for better understanding these
complex interactions. These models simulate the thermal-hydrological behavior
of pavement under different environmental scenarios by using heat conduction
equations, moisture transport mechanisms, radiation balances and atmospheric
boundary conditions [34], [35]. Coupled simulations allow researchers to
evaluate the combined impact of permeability, thermal conductivity, water
retention, and weather parameters on urban pavement temperatures and stormwater
processes.
Recent advances in smart pavement technology have driven
increased research on hydrothermal. Nowadays, the real-time distribution of
pavement moisture and temperature is monitored by means of embedded thermal
sensors, infrared thermography, remote sensing systems, Internet-of-Things
(IoT) monitoring platforms and machine learning algorithms [36], [37]. These
technologies enable predictive maintenance, improved performance optimisation
and adaptive climate responsive pavement management systems.
Hydrothermal coupling is one of the key processes for
permeable pavements to enhance the urban climate resiliency. Permeable pavement
systems have many environmental benefits such as stormwater infiltration,
moisture retention, evaporative cooling and thermal energy exchange. These
systems can contribute to the reduction of urban heat islands, flood risks and
thermal discomfort, and to sustainable urban development.
3. Permeable
Pavement Systems
Permeable pavement technologies have emerged as important
elements of sustainable urban infrastructure that can address the issues of
stormwater management, urban heat island (UHI) reduction and environmental
sustainability. Permeable pavements, in contrast to conventional impervious
pavements, are fabricated with interlinked pore networks that allow
infiltration of rainwater through the surface layers and into underlying
storage reservoirs and subgrade soils. It reduces surface runoff, delays peak
discharge, increase groundwater recharge and participate in evaporative cooling
processes, which help to reduce urban heat [38], [14]. Besides their
hydrological functions, permeable pavements also have an important impact on
urban surface energy balances. Such technologies improve water infiltration and
short-term water retention, which increases latent heat exchange via
evaporation and reduces sensible heat fluxes emitted to the urban atmosphere.
Therefore, permeable pavements can control the surface temperature, decrease
the heat retention in the metropolitan area, and improve the thermal comfort
outdoors under favourable meteorological conditions [15]. The overall thermal
and hydrological performance of permeable pavements depends on several
structural, material and environmental factors, such as:
- Porosity and pore connectivity
- Aggregate gradation and mineral composition
- Pavement layer thickness
- Hydraulic conductivity and infiltration capacity
- Thermal conductivity and heat capacity
- Moisture retention characteristics
- Climatic conditions and rainfall patterns
- Traffic loading and maintenance practices
The four main types of modern permeable pavement
technologies are porous asphalt pavements, pervious concrete systems, permeable
interlocking concrete pavements (PICP), and water-retentive pavements. Each
system has unique structural characteristics, thermal performance, infiltration
characteristics, durability performance and environmental benefits.
3.1 Porous
Asphalt
Porous asphalt is a widely used permeable pavement
technology for urban roads, parking lots, pedestrian facilities, bike lanes,
and low traffic transportation infrastructure. Open-graded asphalt mix is an
open graded asphalt mix with interconnected air voids that allow for rapid
infiltration of stormwater through the pavement surface and into underlying
aggregate reservoirs. Average typical values of porosity are around 15% to 25%
which is significantly higher than typical dense-graded asphalt pavements [38].
The normal structure of porous asphalt pavement is usually:
- Open-graded porous asphalt surface layer
- Crushed stone aggregate reservoir
- Permeable base and subbase layers
- Geotextile filtration layers
- Optional underdrain systems for excess water
removal
The interconnected void structure provides considerable
hydrological benefits, allowing for rapid rainfall infiltration, thereby
reducing surface ponding and minimising the risk of hydroplaning during storms.
Infiltrated rainwater is temporarily held in subsurface aggregate layers where
it either slowly percolates into underlying soils or is discharged to a
controlled drainage system. This method decreases peak runoff values, and
improves the efficiency of urban drainage, and helps groundwater recharge [39].
From a thermal point of view, the porous structure and water movement in porous
asphalt are responsible for relevant cooling mechanisms. Rainwater collected in
the interconnected pore spaces evaporates slowly, absorbing latent heat from
pavement surfaces and the surrounding air. Therefore, porous asphalt systems
tend to have lower surface temperatures than conventional dense asphalt
pavements during day [13]. The main thermal and environmental benefits of
porous asphalt are:
- Enhanced evaporative cooling capacity
- Reduction of stormwater runoff generation
- Increased latent heat exchange
- Lower sensible heat fluxes to surrounding air
- Reduced peak surface temperatures
- Improved pedestrian thermal comfort
- Mitigation of urban heat island intensity
Research in the field and laboratory has shown that under
similar climatic conditions, the surface temperature during daytime is about 5
°C to 12 °C lower than that of conventional asphalt pavements [13], [29]. The
cooling effects are especially noticeable after rain, when the pores of pavements
trap moisture that continuously evaporates under the sun’s radiation [40].
Due to the air voids, the porous asphalt is generally less thermally conductive
than dense asphalt mixtures, which reduces the conductive heat transfer in the
pavement structure. When pavement pores are saturated with water, thermal
conductivity increases significantly, as water transfers heat more efficiently
than air [22]. The dynamic hydrothermal interaction is of great importance to
pavement temperature distributions, heat retention properties and cooling
effectiveness.
However, porous asphalt systems have many operational and engineering problems,
despite their benefits. The main limitation is clogging caused by accumulation
of sediments, dust, organic matter and vehicle pollutants in surface pores. Temporary
blockage leads to a time dependent decrease of infiltration capacity and a
degradation of the evaporative cooling performance [41]. Other limitations are:
·
Reduced
structural durability under heavy traffic loading.
·
Moisture lost
during prolonged dry periods.
·
Asphalt binders
oxidation and aging potential.
·
Freeze-thaw
vulnerability in temperate climate.
·
More maintenance
than dense pavements.
In hot-arid climates, stored moisture is rapidly depleted
by fast evaporation, which limits the latent heat exchange and thus the cooling
effectiveness [42]. Thus, the long-term thermal behaviour of porous asphalt is
highly dependent on the climatic conditions, the frequency of rainfall and the
quality of the maintenance.
Recent
research has focused on improving porous asphalt performance using advanced
materials and innovative design strategies, including:
- Polymer-modified asphalt binders
- Recycled and lightweight aggregates
- Nanomaterial-enhanced mixtures
- Hydrophilic additives for moisture retention
- Reflective aggregates for increased albedo
- Integration of phase-change materials (PCMs)
These
technologies aim to simultaneously enhance permeability, structural durability,
and thermal mitigation performance under increasingly severe urban climate
conditions [26], [27].
3.2 Pervious
Concrete
Another
widely adopted permeable pavement technology is pervious concrete, also known
as porous concrete or no-fines concrete, which is increasingly being adopted in
sustainable urban drainage systems and climate-resilient infrastructure
projects. Pervious concrete differs from conventional dense concrete in that it
contains little or no fine aggregates, and thus has a highly interconnected
pore structure, which allows for rapid water infiltration [43]. Usually the
material is:
•
Grains are
large clusters
•
Cementitious
materials
•
Water
•
Admixtures
and stablizers
The
open-pore structure of pervious concrete allows infiltration rates of more than
500 L/min/m2 , which makes it very effective in stormwater management
applications [43]. Rainwater percolates directly into the underlying aggregate
reservoirs through the pavement surface, leading to a significant reduction in
runoff generation and peak discharge rates. Pervious concrete is normally used
for:
Parking
facilities
•
Sidewalks
and pedestrian pathways
•
Residential
area streets
•
Bike paths
•
Recreation
facilities
•
Transport
corridors of low volume
The thermal
behavior of pervious concrete is significantly different than conventional
dense concrete pavements because of its high porosity and moisture retention
properties. The increase in pore volume reduces the density of the material and
the thermal conductivity is reduced which limits the conductive heat transfer
and reduces the heat storage in the pavement structure [19].
The primary
thermal benefits of pervious concrete include:
•
Lower
thermal conductivity
•
Low
volumetric heat capacity
•
Higher
moisture retention
•
More
opportunities for evaporative cooling
•
Reduces
surface heat build-up
•
Better
thermal regulation after rainfall
Pervious
concrete usually has lower surface temperatures during the daytime compared to
dense concrete pavements, because the material stores less heat energy [44].
Moreover, residual moisture in pore networks prolongs the evaporative cooling
after rainstorm events, especially in areas with moderate humidity and intense
sun radiation.
The
hydrothermal properties of pervious concrete are significantly influenced by
factors including pore size distribution, aggregate gradation, saturation
conditions and environmental exposure. The increase in porosity improves the
infiltration and evaporation but may decrease the compressive strength and the
structural stability. Therefore, the pavement design has to consider the
hydraulic performance as well as the mechanical stability. Environmental
Advantages of Pervious Concrete
•
Reduction of
urban flood risks
•
Improvement
of groundwater recharge
•
Improvement
of stormwater quality by filtering pollutants
•
Lowering
pavement surface temperatures
•
Reducing the
urban heat island intensity
Despite
these advantages however, there are many engineering challenges. Higher
porosity levels generally lead to lower mechanical strength of the material
compared to conventional dense concrete pavements, limiting its application for
heavy traffic conditions. Freezing and thawing durability is a concern in
colder climates as the presence of residual moisture can be conducive to
cracking and degradation of the structure [33].
Clogging is
a serious problem; it has long-term implications. Fine sediments transported in
rainwater enter surface pores and are gradually filling them up thus reducing
permeability and hydraulic efficiency. Routine maintenance operations such as
vacuum sweeping, pressure cleaning and sediment extraction are essential to
maintain infiltration and thermal efficie ncy [45]. More recent studies have
looked at new ways of improving the performance of pervious concrete, including
the addition of:
•
Supplementary
cementitious materials
•
Aggregates
from recycling
•
Nanofibers
and Nanomaterials
•
Internal
curing agents
•
Moisture-retaining
agents
•
Reflective
mineral matters
These
advanced mixture designs target enhancing structural strength without
sacrificing permeability and thermal regulation capabilities.
3.3
Permeable Interlocking Concrete Pavements (PICP)
Permeable interlocking concrete pavements (PICP) are made
up of individual concrete paving units with permeable joints in between. The
joints are filled with aggregate materials that allow stormwater to infiltrate
into the storage layers below. PICP structures are modular and flexible, unlike
monolithic pavement systems which facilitate maintenance, replacement and
structural adaptation [46]. A typical PICP structure contains:
- Interlocking concrete paving blocks
- Aggregate-filled permeable joints
- Bedding layer
- Open-graded aggregate base
- Subbase storage reservoir
- Optional underdrain systems
PICP systems
are widely implemented in:
- Urban sidewalks
- Parking facilities
- Public plazas
- Residential developments
- Commercial landscapes
- Low-speed roadways
A very important advantage of PICP systems is the very high infiltration
capacity. Stormwater infiltrates through joint openings, is temporarily stored
in the aggregate base layers, and slowly seeps into the subsoil or is
discharged via controlled drainage [46]. This approach reduces the generation
of urban runoff, and is consistent with sustainable stormwater management
principles.
The heat storage capacity of PICP systems is usually lower
than that of traditional asphalt pavements because concrete paving blocks have
higher albedo and lower absorption of solar heat. The retained moisture in the
joint aggregates helps in evaporative cooling and latent heat exchange [47]. The principal environmental and thermal advantages of
PICP systems encompass:
- High stormwater infiltration efficiency
- Improved urban drainage performance
- Reduced surface heat accumulation
- Enhanced evaporative cooling
- Modular maintenance capability
- Reduced nighttime heat storage
Studies have shown that PICP surfaces have lower evening
temperatures than dense asphalt pavements because of lower thermal mass and
higher heat dissipation [46]. The modular structure decreases continuous
accumulation of heat on the pavement surface and enhances thermal adaptability
under various weather conditions. However, the long-term hydraulic and thermal performance
of the system strongly depends on the quality of maintenance and the condition
of the joints. The long-term effect of sediment accumulation in joints can be a
significant reduction in infiltration capacity. The effects of the traffic
loading on the long-term pavement performance are joint deformation, aggregate
movement and structural stability. Recent developments to PICP systems are:
·
Reflective concrete materials
- Recycled aggregate incorporation
- Water-retentive joint fillers
- Smart sensor integration
- AI-assisted pavement monitoring systems
These
innovations aim to improve multifunctional pavement performance and enhance
urban climate resilience capabilities [37], [48].
3.4
Water-Retentive Pavements
Water-retentive
pavements are an advanced class of climate responsive pavement systems designed
to maximise moisture retention and enhance efficiency of evaporative cooling.
These pavements are made from highly absorptive materials that can store large
amounts of water in the surface and subsurface layers for long periods of time
after rain or artificial irrigation [49]. Water-retentive pavements are
particularly efficient in the hot-arid urban climate with high evaporative
cooling potential and extreme urban heat island intensity. Such systems extend
latent heat exchange processes by retaining moisture during dry periods, and
they have a considerable impact on pavement surface temperatures.
Typical
water-retentive pavement materials include:
- Porous ceramic aggregates
- Absorptive cementitious materials
- Hydrophilic polymers
- Recycled porous materials
- Water-storage layers
- Moisture-retaining fillers
Water-retentive pavements cool primarily through the
continuous evaporation of the moisture they hold. Water evaporates, drawing
latent heat from pavement surfaces and the surrounding air, thus reducing
pavement temperatures and sensible heat fluxes to the urban atmosphere.
Water-retentive pavements can reduce pavement surface temperatures by about
15-20 °C from conventional asphalt pavements, under favourable environmental
conditions [49], [31]. Cooling effects are most pronounced shortly after
rainfall events and during periods of high solar radiation.
The thermal performance of water-retentive pavements
depends on a number of interacting factors, for example:
- Water storage capacity
- Evaporation efficiency
- Climatic conditions
- Solar radiation intensity
- Relative humidity
- Wind velocity
- Surface albedo
- Material porosity and permeability
In addition
to thermal mitigation benefits, water-retentive pavements contribute to:
- Reduction of stormwater runoff
- Improved pedestrian thermal comfort
- Reduced atmospheric heating
- Enhanced urban climate resilience
- Lower building cooling energy demand
Nevertheless there are many problems with these systems. When the stored
moisture is depleted, as during prolonged dry periods, the cooling
effectiveness is sharply reduced. If precipitation is limited, additional water
supply systems might be required to support the long-term thermal benefits
[31]. Structural durability can be affected by repeated wetting and drying,
material degradation and traffic stress. Installation and maintenance costs may
be higher than traditional pavers. Such as the new ways to improve the
performance of water-retentive pavements have been studied recently: Integration of phase-change materials
(PCMs)
- Smart moisture-regulating systems
- Nanomaterial-enhanced absorptive layers
- Hybrid reflective-permeable pavement designs
- AI-assisted moisture management technologies
These
emerging technologies aim to optimize hydrological and thermal regulation
performance while improving long-term pavement durability, sustainability, and
climate adaptability.
4.
Stormwater Infiltration and Temperature Buffering
4.1
Infiltration Mechanisms
Permeable pavements manage stormwater through infiltration,
detention, filtration, and the slow release of runoff. Water fills the pores at
the surface and is held for a short time in the lower layers before it soaks
away into the subsoil or drainage system. The performance of infiltration
depends on:
- Porosity
- Pore connectivity
- Surface permeability
- Clogging condition
- Rainfall intensity
- Base layer design
Enhanced
infiltration increases latent heat exchange and decreases surface temperatures.
4.2
Evaporative Cooling Processes
Evaporation
is a dominant cooling mechanism in permeable pavements. The latent heat of
vaporization extracts energy from pavement surfaces, thereby lowering
temperature. Experimental studies reveal that:
- Wet permeable pavements exhibit significantly
lower midday temperatures.
- Cooling duration depends on retained moisture
volume.
- High humidity reduces evaporation efficiency.
- Wind speed enhances convective evaporation.
Evaporative
cooling is especially important in hot-dry climates.
4.3 Moisture
Retention and Thermal Regulation
Moisture
retention capacity strongly influences thermal buffering performance.
Water-retentive pavements sustain evaporative cooling for longer periods
following rainfall events. Key controlling factors include:
- Void ratio
- Aggregate absorption
- Surface roughness
- Subsurface storage capacity
- Drainage characteristics
Hydrophilic
additives and recycled porous materials have recently been introduced to
improve water retention.
5. Thermal
Conductivity and Heat Transfer in Pavements
Thermal
conductivity and heat transfer mechanisms are key factors determining the
thermal behaviour of pavements and their contribution to the urban heat island
(UHI) intensity. Pavement materials continuously transfer heat to the
atmosphere through radiation, conduction, convection and evaporation. The speed
and direction of heat transfer in pavement systems are important factors in
surface temperature fluctuations, heat retention in the subsurface, heat
dissipation in the night and the overall urban thermal environment.
In typical
metropolitan environment, pavements absorb great amount of solar radiation
during daytime and later release the stored heat energy at night. Delayed heat
release plays a major role in the higher night temperatures typical of urban
heat islands [12]. The thermal behaviour of pavements is mainly determined by
the material properties such as thermal conductivity, thermal diffusivity, heat
capacity, density, porosity, moisture content and surface reflectivity.
The porous nature of permeable pavements leads to different thermal performance
compared to conventional impervious pavements, which will change conductive
heat transfer and moisture-related thermal processes. Permeable systems
generate complex hydrothermal interactions through infiltration and evaporation
of water that can significantly change the dynamics of pavement temperature and
urban energy balances [28].
5.1 Heat
Conduction Mechanisms
Heat
conduction in pavement systems is mainly characterized by Fourier’s law of heat
transfer where the thermal energy flows from the areas of higher temperature to
the areas of lower temperature in accordance with the thermal conductivity of
the material and the temperature gradient. In pavement structures, the solar
radiation absorbed at the surface propagates downwards through the several
layers of the pavement, such as the wearing course, base layer, subbase and the
underlying soil [25]. The general conductive heat transfer relationship can be
expressed as:
The effectiveness of conductive heat transfer is very much
dependent on the composition of the pavement material and its structural
configuration. Dense asphalt pavements are generally characterized by a
relatively high thermal storage capacity and a moderate thermal conductivity,
which makes it possible to absorb and store a large amount of solar energy
during the daytime periods [18]. That stored heat is released at night, helping
to increase urban temperatures after the sun sets.
Heat conduction in pavement systems is realized by three
major pathways:
- Surface-to-subsurface conductive transfer
- Inter-particle heat transfer between aggregates
and binders
- Heat diffusion through pore spaces and
moisture-filled voids
In traditional thick pavements, heat transport is mainly
through the contact networks of solid aggregates and matrices of asphalt
binder. The mechanisms of thermal transfer are significantly influenced by the
air voids of permeable pavements. Air-filled pores are thermal insulators,
since air has relatively low heat conductivity compared to mineral aggregates
and water. Hence, for dry conditions, the total conductivity of porous
pavements is normally less than that of dense pavements [20]. Thermal reactivity
of pavements is very different during day and night. The high solar radiation
intensities during daytime periods cause large surface heating and strong
temperature gradients between surface and subsurface layers. Heat is conducted
downward into deeper pavement layers where heat energy is stored temporarily.
At night, the direction of heat transfer is reversed, as the subterranean heat
which has been stored up escapes and is dissipated into the atmosphere [23]. The extent of heat conduction is affected by various
material and environmental conditions, including:
- Aggregate mineral composition
- Asphalt binder characteristics
- Pavement density and compaction
- Surface albedo
- Air void distribution
- Layer thickness
- Solar radiation intensity
- Ambient air temperature
- Wind speed and humidity
Pavements with low thermal conductivity reduce heat flow
downwards and may limit deep thermal storage. However, lower conductivity can
lead to more intense buildup of heat at the surface during the middle of the
day as the energy absorbed is retained on the surface of the pavement. In case
of high solar radiation, surface temperatures could significantly increase
[24]. High thermal conductivity pavements transfer heat more efficiently to the
underlying layers and therefore reduce the peak surface temperatures during
daytime. However, these pavements have the potential to release higher amounts
of accumulated heat during overnight, thus prolonging the effects of urban
warming [25]. Thus, the thermal conductivity of pavement should be optimised
between cooling benefits during the day and heat dissipation properties at
night. Recent studies have emphasised the significance of multifunctional
pavement materials that can control the transmission of conductive heat while
improving the stormwater infiltration and evaporative cooling. Innovative
pavement systems like reflective aggregates, recycled materials, phase-change
materials (PCMs) and nanomaterial-enhanced binders have shown considerable
promise for thermal regulation [26], [27].
5.2
Influence of Moisture Content
Moisture
content is an important factor influencing thermal conductivity and heat
transfer dynamics in permeable pavement systems. Water in pores of pavement
changes the thermal transport mechanisms significantly because water has much
higher thermal conductivity than air. Thus, the effective heat conductivity of
saturated or partially saturated pavements is usually higher than that of dry
pavements [21]. The heat transmission is reduced by the interconnected air
spaces in dry permeable pavements, as air is a thermal insulator. When the rain
enters the pores of the pavement, the air voids will be partly or filled with
water, hence facilitating the conductive heat transfer of the pavement
structure. The thermal performance of pavements is significantly influenced by
moisture saturation, and the heat conductivity of water is 20-25 times higher
than that of air [22]. Moisture affects heat transfer in pavement via multiple
interrelated mechanisms:
- Increased conductive heat transfer
- Enhanced thermal storage capacity
- Promotion of evaporative cooling
- Modification of surface energy balance
- Regulation of temperature fluctuations
- Delayed heat release cycles
While moisture increases the conductivity of the pavement,
the increase in evaporative cooling usually dominates over the warming effects
associated with increased heat conduction. The evaporation takes up large
amounts of latent heat from the surface of the pavement and the surrounding
air, thus reducing the surface temperature and limiting the sensible heat
fluxes to the urban atmosphere [29]. The exchange of latent heat can be
conceptually described as:
Q=mLv
where:
- (Q) represents latent heat energy,
- (m) is the mass of evaporated water,
- (Lv) is the latent heat of vaporization.
The evaporation process is important for permeable and
water-retentive pavements because these systems can store large amounts of
stormwater in their porous matrices. Stored moisture evaporates slowly under
sun radiation after precipitation events, continuously removing heat from the
pavement surface [32]. Field experiments have shown that wet permeable
pavements often retain a much lower temperature than dry impermeable pavements
in hot weather conditions. It has been reported that the surface temperature can
be reduced by about 10-15 °C by the prolonged evaporative cooling after the
rainstorm events [30]. Some advanced water-retentive pavement technologies have
been proven to have cooling effects of more than 20 °C under high sun radiation
conditions [31]. The effect of moisture content on thermal behaviour of
pavement is due to a number of interrelated factors including:
- Degree of saturation
- Pore size distribution
- Water retention capacity
- Climatic conditions
- Relative humidity
- Solar radiation intensity
- Wind speed
- Drainage characteristics
Permeable pavements are an important source of thermal
mitigation in dry warm climates due to high evaporation rates. Moisture
depletion may occur rapidly under long dry periods, thereby reducing cooling
efficiency and increasing pavement temperatures [50]. In humid areas, the
smaller vapour pressure gradients may reduce the efficiency of evaporation and
thus limit the cooling potential of the retained moisture. Wet-dry cycles also
affect the durability of pavement and material properties. Changes in moisture
content may affect thermal expansion properties and bonding properties between
aggregate and structural stiffness. Long-term hydrothermal cycling may affect
the performance and durability of the pavement [33]. Recently, new approaches
have been investigated to enhance the moisture-related thermal regulation of
pavements. These are:
- Hydrophilic additives for enhanced water
retention
- Porous ceramic materials
- Moisture-regulating polymers
- Phase-change materials
- Smart irrigation-assisted pavement systems
- AI-based moisture management technologies
Such
technologies aim to maintain optimal moisture levels within pavement structures
and maximize long-term evaporative cooling performance under changing climatic
conditions [37].
5.3 Thermal
Diffusivity and Heat Capacity
The
important thermal properties that have significant influence on the behavior of
temperature response of pavements and heat storage dynamics are thermal
diffusivity and volumetric heat capacity. Thermal conductivity governs the rate
of heat transfer through a material while thermal diffusivity governs the rate
of temperature change through the pavement structure.
Thermal
diffusivity (
Essentially, thermal diffusivity is the combination of the material's
ability to conduct (k) and its ability to store heat (ρc-p). Materials with
high and low ρc-pwill have high thermal diffusivity and respond fast to
temperature changes. Materials with high cp and moderate k, such as water, have
low thermal diffusivity and heat up slowly. This relation finds wide
application in heat transfer analysis. Especially in transient conduction
problems such as the one-dimensional heat equation:
High thermal diffusivity materials respond quickly to
temperature change as heat is quickly transmitted through the material.
Alternatively, materials with low diffusivity show lower thermal reaction rates
and lower temperature variability [51]. Volumetric heat capacity is a measure
of thermal energy that a material can store per unit volume and is dependent on
the density and composition of the pavement. Dense asphalt pavements usually
have a high volumetric heat capacity, which can absorb and store a large amount
of solar energy during daytime [12].
The accumulated heat is slowly released during the night,
worsening the urban warming and sustaining the effects of the urban heat island
(UHI) phenomenon. Permeable pavements generally have:
- Lower thermal diffusivity
- Reduced volumetric heat capacity
- Lower material density
- Greater porosity
- Enhanced evaporative cooling potential
- Reduced long-term heat storage
Because
permeable pavements contain substantial interconnected pore spaces, their
effective density and heat storage capacity are reduced compared with dense
pavements [24]. Consequently, these systems often experience more rapid cooling
after sunset and lower nighttime heat release. Reduced thermal inertia also
enables permeable pavements to respond more dynamically to changing
environmental conditions.
The thermal
diffusivity of pavement materials is strongly influenced by:
- Air void content
- Moisture saturation
- Aggregate mineralogy
- Pavement thickness
- Surface aging
- Material Composition
Generally, thermal diffusivity increases with moisture
saturation due to the enhancement of heat transfer efficiency by water in the
pore network. Simultaneous actions of evaporative cooling can decrease
temperature rises and minimise overall heat accumulation at surface [59]. In
the context of urban climate mitigation, low thermal diffusivity is commonly
desirable, as it decreases the depth of heat penetration, and thus reduces the
night-time release of heat. However, too low diffusivity can lead to increased
surface temperatures during the day as solar energy absorbed cannot be
sufficiently transported to the layers below [25]. Therefore, the enhancement
of thermal behaviour of pavement requires balance among the conductivity,
diffusivity, heat capacity and moisture dynamics. Advanced pavement materials
are being increasingly engineered to more efficiently enhance thermal
diffusivity and heat storage properties. Phase change materials (PCMs) are
especially beneficial since they can store and release latent heat during
melting and solidification processes, thus, they can help to stabilise the
temperature of pavement in extreme climatic conditions [26]. Thermal
performance can also be enhanced by nanomaterials-enriched binders and
reflecting mineral additives, while maintaining structural integrity [27].
Recent urban climate studies have highlighted the need to incorporate thermal
diffusivity factors into holistic urban planning and pavement design. Thermal
storage reduction and enhanced evaporative cooling pavement systems can
significantly mitigate urban heat island effects, improve pedestrian thermal
comfort, reduce energy demand of the building stock, and improve climate
resilience in fast-growing urban areas [52], [53].
6.
Comparative Performance of Pavement Technologies
There is a
large variation in the hydrological, thermal, structural and environmental
performance attributes of the urban pavement technologies. Traditional
impermeable pavements have been mostly designed to sustain traffic loads and
improve longevity, whereas less concern has been given to their thermal effects
on metropolitan climates. However, the growing concerns about urban heat
islands (UHI), stormwater flooding and climate resilience have accelerated the
development of advanced pavement technologies that can control heat transfer
and hydrologic processes simultaneously. Current pavement systems are generally
classified into conventional asphalt pavements, cool pavements, permeable cool
pavements and green pavement systems. Each group has unique thermal
characteristics, penetrative efficiencies, evaporative cooling potentials, and
ecological consequences. Another factor affecting their relative performance is
a number of parameters such as weather, material, moisture, traffic stress,
urban layout, maintenance procedures and surface ageing [54], [52].
Understanding the relative pros and cons of different pavement systems is
important for selecting climate-adaptive urban infrastructure options that can
mitigate UHI intensity while maintaining structural integrity and long-term
viability.
6.1
Conventional Asphalt Pavements
Traditional asphalt pavements are the most commonly used
road surfacing material in urban transportation infrastructure throughout the
world, due to the relatively low construction cost, high structural
flexibility, ease of maintenance and ability to withstand traffic loads. Such
pavements are generally constructed with dense-graded asphalt mixtures
consisting of mineral aggregates bonded together with asphalt cement. Despite
their engineering benefits, conventional asphalt pavements are the primary source
of heat accumulation in the urban environment due to their adverse thermal and
hydrological properties. The very low albedo values indicated by their dark
surface colouration, usually between 0.05 and 0.20, result in the absorption,
rather than the reflection, of a large fraction of incoming solar radiation
[12]. As a result, asphalt pavements can store a substantial amount of thermal
energy during the day and release the stored heat at night, which may aggravate
urban heat island effects. The conventional asphalt pavements are mainly
characterised by:
- Low solar reflectivity (low albedo)
- High thermal storage capacity
- Limited moisture retention
- Low permeability
- Rapid stormwater runoff generation
- High sensible heat flux to the atmosphere
Solid asphalt pavements can reach surface temperatures
higher than 60-65 °C under intense solar radiation for hot summer afternoons
[12]. In some arid and semi-arid locations, pavement temperatures can be even
higher, greatly increasing pedestrian discomfort, air temperature and the need
for cooling energy in nearby buildings. Hydrologically speaking, asphalt
surfaces are virtually impermeable. Rainwater is blocked from entering the
pavement structure and is rapidly diverted into drainage systems, which increases
peak runoff rates and urban flooding hazards [39]. In the absence of moisture
penetration, latent heat exchange and evaporative cooling are limited, and a
larger proportion of the absorbed solar energy is transformed into sensible
heat. Asphalt pavements also have high volumetric heat capacity and high
thermal inertia.
6.2 Cool
Pavements
Cool pavements are intended to reduce the urban heat island
effect and surface heat storage by increasing solar reflectance and thermal
emittance. Usually, these pavements incorporate the application of reflective
aggregates, light-coloured surface coatings, modified asphalt binders or
certain cementitious materials for reducing solar heat absorption and lowering
the roadway surface temperature [5]. The primary thermal process in cool
pavements is the increase in surface albedo, which reflects a greater portion
of incident solar radiation to the atmosphere, rather than absorption into
pavement layers. Various advanced pavement materials also improved infrared
emissivity, supporting better longwave radiation emission and cooling at night.
The main benefits of cool pavements are:
• More solar
reflectivity
• Reduced daytime
surface temperatures
• Less heating of
the urban atmosphere
• Lower cooling
energy use
• Improved
thermal comfort in the surrounding areas
• Reduction of
greenhouse gas emissions potential.
Field studies
have shown that cool pavements can decrease peak surface temperatures by 10-20
°C compared to conventional asphalt pavements under comparable environmental
conditions. Lower surface temperatures reduce the sensible heat transfer to the
air surrounding the surface and can help to reduce local ambient air
temperatures, especially when deployed at large urban scales [16]. There are
different types of cool pavements:
1. Reflective
asphalt roads.
2. Reflective
concrete pavements.
3. Reflective
surfaces that are coated or painted.
4.
Composite
pavements (modified).
5.
Hybrid systems
with thermal optimization
Due to their
lighter colour and mineral composition, reflective concrete pavements tend to
have higher albedo values than asphalt pavements. Modified asphalt pavements
can incorporate reflective aggregates, polymer additives or specialised
coatings to improve thermal performance, while maintaining mechanical
durability. Cool pavements do offer thermal benefits but there are also several
limitations and potential trade-offs. The main issue is the increase in
reflection of shortwave radiation back to pedestrians and adjacent buildings.
Other challenges with cool pavements include:
·
Surface
aging and albedo degradation over time
·
Increased
glare under intense sunlight
·
Higher
installation and maintenance costs
·
Potential
winter icing concerns in cold climates
·
Variable
performance depending on urban geometry and shading conditions
The
effectiveness of cool pavements additionally depends on climatic and
environmental conditions. In hot-arid regions, enhanced reflectivity generally
provides substantial cooling benefits. However, in humid climates where latent
heat exchange plays a larger role in thermal regulation, reflective cooling
alone may provide more limited benefits [17].
Recent
advancements in cool pavement technologies have focused on integrating
multifunctional properties such as permeability, moisture retention, and
phase-change materials to improve overall climate adaptation performance [26].
6.3
Permeable Cool Pavements
Permeable cool pavements are an advanced form of hybrid
pavement technology that combines the hydrological benefits of permeable
pavements with the thermal benefits of cool reflective surfaces. The systems
simultaneously aim to reduce storm water runoff, increase evaporative cooling
and reduce solar heat absorption. Conventional cool pavements rely mainly on
reflection-based cooling. Permeable cool pavements utilise both reflective and
evaporative cooling mechanisms. Rainwater infiltrates into porous networks and
is temporarily stored in the underlying layers, where latent heat is extracted
from the pavement system and the ambient atmosphere through gradual evaporation
[14]. The mechanisms of thermal regulation are combined as follows:
- Increased solar reflectivity
- Enhanced moisture evaporation
- Reduced sensible heat flux
- Lower thermal storage capacity
- Improved heat dissipation
- Reduced stormwater runoff generation
Recent
studies have demonstrated that permeable cool pavements often achieve superior
thermal performance compared with single-function pavement technologies because
they simultaneously address both radiative and hydrological heat transfer
processes [54]. Permeable cool pavement systems may incorporate:
- Reflective porous asphalt mixtures
- Light-colored permeable concrete
- Water-retentive reflective aggregates
- High-albedo permeable coatings
- Hybrid porous ceramic materials
One of the
major advantages of these systems is their ability to maintain lower
temperatures during both daytime and nighttime periods. Reflective properties
reduce daytime solar heat absorption, while moisture retention and evaporation
enhance cooling following rainfall events.
Hydrologically,
permeable cool pavements provide significant stormwater management benefits
through:
- Rapid infiltration capacity
- Peak runoff reduction
- Improved groundwater recharge
- Pollutant filtration
- Delayed runoff discharge
These systems are increasingly being recognized as
promising components of climate resilient urban infrastructure and sponge city
initiatives. However, permeable cool pavements have many engineering and
operational limitations. Materials of high reflectivity and permeability may be
difficult to realize, since the increase of the porosity may damage the
mechanical strength and durability. Also, surface obstruction may decrease the
infiltration efficiency and evaporative cooling efficiency with time [45]. The
maintenance requirements of the permeable cool pavements are generally higher
than traditional pavements as regular cleanings are needed to maintain
permeability and reflective performance. Long term durability under heavy
traffic load is still a major research topic. However, hybrid permeable cool
pavements are increasingly recognized as one of the most efficient
multifunctional measures to simultaneously mitigate urban flooding and urban
heat island effects in future climatic scenarios despite these limitations.
6.4 Green
Pavement Systems
Green pavement systems represent a novel category of
environmentally integrated pavement technologies that integrate permeable
pavement structures with vegetation, ecological components or biologically
active materials. These systems also seek to enhance urban thermal regulation,
stormwater management and environmental sustainability, and maximize ecosystem
services. Green pavements can consist of:
- Vegetated permeable pavements
- Grass pavers
- Bio-integrated pavement systems
- Green parking surfaces
- Ecological infiltration pavements
- Vegetation-supported permeable concrete systems
The primary
thermal advantage of green pavement systems arises from evapotranspiration,
whereby vegetation absorbs water from the soil and releases moisture vapor into
the atmosphere. This process consumes latent heat and substantially cools
surrounding surfaces and air temperatures [3]. In addition, vegetation provides
shading that reduces direct solar radiation exposure on pavement surfaces. Major
environmental and thermal benefits of green pavement systems include:
- Enhanced evapotranspiration cooling
- Carbon sequestration potential
- Reduction of urban heat island intensity
- Improved stormwater infiltration and retention
- Enhanced biodiversity and habitat creation
- Improved air quality
- Reduced surface runoff generation
- Increased urban aesthetic value
Vegetated systems can have considerably lower surface
temperatures in the summer compared to conventional pavements, due to shading
and evaporative cooling effects. In arid urban areas vegetation improves
micro-climatic comfort by reducing ambient air temperatures and increasing
atmospheric humidity. Green pavement systems improve ecological sustainability
via urban biodiversity and soil-water interactions. Vegetated pavements are
being incorporated in multifunctional green infrastructure networks to improve
urban resilience to climate change in sponge city and low-impact development
(LID) contexts. However, there are some technological and operational barriers
that prevent the widespread implementation of green pavement systems. In hot
and dry areas, vegetation requires frequent watering, upkeep, and fertiliser
management. Structural strength limitations may further limit suitability for
heavy traffic applications. Additional
challenges include:
- Higher maintenance complexity
- Vegetation degradation during drought conditions
- Root intrusion and structural instability risks
- Seasonal performance variability
- Increased installation costs
- Limited applicability under high traffic loads
The hydrological performance may be reduced if the
vegetation becomes compacted or if the permeability of the soil is reduced over
time. As a result, long-term management strategies are critical to maintaining
ecological and engineering effectiveness.
Recent studies have investigated novel concepts of green pavements, such
as smart irrigation systems, drought tolerant plant species, recycled organic
matter and environmental monitoring technologies based on artificial
intelligence [37]. The developments seek to improve the balance between
ecological functionality, thermal regulation and structural performance. Green
pavement systems are a very sustainable response to urban climate change,
offering advantages in terms of heat mitigation, stormwater management,
ecological restoration and carbon reduction. It is expected that the
integration of these in future urban development approaches will considerably
boost urban resilience to increasing climatic stressors.
7. Numerical
Modelling and Simulation Approaches
Numerical
modelling and simulation methods are important tools for understanding the
complex thermal and hydrological behaviour of pavement systems under various
environmental conditions. Pavement performance is influenced by a series of
interdependent processes including the absorption of solar radiation, heat
conduction, infiltration of moisture, evaporation, runoff production and
exchange with the atmosphere. Therefore, experimental studies are often not
enough to fully characterize the behaviour of pavement in space and time. The
use of sophisticated computational models enables researchers to simulate
pavement temperature dynamics, stormwater transport, moisture retention and
urban climate interaction under different climatic conditions and material combinations.
Such models are useful for optimisation of pavement design, predicting
long-term hydrothermal activity, and evaluation of urban heat island mitigation
methods [34], [35]. Increasingly, modern pavement models are adopting
multi-disciplinary approaches such as thermodynamics, hydrodynamics,
atmospheric physics, computational fluid dynamics (CFD), and artificial
intelligence.
7.1 Heat
Transfer Models
Numerical
heat transfer models have been extensively used to predict pavement temperature
distributions, heat fluxes, thermal storage properties and urban surface energy
balances. These models simulate the transport of thermal energy in pavement
layers and between pavements and atmosphere under different meteorological
conditions.
The
governing thermal processes in pavement systems include:
- Solar radiation absorption
- Longwave radiation emission
- Conductive heat transfer
- Convective heat exchange
- Latent heat transfer through evaporation
- Subsurface heat storage and release
Most
pavement heat transfer simulations are based on transient heat conduction
equations derived from Fourier’s law. The general heat diffusion equation used
in pavement thermal analysis can be expressed as:
Several
numerical techniques are commonly applied in pavement thermal simulations,
including:
- Finite element modeling (FEM)
- Finite difference methods (FDM)
- Finite volume methods (FVM)
- Computational fluid dynamics (CFD)
- Energy balance models
- Coupled hydrothermal simulations
Finite difference models are computationally efficient and
widely used for analysis of transient heat conduction in pavement systems.
These models discretize the pavement layers at grid points and compute the time
variation of temperature. Finite difference methods, easier than FEM, are still
popular for one-dimensional thermal simulation and long-term prediction of
pavement temperature [51]. The energy balance models are widely used for urban
climate research as they take into account radiative, conductive, convective
and evaporative heat fluxes on pavement surfaces. They evaluate the impact of
pavement on the intensity of urban heat islands and quantify the cooling effect
of permeable or reflective pavement solutions. Coupled hydrothermal simulations
represent a significant improvement in pavement modeling. These models
reproduce the moisture transport and heat transfer processes simultaneously,
allowing an in-depth investigation of the evaporation, infiltration, saturation
dynamics and thermal regulation [28]. Integrated techniques are very important
for permeable pavements as moisture plays an important role on the thermal
conductivity and latent heat exchange.
Recently, thermal simulations of pavement have been increasingly taking into
account interactions of atmospheric boundary layer and urban canyon effects.
These models take into account shading, radiative trapping, architectural
geometry and urban airflow circulation [11]. Models at the scale of cities can
tell us important things about how large amounts of pavement might change the
local microclimate and the temperature of the city. Furthermore, advanced thermal modeling approaches now
include innovative materials such as:
- Phase-change materials (PCMs)
- Reflective aggregates
- Nanomaterial-enhanced binders
- Moisture-retentive materials
- Recycled porous materials
These
simulations help optimize material composition and pavement structure for
climate adaptation applications [26], [27].
7.2
Hydrological Models
Hydrological
models are vital tools for evaluating the performance of permeable pavement
systems in stormwater management. These models reproduce infiltration
mechanisms, runoff generation, water storage behaviour, drainage dynamics and
groundwater recharge under different rainfall situations. Permeable pavements
are designed to accommodate urban hydrology by infiltration and detention.
Hydrological simulations are necessary to assess the effectiveness of permeable
pavements in mitigating urban flooding and improving stormwater management
[39].
Hydrological
pavement models typically examine:
- Rainfall-runoff relationships
- Infiltration capacity
- Surface ponding
- Water retention behavior
- Drainage efficiency
- Subsurface flow dynamics
- Pollutant transport and filtration
Several
widely used hydrological modeling tools include:
- Storm Water Management Model (SWMM)
- HYDRUS
- MIKE URBAN
- COMSOL Multiphysics
- MODFLOW
- EPA urban drainage models
SWMM is a widely used urban hydrological simulation program
for the analysis of stormwater runoff and drainage systems. It can simulate
permeable pavements as low-impact development (LID) measures and evaluate
infiltration, runoff mitigation and detention effectiveness under different
precipitation scenarios. HYDRUS is a general software package for modelling
water, heat, and solute movement in variably saturated porous media. The model
is especially suited to study moisture migration in permeable pavement layers
and coupled hydrothermal behaviour [50].
MIKE URBAN has full urban drainage modelling capabilities and is widely applied
in mega urban stormwater management studies. COMSOL Multiphysics allows
detailed coupled simulations of heat transfer, fluid dynamics and moisture
transport in pavement systems. Despite great advances in the modelling of
pavement hydrology, a complete hydrothermal simulation is still a relative
rarity. Much of the current research still treats thermal and hydrological
processes separately, even though these processes are strongly coupled in
permeable pavements [35]. One of the big problems in pavement hydrology
modelling is the representation of clogging behaviour and the long-term
permeability degradation. Deposition of sediment in pore structure leads to
gradual reduction of infiltration efficiency and change of water storage
behaviour [45]. There is still a lot of research to do on long-term field
calibration and maintenance modelling. There
is increasing coupling of hydrological simulations with climate change
projections to assess the resilience of pavements to future extreme weather
events. These models are used to evaluate pavement performance under:
- Intense rainfall events
- Prolonged drought periods
- Heat waves
- Flood scenarios
- Seasonal climatic variability
These
simulations support the development of climate-resilient urban drainage
infrastructure and sponge city planning strategies.
7.3 Machine
Learning Applications
Machine
learning (ML) and artificial intelligence (AI) [55], technologies are emerging
as transformative tools in the field of pavement engineering and urban climate
research. In many cases conventional numerical models need detailed material
characterisation and complex calibration for the boundary conditions, which is
computationally expensive. Machine learning approaches offer alternative
data-driven approaches that could identify nonlinear relationships between
environmental variables, pavement properties and thermal-hydrological
performance. Field monitoring systems, remote sensing platforms, infrared
thermography, and embedded pavement sensors are generating large datasets,
which are increasingly being analysed using machine learning algorithms. Current
machine learning applications in pavement systems include:
- Pavement temperature forecasting
- Permeability degradation prediction
- Clogging detection and assessment
- Structural health monitoring
- Moisture retention prediction
- Traffic-induced thermal stress analysis
- Maintenance optimization
- Urban heat island forecasting
- Climate-responsive pavement design optimization
Commonly
used machine learning techniques include:
- Artificial neural networks (ANNs)
- Support vector machines (SVMs)
- Random forest algorithms
- Deep learning models
- Convolutional neural networks (CNNs)
- Long short-term memory (LSTM) networks
Even with their advantages, the machine learning methods
have some limitations like:
- Dependence on large high-quality datasets
- Limited interpretability of complex models
- Difficulty generalizing across climatic regions
- Sensitivity to data uncertainty
- Challenges in long-term prediction reliability
Future
research is expected to increasingly integrate physics-based simulations with
AI-driven predictive models, creating hybrid modeling frameworks capable of
improving both computational efficiency and predictive accuracy.
8.
Sustainability and Environmental Implications
Besides
temperature regulation in urban regions, the advantages of permeable as well as
thermally optimized pavement systems are also notable from environmental and
sustainability perspectives. These pavements also contribute to the creation of
more resilient and sustainable urban environments, while enhancing stormwater
management, reducing energy use, improving air quality and enhancing climate
adaptation strategies. Climate change is leading to more frequent extreme
rainfall events and urban flooding, and sustainable pavement technologies are
increasingly being recognized as important components of green infrastructure
and low-impact development (LID) frameworks [6].
8.1 Urban
Flood Mitigation
One of the
main environmental benefits of permeable pavements is the reduction of urban
flooding through the mechanisms of infiltration and storm water detention.
Conventional impervious pavements quickly transport rainfall to drainage
systems, resulting in increased peak runoff rates that can overwhelm urban
stormwater infrastructure during intense precipitation events.
Permeable
pavements mitigate these problems by:
- Enhancing rainfall infiltration
- Reducing peak runoff discharge
- Increasing groundwater recharge
- Delaying runoff timing
- Improving urban drainage efficiency
Many
permeable pavement systems can reduce runoff volumes by approximately 50-95%
depending on rainfall intensity, pavement structure, and maintenance conditions
[14]. These capabilities are particularly valuable in highly urbanized regions
where conventional drainage systems are vulnerable to overload during extreme
weather events.
Sponge city
initiatives in China and similar climate-resilient urban programs worldwide
increasingly incorporate permeable pavements into integrated stormwater
management strategies.
8.2 Energy
Consumption Reduction
Urban pavement temperatures exert a strong influence on
ambient air temperatures and building cooling energy demand. Conventional
pavements absorb a lot of solar heat and contribute to warming the atmosphere,
thus increasing the need for air conditioning in nearby buildings. Cool and
permeable pavements can decrease the surface temperature and sensible heat
fluxes and thus reduce the urban ambient temperatures, and the building cooling
loads [16]. Thus, large scale implementation of thermally optimized pavements
may contribute to:
- Reduced electricity consumption
- Lower peak energy demand
- Improved urban energy efficiency
- Reduced greenhouse gas emissions
Studies have
demonstrated that reductions in pavement surface temperatures can indirectly
improve urban thermal comfort and decrease heat-related energy consumption
during summer periods [52].
8.3 Air
Quality Improvement
Urban heat islands also degrade air quality by speeding up
the photochemical reactions that form ground-level ozone. Higher temperatures
of pavement and air can increase the formation of secondary air pollutants and
worsen urban smog conditions. Permeable and cool pavements can help to mitigate
urban heating by:
- Reduced ozone formation rates
- Lower atmospheric pollutant concentrations
- Improved pedestrian comfort
- Reduced heat-related health risks
Vegetated
and green pavement systems can additionally improve air quality through
particulate matter capture, carbon sequestration, and increased oxygen
production [16].
8.4
Life-Cycle Assessment
Life-cycle
assessment (LCA) has become an important tool for evaluating the overall
environmental impacts of pavement technologies throughout their service life.
LCA studies consider:
- Raw material extraction
- Manufacturing energy consumption
- Construction impacts
- Operational performance
- Maintenance requirements
- End-of-life disposal or recycling
Many LCA
investigations indicate that permeable pavements may reduce long-term
environmental impacts when stormwater management benefits and urban climate
mitigation effects are considered.
Potential
sustainability advantages include:
- Reduced flood damage costs
- Lower energy consumption
- Improved groundwater recharge
- Enhanced urban climate resilience
- Reduced Environmental Pollution
9.
Challenges and Limitations
Permeable and thermally optimized pavement systems offer
significant environmental and thermal benefits, but they also face a number of
technical, operational and economic barriers to broad implementation. One of
the more critical limitations is clogging caused by the accumulation of fine
sediments, organic debris and vehicular pollutants in the pores of the
pavement. Over time, clogging decreases permeability and therefore decreases
infiltration and evaporative cooling efficiency [41]. Other major challenges
are:
- High maintenance requirements
- Seasonal variability in thermal performance
- Structural strength limitations under heavy
traffic
- Freeze-thaw durability concerns
- Limited long-term field performance data
- High installation costs in some applications
- Lack of standardized testing methods
- Uncertainty in climate-specific performance
The thermal performance is very sensitive to environment
conditions and water availability. In dry areas, moisture loss can be rapid and
thus reduce the benefits of evaporative cooling. In wet climates, reduced
evaporation efficiency may limit the ability to mitigate heat for the same
reasons. One of the main issues is the durability of the structures, especially
for porous asphalt and pervious concrete structures under heavy traffic loads.
The increased porosity often decreases the mechanical properties and can result
in an increased material degradation under cyclic loading conditions [33].
Furthermore, many climatic areas, especially tropical, arid and frigid zones,
lack adequate current pavement design recommendations. Current standards are
usually related to hydrological or structural performance, and do not consider
the integrated assessment of combined hydrothermal activity. There are
relatively few monitoring studies over long periods of time, which makes it
difficult to accurately assess the pavement performance over the entire service
life cycle.
10. Future
Research Directions
Future research on permeable and thermally optimized
pavements should focus on the design of integrated multifunctional systems that
address urban floods, thermal mitigation, structural durability, and climate
resilience simultaneously. In the last few years many important research goals
have emerged:
1. Creation of integrated hydrothermal
modeling frameworks that encompass heat transfer, moisture transport, and
interactions with urban climate.
2. Intelligent sensor-driven pavement
monitoring systems for instantaneous evaluation of thermal and hydrological
performance.
3. Incorporation of phase-change materials
(PCMs) for thermal energy storage and temperature regulation.
4. Binders and aggregates enhanced by
nanotechnology for greater thermal regulation and durability.
5. Prolonged field surveillance throughout
various climatic conditions.
6. Design techniques for climate-resilient
pavements tailored to specific regional environmental circumstances.
7. AI-enhanced predictive maintenance and
obstruction management solutions.
8. Multi-objective optimization of thermal,
hydraulic, structural, and economic efficiencies.
9. Enhanced utilization of recyclable
materials and circular economy principles in pavement building.
10. Integration of pavement systems at an urban
scale within smart cities and sponge city frameworks.
Emerging
sponge city initiatives offer significant opportunities for extensive
experimental execution and cohesive urban climate research. Future pavement
technologies are anticipated to be progressively adaptive, multifunctional, and
data-driven by integrating artificial intelligence, remote sensing, smart
materials, and climate-responsive design concepts. The effective advancement of
sustainable pavement systems necessitates interdisciplinary collaboration among
pavement engineers, urban climatologists, hydrologists, environmental
scientists, urban planners, and policymakers.
Conclusion
Permeability
and thermal conductivity are important parameters influencing hydrothermal
performance and environmental effectiveness of urban pavement systems. However,
with the rapid urbanisation and the growing occurrence of extreme heat and
stormwater floods due to climate change, permeable and thermally optimised
pavements have emerged as a feasible multifunctional solution for sustainable
urban infrastructure development. Such pavement solutions can significantly
affect urban microclimates and reduce urban heat island (UHI) intensity by
jointly regulating stormwater infiltration, moisture retention, heat
transmission, and evaporative cooling processes. The findings of this study
suggest that permeable pavements, porous asphalt, pervious concrete, permeable
interlocking concrete pavements (PICP), cool pavements, and water-retentive
systems greatly improve urban hydrological performance and thermal regulation.
The interplay of infiltration processes, moisture retention, latent heat
transfer, evaporation, conductive heat transfer and thermal storage establishes
dynamic hydrothermal buffering mechanisms that can reduce pavement surface
temperatures by around 5-20 °C under optimal climatic and moisture conditions. In
particular, augmented permeability enhances stormwater infiltration and
evaporative cooling efficiency, whereas thermal conductivity governs subsurface
heat dissipation, heat storage capacity and nighttime heat release dynamics.
The thermal and hydrological behaviour of pavement is complex, and it is
greatly affected by several interacting parameters such as pore structure
characteristics, material composition, moisture availability, meteorological
conditions, traffic loading, pavement ageing and maintenance procedures. High
permeability often helps with evaporative cooling and runoff reduction. But
excessive porosity can undermine the structural integrity and long-term
functional performance. The higher thermal conductivity can lead to less
accumulation of surface heat during the day because of increased heat spreading
between pavement layers, but it can also increase the release of heat at night.
Thus, a good pavement design should balance hydraulic efficiency, heat
mitigation capacity and structural integrity and long-term durability. The
assessment highlights important research gaps and technical barriers to the
large-scale adoption and improvement of climate-responsive paving systems. Some
studies have looked at hydrological and thermal performance separately, but
little has been done to study the fully coupled hydrothermal dynamics in real
environmental scenarios. In addition, long-term field monitoring data are
scarce, especially for permeability deterioration, clogging behaviour,
maintenance effects, seasonal heat variations and climate-specific pavement
performance. The lack of uniform testing methods and coherent frameworks for
performance assessment hinders the comparison of different pavement
technologies and geographic areas. It is expected that numerical modelling,
remote sensing, embedded sensor technologies and machine learning approaches
will greatly improve pavement research and infrastructure management
capabilities in the future. Innovative technologies including phase change
materials (PCMs), nanotechnology-enhanced binders, intelligent
moisture-regulating systems, reflecting permeable surfaces and AI-assisted
predictive maintenance models offer significant opportunities for improving the
thermal and hydrological performance of pavements. The use of permeable
pavements in sponge city projects, sustainable urban drainage systems (SUDS),
low impact development (LID) frameworks and climate adaptation methods is
likely to be more important for resilient urban planning in the future. This
move towards multifunctional sustainable pavement systems is an important
component of future climate-resilient urban infrastructure. Permeable and
hydrothermally optimized pavements contribute to stormwater management and
urban heat reduction while fostering environmental goals, such as energy
efficiency, thermal comfort, flood resilience, groundwater recharge and urban
sustainability. The development of pavement technologies that respond to the
increasing environmental and climatic challenges of the rapidly urbanizing
cities around the world requires interdisciplinary research in pavement
engineering, urban climatology, hydrology, materials science and artificial
intelligence.
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