Cooler Pavements. Commonplace paved features
of urban areas, which include streets, parking areas, sidewalks, plazas, and playgrounds,
typically cover a significant percentage of a city’s overall land area. For example, a study
of Sacramento, California in 2000 estimated that roughly 39 percent of the city’s urban land
area was covered in pavements when seen from above the vegetative canopy . The thermal
characteristics of these paved surfaces—their albedo, heat capacity, conductivity—and
their interaction with solar radiation are among a number of causal factors that affect
a phenomenon referred to as “urban heat island” (UHI). (Other factors affecting UHI include
the area’s population and population density, its building materials, the spacing and height
of buildings, waste heat generated by vehicles and building equipment, vegetative cover, and
geographic location .)
UHIs can be broken into two distinct types: surface heat islands and atmospheric
heat islands. A surface UHI is defined as the increased temperature of exposed urban surfaces
heated above the ambient air temperature . An atmospheric UHI is the increased air temperature
of an urban area compared to the surrounding rural area . By extending these phenomena,
a third type of UHI called “near-surface heat island,“ could be defined as the increased
air temperature above the ambient air temperature at a greater height above the surface.
Depending on characteristics of an urban area, all three UHI types can have
important consequences for city dwellers. Atmospheric UHIs can potentially cause poor air
quality, increased energy consumption, and have other negative effects. Surface and
near-surface UHIs can potentially affect human thermal comfort, air quality, and
the energy use of nearby vehicles and buildings.
So-called “cool pavements,” which reduce urban heat islands, could potentially
help mitigate heat island effects and reduce their negative impacts, but use of these
alternative pavements does not guarantee benefits. How beneficial they are depends in part
on the size and characteristics of the urban area and climate region, the extent and behavioral
characteristics of air conditioner use, the materials and construction impacts of different
pavement types, and a number of other considerations. Cost must also be considered and compared
with other alternatives for addressing problems associated with UHI. Finding the answers to
questions about when and where, and what kinds of cool pavements can provide how much net
positive impact over the entire lifetime of pavements is an area requiring research.
With funding from the University of California, Davis Sustainable Transportation
Center (through the University Transportation Centers program funded by Caltrans and the US DOT) and
additional support from Caltrans, the UCPRC has undertaken a research investigation that has
two goals: (a) to explore and evaluate the effectiveness and applicability of several cool
pavement technologies and strategies, and (b) to quantify the environmental impacts of these
cool pavements as they might occur in several urban climate regions in California, focusing
on the near-surface heat island environment, which affects humans and vehicles on the pavement
and nearby buildings. These results will also provide input for future investigations of
atmospheric heat island.
In conducting this doctoral research study, a UCPRC graduate student is
completing tasks in five main areas:
- Laboratory testing of thermal properties of pavement materials.
- Field measurement of the thermal performance of different cool pavement technologies and strategies, using both controlled test sections and uncontrolled existing pavements.
- Development, validation, and application of local microclimate models in order to evaluate the lifetime impacts of varying cool pavement types in different climates.
- Assessment of the impacts of cool pavements on (a) human thermal comfort and (b) energy use by air conditioning of vehicles and buildings during peak demand for the selected urban areas in three climate regions.
- Development of a framework for consideration of near-surface heat island in Life Cycle Assessment (LCA) studies to examine the environmental impacts of cool pavements.
Three primary types of pavement are being investigated: concrete, asphalt,
and integrated concrete pavers (concrete blocks). Within each of these three types, traditional
impermeable pavements are being used as controls for comparison with one or more types of
fully permeable pavement, which may provide cooling benefits through evaporation of water
stored in them on hot days.
It is expected that the completed study (projected for completion by early 2013) will
yield insights into whether and how cool pavement technologies and strategies may be deployed
most advantageously. The research program will potentially provide a greater understanding of
these pavements’ thermal behavior and their impact on local microclimates, as well as their
life cycle effects on the broader environment.
Important Note for Caltrans Users: Prior
to scheduling pavement preservation (preventive maintenance or CAPM) or roadway
rehabilitation work on flexible pavement highway sections, the District Materials
Engineer and/or the Project Manager should review
this spreadsheet to
ensure that the proposed project does not include sections active in
the “Quieter Pavement Research” (QPR) testing program. If the proposed
project is within a QPR test section, please
contact Linus Motumah of the
Caltrans Office of Pavement Design before scheduling the work.
To view maps that show where the sections are located, click the following
(or copy-and-paste it into a new browser window for a slightly larger view).
For more information,
contact John Harvey of the UCPRC or