Urban heat islands studied within a pavement LCA framework

Urbanization has caused artificially created surfaces to replace open land and vegetation. This led the once permeable and soft surfaces to absorb and store more heat while retaining very little moisture, thus raising their surface temperatures and causing urban areas to become warmer than the surrounding rural area. This phenomenon has been called the Urban Heat Island (UHI).

According to the United States Environmental Protection Agency (EPA), on a hot summer day, the sun can heat urban surfaces to temperatures 50–90°F hotter than the air, while natural surfaces—often in more rural surroundings—remain closer to the air temperature. At night, these urban surfaces release their stored heat and warm up the air significantly, which has serious implications for human health, comfort, and building energy.

Several factors play a role in the spread of UHI worldwide, including pavements, which can occupy a significant proportion of the urban surface area, sometimes over 50% in major cities. To quantify the effect of pavement infrastructure on UHI, Civil and Environmental Engineering Professor Jeffery

Roesler and Ph.D. student Sushobhan Sen have been researching the “Impact of Pavements on the Urban Heat Island.”

The study aimed to investigate the effects of pavement layers and their thermal properties on surface temperature, heat flux, and the resultant UHI, using a microscale approach, which resolved the temperature of the entire pavement including its sub-surface layers. Furthermore, UHIs were studied under the use phase of a pavement life-cycle assessment to quantify their impact within the overall environmental impact of the pavement.

The researchers thermally modelled pavement layers over their design life and developed new metrics that could capture the microscale impact at various time and spatial scales. A thermal analysis program, called the Illinois Thermal Analysis Program (ILLI-THERM), which uses weather data for modeling and analyzing a pavement, was developed and applied to a variety of pavement sections. The analysis was performed for weather conditions both in Chicago, IL, representing a cooler climate, and Austin, TX, representing a warmer climate. ILLI-THERM considered the average net ground heat flux over the design period to estimate the radiative forcing of the pavement, which is a new microscale approach to the radiative forcing concept commonly used in climatic studies. Furthermore, the researchers also developed a new metric – the annual seasonal day temperature of the pavement – to show the diurnal effects of different pavement materials and layers on the UHI.

In addition, the research team developed one of the first aging models for the albedo of asphalt pavements, and determined its effect on UHI using the ILLI-THERM model. The radiative forcing metric showed that albedo was the most significant property affecting pavement UHI, but its aging behavior can change the magnitude of the effect by about 25%. However, the researchers observed that radiative forcing does not consider diurnal lags in the temperature field and heat flux that have the potential to mitigate UHI during certain hours of the day. The alternative annual seasonal day metric could take this effect into account.

 “The UHI is an extremely important area to be studying because civil engineers design and construct the city infrastructure, which means they can mitigate the UHI problem in cities through proper selection of construction material’s thermal and optical properties, design of the urban form, and landscaping considerations,” Professor Roelser said.  

The scope of the study also included an investigation of the potential role of multi-functional inlays for pavement preservation. These inlays were made of a special, and relatively new, type of white cement containing TiO₂ nanoparticles, which created a reflective surface while also increasing the pavement’s thermal inertia. Therefore, the multi-functional inlays were found to be beneficial in mitigating surface UHI.

The research team also introduced a new method to evaluate the albedo of a concrete sample of any dimension using an albedometer and the concept of radiative view factors. The proposed method combines measurement of the apparent specimen albedo at different heights with view factor analysis of the experimental conditions. The advantage of this method is that it can be used for lab-sized concrete specimens, such as beams and cylinders; further, the method explicitly considers the testing location and thus gives engineers more freedom to choose less-than-ideal sites, enabling agencies to measure albedo right at the project site.

The project’s final report, available on ICT’s Publications Page, details the findings of the research team.

This project was sponsored by the University Transportation Center for Highway Pavement Preservation (CHPP). The mission of CHPP is aimed at providing a new platform for accelerating innovation in highway pavement preservation. More information about the Center can be found here.