Impact of pavements on the urban heat island

Sen, Sushobhan

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https://hdl.handle.net/2142/88095 Copy

Description

Title

Impact of pavements on the urban heat island

Author(s)

Sen, Sushobhan

Issue Date

2015-07-20

Director of Research (if dissertation) or Advisor (if thesis)

Roesler, Jeffery R.

Department of Study

Civil & Environmental Engineering

Discipline

Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Urban heat island
• pavement life cycle assessment
• albedo
• concrete pavements

Abstract

Globally, the rapid pace of urbanization has led to several environmental challenges since the time of the Industrial Revolution. Recently however, unhindered use and modification of natural materials and ecosystems and the resulting consequences have led to a renewed focus on sustainability. Among the challenges faced, the Urban Heat Island (UHI) is caused by the replacement of natural materials with man-made ones such as concrete and asphalt, among other factors. UHIs can be studied at three elevations – surface, canopy layer and boundary layer – with Canopy Layer UHIs (CLUHIs) being the most significant as most human activity takes place in this layer. CLUHIs are pronounced during nighttime and smaller during daytime. In the study of sustainable pavements, UHIs are studied under the use phase of a pavement Life Cycle Assessment (LCA) to quantify the impact of the pavement on the local heat budget and microclimate. To study UHI within a pavement LCA framework, two challenges were tackled. The first was to develop suitable models and metrics to characterize UHI. A thermal analysis program that uses weather data to model and analyze a pavement including its sub-surface layers, called the Illinois Thermal Analysis Program (ILLI-THERM), was developed and applied to a variety of pavement sections in Chicago, IL and Austin, TX to represent two different climatic conditions. A new method to calculate the Radiative Forcing (RF) and the corresponding Global Warming Potential (GWP) over the analysis period was proposed. The RF of a pavement was found to be most strongly affected by its surface albedo, with only small variations caused by changing other layer properties such as density, thermal conductivity and heat capacity of the pavement layers. While RF is a measure of the overall environmental impact of a pavement, it fails to take account of diurnal and seasonal variations in heat flux due to differences in thermal properties of the pavement structure. These variations are the key to managing UHI at critical times of the day. The Average Seasonal Day metric was developed to obtain a snapshot of these variations over a given season for the analysis period. Two strategies were developed to mitigate UHI in concrete pavements: using a cement-treated base as a thermal storage layer to induce a thermal delay in the pavement and thus mitigate the nighttime CLUHI by passing on a part of the burden to the less-pronounced daytime CLUHI; and using Lightweight Aggregates (LWAs) in the surface course to quickly emit absorbed heat and thus reduce the amount of time during which CLUHI is prominent. Finally, the percentage of effectiveness metric was developed as a quick method to compare different pavement options by considering the relative difference in their hourly surface temperatures over the analysis period. As albedo has the most significant impact on RF of a pavement, the second challenge was in determining the albedo of paving materials and incorporating it into the thermal analysis program, ILLI-THERM. For this study, a relatively new material, cement with titanium dioxide nanoparticles, was selected and a 1 m x 1 m concrete slab cast with it. An albedometer was used to measure the composite albedo of the slab and background surfaces. Geometric analysis to determine the relative contribution of each surface (called the view factor of the surface with respect to the albedometer) together with a least square regression analysis to take care of surfaces that were not explicitly considered. This new method estimated the albedo of the 1 m2 concrete slab to be 0.54 with the background albedo found to be 0.21. After being allowed to weather over one winter cycle, the albedo was found to decrease to 0.50. However, this decrease was determined to be on account of dust and dirt accumulating on the slab and could be restored to its original value by washing thoroughly, as was determined by taking cores and testing them on a UV-Vis-NIR spectrophotometer. The albedo of several existing asphalt and concrete test sections of various ages was also determined using an albedometer. The results from the asphalt sections were used to calibrate an aging albedo model. Most of the aging due to environmental weathering in asphalt was found to take place within the first two years, with very little change thereafter. This model was implemented in a pavement LCA using ILLI-THERM and compared to the existing practice of using a static albedo over the analysis period. Over a five-year analysis period, the static models were found to overestimate the GWP of a hypothetical pavement section in Austin, TX by 25% as compared to a more realistic case with an aging albedo. In addition, the albedo of one of the concrete sections made of Flowable Fibrous Concrete was measured. Fibers that were visible on the surface were covered by the cement paste and not expected to change the overall albedo of the section. However, if they became exposed on account of abrasion, their albedo would have an impact. Therefore, the albedo the fiber was determined using a spectrophotometer as 0.07, which would have an impact on the overall albedo depending on the surface area exposed.

Graduation Semester

2015-8

Type of Resource

text

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Copyright and License Information

Copyright 2015 Sushobhan Sen

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