The 21st century is the century of urbanization. Along with rapid urbanization, the century is observing the biggest increase in the world’s population in human history. As of 2006, the world population had reached 6.5 billion. Urbanization is quickly transitioning communities from native vegetation to an engineered infrastructure. The result is an increased thermal-storage capacity of the urban infrastructure from the use of the materials. This regional impact is known as the urban heat island effect where urban temperatures are elevated in comparison with their adjacent rural surroundings.
Rapid global urbanization and explosive overall population increases are generating high demand for new road networks. Paved surfaces can comprise up to 45% of an urban region fabric in the U.S. and are designed with energy-intensive products composed of either portland cement or petroleum-based asphalt. Both of these products contribute to greenhouse-gas emissions and climate change at both the urban and global scales. There is a need for quantifying the impacts of pavement materials on climate change.
Climate change can be defined as the variation in meteorological patterns that can range from a local to a much larger multinational scale. The Intergovernmental Panel on Climate Change (IPCC) developed the Global Warming Potential (GWP) protocol to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. The GWP of a greenhouse gas is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas. Direct radiative effects occur when the gas itself is a greenhouse gas. The reference gas used is CO2, and therefore GWP-weighted emissions are measured in teragrams of CO2 equivalent (Tg CO2 Eq.).
The total U.S. emissions have risen by 16.3% from 1990 to 2005. In 2005, total U.S. greenhouse-gas emissions were 7,260.4 Tg CO2 Eq. Two of the top three categories for CO2 emissions are related to pavements. This includes asphalt as well as cement manufacturing.
This article is aimed at presenting a methodology for road designers and transportation officials to model the impact of different pavement types on climate change potentials in terms of CO2-equivalent emissions. The process presented employs variables that can be modified by the designer to customize for their specific road configuration and materials type.
What’s the equivalent?
Figure 1 shows some of the major elements needed to model the annual CO2 Eq. per length of roadway section. These specific elements were selected because they provide a quantifiable input to perform an analytical comparison. Other elements, such as rolling resistance, were found to be difficult and challenging to consider in the modeling approach.
Table 1 provides a summary of components used to model estimates of Kg CO2 Eq. produced per kilogram of the two pavement types: portland cement concrete (PCC) and hot-mix asphalt (HMA). The CO2 Eq. data came from high-quality inventory output data of European origin. This data was used because of its availability and considerable details. In addition, it is noted that this assessment is limited to the first life cycle of the pavement. It does not take into consideration the resources for emissions or lifetime created by secondary operations to recycle the respective types of roads.
Figure 2 shows typical proportions of pavement materials and their respective CO2 Eq. values per kg for production (Pn). The transportation (Tp) values for these pavements, including sand and gravel, are kept the same for simplicity. They were calculated based on a 20-ton diesel truck (0.2821 Kg CO2 Eq. / ton-km or 0.0002821 Kg CO2 Eq. / kg – km). Figure 3 shows the mixing values (Mn), which were calculated utilizing CO2 Eq. for the fuels utilized for each of the pavement structures. The CO2 Eq. impacts from mixing the concrete assume a 355-kilowatt diesel truck running 36 minutes/batch, with 0.76 cu m/batch, which is equivalent to 18,420 kg/batch. Based on interviews with regional companies, the processing data for a 2-ton asphalt/hr-capacity system was determined to require 24.6 L of No. 2 fuel oil and 0.269 cu m of natural gas per hour. The electricity process inventory data source is a North American average.
Table 2 summarizes the density and CO2 equivalents for the production, transportation and mixing stages for both pavement types.
Climate change values are modeled with IPCC 2007 CO2 characterization values. Five pavement design scenarios are created as shown in Table 3. Note that these pavement designs are presented for demonstration purposes, and obviously they are the user’s specific input. The thicknesses were selected to represent common designs practiced by transportation agencies. Two designs are designated with moderate traffic volume, whereas the other three were designated as high traffic volume designs. The estimated life for each pavement was selected based on practical pavement performance experience of the authors (again, this is a user input). UTW is an ultrathin whitetopping PCC pavement; whereas TW is a thin whitetopping PCC pavement design.
A functional unit of 1/km-year and a damage score unit of kg CO2 Eq./km-year are used. Pavement width is assumed to be two 12-ft-wide lanes for all case scenarios. The distance from material production site to application site (Di) for aggregates was assumed to be 25 km. The distance from material production site to placement site (Di) for HMA or PCC was assumed to be 50 km. The values of total CO2/km were calculated for each pavement layer individually, top surface layer, middle layer (if any) and a bottom layer, which is normally an aggregate base.
Figure 4 presents the results for the five pavement design scenarios. The results show that this approach provides a distinction of the total annual kg CO2 Eq. for each lifecycle component and pavement structure type.
Rapid urbanization will continue to place increased demands for transportation infrastructure requiring additional pavement construction. This article introduced a process on how pavement construction contributes to climate change in terms of annual kg CO2 Eq. emissions. The methodology should prove to be a useful tool for engineers and planners to examine the direct CO2 emissions related to the selection of alternative pavement designs. By adjusting the model parameters, users can optimize a pavement design based on organizational needs as well as regionally different climatic conditions, traffic volumes, road maintenance and energy needs.