Industrial byproducts such as coal fly ash, coal bottom ash, and steel slags are being used in considerable amounts as a full or partial replacement of the natural aggregate. However, higher concentrations of heavy metals and other toxic organic pollutants in these byproducts raise concern among policy makers and stakeholders. The goal of the present study was to develop a decision making tool for comparison among industrial byproducts and conventional materials used in the embankment construction of a road. In the present study, a comparison of these byproducts with conventional materials was carried out with respect to cost, environmental pollutants generated, and energy consumption. For this purpose, an Excel based model was developed and used for the analysis. A Cradle to grave approach was taken for data collection. Loading of different materials was then subdivided into different impact categories, such as energy consumption, global warming potential, acidification potential and various toxicity potentials. For assessment of toxicity potentials, all the toxicities were represented with respect to 1, 4 dichlorobenzene using characterization factors developed by Huijbregts. An uncertainty analysis was also undertaken due to considerable variability in heavy metals leaching from industrial byproducts and variability in characterization factors of heavy metals. It was found that uses of byproducts, especially fly ash and bottom ash, reduced the green house gas emission and acidification potential for 20 year, 100 – year, and 500 year time periods. However, the use of fly ash and steel slags increased some of the toxicity potentials, such as fresh aquatic ecotoxicity and fresh sediment ecotoxicity, because of the higher concentration of metals in leachate generated from byproducts and their higher toxicity for aquatic animals. However, in 20 year and 100 year time periods, fly ash and bottom ash have lower human and terrestrial toxicity compared to natural aggregates. A sensitivity analysis revealed that characterization factors for cadmium, cobalt, and copper were the most sensitive parameters for fresh aquatic ecotoxicity and fresh sediment ecotoxicity for most of the industrial byproducts. TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal. Raja Chowdhury and Defne Apul 2 INTRODUCTION Extensive use of natural aggregates in various purposes has been gradually depleting the best available material near the demanding areas. The need for resource conservation and lengthened transportation distances has increased the demand to introduce substitute materials for natural aggregates. At the same time, industry, construction and other similar activities produce large quantities of industrial byproducts such as coal combustion byproducts, foundry sand, construction and demolition waste, and steel slags which cause a heavy burden on landfills. In recent years, a scarcity of dumping ground in urban areas has increased the landfill tipping fee many fold. Hence, researchers and engineers have tried using industrial byproducts for various beneficial reuses so as to bypass the landfill costs as well as to find secondary sources for different conventional materials. Road construction is the primary beneficial reuse application where industrial byproducts can and have been used (1). Industrial byproducts can contain trace concentrations of various pollutants that may potentially leach and contaminate the underlying soil and groundwater. Currently there are no universal specifications for addressing the environmental impact of byproduct reuse in road construction. Faced with this challenge, each state has a different approach for decision making on byproduct reuse. The Recycled Materials Resource Center (www.rmrc.unh.edu) has developed and published a framework for screening the industrial byproducts for beneficial uses (2). The framework consists of lab scale leaching test followed by field scale long term monitoring of groundwater, surface water and soil quality surrounding the area where the byproduct was used. An industrial byproduct can be used for a beneficial purpose if the byproduct passes these screening tests. Lab scale leaching tests have been a topic of research for many years. Different types of leaching tests have been developed to assess the extent of long term pollution from subsurface use of byproducts for conditions where environmentally relevant parameters such as pH, redox potential, and liquid to solid ratios may change (3 – 5). The leaching test protocols did not always simulate the proper environmental conditions and thus underestimated or overestimated the mobility of a pollutant from an industrial byproduct (6). While much emphasis has been placed on leaching test protocols and risks of contaminant leaching, little attention has been paid to other kinds of impact, such as energy consumption and emissions that do not directly originate from the road materials. To discuss the use of TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal. Raja Chowdhury and Defne Apul 3 energy consumption and environmental impact from a wider perspective, a Life Cycle Analysis (LCA) approach can be used. An LCA is a method of accounting for the environmental impacts associated with a product or a service. The method takes into account various upstream processes and emissions generated during the lifetime of a product. Therefore, the primary task in an LCA study is the estimation of emission factors (emissions generated from the production of unit mass of product) from various upstream processes directly and indirectly linked to the manufacturing of the products. The processes one would include to calculate emissions depend on the system boundary of the project, which needs to be determined upfront. There are a few examples of the use of LCA for evaluation of alternative materials in road constructions. In Denmark, a model for LCA of road construction and disposal of waste generated from municipal solid waste incinerator was developed (7). In Finland, Mroueh and his coworkers (8) used an LCA model for road construction to assess the environmental impact from several alternative materials. They found that the use of the industrial byproducts as a substitute for natural aggregate could reduce the environmental impact for some of the impact categories. However, these studies have not taken into account an extensive toxicity assessment approach along with the effects of variability of environmental loadings on toxicity assessment. In the USA, PALATE (9), an Excel based life – cycle assessment tool for road construction was developed recently. PALATE is an extensive tool for pavement life cycle analysis, which includes a more detailed toxicity assessment approach compared to other studies. However, users need to spend time to learn the tool and it is also cumbersome to compare two materials using PALATE. In addition, PALATE data is based on an economic input output analysis through use of EIO-LCA model as opposed to a process based LCA approach (10). Reuse of industrial byproducts in road construction is promoted by the US Federal Highway Administration (FHWA) and its State counterparts. FHWA encourages the appropriate and economical use of recycled materials where engineering performance is equal to or exceeds traditional materials and where the materials do not contribute to current or future environmental problems (2). However, there is no tool available which can be used for comparison of different industrial byproducts with conventional materials taking various environmental criteria and explicitly considering the uncertainty in data. TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal. Raja Chowdhury and Defne Apul 4 Therefore, there is a need to develop a user – friendly Excel based tool, which can be used to compare various industrial byproducts with conventional materials such as natural aggregates, asphalt, and cement using an LCA approach. The aim of this study is to compare the environmental impacts of constructing an embankment from natural aggregates versus industrial byproducts such as coal fly ash, coal bottom ash, and steel slags, using the developed Excel – based tool. An additional primary question that this study addresses is the effect of variability in environmental emissions on the various toxicity impacts.
