A life cycle assessment (LCA) model was developed to compare the sustainability of alternative concrete bridge deck designs: one a conventional steel reinforced concrete (SRC) deck with mechanical steel expansion joints, and the other an SRC deck with engineered cementitious composite (ECC) link slabs.
The model was developed in Microsoft Excel based Visual Basic macros and includes five modules: materials, construction, traffic, distribution, and end-of-life. With over 100 flexible parameters programmed in the LCA computer model and the integration of three other computer programs, US EPA’s MOBILE6.2 and NONROAD emissions models, and the KyUCP traffic model, users are able to explore the impacts of changes in material formulation, bridge deck design, and traffic flow. The two design alternatives were evaluated over a 60-year time horizon: the ECC link slab system was modeled with a 60-year service life, while the conventional joint system required two bridge decks each lasting 30-years. Over this time period the ECC link slab system showed significant benefits in environmental performance relative to the conventional joint system, despite that ECC material is more energy intensive than conventional concrete. The ECC link slab system consumed 40% less total primary energy, produced 39% less carbon dioxide, and consumed an average of 38% less of key natural resources such as coal, limestone, and water. The most influential parameter in the model proved to be construction related traffic. For a 0% traffic growth scenario, construction related traffic energy comprised 80% of total primary energy consumed by the conventional system and 85% of total primary energy consumed by the ECC system. Construction related traffic also dominated results for the majority of air emissions including; hydrocarbons, carbon monoxide, methane, and greenhouse gas emissions.
Results are broken down into their life cycle stage – construction, distribution, materials, traffic, and end-of-life (EOL). Traffic is shown in all diagrams with a ? preceding it. This symbol indicates that traffic burdens are calculated as the difference between normal operating conditions, and traffic conditions resulting from construction related congestion. Thus, traffic values represent only the impacts of construction, and not normal flow over the bridge deck. Figure 1 shows the total primary energy consumption for the two deck systems, and figure 2 shows how total primary energy varies based on the assumed traffic growth rate. Note how sensitive total primary energy consumption for the entire life cycle is to this key parameter.
University of Michigan
Page Last Updated
October 16, 2006
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