This literature review gathered different approaches of bridge environmental assessment. Some references focused on the comparison of the life cycle environmental performance of several bridges, others simply gave recommendations regarding environmental - or even sustainable - issues related to bridge projects, and others did both. Even if different assumptions were made by the authors, some common recommendations and results can be found. Here is a (non-exhaustive) list of the main issues discussed in the articles:
Goal of the study
Among all the references, 12 perform an LCA analysis in order to compare the environmental impacts of different bridge designs and 2 realize a comprehensive general analysis of bridge LCA with a case study. This shows that LCA methodology is often used as a comparative tool in order to choose the best environmental solution, but it can also be performed in order to assess the overall environmental performance of one type of bridge, which is for example the intended goal of this thesis.
Types of bridges
45 bridges are assessed within the 14 references selected, including 22 concrete, 13 steel (includ- ing steel-concrete non-composite), 5 steel-concrete composite, 4 wood and 1 brick bridges. This shows that concrete bridges are predominant in bridge life cycle assessment and that concrete and steel represent the two principal materials used in bridge design, compared to wood or brick.
Functional unit
Only 5 authors clearly define their functional unit. Most of the time, the results are given considering the overall impact of the bridge or some of its elements, according to the goal and scope definition of the study. This choice of functional unit makes difficult the possibility of having comparisons between the results of different studies, as bridge geometry or selection of the elements assessed differ, for example. However, one reference [10] defined its functional
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unit as “1 square meter effective bridge surface area” (i.e. approximately the upper deck surface area), which can be a good functional unit to compare analyses dealing with different bridge geometries.
Life cycle phases considered
Due to limitations in data availability and accuracy, some authors do not include all life cycle stages in their analysis. Material production, transportation, construction and maintenance phases are always taken into consideration. 6 references consider end-of-life treatment. Traffic disruption due to maintenance activities is evaluated in 2 studies, but regular traffic is never con- sidered. The assumptions regarding which processes are included or not in the life cycle phases differ a lot according to the references. However, a majority of the studies consider the impacts from main material processes (concrete, steel or wood production, as well as the construction and maintenance activities related) including the amount of energy required, transportation to the site, assembly at site as well as repair, maintenance and replacement activities.
Design service life
8 references consider a design service life of 100 years or more (120 years for 4 references), which is the average design service life in bridge management. The other studies do not consider the entire bridge (e.g. only a bridge deck replacement) and hence use a shortened design service life.
Emissions and impact categories considered
5 references consider more than 6 impact categories in their studies, usually accompanied with weighting and normalization of the results. 8 references use global warming potential in their studies while the others just consider CO2 emissions or discarded air pollutants. Energy use
is explicitly calculated in 6 references. This is not an impact category as defined in LCA ISO standards, but it gives representative information about the environmental performance of a bridge project.
Life cycle phases contributing to major environmental impacts
For all references, material production (sometimes included in the construction phase of the bridge) is the biggest contributor. The second most contributing phase is either maintenance and repair (3 references) or traffic disruption due to construction or maintenance and repair activities (2 references).
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Bridge components contributing to major environmental impacts
Most of the time, only the bridge superstructure (and more precisely its structural elements) is considered in the analyses. Among the 2 references considering a complete bridge assessment, only one finds out that temporal form work and bridge substructure lead to more environ- mental impacts than the bridge superstructure. Most often, as the structural elements of the superstructure contain the most important part of total material use, and as material phase is related to the biggest share of the environmental impacts, only bridge decks and their support- ing features (girders, arches, cables, etc.) are considered.
Best environmental choice in comparative studies
Among the 12 comparative studies of the literature review, 3 references highlight innovative bridge deck design (ECC link slabs, HPC concrete, minimized number of girders) over conven- tional bridge deck design. Regarding the studies assessing different material choices, concrete is often the best environmental solution regarding the overall impact (5 references). However, for greenhouse gases (GHG) emissions; steel, composite or wooden alternatives are sometimes preferred (3 references), especially if recycled material are used (1 reference).
Other recommendations
A few similar recommendations are given by the authors, mainly considering material and design quality, but also LCA recommendations for further studies. It is often stated that use of recycled or sustainable material is strongly recommended as it can lower efficiently burdens from material phase, as well as use of renewable energy. For LCA practitioners, it is important to consider transportation distances and traffic disruption during construction or maintenance, as they can represent an important share of the environmental impacts. Hence, as this is directly related to bridge owners, designers and contractors, local material source is important to minimize transportation related emissions, so are maintenance schedules in order to limit traffic disruption related emissions as much as possible. 2 references considering timber bridges insist on the positive impact of CO2 uptakes during wood life cycle. Another
one indicates that more architectural solutions (cable-stayed bridges, for example) often lead to more environmental impacts, as they require more material and construction features to be realized.
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