**3. Life-Cycle Assessment**

Life-cycle assessment (LCA) is one of the most important and accepted methods of evaluating the environmental impact of a product, process, or service during its whole life-cycle, taking into account all the activities involved, which are defined as inputs and outputs. The limits defined for these inputs and outputs are the boundaries of the system and represent the scheme to be considered. The LCA must be complete and thus it should consider all the activities needed for the achievement of the product, process, or service. Therefore, focusing on the construction sector, a full LCA of structures must consider all the activities from the acquisition of the raw material to the end of life. These activities associated with the whole life-cycle of the structures are grouped into the manufacturing phase, construction phase, use and maintenance phase and end of life phase. The LCA makes it possible to carry out an environmental impact assessment of a set of activities associated with the different stages of a structure's life-cycle and the global environmental impact by adding these phases. For all that, the LCA is an excellent tool to evaluate the environmental impact of structures. ISO 14040:2006 [22]

provides guidance on carrying out the LCA, divided into four steps: (1) definition of goal and scope; (2) inventory analysis; (3) impact assessment; and (4) interpretation.

The first step defines all the specifications that will be considered in the LCA. This involves other features besides the definition of the goal and scope, such as the life-cycle inventory to be taken into account, the life-cycle assessment methodology considered, the functional unit and the assumptions and limitations that have been considered in the LCA. According to the guidance defined by ISO 14040:2006 [22], the characterization defines some assumptions and limitations of the LCA that condition the following life cycle inventory and life cycle assessment. Another important feature is the functional unit that represents the unit in which the assessment will be referred.

The inventory analysis is the collection of the data needed to define the inputs and outputs that represent the system studied. This data can be obtained in different ways: from direct measurements, literature, or other sources such as databases. The most common way to obtain data is from databases.

Once these first steps have been defined, the environmental impact assessment is used to evaluate the result of the inventory analysis to obtain a set of environmental indicators that represent the environmental profile of the product, process, or service. There are different methods of representing the environmental profile. These methods can be grouped into two different approaches: midpoint and endpoint assessments. The midpoint approach defines the environmental profile by means of a set of impact categories and the endpoint approach defines the environmental profile by means of a set of damage categories. There are three damage categories (human health, resource depletion and ecosystems) into which the impact categories are clustered. Therefore, although the midpoint approach provides a complete environmental profile, it is more difficult to interpret [23]. Conversely, the endpoint approach does not provide a detailed environmental profile like the midpoint approach but is easier to understand.

Finally, the information obtained must be interpreted. For this purpose, an analysis of the different stages of life-cycle of the bridge is carried out. In addition, a study of the environmental impact of a product, process, or service can be made to improve the environmental impact associated with its activities.
