**1. Introduction**

The basis for the definition of sustainable development lies in the Brundtland Commission's report [1], which describes it as "*development that meets the needs of the present generation without compromising the needs of the future generation*". This idea implies the consideration of different aspects of three main components: economic, environmental and social. Therefore, achieving sustainable development implies a consensus among these three main pillars, which usually have different goals. Wass et al. [2] stated that sustainable development implies that a decision-making strategy must be considered. Decision making is a process that can help to find a solution that provides a compromise between different aspects and therefore achieves a sustainable solution [3,4].

The construction sector is one of the most active sectors and one of the ones with a greater influence on the economic, environment and social aspects of the world. This indicates a need for a trend toward sustainability of buildings and structures. One of the most important structures in this sector is bridges. The construction and maintenance of bridges are crucial to generate and keep the best transport possible between different places. For this reason, the assessment of sustainable development during the whole life-cycle is necessary. Of the three main components of sustainable development, the social aspect is the least studied and there are more doubts about its assessment. On the contrary, the economic and environmental aspects have been studied more intensively and it is convenient to assume that their consideration is sufficient. Considering the evaluation of these two components to achieve sustainability of bridges, the objective is to design the bridge with the lowest cost and lowest environmental impact. Although these two pillars of sustainability have different goals, some works have stated that there is a relationship between the cost and CO2 emissions of structures [5,6]. Therefore, reducing the cost implies a reduction of CO2 emissions.

Obtaining the lowest cost or CO2 emissions have been studied by several works. Optimization algorithms are most often used to reduce the cost or CO2 emissions of structures. In some cases, this involves a mono-objective optimization of cost and CO2 emissions [5–7], whereas other works carry out multi-objective optimization to achieve both objectives at the same time [8,9]. Despite the relationship between cost and CO2 emissions, the environmental impact cannot be assessed by taking into account CO2 emissions alone [10]. For this reason, the environmental impact assessment must achieve a complete environmental profile. This complete environmental profile can be obtained using the life-cycle assessment (LCA) process. LCA is one of the most important and accepted methods of assessing the environmental impacts [11–16], making it an excellent tool for assessing the environmental impact of a bridge.

In this paper, a prestressed concrete precast 40 m bridge is selected as the subject of an optimization-LCA. The optimization of the cost will reduce the cost of the bridge directly and the associated CO2 emissions indirectly. This process makes it possible to obtain a cost-optimized bridge with a low environmental impact. After finishing the optimization, all the features of the cost-optimized bridge will be known, including its cost but the environmental impact will not ye<sup>t</sup> have been obtained. The LCA makes it possible to obtain a complete environmental profile of this cost-optimized bridge. With this methodology, a bridge whose costs have been optimized directly and whose environmental impact has been improved is obtained and finally the LCA for the whole life-time can be performed. For this purpose, a hybrid memetic algorithm is used to carry out the cost-optimization of the bridge. Then, the Ecoinvent database [17] and the ReCiPe method [18] are used to conduct the LCA process of the bridge.
