**5. Conclusions**

Reduction of the environmental impact is a trend that must be taken into account due to the environmental problems that exist nowadays. In this respect, the construction sector has a large margin for improvement. The design of structures or buildings must consider the aspects of three pillars of sustainability. The assessment of the environmental impact during the whole life is a factor that must be taken into account in the design of structures or buildings. Although CO2 emissions are not the only indicator to be considered in the environmental assessment, due to the relationship of this indicator with the cost, it is used to obtain a bridge with the lowest cost and a low environmental impact. Once this bridge has been obtained, a complete environmental assessment is carried out. For this purpose, a heuristic optimization by means of a hybrid memetic algorithm is used to obtain a cost-optimized prestressed concrete precast bridge and thus a low amount of associated CO2. Then, the midpoint and endpoint approaches of the ReCiPe method are used to obtain a complete environmental profile of the bridge. These different approaches make it possible to obtain complementary data that provide different information. While the midpoint approach provides detailed information, the endpoint approach provides more concentrated information so it is possible to obtain only one score to assess all the environmental impacts.

Regarding the results of the midpoint approach, the manufacturing phase and use and maintenance phase are the phases with the higher environmental impact. With this knowledge, it is interesting to determine the processes that make the biggest contributions in these phases to try to reduce the environmental impact. Cement production and steel production are the processes with the highest environmental impact in the manufacturing phase, while the maintenance activities have the most environmental impact in the use and maintenance phase. Therefore, the midpoint approach indicates the process with the highest contribution in each impact category and in this way, it is possible to know which process to modify depending on the impact category to be improved. The midpoint approach provides detailed information but does not offer a single score that represents the global environmental impact of the bridge. For this purpose, the endpoint approach is used. As can be deduced, in the midpoint approach, the manufacturing phase and the use and maintenance phase are the ones with the higher environmental impact.

After studying both the midpoint and endpoint approaches, the results show the need for a complete environmental profile to evaluate the environmental impact of the bridge. The midpoint approach provides information that makes it possible to identify the processes in which improvements should be carried out to improve specific impact categories of the bridge but the endpoint approach provides a single score that is able to evaluate the global environmental impact of the bridge. Furthermore, although CO2 emissions are an important indicator in the environmental impact assessment, in some cases it is not sufficient to obtain an accurate environmental evaluation and it is necessary to take into account all the other impact categories.

**Acknowledgments:** The authors acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness, along with FEDER funding (Project: BIA2017-85098-R).

**Author Contributions:** This paper represents a result of teamwork. The authors jointly designed the research. Vicent Penadés-Plà drafted the manuscript. Tatiana García-Segura, José V. Martí and Víctor Yepes edited and improved the manuscript until all authors are satisfied with the final version.

**Conflicts of Interest:** The authors declare no conflict of interest.
