## 2 Current Approaches to Optimize the Efficiency of Construction Projects
### Synergies between the BIM and LEAN Paradigms  
BIM and Lean are the most representative emerging approaches of the current panorama in the construction sector, but they have been developed in different fields independently. However, their joint application allows to achieve a complete implementation by preserving an entire operation throughout the project, since they act, as mutual facilitators and catalysts. This mutual synergy occurs throughout the project life cycle from the initial design to the building maintenance [42]. In all these stages the use of BIM contributes directly to the achievement of Lean objectives, and at the same time Lean processes facilitate the adoption and the continued use of the BIM model throughout all stages of the project. The synergy of both paradigms has strategic advantages in the organizations that implement it, materialized in customer focus through an early identification of value, conflict detection, fast assessment of alternatives, and collaboration, among others. Some authors have conducted theoretical studies on the integration of BIM and Lean in the design phase [43], in particular the capacity of BIM to reduce waste in the design phase. The authors identified that the most important causes of waste in the design phase are related to design changes, poor decision making, lack of information exchange, lack of communication and lack of knowledge.Given this situation, the integration of both disciplines is emerging as the appropriate solution in this design stage to reduce design times, reworks and conflicts [44], at the same time that increases predictability, and the interaction of stakeholders [45]. Within the design phase, other authors analyze how the BIM-Lean synergy allows to reduce design errors by improving communication between all agents as well as the delimitation of roles and reorganization of the work structures. This lets to identify omissions and errors before they have a late impact on the construction phase. However, all the above literature focuses only on synergies in the design phase and does not cover other phases of the project life cycle [46]. Other authors develop the synergy in other phases such as the construction stage. In this phase Lean acts as a BIM facilitator to promote collaboration through Lean tools such as large room, knotwork or Last planner, improving stakeholder's communication, setting up responsibilities and establishing adequate and concise instructions, fostering a correct implementation of BIM [47]. Also in the construction stage, it is remarkable the benefits obtained from the result of the visualization with 3D modelling. Visualization beyond the design phase, that is, during the construction process, will allow Lean principles to achieve better project performance, increasing productivity and quality and reducing the cost and delivery time of the project. However, the literature shows the low volume of papers addressing the synergy of both paradigms when compared to the large volume of literature on Lean and BIM independently [48]. This is further accentuated if we go beyond reviews of theoretical literature. Authors have verified that there is hardly any research in indexed journals that focus on the integration of these paradigms to validate and demonstrate the viability of existing theoretical hypotheses through case studies. Some of the few studies that address this issue only focus on the facilities chapters, as in Reference [49] where BIM and Lean synergies are validated through a case study developed in the construction phase, but which only focuses on the management of Mechanical, Electrical and Plumbing (hereinafter MEP) works, such as plumbing and electrical facilities. Similarly, in Reference [50] some authors analyse the implementation of both paradigms, but only focus on the facilities management phase. The same is presented in Reference [51] where the integration of BIM and Lean concepts such as the Last Planner System is validated in the operation and maintenance phase.