## 6 Discussion  
The literature review in this research encompassed more than 200 referenced sources. Selecting the most relevant literature was a challenge given the large number of published papers. According to the tool for viewing indexed academic publications, VOSviewer, there at 1789 keywords in all the referenced articles. If we look at the repeated references in at least ten articles, the number of keywords is reduced to 30, which are related as can be seen in Figure 3.  
The most recent research in Construction 4.0 aims at the integration of BIM with ICTs. The IoT and Big Data and their associated technologies allow the construction industry to make the jump to smart construction [10], as can be seen in Figure 4.  
The most recent articles (more yellow) discuss DT. They rely on other technologies such as IoT, Big Data, Blockchain and O&M (green). Big Data may facilitate data management and the CC [76; 112], while the Blockchain optimizes security in all processes [79]. The DB may be increased by IoT devices [75] or unmanned vehicles [78; 112], along with existing traditional documentation and complemented with the analysis of the territorial information provided by GIS [81]. In addition, all the information may be visualized and  
Figure 3: Network of keyword connections of referenced articles. Made with VOSviewer. compared in the BIM model through immersive technologies [108; 109; 111] to ease and optimize their access to ultimate users. The adoption of all these technologies poses numerous challenges and requires a complex, time-consuming, and expensive process, which is why it has not been implemented in the AEC industry yet [7; 80]. It must be considered the initial investment to create reliable 3D information models of existing infrastructure and the linking of documentation and information in real time. In addition, it is necessary to consider the training of specialized personnel in all phases of the project, the conservation of the data centre or its protection against external attacks [235]. In this regard, recent research focuses on the capacity to relate the three-dimensional models with DB using a single parameter for each element that composes it [65]. This allows all the information required at any given time to be displayed and updated [49; 89]. Thus, the BIM model becomes a repository of information, which serves as an access point to the DB. Programming of the mobile app and web platforms that can be customised for each specific case allows access and visualisation from the digital twin with all its information [30; 96; 97; 98; 120; 121]. On another note, studies for transport infrastructure O&M using the advantages that BIM can provide are very limited [151], being reduced in most cases to theoretical developments in FM [65; 103]. Among the studies that apply new technologies, there are a large number of articles that study the sensorisation of spaces with IoT devices. Most of them are developed in buildings, although in this case, their extrapolation to infrastructure is relatively simple, as the integration of data is the same. Most authors agree that the limitations of linear infrastructure modelling, the volume of data and the lack of support for research into FM processes using BIM are the main impediments to the development of viable applications [10; 91; 156; 157]. Table 2 shows a categorisation of the most relevant articles reviewed in the main research topics, comparing the type of infrastructure with the ICTs studied. One out of every four articles are review articles or general case studies, which is why they have not been included in Table 1. Of these, 38 are dedicated to O&M compared to 13 that are dedicated to design and project phases. Figure 4: Keywords referenced by year. Made with VOSviewer. As it is shown in Table 1, the ICTs implementation in building is faster than in other infrastructure. In addition, it can be established that those publications which study the use of IoT devices tend to develop the information visualisation from web platforms or APIs for mobile devices.