# Seeking a Definition of Digital Twins for Construction and Infrastructure Management
## 1 Introduction  
Virtual models, mock-ups, and simulations, as well as the use of scale models of any construction project has been part of the design phase of most relevant buildings and infrastructure. In recent years, regulation in the use of building information modelling (BIM) for construction has updated this concept, raising new possibilities for its implementation and development in infrastructure management throughout the life cycle of the asset. Oncea digital representation of the project and the construction information are obtained, a range of other technologies can be used for different purposes and become what has been identified as a digital twin (DT). DTs have been applied successfully in relevant fields, such as aeronautics [1], medical applications [2], agriculture [3], and industrial processes [4]. The first definition of a DT is attributed to [PERSON] [5; 6] and was applied to industrial processes. Its main goal was focused on the possibility of product lifecycle management (PLM) as an integrated information-driven approach concerning all aspects of a product's life, from its design to its removal from service and disposal. This study shows that the advances in other fields have shown potential benefits for the construction industry which, if they are merged with BIM, can become of significance in the following years. According to [7], DT technology applied to the retrofitting buildings has the potential of reducing greenhouse gas (GHS) emissions during the operational stage by up to 30%. However, the construction industry has important differences with other engineering fields where DTs have been implemented. Both building and infrastructure projects are performed only once with few repetitive processes as they usually depend on the topography of the terrain, characteristics of the soil, and climate and social needs, among other factors. The construction of two identical projects would not be performed in the same way for two different worksites, or by two different companies. These singularities have made the industrialisation of the sector a challenge in every country [8]. BIM has been a good step forward in this regard and the next expected step [9] seems to be the advancement of a DT for the built environment encompassing the use of DTs for building and infrastructure. Having said that, this definition is still a matter of research. DTs originated in aerospace engineering and were defined, in the field of aerospace systems, as \"a set of virtual information constructs that mimic the structure, context, and behaviour of an individual unique physical asset, or a group of physical assets, is dynamically updated with data from its physical twin throughout its lifecycle and informs decisions that realize value\" [10]. A previous definition in the aerospace sector was provided by NASA in 2012 [11]: a Multiphysics, multiscale, probabilistic simulation of its corresponding flying twin, utilising the best physical models, sensor updates, fleet updates, history, and so forth. ISO 23247-1 [12] refers to digital twins in the field of manufacturing as a fit for purpose physical element representing a set of properties of an observable manufacturing element with synchronisation between the element and its digital representation, but the definition is not yet consistent across the various standards that have been developed (or are under development) in different fields. [PERSON] et al. [13] reviewed the standards and the definitions applicable to DTs until 2023, concluding that the standardisation of DT technology is vital for its implementation. This study seeks to address and advance the definition of DTs for the built environment by gathering the set of information and initiatives that are taking place and discuss the most important matters of disagreements with the aim of achieving a certain initial consensus.