Infrastructure is defined as physical assets that are capable of an intended service delivery and comprised of rigid assets such as buildings, bridges, and roads and flexible assets such as utilities and facilities related to water, sewage, and power. Critical infrastructure spans across a number of key sectors including energy, finance, and information technology. An infrastructure system is an integrated structured network of interdependent entities that aid in the service delivery capability of rigid and flexible assets.
Infrastructure evolved with society and technology and is a major key in boosting both the economy and living standards of a population. Critical infrastructure systems are therefore built to provide services and, in some cases, jobs for several generations (National Research Council 2009). Critical infrastructure is almost always subject to political and social pressures. The economic pressures in most cases result in delayed maintenance and rehabilitation. Recently, in the face of terrorists and other saboteurs, infrastructures that symbolize a nation's pride—for example, the Statue of Liberty—are also considered as critical infrastructure (Chai et al. 2011).
The Royal Academy of Engineering (2011) extensively discussed the meanings and principles of “smart structure.” It defined smart structure as a system that uses a feed loop of data, providing evidence about the state of the infrastructure for effective maintenance and rehabilitation decision making. Therefore, the system can monitor, measure, analyze, communicate, and react based on information and data captured by multiple sensors. The principles of the smart structure include (1) large data acquisition and collection, (2) complex mathematical modeling for decision making and engineering assessment, (3) feedback systems that control collected information and improve infrastructure system operations, and (4) adaptability. The system is versatile enough to incorporate new communications and other technologies.
There are five criteria used by different nations and jurisdictions to determine critical infrastructures: (1) health, (2) safety, (3) economy, (4) continuous functioning of government, and (5) national/society morals. The following definition of critical infrastructure was presented by the National Research Council (NRC 2002): critical infrastructure systems are defined as the water, wastewater, power, transportation, and telecommunication systems without which buildings, emergency response systems, and other infrastructure cannot operate as intended.