**2. Overview of structural health monitoring**

Designed principally to determine and assess structural integrity, structural health monitoring has continued to evolve with improvements in the technology and processes used in achieving the said goals. Some fundamental bases of SHM include an assumption that all materials have inherent flaws, and the necessity of at least two system states in order to assess damage. As an important tool for assessing the condition and lifetime performance of a structure, Ref. [9] opined that systems which only include sensors deployed on structures without a definition or classification system for the damage cannot truly be classified as SHM systems. The study stated that a true SHM system would include a quantifiable and pre-established definition of damage to be detected by the sensors. Also, in the development and deployment of a SHM system, a classification process for the identification of damage and assessment of its extent needs to be defined [9]. To improve the practice of SHM for infrastructure, Ref. [3] proposed a condition-based assessment framework for the management of bridges. This process, would ideally provide information on damage to the structure and erosion of structural resistance, as well as the probability of the structure's performance falling below a set standard and an estimation of the remaining useful service life. An SHM system encompassing these parts would arm inspectors with adequate information on the service condition of these infrastructural assets, allowing them to make decisions on the repairs/retrofitting, while drastically cutting down on the need for periodic manual inspections. There are five principal steps in a SHM process. These are detection, localization, classification, assessment and prediction [9, 10]. While the first two steps involve utilizing sensors and other equipment for monitoring via non-destructive evaluation of the structure under investigation, the last 3 steps involve analyses of the data collected from the monitoring process. Thus, it can be said that an optimal SHM process consists of monitoring of structural behavior, and analyzing this data for a proper prognostication of structural health.

#### **2.1 System monitoring**

Monitoring of structures for SHM involves use of instrumentation and processes for the detection and localization of damage. With the inability of sensors to measure

#### *Remote Assessment of the Serviceability of Infrastructural Assets DOI: http://dx.doi.org/10.5772/intechopen.109356*

damage, the data collection at the system monitoring stage is qualitative in nature, giving useful information on the presence and type of damage present in a structure, but not offering quantitative information on the extent of the damage and the remaining useful life of the structure or structural member. Most sensor systems deployed for the purpose of SHM consist principally of this monitoring process, without going further to quantify the damage and predict remaining useful life of the structures [9]. Although only one part of a holistic SHM process, the importance of proper deployment of sensors and instrumentation towards damage detection and monitoring cannot be overestimated. With sensing systems, there is a trade-off between sensitivity to damage of a sensing system and its noise rejection capability, and also the size of damage detectable is inversely proportional to the frequency range of excitation [9]. Generally, the length and time associated with the initiation and evolution of damage dictates the properties of the sensing system to be used. Nondestructive evaluation techniques have been widely investigated for this purpose. These techniques can be deployed in an online manner for continuous monitoring, or in an offline manner for inspection purposes. NDE methods researched and deployed for SHM include vibration-based methods [11–13], optical based methods [14], radiography [15], ultrasonic testing [16, 17], acoustic emission [18, 19], electromagnetic methods [20–22], magnetic particle inspection [23, 24] and thermographic methods [25–28]. These methods are usually used to capture damage in either continuous monitoring schemes or in inspection regimes. Several types of damage are usually the targets of such non-destructive evaluation processes. Ref. [13] in a follow up of a study by Ref. [29], determined that the material and geometric changes that can be characterized as damage by these systems include cracks, corrosion, buckling, creep, fastener loosening and loss of preload, debonding, delamination, microstructural degradation, and pull-out.
