4.1. Bridges
As the vital hinges of the transportation lines, the health conditions of the bridges have always been concerned by the bridge owners and managers. Continuous real‐time monitoring of the environmental and operational loadings as well as the structural responses and behaviors of the bridges has been proved to be a promising and effective way for system identification, damage detection, safety condition assessment, and structural performance prediction. Due to the nonsubstitutable capabilities and unique advantages, the optical fiber sensing technology has been served as an effective tool for the monitoring of each phase of the bridge life-cycle (construction, operation, reinforcement, and rehabilitation), of various structural components of bridges (decks, towers, stay cables, suspenders, girders, piers, piles, and abutments), and of different measurands (strains, temperatures, accelerations, deflections/displacements, cracks, and corrosion). There have been a lot of investigations on bridge health monitoring and structural condition assessment based on the optical fiber sensing technology as detailed in the following sections.
4.1.1. Integrated Bridge Monitoring System
A considerable number of optical fiber-based integrated SHM systems deployed on various types of bridges have been developed worldwide [79]. In USA, Mehrani et al. [80] developed a remote monitoring system based on optical fiber sensors for condition assessment of bridges and the performance of the developed system was validated through field instrumentation on a bridge in Florida, USA, during its construction stage. Glisic and Inaudi [81] developed a method for integrity monitoring of fracture critical bridges using simulated Brillouin scattering based on a crack or local deformation identification algorithm and a sensor delamination mechanism. Talebinejad et al. [82] developed an FBG-based accelerometer by use of the stiffness of the optical fiber and a lumped mass and the performance of which was evaluated during ambient vibration tests of a real bridge. In Canada, a total of 16 bridges have been instrumented with long-term SHM systems by intelligent sensing for innovative structures (ISIS) with various combinations of optical fiber sensors [83].
Brönnimann et al. [84] investigated the reliability and long-term stability of an FBG-based sensing and surveillance system through a monitoring period of six months during construction of a stay cable in Switzerland. In Portugal, Rodrigues et al. [85] developed an FBG-based system with embedded displacement and strain transducers for long-term monitoring of structural performance of concrete bridges which was applied to a concrete bridge. Barbosa et al. [86] developed a novel weldable FBG sensing system for strain and temperature monitoring of steel bridges and for loading tests and health monitoring of a circular pedestrian steel bridge. In UK, Kerrouche et al. [87] developed a relatively cheap and effective sensing system using a compact FBG-based monitoring system incorporating a scanning Fabry-Perot filter, and the performance of the system was validated through laboratory experiments and field tests in a real bridge. Kister et al. [88, 89] conducted the research on structural monitoring of a composite road bridge by use of FBG sensors, and the performance of the adhesives and the protection system of the sensors were evaluated through field pullout tests. Mokhtar et al. [90] created an innovative FBG-based sensor system for accurate strain measurement with full temperature compensation towards condition monitoring and assessment of arch bridges. Surre et al. [91] developed an optical fiber sensor system for long-term strain monitoring and condition assessment of a redundant 50-year-old concrete footbridge.
In Hong Kong, the high-speed demultiplexing/interrogation system for FBG sensor arrays and FBG sensor package units were deployed for long-term monitoring on the Tsing Ma Bridge which is the world’s longest suspension bridge carrying both highway and railway traffic [92]. Yau et al. [93] proposed a simple, inexpensive, and practical method for measurement of the vertical displacement of bridges by use of FBG sensors. In Chinese mainland, Zhao et al. [94] integrated the distributed Brillouin optical time domain analysis (BOTDA) technology and the FBG sensing technology for strain monitoring of bridges. In Korea, Chung et al. [95] conducted the experimental study on the applicability of long-gauge optical fiber sensors for the monitoring of the structural defection of the prestressed concrete bridges. Lin et al. [96] developed an FBG-based sensing system for online monitoring of highway bridges during construction to record the hydration effects, curing periods, prestressing responses, and removal of support frames.
4.1.2. Monitoring of Rehabilitated and Antique Bridges
Research efforts also have been devoted to measuring the structural behaviors of old bridges or deficient bridges during rehabilitation by use of the optical fiber sensing system [97]. Jiang et al. [98] applied two types of optical fiber sensors embedded in FRP material to monitor the global and local behaviors of the strengthened bridge structures. Zhang et al. [99] introduced two types of optical fiber sensing technologies (FBG and BOTDR) for health monitoring of rehabilitated reinforced concrete girder bridges, and the static and dynamic loading tests were carried out with a simply supported reinforced concrete T-beam strengthened by externally posttensioned aramid fiber reinforced polymer (AFRP). Costa and Figueiras [100] presented the design of an advanced FBG-based monitoring system which was applied to a century steel arch bridge in Portugal.
4.1.3. Monitoring of Bridge Cables and Suspenders
He et al. [101] carried out an investigation on cable force monitoring by use of the local high-precision FBG sensor in combination with the distributed BOTDA sensing technique. Li et al. [102] developed a smart stay cable assembled with FBG-based strain and temperature sensors which were incorporated into a glass fiber reinforced polymer (GFRP) bar. The efficiency of the developed smart stay cable was proved by application to evaluate the fatigue accumulative damage of a stay cable bridge in China [103, 104].
4.1.4. Bridge Scour Monitoring
Zhou et al. [105] proposed an FBG sensing system for scour monitoring of foundations of bridge piers and abutments. This developed system introduced a uniform-strength FRP beam instrumented with two FBG sensors in two sides of the neutral axis, and the feasibility of the system was validated through laboratory tests. Lin et al. [106, 107] developed two types of FBG-based systems for real-time bridge scour monitoring, which were capable of measuring the process of scouring/deposition and the variation of the water level. The in situ FBG scour monitoring system was demonstrated to be robust and reliable for real-time scour-depth measurement and to be valid for indicating the depositional depth. Xiong et al. [108] developed a bridge scour monitoring system by use of FBG sensors, and the experimental investigations verified that the recommended scour monitoring system was capable of measuring the water level, the scour depth, the entire process of scour development, and the deposition height due to the refilling process.
4.3. Tunnels and Pipelines
Ye et al. [113] addressed two engineering paradigms on safety monitoring of tunnel construction by use of FBG sensors. Metje et al. [114] presented a new optical fiber sensing system for structural displacement monitoring which was successfully applied to measure the displacement of a tunnel lining. The novel system was based on a square fiberglass smart rod which was proved sensitive enough to measure the rotational movement of 0.5° and the lateral movement of 0.1 mm of the fixings. Li et al. [115] developed a metal groove encapsulating technique for the bare FBG sensor to measure the surface strain of the second lining of the tunnel. Li et al. [116] developed a differential FBG strain sensor for monitoring the stability of the tunnel during the backfilling and traffic-operating periods.
Glisic and Yao [117] proposed a method for real-time, automatic, or on-demand assessment of health conditions of buried pipelines after the earthquake based on distributed optical fiber sensors, the research of determination of sensor topologies, selection and development of sensors, development of installation and implementation procedures, and large-scale tests were conducted. Zhang et al. [118] experimentally investigated the prediction of locations and progression sequences of the pipe buckling with the aid of the broadening factor of the Brillouin spectrum width using high strength carbon-coated fibers and standard communication fibers.
4.5. Railway Infrastructure
Recently, the optical fiber sensor-based monitoring system has attracted great interests among the researchers in the fields of railway engineering and optical engineering. Yan et al. [127] proposed three FBG-based methods for strain measurement and axle counting in high-speed railway systems, and the advantages and limitations of these approaches were discussed in terms of feasibility and cost-effectiveness through laboratory verification and evaluation. Wei et al. [128, 129] described a real-time wheel defect detection system through deploying FBG sensors on rail tracks of the Hong Kong mass transit railway (MTR) to gain the track strains upon wheel-rail interaction and generate a reliable condition index reflecting the wheel condition, and the effectiveness of the introduced system was verified by extensive field tests. Filograno et al. [130] implemented an FBG-based railway security monitoring system on the Spanish high-speed line Madrid-Barcelona for train identification, axle counting, speed and acceleration detection, wheel imperfection monitoring, and dynamic load calculation.
Pimentel et al. [131] developed a hybrid fiber-optic/electrical train characterization system with a new weight-in-motion (WIM) algorithm for on-motion determination of the train speed, acceleration and weight distribution for traffic monitoring, and safety evaluation of a railway bridge in Portugal. Kerrouche et al. [132, 133] conducted the research on structural monitoring of a decommissioned concrete railway bridge in Sweden loaded to failure by use of an FBG-based distributed sensor system.
Kang and Chung [134] developed an integrated FBG-based monitoring scheme for a maglev guideway in Korea to measure the parameters involving strains, curvatures, vertical defections, and frequencies which were compared with those obtained from the conventional sensors [135]. Yoon et al. [136] proposed a distributed Brillouin optical correlation domain analysis- (BOCDA-) based sensing system to measure the longitudinal strain distribution of a rail in real time, and the results of a spatial resolution of 3.8 cm and an accuracy of ±15 με were achieved under different loading conditions applied to a 2.8 m rail. Wang et al. [137] utilized A-thermal FBG sensors and electronic sensors to record performance changes in the continuous welded rail, and the monitoring results revealed that the optical fiber sensor was durable and capable of long-term monitoring and was capable of providing sensitive, clear, and stable signals.
Bocciolone et al. [138] presented the application of FBG sensors on a pantograph for monitoring of the contact force and the vertical acceleration of the pantograph head of the pantograph-catenary system in an underground line. Boffi et al. [139] developed an innovative optical fiber sensor-based system for online monitoring of the contact force between the pantograph and the catenary at low and high frequencies.