Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Bridges and Monitoring. <s> Brillouin fiber optic sensing is a promising technology for Structural Health Monitoring (SHM) whose diffusion is however at present reduced by the unavailability of proper sensor products and established installation techniques specifically aimed at the building industry. ::: Due to its intrinsic distributed sensing capability, Brillouin systems can individually measure the deformation of any single segment of considerable lengths of single-mode fiber. In addition, Brillouin retains all the other typical advantages of Fiber Optic Sensors (FOS), such as harsh environment durability and electro-magnetic interference rejection. ::: These advantages, especially considering that the required sensors are really low cost, make the system particularly attractive for periodical ("discontinuous") strain monitoring of unattended infrastructures that might be exposed to ageing and vandalism damages. Despite the high equipment cost, the technique can become economically convenient when the same initial investment can be amortized over a number of applications that can be monitored periodically using the same device. ::: This work presents a comparison between two different Brillouin sensor installation techniques: Near-to-Surface Fiber (NSF) embedding and smart-FRP sensor bonding. Both systems have been experimented in the field on small Reinforced Concrete (RC) bridges subject to a diagnostic load test. The obtained results clearly highlight the advantages of the smart-FRP system, in terms of performance enhancements, installation cost, and time reduction. <s> BIB001 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Bridges and Monitoring. <s> In this paper, we propose and experimentally validate the use of a distributed optical fiber sensor for monitoring pipeline dislocation. The proposed technique is based on the measurement of the normal strain, by use of stimulated Brillouin scattering, along three longitudinal directions running along the pipeline. We demonstrate that, by choosing three lines reciprocally displaced by 120° around the circumference of the pipeline, it is possible to measure the amount and direction of pipeline dislocation. The longitudinal position at which the deformation occurs is also evaluated. The measurements achieved by using the proposed technique can be directly used to monitor the structural integrity of the pipeline, as well as to monitor the deformations of the rock/soil surrounding the pipeline. This can be of importance to monitor geological events. Numerical simulations carried out by the finite-element method have been employed to evaluate the accuracy of the measurements. <s> BIB002 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Bridges and Monitoring. <s> Many structures like bridges are ageing and the necessity to measure the uncertain parameters is relevant. Crack-related parameters can be measured with traditional techniques like crack gauges and displacement transducers. A method that can detect and localise cracks as well as measure crack width is most favourable. Several distributed and quasi-distributed systems were introduced to the market and tested in recent years. This paper presents a large-scale Structural Health Monitoring project based on stimulated Brillouin scattering in optical fibres for an old bridge. The Gotaalv Bridge is a continuous steel girder bridge with concrete bridge deck. Steel girders suffer from fatigue and mediocre steel quality and some severe cracking and also a minor structural element collapse have taken place. The system installed on the bridge measures strain profiles along the whole length of the bridge and detects cracks that are wider than 0.5 mm. Procedures like factory acceptance test, site acceptance test, laboratory testing and field testing are presented and analysed. Innovative technology was developed, tested and applied on the bridge. Heuristic knowledge was collected; conclusions are presented and discussed for future development. <s> BIB003 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Bridges and Monitoring. <s> The Streicker Bridge at Princeton University campus has been equipped with two fiber-optic sensing technologies: ::: discrete long-gauge sensing, based on Fiber Bragg-Gratings (FBG), and truly-distributed sensing, based on Brillouin ::: Optical Time Domain Analysis (BOTDA). The sensors were embedded in concrete during the construction. The early ::: age measurements, including hydration swelling and contraction, and post-tensioning of concrete were registered by ::: both systems and placed side by side in order to compare their performances. Aside from the usual behavior, an unusual ::: increase in strain was detected by several sensors in various cross-sections. The nature of this event is still under ::: investigation, but preliminary study indicates early-age cracking as the cause. The comparison between the two ::: monitoring systems shows good agreement in the areas where no unusual behavior was detected, but some discrepancies ::: are noticed at locations where unusual behavior occurred and during the early age of concrete. These discrepancies are ::: attributed to the spatial resolution of the distributed monitoring system and the temperature influences at early age. In ::: this paper, general information concerning the Streicker Bridge project is given. The monitoring systems and their ::: specifications are briefly presented. The monitoring data are analyzed and a comparison between the two systems is ::: performed. <s> BIB004 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Bridges and Monitoring. <s> Abstract An I type steel girder was assembled with a concrete upper plateand then placed on two roller supports. Four vertical load actuators were laid on the upper plateto add the vertical load. A single mode optical fiberwas attached on the surface of the lower flange of a composite steel girder bridge with a lengthof 39 m. Longitudinal strain distribution of a composite steel girder bridge is monitoreddirectly using optical fibre sensor system based on Brillouin frequency shift dependency of the strain applied on the optical fiber.Brillouin scattering signal is generated through the Brillouin optical correlation domain analysisat a selected location. Strain data wereacquired at 195 points with the interval of 20 cm over the full range.From the strain data over the entire length of the composite steel girder bridge, the strain distribution shape was shownwhen the vertical load was added onthe bridge. <s> BIB005 </s> Brillouin Distributed Fiber Sensors: An Overview and Applications <s> Bridges and Monitoring. <s> Fiber optic sensors can give us valuable data to assess the health of a structure, known as Structural Health Monitoring (SHM). Bridges, being critical structures, are monitored with different sensors such as strain measurements mostly at discrete locations with resistive based sensors. Fiber optic sensors can be used to sense the strain status by using one sensing optical fiber line. Recently, fiber optic Bragg grating sensors are a representative of point sensing technique, and also fiber optic BOTDR or BOTDA sensor is a unique measurement technique to sense fully distributed strain through the sensing optical fiber. Therefore, in this paper, fiber optic Bragg grating(FBG) sensor, fiber optic BOTDR(Brillouin optical time domain reflectometry) sensor, fiber optic BOTDA(Brillouin optical time domain analysis) sensor are installed on 4 span model bridge to study distributed sensing and data analysis to acquire some useful data to identify bridge condition assessment. <s> BIB006
Near-to-surface fiber (NSF) embedding and smart-FRP (fiber reinforced polymer) sensor bonding techniques have been experimented on small reinforced concrete (RC) bridges subject to a diagnostic load test BIB001 . Two bridges with similar dimensions were instrumented and tested under similar environmental conditions. Acryl cyanide and epoxy putty were used, respectively as bonding and encapsulating media for NSF. Being both fragile materials, the crack could easily propagate across the NSF section inducing a strain distribution on the sensing fiber that is mostly concentrated in the small distance between the crack edges. This very short step-like condition in the strain distribution is extremely demanding for the performance of the BOTDR equipment, since the declared accuracy could be obtained only with a strain step length of 1 m. However, the situation is different for smart-FRP sensor, where the bridging effect due to structural fibers "spreads" the strain peak over a certain shear stress transfer length. Both the bigger fiber length that is interested by the phenomenon and the smoother transition in the strain level contribute to enhance the strain sensitivity of smart-FRP in comparison with other fiber installation technique. From the obtained results it must be remarked that NSF took about 28 manhours to be installed on four girders, while smart-FRP took only 9 man-hours to be installed on three slabs. In addition, authors claim that NSF installation resulted to be much more problematic for the amount of dust and noise produced during groove cutting. All the results drive to the conclusion that for this kind of application in terms of performance enhancements, installation cost, and time reduction the smart-FRP system has to be considered as an optical sensor installation in bridges technique BIB001 . On Götaälv Bridge (built in 1939 in Sweden) severe cracking in zones above columns and a minor collapse in a structural element were observed. For safety and security reasons the bridge was monitored continuously for unusual strain changes, as well as for crack detection and localization during the refurbishing works. To do this tasks a largescale distributed fiber optic measuring system developed by Enckell et al. BIB003 was implemented on the structure ( Figure  12 ). The project was carried out with the aims of detecting and localizing cracks that may occur due to fatigue and mediocre quality of steel; to report automatically about high strain values, high strain variation as well as temperature values in short-term and long-term perspective; and to send warnings to the traffic authorities as owner of the bridge. In Figure 13 the Brillouin gain response from a fiber transducer zone with a crack is illustrated. Results of the tests are enlightened, presented, and discussed in the reference by Enkell et al. BIB003 . A good example of truly-distributed sensing, based on brillouin optical time domain analysis (BOTDA)-in addition to a discrete long-gauge sensing, based on Fiber Bragg-Gratings-was installed on the Streicker Bridge at Princeton University campus during its construction. The sensors were embedded in concrete during the construction. The technique allows measurements of hydration swelling and contraction in the first stage, and posttensioning of concrete was registered by both systems and placed side by side in order to compare their performances. Aside from the usual behavior, an unusual increase in strain was detected by several sensors in various cross-sections. The nature of this event is still under investigation, but preliminary study indicates early-age cracking as the cause. The comparison between the two monitoring systems shows good agreement in the areas where no unusual behavior was detected, but some discrepancies are noticed at locations where unusual behavior occurred and during the early age of concrete. According to the authors, these discrepancies are attributed Figure 13 : Illustration, the Brillouin gain response of fiber transducer embedded in a concrete structure with and without a crack, courtesy of Daniele Inaudy. to the spatial resolution of the distributed monitoring system and the temperature influences at early age BIB004 . To measure the longitudinal strain distribution of a bridge a distributed optical fiber sensor is used BIB005 . A single mode optical fiber was attached on the surface using epoxy glue along the length and two ESG (electric strain gauge) were attached at a 1/4 position and 1/2 position of the length of the bridge, near the optical fiber. The optical fiber was connected to a developed fiber optic sensor system based on the concept of the Brillouin optical correlation domain analysis (BOCDA). The longitudinal strain of the bridge was directly measured at 195 points, and a spatial resolution of 14.5 cm. The measurements were made for several values of loads which remains constant during a period of time (from 0 to 50 kN), and a proportional increasing of the load from 0 to 70 kN at a rate of 0.33 kN/s. A remarkable conclusion of this work is that the strain measured by the optical fiber and the strain distribution shape of the girder coincide with the data of ESG (electric strain gauge) within the measurement error range BIB005 . Recently, a study of distributed sensing on 4 span model bridge was carried out. The study consists on data analysis of gathered information from BOTDA and BOTDR systems; the project was addressed to acquire useful data to identify bridge assessment conditions BIB006 . It can also be mentioned that employing Brillouin fiber distributed sensing techniques, the strain distribution (with a spatial resolution of 3 meters and a resolution of 15 με) along a supporting beam of a road-bridge with a span length of 44.40 m has been successfully carried out BIB002 .