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Reinforced concrete structural components such as beams are subject during their service life to different loading conditions that may affect their durability and efficiency. This can reduce the safety level of the structure over time until it degrades completely and reaches its ultimate limit state. In particular, cracking conditions due to tensile conditions can be developed leading to the exposure of the steel reinforcements to the aggression of external agents, such as chlorides. Being able to monitor the development and the evolution of cracking is crucial and this research is aimed at this purpose. In particular, laboratory tests have been performed on reinforced concrete beams equipped with distributed fiber optics sensors to monitor the state of cracking. This technology has been selected because its inherent accuracy and low noise. Furthermore, since technology relies of optical signals, the sensors are not affected by electrical and magnetic noise. They are also resistant to high temperatures and chemically reactive environments. The main objectives of the presented first part of this research are focused on the deformation and temperature assessment after concrete pouring, crack detection, localization and opening quantification.

Miniature optical fiber sensors, based on Fabry-Perot technology, have found numerous applications in both patient monitoring and civil structural monitoring of large and massive infrastructures. Although structural health monitoring and patient monitoring may benefit from the unique advantages of optical fiber sensors (OFS) such as electromagnetic interferences (EMI) immunity, sensor small size and long term reliability, both applications are facing very different realities. This contribution presents the underlying sensing technology, the miniature sensor fabrication technique and numerous application examples in medical and civil engineering-fields. OFS for medical applications are single-point, measuring mainly parameters such as pressure or temperature. In the intra-aortic balloon pumping (IABP) therapy, a miniature OFS can monitor in situ aortic blood pressure to trigger catheter balloon inflation/deflation in counter-pulsation with heartbeats. Similar sensors reliably monitor the intracranial pressure (ICP) of critical care patients, even during surgical intervention or examinations under medical resonance imaging (MRI). Temperature OFS are also the ideal monitoring solution for such harsh environments. The same miniature sensing elements can be packaged differently to allow their use in civil and geotechnical monitoring. In particular, Fabry-Perot optical sensors have been in use for many years as direct replacement of conventional sensors based on the vibrating wire and other electrical technologies. Those sensors are ideal to measure strain, deformation, pressure and temperature in applications subject to strong electromagnetic fields, lighting strikes or requiring long cables. Application examples includes the monitoring of groundwater pressures in tailing dams used for mining applications in Chile, leaks in levees in The Netherlands and displacements in the cryogenically cooled superconducting magnets for ITER project in France.

This paper will discuss the challenges and solutions required for large scale production of more than 250 000 Fabry-Perot sensors per year, demonstrating that automated production of large-volume fiber optic sensing is now a reality. Medical and industrial applications requiring large volume production are also explored.

This paper illustrates an application of Bayesian logic to monitoring data analysis and structural condition state inference. The case study is a 260 m long cable-stayed bridge spanning the Adige River 10 km north of the town of Trento, Italy. This is a statically indeterminate structure, having a composite steel-concrete deck, supported by 12 stay cables. Structural redundancy, possible relaxation losses and an as-built condition differing from design, suggest that long-term load redistribution between cables can be expected. To monitor load redistribution, the owner decided to install a monitoring system which combines built-on-site elasto-magnetic and fiber-optic sensors. In this note, we discuss a rational way to improve the accuracy of the load estimate from the EM sensors taking advantage of the FOS information. More specifically, we use a multi-sensor Bayesian data fusion approach which combines the information from the two sensing systems with the prior knowledge, including design information and the outcomes of laboratory calibration. Using the data acquired to date, we demonstrate that combining the two measurements allows a more accurate estimate of the cable load, to better than 50 kN.

Many bridges worldwide are approaching the end of their lifespan and it is necessary to assess their health condition in order to mitigate risks, prevent disasters, and plan maintenance activities in an optimized manner. Fracture critical bridges are of particular interest since they have only little or no load path redundancy. Structural health monitoring (SHM) has recently emerged as a branch of engineering, which aim is to improve the assessment of structural condition. Distributed optical fiber sensing technology has opened new possibilities in SHM. A distributed deformation sensor (sensing cable) is sensitive at each point of its length to strain changes and cracks. Such a sensor practically monitors a one-dimensional strain field and can be installed over all the length of the monitored structural members, thereby providing with integrity monitoring, i.e. direct detection and characterization (including recognition, localization, and quantification or rating) of local strain changes generated by damage. Integrity monitoring principles are developed and presented in this article. A large scale laboratory test and a real on-site application are briefly presented.

The dynamic monitoring of civil structures such as buildings and bridges is traditionally approached using acceleration and velocity sensors. When a monitoring program is designed to address concerns related to the level of strain in certain members, fatigue or displacements due to extreme loads, it is sometimes advantageous to measure those parameters directly, instead of deriving them from acceleration or velocity data. Inhomogeneous construction materials such as concrete, stone or masonry require the use of long-gauge sensors to measure strain, since local sensors can provide erroneous data due to local material changes. On the other hand, acceleration and tilt sensors are very useful to capture the global deformations and displacements. The development of long-gauge-length fiber optic sensors can be considered as a useful addition to the toolkit of those interested in the structural dynamics. The system can monitor structures over long periods of time at acquisition frequencies up to 10 kHz and with sub-microstrain resolution. The possibility of obtaining static and dynamic measurements from the same sensor is another advantage of this technology. For acceleration and tilt monitoring, MEMS sensors are now recognized as a powerful and low-cost alternative to more traditional sensor types. Fully-integrated sensors with 3-axis accelerometers and two-axis tiltmeters are now available in a compact packaging that also contain all data acquisition electronics and can be easily deployed in new or existing structures, connecting them directly to an Ethernet network. This contribution briefly introduces fiber optic and MEMS sensing technologies and will than illustrate their application to structural health monitoring through several examples, including the new I35W bridge in Minneapolis USA, the Ile d’Orléans suspension Bridge in Québec Canada and the shaking-table test of a full-scale masonry building reinforced with composite materials.

Structural health monitoring is a process meant to provide accurate and real-time information concerning structural condition and performance. Needs for structural health monitoring in the past two decades increased rapidly, and these needs stimulated the development of various sensing technologies. Distributed optical-fiber sensing technology has opened new possibilities in structural monitoring. A distributed deformation sensor (sensing cable) is sensitive at each point of its length to strain changes and cracks. Such a sensor practically monitors a one-dimensional strain field and can be installed over the entire length of the monitored structural members (suspension cables, bridge girders, tunnel vaults, dam basis, etc.). Therefore, the sensor provides for integrity monitoring, that is, direct detection, characterization (including recognition, localization, and quantification or rating), and report of local strain changes generated by damage. An integrity monitoring principle for long bridges and tunnels is developed. Various distributed sensing techniques are summarized, and their potential for the use in integrity monitoring is compared. Finally, the first large-scale, actual on-site application is briefly presented.

Submillimeter crack is detected with a dedicated fiber-optic strain cable, a 1-m-spatial-resolution (w) distributed Brillouin sensor and an advanced signal processing technique. The signal processing approach consists in spectrum shape analysis and multiple peaks detection.

This paper presents the laboratory validation of a prototype optic-fiber instrumented structural element. The element is a reduced-scale reinforced concrete beam, of dimensions 3.8×0.3×0.5m that can be pre-stressed by an internal Dywidag bar. The sensing technology is based on a multiplexed version of the SOFO strain sensor, prepared in the form of a 3-field smart composite bar; in-line multiplexing is obtained by separating each measurement field through broadband FBGs. The experiment aims to identify the response of the sensors to differing damage conditions artificially produced in the element, including cracking and loss of prestressing. A numerical algo-rithm, based on Bayesian logic, is applied to real-time diagnosis: by processing the sensor meas-urements and prior information, the method assigns a posterior probability to each assumed damage scenario, as well as the updated probability distributions for each relevant structural parameter. With respect to classical damage detection approaches, the merit of those based on Bayesian logic is to provide not only information on the damage, but also the degree of confidence in this informa-tion. The paper discusses the ability of the system to identify the differing damage conditions. The reported test clearly shows that an occurrence such as a loss of prestressing can be recognized early with a high degree of reliability based on the strain data acquired.

The paper describes the characteristics of a monitoring system installed on a ca- ble-stayed bridge recently constructed in the Port of Venice. The system is based on the use of the SOFO™ fiber optic sensor family and it has been conceived for both static long-term and periodic dynamic monitoring. The monitoring system has been installed during construction, in order to acquire control over the most significant construction phases, and for permanent static monitoring. Linear deformation sensors and their locations have however been selected in order to allow the execution of periodic dynamic measurements and identification of the structural characteristics. The first analyses of the data collected during the final stages of the construction and just after construction completion are presented. The characteristics of the algorithms that have been selected to interpret the monitoring data are also discussed. The Port Authority of Venice, in the framework of the development of the container and multi- purpose terminals of the Marghera basin, situated at the inner edge of the Venice Lagoon, have decided the construction of a new road link between the national highway system and the port areas. The road link is crossing the "West Industrial Channel" and the railways serving the ter- minals, thus requiring the construction of a long-span bridge. An international competition has been launched for the design of the bridge and related ac- cess viaducts. The preferred design was including a cable-stayed bridge formed by a composite steel and reinforced concrete beam, continuous over two spans of 105 m and 126 m in length, respectively. The bridge axis is a circular segment of 175 m radius. The deck on each of the two spans is supported by 9 cables, composed by 31 to 85 strands, attached to a reinforced concrete

Optical fiber sensor (OFS) technologies have developed rapidly over the last few decades, and various types of OFS have found practical applications in the field of civil engineering. In this paper, which is resulting from the work of the RILEM technical committee “Optical fiber sensors for civil engineering applications”, different kinds of sensing techniques, including change of light intensity, interferometry, fiber Bragg grating, adsorption measurement and distributed sensing, are briefly reviewed to introduce the basic sensing principles. Then, the applications of OFS in highway structures, building structures, geotechnical structures, pipelines as well as cables monitoring are described, with focus on sensor design, installation technique and sensor performance. It is believed that the State-of-the-Art review is helpful to engineers considering the use of OFS in their projects, and can facilitate the wider application of OFS technologies in construction industry.

A full-scale on-site test represents an ideal way to check a hypothesis and to determine the real behavior of structures, especially in cases in which some uncertainties cannot be reduced otherwise. To perform the test successfully it is necessary to monitor the parameters that representatively describe the structural behavior. In the case of piles, axial compression, pullout, and flexure tests cover all load combinations that may appear in service. To assess the foundation performance at a semiconductor production facility, two sets of piles with three piles in each set were tested. The monitored parameters were average strains, registered in several segments over the whole length of each pile using long-gauge fiber optic sensors. This type of sensor, combined in appropriate topologies, gives rich information concerning the piles’ behavior and soil properties. The monitoring method is presented and its performances through the results of the tests are discussed. This method allowed the determination of the Young modulus of the piles, the occurrence of cracks, the normal force distribution, and the ultimate load capacity in the case of axial compression and pullout tests, as well as the curvature distribution, horizontal displacement, deformed shape, and damage localization in the case of the flexure tests. Moreover, the pile–soil friction distributions, the quality of soil, and the pile tip force were estimated. The advantage of the presented method resides in the use of long-gauge sensors, which are insensitive to local structural defects like crack openings or air pockets and allow the collection of data on a global structural level and not on a local material level.

Over the past decades, several major industrial accidents led the chemical industries handling large quantities of dangerous substances and national regulation bodies to reinforce the safety and prevention measures of their installations, in compliance with local laws such as the Seveso II directive in Europe. Indeed, leakages of chemicals can be at the origin of toxic releases, which can have severe consequences on the installations as well as on the environment and nearby inhabitants. Industries are prompted to take all possible measures to reduce the occurrence of such catastrophic events by implementing additional technical safety barriers in order to prevent or mitigate any potential danger on their key structures such as pipelines, reactors, storages, transfer lines, etc. Pipeline leakages may have different origins, such as corrosion, fatigue, material flaws, shocks, abnormal temperatures, extreme pressures, or excessive deformations caused by ground movement. In the case of liquefied or pressurized gases, leakages can be detected by the rapid drop of temperature due to the evaporation of the released liquid and its evaporation gases or due to gas expansion. These local thermal anomalies can be reliably detected by a fiber-optic distributed temperature sensing system able to detect temperature changes of the order of 1°C, with 1m spatial resolution and 10s response time. A fiber optic cable is installed all along the whole length of the pipeline and is connected to a measurement system that can automatically detect temperature anomalies which are telltale of leakages and generate an alert to initiate appropriate response actions on the affected pipeline section. Such a system has been permanently deployed at several industrial and chemicals sites were functional and operational tests have also been carried out. This paper will present the system architecture and installation at an ammonia production, storage, shipping and processing site. Results of simulated leakage detection tests on ammonia pipeline and long-term operation in normal conditions will also be presented.

In many bridges, the vertical displacements are the most relevant parameters to be monitored in both the short and long term. Current methods (such as triangulation, water levels or mechanical extensometers...) are often tedious to use and require the intervention of specialized operators. The resulting complexity and costs limit the temporal frequency of these traditional measurements. The spatial resolution obtained is in general low and only the presence of anomalies in the global structural behavior can be detected and warrant a deeper and more precise evaluation. To measure bridge vertical displacements at low cost and frequently in time, one solution consists of installing a network of fiber optic sensors during concrete pouring or installing them on the surface of the structure. By subdividing the whole structure into structural elements and those elements into cells that are analyzed by the sensors, it is possible to obtain information about the average cell deformation (e.g., mean curvature) that can then be combined to obtain the global structural displacement field. In 1996, a concrete highway bridge near Geneva (Switzerland) was instrumented with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30-cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the interaction between the old and the new concrete, we choose low-coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor the long term deformations of the bridge. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The vertical and horizontal curvatures of the bridge are measured locally at multiple locations along the bridge span by installing sensors at different positions in the girder cross-section. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformations of the bridge. This measurement methodology was also applied to the Lutrive Highway Bridge in Switzerland in order to measure the variation in vertical bridge displacements due to a static load test. The results obtained using the low coherence interferometric sensors of the SOFO system were then compared with the displacements obtained through an optic leveling system. In the case of this cantilever bridge of 60 meters half-span equipped with 30 fiber optic sensors, a discrepancy of less than 7% was obtained between the two measuring systems.

All structures undergo deformations under the effects of loads or degradation of the constituent materials. The deformations of any structure (bridges, dams, frames, shells, tunnels, towers, wings, trusses,) contain a lot of information about its health state. By measuring these deformations it is possible to analyse the loading and aging behavior of the structure. The presented method analyses a structure by subdividing it into sections and cells. The deformation of each of these macro-elements is first analysed separately to obtain local information about the materials, and then combined to provide insight on the global behavior. Examples of these techniques applied to civil engineering structures fitted with long-gage-length fiber optic sensors show the variety of information that can be obtained using this powerful analysis technique.

This contribution illustrates an application of Bayesian logic to monitoring data analysis and structural condition state inference. The case study is a cable-stayed bridge 260 m long spanning the Adige River ten kilometers north of the town of Trento, Italy. It is a statically indeterminate structure, consisting of a steel-concrete composite deck, supported by 12 stay cables. Structural redundancy, possible relaxation losses and an as-built condition differing from design, suggest that longterm load redistribution between cables can be expected. To monitor load redistribution, the owner decided to install a monitoring system that combines built-on-site elasto-magnetic and fiber-optic sensors. In this article, we discuss a rational way to improve the accuracy of the load variation, estimated using the elasto-magnetic sensors, taking advantage of the fiber-optic sensors information. More specifically, we use a multi-sensor Bayesian data fusion approach, which combines the information from the two sensing systems with the prior knowledge including design information and outcomes of laboratory calibration. Using the data acquired to date, we demonstrate that combining the two measurements allows a more accurate estimate of the cable load, to better than 50 kN.

This paper illustrates an application of Bayesian logic to monitoring data analysis and structural condition state inference. The case study is a 260 m long cable-stayed bridge spanning the Adige River 10 km north of the town of Trento, Italy. This is a statically indeterminate structure, having a composite steel-concrete deck, supported by 12 stay cables. Structural redundancy, possible relaxation losses and an as-built condition differing from design, suggest that long-term load redistribution between cables can be expected. To monitor load redistribution, the owner decided to install a monitoring system which combines built-on-site elasto-magnetic and fiber-optic sensors. In this note, we discuss a rational way to improve the accuracy of the load estimate from the EM sensors taking advantage of the FOS information. More specifically, we use a multi-sensor Bayesian data fusion approach which combines the information from the two sensing systems with the prior knowledge, including design information and the outcomes of laboratory calibration. Using the data acquired to date, we demonstrate that combining the two measurements allows a more accurate estimate of the cable load, to better than 50 kN.

This paper reports on experimental and theoretical investigations into the use of optical fiber sensors in the monitoring of civil engineering structures. For this purpose a highly accurate optical fiber deformation sensor based on low coherence interferometry has been developed. This system was conceived in order to monitor a test concrete slab over a period of years. It enabled the measurement of both concrete shrinkage and elastic coefficient in the whole slab with good reproducibility, in agreement with measurements performed on small concrete samples and with theoretical calculations. The thermal effects on the concrete structures are also presented.

The evaluation of the structural performance of marine structures, such as ship hulls, off-shore platforms and risers requires the monitoring of the static and dynamic strain levels undergone during the whole lifetime. In these environments, the use of passive fiber optic sensors presents advantages in terms of reliability and multiplexing ability. Frequently used structural materials, such as steel and composites, exhibit local defects or discontinuities, such as welds, thickness / diameter variations, marine growths and cracks, introducing discontinuities in the mechanical properties of the material at a local level. Yet, the properties of the material at a global level are more indicative for structural behavior. Therefore, for structural monitoring purposes, it is necessary to use sensors that are insensitive to local material discontinuities. A long-gauge strain or deformation sensor, by definition, is a sensor with a gauge-length several times larger than the maximal distance between discontinuities or the maximal diameter of defects in the monitored material. In this paper we will present the design, testing and applications of a long-gauge fiber optics strain sensor for underwater applications.

In the last 15 years, fiber optic sensing has become a useful and increasingly widely used tool for structural health monitoring of bridges and other civil structures. We have been fortunate to participate to their development and introduction to real field applications. This paper is an overview of 40 bridge monitoring projects carried out over the last 15 years in 13 different countries. In particular we concentrate on the analysis of the different types of bridges that were monitored, their situation (new construction, existing structure, refurbishment...) and the main purpose of the installed monitoring system. Two main categories emerge form this analysis: new bridges with innovative aspects or particular relevance and existing bridges with known deficiencies. We will than analyze the most typical sensor architectures used to monitor those bridges, giving statistics on the number and type of installed sensors, their survival rate and the duration of the monitoring project. For some projects, the monitoring concentrated on a specific event in the life of the bridge, for example its construction or refurbishment. Other projects aimed to longterm monitoring and are still running, in some cases after almost 10 years. In the early applications of fiber optic technology the cost of the measurement instruments and their fragility were discouraging permanent monitoring and many projects called for periodic manual measurements. In the last 5 years, significant progresses in the instrument reliability and lower cost have enabled a growing number of permanent instrumentations. Another interesting aspect concerns the entities involved in each monitoring project. We will analyze weather the project originated form a research entity (university, research institute) or was driven more by the bridge owners. Finally we will summarize the main findings of each project and show concrete examples of actionable information that could be gained thanks to monitoring.

Sub-millimeter crack is detected with a dedicated fiber optic strain cable, a 1 m spatial resolution (w) distributed Brillouin sensor (DBS) and an advanced signal processing technique.

This paper introduces a concept of smart structural elements for the real-time condition monitoring of bridges. These are prefabricated reinforced concrete elements embedding a permanent sensing system and capable of self-diagnosis when in operation. The real-time assessment is automatically controlled by a numerical algorithm founded on Bayesian logic: the method assigns a probability to each possible damage scenario, and estimates the statistical distribution of the damage parameters involved (such as location and extent). To verify the effectiveness of the technology, we produced and tested in the laboratory a reduced-scale smart beam prototype. The specimen is 3.8 m long and has cross-section 0.3 by 0.5m, and has been prestressed using a Dywidag bar, in such a way as to control the preload level. The sensor system includes a multiplexed version of SOFO interferometric sensors mounted on a composite bar, along with a number of traditional metal-foil strain gauges. The method allowed clear recognition of increasing fault states, simulated on the beam by gradually reducing the prestress level.

One of the advantages of fiber optics with respect to traditional electrical gauges is they can act both as sensors and as a pathway for signals produced by other sensors. This feature allows adoption of a simple sensor system architecture, even when arrays of many sensors are needed. In this paper, we present the development and laboratory validation of in-line multiplexing for the low-coherence interferometric SOFO standard deformation sensor. A standard SOFO, as developed, produced and commercialized by Smartec SA, employs total reflectors at the end of the measurement and the reference fibers, allowing measurement of the strain over a single field. In the solution presented, broadband Fiber Bragg Gratings (FBGs) are employed as partial reflectors to obtain in-line multiplexing. These FBGs, presenting a 5% reflectivity in their reflection spectra, are produced using a chirped phase mask. An experiment was carried out on a 3-field sensor, to investigate effectiveness, resolution and temperature sensitivity. Outcomes show clear fringe visibility for each pair of gratings and a resolution of about 2.5 mm RMS, of the same order as the single field sensor. Issues regarding the maximum number of measurement fields on a single line and deformation range limits are discussed.

Gotaalvbron, the bridge over Gota river, was built in thirties and is now more than seventy years old. The steel girders were cracked and two issues are in cause of steel cracking: fatigue and mediocre quality of the steel. The bridge authorities repaired the bridge and decided to keep it in service for the next fifteen years, but in order to increase the safety and reduce uncertainties related to the bridge performance an integrity monitoring system has been mandatory. The main issue related to selection of the monitoring system has been the total length of the girders which is for all the nine girders more than 9 km. It was therefore decided to monitor the most loaded five girders (total length of 5 km approximately) and logically a fiber optic distributed sensing system have been selected. For the first time a truly distributed fiber optic sensing system, based on Brillouin scattering effect, is employed on such large scale to monitor new crack occurrence and unusual strain development. The monitoring system itself, the monitoring strategy, challenges related to installation and the data management are presented in this paper.

Composite coiled tubing is an emerging technology in the oil & gas sector that presents important advantages compared to the steel coiled tubing and conventional drilling. The composite tube has reduced weight, allowing extended reach and improved fatigue life. An additional advantage resides in the fact that the coiled tube wall can contain and protect additional functional elements, such as electrical conductors and fiber optics for sensing and data communication. Sensing systems based on Brillouin and Raman scattering can be used to verify the pipe operational parameters, prevent failure, optimize oil production from the well, provide strain distribution along the tubing and detect hot-spots in highpower cables. The integration of such sensing elements into composite tubing presents additional advantages and challenges. On one hand the embedded sensors are protected by the composite material and can be installed during production, avoiding external installation that could interfere with the tubing operations. In the other hand, the integration of optical fiber sensors into the composite structure requires the development of appropriate packaging and installation techniques that allow easy handling during production and avoid and damage to the sensor and the composite structure itself. This contribution presents the sensing cable designs for temperature and strain sensing in a composite coiled tubing as well as testing results form initial field demonstrations.

Fiber optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single fiber is particularly interesting for the monitoring of elongated structures such as pipelines, flow lines, oil wells and coiled tubing. Distributed sensing systems based on Brillouin and Raman scattering are used for example to detect pipeline leakages, verify pipeline operational parameters, prevent failure of pipelines installed in landslide areas, optimize oil production from wells and detect hot-spots in high-power cables. Point sensors based on Interferometric and FBG setups are also effective tools to assess the static and dynamic response of structures such as offshore platforms and risers.This contribution presents different applications of distributed and point sensors to innovative oil and gas structures such as composite coiled tubing, high-pressure composite gas pipeline and deepwater risers.

Distributed fiber optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single fiber is particularly interesting for the monitoring of large structures such as pipelines, flow lines, oil wells, dams and dikes. Sensing systems based on Brillouin and Raman scattering have been used for example to detect pipeline leakages, verify pipeline operational parameters, prevent failure of pipelines installed in landslide areas, optimize oil production from wells and detect hotspots in high-power cables. The measurement instruments have been vastly improved in terms of spatial, temperature and strain resolution, distance range, measurement time, data processing and system cost. Analyzers for Brillouin and Raman scattering are now commercially available and offer reliable operation in field conditions. New application opportunities have however demonstrated that the design and production of sensing cables is a critical element for the success of any distributed sensing instrumentation project. Although standard telecommunication cables can be effectively used for sensing ordinary temperatures, monitoring high and low temperatures or distributed strain present unique challenges that require specific cable designs. This contribution presents three cable designs for high-temperature sensing, strain sensing and combined strain and temperature monitoring.

Since the hydrocarbon reservoirs in Europe are rapidly depleting, there is a need for new 'intelligent' technology for the efficient commercial exploitation of the remaining hydrocarbon reservoirs: an intelligent and high-temperature, corrosion and fatigue resistant thermoplastic composite coiled tubing. This paper describes the polarimetric Fibre Optic Sensor and accompanying advanced transient signal detection techniques for an early detection of damage in this new type of intelligent composite tubing built for drilling and exploiting hydrocarbon reservoirs. The capabilities of the new optical system for detecting growing damage are demonstrated by carrying out bending tests on short test models of the tube.

With increased interest in the use of compressed gas as a vehicle fuel, attention has been focussed on the safety issues surrounding the tanks used to store the fuel. Currently it is necessary to remove the tanks from the vehicle in order to inspect them, which entails a considerable cost in manpower and takes the vehicle being out of service. We have been developing a sensor scheme that can provide in situ monitoring of the tanks condition. This entails bonding optical fibre sensors to the tank and using them to measure the strains experienced by the tank during pressurisation. If the tank is significantly damaged, then the tank will expand in a distorted manner. We therefore measure the strain characteristics of a healthy tank and use them as a reference for future measurements. The method of strain measurement is the well established rf subcarrier phase detection technique, however in this application the changes in optical power caused by microbending of the fibres during pressurisation produces inaccuracies. In order to overcome this problem we use both in- phase and quadrature mixing and then take the ratio of the outputs to obtain a value of arctangent that is independent of amplitude.

®sensors have been in use for the last 10 years for the monitoring of civil, geotechnical, oil & Fiber optic sensing systems are increasingly recognized as a very attractive choice for structural health monitoring. Moving form demonstration project to industrial applications requires an integrated approach where the most appropriate technologies are combined to meet the user’s requirements. In this context it is often necessary and desirable to combine different sensing technologies in the same project. A bridgemonitoring project might for example require long-gauge interferometric sensors to monitor the concrete deck, interferometric inclinometers for the piles and fiber Bragg grating sensors for the monitoring of the strains in the steel beams and for measuring temperatures. Although fiber optic sensors relying on different technologies can easily be combined at the packaging and cable levels, they often require dedicated instruments to be demodulated. A unified demodulation system would therefore be very attractive. This paper describes a technique relying on the analysis of reflected spectra and allowing the demodulation of interferometric (Michelson or Faby-Perot) sensors and fiber Bragg grating sensors with a single measurement system. It also compares the obtained performance in terms of resolution and dynamic range with the available dedicated systems.

Two goals are pursued in this paper. The first goal consists of comparing the performance of the innovative SOFO dynamic system, which uses long-gauge fiber-optic sensors, with the traditional monitoring method based on accelerometers. For this purpose, a dynamic laboratory test was carried out, and measurements were taken from a single-storey threedimensional steel frame model excited at the base by a shaking table. The SOFO dynamic system was installed on one column of the frame structure, while two accelerometers were mounted on the base and on the frame storey, respectively, for comparison. The use of fiber-optic sensors allows to overcome the difficulties associated with the traditional dynamic measurement methods, such as the limitations in the number and in the locations of the monitoring devices. Furthermore, the long-gauge fiber-optic strain sensors show a very high sensitivity and extend the frequency range (1mHz-1KHz). The second goal is to investigate the sensitivity to local damage of a recently proposed method for damage detection and localization. Indeed, the use of better performing long-gauge strain sensors allows the detection of local damage that is hardly visible in the global response of the structure. Damages of increasing intensities are therefore gradually introduced in the structure, and the measurements acquisition is repeated for each of the damaged cases. The SHM-RSM method, which is based on the idea of using a response surface model to approximate the relationship between the measurements collected by different sensors during the same test, is finally applied to the collected data to detect and locate the damages of different intensities.

Owners must manage and ensure the safety of their civil structures even as use of many structures extends well beyond their design lifetime. Traditionally, most structures rely on strict maintenance procedures, visual inspections, and very few sensors. But maintenance is very expensive, visual inspections can miss critical problems, and conventional sensors can fail in harsh environments. Can fiber-optic sensing (FOS) address these issues? This is not a new question, but there are some new answers. This paper highlights several structures where FOS is used, and describes the associated successes and challenges for each application. Many successes are coupled to improved FOS tools: better sensor packages, simpler and less expensive instrumentation, improved installation techniques, and more efficient data analysis tools. Examples of each are provided. Particular attention is given to the economics of instrumenting civil structures - when and how it pays. Conclusions include recommendations for future developments that will further accelerate FOS acceptance and use.

The Compressed Natural Gas (CNG) used as a carburant in automotive industry offers low cost and notably less pollution. Full composite tank used to store the CNG onboard features low weight and extended lifespan. However, the safety issues and maintenance fees remain a challenge for its use in ordinary cars. The structural health monitoring of tanks with accent to damage detection can significantly increase the safety and decrease the maintenance fees. Structural health monitoring and damage detection of composite tanks impose important challenges to the monitoring strategy and monitoring system to be used. The issues of non -intrusive installation of sensors, their topologies and network, and particularly analysis and interpretation of resulting data are very complex. The long-gage interferometric sensors of SOFO type, for direct embedding in the full composite tank during production are developed. The sensor consists of single mode optical fiber embedded into the very thin composite tape. Such packaging offers to optical fiber excellent protection during handling and embedding and makes sensor non-intrusive to the tank material. Appropriate topologies of the sensors are combined in single sensor network used to monitor strain state and damage. The results of monitoring are analyzed at several levels, and the damage is detect ed using algorithms combining the global deformation and changes in both the tank stiffness and sensors cross-correlation. The monitoring strategy, sensors used in full composite tank monitoring, installation issues and the results of the structural health monitoring performed in laboratory are presented in details in this paper.

Thermoplastic and thermoset fiber-reinforced composite materials are well established in aerospace engineering, but also more and more used in the oil and gas industry as well as in civil engineering. In these applications they are mainly used to reinforce, repair or straighten existing structures, but recently full-composite structures have also been built. Independently from the domain of the use, there is a need for these composite structures to be monitored. Since the composite materials are usually applied in form of thin tapes or sheets, sensors have to be embedded within the structure, depending on structural layer that has to be monitored. Embedding the sensors may have as a consequence a significant decrease of the mechanical properties of the composite material due to the dimensions of the sensor. The solution presented in this paper is integration of a fiber optic sensor directly into the main composite component, i.e. into the composite tape. In this paper we present the development of a thermoplastic fiber-reinforced composite tape with integrated long-gage fiber-optic sensors. The fiber-optic sensors are selected due to small transversal dimension and good compatibility with the plastic materials. The tape with integrated optical fiber can be used for tape winding of a structural element, embedded between different layers, but also as a separate sensor – a sensing tape. The optical and mechanical properties of the tapes with sensor are tested. The sensing tape is then installed onto the rail along with standard long-gage fiber optic sensor, additional tests are performed and performance of both sensor compared. The integration of optical fiber into the composite tape, the results of the tests as well as the performances of the tape with integrated optical fiber are presented in this paper.

The introduction of long-gauge fiber optic sensors has opened new opportunities for the monitoring of large structures. Using these sensors it is possible to observe the local elongation of a structure and calculate its global deformations. The use of internal sensors detecting integrated strain is however unable to detect the rigid-body motion of a structure, relative to an external reference system. This would require the installation of long deformation sensors connecting the structure with external reference points. Another possibility is to use gravity as a reference to measure vertical rotations of the structure. A gravity-referenced rotation sensor is conventionally called an inclinometer and measures the angle variations between a nominally vertical reference line in the structure and the direction of gravity. It is possible to use uni-axial inclinometers or bi-axial ones, measuring rotations in the two perpendicular vertical planes. Today's inclinometers are based on a variety of techniques and can achieve sensitivities in the micro-radian range. When using fiber optic deformations sensors in a given structure, it would however be interesting to use the same technology to obtain inclination reading. This prompted us to develop a fiber optic-based inclinometer compatible with the SOFO system. This contribution describes the development of this novel sensor and the first measurements obtained with it.

A new central railway station - Lehrter Bahnhof - is being built in Berlin. Because of construction activities in immediate vicinity and because of difficult soil conditions, different vertical displacements have to be expected. In order to avoid damage to the bridges and to a widely spanned glass roof which will be supported by two concrete bridges these two bridges have to be monitored with regard to their deformation performance right from the beginning of construction until commissioning as well as later on for several years. For this purpose, a monitoring concept has been developed and sensors with excellent long-term stability have been chosen. This paper describes the system for monitoring settlements and heaves by means of laser-based optics and hydrostatic leveling. Additionally, strain and inclination of the prestressed concrete bridges are redundantly monitored by embedded long-gage length fiber-optic strain sensors as well as resistive strain gages, and inclinometers. Measurements on-site are referenced by measurements on two test beams well-defined loaded under laboratory and field conditions. The paper also describes the measuring concept and the sensor techniques as well as installation of the sensor system and first results.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The introduction of long-gauge fiber optic sensors has opened new opportunities for the monitoring of large structures. Using these sensors it is possible to observe the local elongation of a structure and calculate its global deformations. The use of internal sensors detecting integrated strain is however unable to detect the rigid-body motion of a structure, relative to an external reference system. This would require the installation of long deformation sensors connecting the structure with external reference points. Another possibility is to use gravity as a reference to measure vertical rotations of the structure. A gravity -referenced rotation sensor is conventionally called an inclinometer and measures the angle variations between a nominally vertical reference line in the structure and the direction of gravity. It is possible to use uni-axial inclinometers or bi -axial ones, measuring rotations in the two perpendicular vertical planes. Today’s inclinometers are based on a variety of techniques and can achieve sensitivities in the micro-radian range. When using fiber optic deformations sensors in a given structure, it would however be interesting to use the same technology to obtain inclination reading [1]. This prompted us to develop a fiber optic-based inclinometer compatible with the SOFO system [2]. This contribution describes the development of this novel sensor and the first measurements obtained with it.

This paper resumes the nine year long experience of our group in the installation of fiber optic sensors in the most diverse structure types, including bridges, tunnels, dams, piles, anchors, historical monuments, nuclear power plants and many others. To date, we have installed about 1'500 sensors in almost 70 different applications. Statistics show that, by proper handling, it is possible to achieve 95 - 100% survivability during installation and for many years afterward.

This paper gives an overview of the primary issues of structural health and evaluation monitoring for civil structures, such as bridges, dams, buildings and roadways, and role that fiber optic sensors play in the monitoring efforts. Some of the quantities that need to be measured are displacement, velocity, acceleration, jerk, force, stress, strain, temperature, fracture, moisture, fatigue, and chemical state, i.e. corrosion. Fiber optic sensors have the capability to measure most, if not all, of these quantities. Fiber optic sensors exploit a variety of physical principles through which physical quantities are measured. The particular types of fiber sensors that will be discussed in this paper are: intensity-based, modal domain interferometric, Bragg grating, white light interferometric, and Brillouin backscatter. The operating principles and application results from field and laboratory studies are presented.

In 1996, a concrete highway bridge near Geneva (Switzerland) was instrumented with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30-cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the interaction between the old and the new concrete, we choose low-coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor the long term deformations of the bridge. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The vertical and horizontal curvatures of the bridge are measured locally at multiple locations along the bridge span by installing sensors at different positions in the girder cross-section. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformations of the bridge. This paper presents the sensor network design and the measurements that were performed during the construction phases, during the bridge operation since it was reopened and under a recent static-loading test.

The security of civil engineering works demands a periodical monitoring of the structures. The current methods (such as triangulation, water levels, vibrating strings or mechanical extensometers) are often of tedious application and require the intervention of specialized operators. The resulting complexity and costs limit the frequency of these measurements. The obtained spatial resolution is in general low and only the presence of anomalies in the global behavior urges a deeper and more precise evaluation. There is therefore a real need for a tool allowing an automatic and permanent monitoring from within the structure itself and with high precision and good spatial resolution. In many civil structures like bridges, tunnels and dams, the deformations are the most relevant parameter to be monitored in both short and long-terms. Strain monitoring gives only local information about the material behavior and too many such sensors would therefore be necessary to gain a complete understanding of the structure's behavior. We have found that fiber optic deformation sensors, with measurement bases of the order of one to a few meters, can give useful information both during the construction phases and in the long term. In the case of beams and bridges, long-gate sensors can be used to evaluate the curvature variations and calculate the horizontal and vertical displacements by double integration of the curvatures.

In many concrete bridges, deformations due to static and dynamic loading are the most relevant parameters to be monitored in both the short and long term. Strain monitoring give only local information about the material behavior and too many of such sensor are therefore necessary to gain a complete understanding of the bridge evolution. Recent advances in measurement technology have demonstrated that optical fiber sensors are suitable for monitoring full-scale structures. A network of such sensors installed inside a bridges enables the measurement of parameters such as internal deformation and temperature. In the past four years, our laboratory has installed hundreds of fiber optic deformation sensors of varying seizes in concrete and composite steel-concrete bridges. These sensors give useful information during the construction phases and about the long-term geometrical deformations of a bridge under static load. Recently it has been found that these sensors can also be used to measure the quasi-static part of the dynamic deformation of a bridge under traffic load. The measurement technique relies on low-coherent interferometry and it guarantees high resolution, high precision ad long-term stability. Nevertheless each measurement takes a few seconds, thus it is inadequate for monitoring the dynamic behavior of bridges, where measurement frequencies of up to 1 kHz are necessary. This paper also presents a new technique to demodulate at a high frequency of up to 1 kHz are necessary. This paper also presents a new technique to demodulate at a high frequency the signal for the same sensors used for long-term monitoring. With its large dynamic range of several mm it allows for the monitoring of bridges under dynamic loads while maintaining the configuration of the original fiber optic sensor that has been proven to be reliable.

The serviceability of a bridge is generally analyzed by a comparison between the vertical deflections expected by the engineer and those measured during a load test or in the long term. The existing methods do not allow the determination of the vertical displacements from the measurements carried out by a network of deformation sensors placed inside the bridge. The mathematical model presented allows the determination of the displacement field from internal horizontal deformation measurements and helps in the design of the required sensor network. This model was tested on an experimental model and on the Lutrive Highway Bridge in Switzerland by comparing the changes in vertical displacements under daily temperature variations obtained with the proposed method, with those measured directly using an absolute hydrostatic leveling system. Fiber optic deformation sensors and electrical inclinometers were used to carry out the measurements. With this deformation monitoring system, featuring a precision of 10 micrometers on 1 m long deformation sensors, it is possible to retrieve the vertical displacement field of a beam with a global error less than 8%.

In 1996, our laboratory fitted a highway bridge near Geneva with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30 cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the behavior between the old and the new concrete, we choose low- coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor its long term deformations. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The curvature of the bridge is measured locally at multiple locations along the bridge span by installing sensors at different distances from the neutral axis. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformation of the bridge. The choice of the optimal emplacement of the sensor and the sensor network are also presented.

Civil structural monitoring by optical fiber sensors, require the development of reliable sensors that can be embedded or surface mounted in concrete, mortars, steel, timber and other construction materials as well as in rocks, soils and road pavements. These sensors should be rapid and simple to install in order to avoid any interference with the building site schedule and not to require specialized operators to accomplish the task. The sensors have to be rugged enough to withstand the harsh conditions typically found in civil engineering including, dust, moisture, shocks, EM disturbances and unskilled workman. It is also desirable that the instrumentation survives for tens of years in order to allow a constant monitoring of the structure aging. This contribution presents the results of a four-year effort to develop, test and industrially produce a palette of sensors responding to the above requirements and adapted to different applications and host materials. These sensors include a small version (length up to 2 m) adapted for embedding in mortars, grout and glues, an intermediate version of length between 20 cm and 6 m adapted to direct concrete embedding or surface installation and a long version adapted to measure large deformations (up to 2%) over length up to 30 m and especially adapted for geostructures monitoring.

The security of civil engineering works demands a periodical monitoring of the structures. The current methods (such as triangulation, water levels, vibrating strings or mechanical extensometers) are often of tedious application and require the intervention of specialized operators. The resulting complexity and costs limit the frequency of these measurements. The obtained spatial resolution is in general low and only the presence of anomalies in the global behavior urges a deeper and more precise evaluation. There is therefore a real need for a tool allowing an automatic and permanent monitoring from within the structure itself and with high precision and good spatial resolution. In many civil structures like bridges, tunnels and dams, the deformations are the most relevant parameter to be monitored in both short and long-terms. Strain monitoring gives only local information about the material behavior and too many such sensors would therefore be necessary to gain a complete understanding of the structure's behavior. We have found that fiber optic deformation sensors, with measurement bases of the order of one to a few meters, can give useful information both during the construction phases and in the long term.