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SOFO is a fiber optic sensor system that allows the monitoring of micrometer deformations over measurement bases up to a few meters. It is particularly adapted to measure civil structures built with conventional civil engineering materials (concrete, steel and timber). It has been successfully tested in different type of structures such as bridges, tunnels and piles. The application of the system is however limited in some case when unusual materials are used in the construction and in other cases by the dimensions of the standard SOFO sensors. To extend the domain of application of the current system, special sensors have been developed. In this paper we present four special SOFO sensors: long, membrane, thin and stiff sensors. The long sensor has a measurement basis of several tenths of meters and its purpose is the measurement of deformations in massive and large structures (dames, tunnels). The membrane sensor is for use on laminated materials (e.g. membrane roofing) and it is easy to install by simply gluing it to the structure to be monitored. Since standard sensors can not be used for thin mortar layers because of their cross-section, a thin sensor has been developed, too. Finally, the aim of the stiff sensor is to determine the hardening (solidification) time of concrete. This time is determined by comparing the deformations of a stiff and a standard sensor, closely placed in the concrete at the very early age. The design of these sensors is presented along with significant application examples.

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 the frame of the Forum project for the Swiss Expo2001 an experimental program on structural models is in progress. The structures of the Swiss Expo 2001 exhibition will be based on the Tensegrity concept. The behavior of these structures is highly geometrically non-linear and is relatively complicated to calculate and simulate. This pointed to the necessity of extensive testing and permanent monitoring of the structures. The tests are carried out on 1:10 scale models. In the paper the authors shows the application of fiber optic and whole field optical methods to control and monitoring the tests.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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-gage 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.

Introduction: An interesting and increasingly popular method of building steel and concrete composite bridges involves the use of a large steel based structure called a box-girder. These massive structures, which have dimensions on the order of 5m x 5m x 150m with wall thickness sometimes exceeding 5 cm, are assembled by welding together many shorter (e.g. 5m x 5m x 5 m) box-girder pieces near a bridge building site. The massive and long box-girder structure thus created, is then ‘pushed’ from one end of the bridge pier to the other. A concrete deck is eventually built on top of the box-girder to complete the bridge structure. This bridge building approach apparently holds advantages in the sense that it requires far fewer on-site workers and also allows for rapid building of large scale bridges over difficult terrain. One of the more interesting (and potentially dangerous) phases during the construction process of a box-girder bridges is the so-called ‘push’ phase. During this phase, the long box-girder structure, which prior to the push rests on a pier closest to land, is literally pushed from one bridge pile-on to the next, requiring it to travel a distance of over 100 meters. The far pile-on typically is built in a ravine or a deep valley over which the bridge is being built. During the push, the steel box-girder structure is subjected to static strains at various locations, depending upon the position of the box-girder at any given time. For instance, the far end of the box girder would be subject to cantilever motion, which may be driven into oscillations due to winds in the ravine. Monitoring the strain at various locations in the box-girder during the push phase of the bridge construction has potential benefits in future infra-structure modeling efforts as well as in determining construction safety factors. In addition, utilizing enabling technologies (e.g. fiber Bragg grating strain sensors) for strain monitoring during an actual construction phase of a real world structure allows determination of the maturity of: (i) the instrumentation system, (ii) sensor packaging, and (iii) installation approaches [1]. This work reports the results of a 32 element fiber Bragg grating (FBG) array used to monitor strain at various locations in box-girder vaux viaduct being constructed near Lausanne, Switzerland. The entire push phase of the box-girder bridge, which took about a day, was monitored using several FBGs whose responses were interrogated using the scanning FabryPerot (FP) method [2]. Preliminary quasi-static strain sensing data shows fine as well as gross details of the push phase. The sensors are planned to operate during many phases of the boxgirder bridge construction including the push, the concrete deck pouring, and eventually traffic monitoring. We report here on the push phase only.

The Lutrive highway bridges are two twin bridges built in 1972 by the cantilever method with mid-span articulations. The bridge deck consists of a box girder of variable depth. In 1997 this bridge was instrumented with 30 SOFO fiber optic deformation sensors installed inside the box girder. This sensor network is mainly aimed at measuring the short- and long-term spatial displacements of one of the spans. This includes the displacement resulting from the daily variations of temperature and the long-term creep effects. It was found that these same sensors could also be used to capture the quasi-static part of the dynamic deformation of the bridge under traffic load. Although the measurement system can acquire measurements only at intervals a few seconds apart, it was found that these 'snapshots' could give interesting information about the low frequency quasi-static deformations of the bridge. The data from pair of sensors was combined to obtain information about the instantaneous curvature variations of the bridge. This curvature data was then analyzed statistically to extract information about the dynamic traffic loads. This measurement and analysis method was validated in a fatigue test on a concrete slab.

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.

The monitoring of dams represents an important task in the management of hydroelectric systems. Their economic, social and environmental value imposes to know well the real behavior of the structure and its foundations. This paper shows in two practical cases the possibility to improve the quality of deformation measurements by an appropriate fiber optics sensor network. The first case is a study showing the technical and economical feasibility to install an extended, spatial fiber optics deformation sensor network to detect the relative deflection of an entire shell dam. At this purpose the theoretic study has been evaluated on the base of typical load situations with their effective deflections on the Schiffenen dam, a shell shaped concrete structure near Fribourg (Switzerland). The second case concerns the development and realization of two long fiber optics deformation sensors (30 m and 40 m) anchored in the rock to monitor the displacement of the dam relatively to its underground. These sensors have been installed in the Emosson shell dam (Switzerland).

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 Moesa railway bridge is a composite steel concrete bridge on three spans of 30 m each. The 50 cm thick concrete deck is supported on the lower flanges of two continuous, 2.7 m high I-beams. The bridge has been constructed alongside an old metallic bridge. After demolishing this one, the new bridge has been slid for 5 m by 4 hydraulic jacks and positioned on the refurbished piles of the old bridge. About 30 fiber optic, low-coherence sensors were imbedded in the concrete deck to monitor its deformations during concrete setting and shrinkage, as well as during the bridge sliding phase. In the days following concrete pour it was possible to follow its thermal expansion due to the exothermic setting reaction and the following thermal and during shrinkage. The deformations induced by the additional load produced by the successive concreting phases were also observed. During the bridge push, which extended over six hours, the embedded and surface mounted sensors allowed the monitoring of the curvature variations in the horizontal plane due to the slightly uneven progression of the jacks. Excessive curvature and the resulting cracking of concrete could be ruled out by these measurements. It was also possible to observe the bridge elongation under the heating action of the sun.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

A large-scale lifetime building monitoring program was implemented in Singapore in 2001. The monitoring aims of this unique program were to increase safety, verify performance, control quality, increase knowledge, optimize maintenance costs, and evaluate the condition of the structures after a hazardous event. The first instrumented building, which has now been monitored for more than ten years, is presented in this paper. The long-gauge fiber optic strain sensors were embedded in fresh concrete of ground-level columns, thus the monitoring started at the birth of both the construction material and the structure. Measurement sessions were performed during construction, upon completion of each new story and the roof, and after the construction, i.e., in-service. Based on results it was possible to follow and evaluate long-term behavior of the building through every stage of its life. The results of monitoring were analyzed at a local (column) and global (building) level. Over-dimensioning of one column was identified. Differential settlement of foundations was detected, localized, and its magnitude estimated. Post-tremor analysis was performed. Real long-term behavior of concrete columns was assessed. Finally, the long-term performance of the monitoring system was evaluated. The researched monitoring method, monitoring system, rich results gathered over approximately ten years, data analysis algorithms, and the conclusions on the structural behavior and health condition of the building based on monitoring are presented in this paper.

In many concrete bridges, 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 bridge 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 first days after concrete pouring and in the long term. In a first phase it is possible to monitor the thermal expansion due to the exothermic setting reaction and successively the thermal and drying shrinkages. Thanks to the long sensor basis, the detection of a crack traverse to the measurement region becomes probable and the evolution of cracks can therefore be followed with a reduced number of sensors. In the long-term it is possible to measure the geometric deformations and therefore the creeping of the bridge under static loads, especially under its own weight. In the past two years, our laboratory has installed hundreds of fiber optic deformation sensors in more than five concrete, composite steel-concrete, refurbished and enlarged bridges (road, highway and railway bridges). The measuring technique relies on low-coherence interferometry and offers a resolution down to a few microns even for long-term measurements. This contribution briefly discusses the measurement technique and then focuses on the development of a reliable sensor for direct concrete embedding and on the experimental results obtained on these bridges.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new displacement monitoring system based on low coherence interferometry using standard telecommunication fibers is presented. The measuring system is especially designed for the needs and conditions encountered in the field of civil engineering. The system features a precision of 10 micrometers to 100 micrometers (depending on the sensor type) over a measuring length up to 100 m, an operational range of 70 mm, stability over long periods (at least two years) and insensitivity to aging of the fiber and connector losses. The results of laboratory and field tests show that fiber optic extensometers are promising tools for the long term monitoring and survey of civil engineering structures. The theory and the optical set-up are briefly described as well as different sensor types that have been developed for various applications (bridges, tunnels, retaining walls, piles, anchorage cables) and tested in the field and in the laboratory. To validate the technique and calibrate the sensors laboratory testing has been done for short sensors (0.5 - 2 m) at IMM and for long sensors (2 - 10 m) at LMS. The results of these tests are presented and discussed in detail. IMM made several tests with fibers embedded in concrete beams and slabs measuring deformations due to shrinkage, creeping, and imposing traction, pressure and bending to the structure. Field tests on a rehabilitated bridge are also presented. Using a 10 m long calibration bench, LMS tested several fiber set-ups using different fixation techniques and imposed displacements on groups of extensometers with different lengths.

Civil smart structures often require displacement sensors with measurement bases between a few centimeters and a few meters with a precision of the order of 1/100 mm. Low-coherence interferometry offers these performances even for long-term measurements. Being a non- incremental setup, it does not require an uninterrupted monitoring. The main drawback of this technique resides in the fact that a separate sensor is required for each section to be measured. A typical civil structure such as a bridge requires up to 50 sensors for each span, so the complexity for this type of instrumentation and the number of connections often limit its large- scale application. It would be interesting to subdivide the fiber sensor in domains that can be measured separately but have a single lead-out connection. This contribution presents an in- line multiplexing scheme for displacement sensors based on low-coherence interferometry and partial reflectors installed in pairs along the sensing fibers. The multiplexing of up to ten displacement sensors along the same fiber line is demonstrated theoretically and experimentally. Different types of partial reflectors are also compared. The special case of structures that are constructed in sections is also analyzed.

1. ABSTRACT Our laboratories have developed a measurement system called SOFO, based on low -coherence interferometry in singlemode optical fibers and allowing the measurement of deformations of the order of 1/100 mm. This system is especially useful for the long-term monitoring of civil structures such as bridges, tunnels, dams and geostructures. The SOFO system requires the installation of two fibers in the structure to be monitored. The first fiber should be in mechanical contact with the structure in its active region and follow the structure deformation in both elongation and shortening. The second fiber has to be installed freely in a pipe near the first one. This fiber acts as a reference and compensates for the temperature dependence of the index of refraction in the measurement fiber. This contribution presents the design process as well as the lab and field tests of a sensor responding to these requirements and adapted to the installation in concrete structures. The active region can be between 25 cm and 8 m in length, while the passive region can reach al least 20 m. While the reference is free, the measurement fiber (installed in the same pipe) is pre -stressed between two glue-points at each end of the active region. The glue was chosen in order to avoid any creeping problems even at temperatures up to 160°C and elongation up to 2%. The sensor was tested in laboratory and field conditions. The lab tests included survival to concreting, high temperatures, freezing, thermal cycling, vibrations, cracking and corrosion; response to elongation and compression, measurement range and creeping of the glue points at high temperatures and high tensions. The field tests included installation of a number of these sensors in a bridge deck and in a tunnel vault. In these applications we tested the ease of use, the rapidity of installation and the survival rate.

The evolution of the vertical displacements is considered to be a good criterion to assess the condition of a bridge. Most embedded deformation sensors (e.g. inductive sensors, optical fiber sensor...) give the relative displacement between two points in the same structure. Therefore, the vertical displacements of a bridge in an earth-bound coordinate system are not directly supplied by the deformation sensors. This paper presents an algorithm to determine the vertical displacement from internal deformation measurements in the bridge. It is shown that 6n deformation sensors are necessary to draw the exact vertical displacement of a bridge uniformly loaded on n plus 1 supports. A load test has been carried out on a 6 m long timber beam on 3 supports, instrumented with 12 fiber optic deformation sensors. The vertical displacements calculated by the mathematical model on the base of the horizontal measurements are found to be in good agrement with the ones obtained by external dial gages.

In 1995, our laboratory fitted a highway bridge near Lausanne (Switzerland) with low- coherence fiber optic deformation sensors. The engineers, who had designed the steel-concrete composite bridge, were interested in the strain distribution inside the concrete slab and in the effects induced by the concrete shrinkage. More than 30 fiber optic deformation sensors, a few vibrating string deformation sensors, thermoelectric couples as well as resistive strain gages were installed in the concrete deck and on the steel girders of this bridge. Three phases of the bridge life were monitored: concreting and thermal shrinkage, load test with heavy trucks and long term deformations. This contribution presents the fiber optic sensor design, the installation technique and the preliminary results obtained on this bridge.

Long-gauge deformation sensors have opened new possibilities for the health monitoring of civil engineering structures. They are particularly suitable for applications in structures built of inhomogeneous materials, such as concrete, and with complex strain fields, such as bridges, buildings, dams, whenever the global structural behaviour assessment is of interest. Different technologies and measurement principles have been developed for measuring average strains over measurement bases that can reach tens of meters with resolutions in the micrometer range. In this work, the performances of seven commercially available alternative solutions, based on fibre Bragg-grating, Fabry-Perot interferometry, stimulated Brillouin scattering, low-coherence interferometry and traditional vibrating-wire technology, were tested and directly compared both in laboratory and in field conditions. The results are presented and discussed, aiming at the assessment of the main characteristics of each technology, and taking into account the principal requirements of in-field civil engineering applications. The efficiency of a monitoring method based on long-gauge sensors is illustrated through an application at the Ricciolo Viaduct in Switzerland.

A deformation monitoring system, based on low-coherence interferometry in standard telecommunication fibers embedded in concrete structures, is presented. This measuring system, developed for the needs of civil engineering, features a precision of 10 micrometers over measuring length of up to 100 m. Extensive tests have been carried out on a number of 1 m X 5 m X 0.5 m high-performance reinforced concrete ties, within a research on seepage flow through cracked elements. The presented system allowed the internal measurement of the deformations during the whole setting process (including the thermal expansion phase), of the deformations during the tensile test and of the crack openings evolution. This paper presents the operating principle of the low- coherence interferometer, the lessons learned in installing optical fibers in concrete structures, as well as a comparison with the measurements performed by standard sensors installed on the structure surface.

Civil underground works offer good opportunities for fiber optic displacement sensors. This communication presents an overview of possible applications in this field. Low-coherence interferometry appears to be the ideal technique for the long-time monitoring of such structures. The principle of the portable measuring system developed at our institute and based on this technique is presented along with different possible multiplexing techniques. The presented experimental results confirm the precision of 10 μm found under laboratory conditions and the immunity to drift for at least one year. A description of the techniques used to install optical fibers in anchorage cables and piles, as well as the lessons learned from these field tests are also presented.

A measuring system adapted to the needs of civil engineering structural monitoring is presented. It is based on low coherence interferometry in standard singlemode fibers and has a resolution of 5 micrometers , an operational range of 7 cm, stability over long periods (at least 1 year) and insensitivity to variations of the fiber losses. The portable reading unit doesn't need to be connected to the sensing fibers permanently and can thus be used to monitor multiple structures reducing the costs of the instrumentation. This paper presents the operating principle of this system and different techniques used to install the sensing fibers in large concrete structures, timber and mixed timber-concrete structures as well as on metals.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An optical-fiber deformation sensor with a resolution of 10 μm and an operational range of 60 mm has been realized. The system is based on low-coherence interferometry in standard single-mode telecommunication fibers. It allows the monitoring of large structures over several months without noticeable drift. No continuous measurement is needed and the system is insensitive to variations of the fiber losses. This technique has been applied to the monitoring of a 20 m × 5 m × 0.5 m, 120 ton concrete slab over six months. It is possible to measure the shrinkage of concrete and its elastic coefficient during pre-straining, giving reproducible results in good agreement with theoretical calculations and measurements performed on small concrete samples. This paper describes the optical arrangement and the procedures used to install optical fibers in concrete.

In this contribution, we use a Bayesian approach to analyze the data from a 19- storey building block, which is part of the Punggol EC26 construction project undertaken by the Singapore Housing and Development Board in the early 2000s. The building was instrumented during construction with interferometric fiber optic average strain sensors, embedded in ten of the first story columns during construction. The philosophy driving the design of the monitoring system was to instrument a representative number of structural elements, while maintaining the cost at a reasonable level. The analysis of the data, along with prior experience, allowed the engineer to recognize at early stage an ongoing differential settlement of one base column. We show how the whole cognitive process followed by the engineer can be reproduced using Bayesian logic. Particularly, we discuss to what extent the prior knowledge and potential evidence from inspection, can alter the perception of the building response based solely on instrumental data.

A novel demodulation technique for performing dynamic de- formation measurements using a path-unbalanced Michelson interferom- eter is reported. The method is based on the rf amplitude modulation of a low-coherence source, and demodulation is achieved by tracking in the frequency domain the position of the minimum of the detected intensity. This technique is particularly suitable for deformation measurements in civil engineering structures where deformations of the order of few milli- meters over the sensor length are expected. The method features a measurement range of at least 10 mm, sensitivity better than 10 mm, and dynamic deformation measurements with a bandwidth up to 100 Hz. © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1543563) Subject terms: optical fiber sensors; Michelson interferometer; deformation mea- surements; dynamic measurements; amplitude modulation; tracking system.

A novel demodulation scheme for the ranging of reflective markers is presented. This method is useful for implementing in-line mul- tiplexing of displacement and temperature sensors based on low- coherence interferometry. It allows the discrimination of reflective mark- ers such as air-gap connectors or other partial reflectors spaced by as little as 0.1 m and the measurement of deformations occurring between two successive reflectors with a resolution of 2 mm. The multiplexing of two displacement sensors is demonstrated experimentally. This type of sensor is especially useful for the long-time geometrical monitoring of large structures. © 1997 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(97)01409-8)

A novel in-line multiplexing scheme for optical displacement and temperature sensors based on low-coherence interferometry is presented. It allows the measurement of deformations with a resolution of 10 μm and temperatures with a resolution of 1°C m. The multiplexing of two displacement sensors and one temperature sensor along the same fiber line is demonstrated experimentally, and a theoretical calculation for a system with 10 sensors is presented. This type of sensor would be especially useful for the long-time monitoring of large structures.

Fiber optic sensors have already demonstrated to be suitable for the monitoring of full-scale structures. Usually, most effort has been concentrated in improving the different sensing techniques and it remains often the problem of adapting the sensor to the harsh and dusty environment of the building site. In this context, the SOFO deformation sensors have successfully been tested in different types of structures. The current measurement system, based on low -coherence interferometry, is specially conceived for the long-term assessment of structures, where high precision, resolution and stability is required. Since each measurement takes a few seconds, the set-up is not adequate to monito r the dynamic behavior of structures, where measurement frequencies of up to 1 KHz are necessary. In order to take advantage of the good performance of the current SOFO sensors, a new technique to demodulate at a high frequency the signal from these sensors has been investigated. Based in the radio-frequency amplitude modulation of a lowcoherence source, this intensity-based method offers the required dynamic range of several mm to measure the deformations of the structure under dynamic loads. In this paper we present the theoretical background together with the experimental verification of the principle.

In a national and worldwide context, countless reinforced concrete structures are in an advanced state of deterioration. A principal cause of such degradation is chloride induced corrosion of reinforcement bars. This phenomenon is accentuated in countries where de-icing salts are used for road safety, as well as in maritime zones. To date, no non-destructive method quantifying chloride content during the corrosion initiation phase has been established. Measurement of such a parameter is important for the development of a better understanding of the complexity of corrosion phenomena and, more practically, for better management of existing structures. This paper proposes a new method for non-destructive measurement, for monitoring continuously and in real time free chloride content in concrete pores. In this context, a chemical sensor that employs optical fibers was developed and tested. The sensor functions using the fluorescence of an indicator dye that is sensitive to chlorides. Through fluorescence spectroscopy, variations in the concentration of free chlorides are related to intensity fluctuations of fluorescence. The use of optical fibers also provides an advantage compared with existing electric non-destructive detection systems due to superior electromagnetic stability. Theoretical and experimental studies calibrated and validated the sensor for implementation within mortar samples. Free chloride concentrations between 30 and 350 mM can be detected. Two experiments reproduced climatic variations in a controlled environment. The first test simulated a hot maritime climate and the second test simulated a cold continental climate. These tests confirmed that it is possible to determine with precision the free chloride content. Also, fluorescence spectroscopy with optical fibers offers an innovative means for early and non-destructive detection of free chloride content in concrete. As a result, this new method has potential for improving the science of corrosion process understanding and for planning appropriately for preventive action in practical situations.

On-site distributed measurements using a sensor based on stimulated Brillouin scattering are presented. Long fibre length can be used, so that a dense 2D or 3D measurement of strain or temperature can be obtained in large structure.

Structural health monitoring (SHM) has the potential to provide quantitative and reliable data on the real condition of structures, observe the evolution of their behaviour and detect degradation This paper presents two methodologies for model-free data interpretation to identify and localize anomalous behaviour in civil engineering structures Two statistical methods based on (i) moving principal component analysis and (ii) robust regression analysis are demonstrated to be useful for damage detection during continuous static monitoring of civil structures. The methodologies are tested on numerically simulated elements with sensors for a range of noise in measurements. A comparative study with other statistical analyses demonstrates superior performance of these methods for damage detection. Approaches for accommodating outliers and missing data, which are commonly encountered in structural health monitoring for civil structures, are also proposed. To ensure that the methodologies are scalable for complex structures with many sensors, a clustering algorithm groups sensors that have strong correlations between their measurements Methodologies are then validated on two full-scale structures: The results show the ability of the methodology to identify abrupt permanent changes in behavior. (C) 2010 Elsevier Ltd All rights reserved.

Monitoring the structural condition of road and airport pavement is an extremely critical task to ensure the safety and efficiency of teh transportation. The topic is relevant to both civil and military transportation infrastructure. The presence of damage in pavement, including surface cracking, depressions, swells, and wear, is inevitable due to the sever environmental and service loads that these structures must be subject to. Existing NDE techniques aimed at assessing the structural condition of pavement include Falling Weight Deflectometer, Ground Penetrating Radar, and acoustic methods based on surface waves. This paper presents improvements to the traditional surface-wave method for the detection of surface-breaking cracks in pavement. The advances include 1) the modeling of the problem as dipsersive waves propagating in a multilayer system, 2) the inclusion of post-processing algorithms based on the Wavelet Transform to improve the sensitivity and accuracy of the inspection, and 3) the use of non-contact, air-coupled acoustic detectors to enhance the mobility of the inspection unit. The crack detection procedure consists of first generating a dispersive wave with an impulse hammer, and then measuring the changes in velocity, amplitude and/or frequency content as the wave travels across the flaw with the aid of the Continuous Wavelet Transform. Multilayer wave propagation modeling provides a better understanding of the experimental results by predicting how the various frequencies interact with cracks of different depths. The results of field tests will be presented for both rigid (concrete-based) and flexible (bitumen-based) pavement with surface cracks.