Nicoletta Casanova

Structural monitoring by curvature analysis using interferometric fiber optic sensors

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.

Development and field test of deformation sensors for concrete embedding

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 at 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 degree(s)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.

Embedded and surface mounted fiber optic sensors for civil structural monitoring

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.

Bridge monitoring by interferometric deformation sensors

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.

SHM process as perceived through 350 projects

Civil structures are important for any society and it is necessary to monitor their health condition in order to mitigate risks, prevent disasters, and plan maintenance activities in an optimized manner. Structural health monitoring (SHM) recently emerged as a branch of engineering with a great potential for addressing the above mentioned challenges. In spite of its importance and promising benefits, SHM is still relatively infrequently used in real structures. A possible reason for this is a lack of understanding of the SHM process, which is often considered to be a supplemental activity that does not require detailed planning. However, the opposite is true - only proper and detailed development and implementation of each SHM step can ensure its successful and maximal performance. The aim of this paper is to present the SHM process through more than 350 projects. Basic concepts are introduced, and the purpose, requirements and benefits of SHM are discussed. The importance of monitoring over a life span is highlighted. Core activities such as creating monitoring strategy, installation and maintenance of hardware, and data management are presented and discussed. The involved parties are identified and their interaction with the monitoring process is analyzed. Finally, important SHM challenges are identified.

Dam monitoring with fiber optics deformation sensors

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 of theoretical 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. The second case concerns the development and realization of two long fiber optics deformation sensors 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.

Development and laboratory tests of deformation fiber optic sensors for civil engineering applications.

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.

Railway bridge monitoring during construction and sliding

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.

Geo-structural monitoring with long-gage interferometric Sensors

The monitoring of geotechnical structures like piles, anchors and tunnels requires the measurement of deformations over bases of a few meters to a few tens of meters. The SOFO monitoring system, based on the use of long-gage low-coherence interferometric sensors therefore presents interesting application opportunities in this domain. The SOFO system was installed in a number of piles to monitor their short an long term deformations, to evaluate the lateral friction and to assess their ultimate bearing capacity. The sensors were also installed inside anchor cables to measure the deformations of the rock, in the free and in the anchored parts. Additional sensors were installed directly on single cable strands. This paper presents the sensor installation and the results from selected applications in the monitoring of piles and anchors.

Bridge spatial displacement monitoring with 100 fiber optic deformation sensors: sensors network and preliminary results

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.