Pascal Kronenberg

Relative humidity sensor with optical fiber Bragg gratings

A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

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.

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.

A new system for early chloride detection in concrete

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.

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.

Special sensors for deformation measurements of different construction materials and structures

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 types 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 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.

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.

Development of a novel fiber optic sensor for humidity monitoring

This paper describes ongoing research into the development of a fiber optic sensor for humidity sensing. Particular attention is paid to the compatibility of this fiber optic sensor with an existing system which is already in use for structural deformation monitoring. In order to achieve this, a special transducing coating induces a length variation of the optical fiber as a function of the surrounding humidity levels. An advantage of this setup is that the sensor can not only be read by the same reading unit but can also easily be multiplexed with other sensor types to form a multi functional sensor network. This is of a particular interest for monitoring materials such as reinforced concrete, where structural health assessment criteria include deformation, depassivation and humidity. Several sensor configurations have been tested using dry-wet cycles at room temperature. Through this testing a prototype humidity sensor has been developed which responds consistently to humidity. Using pH sensitive coatings, the same design could be used for a fiber optic pH sensor.

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.

Optical setup development for the monitoring of structural dynamic behavior using SOFO sensors

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 monitor 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 low-coherence 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.