Detlef Hofmann

Strain measurements in reinforced concrete walls during the hydration reaction by means of embedded fibre interferometers

In a reinforced concrete wall the deformation during concrete hardening was measured by means of embedded extrinsic fibre-Fabry-Perot-interferometers. The sensors were specifically modified in order to provide a self-calibration cycle and to ensure the functional efficiency under adverse conditions at the building site. The installation was done in the walls cage of reinforcement before its concreting. The measurement was carried out automatically over a period of 35 days. The measuring results are very satisfactory and give a resolution of 0.1 micrometers /m.

Determination of structural parameters concerning load capacity based on fibre Fabry-Perot interferometers

Fiber optical sensors were applied on a cracked prestressed concrete bridge in Berlin to get the static and dynamic structure response under load. Interferometric sensors (extrinsic Fibre-Fabry-Perot) were adhered on prestressing steels of a tendon opened inside the box girder; Intensity- modulated fiber sensors were tightened over the cracked concrete region. The very high resolution of interferometric sensors ((epsilon) equals +/- 0.024 micrometers /m) simultaneously allowed to measure strain of tendon and dynamic (natural frequency) response. This bridge is the first prestressed concrete structure in Germany to have fiber optic interferometric sensors installed. The optimization of the adhesive bonding and embedment technique are problems that need to be addressed as part of the further development.

Structure-integrated fiber-optic sensors for reliable static and dynamic analysis of concrete foundation piless

Assessment of ultimate bearing capacity and bearing behavior of large concrete piles in existing foundations or during and after installation remains a difficult task. A common and widespread test method is high-strain dynamic load testing using the one dimensional theory of wave propagation to calculate bearing capacity. Another method of quality insurance based on this theory is low-strain dynamic pile integrity testing. Both testing methods use sensors attached onto or near the pile head. In order to get more precise information about the pile response over whose length, highly resolving fiber optic sensors based on Fabry-Perot technology have been developed for integration into concrete piles at several levels. Motivation is the monitoring of pile deformations during dynamic low-strain, high-strain and static load testing with only one measuring device. First small scale piles have been tested in model tests. All signal responses from integrated sensors have been recorded and compared to signals obtained from common methods of instrumentation. The paper describes the sensing principle, sensor head installation as well as test results.

Static and dynamic pile testing of reinforced concrete piles with structure integrated fibre optic strain sensors

Static and dynamic pile tests are carried out to determine the load bearing capacity and the quality of reinforced concrete piles. As part of a round robin test to evaluate dynamic load tests, structure integrated fibre optic strain sensors were used to receive more detailed information about the strains along the pile length compared to conventional measurements at the pile head. This paper shows the instrumentation of the pile with extrinsic Fabry-Perot interferometers sensors and fibre Bragg gratings sensors together with the results of the conducted static load test as well as the dynamic load tests and pile integrity tests.

Lightning-safe diaphragm pressure gauge for geotechnical applications using a long-term reliable absolute EFPI sensor

The paper describes a sensor head for long-term high-precision measurements of very small deflections of a diaphragm used for pressure gauges. High precision deformation measurement is assured by using a fiber Fabry-Perot interferometer sensor; identification of zero-point changes, and thus, long-term stable measurement is achieved by a specially designed absolute interferometer sensor. Several fiber optic solutions based on fiber Fabry-Perot technique have been investigated to find out a reliable sensor design. The presented sensor design has reached prototype status and allows to measure unambiguously static deformations with high precision. In order to evaluate repeatability and possible changes of zero-point reference if the head has been disconnected, validation of the described pressure gauge has been started. This validation work includes calibration and enables to evaluate possible drift effects, and to identify mechanical or thermal hysteresis.

Strain monitoring of a newly developed precast concrete track for high speed railway traffic using embedded fiber optic sensors

In a German slab track system (Feste Fahrbahn FF, system Boegl) for speeds up to 300 km/h and more different fiber optic sensors have been embedded in several levels and locations of the track system. The track system consists of prestressed precast panels of steel fiber concrete which are supported by a cat-in-situ concrete or asphalt base course. The sensors are to measure the bond behavior or the stress transfer in the track system. For that, tiny fiber-optic sensors - fiber Fabry-Perot and Bragg grating sensors - have been embedded very near to the interface of the layers. Measurements were taken on a full scale test sample (slab track panel of 6.45 m length) as well as on a real high speed track. The paper describes the measurement task and discusses aspects with regard to sensor design and prefabrication of the sensor frames as well as the embedding procedure into the concrete track. Results from static and dynamic full scale tests carried out in the testing laboratory of BAM and from measurements on a track are given.

Strukturanalyse von Betonpfählen durch eingebettete faseroptische Sensoren Analysis of Concrete Foundation Piles Using Structure-Integrated Fibre-Optic Sensors

Zusammenfassung Zur Gründung von Bauwerken auf weichen, gering tragfähigen Böden werden unter anderem Betonpfähle, einzeln oder in Pfahlgruppen, eingesetzt. Deren Bewertung bzgl. Tragverhalten bzw. Integrität ist bei bestimmten Baugrundverhältnissen und Pfahlgeometrien schwierig. Hierfür werden statische und dynamische Pfahlprüfungen durchgeführt. Aus Messwerten vom Pfahlkopf bzw. vom oberen Pfahlbereich werden üblicherweise die notwendigen Aussagen abgeleitet. Eine genauere Bewertung ist möglich, wenn präzise Messwerte über die gesamte Pfahllänge vorliegen. Deshalb wurden hochauflösende betoneinbettbare Messwertaufnehmer auf Basis von Faser-Fabry-Perot-Sensoren entwickelt, die in Modell- und in realen Rammpfählen getestet wurden. Der Beitrag beschreibt den Sensor, die Installation und exemplarisch einige Testergebnisse zur Erfassung der Wellenausbreitung im Modellpfahl. Feldtests wurden zurzeit der Erstellung des Manuskripts erfolgreich begonnen.

Characterization of bond performance of textiles in cement matrices using fiber optic sensors

The bond behavior of textiles embedded in concrete and consisting of multi filament yarns (rovings) differs from that of homogenous materials such as steel. Test results published in the literature as well as own investigations revealed that, for textile reinforced concrete, it has to be distinguished between external and internal bond of the fibers (filaments). The outer filaments which have direct contact to the cement matrix show a good bonding performance. In contrast to this, the inner filaments (core filaments) of a roving transfer forces only by friction, resulting in a less bonding to the surrounding matrix. In order to confirm such a bonding model, strain and slip measurements at single filaments are necessary in pull-out-samples. However, such measurements are not possible with strain gauges usually used in structural engineering. Therefore, strains in outer and inner filaments as well as in the cement matrix of selected samples are measured by using flexible Fabry-Perot fiber interferometer sensors.

Geotechnical pressure cell using a long-term reliable high-precision fiber optic sensor head

Geotechnical measurements are producing various data which are used for interpretation and e.g. safety calculation. The reliability of such data is of most importance as every decision on civil engineering action needed is based on this data. Known and also unknown influences changing data are a basic demand for deeper investigation and research. In present time we have limited tools only to minimize perturbing influences. One of these demands-the long-term development of data also called long-term stability-is described in this paper. The paper describes a sensor head for long-term high-precision measurements of very small deflections of a diaphragm used for pressure gauges. High precision deformation measurement is assured by using a fiber Fabry-Perot interferometer sensor; identification of zero-point changes, and thus, long-term stable measurement is achieved by a specially designed absolute interferometer sensor. Several fiber optic solutions based on fiber Fabry-Perot technique have been investigated to find out a reliable sensor design. The presented sensor design has reached prototype status and allows to measure unambiguously static deformations with high precision. In order to evaluate repeatability and possible changes of zero-point reference if the head has been disconnected, validation of the described pressure gauge has been started. This validation work includes calibration and enables to evaluate possible drift effects, and to identify mechanical or thermal hysteresis. Thus, the highlight in this paper is the observation and measurement of zero-point development over time.

Modelling and simulation of a fibre Bragg grating strain sensor based on a magnetostrictive actuator principle

A new concept for the self-diagnosis of embedded fiber Bragg grating (FBG) strain sensors was developed, simulated and experimentally tested. This concept is based on a magnetostrictive metallic layer directly coated on the fibre cladding over the grating segment of the FBG sensor, so that an on-demand external magnetic field in a millitesla scale can produce a controllable artificial strain as an indication signal for the remote optical interrogator. The relationship between the pre-defined magnetic field and its induced Bragg wavelength shift characterizes this validation concept. Any deviation of the local bonding state of the interfaces from the initial or/and any change of shear strain transferring mechanism from composite matrix to the optical fibre core will result in alterations in this sensitive relationship, and thus triggers an immediate alert for a further inspection. The finite element method is used to simulate the strain of this configuration as result of different values of the magnetic field in order to optimize the geometrical sensor parameters. The simulations are verified by experiments results.