Detection of Multiple Cracks in Four-Point Bending Tests Using the Coda Wave Interferometry Method

International audienceThe enlargement of the cracks outside the permitted dimension is one of the main causes for the reduction of service life of Reinforced Concrete (RC) structures. Cracks can develop due to many causes such as dynamic or static load. When tensile stress exceeds the tensile strength of RC, cracks appear. Traditional techniques have limitations in early stage damage detection and localisation, especially on large-scale structures. The ultrasonic Coda Wave Interferometry (CWI) method using diffuse waves is one of the most promising methods to detect subtle changes in heterogeneous materials, such as concrete. In this paper, the assessment of the CWI method applied for multiple cracks opening detection on two specimens based on four-point bending test is presented. Both beams were monitored using a limited number of embedded Ultrasonic (US) transducers as well as other transducers and techniques (e.g., Digital Image Correlation (DIC), LVDT sensors, strain gauges, and Fiber Optics Sensor (FOS)). Results show that strain change and crack formation are successfully and efficiently detected by CWI method even earlier than by the other techniques. The CWI technique using embedded US transducers is undoubtedly a feasible, efficient, and promising method for long-term monitoring on real infrastructure

Concrete Crack Monitoring Using a Novel Strain Transfer Model for Distributed Fiber Optics Sensors

International audienceIn this paper, we study the strain transfer mechanism between a host material and an optical fiber. A new analytical model handling imperfect bonding between layers is proposed. A general expression of the crack-induced strain transfer from fractured concrete material to optical fiber is established in the case of a multilayer system. This new strain transfer model is examined through performing wedge splitting tests on concrete specimens instrumented with embedded and surface-mounted fiber optic cables. The experimental results showed the validity of the crack-induced strain expression fitted to the distributed strains measured using an Optical Backscattering Reflectometry (OBR) system. As a result, precise estimations of the crack openings next to the optical cable location were achieved, as well as the monitoring of the optical cable response through following the strain lag parameter

Detection and Measurement of Matrix Discontinuities in UHPFRC by Means of Distributed Fiber Optics Sensing

International audienceFollowing the significant improvement in their properties during the last decade, Distributed Fiber Optics sensing (DFOs) techniques are nowadays implemented for industrial use in the context of Structural Health Monitoring (SHM). While these techniques have formed an undeniable asset for the health monitoring of concrete structures, their performance should be validated for novel structural materials including Ultra High Performance Fiber Reinforced Cementitious composites (UHPFRC). In this study, a full scale UHPFRC beam was instrumented with DFOs, Digital Image Correlation (DIC) and extensometers. The performances of these three measurement techniques in terms of strain measurement as well as crack detection and localization are compared. A method for the measurement of opening and closing of localized fictitious cracks in UHPFRC using the Optical Backscattering Reflectometry (OBR) technique is verified. Moreover, the use of correct combination of DFO sensors allows precise detection of microcracks as well as monitoring of fictitious cracks' opening. The recommendations regarding use of various SHM methods for UHPFRC structures are given

Distributed Fiber Optics Sensing for Crack Monitoring of Concrete Structures

This thesis work aims to develop and validate a method for monitoring crack openings using distributed fiber optics strain measurements. First, the various existing theories on strain transfer from the host material to the optical fiber are presented, with their validity domain. The problem of perfect interfacial bonding is then studied and a three-layer analytical model capable of handling imperfect bonding case is proposed. This model is then generalized to multi-layer systems. Experimental studies validating this new model are presented. They show that it is possible to monitor crack openings up to 1 mm with an error of less than 10% for a fiber optic cable glued on the surface. Cables embedded in concrete show less accurate results. The type of cable, the bonding length and the hardening of the concrete material also influence the accuracy of the estimated crack openings. Finally, the results of case studies on laboratory-size reinforced concrete samples are presented. They show the optical fibers capacity to detect cracks as early as ultrasonic sensors and to monitor the opening of multiple micro cracks.Le travail de thèse présenté dans ce mémoire vise à développer et valider une technique de suivi d'ouvertures de fissures à l'aide de mesures réparties de déformation par fibres optiques. Dans un premier temps les différentes théories existantes sur le transfert de déformation du matériau hôte vers la fibre optique sont présentées avec leurs domaine de validité. Le problème de l'adhésion parfaite entre couche est ensuite étudié et un modèle analytique à trois couches tenant compte d'une adhésion imparfaite est élaboré. Ce modèle est ensuite généralisé aux systèmes multicouches. Les études expérimentales validant ce nouveau modèle sont alors présentées. Elles montrent qu'il est possible de suivre les ouvertures de fissures jusqu'à 1000m avec une erreur inférieure à 10% avec un câble à fibre optique collé en surface. Les câbles noyés dans le béton donnent des résultats moins justes. La justesse des mesures est aussi influencée par le type de câble, la longueur d'ancrage et le durcissement du béton. Enfin, les résultats des études de cas sur des échantillons en béton armé de laboratoire sont présentés. Elles montrent la capacité des fibres à détecter des fissures aussi précocement que les capteurs acoustiques à ultrasons et de surveiller l'ouverture de micro fissures multiples

Mesures Réparties par Fibres Optiques pour le suivi des Fissures dans les Structures en Béton

This thesis work aims to develop and validate a method for monitoring crack openings using distributed fiber optics strain measurements. First, the various existing theories on strain transfer from the host material to the optical fiber are presented, with their validity domain. The problem of perfect interfacial bonding is then studied and a three-layer analytical model capable of handling imperfect bonding case is proposed. This model is then generalized to multi-layer systems. Experimental studies validating this new model are presented. They show that it is possible to monitor crack openings up to 1 mm with an error of less than 10% for a fiber optic cable glued on the surface. Cables embedded in concrete show less accurate results. The type of cable, the bonding length and the hardening of the concrete material also influence the accuracy of the estimated crack openings. Finally, the results of case studies on laboratory-size reinforced concrete samples are presented. They show the optical fibers capacity to detect cracks as early as ultrasonic sensors and to monitor the opening of multiple micro cracks.Le travail de thèse présenté dans ce mémoire vise à développer et valider une technique de suivi d'ouvertures de fissures à l'aide de mesures réparties de déformation par fibres optiques. Dans un premier temps les différentes théories existantes sur le transfert de déformation du matériau hôte vers la fibre optique sont présentées avec leurs domaine de validité. Le problème de l'adhésion parfaite entre couche est ensuite étudié et un modèle analytique à trois couches tenant compte d'une adhésion imparfaite est élaboré. Ce modèle est ensuite généralisé aux systèmes multicouches. Les études expérimentales validant ce nouveau modèle sont alors présentées. Elles montrent qu'il est possible de suivre les ouvertures de fissures jusqu'à 1000m avec une erreur inférieure à 10% avec un câble à fibre optique collé en surface. Les câbles noyés dans le béton donnent des résultats moins justes. La justesse des mesures est aussi influencée par le type de câble, la longueur d'ancrage et le durcissement du béton. Enfin, les résultats des études de cas sur des échantillons en béton armé de laboratoire sont présentés. Elles montrent la capacité des fibres à détecter des fissures aussi précocement que les capteurs acoustiques à ultrasons et de surveiller l'ouverture de micro fissures multiples

Distributed Fiber Optics Sensing for Crack Monitoring of Concrete Structures

This thesis work aims to develop and validate a method for monitoring crack openings using distributed fiber optics strain measurements. First, the various existing theories on strain transfer from the host material to the optical fiber are presented, with their validity domain. The problem of perfect interfacial bonding is then studied and a three-layer analytical model capable of handling imperfect bonding case is proposed. This model is then generalized to multi-layer systems. Experimental studies validating this new model are presented. They show that it is possible to monitor crack openings up to 1 mm with an error of less than 10% for a fiber optic cable glued on the surface. Cables embedded in concrete show less accurate results. The type of cable, the bonding length and the hardening of the concrete material also influence the accuracy of the estimated crack openings. Finally, the results of case studies on laboratory-size reinforced concrete samples are presented. They show the optical fibers capacity to detect cracks as early as ultrasonic sensors and to monitor the opening of multiple micro cracks.Le travail de thèse présenté dans ce mémoire vise à développer et valider une technique de suivi d'ouvertures de fissures à l'aide de mesures réparties de déformation par fibres optiques. Dans un premier temps les différentes théories existantes sur le transfert de déformation du matériau hôte vers la fibre optique sont présentées avec leurs domaine de validité. Le problème de l'adhésion parfaite entre couche est ensuite étudié et un modèle analytique à trois couches tenant compte d'une adhésion imparfaite est élaboré. Ce modèle est ensuite généralisé aux systèmes multicouches. Les études expérimentales validant ce nouveau modèle sont alors présentées. Elles montrent qu'il est possible de suivre les ouvertures de fissures jusqu'à 1000m avec une erreur inférieure à 10% avec un câble à fibre optique collé en surface. Les câbles noyés dans le béton donnent des résultats moins justes. La justesse des mesures est aussi influencée par le type de câble, la longueur d'ancrage et le durcissement du béton. Enfin, les résultats des études de cas sur des échantillons en béton armé de laboratoire sont présentés. Elles montrent la capacité des fibres à détecter des fissures aussi précocement que les capteurs acoustiques à ultrasons et de surveiller l'ouverture de micro fissures multiples

A General Solution to Determine Strain Profile in the Core of Distributed Fiber Optic Sensors under Any Arbitrary Strain Fields

International audienceDespite recent publications, the strain transfer in distributed optical fiber sensors is still often overlooked and poorly understood. In the first part of this paper, strain transfer is shown to be driven by a second-order differential equation, whether the optical fiber is embedded into the host material or surface-mounted. In this governing equation, only the value of a key parameter, called strain lag parameter, varies according to the attachment configuration and the type of optical fiber used as a sensor. Then, a general solution of the governing equation is proposed. It is an analytical expression established from new boundary conditions that are more adequate than those used previously in the literature and allows the determination of the strain profile in the core of a distributed optical fiber sensor under any arbitrary strain fields. This general solution has been validated by two experiments presented in the third part of the paper. A very good agreement between the analytical solutions and measured strain profiles using a high spatial resolution optical interrogator for both uniform and non-uniform strain fields has been obtained. These results highlight the importance of the strain lag parameter which must be taken into account for a correct interpretation of measurements, especially in the case of important strain gradients

Quantification of cracks in reinforced concrete structures using distributed fiber optic sensors

EWSHM 2018, 9th European Workshop on Structural Health Monitoring Series, MANCHESTER, ROYAUME-UNI, 10-/07/2018 - 13/07/2018Even under normal service loads, reinforced concrete structures in the field of civil engineering are full of micro cracks. More than twenty years of development of distributed fibre optic sensing techniques in terms of accuracy and spatial resolution made them capable of not only monitoring strain and temperature changes, but also detecting and localizing cracks. In order to detect safety-related issues and better maintenance and to offer better maintenance and management strategies, continuous monitoring of the crack width is also paramount. The paper focuses on the application of a theoretical mechanical strain transfer model in concrete substrate. The experimental setup involved employment of the Optical Frequency Domain Reflectometry (OFDR) technique with high spatial resolution. Three-point bending tests were performed on 1m length reinforced concrete beams instrumented with embedded and surface attached fibre optics cables. Results showed that the Crack Opening Displacement (COD) in single and multiple neighboring micro cracks cases can be well estimated

Distributed fiber optics sensing and coda wave interferometry techniques for damage monitoring in concrete structures

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