This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non-contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC-based methods may lead to erroneous results. In this study a post-processing procedure is presented to cope with noisy full-field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full-scale rectangular beams and then on mock-ups scaled down by 1/3. In addition, fours RC walls are tested under in-plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in-plane displacement and strain fields. Full-field measurements are post-processed by the noise-filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point-based sensors (linear variable differential transformer LVDT and fibre-optic FO transducers). The proposed DIC-based post-processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale-effect on the cracks width.

Within the CEOS.fr national research project, several experiments on massive concrete structures were conducted to improve the knowledge on the cracking phenomenon. In this paper, experiments where deformations at early age are restrained are presented. Testing bodies are I-shaped and two largely dimensioned steel struts are placed laterally between the two transverse heads to prevent almost any shrinkage. Three testing bodies were realized: RG8, the reference one; RG9, with a reduced reinforcement and RG10, with an increased cover. A full set of measurement was used for auscultation of these beams during early age. Optical long base fibres gave information on the relative displacement of the central part of the beam.Local measurements of strains in concrete were given thanks to Vibrating Wire Extensometers. Gauges on rebars produced data of the strain on the first reinforcement layer, and the force in struts was monitored. With this, the force and stresses in concrete and rebars could be deduced. This huge amount of data allows verifying the phenomenology of the concrete. Various hypotheses were analysed to explain the strain measured and the corresponding forces in each component during specific period of early age. A first analysis of the cracking process shows that the cracks could appear for stresses below the tensile strength.

In order to meet application requirements, dependence of the Brillouin frequency shift to strain should be enhanced. We evaluated the influence of several optical fiber dopants on Brillouin scattering strain sensitivity. Based on a FEM-2D modeling, we developed a model for Brillouin gain spectrum and its strain sensitivity. Geometrical profile, doping composition and drawing conditions are taken into account. We show alumina is a very interesting dopant to enhance strain sensitivity in optical fibers. This result is experimentally validated: up to 0.0639MHz/μƐ is obtained with a 5.2mol% Al2O3 doped fiber. We expect to reach 0.1MHz/μƐ with 25mol% doping level.

The aim of this article is to give an illustration of how numerical simulation is helping EDF to manage its concrete structures in nuclear power plants. Several examples are described in the field of concrete modeling for prediction of material ageing or validation of inspection techniques and in the field of structural computation.

The water content in concrete (nuclear structures and nuclear waste repositories ) is a major topic to understand and predict the behaviour at the end of the operating period. That is th e reason why ANDRA and EDF are involved in research programs dedicated to concrete Thermo-Hydro Mechanical (THM) modeling and to in situ water content assessment technologies [1]. Another example concerns the bridges which incl ude “external” post-tensioned cables to reinforce these structures. These cables are not into the concrete material, hence potential ly accessible for measurement. They are generally placed in High Density Poly-Ethylene (HDPE) ducts, w here the residual space is filled under high pressure with a cement grout intended to preven t corrosion. Nevertheless, in some cases, the cables breaking occur in non-protected zones [2, 3 ] due to the presence of a “white paste” or grout voids. To remote diagnosis anywhere and in real time of post tensioned ducts or to measure the water content in concrete, we propose a structural health monitoring method based on Frequency Dom ain Reflectometry (FDR). Today’s, advanced reflectometry methods provide an efficient solution for the fault-detection and for their diagnosis in electric transmission lines [4 , 5]. This paper presents a direct model of the FDR method based on T elegrapher’s equations. An analysis of these signals, based on scattering theory, enables one to retrieve the impedan ce distribution of the electric line. The impedance distribution depends on damage into the duct or the wa ter content in concrete. An inversion algorithm is realized with software ISTL provided by INRIA. FDR method has been applied to two real cases: measurement of the water content in concrete and the diagnost ic of the external post tensioned duct.

The paper presents an overview of the different attempts undertaken by EDF from the 1980's to now to monitor the water content throughout time in the concrete structures of nuclear power plants. A brief review of existing techniques is proposed, associated to a focus on those chosen by EDF to be tested on site. A feedback on capacitive probes embedded in concrete is provided. An example of metrological requirement for this kind of sensor is presented, in relation with their use in structural analysis. Future research works and considered perspectives are discussed.

Usually, strain and temperature coefficients of Brillouin frequency shift (BFS) of optical fibers result from experimental measurements. For the first time, theoretical strain dependence of BFS is analyzed depending on fiber properties. Based on a FEM-2D modeling, the strain coefficient C-epsilon is determined considering geometrical profile, doping composition and drawing conditions. Theoretical results showed great accordance with measurements for different types of single-mode fibers. The C-epsilon coefficient is evaluated with a relative uncertainty better than 8%. Significant C-epsilon magnitude variation has been observed depending on doping profile. This paves the way towards major improvement of strain optical fiber sensors.

In this study, a truly distributed sensing system was used to monitor the mechanical behavior of a representative-scale reinforced concrete structural element tested in four-point bending. The innovative measuring chain consisted of an OBR interrogation unit (Optical Backscatter Reflectometer, based on relative Rayleigh measurements) paired with fiber optic sensors which were bonded to the surface of the test specimen or embedded within the concrete volume, near and away from the reinforcing steel bars. Such a configuration provides both high strain sensitivity and spatial resolution in the centimeter range.Strain measurements performed on the loaded beam (i) were found to be consistent with both experimental data obtained from conventional vibrating wire gauges and theoretically predicted strain profiles, and (ii) showed no influence of the sensor location on quantitative results (externally bonded sensors, cables embedded near/far from the rebars). Besides, the distributed sensing system under consideration offered the capability to detect and localize cracks, at a much earlier stage than visual inspection. (C) 2012 Elsevier Ltd. All rights reserved.

Structural health monitoring is a key factor in life cycle management of infrastructures. Truly distributed fiber optic sensors are able to provide relevant information on large structures, such as bridges, dikes, nuclear power plants or nuclear waste disposal facilities. The sensing chain includes an optoelectronic unit and a sensing cable made of one or more optical fibers. A new instrument based on Optical Frequency Domain Reflectometry (OFDR), enables to perform temperature and strain measurements with a centimeter scale spatial resolution over hundred of meters and with a level of precision equal to 1 mu strain and 0.1 degrees C. Several sensing cables are designed with different materials targeting to last for decades in a concrete aggressive environment and to ensure an optimal transfer of temperature and strain from the concrete matrix to the optical fiber. Tests were carried out by embedding various sensing cables into plain concrete specimens and representative-scale reinforced concrete structural elements. Measurements were performed with an OFDR instrument; meanwhile, mechanical solicitations were imposed to the concrete element. Preliminary experiments are very promising since measurements performed with distributed sensing system are comparable to values obtained with conventional sensors used in civil engineering and with the Strength of Materials Modelling. Moreover, the distributed sensing system makes it possible to detect and localize cracks appearing in concrete during the mechanical loading.

Structural health monitoring is a key factor in life cycle management of infrastructures. Truly distributed fiber optic sensors are able to provide relevant information on large structures, such as nuclear power plants or nuclear waste disposal facilities. The sensing chain includes an optoelectronic unit and a sensing cable made of one or more optical fibers. A new instrument based on Optical Frequency Domain Reflectometry (OFDR), enables to perform temperature and strain measurements with a centimeter scale spatial resolution over hundred of meters and with a level of precision equal to 1 µstrain and 0.1°C. Several sensing cables are designed with different materials targeting to last for decades, either embedded in the concrete or attached to the surface of the structure. They must ensure an optimal transfer of temperature and strain from the concrete matrix to the optical fiber. Based on the European guide FD CEN/TR 14748 Non-destructive testing - Methodology for qualification of non-destructive tests, a qualification method was developed. Tests were carried out using various sensing cables embedded in the volume or fixed to the surface of plain concrete specimens and representative-scale reinforced concrete structural elements. Measurements were performed with an OFDR instrument, while mechanical solicitations were imposed to the concrete element. Preliminary experiments seem very promising since measurements performed with distributed sensing systems are found comparable to values obtained with conventional sensors used in civil engineering and with the Strength of Materials Modelling. Moreover, the distributed sensing system makes it possible to detect and localize cracks appearing in concrete during the mechanical loading.

Residual stresses inside optical fibers can impact significantly on Brillouin gain spectrum. Based on a 2D-FEM modeling, theoretical results are compared with Brillouin gain measurements for fibers from a same preform with different draw tensions.