Imaging of Early-Stage Cracking on Real-Size Concrete Structure from 4-Points Bending Test

International audienceTraditional ultrasonic imaging techniques encounter difficulty on complexes material such as concrete, which is in part due the use of coherent waves in a very heterogeneous material. From this angle, technique called LOCADIFF has been developed for monitoring heterogeneous media using multiply scattered waves [1, 2]. We consider that modifications in the medium are equivalent to the presence of extra scatterers, which are characterized by their effective scattering cross-section &#963,. Within this view, LOCADIFF allows to locate the modification by measuring the spatio-temporal de-correlation of multiply scattered waves and by solving the corresponding inverse problem. Based on LOCADIFF, a newly developed imaging technique has been reported [3]. By mapping the intensity of modification on localized microstructure, the new technique is able to detect perturbations at multiple locations. Here we present the application of this new technique on a real-size 15 tons concrete structure for imaging early-stage cracking procedure issued from four point bending load, as part of the CEOS.fr project. Experimental results show that this technique can not only locate cracks that appeared simultaneously at multiple locations, but also detect them and observe their developments since an early-stage

Characterization of nonlinear, nonstationary systems in operational modal analysis using wavelet transform

ISMA-USD Noise and Vibration Engineering Conference 2022, Louvain, BELGIQUE, 12-/09/2022 - 14/09/2022This study focuses on the characterization of nonstationary, weakly nonlinear, weakly damped mechanical systems, subjected to an unknown ambient excitation modeled as white Gaussian noise. A theoretical analysis using a novel approach of a one degree of freedom system, whose dynamic response is investigated in time domain as a Markov process, allows simple relations to be established on the expectation and standard deviation of its amplitude and instantaneous frequency. This theoretical study of the system behavior is coupled with the use of the Continuous Wavelet Transform (CWT) ridge, which gives the amplitude and instantaneous frequency of its vibrations. A discussion on the tuning of the mother wavelet function for the analysis of these signals is presented. A simple procedure based on the use of the CWT, characterizing the system's nonlinear, nonstationary behavior, is then proposed. Finally, numerical simulations are carried out, and the proposed method is implemented. Results show good agreement with those of the theoretical study

Numerical modelling of large reinforced concrete specimens based on experimental tests from benchmark Concrack

Most infrastructures are reinforced concrete (RC) structures. And these structures have to be kept operational. For that the stackholders have to watch and observe the noticeable signs of degradation resulting from interactions between the environment and their constitutive materials. Of course, these interactions can reveal cracks which are the major worry with respect to durability and sustainability. If no maintenance or repair actions occur this durability could decrease until the serviceability becomes affected and in some cases, until the structural safety is concerned. In this framework, the French national research programme CEOS.fr (Behaviour and assessment of special construction works concerning cracking and shrinkage) has been elaborated. Its aim is to show the behaviour evolution of special concrete structures particularly their cracking states (opening and space of cracks) and develop tools allowing the structural behaviour prediction (forecast). Associated with the project CEOS.fr, an international benchmark named Concrack (Control of cracking in RC structures) dealt with the modelling of the experimental behaviour of large specimens. In that context, the authors are focused on the modelling of one tested mock-up. It is a large reinforced concrete beam named RL1 subjected to a free shrinkage test followed to a bending test. The authors are concentrated on the mechanical part. In order to model the mechanical test, a 2D plane stress model for the bending beam is performed. In this case, both concrete models are used, Mazars one [Mazars, 1984] and Ricrag [Richard et al., 2010]. The steel model is a classical hardening elastoplastic law. The Ricrag model is a law based on the thermodynamics of irreversible processes coupling elasticity, isotropic damage and internal sliding. Thus, this model could show its capacities to predict the behaviour of the structure according to the experimental results. In this study, the authors show the effect of the boundary conditions on the numerical results and his importance to get a good accordance with the experimental results. Crack openings and spaces are obtained by post processing and compared with the experimental results and the global numerical results are also compared to the experimental ones. At last, the set of the results is encouraging

Stochastic finite element modeling of heterogeneities in massive concrete and reinforced concrete structures

International audienceThe lifespan of a reinforced concrete (RC) structure can be greatly influenced by the spatial variability of its material characteristics which, in particular, explains the observed or measured reduction of the tensile strength at first crack when the volume under tension increases. This paper discusses the ability of accounting for the spatial variability of the tensile strength of concrete in RC structures using a stochastic finite element (SFE) method based on random field simulations. In this work, the generation of random fields on the concrete tensile strength aims at computing the force corresponding to the first crack occurrence, and the reduced tensile strength of the structure. The method can be applied in particular to large‐sized structures, which show a pronounced size effect, for different types of loading. The method consists of, first, estimating the mean of the random field, using the analytical approach of the weakest link and localization method (WL2 A ). Then, the discretized random field is defined on a particular 2D or 3D grid, and it is finally projected on the finite element mesh of the studied structure. The study of the parameters that influence the prediction of the cumulative density functions (CDFs) of the rupture force or the tensile strength is highlighted using experimental series of concrete beams having different volumes and subjected to 4‐point bending loading. Moreover, the SFE method is applied to a RC tie‐beam under tensile loading, characterized by a weak stress gradient, which complicates the prediction of crack positions

Prediction of the size effect in concrete structures using an analytical approach to the weakest link and localization method (WL2)

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Predicting fadeout versus persistence of paratuberculosis in a dairy cattle herd for management and control purposes: a modelling study

Contact: [email protected] Diplôme : Dr. d'UniversitéEpidemiological models enable to better understand the dynamics of infectious diseases and to assess ex-ante control strategies. For Mycobacterium avium subsp. paratuberculosis (Map), possible transmission routes have been described, but Map spread in a herd and the relative importance of the routes are currently insufficiently understood to prioritize control measures. We aim to predict early after Map introduction in a dairy cattle herd whether infection is likely to fade out or persist, when no control measures are implemented, using a modelling approach. Both vertical transmission and horizontal transmission via the ingestion of colostrum, milk, or faeces present in the contaminated environment were modelled. Calf-to-calf indirect transmission was possible. Six health states were represented: susceptible, transiently infectious, latently infected, subclinically infected, clinically affected, and resistant. The model was partially validated by comparing the simulated prevalence with field data. Housing facilities and contacts between animals were specifically considered for calves and heifers. After the introduction of one infected animal in a naive herd, fadeout occurred in 66% of the runs. When Map persisted, the prevalence of infected animals increased to 88% in 25 years. The two main transmission routes were via the farm’s environment and in utero transmission. Calf-to-calf transmission was minor. Fadeout versus Map persistence could be differentiated with the number of clinically affected animals, which was rarely above one when fadeout occurred. Therefore, early detection of affected animals is crucial in preventing Map persistence in dairy herd

Characterization of dissipative behaviour of a reinforced concrete mock-up after soft impact tests through wavelet analysis

40th IABSE Symposium Tomorrow's Megastructures, NANTES, FRANCE, 19-/09/2018 - 21/09/2018The objectives of this communication is to characterize the soft impact response behaviour of a reinforced concrete mock-up (OECD/NEA IRIS Phase 3 mock-up, part of a benchmark), and to evaluate the ability of some mechanical models to represent the energy dissipation during and after the impact. Results from the benchmark show insufficient dissipation in numerical models.Using time-frequency analysis, the dissipative behaviour of the mock-up under impact is evaluated. The stiffness and dissipation associated with the first mode are shown to be amplitude-dependent. This non-linear effect seems to be of a different nature than damage or plastic deformation. A simple rheological model with friction exhibits a similar behaviour

On DEF expansion modelling in concrete structures under variable hydric conditions

Delayed Ettringite Formation (DEF) in concrete is likely to develop in massive civil engineering structures such as bridges, nuclear plants, and dams with major security issues. In many cases, DEF pathology can lead to swelling and cracking which may significantly impact mass transfer and mechanical properties. It is then of major importance to build predictive tools for engineering conceptions and expertises. In thiscontribution, the chemical swelling evolution is integrated within the overall constitutive law of concrete that, besides, can experience other phenomena such like damage, plasticity, and long term creep, not all considered here. On another hand, as DEF is activated by environmental humidity above a certain threshold, we introduce the notion of effective time that takes into account the cumulative exposition abovethis threshold. Hence, a special care is taken with regards to the chemical irreversibility, together with the humidity-drying cycles. Computations are used to calibrate various sets of model parameters with the help of results from the literature, on the one hand, and from an experimental campaign where a calcareous aggregates-based concrete is studied, on the other hand. We show the efficiency of the developed numerical tool through a series of numerical examples

GIEC DES PAYS DE LA LOIRE. 2e rapport — Avril 2023

Le GIEC des Pays de la Loire est un conseil scientifique placé sous l’autorité de l’établissement Grand Ouest de l’association Comité français pour le développement durable, dit « Comité 21 ». Créé en octobre 2020, il a pour missions de : 1. Vulgariser et approfondir les connaissances scientifiques sur la contribution des Pays de la Loire aux changements climatiques et ses impacts pour le territoire ; 2. Évaluer la vulnérabilité du territoire, des populations, des milieux naturels et des activités socio-économiques à ces changements ; 3. Informer les acteurs du territoire sur les évolutions du climat, et les aider à identifier les mesures d’atténuation et d’adaptation les plus efficaces, en proposant des méthodes et en veillant à l’impartialité des informations proposées. Le secrétariat général du GIEC des Pays de la Loire est assuré par le Comité 21 Grand Ouest, qui bénéficie à ce titre d’une subvention du Conseil régional des Pays de la Loire

GIEC DES PAYS DE LA LOIRE. 1er rapport — Juin 2022

UN GIEC DANS LES PAYS DE LA LOIRE Un Groupe interdisciplinaire sur les évolutions du climat dans les Pays de la Loire a été créé en octobre 2020. Il est composé d'une vingtaine de chercheurs qui ont pour mission de: – crédibiliser, vulgariser et approfondir la connaissance des changements climatiques dans les Pays de la Loire, au regard des avancées scientifiques et dans une logique pluridisciplinaire; – identifier et préciser les impacts sur le territoire, ainsi que les vulnérabilités socioéconomiques et environnementales qui y sont liées ; – informer les élus locaux et plus largement les décideurs du territoire sur l’évolution du climat et les aider à identifier, promouvoir et mettre en œuvre des stratégies d’adaptation efficaces. Le secrétariat général du GIEC des Pays de la Loire est assuré par le Comité 21 Grand Ouest, qui bénéficie à ce titre d’une subvention du Conseil régional des Pays de la Loire