Prestressed concrete viaducts have been widely built in France and Europe in the past decades and are often strategic assets for the road networks, which deserve structural health monitoring (SHM) techniques to assess and anticipate the effects of their ageing. The Jules Verne viaduct near Amiens in France has been chosen as a case study to gather high volumes of data on its response to the live loads through a long period of time, in the frame of a common research by OSMOS Group, the Université Gustave Eiffel, the CEREMA, and Le Mans Université. The viaduct is a 943 m long bridge with 19 spans. It is made of two independent parallel decks, the first one built in 1988 and the second one in 2002, with similar geometry but significant differences in terms of prestress design. Both decks have been equipped with strain and acceleration sensors since February 2022, continuously gathering measurements at each passage of heavy vehicles on the bridge. In addition, a campaign of acoustic emission measurements was organized in July 2022. This contribution summarizes the various data analysis methods used on the large quantity of records gathered over almost two years, to obtain synthetic useful information from the combination of diverse monitoring techniques. Results and perspectives are discussed, focusing on this interesting opportunity to compare the behavior of two similar prestressed concrete structures with 14 years age difference, in field conditions and under real traffic loads.

This paper demonstrates the potential of using the Continuous Wavelet Transform (CWT) for modal identification of an existing bridge. The dynamic response of a railway bridge crossed by high speed trains is investigated. First, the theoretical dynamic response of the bridge is studied and found to have a very distinctive shape due to the periodic spacing of the train axles. A CWT-based procedure is then proposed for processing the global signal, with emphasis on the choice of wavelet function. An application is given using experimental accelerometer data collected during the crossing of a high-speed train on the bridge. In a first step, the signal has been properly partitioned since it is composed of different time intervals containing specific modal information (in particular, bridge + train and bridge alone when the train has left the bridge). The dynamic response of the "bridge + train" system was found to have a very distinct non-stationary shape. The modal parameters of the bridge were then successfully calculated and non-linearities in the system behaviour were detected. It illustrates the ability of CWT to detect and characterise non-linear behaviour.

The 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 (WL2A). 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.

Modal-based damage assessment methods are efficient tools to identify location and magnitude of damage associated with change of modal properties (natural frequencies, modal damping ratios and mode shapes). However, while numerous applications of these methods to simple structural systems (i.e. simple shape structure made of homogeneous material, submitted to well-known boundary conditions…) have proved their efficiency, quite a few studies have been conducted to complex civil engineering structures. This paper presents then the experimental modal analysis applied to reinforced concrete beam-column joint. During tests, a full-scale specimen was subjected to cyclic loading to introduce damage. After each loading phase, specimens were excited with an impact hammer to determine the current modal parameters. The main objective of is to identify from the after-impact responses, the changes occurred in the modal parameters due to damage induced by loading. Structural tests and model used to interpret the experimental result are presented. The capability of the proposed approach to detect small level damage as well as the impact of the measurement errors are then discussed.

In real structures, the proportional damping assumption is never strictly verified. Indexes of non-proportionality are then necessary to determine if this assumption leading to real modes still remains valid. If not, complex modes will appear and moreover, if their corresponding natural frequencies are close, their imaginary part can become large. In this paper, a new non-proportionality index, quantifying the "complexity" of mode shapes, is presented, derived from the notion of optimal complex modes introduced by Adhikari. This new index is designed for experimental results, for which the system's parameters are not known, and proven to be equal to the previous one up to the first order on damping. Modal identification based on wavelet analysis is considered promising in this study for processing free responses of non-proportionally damped systems, integrated in noise, to directly obtain complex modes. A procedure for choosing an appropriate quality factor for the time-frequency resolution, necessary to get correct identification results in the case of free responses combined with responses to ambient excitation and/or to additive noise, is detailed. The proposed identification technique based on Continuous Wavelet Transform (CWT) is finally applied on different transient responses of a masonry wall specimen during an experimental campaign comprising simultaneous vibrations and shear-compression tests. The results of the CWT method for modal identification are compared with those obtained by a classical modal analysis technique, called Least Squares Complex Frequency method, by means of the Modal Assurance Criterion and the proposed non-proportionality index. (C) 2021 Elsevier Ltd. All rights reserved.

Within the continuum, the kinematical choice is based on an additive split of the total strain, denoted as usual by the tensor.

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 this contribution, 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 above this 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.

The 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.

Internal Swelling Reactions (ISR) can affect long term duration of Reinforced Concrete (RC) structures by causing cracking and expansion of concrete material. These pathologies mainly consist in Alkali Aggregate Reaction (AAR) and/or Delayed Ettringite Formation (DEF). Thus, it is necessary to provide robust predictive numerical models able to re-assess the mechanical state of affected structures. A new numerical strategy is proposed in this paper to compute the evolution of the ISR reaction from intrinsic parameters to be identified for the reactive concrete. Based on Larive's model, evolution of the chemical strain is written in order to assume the irreversibility of the mechanisms and to take into account time history of the Relative Humidity and Temperature supported by the concrete specimen. An effective time is proposed to store history of the reactive concrete and to be able to compute evolution of the chemical strain.

This paper proposes a refined Weibull effective volume (WEV) approach in order to model the size effect in concrete structures. Both the Highly Stressed Volume (HSV) and Weibull Weakest Link and Localization (WL2) methods are presented. An analytical probabilistic approach to running WL2 is then developed, knowing that this method takes into account the energetic-statistical size effect. The approach employed recognizes the inelastic phase before the peak load; it provides an analytical and fast estimation of the structural tensile strength at various scales. This approach depends on a scale length, which accounts for the spatial randomness of the concrete tensile strength, and is identified on a series of concrete specimens under uniaxial tension by use of an inverse analysis. The Weibull modulus estimation is also discussed herein. Moreover, it is shown that the analytical probabilistic approach to WL2 implementation yields the size effect prediction in both average and dispersion for various experimental series, from laboratory tests to large massive structures. The experimental test series discussed in this paper consists of concrete specimens under uniaxial tension and 3-point bending loading.

This paper describes the use of an ultrasonic imaging technique (Locadiff) for the Non-Destructive Testing & Evaluation of a concrete structure. By combining coda wave interferometry and a sensitivity kernel for diffuse waves, Locadiff can monitor the elastic and structural properties of a heterogeneous material with a high sensitivity, and can map changes of these properties over time when a perturbation occurs in the bulk of the material. The applicability of the technique to life-size concrete structures is demonstrated through the monitoring of a 15-ton reinforced concrete beam subject to a four-point bending test causing cracking. The experimental results show that Locadiff achieved to (1) detect and locate the cracking zones in the core of the concrete beam at an early stage by mapping the changes in the concrete's micro-structure; (2) monitor the internal stress level in both temporal and spatial domains by mapping the variation in velocity caused by the acousto-elastic effect. The mechanical behavior of the concrete structure is also studied using conventional techniques such as acoustic emission, vibrating wire extensometers, and digital image correlation. The performances of the Locadiff technique in the detection of early stage cracking are assessed and discussed.

The work presented is a part of the french ANR (Agence Nationale pour la Recherche) project MACENA (Maitrise du Confinement en Accident), its main objective is to better present the role of concrete heterogeneities in RC structures in the cracking process. This paper aims to develop and use the size effect method (WL2) applicable to RC structures proposed by Sellier and Millard 2014 [1]. The originality of the method lies on introducing a weighting function defined in the direction of the maximum principal stress using a scale length. In this work, an inverse analysis of the method allows to identify this scale length using experimental test series of concrete specimens under tensile load and 3 point bending beams. The approach is then applied to predict the sensitivity of the mechanical behavior of a reinforced concrete tie under tensile load. The method is applied in the elastic phase and allows providing the structural tensile strength corresponding to the first crack which is affected by size effect and plays a key role because cracked and uncracked structures behave in severe environment in a very different way. In FE model, correlated random fields on the tensile strength of the concrete can be generated using the identified scale length to characterize the autocorrelation length.

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.

CEOS.fr research project aims to improve the tools for designing the reinforced and prestressed thick concrete structures. Specific test were performed on various test bodies at full scale and 1/3 scale. This document is an overview of the experimental results obtained from the testing of large reinforced concrete specimen proposed for the benchmark ConCrack, including shear wall at scale 1/3 and tests on full scale prismatic blocks with free shrinkage (RL1 and RL6 beams) tested on bending loading. All the test bodies were fully instrumented in order to follow and locate crack initiation and propagation. A particular attention was paid to the measurement of crack widths and crack spacing, during service limit stage and until the end of loading. A summary of experimental results is provided herein: for reference RL1 and RL6 beams and reference shear wall 3.

Traditional 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 σ,. 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.