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Fontainebleau, France

Idiart A.,Amphos21 Consulting S.L | Idiart A.,Oxand SA | Bisschop J.,ETH Zurich | Caballero A.,BBR VT International | And 2 more authors.
Cement and Concrete Research | Year: 2012

Aggregates in cementitious composites subject to drying lead to mechanical restraint of the matrix shrinkage, which under certain conditions may lead to internal microcracking. In the present work this phenomenon is investigated using a two-dimensional (2D) numerical model and an approximate 2D experimental approach. Experimental and simulated samples with simplified and matching spatial aggregate distributions were produced to make a quantitative comparison between experiments and model predictions. In particular, the effects of aggregate size and volume fraction on the degree of internal microcracking are assessed. The main challenges of performing a quantitative comparison are highlighted and discussed. These are related to: (i) the difficulty of designing experiments without moisture gradient effects; (ii) the experimental crack detection limit; and (iii) the role of the creep response of the matrix phase in the model. The results suggest the existence of a critical aggregate size below which aggregate-restraint does not cause detectable microcracking. © 2011 Elsevier Ltd. All rights reserved. Source

Idiart A.E.,Oxand SA | Lopez C.M.,University of Barcelona | Carol I.,University of Barcelona
Cement and Concrete Composites | Year: 2011

In this paper we focus on the external sulfate attack on concrete specimens at the meso-level. There is nowadays a renewed interest in rationally describing the mechanisms behind expansive processes leading to cracking and spalling of concrete exposed to sulfate solutions. A model is presented for degradation of concrete exposed to external sulfate attack at the meso-level, i.e. representing explicitly in the simulations the main heterogeneities of the material. A previously developed mesomechanical approach has been coupled with a diffusion-reaction analysis at the same scale. It is based on the systematic use of zero-thickness interface elements equipped with fracture-based constitutive laws, and the effect of discrete cracks on the transport of ions is explicitly accounted for. The main results obtained agree qualitatively and quantitatively well with experimental observations in terms of ettringite penetration depth, expansions, crack patterns and spalling effects. © 2011 Published by Elsevier Ltd. Source

Idiart A.E.,Oxand SA | Lopez C.M.,University of Barcelona | Carol I.,University of Barcelona
Materials and Structures/Materiaux et Constructions | Year: 2011

In this paper, an existing mesomechanical model for cementitious materials is extended to the domain of diffusion-driven phenomena. The model is based on the Finite Element Method, and uses zero-thickness interface elements equipped with a fracture-based constitutive formulation to represent cracks. The new developments presented in this paper consist of the application of the model to the hygro-mechanical coupled analysis of drying shrinkage in concrete specimens, explicitly taking into account the influence of (micro) cracks on the diffusion of moisture. In a first part of the paper, the model is presented in some detail, especially the new aspects regarding moisture diffusion including effects of cracks, and H-M coupling. The model predictions are then quantitatively compared with classical drying shrinkage experiments on concrete specimens. The consideration of different assumptions for the relation linking shrinkage strains and weight losses is discussed in some detail. Finally, the effect of size and volume fraction of the main heterogeneities of concrete on the drying process and drying-induced microcracking is also addressed. © 2010 RILEM. Source

Schoefs F.,University of Nantes | Boero J.,Oxand SA | Clement A.,Oxand SA | Capra B.,Oxand SA
Structure and Infrastructure Engineering | Year: 2012

Owners of civil infrastructure base their maintenance decision schemes mainly on both structural integrity assessment and consequence analysis. Some inputs come from information collected by inspections with non-destructive or destructive tools. Uncertainties and errors of measurement can lead to bad decisions but are rarely integrated into the decision process. Currently, risk-based inspection (RBI) provides the basic concepts for optimising the maintenance plans of existing structures while insuring satisfactory safety and availability of the structure during its service life. It relies both on reliability computations and probabilistic modelling of inspection results. This last point leads to introduce the probability of detection and the probability of false alarms that are usually considered as parameters in RBI. However, when data from inter-calibration campaigns are available, the whole receiver operating characteristics curves should be used. Then, the comparison of non-destructive testing tools in terms of cost or benefit is difficult as well as the method for optimising a given technique. This article presents the αβ method that gives a new performance indicator in this context. It is applied to the field of inspection of harbour structures. © 2012 Taylor and Francis. Source

Laudet J.-B.,Total S.A. | Garnier A.,Total S.A. | Neuville N.,Oxand SA | Le Guen Y.,Oxand SA | And 4 more authors.
Energy Procedia | Year: 2011

One of the major technological issues for CO2 injection is the long-term behavior of the cement-based materials used to ensure the overall sealing performance of the storage wells. When water is present, the CO 2 injected can react chemically with the cement (i.e. carbonation). The objectives of our experimental program are to assess the kinetics and phenomenology of the changes that occur in class-G Portland cements exposed to CO2-enriched aqueous fluids at 8 MPa and two different temperatures (90°C and 140°C). The experimental program presented in this paper consists of two carbonation tests (static tests) and a coupled chemo-mechanical test (dynamic test) which were performed on similar class-G cement and similar CO2-rich water. The main preliminary results show a carbonation front progressing from the fluid-sample interface towards the sample centre. The front moves faster at 140°C than 90°C because of the different carbonation process involved at this higher temperature. The results of a coupled chemo-mechanical test with injection of CO2-saturated water show that the CO2 flow rate in the cement rapidly decreases, finally resulting in carbonation clogging of the cement sample. They seem consistent with reported field observations. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd. Source

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