Aggregate behaviour in concrete materials under high temperature conditions

Concrete under high temperature conditions is a topic of wide interest for applications in several engineering fields, from nuclear to civil as well as building engineering

Investigation of stress-strain behaviour in concrete materials through the aid of 3D advanced measurement techniques

This work deals with the investigation of the mechanical behaviour of cementitious materials, following a mesoscopic approach where aggregates, grains and cement paste are explicitly represented, and the strict comparison between the numerical results and the experimental results from uniaxial tests is carried out. For this purpose, solid models are created with the support of advanced techniques of measurement and detection, such as laser scanners or computer tomography (CT). The 3D laser- scanning technique in fact allows to acquire the exact shape of the grains added to the concrete mix design while, through the adoption of an ad-hoc random distribution algorithm, a realistic disposition of the inclusions is guaranteed. The industrial CT instead, is able to reproduce exactly the tested specimens; the geometry of the inclusions and their placement. Once reconstructed realistic geometries for the models, the mechanical behaviour of concrete under uniaxial compression tests is numerically studied. A specific constitutive behaviour is assigned to each component; an elasto-plastic law with damage is assumed for the cement matrix while the aggregates are conceived to behave elastically. The implemented damage-plasticity model consists in the combination of the non-associated plasticity model by Men\ue9trey-Willam, where the yield surface is described in function of the second and the third invariant of the deviatoric stress tensor and the scalar isotropic damage model by Mazars. Comparisons between numerical and experimental results fairly prove the correctness of the suggested approach

Nonlinear Modelling, Design, and Test of Steel Blast-Resistant Doors

The nonlinear dynamic response for steel blast-resistant doors is here described, referring to an innovative experience at both national and international level requiring an ad hoc design and specific numerical simulations. The elements capability to sustain thermal loads due to fire hazards is additionally accounted for. The study has been conducted to define and characterize the nonlinear behaviour of a large number of doors, with the objective of sustaining dynamic loads from explosive hazards of fixed magnitude, as well as variable design and clearing times. The local overcome of the material strength limit (with correspondent plastic response) and possible formation of plastic hinges has been critically discussed. Numerical models have allowed for refining first design sketches and subsequently understanding the real thermomechanical behaviour for the investigated elements. Some experimental tests have been additionally performed, verifying the correctness of the already available numerical results, validating the adopted procedures, and correspondingly guaranteeing the doors' structural efficiency even under dynamic loads higher than design ones

New results in 3D-mesomechanical coupled analysis of external sulphate attack in concrete

External Sulphate Attack (ESA) is one of the main degradation processes affecting concrete structures. It takes place when the concrete is in an environment rich in sulphate ions and with a high humidity index. Once it has penetrated the concrete, sulphate undergoes chemical reactions that lead to the precipitation of expansive ettringite crystals that cause volumetric expansions of the cement paste/mortar, eventually leading to cracking and damage. The FE analysis is undertaken by representing concrete as composed by aggregate pieces inserted in a cement/mortar matrix. Zero-thickness interface elements are pre-inserted to represent potential fractures along all aggregate-matrix as well as along some selected matrix-matrix element contacts. An existing fracture-based non-linear constitutive law is used for the interface elements. Concerning the reactive transport problem, the model follows previous work in the same research group, which combined an older approach from the literature for the continuum medium, with interface elements in the context of meso-mechanical analysis of concrete specimens in 2D, as well as initial work in 3D (but no coupling). In the present paper, the effects of the coupling between mechanical and diffusion/reaction in 3D are introduced and demonstrated. The new results obtained confirm that, also in 3D, penetration of ions, expansive reactions as well as subsequent cracking and degradation, all take place much faster when the coupling effect due to the open interfaces is introduced.This research is supported by grants BIA2016-76543-R from MEC (Madrid), which includes FEDER funds, and 2017SGR-1153 from AGAUR (Generalitat de Catalunya, Barcelona).Postprint (published version

Bifurcation investigations of coupled damage-plasticity models for concrete materials

This communication addresses the localization properties of a coupled damage-plasticity formulation for concrete materials to provide information on the onset of material bifurcation and the critical failure modes. Two separate loading functions are considered, one for damage and one for plasticity. A three-invariant yield surface is used to model plasticity and to consider the significant role of the intermediate principal stress and the Lode parameter on the failure of concrete materials. A non-associated flow rule is employed to control inelastic dilatancy. To model degradation of the elastic stiffness a scalar-valued isotropic damage formulation is introduced based on the total strain energy formulation that is used. Monotonic and cyclic uniaxial compression experiments are performed on concrete cylinders under displacement control and photogrammetric images are collected for Digital Image Correlation Analysis. The triaxial based damage-plasticity model is calibrated based on these experimental observations and is implemented in Matlab. Extensive localization analysis studies are performed at the constitutive level for representative load scenarios in the form of non-positive properties of the elastoplastic-damage localization tensor. The contributions of damage, plasticity and coupled damage-plasticity are explored and compared for classical Boltzmann and Micropolar Cosserat continuum formulations

Integral-type regularization of non associated softening plasticity for quasi brittle materials

Within the framework of infinitesimal elasto-plasticity for describing concrete behaviour, this paper pro-poses a new non-local constitutive model. Particularly, a non-local integral-type regularization techniqueis applied to the three-invariant Men\ue9trey-Willam model, here modified by the inclusion of a non-associated flow rule. The resulting model allows for taking into account the role of the intermediate prin-cipal stress and the Lode parameter, as well as to control inelastic dilatancy. A regularization scheme ishence introduced to circumvent strain localization and resolve the incapability of objectively describinglocalized material failure. One- and three-dimensional Finite Element analyses are developed to prove thesoundness of the novel implemented regularized scheme and the upgraded response with respect to alocal approach

Perforation studies on flat bars for XFEM-based failure analysis

This paper revisits the stress concentration problem in the proximity of circular perforations of different diameters and presents results of an experimental program on flat bars made of high strength steel, mild steel, cast iron and aluminum alloys. A digital image correlation (DIC) system is used to monitor the experiments and provide displacement and strain fields on the surface of the specimens at different stages of axial loading. The extended finite element method (XFEM) is introduced and applied to the inelastic deformation regime by coupling localization analysis and XFEM to discretely follow crack propagation into the ductile response regime