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• Concrete structures: on-site electrochemical techniques and concrete resistivity to predict service life Part 2: Service life of structural concrete, present and future challenges

  C. Andrade

  Ochrona przed Korozją (2019). Vol. 2019 (2)

  
  

  Abstract

  Concrete protects the steel of the reinforcements in a similar manner than do the paints but with the particularity that it is a material chemically active and evolving for decades. Concrete provides a very alkaline electrolyte encapsulated in its pore network. The steel may remain passive indefinitely but chlorides and atmospheric carbon dioxide penetrate the pores and reaching the reinforcement promote corrosion, which impairs the concrete structural properties inducing the cover cracking and the loss in steel-concrete bond. Calculation of service life is becoming an important subject in engineering terms due to the increasing demand to produce buildings and infrastructures with service life beyond 100 years. In the paper focus will be devoted to two main areas: 1) the measurement techniques in concrete and permanent probes, including on site methods for large structures and contactless (inductive) methods 2) the illustration of the corrosion resistance control exerted by concrete resistivity which enables the calculation of the diffusion of ions through the concrete and the prediction of the corrosion rate in function of the climatic conditions.

  Abstract
  Concrete protects the steel of the reinforcements in a similar manner than do the paints but with the particularity that it is a material chemically active and evolving for [...]

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• Porosity and electrical resistivity-based empirical calculation of the oxygen diffusion coefficient in concrete

  P. Linares-Alemparte, C. Andrade, D. Baza

  Construction and Building Materials (2019). Vol. 198, pp. 710-717

  
  

  Abstract

  Concrete carbonation induced by gas transport has an adverse effect on the service life of the steel-reinforced material. Gas tightness is crucial where concrete is used to contain radioactive materials and of utmost importance where acting as a radon barrier. Since as a rule gas travel inside concrete is governed either by diffusion or permeability, the material’s performance in that respect is assessed by analysing those two parameters. The gas diffusion coefficient in concrete is not readily found, however, due to the practical difficulties involved in preventing gas from leaking out of standard diffusion cells. An alternative approach is to measure permeability at high gas concentration and pressure. The existence of a relationship between the oxygen diffusion coefficient and permeability in OPC concrete was established by the authors in a previous paper. Diffusion has also been shown to be related to porosity and, in solutions, to electrical resistivity. Little is known, however, about the relationship between gas diffusion and resistivity where the pores may be filled not with a liquid, but with air. Inasmuch as resistivity is a non-destructive technique and porosity a property that can be readily measured, these two parameters could be used to directly assess concrete durability, its performance in nuclear facilities and its suitability as a radon barrier. This paper describes a study conducted to formulate equations from which to analytically determine the oxygen diffusion coefficient. These equations are derived from empirical measurements of oxygen diffusion coefficient, concrete porosity and electrical resistivity. The findings, which corroborate the existence of such relationships, were used to formulate an equation to calculate the diffusion coefficient directly from the experimental values of concrete porosity and resistivity. Since porosity depends primarily on concrete batching, curing and moisture content, two OPC concrete mixes were prepared using different water/cement ratios and two curing conditions. The combination resulted in four types of concrete. Pre-conditioning at three values of relative humidity was subsequently deployed.

  Abstract
  Concrete carbonation induced by gas transport has an adverse effect on the service life of the steel-reinforced material. Gas tightness is crucial where concrete [...]

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• High performance reduced order modeling techniques based on optimal energy quadrature: application to geometrically non-linear multiscale inelastic material modeling

  M. Caicedo, J. Mroginski, S. Toro, M. Raschi, A. Huespe, J. Oliver

  Archives of Computational Methods in Engineering (2019). Vol. 26 (4), pp. 771-192 (Preprint)

  
  

  Abstract

  A High-Performance Reduced-Order Model (HPROM) technique, previously presented by the authors in the context of hierarchical multiscale models for non linear-materials undergoing infinitesimal strains, is generalized to deal with large deformation elasto-plastic problems. The proposed HPROM technique uses a Proper Orthogonal Decomposition procedure to build a reduced basis of the primary kinematical variable of the micro-scale problem, defined in terms of the micro-deformation gradient fluctuations. Then a Galerkin-projection, onto this reduced basis, is utilized to reduce the dimensionality of the micro-force balance equation, the stress homogenization equation and the effective macro-constitutive tangent tensor equation. Finally, a reduced goal-oriented quadrature rule is introduced to compute the non-affine terms of these equations. Main importance in this paper is given to the numerical assessment of the developed HPROM technique. The numerical experiments are performed on a micro-cell simulating a randomly distributed set of elastic inclusions embedded into an elasto-plastic matrix. This micro-structure is representative of a typical ductile metallic alloy. The HPROM technique applied to this type of problem displays high computational speed-ups, increasing with the complexity of the finite element model. From these results, we conclude that the proposed HPROM technique is an effective computational tool for modeling, with very large speed-ups and acceptable accuracy levels with respect to the high-fidelity case, the multiscale behavior of heterogeneous materials subjected to large deformations involving two well-separated scales of length.

  Abstract
  A High-Performance Reduced-Order Model (HPROM) technique, previously presented by the authors in the context of hierarchical multiscale models for non linear-materials undergoing [...]

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• Implicit–explicit integration of gradient-enhanced damage models

  T. Titscher, J. Oliver, J. Unger

  Journal of engineering mechanics (2019). Vol. 145 (7), pp. 1-13 (Preprint)

  
  

  Abstract

  Quasi-brittle materials exhibit strain softening. Their modeling requires regularized constitutive formulations to avoid instabilities on the material level. A commonly used model is the implicit gradient-enhanced damage model. For complex geometries, it still shows structural instabilities when integrated with classical backward Euler schemes. An alternative is the implicit–explicit (IMPL-EX) integration scheme. It consists of the extrapolation of internal variables followed by an implicit calculation of the solution fields. The solution procedure for the nonlinear gradient-enhanced damage model is thus transformed into a sequence of problems that are algorithmically linear in every time step. Therefore, they require one single Newton–Raphson iteration per time step to converge. This provides both additional robustness and computational acceleration. The introduced extrapolation error is controlled by adaptive time-stepping schemes. This paper introduced and assessed two novel classes of error control schemes that provide further performance improvements. In a three-dimensional compression test for a mesoscale model of concrete, the presented scheme was about 40 times faster than an adaptive backward Euler time integration.

  Abstract
  Quasi-brittle materials exhibit strain softening. Their modeling requires regularized constitutive formulations to avoid instabilities on the material level. A commonly used [...]

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• Preface: Computational multiscale modelling and design of new engineering materials

  T. Burczynski, J. Oliver, M. Pietrzyk, A. Huespe

  International Journal for Multiscale Computational Engineering (2019). Vol. 17 (3), pp. V-VI

  
  

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• Making use of symmetries in the three-dimensional elastic inverse homogenization problem

  C. Méndez, J. Podestá, S. Toro, A. Huespe, J. Oliver

  International Journal for Multiscale Computational Engineering (2019). Vol. 17 (3), pp. 261-280

  
  

  Abstract

  The objective of this paper is the design of three-dimensional elastic metamaterials with periodic microarchitectures. The microarchitectures of these materials are attained by following an inverse design technique jointly with an homogenization-based topology optimization algorithm. In this context, we have particularly studied the connection between the symmetry of the material layout at the microscale of 3D periodic composites and the symmetry of the effective elastic properties.We have analyzed some possible Bravais lattices and space groups, which are typically associated with crystallography, to study the way in which the symmetries of these geometrical objects can be usefully used for the microarchitecture design of 3D elastic metamaterial. Following a previous work of the authors for two-dimensional problems, we suggest adopting the design domain of the topology optimization problem coincident with the Wigner-Seitz cells of specific Bravais lattices having the same point group to that of the target elasticity tensor. The numerical assessment described in this paper aims at the design of an extreme material. The solutions obtained with this procedure show that different composite microarchitectures emerge depending on the cell shape selection.

  Abstract
  The objective of this paper is the design of three-dimensional elastic metamaterials with periodic microarchitectures. The microarchitectures of these materials are attained [...]

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• Material design of elastic structures using Voronoi cells

  J. Podestá, C. Méndez, S. Toro, A. Huespe, J. Oliver

  International Journal for Numerical Methods in Engineering (2018). Vol. 115 (3), pp. 269-292 (Preprint)

  
  

  Abstract

  New tools for the design of metamaterials with periodic microarchitectures are presented. Initially, a two‐scale material design approach is adopted. At the structure scale, the material effective properties and their spatial distribution are obtained through a Free Material Optimization technique. At the microstructure scale, the material microarchitecture is designed by appealing to a Topology Optimization Problem (TOP). The TOP is based on the topological derivative and the level set function. The new proposed tools are used to facilitate the search of the optimal microarchitecture configuration. They consist of the following: (i) a procedure to choose an adequate shape of the unit cell domain where the TOP is formulated and shapes of Voronoi cells associated with Bravais lattices are adopted and (ii) a procedure to choose an initial material distribution within the Voronoi cell being utilized as the initial configuration for the iterative TOP.

  Abstract
  New tools for the design of metamaterials with periodic microarchitectures are presented. Initially, a two‐scale material design approach is adopted. At the structure scale, [...]

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• Reduced order modeling strategies for computational multiscale fracture

  J. Oliver, M. Caicedo, A. Huespe, J. Hernández, E. Roubin

  Computer Methods in Applied Mechanics and Engineering (2017). Vol. 313, pp. 560-595 (Preprint)

  
  

  Abstract

  The paper proposes some new computational strategies for affordably solving multiscale fracture problems through a FE2 approach. To take into account the mechanical effects induced by fracture at the microstructure level the Representative Volume Element (RVE), assumed constituted by an elastic matrix and inclusions, is endowed with a large set of cohesive softening bands providing a good representation of the possible microstructure crack paths. The RVE response is then homogenized in accordance with a model previously developed by the authors and upscaled to the macro-scale level as a continuum stress–strain constitutive equation, which is then used in a conventional framework of a finite element modeling of propagating fracture. For reduced order modeling (ROM) purposes, the RVE boundary value problem is first formulated in displacement fluctuations and used, via the Proper Orthogonal Decomposition (POD), to find a low-dimension space for solving the reduced problem. A domain separation strategy is proposed as a first technique for model order reduction: unconventionally, the low-dimension space is spanned by a basis in terms of fluctuating strains, as primitive kinematic variables, instead of the conventional formulation in terms of displacement fluctuations. The RVE spatial domain is then decomposed into a regular domain (made of the matrix and the inclusions) and a singular domain (constituted by cohesive bands), the required RVE boundary conditions are rephrased in terms of strains and imposed via Lagrange multipliers in the corresponding variational problem. Specific low-dimensional strain basis is then derived, independently for each domain, via the POD of the corresponding strain snapshots. Next step consists of developing a hyper-reduced model (HPROM). It is based on a second proposed technique, the Reduced Optimal Quadrature (ROQ) which, again unconventionally, is determined through optimization of the numerical integration of the primitive saddle-point problem arising from the RVE problem, rather than its derived variational equations, and substitutes the conventional Gauss quadrature. The ROQ utilizes a very reduced number of, optimally placed, sampling points, the corresponding weights and placements being evaluated through a greedy algorithm. The resulting low-dimensional and reduced-quadrature variational problem translates into very relevant savings on the computational cost and high computational speed-ups. Particular attention is additionally given to numerical tests and performance evaluations of the new hyper-reduced methodology, by “a-priori” and “a-posteriori” error assessments. Moreover, for the purposes of validation of the present techniques, a real structural problem exhibiting propagating fracture at two-scales is modeled on the basis of the strain injection-based multiscale approach previously developed by the authors. The performance of the proposed strategy, in terms of speed-up vs. error, is deeply analyzed and reported

  Abstract
  The paper proposes some new computational strategies for affordably solving multiscale fracture problems through a FE2 approach. To take into account the mechanical effects [...]

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• On the numerical modeling of granular material flows via the Particle Finite Element Method (PFEM)

  C. Dávalos, J. Cante, J. Hernández, J. Oliver

  International Journal of Solids and Structures (2015). Vol. 71, pp. 99-125 (Preprint)

  
  

  Abstract

  The aim of this work is to describe a numerical framework for reliably and robustly simulating the different kinematic conditions exhibited by granular materials while spreading ---from a stagnant condition, when the material is at rest, to a transition to granular flow, and back to a deposit profile. The gist of the employed modeling approach was already presented by the authors in a recent work (Cante et al., 2014), but no proper description of the underlying numerical techniques was provided therein. The present paper focuses precisely on the detailed discussion of such numerical techniques, as well as on its rigorous validation with the experimental results obtained by Lajeunesse, et al. in Ref. ( Lajeunesse et al., 2004). The constitutive model is based on the concepts of large strains plasticity. The yield surface is defined in terms of the Drucker Prager yield function, endowed with a deviatoric plastic flow and the elastic part by a hypoelastic model. The plastic flow condition is assumed nearly incompressible, so a u - p mixed formulation, with a stabilization of the pressure term via the Polynomial Pressure Projection (PPP), is employed. The numerical scheme takes as starting point the Particle Finite Element Method (PFEM) in which the spatial domain is continuously redefined by a different nodal reconnection, generated by a Delaunay triangulation. In contrast to classical PFEM approximations ( Idelsohn et al., 2004), in which the free boundary is obtained by a geometrical technique (a-shape method), in this work the boundary is treated as a material surface, and the boundary nodes are removed or inserted by means of an error function. One of the novelties of this work is the use of the so-called Impl-Ex hybrid integration technique to enhance the spectral properties of the algorithmic tangent moduli and thus reduce the number of iterations and robustness of the accompanying Newton-Raphson solution algorithm (compared with fully implicit schemes respectively). The new set of numerical tools implemented in the PFEM algorithm – including new discretization techniques, the use of a projection of the variables between meshes, and the constraint of the free-surface instead using classic a-shape – allows us to eliminate the negative Jacobians present during large deformation problems, which is one of the drawbacks in the simulation of granular flows. Finally, numerical results are compared with the experiments developed in Ref. (Lajeunesse et al., 2004), where a granular mass, initially confined in a cylindrical container, is suddenly allowed to spread by the sudden removal of the container. The study is carried out using different geometries with varying initial aspect ratios. The excellent agreement between computed and experimental results convincingly demonstrates the reliability of the model to reproduce different kinematic conditions in transient and stationary regimes.

  Abstract
  The aim of this work is to describe a numerical framework for reliably and robustly simulating the different kinematic conditions exhibited by granular materials while spreading [...]

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• Continuum approach to computational multiscale modeling of propagating fracture

  J. Oliver, M. Caicedo, M. Roubin, A. Huespe, J. Hernández

  Computer Methods in Applied Mechanics and Engineering (2015). Vol. 294, pp. 384-427

  
  

  Abstract

  A new approach to two-scale modeling of propagating fracture, based on computational homogenization (FE2), is presented. The specific features of the approach are: a) a continuum setting for representation of the fracture at both scales based on the Continuum Strong Discontinuity Approach (CSDA), and b) the use, for the considered non-smooth (discontinuous) problem, of the same computational homogenization framework than for classical smooth cases. As a key issue, the approach retrieves a characteristic length computed at the lower scale, which is exported to the upper one and used therein as a regularization parameter for a propagating strong discontinuity kinematics. This guarantees the correct transfer of fracture energy between scales and the proper dissipation at the upper scale. Representative simulations show that the resulting formulation provides consistent results, which are objective with respect to, both, size and bias of the upper-scale mesh, and with respect to the size of the lower-scale RVE/failure cell, as well as the capability to model propagating cracks at the upper scale, in combination with crack-path-field and strain injection techniques. The continuum character of the approach confers to the formulation a minimal invasive character, with respect to standard procedures for multi-scale computational homogenization

  Abstract
  A new approach to two-scale modeling of propagating fracture, based on computational homogenization (FE2), is presented. The specific features of the approach are: a) a continuum [...]

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