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Jeong J.,Special School for Public Works, Building and Industry | Neff P.,TU Darmstadt
Mathematics and Mechanics of Solids | Year: 2010

We investigate the weakest possible constitutive assumptions on the curvature energy in linear Cosserat models still providing for existence, uniqueness and stability. The assumed curvature energy is μL2c ∥dev sym ∇axl A∥2 where axl A is the axial vector of the skewsymmetric microrotation A ∈ so(3) and dev is the orthogonal projection on the Lie-algebra sl(3) of trace free matrices. The proposed Cosserat parameter values coincide with values adopted in the experimental literature by R. S. Lakes. It is observed that unphysical stiffening for small samples is avoided in torsion and bending while size effects are still present. The number of Cosserat parameters is reduced from six to four. One Cosserat coupling parameter μc > 0 and only one length scale parameter L c > 0. Use is made of a new coercive inequality for conformal Killing vectorfields. An interesting point is that no (controversial) essential boundary conditions on the microrotations need to be specified; thus avoiding boundary layer effects. Since the curvature energy is the weakest possible consistent with non-negativity of the energy, it seems that the Cosserat couple modulus μc > 0 remains a material parameter independent of the sample size which is impossible for stronger curvature expressions. © 2010 SAGE Publications.

Jeong J.,Special School for Public Works, Building and Industry | Sardini P.,University of Poitiers | Ramezani H.,CNRS Center for Research on Divided Matter | Siitari-Kauppi M.,University of Helsinki | Steeb H.,Ruhr University Bochum
Computational Materials Science | Year: 2013

In the present paper, we focus on the modeling of the CO2 carbonation phenomenon through the cement mortars. The CO2 carbonation will modify not only the pH value of the cement mortars but also the mortar specimen size, i.e. mortar shrinkage. The aforementioned issue is driven via the chemical reactions and it affects the micro-structure and particularly the porosity. Based upon the micro-dilatation theory which was originally proposed by Cowin and Nunziato [1,2], one of the four material moduli dealing with the micro-structure especially, β-ξ relation, has been determined for the first time. To pursue this goal, the carbonation experiments on the cement mortar (CEMI) have been achieved to determine CO2 affected zones by the carbonation process and to measure the corresponding deformations caused by the shrinkage phenomenon. According to our proposed semi-empirical method, it is possible to compute the induced chemical stress as well as the porosity gradient through the mortar. The numerically-obtained porosity distribution across the cement mortar has been afterwards compared to the outcomes coming from the 3D-connected porosity profile which is extracted by means of 14C-PMMA experiments. The latter experiments allow us not only to evaluate the computations for the porosity, porosity gradient and the induced stress but also to determine the micro-dilatation material moduli (coupling number, N = -0.316), in conjunction with the pore stiffness or the so-called void stiffness of the cement mortars. © 2012 Elsevier B.V. All rights reserved.

Neff P.,Universitatsstr 2 | Jeong J.,Special School for Public Works, Building and Industry | Fischle A.,Universitatsstr 2
Acta Mechanica | Year: 2010

We describe a principle of bounded stiffness and show that bounded stiffness in torsion and bending implies a reduction of the curvature energy in linear isotropic Cosserat models leading to the so-called conformal curvature case μ L c 2/2 ∥ rm dev ∇ axl ̄ A 2∥ where ̄ A ∈ so (3) is the Cosserat microrotation. Imposing bounded stiffness greatly facilitates the Cosserat parameter identification and allows a well-posed, stable determination of the one remaining length scale parameter L c and the Cosserat couple modulus μ c . © 2009 Springer-Verlag.

Ramezani H.,CNRS Center for Research on Divided Matter | Mounanga P.,University of Nantes | Jeong J.,Special School for Public Works, Building and Industry | Bouasker M.,CNRS Center for Research on Divided Matter
Cement and Concrete Composites | Year: 2013

The self-induced stress in mortars caused by autogenous shrinkage phenomenon at very early age has been investigated by taking advantage of the experimental results and application of hygro-Cosserat theory. Three cement matrices, prepared with three different types of cement, have been considered to analyze the role of cement paste composition on the development of these initial stresses surrounding the aggregates. The induced bulk deformations around the aggregates have been thereafter scrutinized by means of hygro-Cosserat elasticity taking into account the size effect in an explicit manner and a newly defined parameter called Cosserat size effect number (CS). By taking advantage of the autogenous shrinkage experiments on the cement pastes, the parallel time-dependent finite element analyses have been achieved considering the aggregate-to-cement ratio. The aforementioned parameters are put into practice to create an analytical pseudo-stochastic geometry using a spherical-packing algorithm. The numerical outcomes were analyzed and compared to the experimental outcomes coming from the Scanning Electronic Microscopy observations at 48 h after hydration. © 2013 Elsevier Ltd. All rights reserved.

Ramezani H.,French National Center for Scientific Research | Jeong J.,Special School for Public Works, Building and Industry
Acta Mechanica | Year: 2011

In the present paper, environmentally motivated numerical modeling for the limestone layered masonry structures has been investigated involving the continuous hygric state field variable, i.e. relative humidity Π. By taking advantage of the Lin et al. assumption pertaining to the relative humidity (independency of the relative humidity and temperature) (Lin MW et al. Build. Environ. 41(5):646-656, (2006); Khoshbakht M et al. Finite Elem. Anal. Des. 42(5):414-429, (2006); Khoshbakht M, Lin MW Meas. Sci. Technol 2989-2996, (2006); Khoshbakht M Finite Elem. Anal. Des.; 45(8-9):511-518, (2009)), we have provided a mathematical model involving hygro-thermo-mechanical aspects as well as the water vapor transfer across the porous limestone masonry walls. The numerical study substantiates the impact of hygric effects as the major key point in the thermo-hygro-mechanical degradation and effect of geometry in the real brick-line of mortar assembly. Furthermore, we have obtained the moisture entrapment at the intersection of the lines of mortar through the layered masonry wall by means of the multidisciplinary nonlinear finite element method (NFEM) for variably saturated porous media. The new outlooks and fresh departure in durability and aging have been briefly discussed. © 2011 Springer-Verlag.

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