a b s t r a c tThis study examines the performance of concrete under elevated temperatures at the meso-scale level of observation where aggregate particles and the embedding hydrated cement paste form interacting continua. Decomposing concrete into these two constituents leads to mismatch of the thermal and hydraulic transport properties and hence to self-equilibrating internal stresses introducing progressive damage of the mechanical response behavior of concrete. Thereby the internal stresses are disregarded by the macro-scale level approach when the heterogeneities are replaced by equivalent effective material properties using homogenization. In other terms, the macroscopic approach eliminates the contrast among the individual constituents and consequently negates the development of stresses causing pervasive microcracking in concrete.The current study resolves concrete into its main components, the aggregate particles and the cement paste, bonded by a weak interface transition zone that reduces to some extent the mismatch between the two constituents. The study illustrates the magnitude of the stress state in representative concrete specimens and the resulting damage evolution under high temperatures.
The overall thermo-hygro-mechanical behavior of concrete is to be investigated, because its bearing capacity is required together with its shielding properties, specifically when concrete structures are exposed to high-energy neutron fluxes, which represent the next generation facilities designed for the production of high energy radioactive ion beams in physics research. Irradiation in the form of either fast and thermal neutrons, primary gamma rays or gamma rays produced as a result of neutron capture, are learnt to affect concrete as well as neutron fluences of the order of 10^19 n/cm^2 and gamma radiation doses of 10^10 rad seem to become critical for concrete strength. The collection of data on concrete samples, variously exposed to neutron radiation, has allowed for defining a law for radiation damage within the FEM research code NEWCON3D, assessing the 3D coupled thermo-hygro-mechanical behavior of concrete, modeled as a multiphase porous medium, both at the macroscale and the mesoscale level. The required damage law is thought to be a function of the neutron flux impinging the concrete shielding wall, and a good estimate of this quantity has been provided by means of a Monte Carlo code developed by CERN and the National Institute of Nuclear Physics of Milan, Italy; this code handles radiation transport calculations and represents at this day one of the most reliable procedures for dealing with the interaction of radiation and matter. The suggested procedure for the radiation damage evaluation has allowed for discussing on differences between mesolevel and macrolevel approaches. Stochastic contour maps of the expected radiation field, properly interfaced with the numerical FE code, have allowed for obtaining a more precise evaluation of the radiation damage front as well as its evolution in time
This communication addresses the localization properties of a coupled damageplasticity formulation for concrete materials to provide informations 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 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
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