8 th GRACM International Congress on Computational Mechanics Volos, 12 July – 15 July 2015 !"#$ % "&’( & !"(!" Department of Technical Physics and Engineering Mechanics Belarusian State University of Transport Gomel, Kirova, 34, Belarus e.mail: tm.belsut@gmail.com ; web page: http://engmech.by )#*!+ Computer Modelling, Concrete, Contact Interaction, Grains. ’!,(, The finite element analysis of the concrete structural element was made with taking into account internal contact interactions between cement matrix and filler grains. Two models of composite material were analyzed: the model with brick filler grains and the model with spherical filler grains. The contact interaction takes place between filler grains and matrix. The dry friction and the presence of cohesion are considered. The obtained results both for brick and spherical filler grains show that when the friction coefficient and cohesion values increase the stiffness of the structural element increases either. At the same time the deformation has minimal changes. The maximal stresses in the reinforcing grains are 30–50 % higher than in the composite matrix. Also there are local areas in the matrix which are stretched. Computational results also demonstrated that for some friction coefficient values there is no slipping between grains and matrix under the applied load. The coupling between brick grains and matrix is ensured by less cohesion and friction coefficient values in comparison with the spherical ones. At the same time significant stresses appear in the areas near brick edges and they can cause cracking of the material structure. The stress distribution for the matrix with spherical filler grains is more even. - . Nowadays the development of computer hardware and software designed for the stress.strain state analysis of materials and structures causes the significant increase of investigations related to the modeling of composite materials deformation. In paper [1] advanced finite element techniques in simulation of the materials behavior under mechanical loading are reviewed. Advantages and perspectives of different approaches to the simulation of deformation, damage and cracking of materials are analyzed with taking into account their micro.and mesostructure. Unit cell simulation stand out as one of the approaches for materials modeling. Fang et al in [2] used 3D hexagonal and cubic unit cells with spherical, cylindrical, cubic and rectangular particles with varied orientations to study the effect of particle shapes, orientation and volume fraction on the elastic modulus and stress.strain curves of Al alloy with Al 2 O 3 particles. It was shown that the higher is an aspect ratio of particles in a given direction, the more effective reinforcement in that direction is. Iung et al in [3] compared the stress and strain distributions in a cube and a quadrate randomly distributed properties of either first or second phase. It was shown that a 2D approximation gives results which are sufficiently different from the 3D solution. In paper [4] a mesomechanical approach for simulating the mechanical performance of non.crimp fabric composite structural parts is presented. The approach is based on the use of representative volume elements fully characterized using homogenized by the progressive failure analysis. Results of applications of unit cell approach show that it is quite effective for hierarchical models of materials. When the periodicity and regularity of material structure is observed only on one scale level, the unit cell approach is used together with other models for other scale levels (layered materials [5], "super.elements" [6], etc). Reinforced and ordinary concrete can be considered as material organized like the “composite in the composite” type [7]. At the bottom of the hierarchy there are polycrystalline and amorphous forms of calcium silicate which are indicated as C.S.H due to their composition uncertainty. They can be divided into two phases with different mechanical properties [8]. Reinforced and ordinary concrete (depending on availability of design reinforcement armature) are at the top of the hierarchy. C.S.H.crystal properties were analyzed both by the experimental methods of nanoindentation and by molecular dynamics approach [9, 10]. This analysis allowed to determine their elastic constants. For the elastic