Investigation of aggregate distribution in mechanical behavior of concrete by using finite element method based on representative volume element (RVE) modeling of concrete Hamed mohammad sharifi zamani hamed.mohammad@cv.iut.ac.ir October 21,2020 Department of Civil Engineering Isfahan University of Technology, Isfahan 84156-83111, Iran Degree: M.Sc. Language: Farsi Supervisor: Dr. Eftekhar Mohammadreza, eft@cc.iut.ac.ir Due to the construction costs and time consuming laboratory research of concrete, Researchers in the last half century have mostly simulated the finite element of concrete. In this study, by random distribution of coarse aggregates and considering the effect of fine aggregates on the mechanical properties of mortar in different porosity percentages, The effect of porosity on the tensile strength of concrete has been investigated. For this purpose, in solving the finite element of concrete, it is necessary to define the representative volume element (RVE) under periodic boundary conditions (PBC). In addition to the boundary conditions, it is necessary to periodically create the geometry and meshing of the Representative volume element. The reason for this is to establish a relationship between the micromechanical and macromechanical properties of a heterogeneous composite body. In this study, in order to study the steps of the work as accurately as possible, first, the effect of the arrangement of aggregates and the size of the representative volume element on the concrete stiffness matrix has been investigated. This research has been evaluated in two study phases by the finite element method. In the first phase of this research, after accurate simulation of the representative volume element with periodic conditions, the concrete stiffness matrix is obtained. In the first step of the first phase, the results of this phase have been compared with the laboratory and finite element results of valid references. In this verification, first with a fixed volume percentage of 20% aggregate, with five different arrangements of aggregate, the results have been validated by valid researches that in most cases, the error rate is less than 0.3%. in the next step, with five different volumetric percentages of porosity, ie 0.5, 1, 1.5, 2 and 2.5%, with a constant volume percentage of aggregate of 20%, once again the results are compared with valid studies and the amount In most cases, the error was less than 0.3%. Then, at a constant porosity of 2% and a volume percentage of 20, 25, 30, 35 and 40% of coarse aggregate, the stiffness matrix is calculated. In the second phase, to investigate the effect of porosity on the tensile strength of concrete, first the tensile strength of concrete with a porosity percentage of 10, 20 and 30% is compared with the results of various references and existing theories and the error percentage is 1.15% on average. In the next step of this phase, the tensile strength of concrete was investigated and observed in the porosity percentages of 0.5, 1, 1.5, 2, 2.5 and 3% and the constant volume percentage of aggregate 30%, which increased the porosity from 0.5 Up to 3%, tensile strength is reduced by 10%. In the third step of this phase, assuming the porosity of 2% is constant and the volume percentage of aggregate is changed with volumetric percentages of 20, 25, 30, 35 and 40%, the tensile stress-strain curve of concrete was drawn and shown that By increasing the volume percentage of aggregate from 20 to 40% of the tensile strength of concrete at a porosity of 2%, it increases by 14%. The results also showed that concrete failure occurs due to tensile stresses around the aggregates, which is consistent with empirical evidence and the contents of other sources. In general, the innovation of this research is in using the representative volume element to investigate the effect of porosity on the tensile strength of concrete under periodic conditions. Key words : Representative volume element, Finite Element Solving, Concrete, Porosity, Periodic Boundary Conditions, Mechanical Properties, Tensile Strength