Size Effect Of Bending Performance Of Reinforced Concrete Based On Cosserat Theory

In the field of civil engineering,reinforced concrete materials are widely used.The study of size effect law is of great significance for revising the current design theory and method of reinforced concrete structures based on the results of small size test.The experimental results show that the size effect exists in the flexural behavior of reinforced concrete members,but the classical continuum theory cannot explain this phenomenon.Based on the continuum mechanics method,Cosserat theory takes into account the influence of the microsize of particles that make up the structure,and can explain the size effect phenomenon of structure.Therefore,based on Cosserat theory,the size effect of flexural behavior of reinforced concrete members is studied theoretically in this paper.The main research contents and results are as follows:（1）The separated finite element model of reinforced concrete based on Cosserat theory is established.The 3-node one-dimensional rod element,Cosserat theory plane quadrilateral 8-node element,and the constitutive relationship model of concrete and steel materials were programmed by FORTRAN language,and were implanted into ABAQUS through the user subroutine interface of ABAQUS.Finally,the nonlinear finite element model of reinforced concrete based on Cosserat theory is established.The correctness of the model is verified by comparing with the experiment.（2）Determine the value method of concrete material characteristic parameters based on Cosserat theory.Through the finite element analysis of concrete flexural test based on Cosserat theory,and then comparing the finite element analysis results with the test results,the value method of characteristic size and ultimate tensile stress is determined.The mathematical relations between the characteristic size and the compressive strength of the concrete cube,the ultimate tensile stress and the compressive strength of the concrete cube are established.（3）Based on Cosserat theory,the size effect law of pure bending failure of reinforced concrete members is studied.The size effect of pure bending failure of reinforced concrete members is studied theoretically by the way of finite element analysis of four-point bending symmetric loading reinforced concrete beams.It is found that the dimensionless bending moment and the dimensionless stiffness of the beam decrease gradually with the increase of the macroscopic size of the member,and the speed of decrease slows down gradually.With the increase of reinforcement ratio,the size effect is less and less obvious.The calculation method of flexural bearing capacity and flexural stiffness of reinforced concrete beam with four points flexural symmetry loading considering size effect is established.（4）Based on Cosserat theory,the size effect law of compression-flexure failure of reinforced concrete members is studied.Through the finite element analysis of reinforced concrete columns under eccentric compression,the size effect of compression-flexure failure of reinforced concrete members is studied theoretically.It is found that the dimensionless ultimate bearing capacity of the column decreases gradually with the increase of the macroscopic size of the member,and the speed of decrease slows down gradually.The calculation method of the ultimate bearing capacity of eccentrically loaded reinforced concrete columns considering the size effect is established.