A.M Goltabar - Assistant Professor, Civil Engineering Department, Babol Noshirvani University of Technology
R Shamstabar Kami - M.S. Student, Civil Engineering Department, Babol Noshirvani University of Technology, Iran
S.M Rezvani Divkolaee - M.S. Student, Civil Engineering Department, Babol Noshirvani University of Technology

For considering the design considerations and development of retrofitting of structures, exact identification of materials' behavior and their interaction, is an important subject. In concrete structures that include a high percentage of existent and under construction structures, for calculation of anchorage length of bars and investigation of sufficiency of anchorage length of bars, the state of stress distribution in bar and joint surface with concrete and their effective parameters are important. One of the important parameters in modeling the joint surface between bar and concrete is equivalent spring constant between two materials. Most researchers suggest experimental test to obtain actual value of stiffness between two materials. In this study, for obtaining the exact values of stiffness, formulation on the base of equilibrium of elements and investigation of governing differential equations and experimental method were used. In experimental tests, slips and strains, pertinent to applying load were measured by strain gauges and displacement transducers that were installed between 4 stations over concrete and bar surface with an acceptable accuracy

concrete, bar, axial tension, friction, experimental model, displacement