Rate and state friction has been extensively used to explain many features of the seismic cycle but the scaling of the experimentally derived parameters a, b and dc for real faults is problematic. The purpose of this paper is to present a micromechanical model for rate and state friction in which the contact between the two surfaces occur via plastic and elastic contacts. Shear deformation is accommodated in the bulk of cylindrical contacts rather than at the surface of the contact, as done classically. Assuming that the viscoplastic response is governed by the J2 plastic flow theory, we retrieve the rate and state framework. Unlike previous works, we identify the state variable as representing the changes of plastic contact area. In our model, all macroscopic frictional parameters of the rate and state framework are related to the parameters of the elementary contacts. We provide a derivation of the aging evolution law for the state variable and propose a new evolution law that reconciles the aging, Linker-Dieterich and Nagata evolution laws. We discuss the scaling of the frictional parameters for active faults and landslides. The a and b parameters should have comparable value at fault scale since friction is mostly controlled by plastic contacts at large normal stress (typically hundreds of MPa). Our model predicts that the critical slip distance dc should be scale independent and controlled solely by the plastic contacts.