The roughness coefficient is an important influencing factor of the joint shear strength. However

the research of size effect on joint roughness coefficient has not gained great progress due to surface morphology complexity of rock joint. This paper summarizes three methods to obtain joint roughness coefficient

i.e. standard profile comparison method

theoretical equation method and inverse analysis method

and then analyzes some existing problems and difficulties on studying size effect on joint roughness coefficient. In order to achieve the relationship between joint roughness coefficient and sample size

the ratios of raw materials such as medium sand

silica fume

cement

non-air entraining naphthalene super-plasticizer

are firstly investigated. Then rock materials for simulation are obtained

in which physico-mechanical properties are similar to calcareous slate. Finally

eight groups of rock joints with 176 pairs of different sizes and roughness are made through molds in preparation process. In addition

the roughness coefficients of rock joints are tested by homemade push-pull shear apparatus and experimental data are further analyzed. The results show that statistical mean values of joint roughness coefficient generally decrease with the increase of sample size. However

the size effect tests on the specific rock joints need to be performed through push-pull shear strength testing method. It is found that size effect variations calculated by Barton formula are in good agreement with experimental results

whereas the theoretical and experimental values are different. In particular

when the size of sample is smaller

the differences are greater. Furthermore

rock roughness coefficients even with specific joint surfaces are various and the roughness coefficients of large size rock joint should be comprehensively judged according to the surface morphology and distribution.