if there is the possibility to weld 2 rebars of different diameters (with an overlap), is there any rule/spec/standard that states the max difference between the 2 diameters ?
For rebar, there are mostly regulations per country, in Europe.
Eurocodes say something about cross joints (4x D = max difference), but I believe nothing about lap joints.
Neither EN 10080 nor EN ISO 17660 -1/-2 mention this either.
In the US, AWS D1.4 provides the requirements for welding rebar. I don't believe that it has a requirement for a maximum/minimum difference between the two bar diameters.