rent a 'cut-off saw', some are hand held, some they`ll have on a cart. Drop a chalk line where ya want to cut `n yer off. Wear safety glasses,small pieces might fly while yer cutting.

It is hard work, dusty, you might run into problems with rebar or wire mesh in it, and you have to patch it when done. And it will look like a patch.

Instead burrow under the sidewalk.

Dig a hole on each side.

Then take a piece of pipe and connect it to water hose and Jet it through.

Depending on the size, type of pipe and application then the pipe that you jet through can be the directly used or use a bigger pipe and then put the actual pipe through the jetted pipe.