Some 25 years ago, I had a concrete driveway poured. Of course this is much larger than a sidewalk, but the contractor separated the sections with outdoor treated 2x4s. They lasted pretty well until I replaced them this past summer. While it was a miserable job to chisel the old boards out, it's only a once in 25 years job. I used a sacrificial 1" wood chisel, crow bar, a couple of hammers, and several gallons of sweat. When I put the new boards in the slots, I first used my planer to shave 1/16" off of each side of the new boards. I think the well defined sections look very nice.