We have just replaced the original wood expansion joints in an exposed aggregate patio with new pressure treated pine 2 x 4's. All was well for the first day or so, but today they were beginning to warp. How should the boards, which are placed vertically on the
2" side, be made to remain flat? As it is, they either rock from end to end or bow up in the middle posing a tripping situation. Thanks

Is it possible to pull them back out without damage? Even though they are pressure treated to prevent rot you still need to protect them from moisture changes from sitting on the ground. Usually when you use wood and concrete together like this you apply oil or grease to the wooden forms or paint them with black tar like roof and foundation sealer. Can use axel grease or even a rag dipped in used motor oil. Also I would recommend using 2x6 if at all possible, pound them down or dig out a little to sink them in. Or you could buy j shaped anchor bolts or make some u shaped ones out of rebar or scrap metal and pound them in alongside the 2x4 hooking them over the top of it.

Not all pressure treated lumber is the same. Are you sure yours is rated for direct contact with the ground. Reread the little vinyl label stapled to the end of the board.