"CWLee" writes:

27 years ago my concrete patio was poured. 2" x 4"s were
used to divide it into four large areas. Patio has served
me well. 2" x 4"s are beginning to rot, and slots are
developing in the patio where the 2" x 4"s are. I'm
considering just pulling up the remaining 2" x 4"s and
filling the cracks with some sort of "gunk" I could purchase
at HD or Lowe's. The plan is that the "gunk" would
generally match the color of concrete, be smoothed to be
level with the rest of the patio, and that it would harden
in a few hours/days. Question, what is the best
type/style/brand of "gunk"?

Pull out the 2x4s and pound new pressure treated ones in
place. I've done it. That worked reasonably well.

Other ideas?

Same patio eventually had the blocks go out of level.
The real cure is a jack hammer and a new patio using
pavers.