Exposed Aggregate patio......
 

I have an exposed aggregate patio that was poured in sections using what I guess was some sort of wood to be a divider. Over the years the wood has rotted away and I was curious what to use to fill that gap. It appears the wood was a 1 by 4 or something similar.

Buy a 1 x 4 of recycled plastic stock (when I was buying the stuff in NM, it came in black, gray and brown), cut to length, and glue it into the opening(s) flush to grade using polyurethane adhesive. After you've cleaned any dirt/debris out of the gaps first, of course. The stuff weathers well (one place made picnic tables out of it), and will never rot or get slippery when wet.