I've taken out a concrete patio that didn't fully cover the floor joists.

I'm going to pour a concrete header as a water barrier and as a termite barrier. Of course a new patio could poured against the header at a later time.

The concrete header will be about 4" thick and about 16" high and working up from what might be called the brick ledge. I would like to pour the concrete header in four sections and so I am thinking about 1/8" thick 316 stainless steel plates perpendicular
into the header to make the sections.

Would the plates be better longitudinally into the header making a concrete seam (of new pour against prevous pour) or
perpendicular into the header with the plate as the seam ?

Leave gaps between the wall sections and use non-shrink grout in the gaps