Dox plank was probably the first precast-prestressed plank that was manufactured in this country (USA). I believe it appeared in the early to mid 1950's. The plank were 16" wide, and were mostly 8" depth (but I believe, 6", 10" and 12" depths may have been available). It was a decent prefabricated concrete material (used primarliy for motels, and other repetitive structures in conjunction with CMU bearing walls and some steel framing) because it was relatively light weight, and had a flat soffit. It consisted of individual concrete blocks, strung together over reinforcing steel that was then prestressed to provide some initial compressive stress.
The major problem with Dox plank was that when it came onto the market, reinforcing steel was not yet high strength, and most of the prestressing forces were lost due to concrete shrinkage and other initial losses. The theory was much better than the practice at the time.
Mildly reinforced channel slabs (at that time) were much more durable, and in my opinion more functional, but they lacked the flat soffit of Dox plank.
When extruded (and some wet cast such as Flexicore) plank came onto the market (today's hollowcore plank), at about the same time as higher strength reinforcing, Dox became quickly defunct, as Dox was more costly to manufacture.
If you are looking at a building with Dox plank, it should be analyzed as a mildly reinforced member. If Dox is used on exterior balconies, or any other use exposed to the weather, it is time to demolish them, as the reinforcing is almost undoubtedly corroded severely. Dox plank offered little concrete protection because they were very porous, and allowed considerable migration of moisture, etc.
