MrPete70:

I'm not sure if this helps, but I once repaired a Dox plank floor that had some bottom cracking and spalling of the masonry near midspan, most probably due to overloading in the past.  (Our capacity check calculations showed that the Dox plank should have worked for the intended light storage loading, but not for the forklift truck that the Owner was driving on the slab, which was next to a slab designed to accommodate it).

We repaired the cracked/spalled floor as follows:
1.  Shoring and chipping out the loose, cracked, spalled masonry on the bottom;
2.  Adding bottom rebar into the open cores in two pieces - inserting each direction, and splicing in the middle;
3.  Forming the bottom side;
4.  Wetting the cores;
5.  Pouring concrete from above thru access holes drilled into the appropriate cores of the Dox plank.

We made sure we notified in writing both the Owner and the Contractor we were working with that this was NOT INCREASING THE FLOOR CAPACITY, but only cosmetically repairing the cracks.  We also cautioned the Owner about repeating the overload conditions on the floor.

I haven't heard about any additional problems on that project since doing the repairs.

Hope this helps.  Good luck.

MrPete70,
I am familiar with a method of structurally repairing dox plank full depth from the underside using Gunite.  This repair method maintains the hollowcore integrity and can address any broken prestressed tendons.  I can look for a brochure sent to me by a contractor who is well versed in this type of repair.  Please let me know if you would like for me to find and copy you with that information.  
Yukshemuch

That would be great if it would be too much trouble.