I have repaired/strengthened a lot of 'structurally sick' PT buildings and purposely severed hundreds of UNBONDED tendons in a controlled manner which can vary from heating exposed strand, oxy-burn the anchorage wedges, or my preferred method of 5" angle grinder cutting to exposed strand.
In my experience, the manner in which a partially cut monostrand tendon 'takes-up' the total remaining prestressing force depends on whether the strand has corrosion damage, the grease condition and the sheath type.
For example, if a 7-wire strand has no corrosion damage and the grease has not dried, when say 3 of the 7 wires are cut those three cut wires are free to recoil (strain is released to those cut wires), and the remaining uncut 4 wires tend NOT to 'take-up' the force allocation of the 3 cut wires, so in effect the capacity of the partially cut tendon is 4/7th (not quite, as the central king wire is of larer diameter). Sort of like as if the 7 wires are not stranded, but parallel wires.
For a strand that has corrosion damage/byproduct and the grease has dried out, and of older type sheaths (Heat sealed or kraft paper) when a few wires are cut the friction/binding of the corrosion/dried grease tends not to enable the cut wire to recoil, so there is no appreciable strain relief of the cut wires, and therefore the pre-existing prestress force is taken up on the remaining uncut wires and the uncut wires are then overloaded and failure is eminent.
Every strand tendon that I have repaired is always re-stressed - 99% of the time via hydraulic jacks/rams, but SURELOCK do have a 'Grabbit' turnbuckle type splice for strands, but they are a pain to apply suitable prestress force.
As for repairs in tight spaces, seldom are splice chucks placed at the location of the cut, unless it coincides with approx mid depth of slab to accommodate PT splicing hardware. If access is available to exterior anchorage pockets it is possible to remove and replace the full strand (if double live end) or remove and replace a strand segment and splice at a convenient interior location.
Or do what HOKIE66 suggested, and run a calc assuming this strand provides no structural contribution and see its effect.
For UNBONDED slabs built in the 1970's, many were not constructed with bonded mild steel rebar and the permissible stresses for flat plate banded-uniform placement patterns were/are questionable. Crack control, serviceability, and ultimate flexural capacity of these structures need to be carefully examined.
