It's hard to tell if that actually is Oldhamii, but it does resemble it.

My Oldhamii has roots that extend several feet beyond the clump in all directions. It may be rootbound if it IS actually surrounded on all sides by concrete and it is deep enough down.

The roots need plenty of room to spread. From what you explained, it probably is rootbound and needs 'food' which means it need more room to grow.

I've had some luck(short-term) in topping the ground with fertile soil but that will help the plant only for a short while.

I would clear out all of the old dead culms and see what is left. It may never recover without replacing the existing plant and soil, but since you already have the bamboo, you could take a few divisions and keep it in pots, then replace the soil and replant the new plant in the new soil.

Kt