If I'm reading this right, the driveway is splitting longitudinally at the center saw cut joint? Driveway is only 10' wide? And getting wider? Or is it an expansion joint? How wide is the gap now? Expansion joints normally are cut across the narrow portion, not the longitudinal direction. Hindsight being what it is, post tension (VERY unusual if not never for a driveway) is how it should have been done. Retrofitting a half way post tension system would be terribly expensive and may not be successful anyway. Or terribly expensive squared, LOL

Some good info at the link below.

https://www.concreteconstruction.ne...etrofit-using-external-post-tensioning_o

I'm sorry, I really don't have an answer. But rebar would have been better, might not have prevented it though.