How do I extend a 8" clay yard vent that is 3 inches below the concrete in my parking pad?

The short story is that the previous owners covered the vent with 3" of concrete sometime ago. I had the vent riser located and I broke up a section of the pad and found the vent opening. Now I need to extend that clay section about 3 inches to flush up to the concrete surface. Any ideas what to use?