Tracking Steel

We're looking to track a lot of our small equipment but the real issue is tracking our expensive steel plates. You know them by driving over them during underground construction on the road ways. We're looking to counter sink (almost flush) a small tag and epoxy over it in the side of the 1" thick plates. I'm finding the readers have to be very close (a couple inches). Where is the technology at now? If the industry can come up with a 6' to 10' range there's a huge untapped market out here in construction. We're looking to start testing but I'm having trouble finding product. I'm going to get a couple tags from a couple companies and see what's really going on.
Something for you engineers to work on.