This is a bunker, probably build by the French, and part of a ring of bunkers on the southern edge of Phu Bai. It covers the road from Danang. It was there in the 60's and still sits on the road.
This bunker is just off the road about halfway down the old French-built road that runs out of the Highlands between DaLat and Phan Rang. I would have taken a closer look but there was an old mine warning sign laying in the grass so it was not worth the risk.