When cutting slots in the pavement with a water-cooled diamond blade saw for inductive vehicle detector loops at the traffic signals, we'd shove a 18" piece of the 1/2" backer rod into the slot around the corners to keep the wire from popping out when completing 3 turns of wire; the corners were saw cut with two adjacent 45's to make a easy 90 so as to not stress the wire. Since they weren't wound too tightly the wire would try to pop out as the sun heated up the pavement, especially if the pavement mix was high in asphalt content.
You'd get to a corner with the second turn of wire, pull out the backer rod, push in the wire with a wooden paint stir stick, shove the rod back in, etc. until all 3 turns were in. Then we'd read the loop's continuity and meg it to ground with 500V to insure it was good and no leakage to earth then seal the saw slot with an emulsion-based caulk.
The larger 6 ft. X 50 ft. or 6 ft. X 100 ft. loops were the ones that often needed the backer rod technique; the smaller 6 ft. X 6 ft. loops seldom did.