Thread washers and nuts onto both ends of the rod and tighten them up. Because the forms stay in place, you'll never see voids that remain in the wall. So, you should invest some extra effort to prevent voids. First ask the concrete supplier to add plasticizer to the mix. This makes it slippery, so it flows better around obstructions.
Some manufacturers suggest vibration to eliminate voids.
Corners may need special attention, because extra spacers can sometimes create voids. One builder uses a special needle-shaped vibrator. Following the pumper, a worker sticks the vibrator into the concrete every 3 to 4 ft. It only needs to run for a couple of seconds. Too long will bring water to the top.