Throws Ring Construction
Here is something that may help with pouring a recess circle. 

1) The shotput circle is 7' in diameter where the discus circle is 8' 2 1/2".
2) You should use a metal ring such as aluminum.
3) The shotput and discus pads should be 10' square with the rings in the middle. As far as the area dimensions, it depends on what meets you are planning to host now and in the future.  I know many colleges which can get away with enough distance that a 55' throw lands in the area.  I would suggest at least 60'-65' of landing area.  In 2004 the NCAA changed the sector angles. The sector now is 34.92 degree instead of the original 40 degrees.
4) I suggest a stone dust. Whatever substance that you use it needs to be able to form a nice mark. Larger stones generally do not work. Stone landing areas just spread stones into the front of where the shot lands and the official can easily make a mistake in the measurement.
5) The best circle to use is the Web ring. This ring forms the best circle because the web is used as reinforcement rods in the cement.