As to grounding..... The reason to keep the rebar 3" inside the perimeter of the foundation is to prevent oxidation of the rebar and subsequent weakening of the foundation.
Concrete 'breathes' water in and out normally and retains a fair amount of moisture under its mass. The Moisture in concrete and the surrounding surface, and the contact with that surface actually produce a reasonable ground even before the addition of ground rods. This is called a UFER ground.
Additionally, you should have a perimeter ground around a tower of either driven rods or direct contact subsurface copper that is bonded to the lowest portion of the tower above the foundation using a 'CAD' weld for lowest impedance.
Antenna cables should come off the tower at as near 90 degrees as possible and from the lowest point possible. If you can come off the tower at floor level rather than ceiling level so much the better. Just don't let the cable traverse vertically near the tower as you will re-induce a current into the cables.
Generally 3 - 6 rods surrounding a tower driven to a depth exceeding the foundation depth and bonded with 6 ga. or larger copper is sufficient for a modest tower in average soil conditions, more never hurt.
Cables should be bonded immediately before they turn and leave the tower, and should be grounded to a point that is isolated from the tower at the ground point and grounded with a seperate large ground to the lowest ground take off for the tower. Again it is to seperate the most current flow off off the down conductors.
Cables should be bonded where they enter the building, again with an isolated bar that is grounded to the tower ground system, always downward.
All equipment in the building, including electrical panels and phone protection should be grounded to this building entry point. Do not use a ring ground in a building unless the ring is grounded only at this one point. Grounding to rods or water pipes allows current to flow from the struck tower, through the cables, through the radio equipment and out through the stray grounds. Current flow kills, not electric potential.
Think of your ground system as a you would an electric circuit. You can grab a high voltage line and not be shocked as long as you touch in only one point and there is no current flow. Raising your equipment to a million volts will do no harm as long as all the equipment rises together at the same level and time.
Use lightning arrestors, gas tubes and spark gaps wherever appropriate, always grounding to the common ground bond.
You are not preventing lightning strikes with grounding, you are just controlling the current flow as best as possible. My commercial towers are flashed 300 - 400 times a year, and struck with a signifigant hit 50 - 100 times a year with no damage due to good control of strike current. The only damage that ever occurs is when antennas are directly struck.
And as to amount of Concrete, they are all engineered to signifigant safety factors from many, many factors. Tower girth is just one of them.
Good luck, and no, I'm not a ham anymore, Extra Class expired in 1982, no desire to play radio since I work this for a living.
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