I remember reading Mr. Ufer's reports on his grounding electrode. Under extremely adverse conditions, his method always performed far better than the 25 ohms code calls for. This was confirmed by Max Macomb, in his documentation that finally persuaded the code panels to accept the "Ufer ground."
If you're measuring over 25 ohms, I would suspect a problem with the measurement method.
But, there is no rule agains adding an electrode if you want to....but it needs to be six feet, or more, from the Ufer...which, in this case, means the entire slab. With a "Ufer," your slab is the electrode- not just that little copper wire!
While many measure the resistance of the ground by simply sticking their meter probe in the dirt....there has to be a reason for those pricey clamp-on meters, and other fancy equipment, used to measure grounding impedence.
