I replace a ground rod when the length is suspect or the diameter is too small. For instance: When upgrading a service for a residence and the existing ground rod appears to have been cut or otherwise damaged on the visible top end, I drive a new one. Likewise, if the existing rod is less than 1/2" in diameter, made of bare or galvanized steel or pipe and has some age on it I will drive a new one. Basically if I have any doubt about an existing rod I will spend the $8.00 and add another one. Can't hurt a thing.
