Can I use #8 bare aluminum wire for a ground with 3 insulated #8awg wires in PVC pipe? I have this wire just lying around. Or should I use all insulated wires?
Assuming you are in the US, the National Electric Code (NEC) allows bare aluminum equipment grounds and grounding electrode conductors.
For equipment grounding the relevant section is NEC 250.120, which prescribes corrosion protection for bare, covered, or insulated aluminum or copper-clad aluminum:
For grounding electrode conductors, NEC 250.62 explicitly allows bare aluminum grounding electrode conductors:
with NEC 250.64 prescribing corrosion protection that doesn't differentiate insulated ground wire from uninsulated ground wire: