It sets up a galvanic couple with iron rebar, with the result that the rebar is rapidly corroded away, weakening the concrete structure. Concrete is alkaline, and it never really completely dries, so it acts as the electrolyte of the corrosion cell.
Gary
Aluminum is lower in the galvanic series, so it's more likely the aluminum expands as it corrodes, stressing the concrete.
Ned Simmons
Yep. Still remember the windows in our former embassy in Cuba, where steel screws had been used to attach aluminum. The more electronegative was eaten up to an inch clear away, leaving the screws to finally rust in the salt air while we were away.
That's why they galvanize instead of use lead paint under your car.
I'm going to bet, however, that the interior of the conduit has enough room for the developed aluminum oxide.
It is _always_ appropriate to ask for a statement in writing--it eliminates potential for future misunderstandings. Any inspector who has a problem with writing down his interpretation is already your enemy. Just ask him politely and make it clear that you want to have it down so that you don't forget something important later.
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