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Take whatever your using out of the holes. Go to home depot and buy appropriate sized Redhead wedge anchor bolts. Go home and tap them all the way in with a sledge hammer. Before you hit the Redhead's back the nut off the threads a bit before hitting. Position the nut so the hammer cannot hit the threads, if the threads get mangled you have a heck of a time getting the nut on or off again. Tighten the nut down with a ratchet or wrench until you cannot tighten it anymore. 2 to 3 inch anchor bolts will more than likely be enough maintain railing post integrity if someone put their entire weight on it in any direction.

Retail hardware stores don't carry industrial epoxies, I've tried multiple retail box store epoxies and they all failed in short time (excluding JB Weld... good stuff, unfortunately they don't make concrete epoxies that I'm aware of. Unless you have access to industrial grade epoxies go with the wedge bolts.