The small holes you are referring to are considered penetrations for plumbing or mechanical piping. If possible, these holes should be located about 1/4 – 1/3 of the joist spacing from a joist. For example, if the joist spacing is 6 feet (72″), the holes should be 18″ – 24″ from the joist. If the holes are core-bored after the concrete slab is cured, they must be located far enough away from the joist to ensure the boring does not cut the joist top chord.