At a seminar on floor construction it was said that diamond shaped boxouts around columns are better than circular. Why should this be so?
By diamond-shaped boxouts we presume you mean square boxouts rotated 45 degrees with respect to the column faces. We don't know any reason why these should be considered better than circular boxouts. Both types are recommended by ACI Committee 302 in "Recommended Practice for Concrete Floor and Slab Construction, ACI 302-69." We could add that square boxouts rotated 45 degrees are better than square boxouts that are not rotated, since the latter produce reentrant corners in the panels formed by control joints; these are almost certain sources of cracking. Re-entrant corners are avoided with boxouts that are rotated 45 degrees because floor joints meet the corners of the boxouts .