An duct coupler apparatus for sealing a bore of an anchor includes a coupler having an inner wall and a sealing mechanism affixed to the inner wall of the coupler. The sealing mechanism includes a shaft and sealing element affixed adjacent an end of the shaft. The sealing element is selectively movable between a retracted condition and an extended condition. The extended condition is adapted to be in sealing relation with an inner wall of the bore of the steel structure. The apparatus includes a nut threadedly affixed to the shaft so as to bear against the sealing element. The nut is rotatable on the shaft so as to urge against the sealing element so as to cause the sealing element to be in the extended or expanded condition.