A connection surface disposed on a face of a structural unit comprises a first segment having a three dimensional surface profile including a plurality of positive outer surfaces extending outwardly along a normal direction from a plane and a plurality of negative outer surfaces extending inwardly along the normal direction from the plane, wherein at least two of the plurality of positive outer surfaces are separated from one another along both vertical and horizontal directions and at least two of the plurality of negative outer surfaces are separated from one another along vertical and horizontal directions. A second segment opposes the first segment with respect to an axis, wherein the second segment is a substantial reflection of the first segment across the axis, but reversed along the normal direction.