How is the structure of bone similar to reinforced concrete?
It is composed of a porous solid network filled with different cellular materials that contribute to the strength and other functions of the bone.
It is not a continuous, homogeneous casting of material. Just as reinforced concrete contains an interlinked network of steel bars around and through which the concrete is set, bones are a network of hard calciferous cells supporting and reinforcing the spongy (cancellous) bone and the Medullary cavity.
Good graphics of the different parts of a bone structure are here: