Conduit sections are disclosed for forming pipe lines. Sections have preformed core members of variable shapes and wall thicknesses and cast-in-place concrete components formed selectively on external surfaces and bonded to the core by mechanical and/or chemical means, thereby forming an integral structure. The chemical bonding utilizes adhesives and the like to secure the outer component on core surfaces while the mechanical bonding may utilize various structural shapes to secure the outer component to the inner core. A narrow trenchlike form in selected contours and dimensions may be utilized as a side and bottom forming member into which concrete or other plastic mix is placed before it is cured. Preferably only slight clearance is provided between the core and the trench sides to allow placement and consolidation of the concrete mix. Components of variable thicknesses may be so formed, thereby providing greatest strength in areas where maximum support of the load is required.