Concrete finishes are sometimes described by means of the letter F followed by a number. When are the various finishes called for and how are they produced?
The F designations are used in Bureau of Reclamation specifications and the following descriptions are quoted directly from the Bureau's Concrete Manual: Finish F1.--This finish applies to surfaces where roughness is not objectionable, such as those upon or against which fill material or concrete will be placed, the upstream faces of concrete dams that will normally be under water, or surfaces that will otherwise be permanently concealed. The only surface treatment required is repair of defective concrete, correction of surface depressions deeper than 1 inch, and filling of tie-rod holes where the surface is to be coated with dampproofing or where the holes are deeper than 1 inch in walls less than 12 inches thick. Form sheathing may be any material that will not leak mortar when the concrete is vibrated. Forms may be built with a minimum of refinement. Finish F2.--This finish is required on all permanently exposed surfaces for which other finishes are not specified, such as the surfaces of canal structures; the inside surfaces of siphons, culverts, and tunnel linings; outlet works and open spillways; small power and pumping plants; bridges and retaining walls not prominently exposed to public inspection; galleries and tunnels in dams; and concrete dams except where F1 finishes are permitted on the upstream faces. Form sheathing may be shiplap, plywood, or steel. Thin steel sheets (steel lining) supported by a backing of wood boards may be used on approval, but the use of steel lining is not encouraged. To obtain an F2 surface the forms must be built in a workmanlike manner to the required dimensions and alignment, without conspicuous offsets or bulges. Finish F3.--This finish is designated for surfaces of structures prominently exposed to public view where appearance is of special importance. This category includes superstructures of large power and pumping plants; parapets, railings, and decorative features on dams and bridges; and permanent buildings. To meet the requirements for the F3 finish it is necessary for the forms to be built in a skillful, workmanlike manner, accurately to dimensions. Occasionally tongue-and-groove boards or plywood sheets may be required for specific F3 surfaces. However, specifications usually permit either tongue-and-groove boards or plywood at the contractor's option. Steel lining is not permitted. There should be no visible offsets, bulges, or misalignment of the concrete. At construction joints the forms should be tightly reset and securely anchored close to the joint, as described in section 97. Finish F4.--This finish is required for formed concrete surfaces where accurate alignment and evenness of surface are essential for prevention of destructive effects of water action. Such surfaces include portions of outlets, draft tubes, and spillway tunnels of high dams. The forms must be strong and held rigidly and accurately to the prescribed alignment. Any form material or sheathing that will produce the required surface (such as close-fitting shiplap, tongue-and-groove lumber, plywood, or steel) may be used. For warped surfaces, the forms should be built up of laminated splines cut to make tight, smooth form surfaces after which the form surfaces are dressed and sanded to the required curvature. Finish F5.--This finish is required for formed concrete surfaces where plaster, stucco, or wainscoting is to be applied. Since a coarse-textured surface is needed for bond, the concrete should be cast against rough-faced (S1S2E) form boards. Form oil should not be used. Steel lining or steel sheathing is not permitted. Incidentally, copies of the seventh edition of the 642-page Concrete Manual are again available from the Book Department, CONCRETE CONSTRUCTION Publications.