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SECTION I. PRINCIPLES

IMPORTANCE OF FORM DESIGN

4-2. Formwork holds concrete until it sets, produces the desired shapes, and develops a desired surface finish. Forms also protect concrete, aid in curing, and support any reinforcing bars or conduit embedded within the concrete. Because formwork can represent up to one-third of a concrete structure's total cost, this phase of a project is very important. The nature of the structure, availability of equipment and form materials, anticipated reuse of the forms, and familiarity with construction methods all influence the formwork design. To design forms, you must know the strength of the forming materials and the loads they support. You must also consider the concrete's final shape, dimensions, and surface finish.

FORM CHARACTERISTICS

4-3. Forms must be tight, rigid, and strong. Loose forms permit either loss of cement, resulting in honeycomb, or loss of water causing sand to streak. Brace forms enough to align them and to strengthen them enough to hold the concrete. Take special care in bracing and tying down forms used for such configurations as retaining walls that are wide at the bottom and taper toward the top. The concrete in this and other types of construction, such as the first pour for walls and columns, tends to lift the form above its proper elevation. To reuse forms, make them easy to remove and replace without damage.

FORM MATERIALS

4-4. Forms are generally made from four materials--wood, metal, earth, and fiber.

WOOD

4-5. Wood materials are the most common form materials by far, because they are economical, easy to produce and handle, and can adapt to many shapes. For added economy, you can reuse form lumber for roofing, bracing, and similar purposes. Soft wood, such as pine, fir, and spruce, makes the best and most economical form lumber because it is light, easy to work with, and available almost everywhere. Form lumber must be straight, structurally sound, strong, and only partially seasoned. Kiln-dried lumber tends to swell when soaked with water from the concrete. If the form is tightly jointed, the swelling can cause bulging and distortion. When using green lumber, either allow for shrinkage or keep the forms wet until the concrete is in place. Lumber that contacts concrete should be finished at least on one side and on both edges. The edges can be square, shiplap, or tongue and groove. The last finish makes a more watertight joint and helps prevent warping. Use plywood identified for use in concrete forms for wall and floor forms if it is made with waterproof glue. Plywood is more warp-resistant and can be reused more often than lumber. It is available in thickness of 1/4, 3/8, 9/16, 5/8, and 3/4 inch, and in widths up to 48 inches. The 5/8 and 3/4 inch thicknesses are more economical because the thinner sections require solid backing to prevent deflection. However, the 1/4 inch thickness is useful for curved surfaces. Although longer lengths are available, 8-foot plywood is the most common.

METAL

4-6. Metal materials are used for added strength or when a construction will be duplicated at more than one location. They are initially more expensive than wood forms but are more economical in the long run if reused often enough. Steel forms are common for large, unbroken surfaces such as retaining walls, tunnels, pavements, curbs, and sidewalks. They are especially useful for sidewalks, curbs, pavements, and precast-concrete operations because you can use them many times.

EARTH

4-7. Earth materials are acceptable in subsurface construction if the soil is stable enough to retain the desired concrete shape. The advantages of earth forms are that they require less wood construction and resist settling better than other types of forms. Their obvious disadvantages are rough surface finish and the requirement for more excavation. Therefore, earth forms are generally restricted to footings and foundations.

FIBER

4-8. Fiber forms are prefabricated and are filled with waterproofed cardboard and other fiber materials. Successive layers of fiber are first glued together and then molded to the desired shape. Fiber forms are ideal for round concrete columns and other applications where preformed shapes are feasible because they require no form fabrication at the job site and thus save considerable time.

FORMING

4-9. Forms are assembled and used to make walls. Figure 4-1 shows the basic parts of a wood form for panel walls.

Figure 4-1. Wood form for a concrete panel wall

SHEATHING

4-10. Sheathing forms the vertical surfaces of a concrete wall but runs horizontally itself. The sheathing edges and the side that contacts the concrete should be finished as smooth as possible, especially for an exposed final concrete surface. The sheathing must also be watertight. Sheathing made from tongue-and-groove lumber gives the smoothest and most watertight concrete surface. Plywood or fiber-based hardboard can also be used.

STUDS

4-11. The weight of the plastic concrete will cause the sheathing to bulge if it's not reinforced by studs. Therefore, vertical studs add rigidity to the wall form. The studs are made from single 2 by 4s or 2 by 6s.

WALES (WALERS)

4-12. Wales reinforce the studs when they extend upward more than 4 or 5 feet. They should be made from doubled 2 by 4s or 2 by 6s, and are lapped at the form corners to add rigidity. Double wales not only reinforce the studs but also tie prefabricated panels together and keep them aligned.

BRACES

4-13. Although braces are neither part of the form design nor considered as providing any additional strength, they help stabilize the form. The most common brace is a combination of a diagonal member and a horizontal member nailed to a stake at one end and to a stud or wale at the other end. The diagonal member makes a 20- to 60-degree angle with the horizontal member. To add more bracing, place vertical members (strongbacks) behind the wales or in the angle formed by intersecting wales.

SPREADERS

4-14. Spreaders are small pieces of wood placed between the sheathing panels to maintain the proper wall thickness between them. They are cut to the same length as the wall thickness. Spreaders can be removed easily before the concrete hardens since friction, not fasteners, hold them in place. Attach a wire securely through the spreaders, as shown in Figure 4-1 to pull them out when the fresh concrete exerts enough pressure against the sheathing to permit removal.

TIE WIRES

4-15. Tie wires secure the formwork against the lateral pressure of the plastic concrete. They always have double strands.

TIE-RODS (SNAP-TIES)

4-16. Tie-rods sometimes replaces tie wires in the same function because working with rods are easier.

COLUMN FORMS

4-17. Figure 4-2 shows the basic parts of a wood form for a concrete column.

Figure 4-2. Form for a concrete column and footing

SHEATHING

4-18. Sheathing runs vertically in column forms to reduce the number of saw cuts. Nail the corner joints firmly to ensure watertightness.

YOKES

4-19. A yoke is a horizontal reinforcement in the form of a rectangle that wraps around a column to prevent the plastic concrete from distorting the form. It serves the same purpose as a stud in a wall form. You can lock yokes in place using the sheathing-, scab-, or bolt-type yoke lock. The small horizontal dimensions of a column do not require vertical reinforcement.

BATTENS

4-20. Battens are narrow strips of boards that are placed directly over the joints to fasten several pieces of vertical sheathing together.

FOOTING FORMS

4-21. Footing forms are shown in Figure 4-2 and are discussed in paragraph 4-49.

David L. Heiserman, Editor

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Revised: June 06, 2015