Form Materials

Forms are generally constructed from earth, metal, wood, fiber, or fabric.

Earth

Use earthen forms in subsurface construction where the soil is stable enough to retain the desired shape of the concrete. The advantages of earthen forms are that less excavation is required and there is better settling resistance. The obvious disadvantage is a rough surface finish, so the use of earthen forms is generally restricted to footings and foundations. Always take precautions to avoid collapse of the sides of trenches.

Metal

Use metal forms where high strength is required or where the construction is duplicated at more than one location. They are initially more expensive than wood forms, but may be more economical if they can be reused repeatedly. Originally, all prefabricated metal forms were made of steel. These forms were heavy and hard to handle. Aluminum forms, which are lightweight and easier to handle, are replacing steel.

Prefabricated metal forms are easy to erect and strip. The frame on each panel is designed so that the panels can be easily and quickly fastened and unfastened. Metal forms provide a smooth surface finish so that little concrete finishing is required after the forms are stripped. They are easy to clean, and maintenance is minimal. Care should be taken when stripping metal forms to ensure they are not damaged.

Metal wood forms are just like metal forms except for the face. It is made with a sheet of B grade exterior plywood with waterproof glue.

Wood

Wooden forms are by far the most common type used in building construction. They have the advantage of economy, ease in handling, ease of production, and adaptability to many desired shapes. Added economy may result from reusing form lumber later for roofing, bracing, or similar purposes. Lumber should be straight, structurally sound, strong, and only partially seasoned. Kiln dried timber has a tendency to swell when soaked with water from the concrete. If the boards are tight jointed, the swelling will cause bulging and distortion. When green lumber is used, make allowance for shrinkage, or keep the forms wet until the concrete is in place. Soft woods, such as pine, fir, and spruce, make the best and most economical form lumber since they are light, easy to work with, and available in almost every region.

Lumber that comes in contact with concrete should be surfaced at least on one side and both edges. The surfaced side is turned toward the concrete. The edges of the lumber may be square, shiplap, or tongue and groove. The latter makes a more watertight joint and tends to prevent warping.

Plywood can be used economically for wall and floor forms if it is made with waterproof glue and identified for use in concrete forms. Plywood is more warp resistant and can be reused more often than lumber. Plywood is made in 1/4, 3/8, 1/2, 9/16, 5/8 and 3/4 inch thicknesses and in widths up to 48 inches. Although longer lengths are manufactured, 8 foot lengths are the most common. The 5/8 and 3/4 inch thicknesses are most economical; thinner sections require additional solid backing to prevent bulging. The 1/4 inch thickness is useful for forming curved surfaces.

Fiber

Fiber forms are prefabricated from impregnated waterproofed cardboard and other fiber materials. Successive layers of fiber are first glued together and then molded in the desired shape. Fiber forms are ideal for round concrete columns and other applications where preformed shapes are feasible since they require no form fabrication at the job site. This saves considerable time and money.

Fabric

Fabric forming is made of two layers of nylon fabric. These layers are woven together, forming an envelope. Structural mortar is injected into these envelopes, forming nylon encased concrete pillows. These are used to protect the shorelines of waterways, lakes and reservoirs and as drainage channel linings.

Fabric forming offers exceptional advantages in the structural restoration of bearing piles under waterfront structures. A fabric sleeve with a zipper closure is suspended around the pile to be repaired, and mortar is pumped into the sleeve. This forms a strong concrete jacket.

Stay-in-Place

Systems of finished stay-in-place concrete forms are used for load bearing and nonload bearing concrete walls, above and below grade applications. These systems are very versatile and can be used for a wide variety of applications. Forms are available in different wall types and can incorporate integrated adjustable door jambs, window jambs, and trim.