CHAPTER 7 WORKING WITH CONCRETE

Concrete is the principal construction material used in most construction projects. The quality control of concrete and its placement are essential to ensure its final strength and appearance. Proper placement methods must be used to prevent segregation of the concrete.

This chapter provides information and guidance for you, the Builder, in the forming, placement, finishing, and curing of concrete. Information is also provided on the placement of reinforcing steel, and the types of ties required to ensure nonmovement of reinforcing once positioned. You will also be provided necessary information on concrete construction joints and the concrete saw. At the end of the chapter, you will find helpful references. You are encouraged to study these references, as required, for additional information on the topics discussed.

FORMWORK

LEARNING OBJECTIVE: Upon completing this section, you should be able to describe the types of concrete forms and their construction.

Most structural concrete is made by placing or "casting" plastic concrete into spaces enclosed by previously constructed forms. The plastic concrete hardens into the shape outlined by the forms. The size and shape of the formwork are always based on the project plans and specifications.

Forms for all concrete structures must be tight, rigid, and strong. If the forms are not tight, there will be excessive leakage at the time the concrete is placed. This leakage can result in unsightly surface ridges, honeycombing, and sand streaks after the concrete has set. The forms must be able to safely withstand the pressure of the concrete at the time of placement. No shortcuts should be taken. Proper form construction material and adequate bracing in place prevent the forms from collapsing or shifting during the placement of the concrete.

Forms or form parts are often omitted when a firm earth surface exists that is capable of supporting or molding the concrete. In most footings, the bottom of the footing is cast directly against the earth and only the sides are molded informs. Many footings are cast with both the bottom and the sides against the natural earth. In these cases, however, the specifications usually call for larger footings. A foundation wall is often cast between a form on the inner side and the natural earth surface on the outer side.

FORM MATERIALS

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

Earth

Earthen forms are used 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. Precautions must be taken to avoid collapse of the sides of trenches.

Metal

Metal forms are used 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. Currently, 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 easily cleaned, and maintenance is minimal.

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, an allowance should be made for shrinkage, or the forms should be kept 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 is 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. However, 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.

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