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