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