BUILDING
THE BASE:
Base materials of sand, gravel, and rock are placed at the bottom
of the excavation to prevent settling and heaving and to aid drainage.
They also provide a level surface on which to place the concrete.
The depth
of the base is often regulated by codes. Find out how much base
you need, as you would for concrete, but order twice as much as
your figures indicate. This is because the base materials will compress
to half of their original size when they are tamped. The order of
placement:
- Place
the gravel in at least 2 layers. Tamp each layer firmly so that
the top of the final layer is 2 inches below the bottom edge of
the concrete placement.
- Add
sand. Keep adding and tamping until you have a level layer that
reaches to the top of the concrete pour. Follow this order no
matter whether your installation requires a Setting the Grade
footing or not. The depth of any given layer may vary by code.
The level can be checked with a strike board called a drag board.
BUILDING
THE FORMS:
Concrete takes the shape of the mold in which it is placed. In most
cases, the mold is made of 2X4, 2X6, or 2X8 lumber supported by
1X2, 1X4, or 2X4 stakes.
The forms
must be substantial or they can't support the weight of the concrete
without bulging. Always set form boards on a true perpendicular
to the subgrade. Set all stakes straight up and down so that the
edges of the cured slab are plumb. If they are not, the slab will
be substantially weakened. The forms are sawed in a series of saw
kerfs so the boards may be curved.
Isolation
joints-
Isolation joints are used to separate a new concrete placement from
other already existing materials such as wood, brick, old concrete.
The joint is preformed material that is about 1/2-inch wide. It
allows for differing rates of expansion and contraction.
Construction
joints-
In some installations, the entire slab is not placed at once. The
forms are built. Then a section is closed off with a temporary form
board called a stop board. The section is filled with concrete,
screeded (leveled), and finished. Once the concrete has set, but
not before it has cured, the stop board is removed and the joint
edge is oiled. Then either the rest of the form-or another portion
of it-is filled with concrete.
The joint
between the initial and the later sections is called a construction
joint. This joint also can function as a control joint, so plan
the position of all construction joints to correspond to control
joint dimensions dictated by your specific project.
Oiling
form boards-
You can oil the form boards by brushing on old crankcase oil with
an old paintbrush. This way, the form boards will not stick to the
concrete when you remove them, and the oil on the boards seals the
wood so the water from the concrete is not absorbed into the wood,
weakening the concrete at this point.
