Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

208 Chapter Eight
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The lower form is filled, and the concrete is allowed to set until it
is stiff enough to resist the hydrostatic pressure from the concrete in
the upper form, after which time the upper form is filled. Caution
must be taken to prevent an unwanted cold joint.
Forms for Sloped Footings
Footings with sloped surfaces sometimes are used instead of
stepped footings, primarily to save concrete. Considering the higher
cost of forms, it is questionable whether an overall economy is realized.
Among the disadvantages are the following:
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1. Possible increase in the cost of material
2. Increase in the cost of labor for fabricating forms
3. Difficulty in placing and consolidating concrete
4. Necessity of anchoring the forms down to prevent hydrostatic
uplift when the forms are filled with concrete
For a sloped footing, considerable uplift pressure is exerted against
the form, as much as 6,000 lb of vertical pressure for a 6-ft-square footing
with 1:1 side slopes. Provisions must be made to resist this pressure,
for which several methods are possible, including the following:
1. Attach the panels to the reinforcing steel in the lower 11½ in.
of the footing, using wire or modified form ties. Place this
concrete first, then let it set before filling the remaining portion
of the form.
2. If steel dowels extending from the base of a footing into the
bottom of a concrete column are used, let the lower concrete
set, and then attach the top panels to the dowels.
3. Install a horizontal platform around the top panels and load
it with ballast.
4. Drive stakes or steel pins well into the ground on the outside
of the footing, and anchor the forms to the stakes.
Forms for Round Footings
For relatively small diameters, less than 48 in., laminated fiber tubes
may be used for forms for cylindrical-type round footings. Several
manufacturers provide circular forming tubes. These tubes can also
be used for stepped footings, provided the diameters of the footings
correspond to the diameters of the available tubes.
Forms for round footings whose diameters do not correspond to
the available diameters of fiber tubes or whose diameters exceed the
maximum size fiber tubes may be made from sheets of steel or from
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