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

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Pressure of Concrete on Formwork 39
Placement and Consolidation of Freshly Placed Concrete
Concrete may be placed in the forms directly from a concrete delivery
truck, dropped from crane buckets, or pumped from a concrete
pumping truck or trailer. If concrete is pumped from the base of the
form, ACI Committee 347 recommends that the form be designed for
a full hydrostatic head of concrete wh plus a minimum allowance of
25% for pump surge pressure if the concrete is pumped from the base
of the form. In certain instances, pressures may be as high as the face
pressure of the pump piston.
Cautions must be taken when using external vibration of concrete
with shrinkage-compensating or expansive cements because the pressure
on the forms may be in excess of the equivalent hydrostatic pressure.
Caution must also be taken with internal vibration of freshly
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placed concrete. Consolidation techniques may be responsible for
formwork failures, either by forcing the concrete to remain semiliquid
longer than anticipated, or by excessive shaking of the forms.
Wind Loads on Formwork Systems
The design of formwork systems must include horizontal wind forces
in addition to the vertical weight of concrete and live loads that are
placed on the formwork. Horizontal wind applies loads against the
side of shoring and the formwork on top of the shoring. The wind can
also apply uplift forces against the underside of the formwork.
The impact of wind is influenced by the location, width, length,
and height of the formwork system. The impact of wind increases
with height. For example, the wind force on the formwork and shoring
on the 20th floor of a building will be higher than on the 1st floor
of the building. It is necessary for a qualified engineer to properly
design the formwork system to adequately resist wind forces.
References
1. ACI Committee 347, “Guide to Formwork for Concrete,” American Concrete
Institute, Detroit, MI, 2004.
2. APA-The Engineered Wood Association, “Concrete Forming,” Tacoma, WA,
2004.
3. J. M. Barnes and D. W. Johnston, “Modification Factors for Improved Prediction
of Fresh Concrete Lateral Pressure on Formwork,” Institute of Construction,
Department of Civil Engineering, 1999.
4. N. J. Gardner, “Pressure of Concrete against Formwork,” ACI Journal Proceedings,
Vol. 77, No. 4, 1980.
5. N. J. Gardner and P. T Ho, “Lateral Pressure of Fresh Concrete,” ACI Journal
Proceedings, Vol. 76, No. 7, 1979.
6. N. J. Gardner, “Lateral Pressure of Fresh Concrete—A Review,” ACI Journal
Proceedings, Vol. 82, No. 5, 1985.
7. R. L. Peurifoy, “Lateral Pressure of Concrete on Formwork” Civil Engineering,
Vol. 35, 1965.
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