Importance of Concrete Pressures

The first step in designing forms is to determine the maximum concrete pressure.  This pressure is affected by the rate of pour, the height of pour, the concrete temperature, type of cement used, the method of vibrations used to compact the concrete and concrete slump.

APA Plyform is stronger than conventional plywood as it has to withstand these combined pressures.  APA’s Concrete Forming Design/Construction Guide gives the recommended maximum pressures for Plyform Class I and Structural 1 Plyform. Calculations are based on either a deflection limitation of 1/360th or 1/270th of the span, or the shear or bending strength, whichever is the lowest load value.

While  each country will have its own design guides, it is important to be able to determine concrete pressure for any design specification.  As an example of how concrete pressures are dealt with in form design, APA’s latest guide uses the approach taken by the American Concrete Institute (ACI).

Pressure on Column and Wall Forms

Lateral pressures for newly placed concrete deals with rate of pour, column/wall height and temperature. Where exterior vibration is involved or a pumped placement system is used, the pressures given in this table need to be increased.

Concrete pressure is directly proportional to its density.  The pressures  in the APA guide  are based on a density of 150pcf (pounds per cubic foot), and appropriate for the standard range of concrete poured.

Numerous variables are used in formwork design such as cement types, admixtures, design slump, placement systems etc. The effect some of these variables have on the pressures is addressed by the unit weight coefficient, Cw  and the chemistry coefficient, C c.

Concrete forming case study: the San Diego Library, California, used ¾” plywood substrates and ¾” plywood with film fascia to form the columns, walls and beam forms.

Loads on Slab Forms

These forms have to support construction workers and equipment as well as the weight of  freshly poured concrete.  Normal concrete weight of 150pcf will place a load on the forms of 12.5psf (pounds per square foot) for each inch of thickness.  Our guide gives the minimum design loads which represent average practice when using either motorised or non-motorised vehicles for placing the concrete.

Concrete forming case study showing the Pacifica Apartments used Plyform  ¾” MDO to form the reinforced concrete decks.