Re: Concrete Formwork

Terrence Holder wrote:

> Have there been any studies carried out to ascertain the pressure
> levels of reinforced concrete in/on formwork (i.e. at the bottom,
> middle, top)?  Where is the pressure the greatest?  Are there any
> variables that directly influence the pressures in the formwork (e.g
> frictional forces)?

The ACI publishes values; the full information (and much more) is in SP-4,
"Formwork for Concrete". 
Please note that the latest version (rev 6) does not incorporate Cc and Cw
factors.

In general, liquid concrete is a hydrostatic load, pressure = depth *
density, giving you a triangular pressure distribution. This is the maximum
form pressure from liquid concrete.

In practice, the pressure reaches a maximum (less than depth * density)
based on pour rate and temperature - it begins setting up in the form. This
gives you a rectangular pressure curve with a triangle on top, tapering to
zero. the height of the triangle is (max pressure)/ (density).

As far as variables,
pour rate - the faster you go, the less time it has to set up. 
temperature- high temperatures set up quicker, less form pressure.
density-pressure is directly related to density. the Cw factor is used to
vary the pressure for non-standard weights (lightweight concrete, ballast
concrete)
chemistry - the use of retarders and low-heat cements (among other things)
reduces the rate of cure, and increases the pressure. 

Also, forming system mfr's will supply charts based on these things. 

Rich