Concrete Formwork – Pressure not a Function of Wall Thickness

Just a quickie on formwork pressures.  One of my foremen once asked me if we needed more form ties in a 24″ thick wall versus a 12″ thick wall.  That answer is “no”.

Concrete, when being placed, is liquid.  Just like water.  And as all carpenters know, form ties are used to resist pressure pushing against the forms.  The difference in pressure is only a function of two variables:  the density of the liquid and the height of the liquid.  The height of a liquid is called the “head”.

The denser a liquid, the more pressure it delivers:

  • density of concrete is about 150 pounds per cubic foot
  • density of water, for example, is about 62 pounds per cubic foot
  • therefore, the pressure exerted by concrete over water is about two and a half times (150 divided by 62)

Regarding the height of the liquid, if you are pouring a 1′ tall wall versus a 15′ tall wall, the pressures are much different.  The pressure in the 15′ tall wall is fifteen (15) times greater.

Think of these couple examples.  Think of jumping in your pool.  If you submerge your head just below the surface a foot or so:  no pain in your head because the pressure is low; you’re not deep in the pool.  If you jump in the deep end, the pressure on your head is ten times greater (ten feet divided by one foot).

Regarding the thickness of the wall example – which of your walls needs to be stronger?  Which resists more pressure/force?  Your choices:

  • a one-foot tall wall built in New York which is holding back the Atlantic Ocean from New York to London
  • a one-foot tall wall in your yard holding back the local creek from coming into your front door

The answer: the pressure is the same because the distance from New York to London versus the distance across your basement does not contribute to the pressures in your formwork.