Wall Forms & Hydrostatic Pressure Paradox Pascals Law

[SLPSTRUCT007]

On a wall form with varying height and a “Bulkhead Lid” I know that the pressure at 1 and pressure at 2 must equal each other, due to the hydrostatic paradox (Pascals law), assuming full liquid head. My question is what happens if this “Bulkhead Lid” is removed and the concrete is exposed?

Does the hydrostatic paradox still apply or is the pressure at point 2 something lower than the pressure at point 1. See the snippet below for reference.

 

[CJLCivilStruct]

If the bulkhead is removed then the surface at point 2 is free to air so P must equal 0 - that's my understanding anyway.

Also, worth noting that concrete pressures are not fully hydro static - refer to CIRIA 108.

 

[SLPSTRUCT007]

Thanks for your response,
This problem remindes me of a retaining wall design with a sloping back slope. If you were to analysis a retaining wall you would consider an additional "surcharge" load due to the back slope. I can image this being something similar.

If there is an additonal pressure applied to the vertical face, and this pressure acts in all directions, then this same pressure acts on the walls of the forms.

Yes, you are correct concrete pressures can vary depending on pour rate, chemistry coeff., temp, etc... to simplify the problem I did not want to consider those factors in this conversation.

 

[rb1957]

if you remove the lid then the pressure at 2 is not equal to the pressure at 1, as the head of stuff above 2 is less than 1. no?

 

[JStephen]

If I'm understanding your situation correctly-
You're filling a form with concrete, and to calculate pressure on the form, you treat it as a liquid. But it isn't entirely liquid.
If it was a liquid, then with or without a lid, pressure at points 1 and 2 would always be the same. If you took the lid off, depth would drop but would still be the same at both points also,
But if the concrete is stiff enough that without a lid, it can hold that slope on top, then I would assume that pressures at points 1 and 2 were different also.
That might also be the case with the lid, but you just don't know what the pressure is, so you assume it's the same.
Actual pressures might vary a lot, depending on concrete properties and width of the wall, etc, but treating as a liquid would be reasonable for that pressure.
I'm not familiar with a hydrostatic "paradox" here.

 

[rb1957]

if there were no lid, and if the contents were liquid, then the tank would need to have the "top" (the lid face) horizontal (and the base would be inclined) and then "clearly" the pressure at 2 would be different to the pressure at 1.