Floatation of a plastic Mould in Concrete

[slippydog]

Hi I've a plastic/polystrene mould to place in a concrete tank. The volume of the mould is approximately 15m3 and I intend to pour concrete from the bottom of this mould to the top (level with the top). Concrete will be placed evenly all around (total height of approximately 1.3m). When the concrete sets I will subsequently remove the mould. However for floatation I was wondering what sort of weight I will need to hold this mould in place, i.e. stop it floating. Is it different than density of water x volume seen as it's in a concrete base or how should I approach this. Any help would be much appreciated!

 

[cvg]

you are pouring concrete not water, so why would you use the density of water in the calculation?

 

[dhengr]

slippydog:
What would happen if you cast this tank up-side-down, so that you cast the bottom conc. last over the void form? Then, when it cures, roll the tank and remove the void form. If you were going to make a bunch of these you might want some sort of a smooth, collapsible void form which could be reused and would give you a smooth interior wall surface.

 

[cvg]

the whole idea sounds a bit iffy to me. unless this is some special, high strength polystyrene, I can visualize this mold being flattened by the weight of the wet concrete pushing in from all sides. I can't think of a good way to brace it for these forces, polystyrene is just not stiff enough. most concrete forms are braced steel.

 

[concretemasonry]

If you can, run braces over the top to restrain it from the flotation forces. Then pump in water as the concrete comes up to hold the mold down somewhat to offset the movement/force of the mold trying to rise.

The amount of force to hold the mould down will depend on the geometry and difference in volumes of the geometry and the densities of the the two different materials (concrete and water). The concrete will be approximately 2.5 times as heavy as the concrete during the placing process. Keep in mind the consolidation and vibration (if any) of the concrete will add some additional unbalanced loads.

Pump the water out after the concrete has gained some strength.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.