Apply vacuum and then pressure to a concrete mold

[StoneMaker]

I have considered making my own spherical molds to be easier to manage. I have wondered what would happen if I made them so that I could hook it to a vacuum pump and air compressor. My thought is if the mold was vibrated and had a vacuum applied at regular intervals while filling it, it would compact the concrete and have a smooth surface. Then when it was full, if pressure was a applied, concrete at the top half of the mold would be pressed against the mold and hopefully result in the top of the sphere having the same surface as the bottom of the sphere. As it is now, since the weight of the concrete doesn't push against the top half as much as the bottom half, the top half is always very noticeably rougher and I thought the idea could result it that problem being eliminated. Would it work or not?

 

[TehMightyEngineer]

Why not just set the form up to be rotated during the pour? Seems like it would be easier. How big of a sphere are you casting?

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[concretemasonry]

The pressure will have little effect on the concrete finish.

You will always have some consolidation and shrinkage as the entrapped air is released over time. You will always have some voids even with both vibration and rotation (even biaxial).

Is your aggregate pre-wetted/soaked before weighing and mixing?

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

 

[DETstru]

I doubt the vacuum will solve your problem. You probably want to look at your mix and make sure you're using enough cement, small aggregate, the right admixtures, etc.