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Is Buoyancy of Wet Concrete an issue?

Is Buoyancy of Wet Concrete an issue?

(OP)
Hi, I found this thread that speculated on the effective buoyancy of a vessel floating in wet freshly poured concrete:

thread167-14699: Buoancy Calc in Wet Concrete

though no final conclusion.

I have setup a 4 inch pvc pipe in a trench with a specific fall which i want to maintain. Pipe is propped on bricks.
I am going to fill the trench to the top of the pipe with concrete, so just the very top of the pipe shows through the concrete.
My question is will I have any issues with the pipe trying to float up due to the Archimedes' principle, ie displaced fluid giving a force of lift equal to weight of fluid displaced?
I also intend to vibrate the concrete to ensure a good water proof result.

Any advice is appreciated.

Ray

ps if this is posted in wrong section, plse advise.

RE: Is Buoyancy of Wet Concrete an issue?

Yes. It will want to float. Can you temporarily fill it with water during the pour? That will help to lower the uplift. Best to tie it down though.

Mike McCann, PE, SE (WA)


RE: Is Buoyancy of Wet Concrete an issue?

(OP)
Thanks Mike,
all I can do is put some concrete blocks at set intervals to keep it down, i will try to block outlet and fill with water.
Any idea how much it will tend to lift?

Given the high viscosity of the concrete - 80 slump, I thought the uplift effect may not be significant? ie the concrete will not act as a true liquid??

Maybe should fill trench to just under the pipe on the first run then go back and do the last bit - but makes the job a bit harder?

Ray

RE: Is Buoyancy of Wet Concrete an issue?

rayhines -

Why are you concerned about just pouring concrete just to the top of the pvc and why are you concerned with trying to get "waterproof" construction?

Just support the pipe for construction purposes. Get the pipe heavier by getting it full of water and pour concrete under, along and over the pipe to create a bonded heavy mass. Just get enough (a few inches to a foot or so) over it and then backfill with native soil.

Concrete weighs far more than the soil could ever weigh and provides stability. You could also use low strength concrete since it will probably have the necessary strength.

Putting concrete block for weight on top before the pour will decrease the flotation during the construction somewhat because the density of the hollow block is about the same (light weight block at 100 pcf) and normal weight hollow block are over 130 pcf.

Allow the cores of the block to be filled somewhat by the concrete. The concrete placed over and around the surrounding soil and backfill with have more strength and weight than the existing and surrounding soils.

This could be an application for flowable fill concrete that is heavy enough plus it gains strength and is far heavier than either soil or water, so floating is real problem unless there are other issues/requirements.

Some idea of the dimensions and scope of the situation would help for specific answers for you speculated project.

Dick

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

RE: Is Buoyancy of Wet Concrete an issue?

(OP)
Thanks Dick,

the reason the concrete is only to the top of the pipe is that the pipe will act as a spoon drain, after I cut some slots in the top of it. It is behind a basement retaining wall. The idea is that any ground water that meets the wall is directed down to the pipe and runs away to a sump before it can seep to the bottom of the wall and into the basement via the wall slab joint. After placing the concrete I will cut slots, place geo fabric over, deposit gravel and then backfill earth against the retaining wall.

cheers,
Ray

RE: Is Buoyancy of Wet Concrete an issue?

I agree with M^2. Voids lifting is a frequent problem in voided slab bridge decks, as is cast-in ducts moving. If you tie the pipe down securely to the support blocks, then place concrete to the underside of the pipe and allow it to start to harden, before placing around the pipe, that should provide sufficient restraint. Filling it with water will also help.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

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