Bouyant Force of Wet Concrete

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One of my favorite books is my 'pocket reference', it has just everything in it. I got it at Ace Hardware, it's by Thomas J. Glover.

It shows water @ 62.4 lbs./cubic foot

dry sand @ 100 lbs./cubic foot

crushed stone @ 100 lbs. cubic foot

portland cement @ 94 lbs. cubic foot

Now you still have to work out the proportions to get the actual buoyancy.

I think it's homework, but, you are right about the Pocket Ref.

It's without a doubt the best $10.000USD book I've ever owned.

If it isn't in that book, you don't need to know it!

The buoyant force on any particle submerged in water is 62.4*V where V is its volume in cubic feet. Concrete has nothing to do with the question posed.

Can you answer this question?

Say you have a 54" Diameter Concrete pipe .

Encased in a 6'x6' form. How much uplift force can you expect?

The answer to your question is...yes, I can answer your question. A 54" dia. concrete pipe encased in a 6'x6' form would have no uplift at all by simply being encased in a form. If the form is subsequently filled with a liquid, the uplift force on the pipe would be the weight of the liquid displaced by the pipe.

If the liquid placed in the form is cast-in-place concrete, the uplift force would depend on the speed of placement. As you all know, cast-in-place concrete does not remain liquid for very long. If the placement is slow enough, there may be minimal uplift on the pipe, but if the placement is very rapid, the uplift force could be as high as 150V where V is the volume of the pipe in cubic feet.

The particles are not submerged in water. They are in a cement and aggregate slurry with an effective density of 145 lbs/cubic foot.

Therefore any object that you are trying to encase in concrete would have a buoyant force of 145*V where V is the volume of the object.

Read the bloody OP. The particle IS submerged in water.

I did read the OP and my response was basically the same as your post #8 except that I did not delve into the rate of pour issue. I point out that all concrete is usually vibrated which will result in the full liquid head acting on the submerged element during vibration no matter what the rate of pour is.

The OP asks about the buoyant force on the particles which are in wet concrete. These particles are not submerged in water. They are coated with a water and cement mixture. If they were truly submerged then hey would behave more like a layer of sand and gravel in the bottom of a bucket filled with water. If the buoyant force on the particles was only that due to water then all the sand and gravel would immediately settle to the bottom since its density is much higher than water and the cement paste would flow away.

Not the way I read it. He is asking about the buoyant force on a single sand particle or stone particle or cement particle IN WATER.

If he means to ask about the buoyant force on a single sand particle or stone particle contained within a concrete mix, then I guess I agree with your interpretation. I'm not sure that you are correct, however in the case of a single cement particle as cement has a lower density than concrete. How would a chunk of course aggregate affect the buoyant force on a cement particle? I believe other forces are at work within the cement paste between chunks of aggregate.

"If the buoyant force on the particles was only that due to water then all the sand and gravel would immediately settle to the bottom since its density is much higher than water and the cement paste would flow away".

But cement is not less dense that water

(see post #2)

What Bruce mentioned is the way to go