Buoyancy

Hello Guest,

Think about the situation logically.

I think you have mis-stated your question.

<".....what is the buoyancy of a section of 6" GRE pipe weighing 12.5 kg/m in encased wet 2200kg/m 'fluid' concrete.">

You state that the pipe weighs 12.5kg/M in encased wet concrete...

That means that somehow you have managed to weigh that pipe while it is already in the concrete, does it not.

Please advise further, to clarify, please re-form your question to eliminate possible variables, with please.

Kind Regards....

If my statment meant as you suggest I wouldent be asking for the buoyancy. The weight of the pipe in air is 12.5 kg/m. What wet concrete density do you use to determine the buoyancy of the pipe?

Sorry!!!

You've given the concrete as 2200 kg/m and I think that must be the density 2200 kg/m^3. So, what you need is the volume of 1 meter of the GRE pipe (I think the 12.5 kg/m is the linear weight). Let's just say, for illustration, that a one meter piece has a volume of 0.01 m^3. Then the displaced concrete would have a weight of 2200 kg/m³ * 0.01 m³ = 22 kg. Then the buoyancy per meter would be 22 kg - 12.5 kg = 9.5 kg. I think you should convert that to Newtons (using kg for weight makes my last neuron wiggle and whine) which would give you 93 N/m.

Can't you just tie the stuff down?

Need to know the buoyant force to design the tiedown.

I've seen pipe pop out of the water because tiedowns (temporary precast concrete saddle collars) were not sufficient. It was not a good day.

Hi TVP45,

The concrete is 2200 kg/m3.

My question is do you use the full density of the concrete to determine the buoyancy? Does the rock part of the concrete add to the buoyancy?

Take the same pipe and bury it in pea gravel saturated with water. Do you use the combined densities of the gravel and water to determine the buoyancy?

I think that a reduced density of the concrete should be used. Should it be the densities of the water, sand and cement? I don't have any documented tests as to what the actual 'buoyancy density' should be.

Many thanks,

chama2633

I don't know enough to discuss the details of which part of the aggregate counts. I only know that the standard for buoyancy is 150 pounds per cubic foot which is about the same as your 2200 kg/m^3. You'd have to get a civil engineer to further refine that.

BTW, when I said you needed the volume of the pipe, that was a real Duh! moment even for me. Of course the pipe is closed.

Take the same pipe and bury it in pea gravel saturated with water. Do you use the combined densities of the gravel and water to determine the buoyancy?

The pea gravel would have some effect on the pipe. Even in the dry state, it causes lateral pressure on a retaining wall. In a saturated state, the water may act as a lubricant. I would guess that you would be in the right ballpark by taking the combined density of pea gravel and water to calculate the buoyant force. It is a relatively easy experiment to conduct, so why don't you give it a try and report your findings back to CR4?

It is generally believed that the buoyant effect of quicksand can be found by considering the displaced volume with the full density of quicksand, not some reduced value. You cannot drown in quicksand because your density is approximately the same as water while the density of quicksand may be twice as much.

Perhaps wet concrete behaves like quicksand. It doesn't last very long because it doesn't remain wet for very long, but certainly long enough to pop the pipe out of the concrete if it is not tied down.

Hey Bruce,

I would suggest that the lateral force you mentioned is from the weight of the gravel and nothing to do with buoyancy. I am still not convinced that you use the full density of the concrete.

Take the pea gravel case, if the pea gravel was not saturated with water would there be a buoyancy effect? I think the word fluid is the answer. What is a fluid? is wet concrete a fluid? I think sand, cement and water could be considered a fluid. The gravel doesn't seem to fit in.

I think you are right. An experiment is the answer.

Many thanks

Chuck

Hi Chuck,

There may be a difference between the behavior of pea gravel in water and wet concrete. With pea gravel, the particles are making direct contact and in fact the weight of each pea is carried, partly by buoyancy and partly by bearing on other particles.

In concrete, that is not the case. The particles of rock are more sparsely distributed throughout the mix and do not necessarily make direct contact with other similar particles.

In the case of quicksand, the particles of sand, silt and clay are in suspension, so they are probably acting more like a fluid than the other two cases.

You may have a point, Chuck. The test will tell.

Dear chama2633

The gravel when located only with water, it shall be deposited into the bottom of container, and in this case the specific gravity (or density) of fluid shall be that of water only. But when the gravel becomes a component like water, cement and sand, the gravel shall be suspended in the mixture, and I think the density of mixture shall be a function of density of every component in the mixture. For other details, please refer to my post #16.

6" is 0.1524 m

so r is 0.0762 m

so pi*r² is 0.01824 (and, when you multiply this by 1 meter you get the same thing). So that's the volume of concrete displaced.

multiply by 2200 and you get 40.13 (that's the mass of concrete displaced)

subtract 12.5 gives 27.63 Kg and multiply by g

271 Newtons per meter of pipe.

Case A. Pipe segment is closed from its both sides:

♥ Volume of 1 m of pipe 6" NPS, V = (3.14/4) OD² = (3.14/4)(0.1683 m)² x 1 m = 0.022235 m³, this volume of pipe weighted, W_pipe = 12.5 kg.

♥ The weight of the same volume as a displaced wet concrete shall be, W_{wet con} = V x Density of wet concrete = (0.022235 m³)(2200 kg/m³) = 48.917 kg

♥ By comparing between W_pipe (12.5 kg) and W_{wet con} (48.917 kg), we find that weight of pipe is less than the weight of displaced same volume of concrete, so the pipe shall float.

Case B. Pipe segment is opened and wet concrete shall go through inside the pipe :

In this case the weight of pipe shall be increased by weight of wet concrete inside the unit length of pipe :

♥ Weight of concrete inside pipe, W_{con inside} = (3.14/4)(ID²) x 1 m x Density of concrete = (3.14/4)(0.1483 m)² x 2200 m³/kg = 37.982 kg (by assuming the pipe wall thickness = 10 mm)

Total weight of pipe and concrete inside, W_{pipe & con inside} = 12.5 + 37.982 = 50.482 kg

♥ By comparing between W_{pipe & con inside} (50.482 kg) and W_{wet con} (48.917 kg), we find that weight of pipe and concrete inside is larger than the weight of displaced same volume of concrete, so the pipe shall immersed.

Mny thanks Abdel,

I dont think you use the full density of the concrete to determine the buoyancy. However I dont know what density one would use. When you classify concrete as 2200 kg/m3 is that wet or after set?

Chuck

Dear chama2633 thank you very much

The density of concrete shall be taken into consideration in buoyancy calculation, and I don't know exactly how much is it (2200 kg/m³ ?).

From my point of view, the density of concrete used in buoyancy calculation must multiplied by a factor K which depends on the degree of wet of concrete, volume of mixed water per unit, type of cement and its density, type and size and density of gravel, ... etc.

Density of wet concrete used in buoyancy, D_wet = Actual Density, D_actual x Constant, K

Determining that constant K can be a practical search for a civil engineer.

Calculate the displacement (volume) of the pipe section and its weight including the air inside.

Calculate the weight of the wet concrete that will be displaced by the pipe section.

The difference is the buoyant effect - Archimedes's principle.

If the buoyancy of the pipe is a problem in the application - look at filling it with water. The will reduce the buoyant effect a lot

Sincerely,

Mr. Gee

Good idea, but with the concrete being more than twice the density of the water, there will still be a lot of buoyancy. What about filling it with gravel, as long as you can clean it out with fast water after the concrete sets.