Thread: pressure from poured concrete vs. pressure from water

Let's say you construct two cubic boxes (with open tops, and of equal dimensions) and pour concrete into one box (like concrete right after it is mixed), and then you pour water into the other box. You then analyze the total pressure on a wall from each box.

Which box would have a larger outward pressure on the wall. I was thinking that because concrete sticks together, it may end up exerting less pressure on the wall.

Doesn't a calculation for pressure on a wall involve much more than simply "How heavy is the liquid like substance."

I am thinking viscosity must play a very large role here.

Or we could assume that we just built a dam. Would the dam that holds back 50,000 gallons of freshly poured concrete need to be any stronger than a dam that hold backs 50,000 gallons of water?

I think the only difference would be how quickly the pressure from the liquid spread out over the wall. Other than that it is the gravitational force causing the pressure, so the denser liquid exerts more presure.

Put it this way: if the only force acting on the liquid is gravity, and there is a greater force acting on the concrete as it is denser, then the pressure caused by the concrete must be greater, since the area is the same.

Originally Posted by SteveC

Doesn't a calculation for pressure on a wall involve much more than simply "How heavy is the liquid like substance."

no

You are talking about hydrostatic pressure, which at a certain height from the bottom of the box, h, is given by , where is the fluid density, and g is the acceleration due to gravity.

Since the fluid pressure is a function of height (increases linearly with depth) the total pressure on a wall will be the integral of pressure over the surface area of the wall.

If the boxes are equal in dimension and size, the higher density fluid will exert the overall higher pressure.

Pressure drops due to viscous effects of a fluid need only be considered in moving fluids (see Poiseuilles law), whereas you are considereing fluid statics.

Originally Posted by DrRocket

Originally Posted by SteveC

Doesn't a calculation for pressure on a wall involve much more than simply "How heavy is the liquid like substance."

no

Thanks for clarifying my post.

So as long as the substances are liquids, then the denser one exerts more pressure. I can't argue with that. However, if we were to consider a situation such as sand vs. water, or clumpy mud vs. water, then the situation would get much more complex, becuase sand or clumpy mud aren't neccesarily liquids and frictional forces will make those substances stick together much more.

However, I still feel that viscosity would play a more significant role in the conrete versus water than you guys seem to think. If we assume that concrete is 2.5 times as dense as water, well, I still feel that it would only exert 2 times as much pressure. I have not tested this.

I am not even sure if I would call concrete a liquid after all. Its not even one single substance anyway.

Thanks guys

Originally Posted by SteveC

So as long as the substances are liquids, then the denser one exerts more pressure. I can't argue with that. However, if we were to consider a situation such as sand vs. water, or clumpy mud vs. water, then the situation would get much more complex, becuase sand or clumpy mud aren't neccesarily liquids and frictional forces will make those substances stick together much more.

Yes, but since momentum is conserved, the forces in friction act both ways and have no effect on the overall force causing the pressure. Otherwise, gravity is not the only force acting.

Originally Posted by SteveC

However, I still feel that viscosity would play a more significant role in the conrete versus water than you guys seem to think. If we assume that concrete is 2.5 times as dense as water, well, I still feel that it would only exert 2 times as much pressure. I have not tested this.

No, over time as long as it can act fluidly, the concrete will spread out and exert 2.5 times the pressure. The water will simply get to relative pressure of 1 much quicker than concrete gets to 2.5

Originally Posted by SteveC

I am not even sure if I would call concrete a liquid after all. Its not even one single substance anyway.

True. But it acts fluidly, and there are no forces other than gravity acting on the system, so it doesn't matter.

Originally Posted by SteveC

So as long as the substances are liquids, then the denser one exerts more pressure. I can't argue with that. However, if we were to consider a situation such as sand vs. water, or clumpy mud vs. water, then the situation would get much more complex, becuase sand or clumpy mud aren't neccesarily liquids and frictional forces will make those substances stick together much more.

However, I still feel that viscosity would play a more significant role in the conrete versus water than you guys seem to think. If we assume that concrete is 2.5 times as dense as water, well, I still feel that it would only exert 2 times as much pressure. I have not tested this.

I am not even sure if I would call concrete a liquid after all. Its not even one single substance anyway.

Thanks guys

Viscosity is only an issue when you are talkng about fluid flow problems. Pressure does not require flow.

A liquid is by definition a material that cannot sustain a shear stress without flowing. That is all that is necessary for pressure to be a scalar and for density and height (aka "head") to determine the pressure.

If the concrete is suficiently heavily loaded with solids that the solids can form networks that can carry load then you no longer have a liquid, but rather a complex structure and things are more complicated. Viscosity won't help there either.

Just to add, if you are interested in the behaviour of flow of materials such as sludges or poured concrete (Non-newtonian fluids), there is an area called rheology which studies it. It also deals with solids which exhibit plasticity (permanently deform under shear stress).

Originally Posted by Nabla

Just to add, if you are interested in the behaviour of flow of materials such as sludges or poured concrete (Non-newtonian fluids), there is an area called rheology which studies it. It also deals with solids which exhibit plasticity (permanently deform under shear stress).

Yes, it is also fairly difficult stuff since it basically covers everything that is not Newtonian rather than dealing with a specific material model.

For those who have not been exposed to this science, a Newtonian fluid is one in which the shear stress is proportional to the velocity gradient, and that proportionality constant is called viscosity. Non-Newtonian fluids can have either increased or decreased shear stress with increasing velocity gradient. Raw silicone rubber, for instance exhibits "shear thinning" which means that the "viscosity" decreases as the flow rate increases -- iimportant in injection molding applications.

Modern solid rocket proopellants are slurries of solid particles in a polymer binder and knowledge of the rheology is important in the casting process. Particle concentrations and alignment are important factors in determinng the burning characteristics at a fine scale.

http://en.wikipedia.org/wiki/Newtonian_fluid

http://en.wikipedia.org/wiki/Rheology

http://en.wikipedia.org/wiki/Bingham_plastic

Rheology (And fluid dynamics) in general are fascinating subjects

Originally Posted by SteveC

So as long as the substances are liquids, then the denser one exerts more pressure. I can't argue with that. However, if we were to consider a situation such as sand vs. water, or clumpy mud vs. water, then the situation would get much more complex, becuase sand or clumpy mud aren't neccesarily liquids and frictional forces will make those substances stick together much more.

However, I still feel that viscosity would play a more significant role in the conrete versus water than you guys seem to think. If we assume that concrete is 2.5 times as dense as water, well, I still feel that it would only exert 2 times as much pressure. I have not tested this.

I am not even sure if I would call concrete a liquid after all. Its not even one single substance anyway.

Thanks guys

Concrete contracts and expands, because cement is the binding agent, so you can not compare it to a mixture of pure sand dipped in water as it will never exert any pressure on the box save for Gravity. Concrete otoh is on the way to become a solid and Gravity is not the only force working on the box.

Originally Posted by DrRocket

the solids can form networks that can carry load then you no longer have a liquid, but rather a complex structure...

The concrete jargon is "slump".