Buoyancy in Wet Concrete

Hello chama2633

Perhaps you could refer here: http://cr4.globalspec.com/thread/18289/Buoyancy

Trust that assists you.

Kind Regards....

Water in concrete is neutral bouyant, and not considered. You use the weight of concrete usually 150 to 155 pcf to determine the object weight, and the weight for the volume of water displaced by the object (water is 62.4 pcf). If the pipe is fully encapsulated and the water is only outside, use the outside diameter of the concrete to determine the cross-sectional area, determine the water level outside the pipe, calculate how much water is displaced by the submerged cross-section of the pipe. This water weight displaced is the bouyant force. Now you calculate the weight of water displaced, based on the volume displaced. Next calculate the weight of the pipe (concrete , inside liner, any fluid inside the pipe all add to the weight). The change in the loading from dry to submerged condition is what needs to be considered. You do not want to go from a positive downward load to a negative downward load, or you could potentially float the pipe if it is great enough to overcome the overburden.

RCE,

Forget the water outside the pipe. What I am looking for is the buoyancy of a piece of pipe fully encased, to a level 500mm above the top of the pipe, in wet concrete. What is the buoyancy prior to the setting of the concrete?

Sorry for the confusion.

Chuck

Ok then cross-sectional area of the pipe (to the depth submerged in concrete) times the unit weight of concrete (standard practice is to use 150 pcf) gives you the bouyant uplifting force.

RCE,

Thanks for your reply.

How do you determine the water level outside the pipe. Concrete is a homogeneous mixture of cement, sand water and gravel.

chama2633

Actually it is not homogeneous, it is a conglomerate of the components, but they are not homogeneous through out. To determine the water level outside of a pipe you can install piezometers and measure it, or dig a hole and measure it. If you know the depth the pipe is being installed and the depth of the water, you can calculate the submergence depth of the pipe.

In my area, it is approximately 2300 kg/m³ or 145 #/cf. To be conservative, use 2400 and 150 respectively. Until the concrete sets, it acts as a fluid with that density. After initial set, the buoyancy is no longer a factor.

Are we discussing the bouyancy of the pipe in water (potential uplift on the pipeline), or the bouyancy of a pipe being cast in wet concrete (temporary construction uplift). Bouyancy in water is a longterm design along with construction consideration, bouyancy during construction is a contractors consideration. Long term bouyancy in water, especially where water table will rise above the pipeline, should be considered in the design of the pipeline, especially gravity pipelines where maintaining the invert elevations are very important to the conveyance of fluids.

RCE,

My concern was the buoyancy of pipe cast in wet concrete. Prior to the concrete setting there will be uplift forces acting on the pipe. Buoyancy is calculated as the displacement of the fluid times the density of the fluid. If the fluid is wet concrete what density do you use?

Thanks for your reply.

Chuck

Typically150 pcf

Another question - What is the density of concrete /water mixture?

A good stiff mix of concrete is still about 150 pcf. I uncerstand that you are attempting to be accurate and want a precise density, but the mineralogy and porosity of the rock and sand can effect the density as much as the water added in the range of acceptable mix designs (usually 145 to 155 pcf is the range for a standard concrete mix). So it is typical to use 150 pcf, and make sure you have sufficient factors of safety in accordance with the codes. The required factors of safety will be much larger than the variation in the density of the concrete.

Is it safe to say a 54" pipe 60' long will have a bouyant force of 143,100 lbs?

Jonathan

Could not say, what is the wall thickness and what is the weight of the fluid inside the pipe when the maximum bouyancy would be encountered. A RCP 54" in diameter could be about 4 to 7 inches thick depending on design loads the pipe has to resist. This thickness makes a big difference in the weight and increases the actual outside diameter by double that wall thickness, such that a 54" pipe might actually be 62" to 68" in diameter with respects to displacement (which can change your increase your displacement by 30% to 60% greater).