Buoancy Calc in Wet Concrete 1

[krd]

For a typical concrete mix weighing say 148 pcf I generally use 100 pcf for the unit weight of concrete causing uplift on a submerged form (buoancy = displaced volume x unit weight of "fluid&quot. The difference being that the rock part of the concrete doesn't really cause uplift. While this has worked so far, does anyone know of any real research or data that would support this or another value?

 

[ishvaaag]

No research paper seen on my part but it is quite likely as well that the solid part (gravel etc) bear on the submerged part counteracting the buoyancy (aggregates we don't see being displaced by buoyancy effects normally, even if due to proper anchorage), hence part of the buoyancy value is directly canceled by this weight.

 

[JAE]

I may be a little dense here today (last day of 2001), but to properly do buoyancy calculations on a concrete structure, say a concrete box of some kind, you calculate the displaced portion of the fluid x unit wt. of fluid (as you stated above krd).

I don't get what you mean by "the rock part of the concrete doesn't really cause uplift". It is not the concrete or rock that "causes" uplift, but the displacement of the fluid due to the shape of the object.

The net downward force resisting this uplift is the raw weight of the object. I've never heard of using 100 pcf for this purpose, although granted: its conservative.

 

[krd]

JAE, that's okay - the concrete is the fluid, ie before it has set up. A case would be embedding a 10 ft diameter pipe in concrete. What is the buoancy of the pipe in "fluid" concrete.

 

[ishvaaag]

I think Jae has been thinking on the buoyancy of some construction...it seems krd is referring for example to the buoyancy of a polyestirene foam voiding part to make joists or structurally waffer slabs on a soffit. Of course buoyancy is volume multiplied by specific weight of the liquid, but what is the specific weight of the liquid?

krd seemingly things the coarse aggregate (itself a solid part submerged in the liquid concrete soup) can't -since not a liquid- generate buoyancy. If you say, well, in any case, the rest has the same average specific weight than concrete and is liquid, maybe it is neither so, it is just that the fine aggregate or even the concrete fines is just smaller, but neither a liquid.

Maybe only the gelified part we can count as generating buoyancy, but at what specific weight?

Furthermore I add that extant weight of coarse aggregate and through viscosity and friction partly that of fines opposes full development of the gel buoyancy...hence only tests (may be already done somewhere) can give a correct answer to what krd asks, I think.

 

[JAE]

Like I said...it was that last day of 2001. I see what you are asking. Unfortunately, I don't know how concrete behaves in its fluid state. One place to look might be the void form people (Sonotube?) as they market their cardboard tubes as horizontal elements in thick slabs....kind of like making a cast-in-place hollow core plank. They require straps to tie down the tube forms and may have some info on the uplift produced by wet concrete.

 

[RiBeneke]

For calculation uplift forces on void formers embedded in concrete I have always used the full mass density of the concrete (25 kN/cu m = 160 lb/cu ft typically).

The logic is that the stone and sand particles are kept in suspension by the cement gel. For force equilibrium the pressure in the gel is derived from the total mass.

Now this is strictly only true while the concrete is experiencing vibration (poker or formwork vibrators).
However in most practical cases the depth of vibration can equal the full depth of the concrete and the embedded void, for example as the poker vibrators are withdrawn.

If you have a really deep concrete pour, you could use the reduced pressure graphs that are used for formwork design. In this assumption the pressures peak at 1.5 to 3 m depth below the concrete surface, depending on circumstances.

 

[sound789]

While we are aware that the active pressure on the cohesive soil is gh-2c, where g is unit weight, h is head and c is cohesion, if we apply this philosophy to the concrete we find that c is zero initially and increases gradually as the setting time increases. Hence we should worry only for the layer that is under vibration and the age is less than initial setting time, for the buoyancy pressures. Hope i am correct. Kindly support if i am either.

 

[krd]

sound789, as you and others have said, you can get a value for buoancy using unit wt of concrete times head, although I don't think your model proves the point. My point was that this value is conservative and is probably 50% more than the actual buoyant force, not considering vibrators. This can be ignored in many cases and should be. However in particular instances the cost impact of designing for 50% more load than is real is hard to ignore.

My model would be: consider burying a square box 10 feet each side x 10 ft deep so displaced volume is 1,000 cf.
- If you place gravel under the box and 10ft up the sides most would agree that there is very little buoyant force.
- Flood the gravel with water 10 ft up sides and we would agree buoyant force using unit wt water is 62.4 kips.
- Instead of water, flood the gravel with a cement and water mixture, ie place concrete under the box and 10ft up the sides. As RiBeneke said, now the gravel is in suspension. However I say it still doesn't exert a significant upwards force on the bottom of the box. I estimate the fluid part of the concrete that does cause uplift weighs about 100 pcf so my buoancy is 100 kips.
- If the aggregate under the box were to exert an upward force equal to it's weight then the total unit weight of concrete would be applicable and the buoancy would be about 150 kips.

 

[sound789]

Dear krd,

I agree that the unit weight of concrete to be considered for the subject calculation shall be in the range of preferably 1MT/CUM (10KN/CUM) to conservatively 2.5MT/CUM (25KN/CUM). My opinion is that the liquid pressure height should be restricted to only the layer under vibration or under initial set. However we are trying to experimentally explore the facts for the above theories which we shall let you know as soon as we succeed. Thanks.

 

[trolleybob]

A very fascinating concrete subject is the "Mulberry"- essentially floating docks for Allied supply ships during the invasion of Normandy in 1944.

Constructed out of a huge concrete casting "tub" they were solidly filled in with "ballast"- the non flamable, non organic debris (concrete, brick, etc), with crushing and pulverizing services provided by the Henkel, V1 and V2.

First example of urban recycling.

Sorry, no puns intended- practicing my old stand up comedy routine.

Bob D.

PS- Construction design equations for the ultimate concrete floating cube are probably available at the UK Defense Ministry historical archives Dept.

Dont reinvent the wheel- glom their equations!

 

[trolleybob]

I think I recall it has to do with "displacement".

Lord Brunel- built the first iron/steel ship, the "Great Eastern", ca 1854, "Displacement", maybe 200 Tons of Steel and Iron.

I recall skimming through something, the trick has to do with this- the weight of the building material has to be distributed in such a manner over the total area of the vessel BELOW THE WATER LINE (including the sides of the vessel below the waterline). This value needs to be less than the expression value of the displaced water. I think the ratios are supposed to be stated terms of hull pressure in "two dimensional density" OR "PSI"- pressure (ponds)per square inch. I think using Volume (cubic inches) can result in lots of errors.

The way you get out of using "3 dimensional" arithmetic in this case, is you look up a Table of weights for materials, I forget the value for water, but its alot, the value for concrete is somethng like 2800 pounds/cubic foot. However, for the purpose of a canoe, you convert to "PSI".

For example, if your canoe had a hull 3/16" thick, you would do a conversion factor to figure out the weight of a hull area of 9 square feet x 3/16" thick, etc.

Bob D.

 

[trolleybob]

Great materials "displacement" table.

http://www.pembroketrading.com/conversiontables.htm

Bob

PS- Read some good manuals on the art/science of "Rigging"- especially House Moving, unusually shaped cargo, cables, chainers, binders, nylon straps, shrink wrapping, the way to use a boom hoist- the longer you make your boom, the less weight it will lift! Function of arm capacity a function of arm radius- function of boom angle.

Bob D.

 

[trolleybob]

Pouring Pre-Stressed Concrete...

I used to enjoy mixing up Portland Type II in a wheel barrow...

Another trick is to build your "reinforcement matrix FIRST- It must be a self supporting structure. Keep in mind though, that a self supporting structure can be built out of silk thread- if its formed into cables, tensioned, and anchored to some half way decent anchor members at each end...

Anchor members could be any rigid form of tubing, as long as it:

1. Cannot Corrode by oxidation (loose hydrogen atoms on surface of part at time of pour.

2. Cannot be subject to "bi metalic contact"- rust out by Electrolysis.

Think of the cables in the Brooklyn Bridge- 1/16" solid wire, bundled and CAREFULLY WOVEN (looking at 19th century hemp rope manufacturing is interesting) into 18" cables.

Also, Roebling knew that Boss Tweed was going to supply 30% defective (failed PSI rating test) wire for tthe main cables, so the Tammany Hall suppliers could "make a living".

You cant stop "corruption"- just include it as another coefficient in equations.

Wise to include a "suppliers making a living" margin of safety in all critical calculations. Especially in anything that can snap or explode!

Bob D.