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Unusual Method of Prestressing Concrete

02/21/2016 10:41 PM

I found a very short paper on an unusual (to me) method for prestressing concrete published by a researcher in what was then (1972) the Soviet Union.

It involves welding a sheet metal plate to all edges of a concrete part (the edges being prepared for this by incorporating steel connectors made up of angle iron welded to loops of rebar). Once the concrete is poured and the cure is well along, grout is injected into the space between the sheet and the concrete part to pre-stress it. The grout sets and the concrete part is pre-stressed by the internal pressure retained by the set grout.

According to the paper, only ordinary steel is required, and only ordinary concrete, which makes a big contrast to western pre-stressing practice which requires both high-strength steel and ultra-strong concrete mixes.

Naturally, this would seem to lend itself to a "poor man's pre-stressing" for the Third World (where I happen to live). But I can't find any further information. The references in the paper are to Soviet publications, except for one which is East German. The back numbers of the last are maintained only in libraries in the old Eastern Zone - none in the West.

So as a last resort I am hoping to find an expert in this forum who can tell me more about this technique and perhaps steer me toward a source of additional information.

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#1

Re: Unusual method of prestressing concrete

02/21/2016 11:27 PM

I see no way this can work. To be feasible the grout would need very high tensile strength, otherwise it just cracks and relieves any stress in may have built up when it shrinks as it cures.

Now, if you can find a way to make the grout expand when it cures..........................

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#2
In reply to #1

Re: Unusual method of prestressing concrete

02/22/2016 12:22 AM

Obviously I can't have an opinion about this, but this was not offered as a proposed method - it was claimed to have been widely and successfully applied. It did state that the steel was not stressed beyond its elastic limit, which suggests elastic recovery if there was shrinkage in the grout.

Either Voronkov lied, or there is a way to make this work.

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#14
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Re: Unusual method of prestressing concrete

02/24/2016 10:25 AM

Can you describe it in more detail? Prestressing I'm aware of is for reinforced concrete beams, prestressed lengthways, so that when a bending load is applied the concrete doesn't go into tension immediately (tensile strength in concrete cannot be relied on) and a greater load can be taken.

Yours sounds like a concrete block encased in steel plates. How is the concrete stressed in tension under an applied load, so prestressing gives a benefit?

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#15
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Re: Unusual method of prestressing concrete

02/24/2016 7:43 PM

This method inherently pre-stresses the concrete part in two directions - which can only be done in conventional pre-stressing by running tendons in both directions. Basically, the sheet is welded to anchorage members all around the edge of the part, and when the sheet is put into tension it pulls in both directions. The examples in the paper all take advantage of this feature in particular, but it's difficult to think of any situation in which it would be a DISadvantage. Take a simple beam - conventional pre-stressing compresses the tension side in only one direction, but then passive steel has to be present to hold tension in the other direction or there will be failure in the other direction due to necking under load. This system takes care of both.

But what intrigues me is that - apparently due to the large areas over which these internal loads are applied to both the metal and the concrete - only ordinary steel and concrete are needed to make this work. In conventional prestressing, high strength and high stiffness are both needed.

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#16
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Re: Unusual method of prestressing concrete

02/24/2016 8:42 PM

"Invented in the Soviet Union" says it all for me.

"when the sheet is put into tension it pulls in both directions" also leaves me wondering.

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#17
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Re: Unusual method of prestressing concrete

02/24/2016 8:58 PM

Pulling in only one direction would be even more wondering.

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#18
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Re: Unusual method of prestressing concrete

02/24/2016 9:05 PM

Pulling in one direction away from the slab when the steel rusts away is within my comprehension.

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#19
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Re: Unusual method of prestressing concrete

02/27/2016 11:42 AM

No doubt the Russians did this for a reason, but I still can't picture the situation. What shape is the "part"? When you say edges, do you mean faces? as it would have to be an enclosed volume if you're going to pump grout in.

I'm not an expert on reinforcement, but I don't see what you mean by passive steel has to be present to hold tension in the other direction. In a beam, as far as I'm aware re-bars go in just one direction, lengthways. As beams usually take mainly a weight load, in one direction, downwards, in principle most of the bars could go in the bottom side, but if it's a symmetrical section would have to make sure the guys on site put it the right way up. If it's a T-shaped beam, eg for a floor, that's not an issue.

I still don't see how your concrete is put into potential tension by an external load. Also other advantages of conventional reinforcement is that the concrete cover protects the steel from corrosion and fire.

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#20
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Re: Unusual method of prestressing concrete

02/27/2016 12:03 PM

Various examples are given. One is a simple slab, simply supported at two edges, another is an underground box with each inner face prestressed, the third example demonstrates the use of the method of sections with this technique.

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#3

Re: Unusual method of prestressing concrete

02/22/2016 12:45 AM

The grout would be in compression rather than tension. Moreover, the whole idea of grout is that it shrinks minimally if at all, and I think some elastomer-type grouts expand without shrinking afterward. "Grout-Fast" being one example, if that brand is still in use.

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#5
In reply to #3

Re: Unusual method of prestressing concrete

02/22/2016 1:31 AM

Correction: the name was "Chock-Fast".

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#6
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Re: Unusual method of prestressing concrete

02/22/2016 9:11 AM

How can the grout be in compression? The plate is on the outside of the pour. The pour has not cured when the grout is injected into the space. The concrete will continue to shrink away from the steel plate and the grout.

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#7
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Re: Unusual method of prestressing concrete

02/22/2016 10:36 AM

You can't push a rope.

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#8
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Re: Unusual method of prestressing concrete

02/22/2016 10:53 PM

Of course the grout is in compression. It is sandwiched between the concrete part and the sheet metal that is bonded to the concrete at its edges. As it is injected into that space, the metal stretches, putting the grout in compression and the metal into tension. That tension is exerted on the edges of the concrete part, putting it in compression.

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#9
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Re: Unusual method of prestressing concrete

02/22/2016 11:23 PM

OK, I missed the part where the forms were removed after pouring (curing well along didn't sink in) and the grout space is totally confined.

The problem I see now is the amount of pressure that can be generated by injecting an incompressible mix into a finite space without blowing the metal "sheet" away from the "steel connectors made up of angle iron welded to loops of rebar".

Drawing on my experience with injection molding, I see no way that this can exert enough pressure on the concrete pour to be worth the effort and expense.

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#11
In reply to #9

Re: Unusual method of prestressing concrete

02/23/2016 2:03 AM

Hmm. Grout - even grout that is not fully confined as this is - can exert huge forces when used to grout a structural column, for example. I've seen that use. That's the so-called dry-pack technique. The grout for this technique would of course have to be much more workable.

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#4

Re: Unusual method of prestressing concrete

02/22/2016 1:10 AM
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#10
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Re: Unusual method of prestressing concrete

02/23/2016 1:44 AM

In PCC pole factory here the contractor had installed crude way of pre tensioning of high tensile bars. He had installed inverted 'U' frame mounted with 2 Tonne capacity Chain Electric Hoist. At hook of the hoist,a sling is passed through vertical pulley fixed in ground to create horizontal pull on the sling. Sling's another end is attached to steel plate which had pullers which are clamped on steel bars to be pre stressed. before pouring concrete steel bars are prestressed to specified length and concrete is poured.

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#12

Re: Unusual Method of Prestressing Concrete

02/24/2016 12:02 AM

All concrete structures are meant to have a minimum of 50mm (2") of concrete between the atmosphere and any rebar. It sounds like this method has a connection from the outside to the rebar inside. This would ultimately lead to concrete cancer and collapse. I expect there are reasons it is not in widespread use.

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#13
In reply to #12

Re: Unusual Method of Prestressing Concrete

02/24/2016 1:25 AM

It makes a connection between the steel sheet and the anchorages, but the rest of the steel (passive steel) could be isolated. In any case, there is a precedent in methods accepted in the West for putting tendons on the outside of the structure, primarily as a retrofit but also as an original construction method.

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#21

Re: Unusual Method of Prestressing Concrete

03/11/2016 12:47 AM

You can also search on google.

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#22
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Re: Unusual Method of Prestressing Concrete

03/11/2016 6:49 AM

I haven't had any luck on Google or EspaceNet (for patents) searching for this method. Which is why I am here.

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