Unstressed Strands as Reinforcing 1

[MarkAllan]

I am researching the use of high strength unstressed wire strands, coiled together, as conventional reinforcing in concrete instead of rebar. I was looking for information on bond strengths, strain compatibility etc, but found nothing substantial as of yet. One bit of information I did find is the CLOSED thread below from Eng-Tips Forums web page.

thread589-156288

In this thread, RAPT said "You cannot generate the full breaking strength of strand if it is simply used as untensioned reinforcement so you cannot substitute it on the basis of equivalent yield force. In fact, even as prestressed strand, you normally cannot generate the full breaking stress of the strand, normally only about 95%. But as unstressed you will only generate about 30-40% maximum. A strain compatibility analysis is required to determine how much stress you can generate in the strand in both cases."

Could someone perhaps point me in the right direction please? Where would I find information on such matters? Even send some reference info?

MarkAllan

 

[swivel63]

do a strain analysis on it and see how close the strand it to yeilding or if it even comes close to 60 ksi.

 

[civilperson]

Note: The E, modulus of elasticity, is NOT 29000 ksi, varies from 25,500 to 27,000. Use the calculated E from the strand steel report. If substituted for Grade 60 reinforcement in a equal area exchange, the reinforced concrete will have very similar strengths if the bonding lengths are correctly proportioned. You can not say 270 ksi steel can substitute for 60 ksi steel in proportion to the yield strengths.

 

[UcfSE]

There are several issues to consider here. Using strands as mild reinforcing will affect the ductility of the member. This could cause issues for you if you are in a seismic region. You should also consider the failure mode of the member if you need it to be a beam since the code typically places limits on reinforcing to ensure ductile failure. Again, even if you assume a low yield strength, you don't want the beam to actually fail in a brittle manner should it ever do so. You will also have to consider the changes to all of your development lengths and lap splices.

 

[mitchelon]

MarkAllan,

You will be wasting your time if you try to do any type of analysis. The whole reason we have prestressed concrete today is because of the use of high strength steel (250K, 270K). You cannot take advantage of the properties of HS steel if it is not pre/postensioned. The prestress phenomenon is a consequence of the tensioning of HS steel, and therefore we cannot ignore it; although it has many advantages.

 

[rapt]

MarkAllan,

Theoretically you can use un-tensioned strand in a concrete member but it is not practical as a standard design procedure as suggested by others above. The benifits of stressing the strand in terms of ductility, crack control and deflection are far to dominant to waste.

For a start, you may be able to achieve the ultimate strength and ductility you require, though the calculation methods required are far more complex than normally used by RC designers, but you will end up with terrible deflections. You would have to do full deflection calculations allowing for cracking, creep and shrinkahge effects properly, not using simplified formulae in codes or text books. The computer software that we develop, RAPT, will actually do all of this for you. But is assumes full bond for the strand (see below).

The second problem is determining what bond can be achieved. As prestressed strand, the stress in the strand before it is bonded to the concrete is normally in the order of 1000 to 1200 MPa and the stress in the strand at ultimate strength is about 1500 to 1650 MPa, so the bond only has to provide a force equivalent to a maximum of about 600MPa of stress in the steel. This is easily achieved in prestressed tendons.
If the strand is unstressed when bonded, the force due to the full stress achieved has to be transferred by bond. The bond stresses would then be in the order of 3 times those achieved as prestressed tendons. I have not seen any test results showing this is possible but I know that someone is currectly doing some work on it to justify some slabs that contain some unstressed tendons.

If the section is lightly reinforced, it is actually theoretically possible to generate the full stress in the strand using strain comaptibility but this is assuming full bond is achieved. This will normally not be possible to achieve.

The end result of this is stress the strand before bonding it. One use unstressed strand when you are trying to justify a stuffup.

 

[MarkAllan]

Thanks to all for your input. By way of experiment, I did an Ultimate (ULS) Strength Analysis on a 200mm wide x 500mm 30MPa beam with 2 x 16mm diameter deformed bars (fy = 450MPa, Es = 200GPa), area of steel = 402mm2. I then did an equivalent area-force substitution with 1400MPa strand (Es = 205GPa), and needed 129mm2 of strand to get same ULS resistance as 2 x 16mm bars.

I then did a strain analysis with the 1400MPa strand, and crack widths were unaccepatable at Service (SLS) Loads - about 70% greater than upper limit allowed, thus highlighting the serviceability concerns.

Of course, I made some "dicey" assumptions for the purpose of this comparison, for example that stress-strain curves for rebar and strand are the same (which they are not), that bond strength for strand is same as rebar (which it is not) etc.

But it did show me the truth of the words from my Pre-stress course notes concerning the use of unstressed high strength steels: "the strain required to develop the strength is so high that the concrete surrounding the steel would be severley cracked at the serviceability limit state."

So the conclusion of the matter...pre-stress high strength strands.
MA

 

[swivel63]

whoa, what units are those?????

LOLOLOL

 

[JLNJ]

It seems to me that if you "coil" the strands your modulus of elasticity will be different than if the strands are "straight".

Maybe you will be closer to the characteristics of wire rope when looking at strain compatibility.

 

[rapt]

swivel63

Real logical computer input friendly units, not like ft, inch's and fractions of an inch.

 

[mitchelon]

Didn't I tell you not to waste your time? Consider precast concrete piles with prestressing strands. Why are the strands tensioned if by doing this you are applying a prestressing force which reduces the capacity of the section to resist external loads? The answer is, because you have to tension prestressing strands. If you find a case where strands were used successfully as conventional reinforcement, please let us know. Another thing you need to remember, you cannot use the code equations (ACI for US and Puerto Rico, and some other countries) for reinforced concrete if you are using strands. You have to use strain compatibility and a stress strain curve obtain from an eccentric compression test (i.e. Hognestad and Rusch). (You cannot use 0.85*f’c*b*a).

 

[swivel63]

who uses those units in america? LOL, i thought we were the only country in the world who didn't use SI.

<=== hasn't used MPa for anything since physics my freshman year, LOL

 

[rapt]

mitchelon,

Bad example, The strands in pretensioned piles are stressed to keep the pile in compression during driving, transportation and in resisting uplift forces.

swivel63,
Yes you and one other very small country in central Africa still use other units. I would not be boasting about it. The rest of us use SI and gain its benefits. I did not realize that we had to be American to use this list or waste our time converting everything to US units because you cannot be bothered to join the rest of the world.

 

[swivel63]

who's boasting? we don't use it here, it is what it is. or at least i've never seen it on any project i've worked on. hell, if they put SI units on the S2 exam, i'm screwed, LOL. if you like it, i love it. if i said something about millimeters to a contractor, he'd look at me cockeyed. that's just how it is in this country.

now if you'd kindly remove your panties out of that bunch, there are plenty of structural discussions to be had here. i assume you have pretty thin skin to get all defensive just because someone said they haven't used SI units in years. i have a tissue for you.

 

[mitchelon]

swivel63,
I grew up and went to college in Puerto Rico where as in the US, we don’t use SI. The US has been trying for decades to change to SI. You cannot deny the fact that SI units are just better. That we are too lazy to change, that is a different story. I was involved in a construction contract in the US where the design, developed in the US, was all in SI. Lots of errors. If you are involved in one, make sure you scrutinize them.

rapt,
Not a bad example. I know why you use the strands in piles (flexural capacity for transportation, erecting, driving, and lateral loads as well as for uplift). The point I was trying to get across was that if you could in fact use unstressed strands, you would see the application at least in piles. Why sacrifice axial capacity when you can use “unstressed strands?” Of course, you cannot. Do you understand what I was trying to say?

 

[swivel63]

personally, it's a non issue to me. units are units, and all it will change is the calculation. the engineering logic and skill is still the same. sum of F and M will always equal zero.

there definitely is an advantage to using SI units....but you'd have to get the old guard to have to develop a new intuition on how large or small some numbers are. 125 psi for a PT slab means a whole lot more to me and a significant portion of the engineering population than whatever unit conversation is into MPa.

 

[Compositepro]

Would carbon fiber strands require prestressing? Carbon fiber strand would have a ultimate tensile strength of 300,000 psi and 1.5% strain to failure. I'm not clear on what the original post meant by coiling the strands. Did he mean making a cable form smaller strands or using coiled, overlapping loops of strands to reinforce concrete(somewhat like chainlink fence mesh)? It seems to me if overlapping loops are used, then the need for a high shear strength bond between the strand and concrete would be greatly reduced.

 

[rapt]

Compositepro

Non-prestressed carbon fibre normally cannot be used at it's full ultimate strength with concrete for the same reason that steel strand needs to be prestressed.

The other problem that you have with carbon fibre with a failure strain of 1.5% would be ductility of the section. If you used the carbon fibre strand to its ultimate capacity, there would be little of no plastic deformation before failure. This applies whether is is prestressed or not.