High strength bars

[structural87]

Does the aci318 specify any upper limit of bars reinforcement ?
The thing is that i have the contractor which is proposing to use steel reinforcement of fy>1000Mpa in some concrete elements ?
The thing is that i think i wont have anymore a ductile.section since concrete will crush ans steel will always be at its elastic phase with that fy which doesnt satisfy the tension controlled criteria
Thanks for the help

 

[jayrod12]

I would expect it to be difficult to develop proper bond to even come remotely close to the strength of the bars. You make another great point about the crushing prior to yield. Perhaps if you adopted some of the FRP design requirements you might be ok.

 

[hokie66]

Is the reinforcement prestressing strands? If it normal deformed bars, I don't think 1000 MPa bars exist, and if they did, they would not meet any concrete material specification of which I am aware.

 

[structural87]

Hi hookie,
It is not prestressing. Apparently the material is being imported from china.and test has been done to validate the fy.

 

[rapt]

Prestressing bar has strengths of over 1000MPa, but nowhere near the 1800MPa strength of prestressing strand, but they have to be stressed to use in concrete members in flexure. You cannot use them as a plain reinforcing bar. And if you are going to stress the steel, it is more economical in most cases to use strand rather than stressing bar.

ACI allowed reinforcing bars are currently less than 500MPa I think.

 

[PSEPK]

Referring to ACI 318M-14 Table 20.2.2.4a, maximum value of fy for non-prestressed deformed bars, is 700 MPA.

A good structural engineer is often a blessing for others.

 

[Ingenuity]

Referring to ACI 318M-14 Table 20.2.2.4a, maximum value of fy for non-prestressed deformed bars, is 700 MPA.

700 MPa (100 ksi) is for usage "in lateral support of longitudinal bars, or concrete confinement."

For flexural applications (which the OP appears to be referring) the max value for f[sub]y[/sub] for design calcs is 80 ksi (550 MPa), or 60 ksi (415 MPa) for special seismic systems.

 

[structural87]

Thank you for the help.
However,these values that ACI is refering to are to be used in design.
But is there any recommendation stating for example that 550MPA bars should not show a fy higher than 20% when tested ?
I dont know if i made myself clear

 

[rapt]

Agreed you will have a less ductile member. Do the ASTM reinforcing material codes give maximum values for strength etc. I know they define minimum levels for the different types of bar but not sure if they define maximum variations.

 

[hokie66]

There are controls on elongation, and that is where your bars will fail. The Chinese are trying to dump this stuff on you because it has failed tests elsewhere.

 

[PSEPK]

structural87,

ACI 318M-14 Section 1.10, provides a procedure for getting the approval of alternate construction materials that not covered by the code at present.

Can it be applicable in your case?


A good structural engineer is often a blessing for others.

 

[BUGGAR]

I would like to know more about this steel. Can you share?

 

[TXStructural]

The reinforcing industry is currently working to develop materials up to fy=100 ksi for flexure. There is now an ASTM that includes grade 100 for non-seismic use, BUT a designer who uses this material current does so of their own accord. ACI 318 does not incorporate grade 100 for longitudinal/flexural reinforcement. The estimate is that there needs to be more than $20 million in research to fully develop design criteria for grade 100 rebar. A few designers currently use grade 100 in columns. In flexural members, you may get excessive cracking since elongation under load relates to modulus NOT strength.
Also, the bars you mention most likely do not have sufficient ductility between yield and tensile. In this case, there will be insufficient warning of impending failure. There are a number of other reasons not to use brittle reinforcement but the main reason is that the codes do not work properly unless the materials conform to the required specs.

(I am no longer associated with the reinforcing industry but will continue to try to answer questions when I can.)

 

[structural87]

I will ask for bars testing : yield and maximum tensile strength as well as elongation.
I will search in the astm if they give any percentage not to go beyond for a specified fy.
The contractor is saying that he may also usr this type of bars in concrete pipes where no bending is present, just axial forces

 

[rapt]

If you do look at using it in flexural members, definitely the deemed to comply (simplified) crack control and deflection rules in the code would not apply. They are even possibly a problem for 500-600 MPa reinforcement.

Development, shear and other areas would also be a problem.

Basically you would have to go back to first principles logic for all aspects of design and not use ACI code simplifications.

You would also have to test the steel for relaxation (steel creep under tension) and performance in fires.

Also check its ductility and all properties (I mean yourself, do not trust their certificates).

 

[structural87]

ok rapt. Noted. thanks a lot.
ACI318-14, in its section 20.2.2.5 limits the actual yield strength of the deformed non-prestressed longitudinal reinforcement resisting earthquake-induced moment, in high seismic areas and states that it should not be higher than the design yield strength by more 124Mpa since a higher fy will lead to higher shear and bond stresses at the time of development of yield moments (assuming that a grade of 420Mpa is used).
This is the only limitation that I found regarding this subject.