rebars nominal strength 1

[StewardMM]

Good day all.

we conduct a test for strength of rebars used in our project. the maximum nominal mass per our standard is 6%. but the result of the test exceeds this maximum value. how will this affect the design?

 

[concretemasonry]

Steward -

Where are you located and what codes and material standards are in effect where the project is located.

The basic criteria for rebars is usually the tensile strength expressed in load per amount of area. The the mass is not a part of it because the steel weight is usually constant within a tight range, independent of the strength.

What is the specific test or standard identification procedure?

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[KootK]

I wouldn't sweat it unless you feel that the weight issue is representative of a general quality control /specification compliance problem.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CELinOttawa]

South East Asia? Vietnam? This is only a concern with Bessemer steels where it can indicate excess COST to the project for over, or insufficient material where under. From what I understand it is not indicative of a technical danger unless low.

Very strange for this to be in a modern code! Be sure that the rest of the requirements are met. Paddle analysis for carbon content is very important for these old style steel processes. Also get your elongation test results.

As to your specific question: The effect on the overall mass of the slab is nominal and can be neglected. The effect on the project budget is far greater, though not a technical danger.

 

[CELinOttawa]

Laddle analysis... Man I really, really have to figure out how to turn off this damned autocorrect!

 

[TXStructural]

ASTM A615, A706, A996, and A955 (the specs used in the US) all require bars to be no less than 94% of nominal weight (they can be rolled up to 6% light, or could have lost section due to corrosion or other factors.) They must still retain the full specified strength, which for grade 60 is 60 ksi times the nominal diameter. This means that after deformations are rolled in and one accounts for the up to 6% light section, the actual steel strength is significantly higher than 60 ksi. This is why you are not permitted to use a partial/reduced section of bar to determine compliance with the specification.
For example, a #4 grade 60 round bar (no deformations) is 1/2" diameter. The cross section is nominally 0.2 sq in and the weight per foot is specified at 0.668 lbs/ft. At grade, this bar is required to hold 12 kips without yielding. This same bar is then rolled to imprint the deformations and retain the same 12 kips capacity and weight per foot (not quite how it is done,but for this example it is close enough.) The specs allow the final bars to be up to 6% light, or 0.628 lbs/ft.
Based on this, the steel would need to be not less than about 64 ksi (and in reality will be a bit higher to account for the narrowing at deformations.) Similarly, a slightly weaker steel can be rolled a little heavier to meet the grade specification. The specs allow bars to be heavier than spec without any limit imposed.
In the case of A706, the maximum yield must be within 18 ksi of grade FOR THE SECTION. This means that the actual steel strength and weight for A706 reinforcement can vary, but a #4 (1/2") deformed bar must yield between 12 kips and 15.6 kips, without respect to the actual steel strength or bar weight.

To answer the OP question, as long as the bar yields at the proper load, and the bar is no lighter than 94% of spec, it has been considered by the design code (in our case, ACI 318.) A lighter section can result in excessive elongation and wider crack widths before yield, because crack width is a function of the steel modulus and the area of steel crossing/restraining the crack.

 

[MotorCity]

TXStructural,

Thank you for the good explanation.

So if I understand correctly, its an all or nothing situation regarding strength of the bar. If I have a bar that's at least 94% of the nominal weight, it can be relied upon to provide the nominal strength. If the bar weighs less than 94% it is considered to provide 0% (no strength), and you cannot pro-rate the strength.

Lets say I have a #8 bar that has been exposed to weather and appears to be corroded. I chip around the bar and measure that 1/8" of the bar diameter has been lost due to corrosion. This means that I now have a 7/8" diameter bar remaining (essentially a #7 bar). The difference in weight between a #7 and #8 bar is .626 lb/ft or a 24% loss in weight. Clearly this exceeds the 6% loss requirement.

Even with the loss, I cannot assume that the remaining bar offers some strength? I could not even assume that I have at least a #4 bar (even though the remaining bar is 7/8" diameter? Seems overly conservative.

 

[MrHershey]

While there would certainly be some strength (I could perhaps see assuming a lower bar size), the issue is that the 94% requirements is explicitly in the ASTMs. If you don't meet that requirement, you don't meet the ASTM. ACI 318 requires you to use deformed bars meeting one of the ASTMs TX cited. If they don't meet the ASTM, then you don't comply with the structural concrete code. Period. Everything in that book assumes you're provided deformed bars meeting the required ASTMs.

I agree that you should logically be able to just step down to a lower bar size and run with it, but the issue is that you're using non-ASTM bars in an application that requires ASTM bars.

And, at least for deformed bars, I don't know think you'd be able to reclassify a #7 as a lower size within the confines of the ASTM standards. While you can drop down a size and the minimum area and mass works, your deformation spacing would be too wide so you still wouldn't meet the ASTM and thus wouldn't meet ACI.

 

[CELinOttawa]

Great thread... Really great post from TX! I wasn't aware of those details for current bars...

 

[PittEng88]

TX Thanks for the great explanation.

Typically when load rating a reinforced concrete beam bridge (non-presstressed) with exposed/deteriorated bars, the standard of practice is to rate the beam based off the remaining area of steel.

 

[TXStructural]

The ASTM specification requirements are for new bars introduced into new concrete.

In the case of in situ corroded bars, you can use the remaining strength. ICRI provides good guidance on this for repair conditions. When I did that for a living, I typically underestimated the remaining section in the design of repairs because you never really know all the conditions in existing structures. ACI 318 is principally for NEW design and under repair situations you frequently cannot rely upon the existence of the assumed conditions required to implement the 318 code.

 

[KootK]

That really was some great info TX. I have a somewhat trivial follow up question. I've used numerous spreadsheets based on rebar area calculated from nominal bar diameters. I always assumed that there was some negligible slop built into the tools because I thought that calculations should rightfully be based on the actual bar diameter and area.

Based on what you've written here, it sounds as though using a bar area calculated from the nominal diameter is the most accurate way to go. Is that right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.