3000 psi vs 4000 psi concrete 2

[lisyuse]

It is said that 3000 psi concrete are more ductile than 4000 psi.. by how many percentage? do you use 3000 psi or 4000 psi in your building (ordinary and special moment frames)? I know so many structural engineers who have never use 4000 psi.. they just use 3000 psi.. how about you?

 

[Ron]

The strength is selected based on the load resistance needs, not by its relative ductility. Concrete, in general, is not a ductile material so we don't worry about that....we add reinforcing steel to force the necessary ductility in the design.......a basic concept of reinforced concrete design.

 

[lisyuse]

Ron, I know that.. i'm talking of the following context.. where for example dik mentions in

http://www.eng-tips.com/viewthread.cfm?qid=255771

"Higher strength concrete is more brittle and less ductile. It is, however, higher strength... It behaves plastically a little better... but brittle none the less.

Dik"

You will notice that in stress-strain diagram.. lower strength concrete has greater strain.. see strain-stress diagram in Figure 1 of

http://www.iitk.ac.in/nicee/wcee/article/0809.pdf

So do you guys prefer 3000 psi than 4000 psi because of greater strain per stress in the 3000 psi concrete in special moment frames?

 

[Guastavino]

I haven't done any special moment frames but I'll say this, if the numbers work I'd go with the 3000psi. A few reasons:

1. Less cost. 3000 psi uses less cement, therefore cheaper.
2. Less shrinkage. Less cement, less shrinkage.


And...let's be honest, 3000 psi concrete is probably closer to 4,500 psi in the real world after a few months.

 

[MacGruber22]

You are wasting your time trying to compare ordinary concrete strengths with respect to ductility.

The coefficient of variation of concrete modulus is very high. The modulus of concrete can vary +/- 20%. Couple that variation with the likelihood that actual strengths will be 1200 psi or more than the specified, and you are left arguing within the margin of error.

Keep the design ductility predictable with steel.




"It is imperative Cunth doesn't get his hands on those codes."

 

[rapt]

The ductility/brittle behaviour is not reduced significantly until much higher concrete strengths than you are talking about. Up to about 7500psi there is no problem with concrete ductility/brittle behaviour. Over this level the maximum strain reduces and the peak strain increases until they are at about the same point at about 1100 to 12000 psi.

However, I would never use 3000psi these days for suspended structures more from a durability viewpoint. In Australia, 4500psi is about the minimum with 6000psi common especially for PT structures and RC structures in coastal areas.

 

[MacGruber22]

Also, 3,000 psi concrete is not economical for framed concrete. The concrete, itself, is not appreciably less expensive than 4 or 5 ksi normal weight concrete. By contrast, with 3 ksi framed concrete your sections need to be much larger and become more expensive because of formwork and greater minimum reinforcement required by the code, which of course is a function of cross-sectional area.

"It is imperative Cunth doesn't get his hands on those codes."

 

[wannabeSE]

I have never designed a concrete special moment frame. But, I always thought the reinforcement and detailing requirements of special moment frames compensated for the concrete's lack of ductility.

 

[lisyuse]

Most I met who designed residential 2 or 3 storey buildings never used 4000 psi. They stick to 3000 psi. Why is that.

 

[TehMightyEngineer]

"Normal" concrete cylinder under compression:

https://www.youtube.com/watch?v=ALizr63yO00

"High-strength" concrete cylinder under compression:

https://www.youtube.com/watch?v=0gXHqKpg__A

Both have a similar modulus of elasticity and both fail in a brittle fashion. About the only major difference is the "normal" concrete at least gives some warning before failure while the high strength concrete fails almost instantly. Wouldn't consider either to be what I would call "ductile" though. Spec the concrete strengths you need or can achieve without much cost or hassle.

Edit: Fixed link.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[IDS]

In my 43 years working with concrete I have never seen a 3000 psi mix used in a structural application. I work in infrastructure rather than building, but even in building 3000 psi (20 MPa) would rarely be used in my experience (UK, Middle East, and Australia). In fact it is only allowed in the mildest exposure classification in Australia.

I agree with rapt's summary. The difference in ductility is only significant for much higher strengths.

MightyEngineer - both of your links were to "normal" strength concrete (and it didn't say what strength that was).

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BAretired]

I can recall in the early 1950's when 2000 or 2500 psi concrete was used in suspended slabs and beams. Durability was not considered much in those days and the saving was deemed substantial (at least by some of the contractors with whom I dealt). The general trend since that time has been toward higher strengths and of course, there are now many additives which were not available in the fifties.

As recently as 2005, I ran into a contractor who wanted to use 2000 psi in a residence. He steadfastly refused to consider anything stronger regardless of any arguments I offered. When I retired in 2008, most people were using a minimum of 25 MPa (3600) psi and many engineers were specifying 30 MPa (4350 psi) minimum in commercial structures to improve durability.

BA

 

[MacGruber22]

As recently as 2005, I ran into a contractor who wanted to use 2000 psi in a residence.

Well, if that doesn't raise a red flag. That is illegal concrete in the US!

"It is imperative Cunth doesn't get his hands on those codes."

 

[TehMightyEngineer]

IDS: Fixed the link in my original post:

https://www.youtube.com/watch?v=0gXHqKpg__A

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[BAretired]

Well, if that doesn't raise a red flag. That is illegal concrete in the US!

I agree that it raises a red flag when used in structural members but I was not aware that it was illegal concrete in the US. Do you have a reference for that? Low strength concrete is used as fill material in Canada and I would imagine also in the US.

BA

 

[mike20793]

ACI sets the lower limit for structural concrete at 2500 psi.

 

[TehMightyEngineer]

I wouldn't use the term "illegal". Not meeting code would probably be a better way to say it.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[Signious]

Most I met who designed residential 2 or 3 storey buildings never used 4000 psi. They stick to 3000 psi. Why is that.

I do residential & spec 20MPa (3000psi) for the vast majority of my RC work. Typically the only time I shoot up to 25MPa+ is when there are sulphates in the soil and you need a type 50 cement anyways. Few reasons:

-An increase in concrete strength rarely changes section sizes. 6" thick basement walls are allowed, but are exceptionally rare, and footing width is governed by the soil properties.
-Cheeper, in this scale builders would rather pour an extra yard or two of 20MPa rather than springing for a 30+.
-(biggest reason) It lets the concrete companies ship off-batches from commercial work to the residential customers. If they were supposed to pour a 35MPa for the new office building downtown but the mix was off they can shoot it down to joe shmo home builder at a discount as a 20MPa batch.

 

[MacGruber22]

I wouldn't use the term "illegal". Not meeting code would probably be a better way to say it.

State and commonwealth legislature's creates statutes (laws) to enforce local building code --> adopts model IBC with modification---> adopts ACI 318 with modification ---> structural concrete shall be 2500 psi ore greater. By definition, every adopted provision (not commentary or appendices) is absorbed into the statute. Now, obviously some are more glaring to those enforcing the intent of the statute. We all generally are aware of things that can be slipped by with no harm. Trying to pose 2000 psi concrete as "structural" concrete is asking for trouble (with the law), whether it works or not.

Regardless, I think the discussion is moving from the OP's original moment frames to residential foundation construction. This is apples to oranges. We all know 3 ksi concrete works all day long for single family residence basement walls.


"It is imperative Cunth doesn't get his hands on those codes."

 

[BigH]

If I can interject a comment - perhaps in North America and other countries when you order 3000 psi concrete - you get 3100 to 3300 psi concrete - economics! I have found over in Asia on project specific batch plants that one might have a 3000 psi mix design but when the concrete is tested at 28 days, it'll be 4000 or more from time to time and usually always more than 3500. Hundreds of tests on several projects have confirmed this. We have had some 20 MPa concrete test out at nearly 40 MPa. Can't blame it on the batch plants' additions of cement as they are computer run.