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4000 psi VS 5000 psi Concrete

nader666 (Structural) (OP) 
7 Mar 13 22:18 
Hi everyone,
I am involved in reviewing shop drawings for a sewage treatment plant project. The treatment reinforced concrete tanks were designed based on 5000 psi concrete, the design plans are showing 16" thick walls and # 7 @ 9" vertical rebars.
A new structural engineer has been assigned to the project. I received the concrete mix design shop drawing for a 4000 psi
and report from the new structural engineer stating that the concrete section capacity is only 1% less if 4000 psi to be used instead of using 5000 psi. He attached a simple calculation of the moment using As x fs value times (da/2) for 4000 then calculate the moment using the same equation to get the value of "a" based on 5000 psi to get new value of
(da/2)then the moment value for 5000 psi indicating that the difference is only 1% in the section moment capacity and can be neglected so 4000 psi concrete is ok to be used for the same section and rebars which have been designed based on 5000 psi concrete.
I do not agree to use 4000 psi because I do not think that his conclusion is right.
I have requested to redesign the tank all over based on 4000 psi. Please advice if you agree or disagree on the conclusion of using 4000 psi VS 5000 psi would decrease the section capacity by 1% and it can be used for the same section and reinforcement.
Thank You


Ron (Structural) 
7 Mar 13 22:21 
There are reasons other than strength to consider 5ksi vs. 4ksi....this is an aggressive environment and durability of the concrete will be critical to longevity. Higher strengths usually beget better durability. 

hokie66 (Structural) 
7 Mar 13 23:31 
Agree with Ron. My minimum for a sewage treatment plant would be 40 MPa, about 6000 psi. Strength of concrete is normally a minor consideration in sewage treatment plants, but durability is paramount. May be a roundabout way, but specifying higher strength concrete goes a long way toward achieving what is really required, i.e. durability. 

JAE (Structural) 
8 Mar 13 0:36 
ACI 350 requires 4500 psi for most structures for water containment.


I agree with the previous answers. The 5000 psi requirement is not for strength  it's for durability. Look at ACI 318, Chapter 4. 

JAE (Structural) 
8 Mar 13 13:46 
cliff234  agree with you but technically a sewage treatment tank wouldn't be under 318 but rather 350.


I would question someone who says that the computational difference between the two strengths results in 1% difference. That is certainly not correct. 

dik (Structural) 
8 Mar 13 15:01 
Agree with the higher strength and also I think TXS has nailed it on the head... For Sewage Treatment Plants, my experience is that you cannot have concrete that is too strong... usually stipulate a minimum cement content as well as max W:C ratio...
Dik 

The difference in ultimate flexural strength calculations by changing from 5000 to 4000 psi is likely to be in the 1 to 2% range. The 5000 psi, as has been stated might be needed for durability and then a maximum water/cement ratio comes along with that. But for structural computations, we are missing looking as shear and deflection. Shear is typically based on the square root (SQRT) of the compressive strength. The SQRT of 5000 psi is 71 psi and the SQRT of 4000 psi is 63. That change 71/63 is about 12%. In addition, deflections are based on the MOE which is also typically based on the SQRT of the compressive strength. The cracking moment is based on the SQRT of the compressive strength. And finally, development lengths are based on the SQRT of compressive strength.
I don't know if the ultimate flexural strength controlled for the tank but changing from 5000 to 4000 psi has dropped the shear capacity, dropped the cracking capacity and increased deflections and development lengths. But maybe none of this is a concern for your tank.
B 

dik (Structural) 
8 Mar 13 18:50 
Bruce... the flexural strength loss is minimal unless you have a very high percentage of steel reinforcing. For small percentages of steel, the loss is negligible.
Shear loss can be an issue... one we have under construction now has about 40' of soil over it... and shear is critical.
As noted, the biggest loss is in the durability of the concrete...
Dik 

IDS (Civil/Environmental) 
9 Mar 13 17:03 
I agree that the concrete durability is the real issue here, but just for the record the changes in design capacity are (change from 4000 psi to 5000 psi, assuming no axial load and equal reinforcement both faces):
ULS bending moment: +2.5%
ULS shear force (no shear reinforcement): +7.7%
SLS bending moment (reimforcement stress limit): +0.5%
SLS bending moment (0.25 mm crack width): no change
Flexural stiffness: +1.7% Doug Jenkins
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