Retaining Wall or U-Frame design with 3,000 psi concrete but 4,000 psi used for construction

[CWEngineer]

Say a retaining wall or u-frame structure are designed using a concrete strength of 3,000 psi, but during construction a higher concrete strength, such as 4,000 psi is used. Do you guys/girls see anything that might be unconservative in using 4,000 psi concrete strength during construction when a retaining wall or u-frame were design using a concrete strength of 3,000 psi? Off hand, I do not see an issue but running it by you guys in case I am missing something.

Thanks

 

[hokie66]

No problem, just better quality than you paid for. I think most of us don't specify anything below 4000 psi anyway.

 

[SlideRuleEra]

Agree with hokie66. When I worked in bridge construction (1970's), the state DOT designs were based on 3000 psi but the concrete specified was 4000 psi. This allowed plenty of leeway for unintended problems in the field. Nobody, neither the DOT nor the Contractor, wanted to remove and replace a bridge pour.

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[Ron]

Agree with hokie66 and SRE.....better durability and hedge against error

 

[bridgebuster]

Ditto to the above. Similar to SRE, NYSDOT cast-in-place designs have a 28-day strength of 3,000 psi but the resulting strength is typically much higher, 5,000+ psi is not unusual.

 

[CWEngineer]

Was doing a little bit more thinking. If I provide a higher concrete strength during construction (say 4,000 psi) than what I designed for (say 3,000 psi), I am getting a greater shear and moment capacity in the field than what I design for, that is good. The maximum reinforcement ratio is larger when using 4,000 psi compared to using 3,000 psi, hence maximum reinforcement ratio check is conservative when providing a concrete strength of 4,000 psi when it was actually design for 3,000 psi.

But the minimum area of steel (3SQR(Fc'))bd/fy value is larger when using 4,000 psi in the field than the 3,000 psi I used during design. So in this sense wouldn't providing a larger concrete strength than what it was design for be unconservative? Since the area of steel provided could potentially be less than the minimum area of steel requirements and not be in accordance to ACI 318.

 

[Ron]

That's only the case if you NEED the 4000 psi.

As others have noted, if you design to 3000 psi, chances are you will get a somewhat higher actual strength, so under your premise, all designs would be non-conservative if the strength went above the design strength.....not so.

 

[Hokie93]

Ron is right. If you want to look at it another way, you do not have to satisfy ACI 318-11 Equation (10-3) if you have provided 1/3 more reinforcing steel than required by analysis (10.5.3). Yet another option for sleeping soundly is to remember that Equation (10-3) only got introduced in ACI 318-95. Prior to that the minimum flexural member reinforcing requirement was 200/f_y; irrespective of f'_c. I have not heard reports of those structures collapsing left and right.

 

[CWEngineer]

In my case, I designed the structural members (concrete thickness and reinforcement) using 3,000 psi. But after I completed the design, the material/soils engineers is telling me that we need to use a concrete strength of 4,000 psi now to meet the ACI 318 Table 4.3.1 Requirements for Concrete by Exposure Class (i.e, sulfate). There is a lot of calculations and I do not want to redo them (change 3000 psi to 4000 psi), if I do not have to. I was planning on providing a statement that the calculations were performed using a concrete strength of 3000 psi, but 4,000 psi concrete strength will be provided during construction. Of course as long as providing 4,000 psi during construction, while using 3,000 psi to design is conservative.

 

[hokie66]

No need to change any calculations. The concrete strength has very little affect on flexural reinforcement requirements at any rate. But if sulfate exposure is the issue, I would be inclined to use at least 5000 psi concrete.

 

[Hokie93]

It turns out that 200/f_y controls for f'_c = 3000 psi and 4000 psi so if you designed per ACI 318-11, you should be fine as far as minimum reinforcing is concerned. Or, again, if you provided 1/3 more reinforcing steel than required by analysis then you are also code-compliant by that provision.