Concrete Cylinder Strength...Such a thing as too Good? 2

[Celt83]

Maybe a dumb question but ACI addresses low break results with some procedures for acceptability or rechecking design.

What happens if you say get 2-3x breaks when compared to design strength, ACI list acceptance as meeting or exceeding the specified design value. Could to high of an achieved strength vs design strength lead to potential ductility issues?

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[r13]

The direct answer to your question is "no". The supplied concrete has a higher strength than specified in the design, in general, will not affect the performance of the concrete member in a negative way, rather, it helps in enhancing shear and compressive strength. Actually, in order to prevent the consequence of low cylinder strength at testing, many suppliers rather to furnish higher strength than the specified, since the cost differential is low compared to the risk. Ductility is affect by steel reinforcement rather than concrete.

 

[BAretired]

I have never worried about concrete tests which came in too high.

BA

 

[Rabbit12]

Technically, you could have an issue if you are teetering on the edge of tension vs. compression controlled.

That being said I've never worried about high breaks. I had a colleague once that did on footings and it didn't go over well with the contractor.

 

[Celt83]

Ductility is affect by steel reinforcement rather than concrete.

Right but this is accomplished by providing enough steel such that the uncracked capacity of the material is less than the cracked reinforced capacity. If, for the sake of the discussion, you had a beam designed with an F'c of 4 ksi and the loading was such that it only needed minimum steel then an increase to 8ksi concrete without an increase to the specified reinforcement would result in a section that is not in compliance with the minimum flexural steel requirements.

As you noted the flexural and shear strength are increased a decent amount, so from a strength perspective I would agree more than likely a non-issue. I work in a low seismic region so don't get into special frame/wall design at all, but would that be an area where the ductility impact of a higher break might become more of a potential issue?

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[CANPRO]

I'd be concerned with the accuracy of the results if you're getting 2-3x the strength.

In my part of the world, minimum concrete strengths are 25-30 mPa - so 2-3x of the minimum strength puts you at 50-90 mPa. I doubt the concrete supplier is going to accidentally provide such high strength concrete.

 

[r13]

Celt83,

I think it will be a good exercise for you to figure out the effect of concrete with different compressive strength, but with the same reinforcement that was designed for the lower class concrete.

a = AsFy/(0.85fc'b). For fc'=4, a₄ = Asfy/3.4b, and for fc'=5, a₅ = Asfy/4.25b, thus a₄>a₅. What is the implication? Appreciate your feed back.

 

[Celt83]

retired13:

ACI 318 Minimum Reinforcement of Flexural Members:
As, min = max[ (3*√F'c / Fy)*bw*d, 200*bw*d / Fy]

4ksi: max[189.74*bw*d/Fy, 200*bw*d/Fy] = 200 bw*d/Fy controlled
5ksi: max[212.13*bw*d/Fy, 200*bw*d/Fy] = 212.13 bw*d/Fy controlled <--- slightly more steel required to satisfy minimum at 5 ksi

CANPRO:
yeah I doubt in practice this would become a real issue, unlikely a GC is going to pay for an 8ksi mix when they only need 4ksi. Have seen upwards of 1.5x design strength come thru though for elements like elevated slabs where for schedule reasons they went with a higher mix design to hit higher early strength.

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[r13]

Celt83,

Since fc'=5 requires higher minimum reinforcement, so it is "under reinforced", since As_min was determined by fc'=4. Am I correct in saying that?

 

[Celt83]

yep, that's my thought. Admittedly in a real world case it is highly unlikely that you would only have enough steel to satisfy minimum requirements in the cross section, revisiting old questions I've had written down but never got around to fully vetting.

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Open Source Structural GitHub Group:

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[STrctPono]

Wow! 2-3x higher strength is pretty overkill. 25-50% higher strength is more typical. Remember, higher strength concrete will require higher cement content. Even if they keep the w/c ratio the same your total free water content is increased which means more shrinkage.

Even in the cases you are mentioning that the higher strength concrete will push you into non-compliance with minimum As, doesn't ACI 318 have a provision that you need not be higher than 1.33 times the moment demand even if you don't satisfy minimum steel requirements. AASHTO does, so I thought ACI might share similar sentiments.

I agree that for a design concerning only vertical loads, there is no cause for alarm. However, there is certainly a cause for alarm if your analysis and detailing of the structure are relying on a certain level of ductile performance from your system under seismic loading. Whether you are running a force based or performance based analysis, the assumption is that plastic hinging will occur and that is why we specifically detail for these higher ductile system. Whether you are assuming your plasticity will occur in the column or the beam may never come to fruition if you are pouring 2-3x higher strength concrete. This significant increase in strength will change your moment-curvature results for a performance based analysis. When we run pushover analyses for existing concrete bridges we follow the recommendations of M.J.N. Priestly and take a 10% increase on steel yield strengths and 30% increase on concrete strengths. Material overstrength factors are something that should be considered as an underestimation of a concrete columns flexural strength could lead to grossly under designed shear and confinement steel detailing.

Short answer, yes, if you specifically designed and detailed for ductility based on expected lateral loads, I would be very concerned if the element intended to undergo non-linear hinging were poured with concrete strengths 2-3x higher than specified. If this issue is only concerning members that were considered for vertical loads only, then no, I would not be concerned.

 

[Agent666]

2x long term concrete strength gain is fairly common. Both because the strength gain over time continues for years and generally for the supplier to ensure they meet the specified 28 day compressive strength, they target a higher strength anyway in their mix design. After all the 28 day specified strength is typically a 5% characteristic strength. Meaning 95% of mixes should have a higher strength.

Celt is correct in terms of the main issue being that you push the balance regarding ensuring the reinforced capacity is greater than the cracking capacity of a cross section if the concrete strength is higher than expected. If it isn't then you risk a single crack scenario and all strains in the reinforcement concentrating in the bars over a very short length resulting in bar fracture due to low cycle fatigue.

I did a post on this a while back on my blog which tended to show even with a substantial gain in concrete strength that the minimum provisions (in my own code at least) still maintained a sufficient margin over and above the cracking capacity.

You can find the comparison here if you are interested, I also discuss a few cases where this bar fracturing behaviour was actually observed in our recent seismic events here in New Zealand .

 

[r13]

High strength concrete does posses design problem that needs to be looked into, as it is stronger but brittle (less ductile). However, besides the potential problem, it is indeed too good to believe it would occur in real fresh concrete application. Let's say design fc' = 4 ksi, then 2*fc' = 8 ksi, and 3*f'c = 12 ksi, both are in the class of high strength concrete, that costs a lot more to produce.

 

[Compositepro]

Is cracking considered ductility? Assuming that modulus is relatively constant, higher strength simply means it cracks at a higher strain. Is that not a good thing?

 

[STrctPono]

Ductility of the material is the ratio of the ultimate strain to the theoretical "yield" strain. Unreinforced concrete has no real meaningful material ductility worth discussing. If any conversation about ductility is to be had it should be regarding the member or system ductility which, although is dependent on the concrete strength, is a very different thing.

 

[Tomfh]

Higher strength concrete needs more reinforcement to meet the minimum bending strength (ductility) requirments. So check that.

Also, some codes have explicit clauses limiting concrete strength for various elements, e.g. ductile shear walls which need to undergo a lot of ductile cycling, where high strength concrete is worse (more brittle).