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I usually design precast concrete members for the common concrete strength of f'c=4,000 psi at 28 days. Occasionally, a mix design will be submitted that has a very high strength. In one case, the mix that was submitted had a history of cylinder breaks above 7,000 psi. This is 175% of the specified strength!

Normally in engineering extra strength is good, but this seems excessive. Steel grades typically specify a strength range (min and max), but I have never seen a maximum compressive strength in a concrete specification.

Should I reject these very-high strength concrete mixes?

What could be the complications of too high of a compressive strength?

There are only two possible areas of concerns that I can think of:

  1. Modulus of Elasticity is directly related to strength. A high strength may cause the concrete to be more prone to cracking (serviceability concern).
  2. Un-designed high strength may be detrimental in seismic design where you want members to fail in certain locations and in certain modes. Note, my situations typically include moments (either beams or beam-columns). There likely wouldn't be any concern in a purely compression member.

The reason for submitting such a high strength mix seems to be that the precaster wants to get the piece out of the form quickly, so they add lots of cement to insure that the strength requirement is met.

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  • $\begingroup$ My understanding was that in steel, an upper limit is important since seismic design is all about ductility, but concrete is always treated as non-ductile - only the steel in it is considered potentially ductile. So a higher strength would be less problematic in concrete than steel. Where I live, it's common for a contractor to use higher strength concrete than specified in hopes that their 7 day break will meet the design criteria. If it does, they often get to remove shoring early and proceed with the next phase of construction. $\endgroup$ – Ethan48 Nov 17 '15 at 5:24
  • $\begingroup$ Concrete strength is directly proportional to the cement:water ratio. Excessive strength means excessive amount of cement used - which is costly. While there are no obvious engineering disadvantages, it raises a red flag about a foul play with the budget. $\endgroup$ – SF. Nov 17 '15 at 9:38
  • $\begingroup$ This is an interesting question, but the answer is no, unless the general or technical specification, which forms a part of the binding legal contract, includes a maximum strength limit. So if you do reject the concrete, the contractor/supplier could make a legitimate claim. If this work is under FIDIC, then you may try to reject it under the general conditions of contract, but one important basis of FIDIC is that the Engineer must be seen to be reasonable, and such a rejection could be challenged by the supplier as being unreasonable. $\endgroup$ – AsymLabs Dec 2 '15 at 17:21
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    $\begingroup$ The issue is not the strength, per se, it is the mixture composition. Some specifications do include maximum cement contents. This could be a possible reason for rejection. But a richer mixture will be at greater risk of to temperature distortion/cracking, shrinkage cracking and creep effects. These are the concerns I would have. Instead of rejecting the mixture outright, I would be inclined to see if there are any shrinkage or creep limits in the specification, and to ask the contractor to verify (with tests) that the proposed rich mixture can meet these requirements. $\endgroup$ – AsymLabs Dec 2 '15 at 17:32
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    $\begingroup$ @asymlabs looks like you have enough words there for an answer.... ; ) $\endgroup$ – hazzey Dec 2 '15 at 22:40
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Reliability of Data

Are you able to repeat the test to confirm the 7ksi break? Did you have a second 28 day cylinder or 7 day breaks which were also higher than typical?

How sure are you that the cylinder was formed properly and followed ASTM?

Application Specific Requirements

Does the specifications from the design engineer call for a max and min ksi or only a minimum strength? I have projects which utilize cement in flexible applications and require max and min range to firstly provide strength and secondly fail in a planned motion to prevent major cracking (paved soil cement).

Have you spoken to the design engineer about the results to see if he/she has any hesitations about high strength concrete for the application?

Customer Satisfaction

Higher strength concrete costs more than a leaner product, the customer or your company are not getting their money's worth.. or the concrete supplier is ripping themselves off. However, these would not necessarily be reasons to reject the concrete just something to keep in mind depending on who you serve.

Less Generally After writing all of that and looking at your profile it seems like you may be the design engineer, doh. I would most likely look at the pros and cons of why you didn't select the higher strength concrete to begin with and throw out the reasons that involve cost. Will the higher strength reduce work-ability of the structure, more apt to chip than absorb when manhandled on site. Is there a strength to thickness ratio that needs to be maintained that would mean the higher strength would require a thicker or thinner design that it no longer meets. Will the higher strength change curing time, increase reactivity of the cement which would change your production rate coming out of the precast plant?

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  • $\begingroup$ I agree that it is strange that the fabricator would spend extra money, but that is out of my hands. $\endgroup$ – hazzey Nov 20 '15 at 1:45
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From a legal liability stand point, you as the design engineer would have to take responsibility for the higher strength concrete. In this sense, I would delay approval until new calculations are run to establish the higher strength concrete will meet the design criteria.

Here are my concerns, in decreasing order of importance:

1.) The higher strength concrete undoubtedly has a higher density and therefore more dead weight, due to cement being 3x the density of water. This will certainly affect the seismic calculations.

2.) Will this product be capable of meeting 7 ksi, but only rejected if it tests less than 4 ksi? In this case, then you don't have 7 ksi concrete, you have 4 ksi concrete with the density of 7 ksi concrete - a real headache for seismic loading. If they submit 7 ksi concrete, they need to show that they are producing 7 ksi concrete, not just meeting the original specification.

3.) Steel work (admittedly not my area of expertise) - will placement of the steel be as consistent in the 7 ksi concrete as in the 4 ksi?

4.) Due to the increased amount of cement, will there be any additional problems with environmental exposure, etc? How does the reliability fare?

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    $\begingroup$ I'm not sure that the density will change that much. The majority of the concrete is still aggregate. Usually the design density of concrete falls into light/normal/heavy based on aggregate, not strength. Point 4 is my biggest unknown. $\endgroup$ – hazzey Dec 1 '15 at 19:54
  • $\begingroup$ While the density of normal concrete does change with strength it does not do so by much, a 100% increase in concrete strength only affects density by 5% or so. In fact the Eurocode simply prescribes a single value for density of concrete (24 kN/m^3) no matter what strength for normal calculations. Since I'm not living in a seismic area I don't know whether this is true for the seismic calculations though. $\endgroup$ – Mr. P Apr 27 '18 at 13:54
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Higher concrete strength also has faster solidifying times. There is a trade-off with all types of concrete so you'd want to know how the resultant of higher strength is achieved and for what purpose they make it higher strength.

You only really need the 90th percentile for regular use even when up to 15% variation of concrete design is above the top 5th percentile of mean production output. Usually this increased strength is a result of poor production management oversight. Check that out before staking your reputation on the final products made by another person.

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