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Specifying 56 day strength for High Strength Concrete 2

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RFreund

RFreund

Structural
Aug 14, 2010
1,880
The contractor has submitted a mix design requesting the use of 56-day strengths for "vertical concrete members" as opposed to 28 days. It seems that this is more common for high strength concrete (which this mix is 6,000psi) and I don't see an issue with it off hand. However, I'm trying to think what this might impact...
[ol 1]
[li]Shores/reshore[/li]
[li]For a podium structure, maybe we would need to check to see what their estimated progress on the superstructure would be. However, I don't know that the impact would be that substantial given the final design includes a much larger load.[/li]
[/ol]
Are there any other characteristics that would be different between a 6,000psi at 28 day concrete and a 6,000psi at 56 day concrete?


 
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There will likely be more performance based concrete specs in future and the extended time will likely be one of these measures... I don't see a problem with it, except that your current expectations of a mix are based on 28 day material. The 6000psi stuff at 28 days will likely be a tad stronger.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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6000 psi isn't that high where I am. It's higher than we generally specify, but it's in range of what we typically see in the compressive tests.
 
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Most design code rules are developed based on the concrete actually being stronger than the 28 day value over time.

Using the 56 day value is using up some of this over strength that is assumed to exist when determining phi factors or material factors and overload capacity.

I would tell them to use 28 day unless they do a complete review of the requirements.


 
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Aren't most shoring specs something like "concrete meets XXX psi min or 75% of 28day strength, whichever is greater"? By that token, I would presume that either:
A. Your shoring specs would need to reflect what that minimum strength needs to be
B. The testing would still need to occur at 28d, so some indication of 28d strength would still need to be clarified

 
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56 day is pretty much all I do for vertical elements. Among other things, it allows for high fly ash content to be used in those members. The only reason that I don't allow it for slabs is concern over the impact this would have on deflection.
 
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Where do you live that 6000 psi is considered high strength?
 
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If Ron's around, maybe he can comment... 28day spec is nearly a 'legal' benchmark. I don't know what added requirements are required to spec 56day stuff... It may simply be a matter of speccing a different time, or there may be more to it.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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Thanks for the replies. Appreciate the info.

Rapt - you make an interesting point. I was curious about this, but I couldn't find anything "concrete" that states this. I could very well be overlooking something though.

6,000psi really isn't that "high-strength", but I think most texts consider anything over 5,000psi "high-strength" or at least that's how I've seen it presented.


 
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Interesting question. Don't have much to add on the 56 day thing other than when cylinders come back low one of the first responses from the ready-mix supplier is...wait longer (and we do). Nothing bad seems to happen so?

Other then that I found a few references maybe worth considering.

From CSA A23.3 we have

Capture_kfduec.jpg


I'm not sure if there's a better clause delineating what CSA considers high-strength vs normal strength. But based on the following I'd think it would be 50 MPa (7200 PSI) or greater since that's when it says to check into QC thoroughly. Also seems to jibe with this.

Capture2_c2k9vt.jpg


The PCA seem to list high-strength concrete as anything greater than 6000 PSI (meaning your 6000 PSI is technically normal strength). More info on 6000 PSI threshold here.

Capture3_ntsquy.jpg
 
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Using fly ash as we have done on a few hydro projects, the strength at 90 days is specified, not 28 days. For Roller Compacted Concrete, it is typically taken at 180 days or 365 days - of course, in RCC, density checks are important to ensure maximum density at placement.

As for the "overstrength" loss alluded to - is this really built in? The design (dating myself) is based on 28 day strength - and one I would think would not "count on" strength gain - it just happens to be a bit of insurance.
 
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Even if a 56 day strength is provided, the contractor should still be able to provide you with the strength gain of the mix at say, 3, 7, 10, 28 days or whatever. They should have historical breaks to document the rate of strength gain. Ask for that information and compare the strength provided to the strength needed at a particular time.
 
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You may have to spec the added tests...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik
 
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I've done what Koot said, for concrete columns 56 day is what we've done before. Had it come up with a project that they made the concrete for the columns a little too workable and needed to get to 56 days until it hit full strength.

It's not so much a gray area as it is that there are a lot of variables (vert vs horiz element, exposure, weather, etc.).

But MotorCity is right, you should still be getting the previous cylinder breaks leading up to 56 days so that you can be sure things are trending in the right direction.
 
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RFruend,

When they calculate Reliability factors to determine Phi factors etc, the overstrength capacity of both concrete (above 28 day) and reinforcement (above yield) are normally considered as contributing in the calculation of the actual capacity available.

So to base the concrete strength on 56 day, you are reducing the actual overstrength available compared to the assumptions used to develop code factors.
 
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I’d only accept it if the final strength is expected to match the 28 day mix’s final strength (ie it’s merely a lower early strength, not low final strength). Otherwise as rapt says you’re simply accepting a lower grade product.

Also as noted it can have issues on deflection etc. You would need to prop it longer to get the same final deflection.
 
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