Cylinder variances

[lokasjoa]

We have had some breaks that have come up short on their compressive strength. Problem is, is that the GC is saying we broke those cylinder 22 hours too early. I know that the ASTM gives you +/- 20 hours, but I was wondering what the variance is for AASHTO?

 

[concretemasonry]

How many days curing? On 28 day breaks, it would really make no difference. If it is quick break 2-3 day breaks and fly ash or some other additives/retarders it technically make a difference. For 7 day the materials and mix design can affect the results in a minor way.

Is this for removal of forms or supports on a time sensitive project with penalties or bonuses for the GC depending on the completion date? - This can be a major factor in attitudes and interests. I have a friend involved in a project where the contractor and supplier jointly set a control on the strength 25% over project specs to allow reliable, quick construction in harsh weather on a bridge and early completion date bonus was over $50,000,000.

Was the time of preparation of cylinders documented before testing?

The first 24 hours of curing and any handling can easily skew the results in the short term.

The technician making the cylinders can easily boost or lower the cylinder strength, especially in the short term.

Does the concrete supplier have any opinion? He would rely on the lab's value more than the short term business from one job since the GC may not be around especially if they quibble about 2 hours. - The concrete supplier should be notified in some way. I would imagine the mix design included some testing for acceptance, so this could be a reference.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[lokasjoa]

Yes, they were 28 day breaks. And the avergae breaks so far during the project have been border line as it is. So when we broke these 28 day'ers, we didnt think much of it. Its a lean mix, so it borders on the line as "cheap". but to say that our plus or minus 20 hrs may have skewed the results? Proposterous i say.

 

[BigH]

You should check out the AASTHO Spec - they used to say at the beginning that the spec is the same as ASTM with the following exceptions . . . . If you are a few hours short of the 28 days, I don't see a big deal - but the question would be "Why did you break outside the time limits set by ASTM?" You only ask for trouble . . . it gives anyone a free shot at the lab.

 

[msucog]

it makes no difference...but you shot yourself in the foot by not following astm a bit closer. and by the way, the supplier will throw the testing firm under the bus...guarenteed. (this is one time i disagree with concretemasonry)

was it short on the design strength or on the aci acceptance criteria? what would the trendline show? why not hurry up and core it? i mention that because it'll come up so you'd be better off suggesting it...BUT keep in mind the compressive strength results between cylinders and cores are NOT the same. the argument is almost impossible to win with an owner so just play the card first acknowledging that they're different and that the strength may come up. if it does, then the structural can hang his/her hat on it and the owner is happy. if it doesn't, then a more in-depth assessment can take place at the direction of the EOR or owner...or tear it out.

 

[Ron]

Sounds like your mix design might be a bit marginal on the moving average 28-day strengths.

As for the 22 hours....I agree with msucog...no effect at all, but any deviation from a standard will bring all your results into question.

If you plot a strength gain curve, you'll see that after about 7 days, the curve flattens significantly. From 14 to 28 days, the curve is very shallow. This is the portion that will prove your point...If only 20 percent of the strength is gained between 14 and 28 days (336 hours), then for each percent of strength gain, it takes almost 17 hours. So your 22 hours might change the results by a little over 1 percent. So for a 3000 psi mix that would be a little over 30 psi...not a big deal.