28 day vs. 56 day concrete breaks

huangyhg

28 day vs. 56 day concrete breaks
i have a contractor that has repeatedly had 7-day and 28-day concrete cylinder breaks significantly lower than the specified strength.  in a meeting on friday, he presented 56-day breaks (from cored samples) that meet or are very close to the specified strength.  my opinion is that the in place data trumps the cylinders.  is there any criteria that i am missing here?
per aci 318 section 5.6.5, i would agree with you.  i would verify that the cylinders came from the area where the low strength concrete was placed, and that the proper amount of cores were taken and tested.
i would not say that the in place data "trumps the cylinders".  it is expected that the 7 day strength should be below specified strength, maybe about 2/3 of it.  the 28 day cylinder strength should be at or above specified strength or your client is not getting what he paid for.  it is expected that the 56 day core tests would be stronger than specified strength.  
there is nothing wrong with accepting the concrete if you are satisfied that it is adequate, but perhaps there should be a financial adjustment to the benefit of your client as he clearly did not get what he paid for.
ba
i agree with you 100% ba.  structurally, the concrete will meet the need, but we will likely advise the owner that they should not pay for the lower strength concrete and/or demand an extended warranty for this portion of the work.
thanks.
this happens occasionally... the only caveat i have is that for durability, you are expecting a similar material to what you are accustomed to. the specified material would be stronger at 56 days... if i spec a 30mpa mix, i'm expecting a certain quality of concrete, not a weaker one. in most cases, it's acceptable.
dik
the 28-day compressive strength is used for design.  when it isn't met, there's an obvious deficiency.  there are two parameters to look at; strength and durability.  low strength results should be evaluated in accordance with aci 318 or an equivalent method if outside the us.  in general, if the core strengths achieve at least 85% of the design compressive strength, it is acceptable.  no single result should be more that 500 psi less than f'c.
durability of the concrete is greatly affected by the strength deficiency.  if the concrete will be subjected to abrasion or surface traffic, you will likely experience long term problems with durability.
before you move forward, you need to find the reason for consistently low strength results.  was it from a marginal mix design (if so, it probably doesn't meet the statistical criteria of aci 318)?  was it from adding water at the site?  was it because of the cement types or blending?
these things need to be known before you make a presumptuous decision based on strength alone.  as seit noted, make sure the cores are representative of the area in question.
seems like every so often i repost the info below:
the following is our in-house guideline that (i believe) follows aci 318 chapter 5:
acceptance of concrete
per chapter 5 of the aci building code requirements for structural concrete
(aci 318)
a strength test is defined as the average strengths of two cylinders tested at 28 days.
a strength test is acceptable if both of the following are met:
1.    every arithmetic average of any three consecutive tests (6 cylinders) equals or exceeds f'c.
2.    no individual strength test (average of two cylinders) falls more than 500 psi. below the required strength, f'c.
if either of the above two requirements are not met, contractor should take steps to increase the average of future concrete tests.
if item 2 above is not met, then the following should also be followed:
1.    with the lower f'c derived from the strength tests above, the structural engineer should review the design of the affected portion of the structure to determine if the lower f'c is acceptable.  if the lower f'c is acceptable, then no further efforts are required and the concrete can be accepted.  steps still should be taken by the contractor to increase the compressive strength of the concrete for future mixes.
2.    if the lower f'c is found to be critical to the performance of the structure, then further testing should be performed using drilled core samples.
core samples should be obtained – three at a time, for each strength test that falls below 500 psi.   if the concrete in the structure will be dry under service conditions, cores shall be air dried for 7 days before the test and tested dry.  if the concrete in the structure will be more than superficially wet under service conditions, cores shall be immersed in water for at least 40 hours and be tested wet.
the concrete can be accepted if both of the following are met:
1.    the average of the three cores is equal to or greater than 85% of f'c.
2.    no single core is less than 75% of f'c.
if the cores don't meet the required levels the following are all options to consider:
1.    perform a load test of the structure in the area under consideration  (per aci chapter 20)
2.    provide additional structural framing to strengthen the portion of the affected structure to meet the required load carrying capacity.  
3.    accept the concrete if acceptable to the owner.
4.    reject the concrete and remove and replace the portion that is considered below strength.

jae...looks like an faq to me. why don't you post it.
ron
i did -  
i would also investigate the concrete sampling method. it could be as simple as that the concrete is good but the taking, storage or transportation of samples is not right.
it has happened to me
jae - i think it might be more clear if you say three consecutive test results (3 x (2 specimens = 1 test result) rather than saying 6 cylinders . . .
  question?  did the mix have fly ash in it?  a lot of it?  this might/would retard 28 day strengths (as the fly ash needs the cement hydration for its own hydration) and yet permit good 56 day results.  on our current job, the mix design, because of the fly ash content, is using 90-day strengths.  might want to determine if spec actually says 28-d strength test value or if the design made a variation subsequent to the contract signing.  on another contract where we did extra cylinders to see what increase of strength we had, we have found that our 90-d strengths have had about a 30% average increase in strength over the 28-d values.