low concrete strength

huangyhg

low concrete strength
a tank foundation was poured and the 28 day compressive strength of some of the test cylinders was less than specified.  4000psi concrete was specified and the test cylinder strengths ranged from 2300 psi to 3500psi.  how concerned should the owner be?  what action should be recommended?
i work for an indepent testing lab that deals with concrete testing. should the owner be concern with 65% concrete, very concern.
first find out if there is a sample problem: curing, transport, making,testing
find out the percent of bad to good with an average
was it a finisher problem
core the slab.
i would be looking at this with a fine tooth comb, usually if it a well establish mix from a good producer, it is usually a sample or finisher problem.
those strengths are significantly lower than would be anticipated, even for an isolated test condition.  the strength would not have anything to do with finishing or placement in this case, as you indicated these were test cylinder specimens (presumably cast at time of placement).  if cores were taken, then placement/finishing would enter into the evaluation.
would need to know a bit more about the mix design, admixtures, information taken at time of placement, storage of cylinders in field, qualifications of technician sampling the concrete, where it was sampled, whether it was a pump mix, the design and actual slump, laboratory capabilities, ready mix supplier history, etc.
some types of cement (ground blast furnace slag)are slow to gain strength, but usually by 28 days they have come up to strength.  excessive air content, alone or coupled with other admixtures or cement types can cause this.
would suggest the following course of investigative action:
get all info as noted in this message
if not cored, core the slab in accordance with astm c42
do compressive strength on at least 3 cores and save a couple untested for petrographic examination.  have petrographic done to check for excessive air content, excessive water added (retempering), aggregate/paste characteristics, and other relevant physical attributes that can be used in the evaluation.
look at all data and the answer to issue will probably be there.
i'm not sure if this is directly relevant to the present discussion.  a concrete grade is specified based on cube tests, and that cube test results are always greater than that of cylinders.  in design, a factor of 0.67 is applied to cube test results and 0.85 to cylinder test results to determine the extreme fiber stress level in compression at failure.  that's all.
azmi ibrahim
i agree with ron that the results are very low.  in addition to ron's advice, i would also introduce some non destructive testing ie schmidt hammer test.  there are new pullout tests on the market where you core into the concrete, put a hydraulically expanding tube into the hole and then jack the tube back out of the concrete.  the pullout force can then be correllated directly to an in-situ strength for the concrete.
regards
andy machon
whether the issue is critical in a structural sense depends on the design. if 4000 psi was specified when 2000 psi would do the job, there would be no structural consequence. of course, the legal consequence is different. if i am paying for 4000 psi, i am entitled to get it or to receive a refund.
now the issue gets interesting. if the job needs 4000 psi, the problem won't be solved by the payment of damages and then doing nothing. in this case, the job needs to be replaced.
as an aside, the word "foundation" refers to the ground that supports the footing. what are we talking about here? a footing slab or a foundation? if it is the latter, how does concrete strength enter the equation? does it refer to concrete that was used to augment the strength of the foundation?
regards, helmut
helmut,
"foundation" is colloquially used to mean the concrete (typically) portion upon which everything else rests.  the bearing soils are not generally regarded as the "foundation" though i suppose semantically you are correct.
agree with your premise of strength.
you might want to review chapter 5 of aci 318 (if you are in a region that uses aci 318).  they have clarified the proper acceptance criteria for concrete with low strength tests.  section 5.6.3 states that an individual class of concrete can be considered acceptable if:
1.  every arithmetic average of any three consecutive strength tests equals or exceeds f'c (a test is the average of two cylinders broken at 28 days).
2.  no individual strength test (again - two cylinders) falls below f'c by more than 500 psi.
if either of the above are not met, for large projects, you should take steps to increase the subsequent concrete strengths.  in your case, this probably isn't possible.  if item number 2 above is not met (which appears so by your numbers) you must then meet section 5.6.5 which requires that three cores should be taken for each strength test below f'c by more than 500 psi.
if the average of the three cores is equal to at least 85% of f'c and if no single core is less than 75% of f'c, the concrete can be considered adequate.
failing all of the above, you could then turn to aci chapter 20 and possibly load test the foundation although for slabs/foundations on grade, this might be somewhat difficult.