concrete testing labs

huangyhg

concrete testing labs
i've been getting reports in on concrete break strengths for a project.  the majority of results of 7 day and 28 day strengths have seemed suspicious.  today, i received a report of 2 sets of cylinder breaks.  the 7 day was 4,950 and 4,990 psi, respectively.  the 28 day strengths are 5,010, 5,020, and 5,060 for set one, and 5,130, 5,200, and 5,090 for set two.
has anyone heard of only a difference in 100 psi for 7 day and 28 day strengths?  this concrete was normal weight, no special type of cement or curing admixtures used.  similarly, on another report, i saw 7 day breaks at 3,100 psi and 28 day breaks at 3,200 psi.
i'm starting to think the testing lab is yanking my chain here.
i would guess that they are using a batch with type iii cement without knowing it.  i would go back to the plant supplying the concrete and verify the mix design.
would ask the testing firm to explain results.  could be simple typo errors or technician not knowing how to fill in the blanks.  their reputation should mean a lot to them and funny numbers are bad for everyone.  depending on size and importance of project, maybe you can make them do some field cores to verify in place strength.
one or more factors could be at play here, such as:
1. an accelerator or type iii cement as suggested by vmirat was used in the concrete mixture.
2. cylinders were not properly cured in the field and or laboratory.
3. mix-up in cylinders tested. this can happen when several classes(mix designs) of concrete are placed on the same day and identificaton of specimens is sloppy. the ability to track specimens with id number and match them with location concrete was placed is very important.
4. use of "magic pencil" to create results. let's keep our fingers crossed that this was not the case.
a few questions:
(i) did this problem happen for more than one different mix? i bring this up because some of the 28 day results are slightly over 5,000 psi and another one is just over 3,000 psi, suggesting more than one mix.
(ii) how do these results compare with results indicated in the mix design submitals? mixes proportioned on the basis of statistical analysis of field data are recommended.
(ii) have you ever visited the testing facility to examine their curing room, testing machines, calibration certs?
(iii) is the testing lab accredited for testing concrete cylinders?