RIZ..that maturity is probably on the edge for potential impact for the cores.  With plenty of cooling water, slowed progress and a relatively new coring bit, it will probably be OK.  Make sure the drill doesn't wobble much if any.  These things usually impact the coring more than the abrasion vibration of the bit.

Be sure to keep the cores wet.  Some specifications allow dry testing of cores if the service environment will be dry.  In my opinion, if you are coring at early times or for low strength confirmation, you don't need the potential drying shrinkage effects, so test them wet.

Ron

Hey Riz.

If you use ACI...check out ACI 318, Section 5.6.5 for their take on field cores.

Thanks Ron & Jae.
Compressive strenght result of the core is 10 N/mm2. Cube strength thus would be  1.25 x 10 = 12.5 N/mm2.
This is lower than anticipated value of 0.67 x 25N/mm2 = 16.7 N/mm2 at 7 days.

Riz...under most core evaluation criteria (ACI included), if the core strength achieves 85 percent of the expected strength, it is considered acceptable.  This generally applies to 28 day and later strengths, but could be extrapolated, at least in part, to include earlier strength parameters.

As an example, you cored at 12 days with, I am assuming, a measured maturity of 67 percent.  If your cores achieved 85 percent of that value (0.67 x 25 N/m^2 X (0.85)=14.2 N/m^2)then you would consider that acceptable.  However, early strength gain is affected by lots of "stuff".  The fineness of the cement, the type and quantity of admixtures, whether the cement was modified ( using fly ash, ground blast furnace slag cement, or materials), curing conditions, aggregate influence, w/c ratio, air content, temperature, and probably a few I have left out.  This is a danger of coring/evaluating too early.  The time between placement and about 15 to 18 days is the steepest part of the strength gain curve, so minor mix issue can significantly affect strength gain.  These tend to moderate with time (20 to 60 days.

Your next step might be to get a petrographic examination of a core to evaluate a few of the hardened concrete characteristics that might lead you to a conclusion.

Good luck.

Ron

I am not sure about your codes but the sample size will affect the confidence you can have in your result. So if one core has been taken from a batch of concrete your 95% confidence limit on actual strength of the batch based on that core will be a lot lower than the result you have. This is why using cores is not often a great way to determine the strength of concrete because you need several to make the results statistically significant

Carl Bauer CEng MICE
Bauer Consult
PO BOX 2224
Gaborone
Botswana