huangyhg

core drill concrete for testing
i need to core drill concrete to test compressive strength for 28 day test. can i core drill after 48 hours and hold core, or should i wait 7 days before coring.
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why not wait for 28 days? then the sample will represent the field concrete. if you core early and then store in optimum conditions, 100% humidity and 20 degree centigrade, then the sample may be stronger than the product.
there is contractor pressure to do this right away. my question really is should i wait at least 7 days, or core after 48 hours. can the core drillers sample the material without it breaking on them. does it matter when they core.
concrete was poured last saturday, with concrete sitting 2 hours in truck mixer while the workers got themselves a vibrator
regarding waiting 28 days, well it the same as cylinders.
cylinders are better cured then the actual work, but that is the protocol.
what is this concrete for?
never, but never question engineer's judgement
what type of strength did you specify and what kind of strength do you absolutely need?
never, but never question engineer's judgement
the cylinders represent the optimum strength delivered by the ready mixed company. everything done or not done by the contractor lessens the strength,(add water, late curing, lack of vibration/consolidation, free fall producing segragation). let the contractor prove to you that the out of spec product is sufficiently strong to remain in the structure. two hours is more than the ninety minutes usually specified from batching to final placement. cores of 1200 psi can be successfully taken by good drillers with new bits using sufficient water.
according to note 1 in astm c 42-99, concrete should be at least 14 days old before coring.
thanks everybody
concrete is for a column pedestal, 3000 psi specified.
when i see results i'll re-calculate.
why 14 days? is it to make sure the core won't break up when drilling?
why 14 days?
i think the 14 days is a timr selected to eliminate much of the short term vaguaries caused by site conditions and curing. the results can vary significantly between a slab and more massive section that is not affected by exterior conditions.
to be unscientific, if the initial aggregate and air temperatures are not elevated the extra time may not have been too detrimental.
ddick
i believe the 14 days is a little safety factor for the slower setting mixes (i.e. slag mixes for example) so that "false" low breaks are not reported.
i might disagree to some extent with a comment above. "the cylinders represent the optimum strength delivered by the ready mixed company"--i disagree. the cylinders are checking the consistency of the mix being delivered. 95% of the time, i see the field cored samples break higher than the cylinders. maybe it's the scatter within the test results or maybe it's intentionally developed to have a little fluff in the cylinder strengths...i do not know for sure. i think it's probably got more to do with the field core samples being retrieved from the mass concrete while cylinders have a finite size....maybe i'm just overthinking the whole thing. however, that might be why only 85% of the design strength is required for cores and why aci has acceptance criteria for lower than specified cylinder breaks.
either way, the contractor placed concrete that was out of spec. i typically see the contractor then being responsible for getting the area cored and verified as well as paying for it. also, 28-days is typically the time at which the strength requirement applies. most accept 56-day breaks if the breaks are too far out of the aci acceptance criteria for lower than specified breaks. also with coring, i believe there is still is a moist curing period (5 days if i re

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