huangyhg

obtaining concrete cores
astm c-42 5.2 core drilling—A core specimen shall be drilled perpendicular to the surface and not near formed joints or obvious edges of a unit of deposit.
suppose you have a 36"x36" column poured from the foundation to the bottom of the first level. is it acceptable to takes cores from the top of column vertically?
the top of the column is not a "surface" as such. how would you extract the core, as the bottom would still be attached? i can see where it would be desirable to take the specimen from this location, but don't know if it would work. possibly there is a way to core deeper, break it out, and then saw the core off to correct length.
hokie66 -
in the past we just anchored the drill to a finished surface and overdrilled the depth. we then snapped off the core sample and took it back in for preparation and breaking. there may be some question whether the top inch or two of a column is really represntative of the rest of the column. - undoubtedly less real consolidation due to surface effects.
if that works, he should be able to get a good sample. may have to saw off both ends. in most 36x36 columns, you should be able to pick a point in the middle where you are confident you can avoid reinforcing.
what are you looking for? the compressive strength of the concrete? there are non-destructive ways such as a windsor probe test for determining concrete compressive strength.
if you're interested in the strength near the top, then i don't see why it wouldn't work. if you must core horizontally, then you've got to avoid a lot more rebar and you would need to take core along the length of the column. you should absolutely discuss it with the structural engineer. i've had structural engineers specifically tell me not to drill due to the risk of cutting rebar (we didn't have access to top or we would've cored there). as far as getting the core, it's easy enough to snap off the core after overcoring as previously described. if the structural is fine with having cores only at the top, that's what i'd shoot for.
if you're possibly looking for voids, try sonic testing since it's non destructive. in my opinion, windsor probe is pointless for checking either strength or voids in a column. at best, windsor probe is only suitable for slab areas where you have lots of probe tests and then it should be correlated back to actual field core compressive strength tests. after the correlation, then you might have an idea of where bad concrete might exist within the slab area. windsor probe appears too hoaky to actually evaluate strength...i think it should remain as an index test only. that's my opinion for what it's worth...and i believe aci has similar opinions on the matter even though i do not know where to find the reference.
concrete repair and maintenance illustrated by peter h emmons (part 2) has some good material on possible evaluation techniques of existing concrete.
you might want to take a look at some of the following aci standards as well.
201.1 guide for making a condition survey on concrete in service
364.1 guide for evaluation of concrete structures prior to rehabilitation.
437r strength evaluation of existing concrete buildings
546r concrete repair guide
see 2.2.2.2 of aci437. it has the best discussion of nde tests.

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