Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations SSS148 on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Core Drill Concrete for Testing 2

Status
Not open for further replies.

PT99

Structural
Apr 24, 2007
62
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.
 
Replies continue below

Recommended for you

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? see Sec 5.1.1

I guess this is to provide assurance that the concrete is mature enough to withstand coring. This is one of the things considered when conducting an investigation into factors influencing concrete core strengths. Several papers out there that have dealt with this.
 
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 remember correctly) once the samples have been cored.
and i don't know where you're located but 90 minutes is not the absolute golden rule...if temps are way up like they are here in the south, that 90 minutes goes down.
bottom line--make the contractor jump through hoops to prove that they provided materials that meet the project's requirements. one more thought, if they are responsible for field evaluating the conditions and providing papers that everything is good, then they will also be responsible for not coring through the rebar (hopefully they have access to the top of the column or have a coring tech that knows when he's beginnging to get in to the rebar). good luck.
 
3000 psi.. I wont sweat too much. More than likely the test will exceed that. But I wouldnt continue working (build anything on it) on it until I get the result. If it was was higher strength concrete with really fast curing rate then I would be worried. Good luck!

Never, but never question engineer's judgement
 
PT99, may I ask why you feel you need to core test at all?

 
Why... because the concrete remained mixed in the truck mixer for 2 hours, maybe even more, while the workers got themselves a vibrator. In fact the little that was poured early on before the pause to get a vibrator, got vibrated 2 hours later

Temp was about 90 degrees (New York City).
 
Sounds like you might have some hot concrete and a right to suspect the concrete quality.

Do you have cylinders taken from the trucks as they emptied late?

If so, you could evaluate the 7 day breaks first and then core later if needed.
 
Cylinders were taken early on before the pause, when the inspector asked "where is the @#$%^%^ vibrator".

So they have no value.

Schmidt Hammer. Is this reliable for final values. or only as a guide.
 
Schmidt Hammer: Only as a guide.

 
Seems to me your contractor can get the results he needs in just two days:
day 1 = chip down offending pedestel
day-2 = reset rebar and place in-spec concrete

BTW,

Coring is done at 14 days to make sure that the coring process, the spinning drill bit and water-pressure, doesn't wallow out or washout the sides of the cores. Cores with straight smooth vertical sides give more accurate results. Upon drilling of the cores they are be wiped of the drilling water and placed in a moisture tight bag for 5 days. This equalizes the moisture gradient from wet surface to dry interior.

So the earliest core results can be obtained is 19 days from placement of concrete. If the contractor doesn't like the 19 day time table advise him to pick from the following:
a) DO NOT place out of spec concrete
b) chip it down and do it over

Also, since he managed to f-up the concrete placement so badly why don't you ask him how the post-placement curing was performed: vertical concrete elements not protected by form-work for at least 7 days require curing (ACI-308). Was the element stripped of form-work the following day and allowed to bake in the hot sun with no curing at all? Well so much for cement hydration, probably don't need that anyhow...

Just for laughs you should order a meeting (you, the arch, the owner, etc) to review job site concrete placement procedures. Once the meeting starts launch into a grueling punishing 4 or 5 hour review of the project manual and all related ACI standards. I mean review standards and specs in such depth and detail that the guy wants to put a gun to his own head!

Also,

SP-15(05) FIELD REFERENCE MANUAL: Standard Specifications for Structural Concrete (ACI 301-05) with Selected ACI References

Section 1.3.3 of ACI 301-05 requires that the contractor keep a copy of this manual [SP-15(05) FIELD REFERENCE MANUAL] in the field office of any project where ACI 301 is specified.

ACI-301-05 Specifications for Structural Concrete
1.3.3 Field references—Keep in Contractor’s field office a copy of the following reference:
ACI SP-15 Field Reference Manual: Specifications for Structural Concrete (ACI 301-05) with Selected ACI and ASTM References.

Is this book in the contractor's field office? If not WHY not? Obviously it begs reading by contractor.
 
Also remember how you will evaluate the core that you get and test. Neville had a nice paper in the International Concrete Journal (I think - ACI) that discusses cores and core drilling. Normally, concrete is acceptable if the average of three cores exceeds 85% of the required strength and no single core is less than 75%. Then there is a question as to whether to test the core in a dry condition or wet condition. Do a search on other similar threads on cores - the reference to Neville's paper is in one of them.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor