Core Drill Concrete for Testing 2

[PT99]

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.

 

[DRC1]

Sounds to me like it was the inspector who stopped the pour for two hours for a vibrator for a pedestal. Very well could land on his door step. Other concerns would be a cold joint and how that was treated and remobilizing false set concrete with a vibrator. Good inspection is more than being able to memorize several ACI guides. A little practical experience goes a long way.

 

[jwilsonpro]

Not to interject rudely, but ....
First off concrete inspectors have no authority over the contractors placement of concrete, and typically both of them know that. So lets say the inspector says yea no problem so you smoked your vibrator just throw that last 4 yd's in the forms and lets go home.

Now does the engineer of record want another set of cyl's cast w/o consolidation to simulate this condition. (real issue is does he have molds and tickets at the ready? probably not.)

If the inspector advises site water to allow the concrete to self consolidate more easily or suggests alternative means of consolidation he's putting himself in the line-of-fire.

What the real question here is what is the difference between the initial discharge and final discharge of that truck.

I'll bet my pay check that "Unknown quantities of site water were added after sampling" applies here.

So did they add 30 50 70 or 90 gallons of water to the last how many yards? What did that do to the water to cement ratio and how far along had hydration gotten when they dumped the end of the load out of the chute and all the bonds which were established are permanently broken.

The real indicators are likely be the temp/weather and what the cyl's off that truck do break at. In the mean time since you seem like your, ready, willing and able to pursue the issue, may I sugguest In Place Concete Testing to establish what the worst of it is (the stuff that sat for hours) Vs. the begining of that load. Throw in some other test to validate other sets of clys poured that day or previously to assure accuacy and also qualify the test method. Ask your Materials Lab if they would run a windsor pin or swiss hammer on the suspect area as well as at least two other locations of known psi. Don't be silly and accept surface hardness readings in place of psi results from cylinders. Instead try to corralte the two in order to satisfy yourself it's all OK.

I personally watched a crew place over 100 yd's of FND wall about 10 to 12 feet high in light snow with out enough extension cord to reach the site power breaker box. So these guys cowboyed up and WORKED the concrete with 2x4's inside the forms and beat the form work with hammers to achive consolidation.

 

[PT99]

Thanks again,

Each of the 4 column pedestals will be tested with 3" diameter x 6" long cores, for compressive strength. It be interesting to see what the results are.

 

[BigH]

Read specifications on coring - I think that they require a minimum 4 inch core except in special instances (hoping my grey cells are not dark grey tonight). If you are going to the trouble of coring, do not open yourself up to any possibility of having arrows slung your way!!

 

[PT99]

Our testing lab says 3" dia x 6" long is standard procedure. The column is 16" square, I want to keep the holes relatively small, if possible.

 

[henri2]

Regarding core diameter, the relevant section in ASTM C 42 is Section 7.1. It basically reads the same as Sec 7.1 in this link

http://www.mdt.mt.gov/other/materials/external/materials_manual/112.PDF

 

[BigH]

henri2 - so my cells weren't too clouded (5mm off - how do they get 95mm? - 3.78" mmmmm (probably 4 inch outer diameter minus the wall thickness}.

 

[msucog]

we often take 3" diameter cores instead of 4". it is permitted as long as the l/d is appropriate. i do not know the section off the top of my head, but i'll try to look next time i'm in the office and have a free moment. i personally prefer the 4" cores whenever possible but have never seen big variances in the end numbers (either due to luck or my own ignorance).

 

[BigH]

According to the spec it is permitted only if the client/owner/engineer permits it (not the testing lab) - but the spec calls for the 95mm (4 inch) cores.

 

[boffintech]

Nice try DRC1. Since the inspector sets the scheduled for neither men, materials, nor equipment don't see how you figure it's his fault that the contractor is incapable of coordinating a simple concrete placement.