Core Break Acceptance

[wallerdf]

I called for a 3000psi mix. 28 day breaks came in really low. 2150psi average. Had them core and the 3 breaks are as follows: 3750, 2310, 2260psi.

ACI-318 - 5.6.5.4 — Concrete in an area represented by core tests shall be considered structurally adequate if the average of three cores is equal to at least 85 percent of fc and if no single core is less than 75 percent of fc. Additional testing of cores extracted from locations represented by erratic core strength results shall be permitted.

I meet both the 85% and 75% but would anyone consider the 3750 to be erratic? I'm thinking of making them take 3 more cores just to compare.

 

[JAE]

Yes - that seems a bit off to me.
So additional cores might be a good option.

However, you also could check your design in that area for the lower f'c. But less than 2500 psi would make me even think twice about that.

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[wallerdf]

That's a good FAQ. Nice link to have if you don't have the ACI available. Thanks.

I did check the design and it will work at 2500psi. Also, the job is in Florida and the FBC requires a minimum 2500psi for foundations per table 1808.8.1. Forgot to mention that before.

 

[BigH]

I suggest that you read Neville's paper in the November 2001 issue of Concrete International about coring - "Easy to perform, Not Easty to Interpret. He lays out some very good observations based on his experience and on ACI, ASTM and BS BS/EN specifications. Basically, before coring, make sure everyone is on the same page as to the plan of coring and the interpretation of the results.

 

[JedClampett]

I'm not sure what your mix design is. But this seems unusual. After 28 days, any decent mix should reach 3000 psi and likely exceed it.
I'd dig into this a little. Did they put in the right amount of cement? Did they reverse the proportion of fly ash and cement? Too much water? Was the cement old? (I've heard all of these happening) I bet if you check, there's a simple explanation.
My point is, maybe this is bad concrete that needs to be removed. The longer you wait for strength results, the harder this news is going to be to give them. If you're interpreting "loopholes" in ACI, maybe you're going to far.

 

[wallerdf]

Sorry I didn't update. Jed you're completely right. After doing the digging, the contractor finally admitted that the result came from 2 different trucks and the lower strengths from a truck in which they added a considerable amount of water to. I had those portions ripped out and replaced.