Concrete Cores vs. Concrete Cylinders 1

[gbkxbb]

Hey all, new to the forums.

I've got a job that we are testing some 7000 psi concrete for. I've had a few pours where the concrete cylinder breaks came back lower than the concrete core breaks. On the range of 1000-2500 psi, give or take.

I'm trying to figure out why. I would think I could exclude operator error for my cylinder breaking guy, because the other 50 cylinders he breaks a day seem just fine. I also assume that my cylinders were made correctly in the field (Good technician, ACI cert) and that the cores were taken correctly (My project engineer did these, whlie under supervision of an PE and the entire managment staff of the contractor). If we agreed on these assumptions, any ideas why there would be a difference?

 

[Ron]

Welcome.

First of all, the cylinders are meant to represent the mix design, not the in-place concrete. Cores are more indicative of the in-place concrete strength than cast cylinders would be.

There are numerous reasons for the disparity, but most likely one of temperature differences. You don't state your location, but if your ambient temperature was higher than about 75 degrees in the field, then you will get some acceleration in the strength gain.

If you are using 4x8 cyls instead of 6x12 cyls, keep in mind that ACI 318 requires that 3 4x8 cyls must be averaged for the 28 day strength or 2 6x12 cyls.

Curing conditions in the field are usually vastly different than in the lab. This can cause field strengths to go up or down. Keep in mind that the laboratory test is done under standardized conditions....not better, not worse...just standardized, so as to achieve repeatability and reliance on their predictability. In many cases, the concrete in the field will gain strength faster and in some cases gain more strength...but that's not predictable.