Concrete Testing HIGH at 7 days

[andrew705]

Question for all you concrete chemistry experts;

I have a job where I'm overseeing construction for the EOR. They're pouring the raft slab now and I'm starting to get concrete cylinder tests. I have a specified strength of 35 MPa (about 5000 psi) and I'm surprised to see tested strengths of 45 Mpa (6500 psi) at only 7 days. I'm not sure I have a problem, but I remember from days past being cautioned against concrete that cures too quickly causing shrinkage cracks.

My question is, at what point would you worry that the mix is coming back too strong? What concerns would you have?

Thanks much. Apologies if this was covered in a previous thread. I tried to search through but I couldn't find anything.

 

[darthsoilsguy2]

there's only 2 types of concrete....
i wonder if the research people here have a comment. i'll just say i've seen what you describe from the testing side often for similar strengths. Since you have a connection to the EOR, you could see if that strength development was typical in the mix design trial batches if they are included in the submittal.... Also would want to look to see how the volume shrinkage performs. the plastic cracking is usually a result of water loss. we have so many admixtures in play now than we did in "days past" and a bunch of those admixtures reduce shrinkage cracking.

 

[Ron]

If your design mix is 35 MPa at 28 days and you're getting 45 MPa at 7 days, you have a batching issue. Get a copy of the mix design and compare the batch tickets with the mix design.

Such overstrength might not be an issue; however, it is likely that you have a much higher cement content than desired, which does several things....

1. It makes the mix more expensive
2. It causes more shrinkage, thus increases the cracking potential
3. It raises the temperature during hydration which can cause additional cracking in mass placements like a mat or raft foundation. Temperature controls will have to be introduced to combat this.

 

[andrew705]

Thanks for the replies.

What I didn't mention is that portions of this raft slab actually make up the bottom of some swimming pools (the job in question is a massive community centre), so cracking is my main concern. I visited the site to inspect rebar for a subsequent pour and 14 days after the pour in question there is no visible cracking. Not really sure what to make of the high tests but I'm guessing it isn't a problem.