Concrete Slump out of Spec but Break Strength Good 2

[JoelTXCive]

I have a concrete pump station project under construction. The material lab has flagged a specific test for failing the field slump test. At the same time, the cylinder passed the 28-day break test.

The mix specification is one of the State highway department's mixes, so it is well documented and very common. There are no fancy additives to my knowledge.

Should I be concerned? The slump was 7.75" and the max allowed was 5.5".

Thank you in advance.

-Joel

 

[Enable]

You should only be concerned if there are no super plasticizers or agent reducing admixtures in the mix. If those are present then slump measured after their addition is meaningless except as a verifier of workability for the contractor. If no such agents were added then I might be a bit more concerned. But even then not really. The slump test was designed to be a measure of the w/c ratio so that strength could be judged prior to placement. If strengths came back okay then I see no reason for immediate concern.

That said, a slump of nearly 200mm without an admixture is....uncommon. I might be inclined to think the tester measured incorrectly (if you trust the testing lab for the cylinders).

 

[ntattose]

Slump is a measure of workability. It's also an indication that there is more water present than you may want. You seem good as far a strength goes, but the additional water may result in more plastic shrinkage cracking than you want. I usually check slump before the concrete is placed. That gives you a chance to reject it before it's in the ground.

 

[JAE]

Look up mix 3151690 and see if there were any additives.

 

[Tomfh]

Sounds like a pretty wet mix. It might shrink more than normal.

 

[AskTooMuch]

This is a cylinder test report and not the concrete mix report from the batch plant.
Get the concrete mix report. I'm sure it has a water reducer it there. This is for sure high slump without water reducer.
Check if the state highway have max slump requirement if water reducer is used.

 

[Smoulder]

Site say why not rejected? Find that out first. Durability reduced by excess water if a concern.

 

[MHobbs]

As structural engineers I think we often only consider the compressive strength requirement of concrete.
That is not the whole story. Tomfh and Smoulder are on the money.
The Durability may have been affected and in this case it is likely durability that is the critical property not the compressive strength.
As mentioned above the slump is mainly used as a w/c ratio measure and one of the key ways to control the durability of the mix is to control this.

 

[JAE]

....unless there are plasticizers added, in which case the slump measure isn't all that valid/useful.

 

[Enable]

I've worked as a tester for a material testing company and I routinely saw testers use concrete that they previously completed an air test on for cylinders (a huge no-no) and for slump (a meh). Again at 200mm slump, with strengths coming back as they should, I am most inclined to think tester error in event of no water reducing admixtures. If those admixtures were present then it's a meaningless test anyways (except for workability).

As a contractor I direct testers to test for air first. If that passes we are pouring regardless of slump (...unless I can look at it and go this shit ain't going through the pump). But my attitude is certainly influenced by our routine use of super plasticizing admixtures and air entrainment.