CONCRETE STRENGTH

[ALN3]

This is a new one on me. Recently I was told that it had been stated at a seminar that concrete cylinder break strengths which were significantly higher than the design strength could indicate some kind of deficiency in the completed structure.

Certainly I can see that the "deficiency" could be that the concrete producer was losing profit in supplying material which is more expensive than specified, or necessary, but I'm stumped to come up with any reason why the designer should be concerned. Am I missing something?

 

[youngstructural]

Very strange. You could certainly make this argument in a seismic environment, where your overstrengths dictate plastic hinge formation (to some extent, less than steel), however otherwise I'm going to be interested to see any justification(s) for typical construction.

What about admixtures? I know sodium chloride is an effective plasticizer and contributes to higher early strengths; It's just TERRIBLE for the rebar.

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[frv]

I imagine this may have something to do with seismic design. Specifically, it may reduce ductility where it is required.

In California, concrete producers have a minimum and a maximum value of f'c that they can deliver to the job site precisely because of this reason.

 

[frv]

Should have read YS's post before I posted..

 

[dik]

Higher strength concrete is more brittle and less ductile. It is, however, higher strength... It behaves plastically a little better... but brittle none the less.

Dik

 

[concretemasonry]

ALN3 -

What are the cylinder strengths you are referring to? - Ones submitted with a proposed mix design or samples from the job site.

If you are looking at submittals for the mix design, look at the testing data. Unless it is a unique project, a special, time consuming set of samples is usually not supplied and older test for the same mix with the same materials are taken from testing files.

For normal jobs, the savings in material substitution is not worth the extra problems since the common admixtures are so cheap when bought by the tank or truckload and dosed automatically.

If you are worried, you can request a lower strength that is closer to the design strength.

As usually, the delivery cost is still one of the two most important costs of a delivered yard of concrete.

If you are concerned with the performance of the completed structure, core samples could be an indicator, but they are often higher than the cylinder breaks, but could indicate placement or curing problems during construction.

 

[Ron]

Overstrength conditions can affect moment distribution and certainly affect strain compatibility. In general it is not a big deal unless the difference is significant and presents ductility issues as noted by others.

 

[dik]

"It behaves plastically a little better... but brittle none the less.", didn't come out right... the lower strength concrete behaves more plastically...

 

[hokie66]

The statement may have been broader than related to seismic ductility. It is possible that it referred to an issue which was discussed here in great length recently, namely the minimum steel requirements for flexural elements. For beams, codes have alway prescribed a minimum reinforcing percentage in order to ensure that the reinforced section is strong enough to prevent sudden failure after flexural cracking. With higher strength concretes and the same steel ratio, the cracking moment increases without a corresponding increase in the ultimate strength. The high strength concretes which would cause this anomaly are not efficiently used in beams, but rather are normally found in columns, so I doubt the concern is valid.