Excess Concrete Strength Concerns

[jimbo1948]

Here's one for you. I've never seen this happen in the 41 years I've been practicing. Just received compression test results for test cylinders taken 28 days ago. The design strength for wall footings was specified to be 3,000 psi. The cylinder test results indicate strengths ranging from 6,000 to 7,000 psi. The results average about 240% above the specified design strength. Should I be concerned about this? The higher strengths would generally affect development lengths but would tend to shorten them. The reinforcing is already fabricated so that is not a concern. Does anyone see this as a concern?

 

[Bagman2524]

Shouldn't be. Reinforced concrete is designed to prohibit failure by crushing of the concrete and force failure to first be initiated by yielding of the rebar.

There is ample precedent in the history of science for the overwhelming bulk of the scientific community strongly believing in imaginary entities postulated by a favovered theory. -Michael Behe

 

[BAretired]

My only concern would be that the concrete supplier doesn't seem to know what he is doing.

BA

 

[WARose]

I think its ok until it hits about 8000 psi. At that point the normal assumptions for ultimate strain become unconservative.

 

[BAretired]

If 3000 psi is all that is required, why would a test strength of 8000, 9000 or 10000 be a concern?

BA

 

[n93s412]

ACI 318 states that there is no upper limit on the strength of normal weight concrete that can be used in structures, inclusive of those exposed to high seismic risk.

 

[EngineeringEric]

I suppose the rho calculations are based on F'c, so a higher value will increase the point at which your concrete governs... but the design should already be based on steel yielding... As long as ductility isn't an issue (which by above comments WARose seems like may happen >8ksi).

Whats a little extra cement & additives among friends?

 

[WARose]

If 3000 psi is all that is required, why would a test strength of 8000, 9000 or 10000 be a concern?

Off the top of my head: I cannot think of too much. I seem to remember it can (under certain circumstances) drive down the effective moment of inertia. It’s also not quite as ductile as it was (for seismic purposes).

 

[concretemasonry]

In many areas, ready-mix will routinely not deliver anything less than 3500 psi. For external slabs, some will not deliver anything less than 4000 psi because it actually cheaper for them because of possible litigations.

Was a mix design required and submitted for approval on this project?

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

There are some cases where this might be a concern, flexural members with minimal reinforcement where the higher strength concrete would mean that the cracking moment is so high that ductility is not assured. For wall footings, this would not be an issue.

 

[JAE]

Another issue with rich (i.e. high cement) mixes is autogenous shrinkage. This would possibly create internal cracking in the concrete.

Having too much cement isn't always a good thing.
But for general footings I wouldn't think there'd be much concern.

 

[TXStructural]

There is no real issue with high breaks. As mentioned, shrinkage could be an issue, but once that addressed you should be fine. If you are concerned, run quick numbers for bending at midspan and shear at supports. (You will find that it all works just fine.)

I agree that the supplier may be having quality control issues that should be addressed.

 

[Ron]

Sounds like the testing lab might have mixed up some samples! What were the 7-day results? Either that or the supplier screwed up.

 

[HermanLJ]

Higher concrete strength could significantly increase cracking.

 

[racookpe1978]

Any chance the supplier screwed up and delivered the wrong mix to two different customers? Any chance he didn't screw up, but delivered somebody's else's mix to you as part of a "dumped" (excess) load that he would have otherwise had to waste?

 

[hokie66]

6000 to 7000 psi concrete is not high strength concrete. I would discount concerns of shrinkage and cracking just because of the strength.

 

[graybeach]

This could be a problem if you're in a high seismic area and your design is based on plastic hinges forming in expected locations. Plastic hinges act like fuses, which limit the forces that can transfer adjacent members.

 

[concretemasonry]

If you are really concerned, it might be best to put both a minimum and a maximum strength in the project specifications.

Also, find a way to get a guarantee that the approved mix design was actually delivered. With the current dispatching of concrete is is usually easy to have the mix design number on the delivery ticket. That has been common for highway work in some states for years, since the batching is usually a part of the information flow to generate a delivery ticket with copies for the contractor also.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.