Concrete cylinder strength exceeds the design mix strength by 200%

[qman54]

I am supervising a concreting control work and responsible in establishing concrete mix designs for a given project and source of aggregates. One of our on-going projects using a mix design of 4,200 psi yielded concrete cylinder breaks (a set of 3 samples)averaging 8,000 psi at 28 days. Assuming the same aggregates source and following the same mix proportion for the 4,200 mix design, without additives.

Question:
1. is this possible? We know that compressive strength varies with w/c ratio and slump but the excessive difference is highly improbable. Please comment.

To verify the actual strength of the poured structures, thre (3) sets of core samples were conducted which produced average core strengths of 5030, 4660 & 4450 psi. Wi th these core test results, it is apparent that the above cylinder samples do not represent the poured structures.

Question:
2. Am I being shortchanged?

 

[Ron]

Did your testing lab mix up samples with a high strength job? Core strengths are believable. Audit the lab.

 

[hokie66]

"exceeds...by 200%" is an exaggeration. I suppose you meant 100%.

2) I don't see how you could say you are being shortchanged. Seems to me like you are getting good value.

 

[JAE]

Did they stick a piece of rebar in the cylinders?

 

[Zambo]

Has the compression test machine been calibrated recently?

 

[frv]

I guess it's just a Texas thing, but I'm surprised that I've now seen two threads relating to high strength. We routinely get very high breaks in my area. Granted, 100% over is a bit excessive, but not unheard of. I'm not sure where you guys practice that concrete suppliers have a better controls.

 

[Ron]

frv....if a ready mix supplier follows ASTM requirements and the National Ready Mix Concrete Association requirements, then control should be much better than what has been described here. Most suppliers don't "juice" their mixes for higher strength because of several things...

it costs money
it cause problems with mix control and statistical validation required by code
it creates placement and finish problems that cause complaints

Over the years we have gotten away from doing independent batch plant inspections. I've done many, but it has been at least 10 years since I did the last one. I have not seen that as a specification requirement in a long time....yet we continue to have concrete problems and wonder why.

Audits of both the ready mix producer and the lab are sometimes necessary to achieve what you want from the delivered concrete. Getting the right results in place is a whole 'nother problem!

 

[dik]

If there is a lot of concrete involved, ie. a standard mix to be used for a long time, then there is cost savings in 'messin with the mix' to get a more economical product... it gives an edge in bidding future jobs and/or increases profits.

Is there a minimum w/c ratio? strength is high for even 0.4...

As a last resort, you can add water to increase the slump and still have the design strength <G>.

Dik

 

[dik]

that should be maximum w/c...

Dik

 

[Zimba1]

audit the lab ( refer to ACI and ASTM stardards) if u are satisfied with the lab then u are getting good value for your money!

 

[karma134]

I have a doubt in your strength conversion from core to cube strength. After applying the L/D ratio correction this value is converted to equivalent cube compressive strength either by dividing by 0.8 or multiplying it by 1.25.

If i assume that your core strength of 5030, 4660 & 4450 psi is after the application of L/D ratio factor then it would be around 5890.0 psi equivalent cube strength.