High Strength Concrete Testing 2

[krd]

Compressive strength testing of 8,000 to 10,000 psi concrete:<br>
1.Would you expect HS concrete test cylinder strengths from cylinders capped with neoprene to be lower than that obtained from cylinders capped with another procedure such as using sulpher compound? <br>
2.Assuming that the use of neoprene caps does allow some premature or incomplete failures of 28 day old test cylinders, thus failing a 28 day acceptance criteria, would it make any sense to specify a 56 or 90 day acceptance age instead?

 

[Ron]

The stress distribution with the neoprene caps is different than with rigid sulfur caps and we have seen more variability in high strength results, though not always lower. The neoprene caps tend to have a &quot;center-weighting&quot; feature which causes the edges of the specimen to flake off.<br>
<br>
In my opinion, if you do not need the compressive strength at 28 days, then specify and design the mix for slower strength gain, say at 56 days or 90 days. This does a lot for the economy of the job (if a large project) and can enhance the durability of the concrete by reducing ASR potential, reducing shrinkage crack potential and other positive attributes for durability.

 

[bb29510]

The neoprene caps that we used do make the clyinder flake on the edges. I feel with concrete in the high psi this would create a danger for premature breakage. Although we use a computer controlled break machine the premature breakage would cause a lower reported psi. Not a true actual psi as the sulfur cap would. Due to the computer shutting off early.&nbsp;&nbsp;What we do is with everyday 3000-4000&nbsp;&nbsp;concrete we use the neoprene. With 56 day breaks and everything over 5000 we use sulfur.<br><br>&nbsp;About the 28 day criteria, this really cannot be change due to it is the widespread accepted method. A contractor cannot wait 90 day for acceptance. Even if you are the big boss, you going to be overwritten by ACI. Also concrete gets 95 % of strength in the first 28 days if clyinders are failing due to bad breaks. another testing lab would be hired before the 90 day acceptance would apply. In other words before the 90 day rule, fire the finisher, fire the lab, fire the engineers.

 

[Ron]

A couple of points of clarification to &quot;Brownbagg&quot;'s comments:<br><br>ACI doesn't &quot;override&quot; individual projects.&nbsp;&nbsp;With the exception of ACI 318 and a few other actual code references (i.e. ACI 530) most other ACI documents are &quot;recommended practices&quot;.&nbsp;&nbsp;There have been numerous projects where extending the acceptance time on concrete strength has saved money and irritation at irrelevant low breaks.&nbsp;&nbsp;Establishing a 90-day strength gain time, particularly on foundation concrete or low in the frame of a high rise building often makes good common and engineering sense.&nbsp;&nbsp;After all, much of the design load does not ever get onto a structure, much less during construction (careful though...there are some significant exceptions to that!)&nbsp;&nbsp;As an example, I believe this process was used on the CN Tower in Toronto, saving somewhere in the range of a million bucks.&nbsp;&nbsp;It is sometimes used on large pavement projects also.

 

[bb29510]

Point Taken, which is correct.<br><br>&nbsp;My point was from the contractor view is, If the bid information and spec required an ACI spec which most are generally copied from past project. It is like pulling teeth to except anything new. With a contractor having his money riding on concrete break, which NORMALLY mature on 28 day, he is not going to set still for 90 days. Even if you request the change , it better be&nbsp;&nbsp;understood in writing in the bid proposal, plain and clear.<br><br>&nbsp;I work for a indepent lab and whenever we have a 56 day break, it is almost a lawsuit in some cases. We have been fired due to holdup on projects, even though it was the finishers fault.<br><br>&nbsp;If ACI said 28 days and you mention &quot; according to ACI&quot; in your spec, it is pretty much law. Although everybody knows it is a reccomended practice. Once you say it will be build to ACI practice you the engineer had made it law. and the engineer must live to the same law, <br><br>&nbsp;I know this does not make sense, but if you change the common you will have a fight. It would be easier to restrict the water content or change the mix to meet spec in 28 days. Maybe lower the spec of the mix to meet maybe 90% in 28 days with the last 10% following to 90 days

 

[Jim6758]

Ron is absolutely right on specifiying &quot;other than 28 day strength&quot; on foundation concrete. If the piers aren't going to loaded for 3 months, why do they have to make strength in 28 days. Specifiying 56 day strengths would allow more use of fly ash in foundation concrete and reduce the propensity to shrinkage cracking. Remember that every pound of cement used puts one lb. of carbon dioxide into the air. We are useing unnecessarily rich mixes in many applications. If we don't get our act together, the federal government may well step in and dictate the standards.