The CONCRETE Times • MAR 2015 MONTHLY COLUMNIST - JAY SHILSTONE 15 tests and why not the sample standard deviation? Also, according to a nice publication from the British Quarry Products Association called “Guidance on the application of the EN 206-1 conformity rules”, which I think is no longer available, the standard deviation should be based on 35 tests, not the 15 tests. Even though the QPA publication doesn’t seem to be available, the British Ready Mixed Concrete Association has a great download section at http://www.brmca.org.uk/downloads. The next question that comes up is, “Where does the 1.48 in the first equation for continuous production come from?” It is much lower than the 1.65 student t-factor described above and from ACI 214, Statistical Analysis of Concrete Tests, Table 5.4. and actually results in about a 7% probability of low strength according to the student t-table. The answer becomes a major “rabbit hole” that the average reader of this blog doesn’t want to go down. In fact, it is one that I don’t think I adequately understand, but I will try to give a “short answer” and references for the reader to find the “long answer” if they really want to pursue it. Short answer: The 1.65 student t-factor comes from an assumption that the data we are examining is “normally distributed” (which means it looks like a bell curve) and that each “datum” (the singular of “data”) point is independent of every other data point other than that they are produced by the same process. It is also based on the assumption that we have a relatively large number of data points. It seems that EN-206 may be assuming at least 35 points over a 3 month period. Apparently the 2013 version of EN-206 allows for either a 3 month period or a 3-6 month period, with different criteria for both. In reality concrete test results are not independent of each other, they are closely related to the test result most recently prior to the current test. This is called a “time series”. If you have a large number of tests taken over a short period this becomes apparent. However, in a situation like in the U.S. where concrete producers rely on sporadic data from outside third party laboratories, the data looks more like independent normally distributed data. Since Europe and many other parts of the world rely on in-house producer-generated test data that is more frequent than in the U.S. it makes sense to use the time-series approach, which allows for a lower safety factor for the probabilities involved. Long answer: If you really want to jump down the rabbit hole most of the references on this that I have seen relate to a publication by Luc Taerwe entitled “A general basis for the selection 33 of compliance criteria” which is available at http://retro.seals.ch/cntmng?type=pdf&rid=bse pe-004:1986:10::38&subp=hires I haven’t given this a hard look yet, but what I have provided in my short answer is based on comments about references to Luc’s paper. You can find the threads on this subject at the following LinkedIn group locations: 1) American Concrete Institute – ACI, 2) Concrete Society Technologist, and 3) The Concrete Producer Network. I looked at posting links, but discovered that my LinkedIn member number would be embedded in all the links and I didn’t think that would work. Besides, some of the groups are “Members Only”, so you probably want to join those groups. My thanks to the following people who contributed to increasing my understanding of the issues involved: • Ioannis P. Sfikas • Claus V Nielsen • Yasin Engin • Selim Y. Many other people contributed to peripheral aspects of the conversation and I would like to thank everyone who offered feedback. This is a great example of how LinkedIn discussion groups can assist in international cooperation. Summary: I wrote a blog post, entitled “Thereby Hangs a Tale”, on the need to understand the story behind a technical presentation before you try to apply “sound bites” from that presentation. You can find the article at: http://www.commandalkonconnect.com/2013/01/14/ thereby-hangs-a-tale/. The case of the overdesign calculations and the use of characteristic strength is a case in point. In order to be able to properly apply EN 206 techniques you must first be operating in an environment comparable to that for which EN 206 was developed. If a concrete producer who did not test as frequently as is assumed in EN-206 attempted to use the EN-206 equation they would achieve a 7% failure rate rather than a 5% failure rate, which could result in a huge financial impact on the company. This posting has become a bit longer than I normally like to do, and is certainly more technical, but I think it is important to understand the “big picture” as well as the details. I hope you find this post meaningful. If anyone has any comments, please let me know. If you want to provide a more comprehensive, but understandable, article that explains the concepts used in EN-206, I will be happy to print it. Until next time, Jay Shilstone

The Concrete Times March 2015

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