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The Concrete Times March 2015

The CONCRETE Times • MAR 2015 monthly columnist - jay shilstone our new columnist - jay shilstone jay will be writing a monthly feature for the concrete times, taking a look at concrete issues the the american perspective but also touching on europe Ja James M. “Jay” Shilstone, Jr. is the third generation of Shilstones to be involved in concrete quality control. A Fellow of the American Concrete Institute, Jay has been widely recognised as an expert in concrete quality control around the world and he is also a member of the American Society of Testing and Materials and the National Ready Mixed Concrete Association. He has been in the concrete industry for almost 40 years, with over 30 of those years involved in concrete quality control software. Jay works for Command Alkon, Inc. and is their technical specialist in the COMMANDqc quality control software program. A compelling writer, Jay is also a keen blogger and his blogsite on www. commandalkonconnect.com gets over 1000 hits per week. 31 I have written about statistical overdesign using ACI 318, but it’s more relevant here to discuss the overdesign technique used in EN-206. After all, EN- 206 uses a similar statistical method to ACI 318, right? Wrong! What started out as a simple 2 hour exercise in paraphrasing EN-206 has turned into a major effort involving about 20 people on 3 different LinkedIn groups. One of my favorite sayings has come to be, “If you want to make God laugh, tell him your plans.” My only consolation from this blog post is that God must have gotten a real belly laugh. On the surface EN-206 use a technique that is slightly different from ACI 318. Instead of determining a minimum recommended average strength, EN-206:2001 uses something called characteristic strength. The characteristic strength is a value determined statistically such that 95% of the strength tests will fall above the characteristic strength and 5% below. In other words, there is a 95% probability that a test for a mix will be above the characteristic strength. The Engineer will specify the minimum characteristic strength of the concrete for the structure, then the concrete producer will select or design a mix that meets or exceeds that minimum characteristic strength. (In case you are wondering why I am using EN-206:2001 when there is a new version out, I think from 2013, the 2001 edition is the only one I have.) I’m going to attempt to explain in plain English why this issue is so complicated, but if anyone out there can do better, I invite you to submit an article in non-statistical jargon to explains this. I will be happy to publish it on my blog. How is the characteristic strength determined? The characteristic strength is determined just as the minimum recommended strength is determined by ACI 318 – from the average strength and the standard deviation of the concrete tests. However, ACI 318 uses the following criteria: • 99% chance that the moving average of 3 tests will exceed specified strength, f’c • 99% chance that an individual test will exceed f’c – 500 psi or, for designs over 5000 psi f’c, that an individual test will exceed 90% of f’c EN-206 uses the following criteria: • 95% of the tests will exceed specified strength Looking at it graphically the 95% confidence limit looks like this for a 30 MPa design strength with a 36 MPa average strength and a 4 MPa standard deviation: In this particular case the concrete mix does comply because the characteristic strength is below the design strength of 30 MPa. To calculate the characteristic strength, you would normally use the following equation: fck = average strength – ( t x o-)


The Concrete Times March 2015
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