The CONCRETE Times • MAR 2015 monthly columnist - jay shilstone 32 Where o- is the population standard deviation and t is the student t-value for the probability you want. (In case you have never heard of a t-value, it comes from a table you can find in most statistics books and it is based on the probability of low tests you want and the number of tests you have already made.) For EN-206 based on a 5% probability of low strength, the equation for fck would appear to be: fck = average strength – ( 1.65 x o-) Right off the bat, we are forced to realize that there are a number of assumptions being made in the above equation. Average strength = the mean strength t is based on the student t-test for 5% below a value (which is called a “single tailed” test as opposed to “double tailed” test. You don’t need to understand this except to know that it is an assumption that is being made.) o- is the population standard deviation and not the sample standard deviation, which is what is used in ACI 318 How does EN-206 actually define fck? As nearly as I can tell it doesn’t. It just defines it as the value below which 5% of the tests will fall. If anyone knows where fck is defined with a real equation in the Eurocode, please let me know, and preferably provide a screenshot of the section. How does EN-206 define strength overdesign requirements? Like ACI 318, EN-206 uses two criteria for determining concrete overdesign requirements, but there are only two situations defined in EN-206. For “Initial” production the concrete must meet the following requirements: • The average of 3 tests >= fck + 4 N/mm2 • An individual test >= fck – 4 N/mm2 For those unfamiliar with the units “N/mm2”, they are the same as MPa, megapascals. Once the producer has accumulated 35 tests note: I originally stated 15 tests, but once the producer accumulates 35 tests they can use the average of 15 tests, they can use the two following criteria: • The average of 15 tests >= fck + 1.48o- • An individual test >= fck – 4 N/mm2 • Even here there are a couple of questions. Why are we using the population standard deviation when we only have HTC SUPERFLOOR™ MORE THAN JUST POLISHED CONCRETE IT’S CONCRETE LOVE Is it possible to love a grey and cold concrete floor? Just give concrete a chance and you will find a strong and beautiful flooring material. HTC Superfloor™ is a revolutionary grinding and polishing concept which is also a good environmental choice. You will discover a shining, easily maintained super strong floor. You will find a treasure that was always hidden and has just been waiting to be refined. HTC Superfloor™ is a floor worthy of love. Yes, it is possible to love a concrete floor. www.htc-floorsystems.com HTC Floor Systems Ltd. Phone +44 (0) 845 460 2500 E-mail info@htc-europe.co.uk

The Concrete Times March 2015

To see the actual publication please follow the link above