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The Concrete Times - November 2015

The CONCRETE Times • November 15 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.  17 You would think that after working with concrete for over 100 years, in a modern, scientific setting, that we would know all there is to know about concrete and it would be totally predictable, but “no”. Every day, designers, contractors and concrete producers are surprised when concrete doesn’t reach design strengths, doesn’t pump, doesn’t finish, doesn’t set and doesn’t last like we expect it to. Haven’t we studied concrete to death? Every year thousands of scholarly articles are published, or presented at conferences, but we still can’t predict something as simple as “how much water is required for a new concrete design to produce a 150mm slump?”. We have super-fast computers that can resolve complicated mathematical models and scanning electron microscopes that can see to the molecular level and smaller., yet despite our best efforts, we can only predict water demand to plus or minus 10-20 liters of water per cubic metre. And speaking of water, why is it that we don’t really know how much goes into a load of concrete? Moisture meters have improved tremendously in recent years, yet I never talk to a concrete producer who is totally confident that moisture corrections for the aggregates are always accurate. Even if the moisture meter is correct, water typically creeps into a mixer truck drum from other sources. Water remains in the drum after washing out a previous load. Water gets into the drum during the washdown period after loading. On the testing side, there isn’t a universally accepted test for determining water, or watercementitious ratio, in a fresh load of concrete. The AASHTO Microwave Test, TP23, in the U.S. “cooks” all the water out of a sample of fresh concrete, but there have been conflicting reports on its accuracy. Concrete isn’t rocket science. Typically materials are batched to an accuracy of +/- 1 to 2%. Strengths are reported to just 3-4 significant digits. People constantly argue over the significance of one of the primary measures of concrete performance – the slump test. It is often said that the only thing about workability that the slump test measures is slump. Even if we accept the usefulness of the slump test, slump can only realistically be measured to a tolerance of 3-5mm. Why is it that the first batch of the day requires manual adjustment? Despite fancy moisture meters and accurate batching equipment, the first batch of the day is typically adjusted manually for slump and for air entrainment. Variations in temperature and humidity affect the amount of water and/or admixtures required in the batch. Variations in the entrained air content require “tweaking” of the air entraining agent dosage. Why can’t we enter information about temperature, humidity, wind speed and aggregate moisture into a fancy computer program and have it adjust for the correct materials quantities? The easy answer to all of the above is that concrete is “complicated”. Unlike many other manufacturing processes, concrete production involves the use of heterogeneous (a mixture of dissimilar elements) materials rather that homogeneous (uniform or similar elements) materials. We are working with materials that Mother Nature gives us with relatively little processing to improve uniformity. Concrete is affected by production variations, such as the timing of the introduction of each raw material. It is also affected by climatic conditions, time in transit to the project and a host of other variables. Finally, it is greatly affected by how test samples of concrete are obtained and how the tests are performed. It is a testament to the forgiving nature of concrete that we can produce even a remotely consistent product. Despite all this, is concrete really so complicated that we can’t account for all the variables? If we can land a man on the moon, why can’t we monthly columnist - jay shilstone WHY DOES CONCRETE CONTINUE TO SURPRISE US? JAY SHILSTONE REPORTS Our AMERICAN MONTHLY COLUMNIST SAYS CONCRETE REALLY ISN’T ROCKET SCIENCE.


The Concrete Times - November 2015
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