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The Concrete Times Summer 2015

The CONCRETE Times • SUMMER 15 monthly columnist - jay shilstone - continued from page 10 12 ods of Determination”. In this document Powers describes how the 0.26 value was determined. A small, measured quantity of cement was mixed with a large quantity of water, then the two materials were ground together for 3 days. The purpose of the grinding was to break up the clumps of cement particles and remove the gel coating from the cement particles as is was formed. ALLOWED THE CEMENT TO BE TOTALLY HYDRATED This allowed the cement to be totally hydrated at the end of the test period so the researchers wouldn’t have to wait the normal 60 years associated with total hydration. After the cement was totally combined with the water, the excess water was evaporated from the sample. The newly dried sample was reweighed. It had gained 26% by weight, hence the 0.26 water/cement ratio figure for full hydration of cement. In reality no concrete producer grinds their cement with water for 3 days in order to make concrete. We mix cement, rock, sand and water in a mixer and send them to the project to be placed. This creates its own set of limits and constraints. There are other important aspects of this story. If you read further in Powers’ article you will read that once the water/cement ratio starts to go below 0.40, portions of the cement will remain unhydrated. CEMENT ACTING LIKE VERY FINE SAND The lower the water/cement ratio, the greater the portion of unhydrated cement, meaning that the cement is acting like very fine sand. This is very expensive fine sand. Of course, things have changed since 1949 and the 0.40 and 0.26 values might have changed as well. Cements are made differently today. Admixtures can reduce agglomeration (the clustering of cement particles) and change the viscosity of the paste. Supplementary cementitious materials have been added to the mix. It would really be nice if someone would reproduce Powers’ work with the latest materials. RESEARCH PAPERS NOT THE ONLY STORY Research papers aren’t the only stories in the concrete industry. Building Codes, such as EN-206, also have a story behind them, although sometimes it is an unspoken story. A few years ago I was called in to troubleshoot a concrete paving mix. The contractor complained, “My aggregates meet Eurocode requirements, but I can’t finish my concrete.” 6


The Concrete Times Summer 2015
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