The CONCRETE Times • MAY 2015 monthly columnist - jay shilstone the 0.26 w/c ratio myth-CONCEPTION THIS MONTH, JAY SHILSTONE EXAMINES A WIDELY ACCEPTED ‘TRUTH’ CONCERNING CEMENT QUANTITIES AND W/C RATIOS AND STRIPS IT DOWN TO IT’S BARE BONES 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. 10 The 0.26 w/c Myth-conception In these days of an ever increasing demand for improvements in concrete performance, it is easy for specifiers to continually decrease the maximum allowable water/cement ratio for concrete they specify. After all, a lower water/cement ratio produces higher performance concrete, right? And everyone knows that it only takes a 0.26 water/ cement ratio to fully hydrate cement, right? WRONG! First, let’s look at the notion that it only takes a 0.26 w/c ratio to fully hydrate cement. Where did that come from? In 1949 Treval Powers authored PCA Bulletin 29— “The Nonevaporable Water Content of Hardened Portland-Cement Paste—Its Significance for Concrete Research and Its Methods of Determination”. This is the first place that I can locate that documents the 0.26 minimum w/c ratio to fully hydrate cement. But how did Powers derive this figure? He placed a pre-weighed sample of cement in a grinding apparatus and ground the cement sample mixed with water for several days. This was to break up the cement particles and make certain that every grain of cement was fully hydrated. He then heated the sample to drive off all the excess water. After that he re-weighed the hydrated cement and found that it gained 26% in weight. Thus, the water consumed in the hydration process was 0.26 times the weight of cement. Power’s procedure differed from production conditions in many ways. However, the primary difference is that in the real world not all the cement grains get exposed to water and become fully hydrated. Another difference is that even if we were able to fully expose the cement grains to water and obtain full hydration, we still have to worry about the workability of our concrete. Before any water can contribute to a measureable slump, all the spaces between the cement particles have to be filled with water (or air bubbles), then a little extra water has to be added to separate the cement particles and allow the paste to flow. Other studies by multiple authors, including Powers, Brownyard and Copeland, indicated that paste had to have a w/c of about 0.40 in order to fill the spaces between cement particles. If we don’t have the minimum required water content, our concrete mix will be too stiff to place. This brings us to another interesting point. Typically the relationship between w/c and strength isn’t linear. That is why they call it a “water-cement ratio curve”. Even so, as we decrease the w/c, we typically increase strength. However, there is a factor called the “cement efficiency” that actually declines as we exceed a certain optimum cement content. The cement efficiency is the psi (or MPa) produced per lb (or kg) of cement. Normally this optimum cement content is at about 550-575 lbs of cement per cubic yard (325-340 kg/cubic meter).
The Concrete Times May 2015
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