Page 12

The Concrete Times May 2015

The CONCRETE Times • MAY 2015 monthly columnist - jay shilstone 12 Adding cement above this point usually increases strength at a slower rate than adding cement below this point. Once the optimum cement content is reached, the only way to increase cement efficiency is through the use of admixtures or supplementary cementitious materials. Now we get to the real world. Today we routinely place workable concrete at w/c ratios below 0.40, sometimes even below 0.30. Changes in admixture, cement chemistry and cement physical characteristics allow us to do things Powers never dreamed of. But just because we can do something doesn’t mean we should do something. Adding cement to a concrete mix might improve the mix’s performance, but there might be better ways to improve performance. For example, adding an admixture to reduce water demand will reduce the amount of cement required to maintain a w/c. Adding cement to a mix will typically reduce permeability, but adding fly ash to the mix can reduce permeability even more. These examples don’t even address the problems associated with adding more cement. Cement is expensive. Adding cement can increase heat generation and subsequent thermal expansion and contraction, resulting in cracks. Adding cement can increase autogenous shrinkage (due to the fact that hydrated cement takes up less volume than the original water and unhydrated cement). My point for today is, “Let’s learn from the past and apply their wisdom to today.” Given the changes in cement characteristics, what is the minimum w/c required for complete hydration? Is it still 0.26? What is the minimum requirement for evaporable waterfilled spaces between cement particles? Is it still 0.40? (I bet that last question is one that many large cement and concrete producers have spent a lot of money on.) Should we be specifying w/c ratios below 0.40, or 0.35? At what point are we hurting the concrete and wasting cement? Given today’s environmental concerns, these are questions that need to be answered. What do you think about optimising cement characteristics and minimizing paste water demand? Until next time, Jay Shilstone 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 May 2015
To see the actual publication please follow the link above