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”. [Ed. Note. To get a copy of this publication, buy the PCA’s Concrete Research Library. This DVD contains hundreds of old research publications from the PCA.] 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). 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 water-filled spaces between cement particles? Is it still 0.40? (I bet that last question is one that Lafarge has 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 optimizing cement characteristics and minimizing paste water demand?
