Thereby hangs a tale
January 14, 2013
Last week I was in Nebraska speaking at the Nebraska Concrete & Aggregate Association’s “Quality Concrete Conference”. (“Thanks” to the members of the NC&AA who showed me the best of Nebraska hospitality.) My topic was “Everything Old is New Again” and I spoke about the “stories” behind some of the innovations in basic concrete. These were things like Duff Abrams and the w/c ratio theory, the origin of the statement “0.26 is the minimum w/c to hydrate cement” and how air entrainment was studied in the early days. My main theme was that concrete history not made up of short and simple sound bites, like “concrete strength is determined by the w/c”, but is made up of “stories” that need to be understood to place their conclusions in context.
For example, I imagine most people in the industry are aware that Duff Abrams developed the “w/c ratio theory” that says that as the proportion of water to cement increases, the strength decreases. But most people don’t know the following:
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Abrams’ original study was based on 50,000 concrete and mortar tests and was published by the Lewis Institute as Bulletin #1, “Design of Concrete Mixtures”
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The study recommended the use of the “Combined Fineness Modulus” method for determining aggregate proportions
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Previous methods based on proportioning aggregate for maximum aggregate or concrete density didn’t work
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Fixed weights of coarse and fine aggregate in a mix were “far from satisfactory”, a statement that can be construed to recommend variable mix designs for performance concrete
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Abrams calculations for w/c were based on dry, loose volume. A 94 pound sack of cement was considered to be 1 cubic foot. An acceptable w/c ratio on his graph was 1.00, which corresponds to about a 0.66 w/c by weight. I recently reviewed an article where it was obvious someone read Abrams’ article and the current authors tried to produce concrete with a w/c of 1.0 by weight. They were surprised to get “soup” instead of concrete.
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The typical sands of the time, on which the study was based, were much coarser than today and had a fineness modulus of 3.1-3.8, compared to today’s values of 2.3 to 3.1. Mixes used in Abrams study would typically have been more well-graded than the gap-graded mixes often used today.
For those that are interested, Lewis Institute Bulletin #1 and many other historical publications can be found on the “Concrete Research Library”, a DVD published by the Portland Cement Association. [Ed. Note – apparently the DVD is no longer available in the Bookstore.]
Other stories often ignored by today’s wisdom include:
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A water/cement ratio of 0.26, often cited as the minimum water/cement ratio needed for full hydration of cement isn’t applicable to field concrete. It was determined by grinding cement and water together for 5 days, drying the cement, then weighing the cement to determine the increase in cement weight. Of much more relevance is the fact that when the w/c goes below 0.40, there is not enough water to reach all the cement grains so some of the cement remains unhydrated. (PCA Bulletin 19)
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Air entrainment in concrete should be based on the paste (16%) or mortar content (9%) and not total concrete. (PCA Bulletin 77)
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Strength and slump often don’t correlate in the field. (ASTM C-143 Note 1)
Why have these stories been lost for the sake of a “sound bite”? If we don’t know the whole story it is impossible to tell if a statement is taken out of context, or flat out wrong. Too many people don’t bother to find out the whole story, probably because they lack time or interest to wade through a complex technical article filled with facts. As I write more articles, and tell more speeches, I realize that people often don’t relate to facts. They relate to stories.
This doesn’t just apply to concrete. In As You Like It William Shakespeare said, “All the world's a stage, and all the men and women merely players.” I take exception to Shakespeare’s word “merely”. We all play leading roles and we all have stories to tell. I know that most days my story isn’t worth repeating, but other days my story is significant. Sometimes it is even momentous. It is up to me to be certain that when I tell my story, I tell it completely without being tedious.
It is also up to me to be certain that when I hear someone else’s story, I hear it completely. Even people with opinions different than mine, or people I don’t like, have a story to tell. I need to listen to their story because I don’t learn anything from people who share my opinion. I can only learn from people who can tell me something I don’t know.
Getting back to concrete, I think we need to look back at the old stories and try to understand them completely. Some of them are still relevant today. Others need to be updated. PCA Bulletin 19 on w/c and evaporable water should be reevaluated using today’s materials. Today we use a much finer cement with a different chemistry. We also use superplasticizers. How have these materials affected the minimum w/c required for full hydration of the cement? How have they impacted the degree of hydration in normal concrete? That is a relatively easy Master’s thesis.
As baseball great Yogi Berra once said, “If you don’t know where you’re going, you might wind up someplace else.” Looking the other direction, George Santayana said, “Those who cannot remember the past are condemned to repeat it.” Let’s learn from the past so we can head toward a better future.
