Concrete Mix Design: Art and Science – w/c curves aren’t just about water and cement

jay_shilstone_HS

Have you ever started a project, then realized that the only way you can begin your project is to do 3 other projects first? Sitting down for the third time to write an article on water/cement ratio curves, I am reminded once again how concrete imitates life. Another thing I have discovered is that when I have trouble writing an article, it is usually because I don’t understand what I am writing about, even if I think I do. I have been having a problem writing about water/cement ratio curves because they aren’t just about paste and water and cement, as I have previously implied, but about an entire concrete mix.

So far I have been distracted by the fact that I started this series talking about aggregates and now I want to talk about paste. The problem is that we don’t construct buildings out of paste, but out of concrete. Sure, I have covered the parts about why we use paste:

  • Strength
  • Durability
  • Workability

But I have neglected the fact that in water/cement ratio curves we aren’t testing paste containing just water and cement, but concrete that contains rock, sand, cement, water and a bunch of other stuff that wasn’t around 100 years ago. (I started to say “20 years ago”, then I realized they were around not only 20 years ago, but also 50. Even I was around 50 years ago.)

As I have said, when we test a water/cement ratio curve series we aren’t just testing paste, but concrete. We have to come up with a series of 3 or 4 concrete mixes, all the while considering:

  • Maximum aggregate size
  • Combined aggregate grading
  • Aggregate composition and shape
  • Freeze-thaw conditions and entrained air requirements
  • Durability conditions such as abrasion, sulfates and chlorides
  • Supplementary cementitious materials
  • Admixtures
  • Desired slump, workability and placement requirements (they are all different)
  • Setting time
  • Other concrete characteristics, such as shrinkage and bleed rate

Once we have done this, we can develop a base concrete mix design, then produce variations on it for our water-cementitous ratio series.

I have developed a truism for my life: “It’s not a good project unless you have had 3 visits to Home Depot.”

  • Go get the part or tool you need for the project
  • Go back and get the right part or tool for the project
  • Go back and get a replacement for the part or tool that you just broke or lost

Unfortunately, by the time the need for a 4 th trip rolls around I usually pick up the phone to call a professional to fix the damage I’ve just done. Hopefully this article marks my 3 rd trip back to the “supply store” and I can proceed with the project I had planned – discussing designing a concrete mix based on a water-cementitious ratio curve.

This article is short, so I’m going to publish it out of my normal schedule and hopefully get back on schedule for next Monday. I hope to see you then.

As always, if you have any comments I would love to hear from you.

Until next time,

Jay Shilstone

P.S. For a list of trade shows I will be attending drop by my personal page at http://www.commandalkon.com/services/jay-shilstone-concrete-technologist/

About Jay Shilstone

I am a concrete technologist for Command Alkon, Inc. and have been in the concrete industry for over 35 years. For 28 of those years I have been working on quality control software for the concrete industry. I am a Fellow of the American Concrete Institute and a member of multiple ACI, ASTM and NRMCA committees. I look forward to talking about concrete mix design and quality control with everyone.

I have been reading about, interested in, and (trying to) use concrete for several years now. First time I recall hearing the term ‘paste’.

JayShilstone

I don’t know if it is a common term outside of the U.S., but generally paste is cementitious materials, total air and water; mortar is paste plus fine aggregate; and concrete is mortar plus coarse aggregate.

Thank you for reading my blog.

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