In 1597 William Shakespeare penned Romeo and Juliet. In that immortal play Juliet opines, "What’s in a name? That which we call a rose by any other word would smell as sweet." I'm certain Juliet wasn't referring to concrete mix designs when she said those words, but if the shoe fits....
Probably the second most often asked question I get asked is, "How should I name my concrete mixes?" I decided to stop responding with generalities and actually put some effort into coming up with a detailed response. As part of that effort I created a survey to get feedback from concrete producers on how they name their mix designs. The survey is still open for responses. A couple of weeks ago I posted a summary of the survey results on my blog. The full response is too long for the TCP newsletter, in fact it is my longest post ever, but I wanted to present an important basic concept here.
The general premise of the blog post is that the concrete mix design name should reflect the concrete mix design. Many companies do this already. A mix design code that starts with "35" indicates a 3,500 psi mix or a 35 MPa mix. Other characters in the mix code reflect air content, maximum aggregate size, the use of fly ash, and so on. The problem begins when you start to look at the different ways people design concrete mixes. In some areas concrete mixes are sold based on "sack content", the quantity of cement in a mix design, expressed in 94 pound sacks (don't laugh, you metric-philes). In this case does a mix code that starts with "55" indicate a 5,500 psi mix or a mix containing 5.5 sacks of cement? The problem is further compounded when some concrete producers have both 5,500 psi mix designs and 5-1/2 sack mix designs. Which is which?
The answer lies in the first character of the mix design. If we use that character to designate the type of concrete mix, then we open a whole world of possibilities. For example, a first character of "P" could indicate a performance mix that is allowed to vary, as long as the performance requirements are met. An "R" could designate a recipe mix that cannot be changed and the performance is allowed to vary. Following is a list of potential characters that could be used to start the mix code:
P – Performance/designed mix design. Proportions of any material can be changed in order to maintain consistent performance
- R – Prescriptive/prescribed mix design (the R stands for recipe). These exact proportions must always be used.
- W – Performance mix design where the water cementitious ratio cannot exceed a prescribed value
- B – Performance mix design where a specified number of “bags” or “sacks” of cementitious material must be used (this is a very common requirement for residential concrete in the U.S.)
- M – Mortar or grout (no coarse aggregate)
- Y – Slurry
- V – Pervious
- S – Self-consolidating concrete
- L – Lightweight concrete
- H – Heavyweight concrete
- G – Government specified
- X – Customer-submitted mix design (the same as a prescriptive mix)
There is an added benefit to using these characters to start the mix code. Most commercial concrete software programs available today have the ability to perform mass updates on mix designs. One of the biggest problems is deciding which mixes to update. If a mix code starts with "P", then anything goes as long as the performance criteria are met. If a code starts with "B" then aggregates or admixtures could be changed, or the ratio between cement and pozzolan could be changed as long as the total weight of cementitious material wasn't changed. Of course mix codes that begin with "R" would never be changed. The initial mix code goes a long way to facilitating the mass update process.
You can read the full blog article about "Naming Concrete Mix Designs," and let me know what you think about the article, especially if you disagree with it. I never learn anything from a person who agrees with me.
Until next time,
Jay Shilstone, FACI
Command Alkon, Inc.