Concrete Mix Design, A Science or an Art?
Once upon a time, concrete used to be a simple material comprising of cement, coarse aggregate, sand (fine aggregate) and water. There were only a few nominal mix proportions like 1:2:4, 1:3:6 etc. While the expectations from concrete were modest (M15, M20 or at best M25), what was needed to produce it was also much simpler. Durability as an important property of concrete was in the domain of research only!
As technology advanced, things began to get gradually more complicated. Different types and grades of cement appeared; crushed aggregate & sand and recycled aggregates started to be used and above all admixtures were developed to modify the properties of fresh as well as hardened concrete. Engineers started to expect higher and better performance from the concrete and concepts of durability came into the realm of practice.
It was no more possible to rely on the nominal concrete mixes and various methods to proportion concrete mixes emerged. While the procedures were named differently and appeared to be different, the core of all the methods remained the same. These revolved around the variables that affected strength, workability and durability. These were presented in different tables and charts and the sequence of using these differentiated one process from the other.
In the early days of concrete mix design, it appeared that the process was moving from being an art to science. All the charts and tables that were part of the steps of a typical mix design procedure could be very easily embedded in software and for a given requirement; concrete mix proportion could be obtained at the press of a key. The charm and expertise of the concrete technologist was threatened to be high-jacked by the software!
However, the wonderful material that concrete is, it had no intention of allowing itself to be captured by computer software. Today, the variables in the concrete mix proportioning are so many that it defies simple programming. Consider this:
- The actual cement strength is not necessarily the one expressed by the ‘grade’ of cement.
- There are now a variety of supplementary cementitious materials like fly ash, ggbs, micro silica and metakaolin
- The supplementary cementitious materials not only influence the strength of concrete, but also the water demand, workability and the ability to retain workability of the mix!
- Admixtures continue to evolve, each generation of these bringing in greater complexity in the interplay of the constituents of concrete.
- The transportation and placing methods have their own demand on the properties of fresh concrete at different stages.
- Designers (both structural and concrete mix) have the confidence, (sometimes a need) to use 56 days or even 90 days strength as against 28 days strength.
- It has never been possible to capture the surface area/mass properties of aggregates in grading curves, maximum nominal sizes, surface texture definitions and angularity numbers.
It is surely within the capability of the modern day IT systems to capture all these variables in a complicated mix design software. However, the trial mix would not necessarily follow the predicted results as the numbers of independent variables contributing to the single outcome are too many! The mix design process with or without software therefore continues to be, and will most probably continue to be, a tool to get the first mix which is very close to the final one. The journey from the calculated mix proportion to the final proportion would require judgment and experience, and laboratory trials. The concrete mix design will thus thankfully remain an art as well as science and the human insight and experience will continue to be valued!