As we continue on our discussion of “Concrete Mix Design: Art and Science” I would like to move on to a discussion of “paste”. We have already discussed paste near the beginning of this series ( ) and, after rereading it, it is a pretty good start, but I would like to add a few things to the discussion.
Before we get into the discussion of paste, in case you are wondering about where the article on Bolomey mix design of aggregates is, I’m afraid I need to defer that article. When I moved into my home office, as you might imagine, some of my research files got misplaced and the references to Bolomey were among them. I will eventually find them and will write the article when I do. If any of you have a copy of the original work “Granulation et prevision de la résistance probable des bétons” from 1935 I would appreciate either a copy or a link as to where I could acquire a copy. While an English translation would be preferred, a copy of the original French would work also.
Back to matters at hand, I would like to expand and revise my previous article a bit.
What is paste?
If you talk to people who work with plastic concrete and develop mix designs, like concrete quality control personnel and consultants like myself, we almost always define paste as including water, cementitious and inert materials passing the #200 (75µm) sieve and air, both entrapped and entrained. If you talk to a petrographer, who looks at concrete and paste at a microscopic level, paste does not include air. Since air bubbles are easily identifiable to the petrographer, they are split out into their own category. This is all very interesting because, if you refer to ASTM C856, “Standard Practice for Petrographic Examination of Hardened Concrete”, you will not find a definition for “paste”, nor will you find one in ASTM C125, “Standard Terminology Relating to Concrete and Concrete Aggregates”. In C856 you will find the term “paste” being used, but you will also see the term “matrix”. In fact, C856 divides concrete into the following components:
- Coarse aggregate
- Fine aggregate
- Matrix, and
ASTM C457, “Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete”, does refer to paste as a combination of water, cement and supplementary cementitious materials, but doesn’t reference inert fines such as finely divided limestone powder.
While I am not an expert on petrography, paste or ASTM, it appears that C125 needs to have two new definitions added: paste and matrix. In my way of thinking, “matrix” should include water and soluble and insoluble fine materials and “paste” should include the matrix and air voids.
I previously divided paste design into two areas: 1) How much paste do we need to provide workability, and 2) What is the composition of the paste to produce the desired strength and durability. While I still agree with the two categories, they really overlap a bit. The amount of paste required to provide workability is dependent on the composition of the paste. The more fluid the paste is, the less we need of it to provide workability (up to a point). If the paste is too fluid then it will segregate from the combined aggregates. If it is too viscous then it doesn’t matter how much paste you add – the mixture still won’t flow.
Over the next several posts I will be discussing proportioning paste from several points of view:
- Workability (and segregation)
There will be some overlap. For example, a mixture with low workability will not consolidate well and so may exhibit low strength and durability. The strength issue should be fairly simple to address as it is relatively easy to measure directly and design a procedure to test for. Durability and workability can be measured, to some extent, but they may mean different things to different people. Instead of “I know good art when I see it.” we may need to say, “I know workable concrete when I see it.”
These discussions can easily become very technical, but I will try to avoid that and relate things to the “touchy-feely” perceptions. Hopefully I will hit a middle ground that is neither too technical nor not technical enough. Let me know if things need further clarification.
Until next time,