Hello, concrete aficionados! (I assume only those who are really interested in concrete will be taking to time to read about something as boring as rocks and sand.) Today’s post will discuss the role aggregates play in concrete. We aren’t going to get into how to calculate rock and sand quantities – that come later. Instead we are going to discuss why aggregates are important in concrete and what role they play. Once you understand that you will understand how to proportion aggregates for a concrete mix.
Before we can talk about aggregates, however, we need to discuss paste briefly. Paste is the mixture of water, cement, air, plus the extras like admixtures, filler and supplementary cementitious materials. There are a few things we need to understand about paste:
- Water and cement in the paste react to form the “glue” that holds everything together
- Paste provides the lubrication in the mix that allows the concrete to flow.
- Paste is usually the most permeable and weakest material in the concrete
- Cement is usually the most expensive component in concrete
Since the discussion of paste can get quite lengthy, we will leave it for a future post. Suffice it to say that when it comes to paste we want to:
- Minimize the amount of paste in a mix, subject to obtaining workable concrete
- Minimize the amount of cement in the paste, subject to strength and durability requirements
- Minimize the total cementitious material in paste, again subject to strength and durability
Notice that the above three items all refer to “minimizing” something subject to a constraint. That means that it can be bad to have too much or too little paste, cement or cementitious material.
Back to aggregates. The primary purpose of aggregates in concrete is to fill the maximum amount of volume in the concrete while still producing an appropriate workability. We want to minimize the number of voids, subject to producing a workable concrete. We have to add the caveat about “producing workable concrete” because if we just minimize voids we will probably develop concrete that is too harsh to place. This was re-discovered at great expense to the U.S. taxpayer back in the 1990’s during the Strategic Highway Research Program, SHRP, when the ternary packing diagram was introduced to the mainstream U.S. concrete industry. Although the ternary particle packing chart had been used in Europe for many years, it didn’t really “cross the pond” until SHRP. In summary the ternary packing chart was used to identify the maximum packing density of three materials – coarse stone, fine stone and sand. However, the resulting mixes were very harsh and were suitable primarily for paving, and sometimes not even for that. As a result of that, and a few other issues, the particle packing chart has never really found its way into mainstream U.S. concrete mix design.
Even Duff Abrams, in Lewis Institute Bulletin #1 from 1918, said, “We have found that the maximum strength of concrete does not depend on either an aggregate of maximum density or a concrete of maximum density…”. Abrams went on to cite his “theory” that strength in concrete was related to the ratio between water and cement. However, Abrams didn’t ignore the aggregates. He relied on the grading of the combined aggregate to produce an aggregate blend with the lowest water demand to produce the highest strength at a given slump. He went on to say, “The aggregate grading which produces the strongest concrete is not that giving the maximum density (lowest voids). A grading coarser [emphasis added] than that giving maximum density is necessary for highest concrete strength.”
Let’s look at what each size of aggregate does in a concrete mix. We will look at three sizes of aggregate – coarse, intermediate and fine.
Coarse aggregate is usually the strongest, densest material in concrete. We want as much of it as we can get in the concrete mix. However, if we take a one cubic meter or one cubic yard bucket, fill it with rock, then fill the remaining voids with mortar and mix everything up, we won’t have a very workable concrete mix. It will be too harsh, since the rock particles are touching each other. We have to add extra mortar to separate the rock particles and provide workability. This is the whole premise of the ACI 211 b/bo concept of mix design. This procedure was developed by Goldbeck and Gray for the National Crushed Stone Association back in 1942. We’ll talk more about this method in a future post. The main point here is that the stone particles must be separated to produce a workable concrete mix. The rougher and more angular the stone particles, the more they must be separated, which means the less we can use of them.
There are other factors relating to coarse aggregate that must be considered. Typically we want to use the largest practical size of stone, subject to limitations in the thickness of the concrete element and the distance between reinforcing steel. However, some materials are subject to “D-cracking” in freeze-thaw environments and we must use a smaller maximum coarse aggregate size. Typically for higher strength concrete we use a smaller size of coarse aggregate as well. Deleterious materials, such as clay or surface deposits on the aggregate, can reduce strength, so we may need to either wash the aggregate or use less of it. Highly angular materials, especially those containing mica, will greatly increase water demand, so we need to use less of them. For a more thorough discussion on aggregates, read ACI’s E-701 documents on “Aggregates”. It is available for free at http://www.concrete.org/general/E1_07.PDF
My father used to comparing designing concrete mixes to laying up a stone wall. First you lay a row of big rocks, then add a layer of mortar. Then you fill in the bigger holes between the big rocks with little rocks. The same factors relating to coarse aggregate relate to intermediate aggregates, which we normally define as material passing the 3/8” sieve (9.5mm) and retained on the #8 (2.36 mm) sieve. We want to fill the bigger spaces between the large rocks with intermediate rocks. However, if we add too much intermediate aggregate, or the intermediate aggregate is too angular, we will have a harsh mix. Also, we must separate the combined coarse and intermediate particles with mortar to produce a workable concrete mix. If we don’t have enough intermediate aggregate we will have to fill the extra spaces with additional mortar, which increases paste and cost and decreases durability. For a further explanation of combined aggregate grading go to my previous blog post at http://www.commandalkonconnect.com/2012/06/11/well-graded-aggregates-vs-gap-graded-aggregates/
This leaves the fine aggregate. Fine aggregate combines with paste to make mortar. We want to minimize the amount of paste we use, so we need the right grading of fine aggregate. Too coarse a sand increases voids, which increase the demand for paste and water to fill voids in the sand. Too fine a sand increases the surface area of the sand, which again increases water and paste demand. Just like Goldilocks, we want something that is neither “too coarse” or “too fine”, but is “just right”. The problem is, “just right” is dependent on something we have already talked about – particle shape and texture. Unfortunately at this point there is no standard way of characterizing particle shape and texture, and relating that value to an optimal mix design. The more voids in the fine aggregate and the greater the angularity of the fine aggregate particles, the more we have to add paste to separate the particles.
So far we have talked about working from the largest particles to the smallest particles. There is a whole other argument for reconsidering things from the other direction. A high cementitious content in the paste will usually increase total paste volume in a mix. Sometimes we need to find a “home” for that paste, so we have to work from smallest to largest particles. Self-consolidating concrete is an example of this that we will take up at a later date.
There are several special conditions relating to aggregate characteristics that require special consideration, such as the use of lightweight or heavyweight aggregate, but I’ll leave those for another time. There are also concerns about friable (breakable) aggregates, aggregates with cleavage planes and reactive aggregates. Those will also have to await further discussion.
If you learn just one paragraph in this post, remember this one. We usually want the largest aggregate size practical with placing requirements and we want the optimal aggregate blend to minimize water demand for workability. If an aggregate or particle size is angular, we need to use less of it. Leaving out two or three consecutive particle sizes in concrete is usually a bad idea. There is an optimum combination of material sizes, not just among coarse, intermediate and fine, but an optimum combination of each particle size that will result in the lowest water demand and highest strength.
That’s all for today’s post. Next time we will talk about paste.
For those who get this far, please leave a comment and tell me what you think about this post or the series. If you have any other insights, please leave them. That is how we all learn.
Until next time,