Water-reducing admixtures have the ability to reduce, by 5% to more than 30%, the expected amount of mix water required to achieve the desired slump.
Adobe Stock / StockMediaProduction Water-reducing admixtures have the ability to reduce, by 5% to more than 30%, the expected amount of mix water required to achieve the desired slump.

The go-to document in the concrete industry for mix design has, for many years, been ACI 211.1, Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete. Although it is not a code, specification, or specifiable standard, it contains a wealth of information and shows the general trends to be expected for relationships pertaining to a wide range of concrete-making materials.

Figure 2: Water content for various aggregate sizes, based on ACI 211.1 for non-air-entrained concrete.
Figure 2: Water content for various aggregate sizes, based on ACI 211.1 for non-air-entrained concrete.

One of the document’s primary contributions is a thought process for mix design, beginning with selection of desired slump and coarse aggregate size, which in turn leads to an estimate of required water content (using data such as that shown in Figure 2 of Part 1). ACI 211.1 alerts the mix designer to a number of other factors that impact workability beyond just water content and aggregate size. My suggested expansion on these notes is shown in Figure 6, indicating relative increases or decreases in suggested water content as percentages of the ACI recommended values.

This chart is particularly handy for estimating the impact of water-reducing admixtures which have the ability to reduce, by 5% to more than 30%, the expected amount of mix water required to achieve the desired slump. This reduction depends on the potency of the water reducer selected and its compatibility with the cementitious materials. Alternatively, using a water reducer without reducing water content is basically the same as increasing the water content by 5% to 30%.

Figure 6. Adjustments to the water content as percentages of those recommended by ACI 211.1. These adjustments show why slump is not a consistent or reliable indicator of water content. At a constant water content, a change in any one or more of these factors will produce a different slump.
Figure 6. Adjustments to the water content as percentages of those recommended by ACI 211.1. These adjustments show why slump is not a consistent or reliable indicator of water content. At a constant water content, a change in any one or more of these factors will produce a different slump.

Supplementary cementitious materials like fly ash can likewise reduce water content because of the particles’ smooth spherical shape, but when the much smaller silica fume is used, surface-area effects become more important and water content has to be increased. This is why silica fume is almost always used in combination with a high-range water-reducing admixture. Note also that the use of an air-entraining admixture is as useful for reducing required water content as a water-reducing admixture operating at about 10% effectiveness.

Still following the ACI 211.1 sequence, after determining water content the mix-designer establishes the required value for the water-cementitious materials ratio (w/cm). This is based on the specified 28-day compressive strength, the precision of concrete production, the specified w/cm, and building code requirements associated with the environmental exposure conditions the concrete will experience (such as risk of freeze-thaw damage, sulfate attack, or corrosion of reinforcing or prestressing steel).

With this rationally selected, and often code-mandated, w/cm in hand, the weight of cementitious materials is then established simply by dividing the water content required for workability by the w/cm required for strength and durability. The wisdom of this step is often overlooked, because in this procedure the mix designer must satisfy two requirements of the concrete:

1. Provide the workability required by the contractor.2. Select the appropriate aggregates and adjust the water content and then establish the w/cm to provide for the strength and durability required by the owner and engineer.

The total amount of cementitious materials is then independently determined as a function of these two steps. The process does not begin by first selecting the total cementitious materials content, but this wisdom brings with it a price.

Concrete mixes are a trade-off among a variety of parameters.
Command Alkon Concrete mixes are a trade-off among a variety of parameters.

Pick Any Two
For example, let’s say we want a 5-inch slump with a 1-inch stone, air-entrained and water-reduced for a 15% reduction from the basic water content recommended by ACI 211.1. That gives us a water content of 85% of 325 pounds of water per cubic yard, or about 276 pounds of water per cubic yard.

Now, if strength and durability requirements are not too severe, we might be able to use a w/cm of 0.50, and our total weight of cementitious materials would be 276 / 0.50 = 552 pounds per cubic yard. If the strength or durability requirements were stricter and a water-cement ratio of 0.45 is required, cementitious materials would go up to 613 pounds per cubic yard, and then jump all the way to 690 pounds per cubic yard for a w/cm of 0.40.

Considering the price of cement, this will have serious cost implications; not surprising since a higher-quality product normally costs more. But going beyond cost to the overall value proposition, increasing the total cementitious materials content will also increase the carbon footprint, the heat of hydration, the potential for deleterious chemical reactions, and the total paste volume. This, in turn, may result in additional shrinkage and may require more air to protect that larger paste volume against freezing and thawing.

But the essential point is that once the water content and the w/cm have been determined, on whatever basis, there is no longer a choice for the total weight of cementitious materials. But this game can be played several ways.

Of the three variables—water content (w), water-cementitious materials ratio (w/cm), and total weight of cementitious materials (cm)—the mix designer or specifier can choose any two independently but not all three.

If we want to achieve higher strength and durability while minimizing the downside implications of increasing cementitious materials, we have no choice but to reduce the required water content by the judicious use of chemical admixtures, supplementary cementitious materials, or favorable blending of coarse, intermediate, and fine aggregates (or try to convince the contractor to place the concrete at a lower slump). If we want to keep from inadvertently bringing on higher shrinkage with higher paste volume, then we may need to turn to shrinkage-reducing admixtures or other strategies for controlling either shrinkage or the inevitable cracks.

So we can choose any two: w, w/cm, or cm. If we don’t like the third one, however, then we are going to have to pay extra to get what we want.

Adjustments and mixture proportions have to be made in pairs with attention to preserving the key parameters that are important to the performance of the particular mix, beginning with yield and including w/cm, cm, total paste content, or mortar fraction. Remember that everything in a mix affects everything else.
Adjustments and mixture proportions have to be made in pairs with attention to preserving the key parameters that are important to the performance of the particular mix, beginning with yield and including w/cm, cm, total paste content, or mortar fraction. Remember that everything in a mix affects everything else.

Wrapping-Up the Mix
When the cement dust finally settles over the great w, w/cm, and cm debate, the mix designer knows the total weight of batch-water and cementitious materials, which can then be converted to paste volume. It can be helpful at this stage to divide that paste volume by the total volume of the concrete (1 cubic yard or 1 cubic meter) to establish the paste volume fraction—we discussed last issue in Part 1. Remember that paste volumes around 25% are likely to lead to manageable shrinkage, while values of 30% or more may lead to shrinkage problems, especially for flatwork.

Next, the mix designer selects air content on the basis of the most restrictive requirements of either the project specifications or the building code requirements, then adds up the total volume of paste plus air. The total aggregate volume is then obtained by subtracting paste plus air volume from the target mix design volume, which in the U.S. is usually 1 cubic yard or 27 cubic feet.

Experienced concrete producers, working with their customers, already have a pretty good handle on the best blend of coarse, fine, and intermediate aggregates used to fill this required aggregate volume to meet the contractor’s needs for placing, pumping, consolidating, and finishing the concrete.

If no such blend is apparent, ACI 211.1 has a suggested approach for estimating coarse aggregate content based on coarse aggregate size, bulk unit weight, and the fineness modulus of the sand. This ingenious method actually compensates for the natural tendency of the coarse aggregate particles to pack efficiently, with a correction that increases coarse aggregate content when the sand is fine, and decreases coarse aggregate for coarser sand. This correction helps to account for the large additional surface area of fine sand that must be coated with cementitious paste, and will always increase water demand.

Caution Flags
As with almost everything else in our industry, there are dozens of local variations on the mix-design steps discussed so far, and the demonstrated performance with local materials and local mix-design methods is generally more reliable for making predictions than nationally published charts, graphs, and tables. But regardless of the method used, whether it be a back-of-the-envelope adjustment to an existing mix or a new mixture generated by a spiffy computer program, the final result is a “trial mix.” It remains a trial mix until there has been the opportunity to test it in the lab (usually for strength and sometimes for durability), and to get it out into the field in large enough batches to assess the suitability of the fresh concrete.

When you couple changing material sources and the day-to-day variability of local materials with the fact that typical mix-design methods simplify complex behavior, you realize why we can’t predict exact behavior in advance without relying on comprehensive and continued testing. Actual concrete performance is the goal; let’s not get the mix-design method ahead of the concrete results.

As the early test results begin to come in, it is absolutely normal and expected that adjustments will be made to the proposed concrete mixture. That’s why we test. But when making adjustments, it is essential to recognize that the concrete mix is a closed system with a fixed volume that is the sum of the volumes of all of the ingredients.

This means that you cannot adjust one and only one constituent of the mix. If you choose to modify aggregate content, for example, you must also modify the paste content so that the total volume is still 27.00 cubic feet, or else your mix will consistently under yield.

If the decision is made to increase the water content (for workability), that increases the total volume of the concrete but does so without increasing cementitious materials content. The result is that the actual weight of cementitious materials per cubic yard has been decreased. At a minimum, adjustments and mixture proportions have to be made in pairs with attention to preserving the key parameters that are judged to be important to the performance of the particular mix, beginning with yield and including w/cm, cm, total paste content, and mortar fraction. My mentor was cement chemist Floyd Slate, who always said, “Remember that in a concrete mix, everything affects everything else.”

Finally, there are some interesting and sometimes troubling connotations to the word “design” when used in the context of mix design. Jim Shilstone, a famous concrete materials expert, was always careful to distinguish between mix-design and mixture proportioning. He defined “mix design” as the selection of six key performance parameters:

  • slump or slump flow
  • maximum nominal aggregate size
  • types of cement and supplementary cementitious materials (SCMs)
  • mechanical or durability-related properties
  • water-cementitious materials ratio
  • air content

Once these parameters are identified by the specifier or the concrete producer, attention is then turned to mixture proportioning, for which any number of fully acceptable concrete mixtures could be developed from a wide array of local materials over a large region to meet the “design” requirements.

Concrete is clearly an engineered material and there is no doubt that the creativity and problem-solving skills that are associated with the design profession can and should be applied to the problem of developing a concrete mixture that will meet the particular demands of a project. But the harsh truth is that we may not yet be able to predict concrete mixture performance to the same level of reliability that we have come to expect in structural design.

With the exception of code-based requirements for concrete acceptance, we do not have code-based procedures for predicting strength or other mechanical or durability-related properties on the basis of mixture ingredients and proportions that are in any way analogous to our time- and research-tested design equations for capacity of structural elements and assemblies. Some agencies are moving towards requiring that mixture designs be prepared by licensed professional engineers, but this could be counterproductive if such requirements would exclude preparation of mix designs by skilled concrete professionals who have amassed years of experience about the complex interactions of their local materials. Those are the folks we want designing our mixes!