Get More from Your Mix – for Less

New material technologies and intelligent mix designs can significantly decrease production and labor costs.

By Chris Von Handorf, P.E.

Reducing material and labor costs makes sense for any manufacturing company, and increasing profits is even more imperative during a slumped economy. Companies in every industry have been forced to look at ways to lower production costs to remain competitive in our rapidly evolving marketplace. The precast concrete industry is no exception.

Concrete constituents are only a portion of the costs incurred by a precast concrete manufacturing facility. This article shows how a smart mix design can help you cut costs. The important point to remember is that a lower-cost concrete does not always result in an inferior product. In fact, in some cases, lowering the cost of your mix design may actually yield a higher-quality concrete product.

Cost of concrete constituents
As you know, concrete is created by mixing coarse aggregates, fine aggregates, cement, water and any number of various admixtures to improve its fresh or hardened-state properties. For the sake of discussion, let’s assume that we are working with 1 cu yd of a standard, 5,000 psi-compressive strength concrete with the mix constituents shown in Table 1.
As can clearly be seen in the table, cement is the most costly material per ton and also makes up about 60% of the cost for the mix design. Since cement is the most costly mix constituent, it makes sense to start here and try to reduce the amount of cement.

Aggregate shape and size affect the bottom line
One of the best ways to reduce the amount of paste (cement and water) required is to optimize the aggregates. Aggregates can be optimized for a variety of properties. One way to optimize aggregates and reduce the amount of paste required in a given mix is to have the lowest surface area-to-volume ratio possible. A lower total surface area of aggregates means less cement paste will be required to cover the aggregates. Less cement paste equals less cement, which means lower material costs.

Surface Area-to-Volume ratio = Surface Area / Volume

When looking at your mix designs, it is important to use the maximum aggregate size practical for the application. Larger aggregates will inherently have a lower surface area-to-volume ratio than smaller aggregates. Section 6.3.2 of ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete,” discusses the considerations and limitations for maximum aggregate sizes.

Round or cubical-shaped aggregates have the highest volume with the lowest possible surface area. Conversely, angular-shaped aggregates, such as crushed aggregates, have a higher volume-to-surface area ratio. Therefore, angular aggregates will require additional cement paste compared with similarly sized round or cubical aggregates. More cement paste means more cost. Additionally, with all other factors equal, concrete using angular aggregates will have a lower workability compared with concrete using round or cubical aggregates. And lower mix workability may lead to additional concrete placement costs (labor).

Aggregate packing factor affects cement content
Another important factor to consider when trying to reduce the amount of cement paste is the aggregate packing factor. The aggregate packing factor is defined as:

Aggregate Packing Factor = Volume of Aggregates /
(Volume of Aggregates + Volume of Voids)

A good way to understand this is to visualize a container full of coarse and fine aggregates. Fill the container with water until no more water can penetrate the aggregates. The aggregate packing factor is the volume of the container minus the volume of water divided by the volume of the container. The less water that fits into the container, the higher the aggregate packing factor. If the water that was poured into the container had been cement paste, the more cement paste that is required, the more expensive the mix. Therefore, when looking to decrease cost, it is desirable to use aggregates with the highest achievable packing factor in order to decrease the amount of cement paste.

ASTM C33, “Standard Specification for Concrete Aggregates,” provides excellent direction to aggregate producers as well as concrete manufacturers regarding optimal gradations for aggregates used in concrete materials. In addition to providing aggregates recommended by ASTM C33, many aggregate suppliers will also include the aggregate packing factor. When used properly, this information can help you obtain aggregates that require the least amount of cement paste per mix.

Besides decreasing costs, it is generally accepted that minimizing the amount of cement paste produces a concrete mix with better durability and reduced shrinkage. Decreasing the cement paste in your product is not only a cost-cutting technique, but the cement reduction will also improve the quality of your product.

Admixtures offer many cost-saving options
The cost of the materials that make up concrete is but one of the many expenses incurred by a precast concrete manufacturing facility. Another major expense is the labor cost to place and finish the product. This cost varies widely depending on a plethora of factors including: the type of product; the climate; the experience level of workers; and reinforcement required.

Here are just a few of the admixtures available that, in the right application, have the potential to significantly lower production costs:

1. Supplemental Cementitious Materials: The use of supplemental cementitious materials, such as silica fume and blast furnace slag, has the potential to enhance the performance of concrete while reducing any bleeding that may occur. When used properly, silica fume can improve concrete’s resistance to chemical attack. It can also increase concrete strength while reducing the permeability of the concrete. And given the fact that silica fume is a recycled byproduct of the energy industry and is considered a cementitious material, it is often relatively inexpensive and may reduce the amount of cement required in a given mix design.

2. Accelerators: The use of accelerators in precast applications has some obvious advantages. The faster concrete reaches the required stripping strength, the quicker the forms can be cleaned, prepped and used again. For a precaster, a quicker strength gain is huge if you are looking to go from pouring once per day in a given form to twice per day. Pouring forms twice per day will translate into the economy and flexibility of double production quantities, expansion to different product lines, and reduction in total days and labor costs for production.

3. High-range water reducers: High-range water reducers (HRWRs) are excellent for nearly every precast concrete application. A good high-range water-reducing admixture will allow you to produce concrete batches with more consistent air entrainment and more consistent ultimate strengths. HRWRs decrease cement content and allow a corresponding decrease in mix cost. HRWRs also allow for greater concrete workability with a decreased water-to-cement (w/c) ratio. Lower w/c ratio produces a higher-quality concrete product while decreasing the labor required for placing and finishing.

4. Release agents: While release agents are not a constituent of the mix, the use of a high-quality release agent is essential for a better-looking, lower-cost, finished product. As with HRWRs, release agent technology has improved significantly in recent history. Although many new release agents are more expensive per unit, most of them do allow for a lighter application than traditional release agents. As a result, the cost per square foot of coverage is often significantly lower using the newer form release agents.

Emerging technology and processes surrounding the concrete industry are rapidly advancing. Admixtures that cost only $1 to $2 per cubic yard may have the potential to save you hundreds of dollars or more in labor and rework. Many of these admixtures were not available a few years ago. Some material testing techniques that were not available years ago, or were very expensive, are now relatively inexpensive. Therefore, it is no longer economical or wise to continue precast production methods with a familiar, long-standing mix design simply because it has been the traditional way of doing things for the last 20 or 30 years.

Chris Von Handorf, P.E., is a technical services engineer with NPCA.