Concrete Mix Design: Proportioning

By Frank Bowen and Paul Ramsburg

Editor’s Note: This is the fifth article in a year-long series that explores the science of concrete to provide a better understanding of mix design. The series is collaboratively written by Paul Ramsburg, technical sales specialist at Sika Corp., and Frank Bowen, business development representative with Rosetta Hardscapes. Click here for the fourth article in the series.

Creating a mix design is more than just proportioning. It involves the whole process from selecting the proper materials and putting them together in a way that satisfies the engineering requirements of the finished product, to meeting the needs of those who are to place, finish and manage the casting(s).

Other than desired hardened precast properties, a perfect mix design includes four key facets: proportioning, sequencing, batch-cycle times and a place-finish-cure plan. A PFC plan is an assemblage of best production practices designed to eliminate the chance of production errors from when concrete plastic stability is optimal through the curing of the structure before it enters its service life. One mix design may have two different PFC plans – one for winter and one for summer – but this should not require additional testing.

In the previous four articles in this series, we’ve discussed the different elements that make up concrete. It’s now time to put them together. Let’s begin with a review of proportioning a concrete mix design. For an understanding of sequencing, batch-cycle times and PFC plans, refer to the respective sections in the provided mix designs.

Proportioning Math for Proper Yielding

Throughout concrete’s history, mixes have been designed with a wide variety of methods. It actually wasn’t that long ago that we were using the 1-2-4 method of proportioning by volume – 1 scoop of cement, 2 scoops of sand and 4 scoops of stone. The Panama Canal was built using this antiquated, volumetric mix design method.

For roughly 100 years, we have been using the absolute volume mix design method. This method differs from the 1-2-4 method by using math to ensure a mix design provides the desired yield no matter what materials are used. Prior to continuing, it is a good idea to read American Concrete Institute 221R, “Guide for Use of Normal Weight and Heavyweight Aggregate in Concrete.”

Here are a few terms that we must understand before we begin designing concrete:

Saturated surface-dry – SSD is the condition of an absorptive material where the material is saturated, but its surface is dry. SSD aggregate neither absorbs water from nor contributes water to the concrete mixture. This is typically only achieved in laboratory conditions.

Specific gravity – The SG of any material is the unit weight of that material divided by the unit weight of water at room temperature. An aggregate with an SG of 2.50 would thus be 2.5 times as dense as water. To understand this concept, consider an iron anvil being dropped into a tub of water and quickly sinking to the bottom. The anvil sinks because the SG of the iron is greater than that of water. Now, if the tub was filled with mercury instead of water, the iron anvil would float because the SG of the iron is less than that of mercury.

Absolute volume – The AV of a granular material is the volume comprised by only the solid matter in a given space. It does not include the volume of the voids between the particles. The AV of a material is computed as follows:

AV = weight of material/(SG of material x unit weight of water)

For example, the SG of a certain oven-dry coarse aggregate is 2.75. The unit weight of water is 62.4 lbs./ft.³. The absolute volume of a 90-pound sample of the aggregate would be:

AV = 90 lbs. / (2.75 x 62.4 lbs./ft.³) = 0.524 ft.³

The AV of a concrete mix can be determined if the weight and SG of the components are known. For a concrete sample mix containing 90 pounds of coarse aggregate with an SG of 2.75, 60 pounds of fine aggregate with an SG of 2.61, 25 pounds of cement with an SG of 3.15 and 12 pounds of water (with an SG of 1), the AV is computed as follows:

Coarse aggregate = 90 lbs. / (2.75 x 62.4 lbs./ft.³) = 0.524 ft.³
Fine aggregate = 60 lbs. / (2.61 x 62.4 lbs./ft.³) = 0.368 ft.³
Cement = 25 lbs. / (3.15 x 62.4 lbs./ft.³) = 0.127 ft.³
Water = 12 lbs. / (1 x 62.4 lbs./ft.³) = 0.192 ft.³
Total Volume = 1.211 ft.³

ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete,” has been adopted by the concrete industry as the standard procedure for mix proportioning. If you design concrete mixtures, it is definitely required reading. This standard provides a starting point, which is a basic design that would need to be tested and adjusted for your specific materials.

Most precast plants have a long history with their materials and understand how they perform in concrete. To begin designing new mixes for your plant, it may be easier to base your designs off the historical data you’ve already gathered. ACI 211.1 may be a better starting point if no previous mix designs have been tested or confirmed for use in your plant. When it comes to adjusting fine and coarse aggregate ratios, approval can only be confirmed through in-plant testing with all of your other local raw materials.

Let’s walk through an example. You need a 5,000-psi mix with a 5% air content, and you need to be able to strip the products from the forms in 15 hours. Historically, for similar mixes, you’ve used 555 pounds of cement and 120 pounds of fly ash per cubic yard of concrete. You typically use around 1,560 pounds of coarse aggregate. With these materials and to reach an adequate stripping strength within 15 hours, you need to have a maximum water-cementitious ratio of 0.40 – all cementitious and pozzolanic materials included.

Let’s say you’re trying a new sand and want to proportion this mix for your plant. First, we need to know the SG of each material. Portland cement’s SG is generally 3.15; however, you would need to obtain the SG of your other raw materials from your material suppliers. For our example, let’s say the fly ash’s SG is 2.23, the coarse aggregate’s SG is 2.75 and the sand’s SG is 2.61. Also, we will need to calculate the volume of the entrained air in the mix. After calculating the volume of each material, we must add them up.

Example problem

Coarse aggregate = 1,560 lbs. / (2.75 x 62.4 lbs./ft.³) = 9.091 ft.³
Cement = 555 lbs. / (3.15 x 62.4 lbs./ft.³) = 2.82 ft.³
Fly ash = 120 lbs./ (2.23 x 62.4 lbs./ft.³ = 0.862 ft.³
Water = 0.40 x (555 lbs. + 120 lbs.) = 270 lbs.; 270 lbs. / (1 x 62.4 lbs./ft.³) = 4.327 ft.³
Air = 5% x 27 ft.³ = 0.05 x 27 ft.³ = 1.35 ft.³
Total Volume = 18.45 ft.³

In our example, the total volume of all materials except sand is 18.45 ft.³. Since there are 27 ft.³ in a cubic yard, you can subtract 18.45 from 27 to determine the volume of sand you need to complete the design. This results in 8.55 ft.³ of sand. To determine the weight of sand, you need to multiply the volume of sand by the sand’s SG and by 62.4 lbs./ft.³. The complete design consists of 535 pounds of cement, 120 pounds of fly ash, 270 pounds of water, 1,560 pounds of coarse aggregate and 1,392 pounds of sand.

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Math Explained

How did we get 270 pounds of water from 0.40 w/c ratio?
Multiplying 675 pounds of total powder by 0.40 w/c ratio equals 270 pounds.

If you batch by gallons, not pounds, covert that as:
270 pounds/8.33 lbs./gal. = 32.41 gallons.

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This is an oversimplification of designing a mix to teach the mathematics of yielding. For information on the importance of various material properties and their effects on the mix design, refer to past articles in this series.

Given Mix Design

When we teach mix design courses, the most frequent comment we get is, “Just give me a mix design.” To do so would be regarded as improper, foolish even. No one does this because a successful concrete mix design at one plant will not always work at another. All concrete is, and always should be, considered localized to a specific manufacturer. Since raw materials, especially aggregates, vary in SG from source-to-source, they can alter the yield of a mix design. Knowing this, we still find it informative to study the mix designs used by other producers. You can learn a lot by seeing how your peers are designing their mixes and trying these ideas yourself. It is important to note you should always ensure your mix yields correctly – 27 ft.³ per yd.³ +/- .01 ft.³ or as otherwise specified.

Also, a mix design should always be tested prior to being used in the manufacture of precast concrete products. That said, shown to the right and below are two perfect precast concrete mix designs. Mix 1 is a traditional mix that typically is used in larger products with normal weight material, while Mix 2 is an accelerated self-consolidating concrete mix that is used in smaller products with tight form spacing (2-inch-wide walls) and no reinforcing.

Mix 1

Proportioning

Cement – 570 lbs.
Fly Ash (SG = 2.23) = 120 lbs.
67 Stone (SG = 2.75) = 1,560 lbs.
Sand (SG = 2.61) = 1,402 lbs.
Water = 276 lbs. / 33 gal.
Air-entraining admixture = 5 fl. oz.
Air target = 5%

Sequencing

1 yd.³ for a counter-current pan mixer that has already mixed a similar batch that day (i.e. preconditioned or buttered):

1. Aggregates/air-entraining admixture
2. Cementitious materials
3. Dry-mix cycle:
   • Dry mix minimum batch time: 60 seconds
   • Dry mix maximum batch time: 200 seconds
4. Water (total weight added to be determined after calculating aggregate-free moisture)
5. Wet-mix cycle:
   • Wet mix minimum batch time: 90 seconds
   • Wet mix maximum batch time: 300 seconds
6. Discharge gate opening size and times at 1 yd.³ size:
   • 20% open for first 10 seconds
   • 100% open for remaining time

PFC plan

Conditions

• Acceptable for use in steel-reinforced products
• Fresh concrete temperature: 75 F +/- 10 F
• Forms intended to be poured with this mix: box culverts, grease interceptors and septic tanks
• Recommended winter aggregate preheat times:
  • 20 – 30 F = 15 minutes
  • 30 – 40 F = 12 minutes
  • 40 – 50 F = 9 minutes
  • 50 – 60 F = 6 minutes

Placement

Place concrete using a 1 yd.³ clam-shell, center-discharge funnel hopper. If the concrete is transported in a hopper by a forklift instead of a crane, extra precautions are necessary to avoid unnecessary consolidation. Concrete should be placed within 20 minutes of discharging from the mixer. To avoid excessive air entrapment, this mix is intended to have a maximum drop distance of 48 inches. Forms requiring a drop distance greater than 48 inches will require proper flow-diverting plates or chutes. Vibration is necessary. Refer to the in-plant casting guidelines by looking up the form’s serial number.

Finishing

This mix should be hand troweled immediately after consolidation, screeding and form-volume yielding have been verified.

Curing

For outdoor and indoor forms, use 6-mil-or-greater, non-transparent plastic tarps for covering during initial cure. Jacket form removal should not take place until test cylinders reach a compressive strength of 1,500 psi. The product should not be removed from the form until test cylinders reach a compressive strength of 2,200 psi.

Indoor curing of castings should take place for a minimum of four hours after demolding when outdoor ambient temperatures average above 55 F and a minimum of 20 hours after demolding when outdoor ambient temperatures are at or below 55 F.

Mix 2

Proportioning

Cement (SG = 3.15) = 575 lbs.
Fly Ash (SG = 2.23) = 150 lbs.
89 Stone (SG = 2.75) = 1,587 lbs.
Sand (SG = 2.61) = 1,208 lbs.
Water = 280 lbs. / 33.6 gal.
Polycarboxylate plasticizer = 33 oz.
Calcium accelerator = 150 oz.
Air-entraining admixture = 6 fl. oz.
Air target = 6%

Sequencing

1 yd.³ for a counter current pan mixer that has already mixed a similar batch that day (i.e. preconditioned or buttered):

1. Aggregates / air-entraining admixture
2. Cementitious materials
3. Dry-mix cycle:
   • Dry mix minimum batch time: 75 seconds
   • Dry mix maximum batch time: 200 seconds
4. Water (total weight added to be determined after calculating aggregate free moisture)
5. Accelerating admixture
6. Polycarboxylate plasticizer
7. Wet-mix cycle:
   • Wet mix minimum batch time: 75 seconds
   • Wet mix maximum batch time: 200 seconds
8. Discharge gate opening size and times at 1 yd.³ size:
   • 20% open for first 6 seconds
   • 100% open for remaining time

PFC plan

Conditions

• Acceptable for use in steel-reinforced products
• Fresh concrete temperature: 75 F +/- 10 F
• Forms intended to be poured with this mix: large precast modular block
• Recommended winter aggregate preheat times:
  • 20 – 30 F = 15 minutes
  • 30 – 40 F 12 = minutes
  • 40 – 50 F 9 = minutes
  • 50 – 60 F 6 = minutes

Placement

Place concrete using a 1 yd.³ clam-shell, center-discharge funnel hopper or 1/2 yd.³ side-discharge hopper, if making a 1/2 yd.³-size batch. Do not transport the hopper by forklift after it has been filled. This mix is only intended to be placed using a crane to avoid unnecessary consolidation. Concrete should be placed within 15 minutes of discharging from the mixer.

To avoid excessive air entrapment, this mix is intended to have a maximum drop distance of 36 inches. Forms requiring a drop distance greater than 36 inches will require proper flow-diverting plates or chutes.

Do not vibrate this mix after placement. Certain forms may need light tapping in critical areas using a rubber mallet. Refer to the in-plant casting guidelines for this recipe found by looking up the form’s serial number.

Finishing

This mix should be screeded immediately after placement. A hand trowel can be used in place of a screed to assist with filling the form’s corners.

Curing

For outdoor and indoor forms, use 6-mil-or-greater, non-transparent plastic tarps for covering the product’s unformed surfaces during initial cure, if applicable to the specified form. Jacket form removal should not take place until test cylinders reach a compressive strength of 1,500 psi. The product should not be removed from the form until test cylinders reach a compressive strength of 2,200 psi.

Indoor curing of castings should take place for a minimum of four hours after demolding when outdoor ambient temperatures average above 55 F and a minimum of 20 hours after demolding when outdoor ambient temperatures are at or below 55 F.

Try, try again

As you experiment and refine your mix design, make sure to use your relationships with admixture and cement technical representatives because most of them can offer some form of training. One-on-one, hands-on training at your own facility is incredibly valuable and often underused.

Try re-proportioning the two mixes given in this article for your own materials. Hit the lab, test and refine your designs. Push the limits, record your findings and results, and learn from your mistakes.

Paul Ramsburg has worked in the prestressed precast concrete industry since 1988 and is currently a technical sales specialist at Sika Corp.

Frank Bowen, a 2013 Master Precaster graduate, received his M.B.A. from Middle Tennessee State University through the Concrete Industry Management graduate program in 2014 and is a business development representative with Rosetta Hardscapes in Charlevoix, Mich.

Resources:

• Chapter 9 in the Portland Cement Association’s “Design and Control of Concrete Mixtures,” reference book explains the absolute volume method of proportioning normal concrete mixtures.
• ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete”
• ACI 201.2R-16, “Guide to Durable Concrete”
• ACI 221R-96, “Guide for Use of Normal Weight and Heavyweight Aggregate in Concrete”
• Concrete Mix Evaluator 2.0, copyrighted software developed by Gary Knight