Aggregates: Total Gradation Curves & Graphs pt 3

Optimizing Total Aggregate Gradation in Volumetric Mixing: Curves & Graphs

Achieving the ideal concrete mix requires more than just selecting good quality fine and coarse aggregates—it demands a strategic gradation approach to balance workability, strength, and durability. For volumetric concrete mixers, this challenge is even greater, as operators must work with only two aggregate compartments while handling multiple aggregate types. This limitation means that either an aggregate compromise or preblending is often necessary to achieve an acceptable combined gradation.

To ensure consistent and high-performing concrete, three key tools can be leveraged:
✔ The Tarantula Curve (for overall gradation balance)
✔ The 0.45 Power Graph (for optimizing particle packing)
✔ The Shilstone Workability Graph (for practical performance evaluation)

By understanding how to apply these tools and adapting them to the unique 53/47 aggregate compartment split in volumetric mixers, operators can create optimized, well-graded concrete mixes for a wide range of applications.

Understanding Total Aggregate Gradation

Total aggregate gradation refers to the proportions of fine and coarse aggregates in a mix, affecting workability, finishability, strength, and durability. A well-graded mix has a smooth particle size distribution, reducing the need for excess cement paste while improving density and minimizing voids.

Challenges in Volumetric Mixer Aggregate Gradation

1. Limited Aggregate Bins: Unlike ready-mix plants with multiple bins, volumetric mixers typically have only two aggregate compartments, meaning multiple materials must be preblended.
2. Fixed Bin Volume Split: The 53% / 47% compartment ratio means that any adjustments to fine/coarse blending must be accounted for in the blend or loading in order to get the mixer to pour off “evenly.”
3. Variability & Availability in Materials: Aggregates from different suppliers often vary in gradation, making regular testing and adjustments essential. The availability and quality of the materials will vary by region. This is the practical reality of the concrete and aggregates business.

 Applying the Tarantula Curve for Optimized Gradation

The Tarantula Curve is a particle distribution guideline designed to improve packing efficiency and concrete performance. It will also point to the viability for the mix to be used in pumping and slip forming applications. It focuses on balancing coarse, intermediate, and fine particles to avoid:
❌ Gap-graded concrete (which results in poor finishing and excessive paste requirements)
❌ Overly fine mixes (which increase water demand and reduce strength)

How Volumetric Mixers Can Use the Tarantula Curve

1. Aggregate Testing: Conduct sieve analysis of both fine and coarse aggregates and compare results to the Tarantula Curve recommendations.
2. Preblend for Proper Distribution: If a third aggregate is necessary to fill a gap in gradation, preblend materials before loading the mixer bins.
3. Adjust Mix Design as Needed: Modify the fine-to-coarse ratio in the preblend to achieve better packing while still fitting within the 53/47 volumetric split.

Using the 0.45 Power Graph for Improved Packing Density

The 0.45 Power Graph is a tool that helps determine ideal aggregate packing by plotting sieve size vs. percent passing using a 0.45 power curve. Well-graded aggregates should closely follow the trendline to maximize packing efficiency and reduce voids.

Steps to Utilize the 0.45 Power Graph in Volumetric Mixing

✔ Plot Sieve Analysis Data: Graph individual aggregate gradations and the combined gradation (including any preblend).
✔ Compare to the 0.45 Line: If the total gradation deviates significantly, adjust the fine/coarse aggregate proportions.
✔ Modify Preblending Process: Adjust blended stockpiles to better align with the ideal curve before loading the mixer.

🔹 Why This Matters?
Aligning with the 0.45 Power Graph minimizes excess cement paste demand, improving strength, pumpability, and finishability while reducing material costs.

Applying the Shilstone Workability Graph for Field Performance

While the Tarantula Curve and 0.45 Power Graph focus on particle size distribution, the Shilstone Workability Graph ensures the mix is practical and field-friendly. This tool evaluates:
🔹 Coarseness Factor (CF) → Balance of coarse vs. fine aggregates
🔹 Workability Factor (WF) → Fine aggregate percentage and mix fluidity

How to Use the Shilstone Graph in Volumetric Mixing

1. Determine Coarseness Factor (CF):
• CF = (Percent retained on #8 sieve / Percent retained on #200 sieve) x 100
• This measures how coarse or fine the overall blend is.
2. Determine Workability Factor (WF):
• WF = Percent passing the #8 sieve
• This reflects the mix’s paste and fine aggregate content.
3. Plot the Mix and Adjust:
• Ideal mixes fall within the workability “sweet spot” on the graph.
• If the mix is too coarse, increase fines in the preblend.
• If the mix is too fine, adjust coarse aggregate proportions.

🔹 Why This Matters?
This step ensures the mix is not only well-graded but also performs well in real-world applications—whether for paving, structural work, or general construction.

 Key Considerations for Preblending Aggregates in Volumetric Mixers

Since volumetric mixers only have two aggregate bins, preblending can be necessary when using more than one type of coarse or fine aggregate.

Best Practices for Preblending

✔ Understand the 53/47 Bin Ratio

• Ensure the preblended aggregate mix fits within this ratio before loading.

✔ Use Stockpile Blending for Consistency

• Preblend materials at the yard using a loader or mixer before loading into bins.
• Preblend aggregates that are close in size to avoid segregation.

✔ Adjust Based on Testing Data

• Run regular gradation tests to maintain consistency in blended aggregates.

✔ Keep Workability in Mind

• Ensure the final blend aligns with Shilstone workability recommendations.

Final Thoughts: Achieving an Optimized Aggregate Gradation

For volumetric concrete mixer operators, mastering total aggregate gradation is an overlooked topic for producing consistent, high-performance concrete. By utilizing:

✅ The Tarantula Curve (for balanced gradation)
✅ The 0.45 Power Graph (for optimized packing)
✅ The Shilstone Workability Graph (for real-world mix performance)

Operators can optimize material selection, improve concrete quality, and reduce material costs while working within the limitations of a two-bin volumetric mixer. As an additional note, getting an exact match to any one of these curves is not necessary. They are a tool and indicator. Not an absolute law of physics. You can use some combination of aggregates that works perfectly fine while not matching an “ideal” version of a curve. It takes making concrete and testing the results with cylinders, placing and finishing to determine the actual performance.

By implementing preblending strategies and regularly analyzing aggregate gradation, volumetric operators can achieve superior concrete performance in every pour.

For expert consulting and volumetric mix optimization, visit www.concretebs.com.