What is the Tarantula Curve?
The Tarantula Curve, illustrated in the graph above, is a graphical tool used in concrete mix design to optimize aggregate proportions, minimizing paste content while maintaining a workable mixture. Developed by Tyler Ley, a Professor at Oklahoma State University, the Tarantula Curve emerged from testing hundreds of concrete mixtures with varying aggregate types and water-cement ratios. This research refined the curve, offering a reliable method for balancing aggregates, lowering water demand, and producing durable, cost-effective concrete.
How to Use the Tarantula Curve?
The Tarantula Curve mixture design procedure consists of an iterative process until the optimal balance of aggregate sizes is achieved. This involves adjusting the proportions of fine, intermediate, and coarse aggregates based on sieve analysis data, plotting the aggregate gradation against the tarantula curve, and making necessary modifications to ensure the gradation falls within the curve’s recommended ranges. Each iteration is followed by testing the mix to confirm that the adjustments have improved the concrete’s performance without causing issues such as shrinkage, segregation, or excessive water demand. The process continues until the mix satisfies all performance criteria.
Being within the curve limits does not guarantee a great concrete mixture; two other important factors to consider are paste content and aggregate shape. You need to ensure there's enough paste to properly coat the aggregates and provide workability. Tyler Ley developed a Mixture Design Procedure that offers a solid starting point for determining the right paste content, helping to guide the mix toward optimal performance. Aggregate shape also influences the water demand of your concrete mixture. Ideally, you want angular, cubical, or rectangular-shaped rocks, as they generally contribute to better packing and reduced water demand. In most concrete mixtures, aggregates do not need to be overly smooth, so this aspect can sometimes be overlooked. To assess aggregate shape, ASTM D4791 provides a standard method to measure flatness, elongation, and the overall geometry of particles, ensuring the mix meets design specifications.
The specification for optimized graded concrete using the Tarantula Curve, as well as a spreadsheet to track the iterative process, can be found at www.tarantulacurve.com/recommended-specification1. This resource provides detailed guidance on how to apply the Tarantula Curve to your concrete mix design, ensuring better control over aggregate proportions and helping you achieve consistent, high-quality results.
Why is Optimized Gradation Important?
Care must be taken in choosing the percentage of fine aggregate in a gap-graded mixture. A poor choice can result in concrete that is likely to segregate or honeycomb because of an excess of coarse aggregate. Also, concrete with an excess of fine aggregate could have a high water demand resulting in higher shrinkage. When properly proportioned, however, these concretes are readily consolidated with vibration.
Benefits of Using the Tarantula Curve
The Tarantula Curve offers a range of benefits when designing concrete mixes by optimizing aggregate gradation. Here are some key advantages of using the tarantula curve:
- Enhanced Workability: By maintaining a balanced aggregate distribution, the concrete will flow and compact more easily, reducing the effort needed to place and finish it.
- Improved Durability: Keeping the gradation within the tarantula curve’s limits helps prevent shrinkage and cracking, enhancing the concrete’s long-term durability.
- Cost Efficiency: Optimizing the gradation allows for reductions in cement content, which lowers the overall cost of the concrete mix while maintaining its quality.
- Reduced Environmental Impact: By using less cement and more locally available aggregates, the environmental footprint of the concrete is reduced, contributing to more sustainable construction practices.
