Introduction to Flakiness & Elongation Index Tests
Shape characteristics of aggregates are crucial factors affecting the workability, strength, and durability of concrete. Flakiness Index and Elongation Index are two important tests that assess the shape properties of coarse aggregates, particularly their dimensional ratios.
These tests are standardized in IS:2386 (Part I)-1963 "Methods of Test for Aggregates for Concrete - Particle Size and Shape" and are essential for quality control in construction projects.
Flakiness Index: The percentage by weight of particles in an aggregate sample whose least dimension (thickness) is less than 0.6 times their mean dimension.
Elongation Index: The percentage by weight of particles in an aggregate sample whose length is greater than 1.8 times their mean dimension.
Why Test for Flakiness & Elongation?
Understanding the shape characteristics of aggregates is vital for several reasons:
Workability
Excessive flaky or elongated particles reduce concrete workability and make compaction difficult.
Strength
Angular, cubical particles create stronger interlocking bonds than flaky or elongated ones, improving concrete strength.
Durability
Flaky and elongated particles tend to break more easily under stress, reducing concrete durability.
Water Demand
Flaky particles have higher surface area, increasing water requirement and affecting water-cement ratio.
Most specifications limit the Flakiness and Elongation Indices to a maximum of 35% for concrete aggregates and 25-30% for road construction aggregates. Higher percentages can lead to poor performance and durability issues.
Required Equipment
Before beginning the tests, ensure you have the following equipment ready:
Balance
A precision balance with accuracy of at least 0.1g for weighing samples.
IS Sieves
Standard sieves of sizes 63.0mm, 50.0mm, 40.0mm, 31.5mm, 25.0mm, 20.0mm, 16.0mm, 12.5mm, 10.0mm, and 6.3mm.
Thickness Gauge
Metal gauge with slots of different widths for Flakiness Index testing.
Length Gauge
Metal gauge with openings of different lengths for Elongation Index testing.
Oven
For drying samples at 110±5°C to constant weight if needed.
Trays & Containers
For handling and separating aggregate fractions.
Flakiness Index Test Procedure
The Flakiness Index test determines the percentage of flaky particles in a coarse aggregate sample. Follow these steps to conduct the test properly:
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Sample Preparation
Collect a representative sample of the aggregate. For accurate results, use at least 2-3kg of material. Ensure the sample is washed and dried to remove any dust or impurities.
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Sieve Analysis
Conduct a sieve analysis using standard sieves (63.0mm, 50.0mm, 40.0mm, 31.5mm, 25.0mm, 20.0mm, 16.0mm, 12.5mm, 10.0mm, and 6.3mm). Discard any material passing through the 6.3mm sieve.
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Fraction Separation
Separate the aggregate into different size fractions (e.g., 63.0-50.0mm, 50.0-40.0mm, etc.). Weigh each fraction and record the weights. Discard any fraction with less than 200g of material.
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Thickness Measurement
For each size fraction, use the appropriate slot in the thickness gauge. The slot width for each fraction is 0.6 times the mean dimension of the fraction. Pass each particle through the corresponding slot of the gauge.
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Weighing Flaky Particles
Collect all particles that pass through the slot (flaky particles) for each fraction separately. Weigh these flaky particles for each fraction and record the weights.
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Calculation
Calculate the Flakiness Index using the formula:
Flakiness Index (%) = (Weight of particles passing through the slots / Total weight of the sample) × 100
The thickness gauge has slots with widths corresponding to 0.6 times the mean dimension of each sieve fraction:
- For 63.0-50.0mm fraction: 33.90mm slot width
- For 50.0-40.0mm fraction: 27.00mm slot width
- For 40.0-31.5mm fraction: 21.50mm slot width
- For 31.5-25.0mm fraction: 16.95mm slot width
- For 25.0-20.0mm fraction: 13.50mm slot width
- For 20.0-16.0mm fraction: 10.80mm slot width
- For 16.0-12.5mm fraction: 8.55mm slot width
- For 12.5-10.0mm fraction: 6.75mm slot width
- For 10.0-6.3mm fraction: 4.89mm slot width
Elongation Index Test Procedure
The Elongation Index test identifies the percentage of elongated particles in a coarse aggregate sample. Follow these steps to conduct the test properly:
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Sample Preparation
Use the same sample preparation method as for the Flakiness Index test. Sieve the aggregate and separate it into different size fractions.
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Fraction Separation
Similar to the Flakiness Index test, separate the aggregate into different size fractions. Weigh each fraction and record the weights.
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Length Measurement
For each size fraction, use the corresponding opening in the length gauge. The length gauge opening for each fraction is 1.8 times the mean dimension of the fraction.
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Testing for Elongation
Place each particle with its longest dimension perpendicular to the length gauge opening. If the particle cannot pass through the opening, it is considered elongated. Collect all such elongated particles for each fraction.
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Weighing Elongated Particles
Weigh the elongated particles (those retained on the length gauge) for each size fraction separately and record the weights.
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Calculation
Calculate the Elongation Index using the formula:
Elongation Index (%) = (Weight of elongated particles retained on the length gauge / Total weight of the sample) × 100
The length gauge has openings with lengths corresponding to 1.8 times the mean dimension of each sieve fraction:
- For 63.0-50.0mm fraction: 90.0mm gauge length
- For 50.0-40.0mm fraction: 81.0mm gauge length
- For 40.0-31.5mm fraction: 64.5mm gauge length
- For 31.5-25.0mm fraction: 50.8mm gauge length
- For 25.0-20.0mm fraction: 40.5mm gauge length
- For 20.0-16.0mm fraction: 32.4mm gauge length
- For 16.0-12.5mm fraction: 25.6mm gauge length
- For 12.5-10.0mm fraction: 20.2mm gauge length
- For 10.0-6.3mm fraction: 14.7mm gauge length
Sample Calculations
Flakiness Index Calculation Example
Let's walk through a sample calculation for the Flakiness Index test:
| Sieve Size Range (mm) | Weight of Fraction (g) | Weight Passing Through Slot (g) | % Flaky Particles |
|---|---|---|---|
| 40.0 - 31.5 | 500 | 125 | 25.0 |
| 31.5 - 25.0 | 700 | 210 | 30.0 |
| 25.0 - 20.0 | 600 | 150 | 25.0 |
| 20.0 - 16.0 | 400 | 120 | 30.0 |
| 16.0 - 12.5 | 300 | 75 | 25.0 |
| Total | 2500 | 680 | - |
Flakiness Index (%) = (Weight of particles passing through the slots / Total weight of the sample) × 100
Flakiness Index (%) = (680 / 2500) × 100 = 27.2%
The calculated Flakiness Index of 27.2% is below the maximum limit of 35% specified by most standards, indicating that the aggregate has acceptable shape characteristics for concrete applications.
Elongation Index Calculation Example
Now, let's calculate the Elongation Index using sample data:
| Sieve Size Range (mm) | Weight of Fraction (g) | Weight Retained in Gauge (g) | % Elongated Particles |
|---|---|---|---|
| 40.0 - 31.5 | 500 | 150 | 30.0 |
| 31.5 - 25.0 | 700 | 175 | 25.0 |
| 25.0 - 20.0 | 600 | 180 | 30.0 |
| 20.0 - 16.0 | 400 | 100 | 25.0 |
| 16.0 - 12.5 | 300 | 90 | 30.0 |
| Total | 2500 | 695 | - |
Elongation Index (%) = (Weight of elongated particles retained on the length gauge / Total weight of the sample) × 100
Elongation Index (%) = (695 / 2500) × 100 = 27.8%
The calculated Elongation Index of 27.8% is below the maximum limit of 35%, indicating that the aggregate has an acceptable percentage of elongated particles for most construction applications.
Combined Flakiness & Elongation Indices
In some advanced applications, a combined index may be required to get a more comprehensive assessment of aggregate shape:
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Flakiness and Elongation Test
First, determine the flaky particles using the thickness gauge.
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Test Non-Flaky Particles
Take the particles that did not pass through the thickness gauge (non-flaky particles) and test them with the length gauge to determine elongated particles.
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Calculate Combined Index
The Combined Index represents the percentage of particles that are either flaky or elongated or both:
Combined Index (%) = (Weight of flaky particles + Weight of elongated non-flaky particles) / Total weight × 100
The combined index is particularly useful for critical applications like high-performance concrete, airport runways, or heavy-duty pavements, where aggregate shape characteristics significantly affect performance.
How the Flakiness & Elongation Index Calculator Works
Our Flakiness & Elongation Index Calculator automates the entire testing and calculation process, making it convenient and error-free. Here's how it works:
Flakiness & Elongation Index Calculator
The calculator streamlines the entire process through a user-friendly interface:
Input Data Entry
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Select Test Type
Choose between Flakiness Index or Elongation Index test using the tab selector.
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Enter Total Sample Weight
Input the total weight of your aggregate sample in grams.
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Input Size Fraction Data
For each size fraction (e.g., 63.0-50.0mm, 50.0-40.0mm, etc.), enter:
- Weight of the fraction
- Weight of particles passing through the slot (for Flakiness Index)
- Weight of particles retained in the length gauge (for Elongation Index)
Calculation Process
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Data Validation
The calculator validates all inputs to ensure they are reasonable and mathematically consistent (e.g., checking that passing weights don't exceed fraction weights).
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Individual Fraction Calculations
For each size fraction, the calculator computes:
- Percentage of flaky or elongated particles within the fraction
- Contribution of the fraction to the overall index -
Index Computation
The calculator aggregates data from all fractions to compute:
- Total weight of fractions used in the test
- Total weight of flaky or elongated particles
- Overall Flakiness or Elongation Index -
Standards Compliance Check
The calculator automatically compares the calculated index against standard limits (typically 35% for general construction) and provides a PASS/FAIL assessment.
Results and Output
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Tabular Results
The calculator displays a comprehensive table showing:
- Details for each size fraction
- Total weights and percentages
- Visual indicators for values exceeding limits -
Summary Card
A summary card prominently displays:
- The calculated Flakiness or Elongation Index
- PASS/FAIL status based on standards
- Interpretive guidance on the results -
Export Options
The results can be:
- Downloaded as a PDF report for documentation
- Saved to the application for future reference
- Shared with stakeholders directly from the application
The calculator offers several advantages over manual calculations:
• Eliminates mathematical errors and ensures consistency
• Saves significant time, especially for large batches of tests
• Provides instant compliance assessment against standards
• Generates professional reports for quality documentation
• Maintains a history of tests for tracking material quality over time
Practical Applications & Specifications
Flakiness and Elongation Indices are used in various construction applications with different specification requirements:
Structural Concrete
Maximum 35% for both indices. Lower values produce better workability and strength in structural elements.
Highway Pavements
Maximum 25-30% for both indices. Stricter requirements due to repetitive traffic loading.
Airport Runways
Maximum 20% for both indices. Demands superior aggregate quality due to high-stress conditions.
Hydraulic Structures
Maximum 30% for both indices. Balanced requirements for strength and permeability.
Mitigation Strategies for High Indices
| Issue | Potential Cause | Solution |
|---|---|---|
| High Flakiness Index | Improper crushing method at quarry | Use impact crushers instead of compression crushers |
| High Elongation Index | Natural elongated rock structure | Secondary crushing to break elongated particles |
| Both indices too high | Unsuitable rock source | Consider alternative aggregate sources or blend with better-shaped aggregates |
| High variability in indices | Poor quality control at source | Implement consistent production and testing protocols |
While flaky and elongated particles should be minimized, complete elimination is neither practical nor necessary. The key is to keep them within specified limits to balance cost and performance considerations.
References & Resources
For further information on Flakiness and Elongation Index testing, refer to the following standards and resources:
- IS:2386 (Part I)-1963 - Methods of Test for Aggregates for Concrete - Particle Size and Shape
- IS:383-2016 - Specification for Coarse and Fine Aggregates from Natural Sources for Concrete
- BS 812: Part 105 - Methods for Determination of Particle Shape
- ASTM D4791 - Standard Test Method for Flat Particles, Elongated Particles, or Flat and Elongated Particles in Coarse Aggregate
- MORTH Specifications for Road and Bridge Works