Understanding Flaky and Elongated Aggregates: Impact and Reduction Strategies

Flaky and elongated aggregates are irregularly shaped particles that can affect concrete strength, workability, and durability.

Flaky aggregates are thin and flat, while elongated aggregates are unusually long. Excessive amounts can reduce compaction and increase voids.

This guide will discuss their effect on concrete, testing procedures, and possible methods of reducing their effect.

What Are Flaky and Elongated Aggregates?

Flaky aggregates are coarse aggregate particles that are unusually thin compared to their length and width. In contrast, elongated aggregates are particles that are excessively long compared to their width and thickness.

Flaky aggregates have a thickness less than 0.6 times the mean sieve size, while elongated aggregates have a length greater than 1.8 times the mean sieve size.

The following are the main differences between flaky and elongated aggregates:

Read our aggregates in construction guide to understand aggregate types, classifications, and selection factors.

Impact of Flaky and Elongated Aggregates on Concrete Properties

Excessive flaky and elongated aggregates can reduce concrete strength, workability, and compaction efficiency. Their irregular shapes affect particle packing and may create weak points within the concrete mix.

The major effects of flaky and elongated aggregates on concrete properties:

• Reduced Concrete Strength: Irregular particles may fracture under heavy loads, increasing cracking risks.
• Poor Workability: The friction between the particles increases, making the concrete more difficult to mix, place, and finish.
• Increased Water Demand: Higher water requirements can reduce concrete strength if the water-cement ratio increases.
• Difficult Compaction: Poor particle packing can trap air and increase voids.
• Reduced Bituminous Mix Performance: Particle breakage and poor packing can lead to pavement durability issues under traffic loads.

Flakiness and Elongation Index: Key Quality Parameters

The Flakiness Index (FI) and Elongation Index (EI) measure the percentage by mass of irregularly shaped particles in an aggregate sample.

• Flakiness Index (FI): Measures the percentage by weight of particles whose thickness is less than 0.6 times the mean sieve size. Engineers use a thickness gauge to identify flaky particles. The index is calculated by dividing the weight of flaky particles by the total sample weight and multiplying by 100.
  
• Elongation Index (EI): Measures the percentage by weight of particles whose length exceeds 1.8 times the mean sieve size. Engineers use a length gauge to identify elongated particles. The index is calculated by dividing the weight of elongated particles by the total sample weight and multiplying by 100.

Testing Procedure for Flakiness and Elongation Index

The flakiness and elongation index tests measure aggregate particle shape using thickness and length gauges. Both of the tests are conducted as per IS 2386 (Part 1):1963, which defines the testing procedure for coarse aggregate.

Apparatus required:

• Standard IS sieves
• Thickness gauge
• Length gauge
• Weighing balance

The following procedure should be used to calculate the flakiness and elongation values:

• Step 1: Prepare the Aggregate Sample: Obtain an aggregate sample that represents a large number of particles of each size fraction to be tested, with a minimum of 200 particles per size fraction.
• Step 2: Sieve the Aggregates: Using the standard IS sieves, separate the sample into different size fractions. Choose the correct thickness and length gauges for each fraction.
• Step 3: Separate Flaky Particles: Pass each size fraction through the corresponding thickness gauge. Collect the particles passing through the gauge and record their count.
• Step 4: Measure and Calculate the Flakiness Index: Weigh the flaky particles and record the total mass of each of the sieve fractions. The formula to calculate FI is below.

Flakiness Index (%) = (Total mass passing thickness gauges / Total mass of tested sample) × 100

• Step 5: Separate Elongated Particles: Test each applicable size fraction using the corresponding length gauge. Collect the particles retained by the gauge.
• Step 6: Measure and Calculate the Elongation Index: Weigh the elongated particles that remain on the length gauge. The formula to calculate EI is below.

Elongation Index (%) = (Total mass retained on length gauges / Total mass of applicable test sample) × 100

• Step 7: Record and Verify the Results: Record the mass of each size fraction and calculated indices. Make a comparison with applicable Indian Standards and project specifications.

What Are the Standard Limits for Flakiness and Elongation?

Acceptable limits depend on the specification and the use, so always check the project specification and the current edition of IS 383 before accepting a load. Many references quote a combined flakiness and elongation figure of around 30 per cent as a common ceiling, and stricter limits apply to wearing courses and high-grade work.

Effects of High Flakiness and Elongation Indices

High indices may minimise the performance of the concrete, decrease the durability of the structure, and increase maintenance costs.

The following are the significant implications:

• Increased voids may allow moisture penetration and reduce durability.
• Heavy loading can cause thin particles to break.
• Early cracking can cause higher repair costs.
• Particle breakage during road compaction may cause ravelling.
• Inadequate packing can lead to higher demand for cement paste and material cost.

Strategies to Reduce Flakiness and Elongation

The right material selection and controlled crushing and screening processes can help to minimise flakiness and elongation in the aggregates. The use of appropriate crushing machinery and periodic quality checks helps produce more cubical, uniformly shaped aggregates.

The following are effective measures to minimise flakiness and elongation in aggregates:

• Choose Quality Aggregates: Use aggregates from trusted quarries known for their consistent rock quality. Select well-graded materials, with acceptable flakiness and elongation index.
• Use Jaw Crushers: Use jaw crushers to break down large rocks into smaller pieces. Additional crushing might be needed to enhance particle shape.
• Use Cone Crushers: Secondarily crush with cone crushers with proper adjustment. These aid in reducing elongated particles and shape.
• Use Vertical Shaft Impact (VSI) Crushers: These crushers impart high-speed impact, which breaks the particles that are irregular in shape and generates cubical aggregates.
• Improve Screening and Reprocessing: Use suitable screens to separate aggregate sizes. Recrush poorly shaped particles where necessary to improve the final product.

Benefits of Proper Aggregate Shape

Well-graded aggregates can enhance the strength, workability, and durability of concrete by providing good particle packing.

The main advantages:

• Enhances load distribution and compression.
• Minimises friction and enhances workability.
• Compacts the mix uniformly with fewer voids.
• Reduces water penetration and premature deterioration.
• Reduces long-term maintenance needs.

Read our angular aggregates guide to understand how particle shape, surface texture, and interlocking affect concrete strength and workability.

Next Steps for Construction Professionals

Controlling flaky and elongated aggregates is essential for maintaining concrete strength, workability, and long-term durability. The quality of construction can be improved by proper selection, testing, and crushing of aggregates.

Select aggregates that meet IS 383 requirements and project specifications. Deal with good sources, check the quality of the aggregate, and seek advice from qualified civil engineers and material testing laboratories as required.

Frequently Asked Questions

What are flaky and elongated aggregates?

Flaky aggregates are thin and flat, with a thickness below 0.6 times the mean sieve size. Elongated aggregates are unusually long, with a length above 1.8 times the mean sieve size.

How do you calculate the flakiness and elongation index of an aggregate?

Pass each size fraction through the thickness gauge for flakiness and the length gauge for elongation. Divide the mass of flaky or elongated particles by the total sample mass and multiply by 100, as per IS 2386 (Part 1).

What are the effects of flaky and elongated aggregates on the quality of bituminous mixes?

Flaky and elongated aggregates can lead to poor compaction and particle breakage, reducing pavement performance and increasing the risks of cracking and ravelling.

How do flaky and elongated aggregates influence the strength of concrete?

Thin or long particles pack poorly and may fracture under load, which leaves voids and weak points. The result is lower strength and a higher risk of cracking.

What is the allowable percentage of flaky and elongated aggregates in concrete mixes?

It depends on the specification and the use. A combined figure of around 30 per cent is often quoted as a ceiling, but check IS 383 and your project specification for the limit that applies.

What are the consequences of using excessive flaky and elongated aggregates in concrete?

Too much flaky or elongated aggregates may lead to compaction problems, low durability, cracking problems, and high maintenance requirements.

How can the workability of concrete be improved when using flaky and elongated aggregates?

Improve workability by optimising aggregate grading, blending better-shaped particles, and using suitable plasticisers or superplasticisers without adding excess water.

What tests are conducted to assess the performance of concrete with flaky and elongated aggregates?

The Flakiness Index and Elongation Index tests under IS 2386 (Part 1) assess particle shape. Sieve analysis checks grading against IS 383.