Have you ever wondered why some roads deteriorate faster or why certain buildings lose strength over time? One often-overlooked factor is the shape of the aggregate particles used in construction. The Flakiness and Elongation Index tests help determine whether aggregates are excessively flat or elongated, shapes that can compromise the strength, durability, and compaction of concrete or asphalt mixes.
Aggregates with high flakiness or elongation tend to create voids and poor interlocking, leading to weaker load-bearing capacity and reduced structural performance. That’s why these tests are essential components of quality control in civil engineering. Understanding them gives better insight into the long-term reliability of roads, bridges, and buildings. Let us explore more about this flakiness and the elongation index of the aggregate.
Overview of Aggregate Testing
Aggregate testing plays a crucial role in assessing its physical and mechanical properties. These tests are essential in determining the suitability of aggregates such as gravel, crushed stone, and fine aggregates for various construction applications, including road construction, concrete production, and foundation laying.
The aggregate testing process evaluates key characteristics of aggregates such as particle size and shape, density, toughness, durability, and resistance to wear and tear. Shape-related properties, like flakiness and elongation, are particularly important for ensuring workability and strength in concrete and bituminous mixes.
Different tests are used based on what property needs to be checked. The primary goal of aggregate testing is to ensure that the materials meet the required standards for strength, durability, and overall quality, thereby contributing to the construction of safe, long-lasting, and high-performing structures.
What is the Flakiness Index?
The flakiness index reflects the mass percentage of particles having a thickness under 60% of their typical dimension. The test identifies thin or flaky particles that could lead to loss of strength and durability in buildings. This evaluation applies only to particles larger than 6.3 mm in size.
For example, a particle averaging 45mm in size is considered thin if its thickness falls below 27mm. A high flakiness value indicates poor form, which affects density and effectiveness in pavement layers or cement-based applications.
What is the Elongation Index?
The elongation value identifies the weight percentage of particles where the length is over 1.8 times their average size. This test identifies elongated particles that can influence workability and concrete or asphalt mixture strength.
Only aggregate pieces above 6.3mm in size are considered for the elongation index check. Aggregates showing elevated elongation values may lead to poor packing and decreased structural resilience during building work. Controlling the elongation index improves aggregate performance and contributes to the stability and longevity of structures.
Impact of Flakiness and Elongation Index on Construction Projects
- Lower Workability
Elongated and flaky aggregates need more water, making the water-cement ratio higher and the workability of the concrete mixture lower.
- Imperfect Compaction
Such aggregates make it difficult to compact the mixture properly, which gives rise to lower density and less structural strength.
- More Voids
Elongated aggregates fill the mixture with more voids, necessitating more binding material and reducing the quality of the concrete.
- Lower Strength
Increased water requirement creates greater voids, reducing the strength of the concrete and weakening its load-carrying capacity.
- Risk of Breakage
The flat particles have a higher risk of breaking when subjected to pressure that lowers the long-term strength of the structure.
- Weak Bonding
Their geometry and surface area prevent the cement paste from bonding properly, weakening internal bonds and lowering durability.
- Not Suitable for High-Load structures
High flakiness and elongation aggregates are especially problematic in heavy-load or high-rise buildings.
Standards and Specifications for Flakiness and Elongation Index
IS 2386 (Part 1): 1963
- Flakiness Index: Refers to the fraction, based on mass, of stone particles whose narrowest side measures under 60% of their average size.
- Elongation Index: Indicates the weight-based share of pieces where the longest edge exceeds 1.8 times their normal measurement.
- Applicability: Neither test is conducted on particles under 6.3 mm in size.
Apparatus Required for Flakiness and Elongation Index Test
- Balance
An accurate balance to weigh up to 1 gram is needed to measure aggregate weight accurately. This allows for the accurate determination of both thinness and length ratios. Any error in measurement can affect the test results and overall concrete quality.
- Sieves
A standard set of sieves with 63, 50, 40, 31.5, 25, 20, 16, 12.5, 10, and 6.3 mm sizes. The choice is based on the required aggregate gradation sample.
- Thickness Gauge
Used for the calculation of the Flakiness index. The openings in the gauge are 0.6 times the mean size of the aggregates. Permissible deviation is ±0.20 mm for sizes equal to or greater than 50 mm and ±0.10 mm for sizes under 50 mm.
- Length Gauge
Used for the calculation of the elongation index. The slot sizes are set at 1.8 times the typical particle size of the aggregate pieces.
Procedure of Flakiness and Elongation Index Test
- Pass the aggregate material through Indian Standard mesh screens based on the specified particle distribution.
- Take a minimum of 200 particles in each fraction retained on individual sieves.
- Weigh each size fraction carefully and note the weight.
- For the calculation of the Flakiness index:
- Use a thickness gauge featuring slots that are 60% of the average particle size of the aggregates.
- Pass individual particles through the appropriate slot as per their size
- Particles that fit through the gap are identified as thin-shaped.
- Weigh them precisely, ensuring the measurement is accurate within 0.1% of the total sample mass.
- For the elongation index:
- Use a length-measuring device with gaps set at 1.8 times the average size of the particles.
- Inspect to determine if there is retention on the gauge.
- Particles retained are categorised as elongated.
- Measure the elongated particles with the same precision.
- Finally, calculate both the thinness ratio and length ratio using the measured values.
- Flakiness Index (%) = (Weight of flaky particles / Total sample weight) ✕ 100
- Elongation Index (%) = (Weight of elongated particles / Total sample weight) ✕ 100
Bottom Line
Flakiness and elongation index of aggregates play a key role in maintaining construction standards. Excessive flat or elongated shapes of aggregates weaken asphalt or concrete mixtures by compromising workability, compaction, and strength. Shape-related defects result in increased void content, poor bonding, and reduced durability in structures.
Following the standards outlined in IS 2386 (Part 1): 1963 supports uniform procedures and dependable analysis results. Using properly formed aggregates improves structural performance, particularly in load-bearing structures such as roads and high-rise buildings. Regular testing allows for unsuitable materials to be detected early on, promoting stronger, more durable construction and lowering long-term maintenance and repair expenses for infrastructure projects.