In the realm of construction, the quality of aggregates used significantly influences the properties of freshly mixed concrete. Particle shape and surface texture play a pivotal role, affecting workability and, consequently, the water-cement ratio. Rough-textured, angular, and elongated particles demand more water to produce workable concrete than smooth, rounded compact aggregates. Therefore, it becomes necessary to increase the cement content to maintain the desired water-cement ratio. Generally, flat and elongated particles are restricted to about 15% by weight of the total aggregate to ensure optimal concrete performance.

Objective of Flakiness and Elongation Index test

The objective of the Flakiness and Elongation Index tests is to assess the shape characteristics of aggregates, specifically identifying particles that are excessively flat, elongated, or angular. These irregular shapes can negatively impact the quality of concrete, making these tests essential in the construction industry.

Apparatus Required for Flakiness and Elongation Index test

The necessary equipment includes:

Balance: A precision balance capable of weighing up to 1 gram is required to ensure accurate measurements throughout the testing procedure.

Sieves: A range of sieves with various mesh sizes is necessary, including sizes such as 63 mm, 50 mm, 40 mm, 31.5 mm, 25 mm, 20 mm, 16 mm, 12.5 mm, 10 mm, and 6.3 mm. The choice of sieves depends on the specific aggregate gradation and testing requirements.

Thickness Gauge: For the Flakiness Index test, a specialized thickness gauge is used. The gauge’s dimensions correspond to 0.6 times the mean dimension Of the aggregate.

Length Gauge: To determine the Elongation Index, a length gauge is employed. The length of this gauge should be 1.8 times the mean dimension of the aggregate under examination.

Procedure:

Sample Preparation: Begin by collecting an adequate quantity of aggregate, ensuring it provides a minimum of 200 individual pieces for testing. The sample should represent the specific fraction under examination.

Sieve Analysis: Conduct a sieve analysis of the aggregate sample using the appropriate sieves based on the gradation requirements. The purpose of this step is to separate the aggregate into different size fractions.

Flakiness Index Test:

  • For each size fraction, gauge the particles for thickness using a specialized thickness gauge. This gauge should have dimensions in accordance with the standards and appropriate for the size of the material.
  • Weigh the total amount of material passing through the gauge with precision.
  • Calculate the Flakiness Index by expressing the total weight passing the gauge as a percentage of the total weight of the test sample. This index reveals the degree of flatness or thickness of the particles.

Elongation Index Test:

  • For each size fraction, gauge individual particles for length using a length gauge designed as per the standards and suitable for the aggregate size.
  • Weigh the total amount of material retained by the length gauge with accuracy.
  • Calculate the Elongation Index by expressing the total weight retained on the length gauges as a percentage of the total weight of the test sample. This index indicates the extent to which particles exhibit elongated characteristics.

Table 1: Dimensions (Length and Thickness) of Gauges

Passing through IS Sieve (mm)Retained on IS Sieve (mm)Thickness Gauge (mm)Length Gauge (mm)
635033.90
504027.0081.0
402519.5058.5
31.52516.95
252013.5040.5
201610.8032.4
1612.58.5525.6
12.5106.7520.2
10.06.34.8914.7

– The “Thickness Gauge” column represents the dimensions for the thickness gauge used in the Flakiness Index test.

– The “Length Gauge” column represents the dimensions for the length gauge used in the Elongation Index test.

– Dimensions are specified for aggregates passing through IS sieves as well as those retained on IS sieves.

– The “Thickness Gauge” dimensions are equal to 0.6 times the mean sieve size.

– The “Length Gauge” dimensions are equal to 1.8 times the mean sieve size.

These dimensions are critical for ensuring that the Flakiness and Elongation Index tests are conducted accurately in accordance with established standards.

Observation and Recording

During the Flakiness and Elongation Index tests, it’s essential to carefully observe and accurately record the measurements. Here’s how to record the observations for both tests:

Flakiness Index (Table 2)

In the Flakiness Index test, record the following observations:

Passing through IS Sieves (mm)Retained on IS Sieves (mm)Flakiness Gauge (mm)
635033.90
504027.00
402519.50
31.52516.95
252013.50
201610.80
1612.58.55
12.5106.75
106.34.89
Total

In this table:

– Record the sizes of aggregates passing through IS sieves and those retained on IS sieves.

– Note the dimensions of the Flakiness Gauge for each size fraction.

– After conducting the test, weigh the total amount of material passing through the Flakiness Gauge for each size fraction.

– Calculate the percentage weight passing for each size fraction by expressing the total weight passing the gauge as a percentage of the total weight of the test sample.

Elongation Index (Table 3)

In the Elongation Index test, record the following observations:

Passing through IS Sieves (mm)Retained on IS Sieves (mm)Elongation Gauge (mm)
6350
504081.00
402558.5
3125
252040.50
201632.4
1612.525.6
12.51020.2
106.314.7
Total

In this table:

– Record the sizes of aggregates passing through IS sieves and those retained on IS sieves.

– Note the dimensions of the Elongation Gauge for each size fraction.

– After conducting the test, weigh the total amount of material retained on the Elongation Gauge for each size fraction.

– Calculate the percentage weight retained for each size fraction by expressing the total weight retained on the gauge as a percentage of the total weight of the test sample.

Accurate and detailed observation and recording are crucial to obtaining reliable results in the Flakiness and Elongation Index tests, providing valuable data about the shape characteristics of aggregates used in construction.

Calculation

Calculation

After completing the Flakiness and Elongation Index tests and recording the necessary observations, the final step involves calculating these indices. Here’s how to calculate both the Flakiness Index and Elongation Index:

Flakiness Index:

The Flakiness Index of an aggregate is calculated using the following formula:

Flakiness Index (%) = (Total weight passing Flakiness Gauge x 100) / Total weight of the test sample

To calculate the Flakiness Index for each size fraction, plug in the values obtained during the test:

– For each size fraction, use the total weight of material passing the Flakiness Gauge.

– Divide this weight by the total weight of the test sample for that size fraction.

– Multiply the result by 100 to express the Flakiness Index as a percentage.

Elongation Index:

The Elongation Index of an aggregate is calculated using the following formula:

Elongation Index (%) = (Total weight retained on Elongation Gauge x 100) / Total weight of the test sample

To calculate the Elongation Index for each size fraction, use the following steps:

– For each size fraction, use the total weight of material retained on the Elongation Gauge.

– Divide this weight by the total weight of the test sample for that size fraction.

– Multiply the result by 100 to express the Elongation Index as a percentage.

These calculations provide valuable insights into the shape characteristics of the aggregates, helping engineers and construction professionals make informed decisions regarding the suitability of the material for concrete production and other construction applications.

(Know about Stripping Value Test of Aggregate .)

FAQs:

What is the significance of flakiness and elongation index?

Flakiness and elongation are properties used to assess the shape and size distribution of aggregates in construction materials like concrete and asphalt.
Flakiness Index helps measure the flatness or thickness of an aggregate particle. Low flakiness indicates that the aggregate has a more cubical or spherical shape, which is desirable for good interlocking in concrete mixes.
Elongation Index assesses the elongated or elongated-to-thickness ratio of aggregate particles. Lower elongation values indicate that the aggregate particles are closer to being equidimensional, contributing to better workability and strength in construction materials.

What is the flakiness index of aggregate?

Flakiness Index of Aggregate:
The Flakiness Index of an aggregate is a quantitative measure that indicates the percentage of elongated and flat particles in a given sample.
It is calculated by measuring the dimensions of aggregate particles passing through specified sieve sizes and comparing the length of the longest dimension to the thickness.

What is the limit of flakiness and elongation index?

The Flakiness Index should not exceed 25%, while the Elongation Index should not exceed 35%.