Table of Contents
In this article, we will discuss the flakiness index.
A. Importance of Flakiness Index
The Flakiness Index of aggregate is the percentage by weight of aggregate particles whose least dimension is less than 0.6 of their mean dimensions. This test is applicable to aggregates having a size larger than 6.3mm.
To determine the flakiness index of the given sample of aggregates, the weight of each fraction of aggregates passing and retaining on the defined set of sieves is mentioned first. The pieces of aggregates are prepared to give through the slot of a specified thickness of gauge and then they are weighed.
Then the flakiness index is determined as the total weight of the material goes through different thickness gauges, defined as a percentage of the total weight of the sample gauged.
Flakiness Index = [W2/ W1] x 100%
Where W2= Weight passed from 0.6 x dmean size
W1= Total weight of aggregates
The Flakiness Index of aggregates employed in road construction must be more undersized than 15% and usually does not surpass 25%.
It is not desirable to use flaky particles in the construction of roads, especially in surface courses. This is because when the load operates along the thin axis (along the axis of minimum moment of inertia) of the flaky flat particles then they result in broken down quickly.
In order to bypass such conditions, the particles have to be tested for their flakiness index values to inspect their suitability in employing for the construction of roads. This is the reason for the test being taken.
| Sl No. | Type of Pavement | Maximum limit of flakiness index in % |
| 1 | Bituminous carpet | 30 |
| 2 (i) | Bituminous / Asphaltic concrete | 25 |
| (ii) | Bituminous Penetration Macadam | |
| (iii) | Bituminous Surface Dressing (Single Coat, Double Coat & Precoated) | |
| (iv) | Built-up spray grout | |
| 3 (i) | Bituminous Macadam | 15 |
| (ii) | WBM base course and surface course |
B. Flakiness Index Test for Aggregate
1. Objective:
To determine the flakiness index test of the given sample of course aggregate.
2. Code Of Reference:
• IS 2386(Part 1):
1963 Methods of Test for Aggregates of Concrete- Particle Size and Shape. Reaffirmed Dec 2016
• ASTM 4791-10:
Standard Test Method for Flat Particles, Elongated Particles or Flat Elongated Particles in Coarse Aggregates, American Society for Testing and Materials.
3. Apparatus Required:
a. Weighing balance
Maintaining the accuracy of calculating 0.1% of the weight of the sample.
b. Metal Gauge
It is made of elongated slots of fixed dimensions. The tolerance of measurements shall be ± 0.2 mm for measurements equal to or more than 50 mm and ± 0.1 mm for dimensions less than 50 mm.
c. Sieves
IS Sieves include sizes of 25 mm, 20 mm, 16 mm, 12.5 mm, 10 mm, and 6.3 mm.
4. Procedure:
1. Provided a sample of aggregates to be determined is sieved through a set of sieves and separated into specified ranges of size.
2. At least 200 pieces of any fraction are considered and weighed appropriately.
3. In order to split the flaky materials, the aggregates which go through the appropriate elongated slot of the thickness of the gauge are available.
4. The width of the appropriate slot would be 0.6 times the standard of the range of sizes.
5. The flaky material passing the appropriate slot from each size range of test aggregates is measured and then weighed.
6. This weight is then divided by the total weight of samples taken from a diverse range of sizes and the ratio is defined in percentage which is the preferred flakiness index.
5. Observation:
| Aggregates passing through IS Sieve (mm) | Aggregates retained on IS Sieve (mm) | The thickness of the gauge (0.6 times the mean size of the two sieve sizes) (mm) | Weight of the fraction (g) | Weight of the aggregate in each fraction passing (g) |
| 25 | 20 | 13.5 | ||
| 20 | 16 | 10.8 | ||
| 16 | 12.5 | 8.55 | ||
| 12.5 | 10 | 6.75 | ||
| 10 | 6.3 | 4.89 | ||
| TOTAL | W= | w= | ||
6. Calculation:
W= Total weight of the fraction,
w= Total weight of the passing fraction,
Therefore,
Flakiness Index= [w/W] x 100%
7. Precautions:
a. Safety shoes must be used along with masks and aprons at the time of the test.
b. Special care must be taken so that no outer air enters when using the balance.
c. All parts of equipment should be kept clean.
d. After the end of the test, the sieve should be cleaned with a smooth brush.
e. Gauge should be cleansed entirely and dried before starting the test.
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