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What is the flakiness index test of coarse aggregates?/ Importance of flakiness index test.

 1. What is the flakiness index of coarse aggregates?

The flakiness index of coarse aggregate is the percentage by weight of aggregate particles whose least dimension is less than 0.6 of their mean dimension. 

What does that mean?

Let us observe the metal thickness gauge as shown below.

The capsule-shaped openings for various sizes are provided over the thickness gauge.

The length of the given biggest opening is 100mm. This opening is made for passing the coarse aggregates having sizes between 50 to 63mm. (elongated size)

Therefore, the mean dimension of the aggregate

   = (50mm. + 63mm.) ÷ 2

   = 56.5mm.

The least dimension is the flattened or flaky size of the aggregate

= [0.6 🇽 mean dimension.]

= [0.6 🇽 56.5mm.]

= 33.9mm.

Therefore, the width side of the opening has a measurement of 33.9mm.

Similarly, all the capsule openings are made having the ratio of 1: 0.6, where 1 stands for the mean dimension of aggregate to be passed & 0.6 for the width or flaky side of the aggregate.

In other words, 

Flakiness index 

= [total wt. (W2) of aggregate sample passing through the various thickness gauge openings ÷ total wt.(W1) of an aggregate sample] х100

FI = [W2 ÷ W1] х 100


The flakiness index test is not applicable to the aggregates retained over 63mm. sieve & passing through the 6.3mm. sieve. 

2. Why do we need a flakiness index test? 

A flakiness index test is conducted to determine the shape of the aggregates. The presence of flaky or flattened coarse aggregates is undesirable in cement concrete, in making bituminous roads, & in the construction of pavements. 

Flaky-shaped aggregates are weak in creating a good bond & compaction in the concretes. They may break due to heavy traffic loads, which may result in failures or forming cracks over the road surfaces.

So, we conduct the flakiness index test to check whether the given aggregates have flaky particles within the specified limits.

3. What is the significance or importance of the flakiness index test?

The flakiness index test helps to maintain the quality of the construction works by minimizing the following defects arising due to the coarse aggregates.

1. The flaky or elongated aggregate particles have a high surface area-to-volume ratio. This reduces the workability of concrete.

2. Flaky aggregates affect the degree of packing or compaction of given mixtures.

3. Flaky particles create a weak bond that reduces the strength of the concrete.

4. Roads or pavements are subjected to regular loads & vibrations due to the moving traffic. Flaky aggregates are not good at taking the desired loads, which eventually break down causing the failure of the structure. 

5. Flakiness index test helps to find out the percentage of undesirable particles present in the sample & to choose the aggregates of the required quality for the particular type of work.

4. Which code is referred to for the flakiness index test?

1. Methods of test for aggregates of concrete.

Part-1, particle size & shape

 IS code 2386 (part 1): 1963

2. ASTM 4791-10 

Standard test method for flat particles, elongated particles, or flat elongated particles in coarse aggregates.

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👀. Flakiness index test for coarse aggregates.

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