Elongation Index Test on Coarse Aggreagtes

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Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than one and four-fifth times (1.8 times or 9/5 times) their mean dimension. It is measured on particles passing through mesh size of 63mm and retained on mesh size of 6.3mm.

Necessity of Elongation Index Test

Presence of elongated aggregates in a mix disturbs the packing of particles and creates more space. Elongated particles have high ratio of surface area to volume which reduces the workability of concrete. If elongated particles are used for pavement base course construction, they may break down easily under heavy loads which will cause damage to the pavement. So, it is necessary to know the elongation index of given aggregate mix.

Apparatus required

The following apparatus is required to determine the Elongation index of the coarse aggregates,

• Aggregate sample
• Digital balance
• Elongation gauge or length gauge
• Test sieves of sizes - 63mm, 50mm, 40mm, 31.5mm, 25mm, 20mm, 16mm, 10mm and 6.3mm.

Test Procedure

The elongation index test procedure contains following steps.

1. Take the aggregate sample which contains minimum 200 pieces of any fraction.
2. Sieve the sample through mentioned sieves i.e., from 63mm to 6.3mm sieves.
3. The aggregate retained on 63mm and passed through 6.3mm should not be considered.
4. The aggregate passed through 63mm and retained on 6.3mm and its above sieves is taken along with sieves. Now we have specified size range for every particle.
5. Now weigh aggregate retained on different sieve and note down the total weight as “W1”
6. The aggregate particles from each of the size range is taken and passed through corresponding gauge of length gauge or elongation gauge with its longest side.
   

   Fig 3 : Conducting Elongation Index Test

7. Each gauge length is 1.8 times the mean dimension of aggregate size range. If particle retained on 10mm and passed through 16mm size is taken, then it should pass through 23.4mm length gauge which is 1.8 times the mean of 10mm and 16mm.
8. To keep accuracy and eliminate testing bias, the particle should be passed multiple times with different facings.
9. The particles which are not passed or retained on the length gauge are weighed which is “W2”.
10. Now calculate the elongation index which is the ratio of weight of particle retained on length gauge to the total weight of the sample gauged and it is expressed in percentage.

Calculations:

Elongation index of aggregate in % is calculated as follows: Where, W₁ = weight of particles retained in length gauge (grams). W₂ = weight of total sample taken for test (grams).

Elongation Index Limits

For pavements either bituminous or Non-bituminous, Elongation index of coarse aggregate should not be more than 15%. Report the Elongation index in percentage rounded off to the nearest whole number. Read More: 1. Tests on Aggregates 2. Material Testing Guide