3. Shape Tests on Aggregates

Shape tests on Aggregates( Flakiness Index and Elongation Index Test)

  • The shape test of aggregates determines the shape and size of particle.
  • Size & shape of aggregates directly has impact on strength of concrete, bond strength, workability of concrete, cement requirement and cost required for concrete.
  • Rounded, elongated and flaky aggregates offer good workability but less bond strength and mechanical strength.
  • Angular & irregular aggregates offers lesser workability but good bond strength and mechanical strength.
  • To know more about shape & size of aggregates click here>>>>
  • There two terminologies related to the shape of aggregates and its test.
    1. Flakiness Index
    2. Elongation index
  • Aggregates are called Flaky & Elongated aggregates when its length is larger than its width and width is larger than its thickness.
  • i.e. length>width>thickness.
  • Flaky & elongated aggregates are not suitable for concreting. It does not form a very good bond with concrete. These are generally obtained from poorly crushed rocks.

Flakiness Index:

  • The flakiness index is defined as the, percentage of aggregates by weight whose least dimension is less than 0.6 times than mean size.  
  • The procedure to find out flakiness index is as per IS: 2386 part 1.
  • The apparatus used for this test is as shown below,
Flakiness Index, Shape test on aggregates
Fig. Flakiness Gauge

Elongation Index:

  • The Elongation index is defined as the, percentage of aggregates by weight whose larger dimension (generally length) is 1.8 times more than mean size.
  • This test is application to aggregates lager than the 6.3mm size.  
  • The procedure to find out Elongation index is as per IS: 2386 part 1.
  • The apparatus (Elongation gauge) used for this test is as shown below,
Elongation Index, Shape test on aggregates
Fig. Elongation Gauge

Apparatus & Equipment:  

  • A standard thickness gauge
  • A standard length gauge
  • IS sieves of sizes 63, 50 40, 31.5, 25, 20, 16, 12.5,10 and 6.3mm
  • A weighing balance of capacity 5kg, readable and accurate up to 1 gm.

 Procedure:

  • 1.      Sieve all the sample through the IS sieves as mentioned in the table.
  • 2.      Take the minimum of 200 pieces of each fraction for testing and weigh them.
  • 3.      For Flaky materials, gauge the fraction for thickness on thickness gauge.
  • 4.      Weigh the flaky material passing through the thickness gauge.
  • 5.      For elongated materials, gauge the fraction for length gauge.
  • 6.      Weigh the elongated materials retained on length gauge.

Data Recording and Calculation:

Size of aggregatesWeight of fraction consisting of at least 200 pieces,gThickness gauge size, mmWeight of aggregates in each fraction passing thickness gauge,mmLength gauge size, mmWeight of aggregates in each fraction retained on length gauge,mm
Passing through IS Sieve, mmRetained on IS Sieve, mm
1234567
6350W123.90X1
5040W227.00X281.00Y1
4031.5W319.50X358.00Y2
31.525W416.95X4
2520W513.50X540.5Y3
2016W610.80X632.4Y4
1612.5W78.55X725.5Y5
12.510W86.75X820.2Y6
106.3W94.89X914.7Y7
TotalW =X =Y =

Record of Shape Test:

Flakiness Index = (X1+ X2+…..) / (W+ W2 + ….) X 100

Elongation Index = (Y+ Y+ …) / (W+ W2 + ….) X 100

Results:

  1. Flakiness Index =
  2. Elongation Index =

 

Recommended Values of Flakiness Index and Elongation Index

  • Flaky & elongated aggregates are not suitable for concreting. It does not form a very good bond with concrete. These are generally obtained from poorly crushed rocks.
  • Thus avoid this kind of aggregates in pavement as well.
  • IRC recommendations for maximum limits of flakiness index are as given.
Sl No:Type of pavementMaximum limits of flakiness index, %
1Bituminous carpet30
2 (i)Bituminous / Asphaltic concrete25
(ii)Bituminous Penetration macadam
(iii)Bituminous surface dressing (single coat, two coats & precoated)
(iv)Built up spray grout
3 (i)Bituminous macadam15
(ii)WBM base course and surface course

 

 

 

 

 

 

 

 

 

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