What is the Shape 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.
  • Flakiness Index
  • 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.

Shape Test: Flakiness and Elongation Index

Flakiness Index:

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

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,

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 aggregates		Weight of fraction consisting of at least 200 pieces,g	Thickness gauge size, mm	Weight of aggregates in each fraction passing thickness gauge,mm	Length gauge size, mm	Weight of aggregates in each fraction retained on length gauge,mm
Passing through IS Sieve, mm	Retained on IS Sieve, mm
1	2	3	4	5	6	7
63	50	W1	23.90	X1	–	–
50	40	W2	27.00	X2	81.00	Y1
40	31.5	W3	19.50	X3	58.00	Y2
31.5	25	W4	16.95	X4	–	–
25	20	W5	13.50	X5	40.5	Y3
20	16	W6	10.80	X6	32.4	Y4
16	12.5	W7	8.55	X7	25.5	Y5
12.5	10	W8	6.75	X8	20.2	Y6
10	6.3	W9	4.89	X9	14.7	Y7
Total	W =		X =		Y =

Record of Shape Test

Flakiness Index = (X₁+ X₂+…..) / (W₁ + W₂ + ….) X 100

Elongation Index = (Y₁ + Y₂ + …) / (W₁ + 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 pavement	Maximum limits of flakiness index, %
1	Bituminous carpet	30
2 (i)	Bituminous / Asphaltic concrete	25
(ii)	Bituminous Penetration macadam
(iii)	Bituminous surface dressing (single coat, two coats & precoated)
(iv)	Built up spray grout
3 (i)	Bituminous macadam	15
(ii)	WBM base course and surface course