To determine the flakiness and elongation index of aggregate.

AGGREGATE SHAPE TEST

(AASHTO T-304, ASTM C-1252)

OBJECT: TO DETERMINE THE FLAKINESS INDEX AND ELONGATION INDEX OF THE GIVEN AGGREGATE SAMPLE

PERFORMANCE OBJECTIVE:

1. To determine the elongation index of the given aggregates.
   

1. To determine the flakiness index of the given aggregates.
   

EQUIPMENT: Sieves, balance, Flakiness Gauge, and Elongation Gauge.

THEORY:

Particle shape is also of prime importance when determining the behavior and suitability of an aggregate. Angular aggregate exhibits increased stability due to internal interlock. The use of flaky or elongated particles should be avoided, as they can be difficult to compact and in situ can present a high air void content. Particle movement under the action of an imposed load is resisted by the internal friction of the aggregates, which is in turn influenced by the shapes of the particles. The presence of flaky and elongated particles is avoided as they may cause inherent weakness with the possibility of breaking down under heavy loads. The shape test is not applicable for sizes smaller than 6.3 mm.

The following tests are used for the determination of particle shape properties.

Flakiness Index (F. I) Test

The aggregate particles whose least dimension (thickness) is less than three-fifths (0.6) of their mean dimension.

The Flakiness Index (F.I) is the total weight of the particles passing the openings of the flakiness gauge, expressed as a percentage of the total weight of the sample. F.I value less than 45% is considered suitable for use in surfacing as well as lower layers.

PROCEDURE:

1. Take 200 pieces of each fraction to be tested and weigh them As W₁ (gm).
   
2. Now pass each stone from the flakiness gauge for its least dimensions.
   
3. Weigh the amount of passing materials from the flakiness gauge to an accuracy of at least 0.1 percent of the test sample as W₂ (gm).
   

CALCULATIONS:

W₂

Flakiness Index = ———- X 100

W₁

Flakiness Index = ___________ %

Elongation Index (E.I) Test

The particles are said to be elongated if the greatest dimension (length) is greater than Nine–a-fifth (1.8) times their mean dimension.

The Elongation Index is the total weight of the particles retained on the openings of an elongation gauge, expressed as a percentage of the total sample weight. In a normal crushed aggregate, the EI is about 1.5 times the F.I.

An aggregate having an EI of less than 45% is suitable for use in surfacing and lower layers.

PROCEDURE:

1. Take 200 pieces of each fraction to be tested and weigh them As W₁ (gm)
   
2. Now pass each stone from the elongation gauge for its greatest dimensions.
   
3. The pieces of aggregates from each fraction tested that could not pass through the specified gauge with its long side are elongated particles and are collected separately to find the total weight of aggregate retained on the elongation gauge from the specified fraction.
   
4. The total amount of elongated material retained by the elongation gauge is weighed to an accuracy of at least 0.1 % of the weight of the test sample W 3,
   

Sieves of different sizes 63, 50, 40, 31.5, 25, 20, 16, 12.5, 10 and 6.3mm.

CALCULATIONS:

W₃

Elongation Index = ———- X 100

W₁

Elongation Index = _____________ %

COMMENTS:

Sieve sizes for Aggregate shape test

RECOMMENDED VALUE:

The shape tests give only a rough idea of the relative shapes of aggregates. Flaky and elongated particles should be avoided in pavement construction, particularly in surface course. If such particles are present in appreciable proportions, the strength of the pavement layer would be adversely affected due to the possibility of breaking under loads. Workability is reduced for cement concrete. IRC recommendations for maximum limits of the flakiness index are as given.