SHAPE TESTS

Flakiness index and Elongation Index of Coarse Aggregates

AIM:

i.             to determine the elongation index of the given aggregates

ii.            to determine the flakiness index of the given aggregates

APPARATUS:

The apparatus for the shape tests consists of the following:

(i)   A standard thickness gauge

        (ii)  A standard length gauge    

(iii) IS sieves of sizes 63, 50 40, 31.5, 25, 20, 16, 12.5,10 and 6.3mm

(iv)       A balance of capacity 5kg, readable and accurate up to 1 gm.

THEORY:

       The particle shape of aggregates is determined by the percentages of flaky and elongated particles contained in it. For base course and construction of bituminous and cement concrete types, the presence of flaky and elongated particles are considered undesirable as these cause inherent weakness with possibilities of breaking down under heavy loads. Thus, evaluation of shape of the particles, particularly with reference to flakiness and elongation is necessary.

       The Flakiness index of aggregates is the percentage by weight of particles whose least dimension (thickness) is less than three- fifths (0.6times) of their mean dimension. This test is not applicable to sizes smaller than 6.3mm.

       The Elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than nine-fifths (1.8times) their mean dimension. This test is not applicable for sizes smaller than 6.3mm.

PROCEDURE:

i)  Sieve the sample through the IS sieves (as specified in the table).

ii)Take a minimum of 200 pieces of each fraction to be tested and weigh them.

         (iii)    In order to separate the flaky materials, gauge each fraction for thickness on a thickness gauge. The width of the slot used should be of the dimensions specified in column (4) of the table for the appropriate size of the material.

(iv)   Weigh the flaky material passing the gauge to an accuracy of at least 0.1 per cent of the test sample.

         (v)    In order to separate the elongated materials, gauge each fraction for length on a length gauge. The width of the slot used should be of the dimensions specified in column (6) of the table for the appropriate size of the material.

         (vi)   Weigh the elongated material retained on the gauge to an accuracy of at least 0.1 per cent of the test sample.

OBSERVATIONS:

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

              Elongation Index   =     (Y₁ + Y₂ + …) / (W₁ + W₂ + ….) X 100     

RESULT:

i)   Flakiness Index     =   

ii)  Elongation Index   =

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 pavement layer would be adversely affected due to possibility of breaking under loads. Workability is reduced for cement concrete. IRC recommendations for maximum limits of flakiness index  are as given.