LEC-28 Civil Engineering By Piyush Sir | Building Material & Construction | Test on Aggregate| #ESE

Shape tests on coarse aggregates such as flakiness index and elongation Index, its importance in concrete construction, methods of determination is discussed.
Angularity Number Test of Aggregate
It is a non- destructive quality test for aggregate. It is the test about the shape of the aggregate. It gives the qualitative representation of the shape of the aggregate. It is measured in terms of percentage of voids in excess of voids in perfectly rounded aggregate(33%). The more angular the aggregate is, the more is the Angularity number.The normal aggregate suitable for making concrete have angularity number lying between 0 and 10. The rounded aggregate has Angularity Number zero.
When rounded aggregate is filled in a vessel, it leaves about 33% void in it. If a well compacted Aggregate in the same vessel of rounded aggregate leaves 43% voids then its Angularity Number is 10. This test is not applied for the aggregates which gets crushed during compaction of this test.

Theory of Flakiness Index and Elongation Index Tests
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.
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

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