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Flakiness and Elongation Test (BS 812: Part III)

The objective of this test is to determine the flakiness index and elongation index of aggregates.

Introduction

The type of rocks and type of crushing machine highly determine the shape and size of the aggregates produced. Elongated and flack stones are normally not very suitable for road works since the shape and size make them difficult to compact. As such the flakiness and elongation test must be carried out to determine the suitability of the material.

The flakiness index of an aggregate is the percentage by weight of particles in it whose dimension (thickness) is less than three fifths of their mean dimension. This test is not applicable to material passing a ¼ inch (6.35 mm) BS sieve.

The flakiness index is the total weight of the material passing the various thickness gauges or special sieves, expressed as a percentage of the total weight of the sample with the sieve analysis. If required the weight of each fraction gauged to the nearest whole number and shall be reported together passing the thickness gauge or special sieve shall be reported as a percentage of the weight of the fraction.

The elongation index of an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than 1 4/5 times their mean dimension. Normally, the properties of interest to the engineer are sufficiently covered by the flakiness or angularity tests. The elongation test is not applicable to material passing a ¼ inch (6.35mm) BS sieve.

The elongation index is the total weight of the material retained on the various length gauges, expressed as a percentage of the total weight of the sample gauged to the nearest whole number and shall be reported together with the sieve analysis. If required the weight of each fraction retained on the length gauge shall be reported as a percentage of the weight of the fraction.

Apparatus

The apparatus used in the flakiness test and the elongation test is:

  1. A metal gauge plate

Working Procedure

This test procedure for flakiness test is explained below:

1.      Three samples of aggregates weighing 2.5 kg each are prepared:

The aggregates of the first sample passing 50 mm BS sieve and retained on 37.5 mm BS sieve, the second sample passing 37.5 mm BS sieve and retained on the 28 mm sieve, and the third sample passing 28 mm BS sieve and retained on the 20 mm BS sieve.

2.      Each sample is gauged in turn of thickness on the metal gauge.

3.      The passing material of each sample is weighed. 

This test procedure for elongation test is explained below:

  1. Three samples of aggregates weighing 2.5 kg each are prepared:

The aggregates of the first sample passing 50 mm BS sieve and retained on 37.5 mm BS sieve, the second sample passing 37.5 mm BS sieve and retained on the 28 mm sieve, and the third sample passing 28 mm BS sieve and retained on the 20 mm BS sieve.

  1. Each sample is gauged in turn of length on the metal gauge.
  2. The retained material of each sample is weighed.
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