Testing of industrial products - Aggregates for construction

Percentage of Crushed and Broken Surfaces Test - Summary

This report gives the results of a cross-testing experiment, involving 17 laboratories from 10 European countries, on a method for determining the percentage of crushed and broken surfaces in coarse aggregates. Three coarse aggregates with differing levels of crushed and broken surfaces were used.

Thirteen laboratories reported percentages of crushed particles that incorrectly excluded totally crushed particles from the calculated results.

The reproducibility coefficient of variation of the test is substantially higher than the level of 8 % that is considered to be desirable for a test that is to be used to check compliance with specifications. In most cases encountered in the experiment, the repeatability coefficient of variation also exceeded 8 %.

Using statistical theory, the repeatability of the test is predicted to be:

r1 = 0.0028 ( X (100 - X) D3 ÷ M )½ %

This equation agrees with the experimental results for the determination of the percentages of crushed, rounded, totally crushed, and totally rounded particles. An equation of the same form

R1 = 0.0051 ( X (100 - X) D3 ÷ M )½ %

was fitted to the reproducibility results for the determination of percentages of crushed, rounded, and totally rounded particles. The estimates of the reproducibility of the determinations of the percentage of totally crushed particles did not fit the above equation for R1 - they were much too high.

It is recommended that laboratories should take part in proficiency tests. Participants in proficiency tests could be sent examples of sorted test portions to help them learn how to classify particles correctly. A number of ways of improving the test method description are recommended, as follows.