Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the micro-Deval test

Laboratories

37 French laboratories took part in the experiments, together with 18 laboratories from other European countries. Two of the French laboratories were chosen and included with the laboratories from the other countries to give a "European" experiment involving 20 laboratories. The data from the 37 French laboratories were treated as a separate "French" experiment.

The laboratories have been given numerical codes used in all the cross-testing experiments. For the purposes of this report they have also been assigned letter-codes (because single-character codes are needed in the histograms and Mandel plots).

Different instructions were given to the laboratories in the French and European experiments. In the French experiment, the laboratories were asked to allocate the aggregates in the order: Level 1, then Level 2, then Level 3, whereas in the European experiment the laboratories were given no instructions on the order of testing of the aggregates. This was done to check if the roughening or polishing action of the aggregates on the drums could influence the results.

Materials

Laboratory samples were labelled Level A, Level B, and Level C, corresponding to Level 1, Level 2, and Level 3 in the present report.

Samples of three materials were prepared and distributed by Partner 1. A report is available describing their preparation (Delalande G. Strength tests programme. Preparation of laboratory samples. LPC d'Angers, Août 1994.). The same three materials are used throughout the four-year programme for all the cross-testing experiments that involve tests of mechanical properties of aggregates.

The samples were prepared, for each level of the experiments, as if they were laboratory samples all taken from one bulk sample, and the participants were required to prepare and test duplicate test portions from each sample. Hence the measures of repeatability and reproducibility given by the experiments are consistent with the definitions of r1 and R1 given above.

Data

The missing results are shown as "--.-" in the data tables.

The micro-Deval test method requires test results to be rounded to the nearest whole number. However, for the purpose of the cross-testing experiment, it was asked that the test results should be reported to the nearest 0.1. This was to prevent rounding of the data affecting the assessment of repeatability and reproducibility of the test method.

Averages and ranges

Laboratory averages are used to calculate the reproducibility of the test method, and to assess laboratory biasses. Between-test-portion ranges are used to calculate the repeatability of the test method, and to assess the repeatability of tests from individual laboratories.

The averages and ranges are shown in the histograms, and standardised values of these averages and ranges are shown in the Mandel plots.

Between-specimen ranges are used to calculate the critical range, and also to assess the repeatability of tests from individual laboratories.

The averages and ranges are also used to test for stragglers and outliers. Where these have been found, they are indicated throughout using a single question mark (?) to indicate a straggler, and a double question mark (??) to indicate an outlier.

The histograms of laboratory averages for the micro-Deval test are asymmetric, with all the stragglers and outliers on the low side. (Lower values indicate a better resistance to wear.) There are two faults which have occurred in the past and which might possibly be causes of the low results in these experiments. One possible cause is using a smaller number of rotations than the specified number of 12000+/-10. (One French laboratory reported using a smaller number of rotations at one level.) The other possible cause is insufficient washing and sieving on the 1.60mm sieve, which leads to the mass retained on the sieve being greater than it should be, and therefore to a lower MDE result.

Sensitivity ratios

It will be of interest to compare the precision of the micro-Deval test with that of other tests of mechanical properties of aggregates. The repeatability and reproducibility standard deviations, or limits, for different test methods cannot be compared directly because they relate to different scales of measurement. Sensitivity ratios are dimensionless, so they do not suffer from this disadvantage, and they may be used to compare the precision of different tests.

It will be seen in the formulae used to calculate them that they involve the average results for the materials used in the cross-testing experiments, so it is essential that different test methods are assessed using the same materials.