Testing of industrial products - Aggregates for construction

Details of the cross-testing experiment on the Sand Equivalent test

Laboratories

34 laboratories took part in the experiment from 12 European countries. The laboratories have been given numerical codes that will be used in all the cross-testing experiments in the current year of the programme. For the purposes of this report they have also been assigned letter-codes (because single-character codes are needed in the graphs). Data from one laboratory (95) arrived too late to be included in the precision calculations, but their data have been added to the tables of data, and to the histograms.

Materials

Samples of three materials were prepared and distributed by Partner 1. The same three materials were used in the cross-testing experiment involving the Methylene Blue test that was carried out at the same time as the experiment reported here on the Sand Equivalent test. The three materials were chosen primarily so that they would give a wide range of results in the Methylene Blue test.

All three levels were crushed sands. Level 1 was a 0/3 mm crushed basaltic sand. The grading curve shows about 20 % of oversized sand on the 3.00 mm sieve, and 11 % of fines (passing 0.063 mm). Level 2 was a 0/3 mm crushed rhyolite sand with up to 30 % oversized at 3.00 mm and 8 % of fines. Level 3 was a 0/2 mm crushed diorite with less than 7 % of particles greater than 2.00 mm and up to 18 % of fines. Note that the high percentage of fines in these sands would have made the preparation of homogeneous laboratory samples difficult.

The samples were prepared, for each level of the experiment, 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 experiment are consistent with the definitions of r1 and R1 used here.

For each level, a bulk sample of about 130 kg was divided by fractional shovelling into about 45 laboratory samples each weighing about 3 kg, and these were sent to the participating laboratories. The participants were required to divide each laboratory sample into two sub-samples, one of about 2 kg for the Sand Equivalent test, and the other of about 1 kg for the Methylene Blue test. For the Sand Equivalent test, the 2 kg sub-sample was allowed to dry to a moisture content of less than 2 %, then divided into three test portions. One of these test portions was used for the determination of moisture content, the other two were each divided into two test specimens for the Sand Equivalent tests.

The Sand Equivalent and Methylene Blue tests were developed to assess natural sands, and most of the cross-testing experiments that have been done in the past have used natural sands. In the experiment reported here, however, three crushed sands have been used, of varying gradings. Crushed sands are more difficult to wash than natural sands - several of the participants reported having such difficulties. The materials contained particles larger than 2 mm, so the participants had to dry-sieve their samples on a 2.00 mm sieve (as required by the test method) to obtain the 0/2 mm fraction for the tests: this is a supplementary cause of variability. These factors would have made the execution of the tests more difficult, especially for laboratories not familiar with the method.

Data

Where a participant failed to reported a determination, the missing value is shown as "--.-" in the data tables.

The Sand Equivalent test method requires test results to be rounded to the nearest whole number. However, for the purpose of the cross-testing experiment, the test results were recorded to the nearest 0.1 %. This was to prevent rounding of the data affecting the assessment of the 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 the laboratory averages are plotted in the Mandel plots.

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.

Standardised values of the averages and ranges are shown in the Mandel plots. These figures are used to identify laboratories that give rise to large laboratory biasses, or large between-test-portion ranges, in more than one level of an experiment. The horizontal broken lines in these graphs show the critical values of the "h" and "k" statistics at the 5 % and 1 % significant levels, taken from the ISO standard on precision (ISO 5725, 1994).