Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the Methylene Blue test

Laboratories

34 laboratories took part in the experiment from 13 European countries. The laboratories have been given numerical codes. For the purposes of this report they have also been assigned letter-codes (because single-character codes are needed in the graphs).

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 Sand Equivalent test that was carried out at the same time as the experiment reported here on the Methylene Blue test. The three materials were chosen in the first instance 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.

The samples were prepared, for each level of the experiment, as if they were laboratory samples all taken from one bulk sample. Thus 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 Methylene Blue test, the 1 kg sub-sample was dried and sieved on a 2.00 mm sieve and used to prepare two test portions, according to the method. Hence the measures of repeatability and reproducibility given by the experiment are consistent with the definitions of r1 and R1 used here.

Data

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

The Methylene Blue test method requires test results to be rounded to the nearest 0.1 MB units. However, for the purpose of the cross-testing experiment, the test results were recorded to the nearest 0.01 MB units. 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 X-Y 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 % significance levels, taken from the ISO standard on precision (ISO 5725, 1994).