Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the Freeze/Thaw test

Laboratories

12 laboratories from 8 European countries reported data in time to be included in this report. 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 histograms and Mandel plots).

Laboratory I (196) reported that they had some troubles with their freezing cabinet in the second run.

Materials

Samples of three aggregates were prepared and distributed by Partners 3 and 4. The three aggregates were all production materials and were chosen so that they would give a wide range of results in both the Freeze/Thaw test and the Magnesium Sulfate test.

The samples were prepared, for each level of the experiment, as if they were laboratory samples all taken from one bulk sample. The laboratory samples for Level 2 were prepared by dividing a bulk sample using a rotary sample divider. The laboratory samples for Levels 1 and 3 were prepared by fractional shovelling. In this process a bulk sample of about 250kg of aggregate was laid out in a line along a rubber mat, and a small flat-bottomed shovel with a capacity of about 0.5kg was used to take increments always from the same end of the line. 15 sample containers were placed in a circle around the line of aggregate, and the increments were placed in these in sequence, until each sample container had received 30 increments.

The participants were required to prepare and test two sets of three test specimens from each sample. Hence the critical range and the measures of repeatability and reproducibility given by the experiment are consistent with the definitions of Wc, r1 and R1.

Data

Where a participant failed to report a determination, the missing value is shown as "-.-" in the data tables. The method for the Freeze/Thaw test requires test results to be rounded to the nearest 0.1%. However, for the purpose of this experiment, the test results were calculated to the nearest 0.01% to prevent rounding of the data affecting the assessment of the repeatability and reproducibility of the test method.

Averages, ranges and standard deviations

Laboratory averages are used to calculate the reproducibility of the test method, and to assess laboratory biasses. Between-test-result ranges and between-test-specimen standard deviations are used to calculate the repeatability of the test method, and to assess the repeatability of tests from individual laboratories. The averages, ranges and standard deviations are shown in the histograms, and the laboratory averages are plotted in the Mandel plots.

Note that between-test-specimen standard deviations are used to calculate the precision values in Table 1. In practice it is more convenient for laboratory staff to check their repeatability by comparing between-test-specimen ranges with the corresponding critical range than to use between-test-specimen ranges. The critical range is related to the between-test-specimen standard deviation by a simple formula.

The averages, ranges and standard deviations 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, ranges and standard deviations are shown in the Mandel plots. These figures are used to identify laboratories that give rise to large laboratory biasses, or large between-test-result ranges, or large between-test-specimen standard deviations, 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 revised ISO standard on precision (ISO 5725 Accuracy (trueness and precision) of measurement methods and results; Part 2 Basic methods for the determination of repeatability and reproducibility of a standard measurement method.).