Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the Chloride test

Laboratories

9 French laboratories took part in the experiments, together with 15 laboratories from other European countries. Two of the French laboratories were chosen randomly and included with the laboratories from the other countries to give a "European" experiment involving 17 laboratories. The data from the 9 French laboratories were treated as a separate "French" experiment.

All the laboratories were given numerical codes that were used in all the cross-testing experiments in the first 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).

Materials

Samples of three materials were prepared and distributed for the experiments by Partner 1.

The source of the aggregates used for both Level 1 and Level 3 was a marine-dredged coarse gravel from the channel. Bulk samples of these aggregates were collected from Graves de Mer (GDM) Dieppe (F).

Level 1 was the 0/14mm fraction of the unwashed gravel. The bulk sample was taken from the stockpile after it had been allowed to dry for one week.

Level 2 was prepared in the laboratory as a solution of sea-water in distilled water.

Level 3 was a 0/5mm sand, prepared by GDM. The bulk sample was taken from the stockpile of washed sand.

For either Level 1 or 3, the samples for the participants were prepared as if they were laboratory samples all taken from one bulk sample. Thus the bulk sample was first mixed in the laboratory, and then laboratory samples were prepared by the same method, fractional shovelling, as was used to prepare the laboratory samples for the mechanical tests (Delalande, 1994. Strength tests programme - preparation of laboratory samples. LRPC d'Angers, Aoüt, 1994.). The materials were such that no crushing of particles greater than 16mm in size was required by the participants.

Because the test method requires two test-portions to be tested, for Levels 1 and 3 the participants were asked to divide each laboratory sample into two sub-samples, and then prepare and test two test-portions from each sub-sample. This allows measures of repeatability and reproducibility to be calculated that are consistent with the definitions of r1 and R1, when the "test method" is understood to mean the whole testing process (including sample reduction, extraction, and Volhard titration), and a "test result" is the average of the determinations on two test-portions of the same sub-sample.

For Level 2, each participant was sent a specimen of about 500ml of the solution, and asked to carry out two determinations of the chloride content of their specimen. The data from Level 2 thus allow measures of repeatability and reproducibility to be calculated that are consistent with the definitions of r and R, when the "test method" is understood to mean just the Volhard titration, and a "test result" is the result of a single determination.

Two different methods of sample preparation were used so that if the experiments showed that the precision of the test was poor, or if some laboratories reported erratic results, then it might be possible to identify the stage of the procedure that was responsible.

Data

Where participants did not report the required number of test results or determinations for a level, the missing results are shown using "---" in the data tables. When some of a laboratory's test results or determinations are missing, the remaining data have been included in the analysis, but there may not be sufficient data to allow the repeatability of the laboratory to be checked.

The method for the determination of water-soluble chloride salts does not contain any indication of how the test results should be rounded, apart from a general requirement in the document (covering all its test methods) that the mean of two determinations should be rounded to the nearest 0.01%. However, for the purpose of the cross-testing experiment, it was asked that the test results should be reported to the nearest 0.0001%. 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 look for biasses that influence the results of a laboratory at more than one level.

Between-specimen, between-test-portion, and between-sub-sample 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 averages are shown 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 using a single question mark (?) to indicate a straggler, and a double question mark (??) to indicate an outlier.

The graphs in Appendix G are used to see if any laboratory suffers from biasses that affect the results at more than one level. Thus, for example, if a laboratory uses the wrong extraction procedure, then this could cause it to obtain results that are biassed in the same direction in Levels 1 and 3, but not in Level 2. If a laboratory uses a standardised solution that has been prepared incorrectly, then this could cause it to obtain results that are biassed in the same direction for all three levels.