Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the Magnesium Sulfate test

Laboratories

9 laboratories from 7 countries returned 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).

Materials

Samples of three materials were prepared and distributed by Partners 3 and 4. The same three materials were used in the cross-testing experiment involving the Freeze/Thaw test that was carried out at the same time as the experiment reported here on the Magnesium Sulfate test. The three materials were chosen so that they would give a wide range of results in both tests.

Figure 1 shows the average Freeze/Thaw and Magnesium Sulfate test results from the two experiments, together with the limits for different levels of environmental conditions given in the draft CEN specification for aggregates for concrete (Proposed prEN Aggregates for concrete including those for use in roads and pavements. CEN/TC 154 draft prepared for public enquiry.). It is clear from this figure that the two test methods are not equivalent: thus the aggregate used for Level 3 just complies with the Freeze/Thaw requirement for environmental condition EC2, but fails by a wide margin to comply with the Magnesium Sulfate requirement for the same environmental condition.


Figure 1. Average freeze/thaw and magnesium sulfate test results for the three aggregates used in the cross-testing experiment, and limits (EC2, EC3 and EC3) for the three levels of environmental conditions given in Table E.2 of the draft specification for aggregates for concrete.


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 50kg of aggregate was laid out in a line along a rubber mat, and a small flat-bottomed shovel with a capacity of about 70g 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 two 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 reported a determination, the missing value is shown as "--.-" in the data tables.

The Magnesium Sulfate test method requires individual determinations to be rounded to the nearest 0.1%. However, for the purpose of the cross-testing experiment, the test results were calculated to the nearest 0.01%. 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-specimen and 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, large between-test-portion ranges, or large between-specimen 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 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. 1994.).