Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the determination of the Particle Density and Water Absorption of Coarse Aggregates

Laboratories

20 laboratories took part in the experiment from 12 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 4. The three materials were chosen so that they would give a wide range of particle densities and water absorptions. 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.

The method used to prepare the laboratory samples was fractional shovelling. A bulk sample of about 200 kg of each coarse aggregate was obtained, then, to prepare the laboratory samples from one of these bulk samples it was tipped out in a line on a rubber mat, and twenty sample containers were arranged around the mat. A small scoop with a capacity of about 200 g for Levels 1 and 2, and 100 g for Level 3, was used to take sampling increments from one end of the heap and to transfer them to the sample containers, working around the containers in order, until each container contained about 6 kg for Levels 1 and 2, or 5 kg for Level 3.

The flint gravel used for Level 3 contained a mixture of very different types of rock, including dense glossy particles and white porous ones.

Data

Where a participant failed to reported a determination, the missing value is shown as "-.----" in the data tables. The methods used for determining the particle density of coarse aggregate require test results to be rounded to the nearest 0.01 Mg/m3. However, for the purpose of the cross-testing experiment, the test results were recorded to the nearest 0.001 Mg/m3. This was to prevent rounding of the data affecting the assessment of the repeatability and reproducibility of the test method. Likewise, the draft Standard requires water absorptions to be recorded to the nearest 0.1 %, but in the experiment they were recorded to 0.01 %.

The data were checked to make sure that the recorded measurements were consistent with the calculated values of the mass, volume and particle density of each test specimen. There were a number of instances in the "pre-dried" particle density test where the laboratory had not used the density of the water in the calculation of the particle density: the correct values were calculated from the reported measurements. Two laboratories mixed up the sample identifications: because the three materials had different densities it was possible to assign their results to the correct materials. The results also contained a few calculation errors. All these instances are noted under the tables of data. Laboratories C (69) and E (95) reported that they had washed their samples on a 0.063 mm sieve prior to doing the pre-dried density tests. Laboratories F (96), I (153), and T (310) divided the test portions for the "SSD" particle density test into two test specimens. Their test results were calculated as averages of two determinations.

Averages and ranges

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 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-result 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 per cent and 1 per cent significance levels, taken from the revised ISO standard on precision (ISO 5725, 1994).