Testing of industrial products - Aggregates for construction

Details of the cross-testing experiments on the Dutch Static Compression test

Laboratories

26 laboratories volunteered to took part in the experiments, including 10 laboratories from the Netherlands. 22 reported results that were used in the analysis. Of these, 6 laboratories have taken part in regular proficiency tests since 1991. The laboratories have been given numerical codes for the programme. For the purposes of this report all the laboratories have also been assigned letter-codes (because single-character codes are needed in the graphs).

Materials

Laboratory samples of three aggregates were prepared and distributed by Partner 1. The samples were prepared, for each level of the experiment, as if they were laboratory samples taken from one bulk sample. A report describing their preparation is available (Delalande, 1994 Strength tests programme; preparation of laboratory samples. LCPC, Paris.). The same three aggregates were used throughout the four-year programme for all the cross-testing experiments that involve tests of the mechanical properties of aggregates. For the DSC test, laboratory samples of the 10/14 mm size fractions of each aggregate were sent to the participants.

Table 1 gives some information on the limiting values for the DSC test that appear in a Dutch specification. Comparison with the average results obtained on the materials used in the experiment shows that the aggregate used for Level 3 would give a result approximately on the limit for its size for a Class 3 aggregate. Further, that used for Level 1 would easily meet the requirement for a Class 1 aggregate. Thus the three aggregates cover the range of strength of interest very well.


Table 1. Extract from Dutch specification for bituminous bound aggregates (crushed rock and crushed gravel).
Aggregate sizeUpper limit to DSC test result
mmClass 3: base layers Class 2: top layersClass 1: special cases *
22.4/31.5564636
16.0/22.5524434
11.2/16.0484232
8.00/11.2444030
5.60/8.00423828
4.00/5.60403626

* Note. For example, high trafficked porous asphalt and stone mastic asphalt.


The participating laboratories were required to prepare four test specimens from each aggregate, so that they were able to derive two test results for each aggregate. The participating laboratories were also required, as part of the test method, to determine the mass of each aggregate required to fill a one-half litre measure and use this quantity as the specimen mass to use in their tests. The methods used to prepare the laboratory samples, and test specimens from laboratory samples, allow the precision of the test method to be described using Wc , r1 and R1 as defined here.

Data

Where a participant failed to reported a determination, the missing value is shown as "--.-" in the data tables. Some participants misunderstood the instructions and prepared only two test specimens at each level. Others used a quantity of one litre, not one-half a litre, for the specimen - their results have not been included in the report.

Laboratory D reported results that are obviously incorrect, but the reason for this cannot be found from their data sheets. Their data have been included in the tables of data, but are classed as outliers for the purpose of calculating precision values.

Laboratory W repeated the test for Level 2 because their first attempt failed the repeatability check in the method. The results obtained in their second attempt were used in the calculations.

The DSC test method requires test results to be rounded to the nearest whole number. However, for the purpose of the cross-testing experiment, the test results were recorded to the nearest 0.1 %. 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 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-specimen or 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 % significant levels, taken from the ISO standard on precision (ISO 5725, 1994).

Sensitivity ratios

It will be of interest to compare the precision of the DSC test with that of other tests of mechanical properties of aggregates. The repeatability and reproducibility standard deviations, or limits, for different test methods cannot be compared directly because they relate to different scales of measurement. Sensitivity ratios are dimensionless, so they do not suffer from this disadvantage, and they may be used to compare the precision of different tests. It will be seen from the formula used to calculate them that they involve the average results for the materials used in the cross-testing experiments, so it is essential that different test methods are assessed using the same materials.