Testing of industrial products - Aggregates for construction

Results of the cross-testing experiment on the Magnesium Sulfate test

Repeatability and reproducibility

Estimates of repeatability and reproducibility are given in Table 1. The standard deviations are also shown in Figure 1. These estimates have been calculated by excluding the data that gave rise to outliers, i.e. the data for Laboratory F in Level 1, for Laboratory H in Level 2, and for Laboratory G in Level 3.

Experiments carried out in the UK during the development of the British Standard method for the Magnesium Sulfate test (BS 812 Testing aggregates Part 121 Method for determination of soundness. 1989) give estimates of the between-specimen and reproducibility standard deviations shown in Figure 2. (Note that these values have been adjusted so that they are comparable with the results of the experiment on the draft CEN method. In the British Standard, the values of repeatability and reproducibility apply when a test result is obtained as a determination on a single specimen, whereas in the draft CEN method, a test result is obtained as the average of the determinations on two specimens.) The functional relations shown in Figure 2 can be seen to fit the experimental results moderately well. Hence the same form of relation has been used to obtain functional relations using the results obtained in the cross-testing experiment: these are shown in Figure 1 and are also given in Table 2.

Comparing the results given in Figures 1 and 2, it is seen that the repeatability in the cross-testing experiment (as measured by the between-specimen standard deviation) is a little better that that reported for the British experiment. However, the reproducibility achieved in the cross-testing experiment is worse than that found in the British experiment, particularly in Levels 1 and 2. It is possible that this is because some laboratories had little experience with the test method before the cross-testing experiment took place.

Assessment of the reproducibility of the Magnesium Sulfate test

It has been argued (J»rck, Sym and Powell, 1994. A study of mechanical tests of aggregates. Green Land Reclamation Ltd Report GLR 3036/03a.) that the reproducibility standard deviation of a mechanical test, when expressed as a coefficient of variation, should be no more than about 8%, if the test method is to be used to assess the compliance of aggregates with specifications. Because specifications for Magnesium Sulfate values impose upper limits, as with mechanical tests, the criterion may also be applied to the Magnesium Sulfate test. The results in Table 3 show that the reproducibility of the Magnesium Sulfate test fails to meet this criterion by a wide margin in Levels 1 and 2.

The high reproducibility coefficient of variation for Level 2 is of concern because this aggregate gave results close to the limits for this test in the draft CEN specification for aggregates for concrete (see Figure 1). The reproducibility results obtained in the British experiment also fail to meet the 8% criterion. Taking all these results into consideration suggests that there is a need to tighten up the method specification. Several possible ways of improving the test can be identified, as follows.

The magnesium sulfate solution should be saturated throughout the test. The CEN method requires the temperature of the solution to be maintained at 20+/-2C. However, if the temperature varies over this range (for example if it drops during the night and rises during the day) then the solution will not stay saturated. There may be a case for tightening up the tolerance to 20.0+/-1.0C or even 20.0+/-0.5C. Another way to ensure that the solution is saturated would be to require the solution to be topped up with saturated magnesium sulfate solution at 25C (whilst the specimens are being cooled after being taken out of the oven).

The CEN method requires the specimens to be cooled "to laboratory temperature" after they have been taken out of the oven and before they are placed in the solution, and, as noted above, it requires the temperature of the solution to be maintained at 20+/-2C. It is thus possible that on some occasions the specimens are one or two degrees hotter than the solution when they are placed in it, and on other occasions they are one or two degrees cooler. This temperature difference could have a significant effect on the crystallisation of magnesium sulfate within the aggregate particles. It is suggested that the method should require that the specimens are not warmer than the solution at the time they are placed in it.

The CEN method does not specify where the specimens should be kept whilst they being cooled. It is suggested that they ought to be kept at a controlled temperature and humidity in the desiccator.

It would be of interest to study the rate at which the specimens loose mass during drying in the oven, to check that they reach a stable mass before the end of the 24h drying period specified in the CEN method, and also to check that the final mass is not sensitive to the chosen drying temperature.

It would be of interest to carry out ruggedness trials to establish the sensitivity of the test results to the factors mentioned above. Such trials would show where the method specification needs to be tightened up to improve the reproducibility of the test. Proficiency testing would allow the benefits of changes to the specification to be assessed, but it could require many rounds to achieve the required degree of improvement.