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 for Laboratory R at all three levels. The data from Laboratory I responsible for the other straggler noted in Figure 1 have been included in the calculations. The reasons for excluding the data for Laboratory R are that they gave laboratory averages that are low in comparison with those from the other laboratories, and because they were the only laboratory where the FD apparatus was not fixed to a concrete block.
In Figure 1 it can be seen that the points representing the repeatability and reproducibility standard deviations fall approximately on straight lines. Figure 2 shows the results of a precision experiment involving the British equivalent of the FD test (BS 812 1990 Testing aggregates: Part 112 Methods for determination of aggregate impact values (AIV).). These results provide rather weak support for the straight line relationship between SR and X in Figure 1, and indicate that Sr does not depend on X.
It has been argued (Jorck, Sym and Powell, A study of mechanical tests of aggregates. Green Land Reclamation Report GLR 3036/03a. 1994) 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.
The results in Table 3 show that the reproducibility of the FD test fails to meet this criterion by a wide margin, so that the test is not suitable to be called up in specifications.
However, it could be used as an indirect test: if U is the upper limit for the FD test equivalent to the upper limit for the reference method in a specification, then test results below U - 2SR could be deemed to indicate a satisfactory level of mechanical strength according to the FD test. Note that the calculation of U would have to allow for uncertainty in the correlation between the FD test and the reference test. As many aggregates meet requirements for mechanical strength by a wide margin, the use of the FD test as an indirect test in this way would allow many aggregate producers to avoid the expense of the reference method. It is only those aggregates that give FD test results above U - 2SR that would need to be tested by the reference method.
Information reported by the participants about their testing machines is given in Table A below. The data were studied to see if there were any correlation between the machine characteristics and the laboratory averages. This analysis is made difficult because some participants did not report all the details requested, and others reported incredible values (see the cells in Table A marked with ?). No significant correlation was found (apart from the observation that Laboratory R obtained low results and that theirs was the only apparatus that was not fixed to a concrete block). These data could be studied further if there is a need to improve the reproducibility of the test.
|Laboratory code||Machine||Hammer||Concrete block fixed to machine|
|letter||number||type||made by||mass kg||drop mm (1)||mass|
|B||3||Automatic||CECP Angers||13.992||392 ?||yes||200|
|D||5||Semi-auto||CECP Angers||2.402 ?||?||yes||80|
|L||17||Automatic||CECP Angers||?||371 ?||yes||125|
|Q||38||Manual||Controls||13.5 ?||414 ?||yes||80|
(1) The drop = the distance between the base of the hammer at the point of release and the surface of the aggregates.
(2) Machine fixed to the concrete floor-slab.
Table B below summarises the results on sensitivity obtained for the four tests for mechanical strength included in the project. Note that:
The results in Table B indicate that, of the four test methods, the Schlagversuch gives the best performance overall, in terms of both repeatability and reproducibility. However, the Los Angeles test gives the best repeatability and reproducibility for low strength aggregates, i.e. over the range of strengths represented by the Levels 2 and 3 (which gave averages of 21.6 and 37.5 LA units).
|Test||Level 2 - Level 1||Level 3 - Level 2||Level 3 - Level 1|