Testing of industrial products - Aggregates for construction

Results of the cross-testing experiment on the Fragmentation Dynamique 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 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.

Assessment of the precision of the FD test

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.

Correlation between results and machine characteristics

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.


Table A. Details of the FD apparatus.
Laboratory codeMachine HammerConcrete block fixed to machine
letternumbertypemade by mass kgdrop mm (1)mass
A2ManualCECP Angers 14403yes50
B3AutomaticCECP Angers 13.992392 ?yes200
C4ManualCECP Angers 14.1401yes60
D5Semi-autoCECP Angers 2.402 ??yes80
E6ManualCECP Angers ?409yes57
F8AutomaticCECP Angers 13.982399yes90
G9ManualCECP Angers 13.99400yes80
H10AutomaticCECP Angers ?398yes25
I13AutomaticCECP Angers ??yes?
J14ManualCECP Angers 14400yes58.6
K16Semi-autoCECP Angers 14.002399yes110
L17AutomaticCECP Angers ?371 ?yes125
M18AutomaticCECP Angers 14.042392yes160
N21ManualEuromatest 14410 ?yes140
O23ManualMAP 14.01400yes6.8 ?
P30Manual 14.002397yes (2)---
Q38ManualControls 13.5 ?414 ?yes80
R43ManualEuromatest 14.1401no---
S1Manual 14403yes40
T42ManualControls 14405yes46

Notes.

(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.


Comparison of the sensitivities of tests for mechanical strength.

Table B below summarises the results on sensitivity obtained for the four tests for mechanical strength included in the project. Note that:

  1. if one test method gives a larger sensitivity ratio for reproducibility than another then it may be said to have a better reproducibility than the other test method, over the range of aggregate strengths represented by the two levels included in the calculation;
  2. a larger sensitivity ratio for repeatability indicates a better repeatability, likewise;
  3. good repeatability is important for process control (when all the tests are carried out in one laboratory);
  4. good reproducibility is important for assessing conformity with specifications.

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).


Table B. Sensitivity ratios.
TestLevel 2 - Level 1Level 3 - Level 2 Level 3 - Level 1
RepeatabilitySchlagversuch 37.218.053.8
Los Angeles 25.021.247.8
Compression 17.18.426.3
FD 16.77.318.1
ReproducibilitySchlagversuch 14.54.512.9
Los Angeles 9.25.912.2
Compression 6.63.310.1
FD 4.72.57.8