The flow coefficient of fine aggregates has been introduced in Switzerland in January 2005. At the present time, no corresponding national specifications can be determined because of a of lack experience. It is needed to provide the basic data to enable a reasonable and coherent definition of national specifications for the flow coefficient in Switzerland.
The objectives of the project are as follows:
- To determine the variability of the flow coefficient of fine aggregates from Switzerland
- To look for a possible correlation between flow coefficient according to EN 933-6 and aggregate shape according to the old SN 670 130
- To provide the basic data to define national specifications for the flow coefficient of fine aggregates
Following activities will be provided during the project:
- Parallel testing the properties of fine aggregates according to the old (particle shape / curve) and a new test method (flow coefficient)
- Clarification of possible correlations between the results according to the new / old test methods
- Production of bituminous mixtures with sand with strongly varying flow coefficient
- Investigation of the deformation behavior of sample mix depending on the flow coefficient of sands used
- Summarisation of recommendations for future demands on the flow coefficient of fine aggregates.
In a first step, a round robin test (RTT) among the three participating laboratories and research institutes was performed. It consisted of the determination of particle shape and particle roundness and the measurement of flow coefficient.
It revealed a lack of correlation for these test results between the different testing facilities. One possible reason could be, that some of the test method rely on visual aspects and, therefore, are based to a certain extent on the experience and specific approach of the tester. In addition, it was impossible to establish a mathematical correlation between the rests of particle shape and particle roundness, and the flow coefficient. The flow coefficient was found to be an unsuitable value to describe either the particle shape or particle roundness.
Analysing the data of the RTT yield in the hypothesis, that the flow coefficient were influenced by the particle grading. The result of a series with artificially combined grading curves, using the same sand source, supports this hypothesis, as here the flow coefficient solely depends on the particle grading.
After the publication of the results of Stütz et al.c which showed that there is no correlation between the flow coefficient and specific values of asphalt concrete, it was decided that such tests were not useful anymore in the present research project and were therefore dropped in the research program. It was found more useful, and therefore decided by the supervising body of the project to focus on the suspected correlation between the grading of a sand and its flow coefficient.
Using the laboratory database of IMP Bautest AG from the years 2005 to 2013, mandates with grading curves of sands 0/2 and 0/4 and its associated flow coefficients were systematically investigated in order to check the interdependency of these parameters.
The review revealed more or less pronounced correlation of data from the investigated gravel quarries for both, the sand 0/2 mm and the sand 0/4 mm. As some of the investigated gravel quarries show a strong correlation between the grading curve and flow coefficient, the latter is not a suitable test to assess particle shape and particle roundness of fine aggregate.
The present project has clearly shown that the flow coefficient test is not a suitable indicator test for the quality of sand. The work of Stütz et al.  in addition could not establish a correlation between the flow coefficient and specific characteristics of asphalt concrete.