Testing of industrial products - Aggregates for construction

The Particle Size Distribution test method

The principle of the method

A test portion of aggregate is oven-dried at 110 ± 5 °C to constant mass, weighed, then washed over a 0.063 mm sieve (protected by a 1.00 or 2.00 mm guard sieve). The material retained on the 0.063 mm sieve is oven-dried again at 110 ± 5 °C to constant mass, shaken through a nest of sieves (manually or mechanically), and then each sieve is shaken individually to check that the sieving is complete. The masses retained on each sieve are determined and used to calculate the percentages passing each sieve. In the calculation, the percentage of fines is obtained by adding together the mass lost during the washing operation to the mass of material passing the 0.063 mm sieve during the dry sieving operation.

The method specifies a minimum for the mass of the test portion as a function of the aggregate size. For sands with a size D < 4.00 mm, such as those used in this experiment, the minimum test portion mass is 0.2 kg. To investigate the possibility that smaller test portion masses can give satisfactory results, the participants were asked to carry out tests according to the CEN method and also using small test portions of about 30 g.

According to the CEN method, if the mass retained on a sieve exceeds 0.005 A × d½ (where A is area of the sieve in mm2, and d is its aperture size in mm), then, to avoid overloading of the sieve, the material retained on the sieve should be divided into sub-samples. One procedure is then to sieve all the sub-samples one after the other. Another procedure is to sieve just one sub-sample, making a due allowance in the calculations. With small test portions on the other hand, the risk of overloading, and the need for sub-dividing material retained on sieves, is much reduced.

The proposed CEN method

The CEN test method is similar to methods used in many European countries. However, all countries do not use the same sieve series, so for the CEN method a compromise had to be reached in which a standard series is always used, together with other sieves needed to satisfy particular requirements. For this experiment, only sieves from the standard series were used. There are also differences between national methods in the techniques used to wash the aggregate. One procedure that is used is to wash the aggregate in a bucket and to pour only the water containing suspended solids over the 0.063 mm sieve. Another procedure is to tip the test portion onto the guard sieve and to spray it with water to try to wash away the fines. The CEN method allows both these procedures to be used.