Reliable results of the sieve analysis (determination of the grain size distribution)
and other examinations require correct sample taking and correct handling of the devices prescribed.
Sample taking with bagged goods
If the entire content of a bag is examined fort he grain size distribution (sieve line) or undersized and oversized grain, the bag content constitutes a representative sample, meaning that the problem of correct sample taking does not arise. The bag content must be narrowed down with sample splitters or correct quartering to the desired analysis size as regards quantity.
A representative sample is carried out as follows: at all four edges and in the middle of the bag that lies flat, from the surface to the deepest position single samples are taken with a spade and combined to a collective sample, which after corresponding narrowing, if applicable, can be examined and allows reliable analysis results.
If several bags are to be sampled, a mixed sample from the representative samples of all relevant bags must be created.
Sample taking from a silo lorry
The taking of samples from the lying silo container with an opened dome lid is not representative. Here, it is irrelevant whether the samples are taken at different heights from the silo container. A single sample is equally unrepresentative, which is taken directly from the silo outlet of the silo container or during unloading.
This has to do with the following physical influences:
When filling the silo lorry, the graining (grain group, grain mix) fall into the silo container in free fall. Here, the homogeneous graining is unmixed – to a greater or lesser extent depending on the grain structure and the width between the bottom and top nominal grain - into individual grain fractions, while the larger rock grains roll further away from the place of impact than the smaller ones. Grainings, which come close to the Fuller parable and therefore have a relatively small cavity content, tend to unmix.
During transport in the silo truck on road, the bulk load heaps of the individual aggregate materials become flatter: the larger grains in the bulk load heaps are pushed outwards by the vehicle movements, while the finer grains are vibrated into the cavities between the larger grains. This results in the larger grains in the bulk load heap moving upwards, and a certain compression of the grain mix occurs.
When unloading from the silo lorry, the graining initially comes from the lower and medium range of the silo, i.e. from the large-size graining and then from the finer graining area. Gradually a flow opening occurs in the middle of the silo – just like with whirling water. The (larger) grains on the outside collapse into the middle and flow out of the silo mixed with the finer ones. The process is not even, but occurs gradually and depends on the actual circumstances, such as the specific graining, the unloading speed, the silo geometry and the silo filling height.
In any event, the grain composition of a delivery sampled at any one position in the silo lorry is random.
A representative sample from the silo is therefore only possible by creating a mixed sample of several samples from the outflowing material flow of the silo, while the samples must be taken even across the entire unloading period. But this method has a high risk of injury.
What does the standard require?
Relevant statements can be found in the standards EN 12904, EN 932 and ISO 2591-1.
The European standard EN 12904 Quartz sand and quartz gravel for water filtering sets out under Item 7.2 Analyses:
"It must be possible to determine the grain size distribution by a sample, which was taken directly from the place of production."
As such, it is guaranteed that the sample taken is representative. On request, we provide the lorry driver with a sample that was taken while filling the silo lorry.
EN 932 Testing procedures for general properties of rock graining, Part 1 Sample taking; similar to ISO 2591-1 sets out that rock graining is preferably taken as a cross-section sample from the still transport conveyor in the processing plant of the manufacturer or from the full material flow. Individual samples for an average analysis should be taken at regular intervals while the batch is moving. A batch is defined as a production quantity, delivery quantity or storage quantity, which was produced at the same time under conditions which are considered to be uniform.
Last revised: March 2011