Deciding the size of material to use on any given project is an issue that causes much head scratching among customers. What is best for my driveway? What size material shall I use for my gabion baskets? etc etc. We will now try and explain in as simple terms as possible how materials are graded and the actual sizes involved.
Aggregates are produced by either crushing or screening (sieving), or a combination of the two. Raw material comes out of the ground in all manner of sizes and must be graded into different fractions before being offered for sale. For those not in trade, you would be forgiven for thinking that when ordering 20mm Gravel for example, that each stone will be a perfect 20mm in size. Correct? No. Graded gravels are produced by screening (sieving) which involves 2 sieves, a top sieve and a bottom sieve. A 20mm Gravel will be produced using a top sieve with 20mm holes and a bottom sieve with 10mm holes. The product produced will be everything small enough to pass through the 20mm sieve and too big to pass through the 10mm sieve. So, in actual fact a 20mm Gravel's true grading is 10-20mm. Similarly, a 10mm Gravel is produced using an upper sieve with 10mm holes and a lower sieve with 4mm holes hence the true grading of a 10mm Gravel is actually 4-10mm. This is the standard method of production throughout the globe.