Crumb rubber is a term usually applied to recycled rubber from automotive and truck scrap tires. During the recycling process steel and fluff is removed leaving tire rubber with a granular consistency. Continued processing with a granulator and/or cracker mill, possibly with the aid of cryogenics or mechanical means, reduces the size of the particles further. The particles are sized and classified based on various criteria including color (black only or black and white). The granulate is sized by passing through a screen, the size based on a dimension (1/4") or mesh (holes per inch : 10, 20, etc.). Crumb rubber is often used in astroturf as cushioning, where it is sometimes referred to as astro-dirt.
Mesh refers to material that has been sized by passing through a screen with a given number of holes per inch. For example, 10 mesh crumb rubber has passed through a screen with 10 holes per inch resulting in rubber granulate that is slightly less than 1/10 of an inch. The exact size will depend on the size of wire used in the screen.
Recycled pavement material
Rubberized asphalt is the largest market for crumb rubber in the United States, consuming an estimated 220 million pounds, or approximately 12 million tires annually. Crumb rubber is also used as ground cover under playground equipment, and as a surface material for running tracks and athletic fields.
Crushing Crumb Rubber
A waste tire processing plant recycles whole car and truck tires into a valuable product called crumb rubber. This crumb rubber can then used in numerous applications including sport fields, rubberized asphalt for paving and a wide variety of molded products. The steel and fiber removed during this process is also recycled into 20mesh-80mesh Crumb Rubber. 
Description of crumb rubber grading
The following are common classifications of crumb rubber:
Retreaders tire buffings shall consist of clean, fresh, dry buffings from tire retread preparation operations.
No.1 - Tire Granule shall consist of granulated tire crumb, Black Only Guaranteed MetalFree, sized. Magnetically separated materials are not acceptable. Fluff from tire cord removed.
No.2 - Tire Granule shall consist of granulated tire crumb, Black & White Guaranteed MetalFree, sized to minus 40 Mesh. Magnetically separated materials are not acceptable. Fluff from tire cord removed.
No.3 - Tire Granule shall consist of granulated tire crumb, Black Only Magnetically Separated, sized. Fluff from tire cord removed.
No.4 - Tire Granule shall consist of granulated tire crumb, Black & White Magnetically Separated, sized. Fluff from tire cord removed.
No.5 - Tire Granule shall consist of unclassified granulated tire crumb, Sized, Unseparated, not magnetically separated, fluff from tire cord not removed.