Crumb rubber

Crumb rubber is recycled rubber produced from automotive and truck scrap tires. During the recycling process, steel and tire cord (fluff) are removed, leaving tire rubber with a granular consistency. Continued processing with a granulator or cracker mill, possibly with the aid of cryogenics or by 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 inch) or mesh (holes per inch : 10, 20, etc.). Crumb rubber is often used in artificial turf as cushioning.

The first synthetic grass was placed over concrete in 1964 and became known as “Chem Grass”. It was later named “Astro Turf” when it was installed in the Houston Astrodome in 1966. Athletes did not like playing on these fields because it was dangerous and not comfortable for their landings. Compared to only a thin layer of Astro Turf on top of concrete, a new artificial turf was developed in the early 2000s and was called styrene butadiene rubber, most commonly known as “crumb rubber.” This rubber was made of small black crumb pieces that came from old tires. The tire crumbs were poured in between the fake grass blades, giving the artificial fields more cushion and support. This was a better solution and satisfied athletes and aided in the prevention of major injuries.

Rubberized asphalt is the largest market for crumb rubber in the United States, consuming an estimated 220 million pounds (100 kt), 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.

...
Wikipedia