Crumb rubber is recycled rubber produced from automotive and truck scrap tires. During the recycling process steel and tire cord (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.
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 which gave the artificial fields more cushion and support. This was a better solution and satisfied athletes and prevention of major injuries.
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.
What it is made of
- 70% recoverable rubber
- 15% steel
- 3% fiber
- 12% extraneous material
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.
Purpose of Crumb Rubber
- Athletic surfaces and fields
- Agrimats and equestrian footing
- Automotive parts and tires
- Landscape, trails and walkways
- Molded and extruded products
- Playground and other safety surfaces
- Rubber modified asphalt and sealants
- Rubber and plastic blends
When dealing with asphalt overlays, reflection cracks can arise and cause an unwanted crack pattern beneath the pavement. Rubber-modified asphalt uses stress absorbing membranes that reduce the reflective cracking because of its elastic properties. With less cracks, there are fewer repairs, so crumb rubber assists in reducing maintenance costs. The pavement has an increased lifespan because after multiple uses and exposure to different elements, regular asphalt loses elasticity over time. The use of the artificial rubber resists the formation of cracks and has an anti-aging effect that keeps the asphalt in a better condition.
Studies have been conducted and scientists have come to the conclusion that there is, in fact, a connection that the chemicals from the tires may be a cause of cancer. Crumb rubber is made up of compounds suspected to be carcinogenic (cause cancer). The International Agency for Research of Cancer has studied the crumb rubber with multiple tests in cancer treatment hospitals and rubber fabrication facilities around the world. Rutgers University did a study in 2006 of the tire crumbs that were being used on synthetic turf fields, and they found six polycyclic aromatic hydrocarbons (PAH). These compounds are most likely carcinogenic to the human body. Due to a solvent extraction used to release the chemicals in this laboratory study, it is difficult to know how much is actually released into the environment and whether this exposure is significant. A study of the chemicals released from crumb rubber found 49 chemicals, with seven of them being carcinogens.
Artificial turf fields are found all over the United States and are most commonly found at major high schools and athletic facilities. Crumb Rubber is used in the “Infill” in artificial Turf systems to make the surface safe. Volumes of research and testing from academics, federal and state governments like New York, California, Massachusetts and Connecticut, and school systems have examined everything called into question about synthetic turf. The conclusions suggest synthetic turf and crumb rubber poses no health risks.
State of Connecticut - Department of Public Health: "Our study did not find a large amount of vapor or particle release from the fields confirming prior reports from Europe and the US. We put these exposures into a public health context by performing a risk assessment. Our risk assessment did not find elevated cancer risk."
Brian T. Pavilonis, Clifford P. Weisel, Brian Buckley, and Paul J. Lioy: “The SVOCs identified based on library matches of their mass spectra were not present in toxicological databases evaluated and many are ubiquitous part of consumer products. Similarly, the metal concentrations measured in field samples indicate that the risk would be de minimus among all populations expected to use artificial turf fields”
FIFA, Prof. Dr. Jiri Dvorak: “The majority of the studies have been on higher surface area particles and have concluded they are currently acceptable. Therefore the larger granules used in artificial turf will have even less potential for emissions. For example a study undertaken by the Danish Ministry of the Environment concluded that the health risk on children’s playgrounds that contained both worn tires and granulate rubber was insignificant. The available body of research does not substantiate the assumption that cancer resulting from exposure to SBR granulate infills in artificial turf could potentially occur.”
The crumb rubber is used as a filling in artificial turf fields. They were used in its capacity that prevented about 300 million pounds of rubber that polluted landfills in 2007. Generally it takes 20,000-40,000 scrap tires to produce enough filling to cover an average football field (City of Portland, 2008)
- Rappleye, Hannah. "How Safe is the Artificial Turf?". NBC.
- EPA. "Management of Scrap Tires". Retrieved 2007-06-12.
- "Granulated Rubber (Tire Crumb)". Retrieved July 15, 2012.
- "Artificial Turf: Exposures to Ground Up Rubber Tires - Athletic Fields, Playgrounds, Garden Mulch". Environment and Human Health Inc.
- "Benefits of Rubberized Asphalt". Clemson.
- "LABORATORY IDENTIFICATION OF COMPOUNDS RELEASED FROM TIRE CRUMBS AND RUBBER MULCH". Environment and Human Health Inc.
- "Review of the Impacts of Crumb Rubber in Artificial Turf Applications". Liberty Tire.