Crumb rubber

Crumb rubber coating on the concrete of the urban beach splash pad within the municipal swimming baths at High Park, Toronto

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 astroturf as cushioning, where it is sometimes referred to as astro-dirt.

Background

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.^[1]

Recycled pavement material

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.^[2] Crumb rubber is also used as ground cover under playground equipment, and as a surface material for running tracks and athletic fields.^[2]

What is it made of

• 71% recoverable rubber
• 14% steel
• 3% fiber
• 12% extraneous material

Description of crumb rubber grading

The following are common classifications of crumb rubber:^[3]

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.

Purposes

• Athletic surfaces and fields (rubber mulch)
• Agrimats and equestrian footing
• Automotive parts and tires
• Construction/indoor
• Landscape, trails and walkways
• Molded and extruded products
• Playground and other safety surfaces
• Rubber modified asphalt and sealants
• Rubber and plastic blends^[4]

Benefits

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 fewer 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.^[5]

Concerns

Scientific Research

Crumb rubber infill has been a widely studied topic. To date, there are nearly 100 studies and reports, from government bodies and independent researchers, that have assessed the potential for health risks based on various pathways of exposure. Connecticut’s Department of Public Health conducted an extensive study and published three peer-reviewed studies on the safety of crumb rubber^[6] and determined that there is “no scientific support for a finding of elevated cancer risk from inhalation or ingestion of chemicals derived from recycled tires used on artificial turf fields.”^[7] The Massachusetts’s Department of Health also studied crumb rubber and came to a similar to conclusion.^[8] Three federal agencies are currently conducting a study as well.

Athletics

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 including New York, California, Massachusetts and Connecticut, and school systems have examined everything called into question about synthetic turf. Many of these studies concluded that no health risks exist. Contradicting studies have found that the raw source material (automotive tires) contains high levels of polycyclic aromatic hydrocarbons and metals such as zinc which have proven dermal, acute, and long-term health affects. The EPA is (as of fall 2015) investigating the health affects of exposure to crumb rubber and the initial results of this study are expected at the beginning of 2016.

State of Connecticut - Department of Public Health: "Based upon these findings, the use of outdoor and indoor artificial turf fields is not associated with elevated health risks. However, it would be prudent for building operators to provide adequate ventilation to prevent a buildup of rubber-related volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) at indoor fields. The current study did not evaluate new fields under hot weather conditions and so the potential for acute risks under this circumstance is another uncertainty. The current results are generally consistent with the findings from studies conducted by New York City, New York State, the USEPA and Norway which tested different kinds of fields and under a variety of weather conditions. Thus, it appears that the current results are reasonably representative of conditions that can be encountered at indoor and outdoor crumb rubber fields, although this tentative conclusion could benefit from the testing of additional fields.."^[9]

Brian T. Pavilonis, Clifford P. Weisel, Brian Buckley, and Paul J. Lioy: “The study demonstrated that for the products and fields we tested, exposure to infill and artificial turf was generally considered de minimus [sic], with the possible exception of lead for some fields and materials.”

FIFA’s Chief Medical Officer Prof. Jiří Dvořák says that “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 styrene-butadiene (SBR) granulate infills in artificial turf could potentially occur.”^[10]

Scientific Instrument Services, Volatile Organic Emissions from Automobile Tires, 1999, Santford V. Overton & John J. Manura : Tire "Brand A were found to contain numerous straight and branched chain hydrocarbons, aldehydes, alcohols, ketones, furans and benzene derivatives." Tire brand B was..."found to contain high concentrations of the compounds sulfur dioxide (DOT), 2-methyl-1-propene, 2-propanone, 2-methyl-2-pentene, 2,4-pentanedione, acetic acid and 2,4-(1H, 3H) pyrimidinedione." Chemicals extracted from raw tires in this study would be found in crumb rubber, which is made by grinding feedstock tires. The chemicals listed above include carcinogens, and other chemicals with dermal and endocrine disrupting impacts.^[11]

Environmental impacts

The crumb rubber is used as a filling in artificial turf fields. It was its use in this capacity that prevented about 300 million pounds of rubber from polluting 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)^[12]

The Connecticut Department of Environmental Protection found in a 2010 study that stormwater passing through crumb rubber regularly exceeded aquatic acute toxicity for zinc. Additionally, copper, barium, manganese and aluminum were found to be at elevated levels after stormwater contacted the materials. Semi-volatile organic compounds and PAHs were found to be elevated as well. The levels of most of these compounds were higher than background but were below levels regulated in waterways for environmental protection.^[13]

References