Recycled Content Flooring Panels

Use of Secondary Crumb Rubber In Compression Molding

Evaluation of CRM Pavement Using SHRP Tests

Performance Testing of Laminated Recycled Rubber Products

Rubber Proportioning and Feed Equipment Modification for CRM Asphalt Project

Engineered Athletic Surface: Shock Absorption Testing

Shredded Scrap Tires As Lightweight Fill In Road Embankments

Technology Brief

Recycled Content Flooring Panels

The SmartCellsTM athletic surface is a unique playing surface system developed by Seamless Attenuating Technologies, Inc., (SATECH), of Kirkland, Washington. The surface system incorporates a cellular cushion pad molded from a mixture of recycled rubber and used athletic shoes blended with a virgin rubber compound. The 1.75" thick pad is then bonded to a base and covered with a specially formulated urethane coating that simulates a traditional wood finish. The result is a softer, more slip resistant floor that retains all the properties of a hardwood surface.

A SmartCellsTM gym floor was installed at the Puget Sound Christian College in Edmonds, Washington in early 1996 as a demonstration project. The recycled content of the floor is 40%, with approximately 15,000 used sports shoes and 525 tires in the full-size gym floor. The college has been very satisfied with the performance of the floor.

SATECH believes the flooring has potential for replacing gym floors, artificial turf for indoor sports such as soccer and football, and non-sport flooring in schools and health care facilities. Successful commercialization of this flooring technology is somewhat dependent on results from comparison of performance data of the SmartCellsTM floor to traditional athletic or non-sport flooring.

To date, testing has included performance data on traction, shock absorption and impact attenuation, and reviews from amateur and professional players (including post-practice evaluations by several NBA teams).

The CWC has funded several empirical tests to collect impact and traction data to compare the SmartCellsTM surface to other athletic surfaces.

The shock absorption and impact attenuation testing measures the ability of the surface to reduce the incidence and severity of stress fractures and serious head injury. The test is conducted per American Society for Testing and Materials (ASTM) test methods for surface systems under and around playground or playing equipment. (ASTM F1292-93 and F355-86). Preliminary results of these impact tests are presented in Table 1.

Additionally, traction characteristics of the SmartCellsTM surface have been tested to determine the translational and kinetic coefficients of friction. Preliminary, qualitative results of these tests are presented in Table 1. Test results indicate that the SmartCellsTM surface will meet or surpass the properties of traditional wood playing surfaces.

When comparing impact data for traditional gym floors and artificial turf surfaces, the SmartCellsTM surface may have potential to reduce certain impact-related injuries such as concussions, and possibly even fatigue-related injuries such as stress fractures,

The friction coefficients of the SmartCellsTM were significantly higher than those of the traditional wood floor. Translational, static coefficients were higher by 25% and rotational, kinetic coefficients were higher by 29%. The resulting affect of this data on player performance and injury potential is unknown.

Additionally, computer models and biomechanic testing at the University of Michigan show that the new floor can increase jumping height and running speed. Finally, written reviews from athletes who have practiced on the court, indicate the same or less fatigue or soreness after practice. In one post-practice evaluation by an NBA team, the results found that 85% of the 13 players felt the traction was the same or better, 69% said the ball bounce was the same or better, and 92% felt their ability to train on the SmartCellsTM was the same or better.

SATECH plans to commercialize the new surface in a variety of professional athletic and commercial venues, and define new safety standards for certain athletic surfacing. Marketing and public relations assistance from the CWC has aided in securing support from adidas Corporation, interest from the NBA, and widespread media exposure.

Fact Sheet Update: November 1997
 
 

CWC Technology Brief

APPLIED ENGINEERING TOOLS FOR RECYCLING: USE OF SECONDARY CRUMB RUBBER IN COMPRESSION MOLDING

This technology brief introduces an applied engineering tool for designing and conducting trials to evaluate the conversion of secondary crumb into conventional compression molded rubber products.

The use of recycled rubber is becoming more important because of stockpiles of spent tires, mandates from large industrial customers, governmental agencies, and consumer demand for recycled content products. In fact, six million scrap tires were ground into crumb for beneficial reuse in 1995. Fortunately, crumb technology has granted a cost-effective means of utilizing spent tires and rubber scrap.

The tool provides essential background information on issues related to compression molding with crumb rubber. Additionally, a structured technique provides guidelines on how to design and conduct trials to evaluate the usability of crumb in compression molded rubber products.

Material and process parameters are both important considerations for converting to crumb content formulations.

Material issues addressed in the tool include:

· Formulating rubber compounds containing crumb.
· Compatibility of different rubber compounds, such as Styrene-Butadiene Rubber (SBR), Natural Rubber (NR), Isobutylene-Isoprene (Butyl), Polychloroprene (CR), Nitrile Rubber (NBR), and Ethylene Propylene Diene Monomer (EPDM).
· Crumb specifications such as particle size distribution, loose contamination (e.g., metals, fiber, other), extract content, ash content, bulk density, carbon black content, moisture content, additives and surface treatment.
· Mixed compound specifications such as viscosity and scorch.
· End-product material properties such as tensile strength, elongation, hardness, tensile strength, modulus, tear strength and potential other properties.

Equipment and processing issues addressed in the tool include:

· Processing crumb from recycled rubber sources.
· Mixing the crumb-content stock.
· Aging and reworking mixed stock.
· Molding and curing
· Expected benefits affects of crumb on processing, such as reduced shrinkage, reduced mold sticking and reduced curing time.
· Process and equipment requirements to compensate for increased compound viscosity, and potential decreased flex and tear strength.

Logsheets are provided to facilitate evaluation of the trial data and comments that were documented during the trials. Troubleshooting recommendations are provided based on deficiencies in required properties of the end-product.

The tool suggests use of each trial result in planning subsequent trials. The findings of these documented and controlled trials should result in the optimum crumb content and processing conditions to produce an acceptable compression molded product.

To order a copy of the tool, or request more information on converting to recycled crumb in compression molding, contact the Center at (206) 464-7040.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: April 1997

Fact Sheet Issue Date: April 1997

CWC Technology Brief

EVALUATION OF CRM ASPHALT USING SHRP TESTS

A project to judge the effectiveness of using newly developed testing procedures in evaluating the use of Crumb Rubber Modifier (CRM) from scrap tires in asphalt pavements was conducted by Terrel Research of Edmonds, Washington and Oregon State University (OSU). The project used performance based testing equipment and protocols developed under the Strategic Highway Research Program (SHRP). Two methodologies of including CRM in asphalt concrete were compared to conventional pavement mixtures as part of the 1993 resurfacing program by the Seattle Engineering Department (SED).

Background

Disposing of scrap tires as CRM in asphalt mixtures has been used on a limited basis for more than 15 years. Several competing technological approaches have been used with successful projects being offset by costly failures that were not expected or predicted using conventional engineering design and testing methods.

Conventional mixture design methods used by highway agencies are usually based on either the Hveem or Marshall procedures. In these procedures, the air voids, stability, and other factors are used to help determine the optimum asphalt content. However, a shortcoming is that the testing procedures and criteria used to judge mixture quality are empirical and do not apply to special mixtures that include modifiers such as CRM. The results of conventional tests cannot be extrapolated to evaluate expected performance with any degree of confidence. Hence, the SHRP performance based testing procedures are used with the expectation of obtaining more useful results.

Technical Problems

Most of the previous CRM projects were designed using proprietary technology without the user agencies being directly involved in the design process. Also, the empirical design procedures provided only limited data upon which to base judgments on expected performance prior to construction, so the user agency had only limited ability to evaluate the technical reasons for success or failure.

The SHRP developed tests and related tests on mixtures follow procedures that are aimed at minimizing specific types of pavement distress, especially:

· thermal cracking (resistance at low temperatures is evaluated using tension tests),
· rutting (resistance is evaluated using shear strain and wheel tracking tests),
· fatigue cracking (resistance is tested using flexural beam specimens),
· aging (potential is evaluated for both short and long term oxidation), and
· water damage (potential is evaluated in an environmental conditioning test).

The SHRP asphalt mixture tests were recently developed at OSU and the University of California, Berkley (UCB) to evaluate properties that could be used to better resist these types of distress.

Mix designs were proposed by the CRM technology suppliers, EnviroTire, Inc. (dry process known as PlusRide™ II) and Eagle Crest Construction, Inc. (wet process known as ARHM-GG) and compared to a conventional paving mixture developed by the Washington State DOT (WSDOT). At OSU, slabs of asphalt concrete were constructed following the designs provided. Various specimens were cut from the slabs for laboratory testing. In addition, core samples were obtained from each test pavement in Seattle.

Performance Test Results

The testing program was successful in demonstrating significant differences among the paving mixtures and it was concluded that the SHRP based tests are appropriate for evaluating CRM mixtures.

Specifically, all the CRM mixtures showed superior characteristics in resisting thermal and fatigue cracking. The PlusRide™ II mixtures did not fare well in resisting rutting, but better mix design procedures should improve this quality. All of the CRM mixtures showed excellent behavior in aging and water sensitivity.

Cost Impact

The cost effectiveness of this CRM project will not be known until the projected performance of the pavements is validated over the next several years. The unit cost per ton of CRM asphalt paving mixtures is higher, but the life cycle cost is expected to be lower. In addition, there is added benefit of recycling scrap tires, approximately 6,000 for this project.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: October 1994

Fact Sheet Update: October 1994

CWC Technology Brief

PERFORMANCE TESTING OF LAMINATED-RECYCLED RUBBER PRODUCTS

Product testing was conducted to characterize the performance of laminated-recycled rubber products. Results of such testing can be used for comparison with performance data from virgin content rubber products.

Schuyler Rubber Company manufactures various fenders for marine applications from used tires. These fenders are fitted on tug boats and around docks and are designed to cushion the initial impact of tug-to-boat, or boat-to-dock. Performance data developed in the laboratories of virgin rubber product makers are used to market similar products. Lacking the testing equipment and expertise to perform these tests, small companies manufacturing competitive products from recycled rubber have not been able to develop comparable data for their products. Schuyler Rubber contributed substantially to the project.

Project Objective

The purpose of this project was to develop a data base on mechanical characteristics of laminated-recycled rubber products. Testing was performed to determine load deflection properties, hardness, loop pull-out strengths and fatigue stress.

This data should prove useful for anyone manufacturing products made from whole used tire sections that have not been chemically treated. More specifically, this data should prove useful for anyone interested in developing cushioning products based on used tires. Applications might include safety barriers along highways and shock absorbing materials near construction/blasting sites, as well as the marine fender products on which the testing was performed.

Description of Testing Specimens

Five types of tests were used for this project, load/deflection, hardness, tensile, loop-pullout and fatigue tests. Laminated rubber products as tested for this project are made from used truck and bus tires. That portion of the tire in ground contact is removed from the tire carcass, milled to a common thickness and cut into layers of the cut rubber between metal plates and clamping the assembly together with large threaded bars.

Specimens provided came in several shapes. All load/deflection specimens came in as blocks. These blocks can be thought of as a deck of cards held together with large bolts drilled through the face of the cards near one edge of the deck. The simple blocks were simply a uniform stack of these cards/sections of tire. Other test blocks had tire sections (cards) of varying length to provide an anticipated higher measure of cushioning, or blocks with built in loops.

Hardness and tensile specimens were simply sections of the tire which were tested analogous to testing one card from the deck.

Testing Methodology

The primary tests were the load/deflection tests. These tests took a nominal 12 inch by 12 inch specimen of varying thickness and simply loaded this 1 square foot surface, monitoring deflection as a function of applied load. This provides the load/deflection and energy absorption/deflection data needed to design marine fenders.

The loop pullout tests to measure the ease with which these loops can be pulled out of the main body of the fender.

A fatigue test was developed which cycled a looped block between 2,000 lbs and 10,000 lbs load per sq.ft. every 54 seconds for 7,850 cycles. Deflection was determined as a function of loads before and after the test.

Results

Results of the load deflection test showed that various block design, whether with internal holes or steps or loops built into the block all provide varying degrees of deflection under load. This provides an ability to design for whatever deflection will be needed while using a common starting material. Data showed that loops were the softest, i.e. provided the greatest amount of cushioning, while the solid block was the firmest.

International Rubber Hardness Degree (ASTM D1415) values shown are the averages of three tests per side per specimen.

Tensile strengths were measured at 535 psi, 871 psi, 2443 psi, and 2817 psi. These values suggesting that the variability of the starting material makes characterization of individual pieces impractical for projecting such information into a fender design. Loop pullout testing showed failure at 9650 pounds with a failure mode that was both a tearing of the loop and the removal of the loop from the block. Fatigue testing showed that after 7850 cycles, the blocks became somewhat softer. Minor delamination occurred.

Conclusions

While no commercial product made with virgin starting materials was tested for this project, a review of the literature provided by these manufacturers shows that the recycled tire based fenders have generally very similar characteristics in load/deflection conditions. Our testing shows that a wide range of load/deflection properties can be engineered into the recycled tire marine fenders using differing manufacturing techniques.

This information should help position laminated-recycled rubber product makers in a more competitive position relative to companies producing marine fenders with virgin starting materials. These other companies can design molds and cross sections to provide a variety of energy absorbing capability. With this basic data and perhaps a little more basic engineering, companies using tires as their starting material should be able to match these virgin-material based products.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: October 1994

Fact Sheet Update: October 1994

CWC Technology Brief

RUBBER PROPORTIONING AND FEED EQUIPMENT MODIFICATION FOR CRM ASPHALT PRODUCTION

A project to resolve technical problems experienced by asphalt companies in the production of crumb rubber modified (CRM) asphalt paving material using the "dry" mix process was conducted by the Asphalt Equipment and Service Company (AESCO) of Auburn, Washington. The project objective was to design a rubber conveying system for introducing granulated rubber to an asphalt plant. The system was retrofitted to existing mix and feed equipment owned by EnviroTire, Inc. The modification streamlines the production process for the manufacture of CRM asphalt, thereby reducing the cost of the product.

Background

Conventional asphalt plant mixing equipment is used to produce crumb rubber modified asphalt. Special feed equipment is needed to control the specified blend of coarse and fine rubber, however, and to introduce the rubber to the plant's mixer. The configuration of asphalt plants varies considerably and installation of rubber proportioning and feed equipment to the asphalt plant can be very time consuming and disruptive to normal plant operations. Most asphalt producers and contractors have not had experience in handling granulated rubber or in the precise calibration of the materials feeding system that is needed to mix the specified formula. Consequently, CRM asphalt production takes longer, is more costly, and product specifications may not be met precisely, causing product failure.

With easy to operate rubber proportioning and feed equipment designed to produce consistent results under a variety of plant configurations, more asphalt companies will be willing to bid a CRM job at costs that are competitive with conventional asphalt concrete mixes.

Technical Problem Actual jobsite experience with the EnviroTire "dry" mix rubber blending equipment showed that, although this equipment provided extremely accurate rubber gradation results (well within jobsite specifications), it presented another problem: How to move the pre-blended rubber into the plant mixer at the required flow rates in an efficient and cost effective manner without disrupting the contractor's existing plant equipment configuration.

A rubber conveying system design was sought to satisfy the following requirements:

· Capable of discharging rubber at the appropriate elevation and the correct flow rates.
· Small, compact, lightweight, and flexible to facilitate transportation and installation.
· Compatible with both drum mix or batch type asphalt plants, requiring a minimum of modifications to existing plant equipment.
· Rugged enough to withstand the conditions typical of construction sites.
· Priced realistically with respect to scope and size of the average project.
· Individual components should be off-the-shelf, reasonably available, and if possible, familiar to asphalt plant maintenance and operating personnel.

Belt conveyors, augers, bucket elevators, and slat conveyors were eliminated early on as not meeting the above criteria. Several types of aero-mechanical and pneumatic conveyors were investigated. Tests were conducted by interested equipment manufacturers in their respective equipment test labs on their system's ability to reach the maximum required flow rate of 20 tons per hour of 35 pounds per cubic foot finely graded granulated rubber.

Performance Test Results

Based on the laboratory testing results, the decision was made to purchase the pneumatic system. While both the pneumatic and the aero-mechanical systems could handle the required flow rates, the aero-mechanical system was judged too fragile for the harsh construction environment. This system required special structural protection to keep it from damage during transportation and installation.

The pneumatic system was field tested at AESCO's plant site in June 1993. Tests were conducted for both the drum and batch plant configurations at flow rates of 5 to 23 tons per hour. No appreciable problems occurred while following operating guidelines.

Cost Impact

Cost has been an impediment in the CRM market, as conventional asphalt is significantly cheaper to produce. With these equipment modifications, the cost of producing dry mix CRM asphalts should decrease. At $2 per ton less, a savings to the state of more than $63,000 could be realized in using dry mix CRM asphalt to meet 1994 federal mandates for CRM asphalt usage.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Issue Date: June 1993

Fact Sheet Update: September 1994

CWC Technology Brief

ENGINEERED ATHLETIC SURFACE: SHOCK ABSORPTION TESTING

Background

About four million scrap tires are discarded each year in Washington State. Research conducted by SATECH (an environmental technology company based in Kirkland, WA) indicates that some of this scrap rubber can be recovered for use in engineered athletic surfaces for sports such as basketball, football, and track racing. One regulation basketball court, constructed with a recycled content material, uses 15,000 used sneakers and 525 scrap tires alone. Tests have shown that basketball courts designed by SATECH exhibit performance characteristics that are superior to those of traditional hardwood surfaces.

SATECH is working with other Washington companies to manufacture and test this new surface technology in a variety of professional athletic and commercial applications. Rubber Granulators of Lynnwood collects the tennis shoes, scrap tires, and other post-consumer rubber, plastic, and textile materials, and processes them into "SmartBitsô." The SmartBitsô are then sent to Scougal Rubber in Seattle where they are blended with a virgin rubber compound and molded into a "SmartCellsô" pad. GACO Western, a urethane coatings manufacturer in Tukwila, designs the urethane/SmartBitsô blends that bond and surface the pad. Application techniques for the "SportPadô" playing surface are being developed by Meidling Concrete in Spokane.

Initial Testing

The Clean Washington Center's ReTAP program has funded a series of three reports that examine the performance characteristics of engineered basketball surfaces containing recycled rubber.

The first report, Engineered Athletic Surface/Shock Absorption Testing of Recycled-Content Flooring Panels (publication T-95-1) provides information on field testing of two-inch flooring samples for shock absorption and impact attenuation. The flooring samples developed by SATECH (called SmartCellsô) contain an engineered pad, one and 3/4-inches thick, containing approximately 25% rubber and fiber from recycled sneakers and scrap tires. The SmartCellsô pad is bonded to a 3/4" plywood panel and surfaced with a urethane to simulate the look and feel of a traditional wood floor.

The next two reports detail test results that enable a comparison between the impact characteristics of traditional hardwood athletic surfacing and a new engineered surface that contains recycled sneakers and scrap tire rubber. Engineered Athletic Surface/PART ONE: Shock Absorption Testing of Traditional Hardwood Floor (publication T-95-2), provides field testing for a standard hardwood basketball court. Engineered Athletic Surface/PART TWO: Shock Absorption Testing of Recycled-Content Floor (publication T-95-3), provides the impact characteristics of the full-size recycled-content surface.

Test Specifications

Each of the three reports presents detailed shock absorption and impact attenuation test results, including maximum impact acceleration (g-max) and head injury criteria (HIC). The testing assesses the ability of each playing surface to protect against serious head injury, such as concussions. Because there is no official safety standard for playing surfaces at this time, the testing has been performed in accordance with ASTM F1292-93 (Standard Specification for Impact Attenuation of Surface Systems Under and Around Playground Equipment) and ASTM F 355-86, Procedure C (Standard Test Method for Shock-Absorbing Properties of Playing Surface Systems and Materials), both of which are standards under the jurisdiction of ASTM Committee F-8 on Sports Equipment and Facilities, and Subcommittee F08.52 on Playing Surfaces and Facilities. This testing work supports the process of data collection that is critical to developing specifications for sports playing surfaces that provide maximum protection for athletes of all types with minimal compromise of playability.

Test Results

Fourteen impact sites were chosen over the surface of an existing, standard hardwood basketball court, to test its shock absorption characteristics. Impact sites were distributed over the entire playing surface, with additional impact sites in areas closest to the baskets. Of the fourteen impact sites, nine did not meet the performance criteria specified in ASTM F1292-93 at the minimum impact height of one foot.

Three two-inch flooring samples with durometer counts of 55, 60, and 65, respectively, were tested using the same shock absorption performance criteria. Test results indicate that the samples with urethane counts of 60 and 65 each met performance criteria specified in ASTM F1292-93 for a maximum impact height of six feet. The 55 durometer count sample met the same criteria for a maximum impact height of five feet.

For the final report, fourteen impact sites were chosen over the surface of a basketball court constructed using panels of the two-inch recycled content material (SmartCellsô). Impact sites were distributed over the entire playing surface, in the same pattern that was used in testing the standard hardwood surface. All fourteen impact sites met the performance criteria specified in ASTM at an impact height of six feet.

SATECH believes that this new playing surface can reduce instances of certain impact-related injuries like concussions, and chronic fatigue-related injuries such as stress fractures. Computer models and biomechanical tests have also suggested that athletes' jumping heights and running speeds increase when playing on the recycled content surface. Tests will also be conducted to measure frictional properties of the surface. These tests are slated for completion this fall.

This technology brief was prepared by the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology.

The support of this project by SATECH is gratefully acknowledged.

Reports Dated: August 1995 (T-95-1), September 1995 (T-95-2), and April 1996 (T-95-3) Fact Sheet Update: July 1996

CWC Technology Brief

SHREDDED SCRAP TIRES AS LIGHTWEIGHT FILL IN ROAD EMBANKMENTS

Each year in the United States land slips occur under road embankments because a weak plane in the in-situ soil is overloaded and fails. One preferred repair technique is to return the road to its original elevation using a lightweight fill instead of no rmal soil. When properly done, this technique prevents the road structure from overloading the weak slip plane again. In the past, sawdust or chipped lumber has been used as the fill material. However, these materials tend to rot in repeated wet/dry cy cles and are becoming more expensive with the rising cost of wood fiber. This technology brief provides information on a viable alternative to conventional lightweight fill æ using shredded scrap tires.

Tires As Lightweight Embankments

 Road embankments have the potential to use an enormous quantity of tires. For example, an eight-mile, 20-foot embankment for a two-lane road would use approximately 50,000 tires. Scrap tires have been tested as lightweight fill for road embankments in W ashington and a number of other states. The Federal Highway Administration sponsored an experimental project in Oregon (1). Some of the valuable attributes of shredded scrap tires documented in this and other studies are described below.

Compaction. Shredded scrap tires are easy to compact and are very lightweight with a density of 24 to 33 pounds per cubic foot (pcf) in haul trucks. They have a compacted density of around 45 pcf (a little less than half the weight of ordinary co mpacted soil) before being surcharged with soil, the pavement structure, and traffic. The maximum density of the shredded tire fill after completion of the pavement structure and several months of traffic is approximately 52 pcf. Vibratory compaction eq uipment does not work well with shredded tires because the material tends to "bounce" rather than compact. One successful compacting technique is to place approximately three feet of loose tire shreds and then compact with three "full coverage" passes of a D-8 or equivalent bulldozer.

Permeability. Voids between tire shreds give shredded scrap tire fill the permeability of clean gravel, but with significantly less weight. This is of particular importance because the light weight and permeability of shredded tire fill minimizes the load increase on the weak slip plane during rainy periods. To prevent the migration of fines into the fill and possible settlement in the surrounding soils, shredded tire fills are commonly encapsulated in geotextile fabric.

Installation. Shredded tires should be dumped from haul trucks and spread by steel-tracked equipment to prevent flat tires, as steel wires protruding from tire shreds can penetrate the inflated tires of loaded trucks.

Leachate. To address concerns about the potential leaching of heavy metals and polynuclear aromatic hydrocarbons (PAHs) from shredded scrap tires, the Rubber Manufacturers Association conducted the EPA's Toxicity Characterization Leaching Procedur e (TCLP) on tires and other rubber products. According to the results, the leachates from rubber products do not exceed federal standards.

 However, recent tests indicate that if tires are submerged in confined, relatively small amounts of water for several days to weeks, the water becomes acutely lethal to some varieties of fish (2, 3 ,4). While not similar to rain water flowing through a s hredded tire fill, this situation could be replicated by slow flowing ground water seeping through a large tire fill. Until ongoing tests determine which compound(s) are causing the lethality, the best practice when using shredded tire fills is to keep t he bottom of the fill above the historic high of the ground water table.

Other Uses For Shredded Tires

 Like rubber, tire shreds have a high coefficient of friction. With the angle of repose for compacted tire chips as high as 85 degrees (5), steep slopes can easily be maintained with shredded tires. Combining both a high coefficient of friction and high permeability, shredded tire scraps will also work well as backfill behind retaining walls (6). Shredded scrap tire fill has also been used as an effective insulating layer below roads to minimize frost penetration. Field tests have shown that a 6-inch t hick tire chip layer can reduce frost penetration by up to 40% (7).

References

1. Experimental Project Use of Shredded Tires for Lightweight Fill. FHWA Experimental Project No. 1 (DTFH-71-90-501-OR-11). Federal Highway Administration, 1991.

2. Abernethy. The Acute Lethality to Rainbow Trout of Water Contaminated by an Automobile Tire. Ontario, Canada, 1994.

 3. Nelson et. al. Identification of Tire Leachate Toxicants and a Risk Assessment of Water Quality Effects Using Tire Reefs in Canals. U.S. Department of the Interior, Bureau of Reclamation, 1993.

 4. Kellough. The Effects of Scrap Automobile Tires in Water. Ontario Ministry of the Environment, 1991.

 5. Edil and P. Bosscher. Development of Engineering Criteria for Shredded Waste Tires in Highway Applications. University of Wisconsin-Madison, 1992. p. 15.

 6. D. Humphrey and T.C. Sanford. "Tire Chips as Lightweight Subgrade Fill and Retaining Wall Backfill." Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials as By-Products for Construction of Highway Facilities. Denv er, CO, 1993.

 7. Humphrey and R. Eaton. Tire Chips as Subgrade Insulation - Field Trial. University of Maine - Orono, 1994.

Additional Resources

A Report on the RMA TCLP Assessment Project, Rubber Manufacturers Association, Washington, DC, 1989.

This technology brief was prepared by the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufact uring Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology.

Fact Sheet Issue Date: August 1995

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