Industry-government partnership issues design guidelines for tire shred fill projects.

A joint industry-government partnership issued technical recommendations designed to minimize internal heating of tire shred fills. The recommendations, "Design guidelines to minimize internal heating of tire shred fills," were developed by the ad hoc civil engineering committee, an industry-government partnership formed to address possible causes of internal heating reactions that occurred at three tire shred fill projects in 1995. The guidelines are the result of investigation into more than 70 successful tire shred fill projects.

"We hope these guidelines will relieve some of the uncertainty about using tire shreds as a fill material, and allow expansion of the use of shredded scrap tires in civil engineering applications as a major market for scrap tires," said John Serumgard, chairman of the Scrap Tire Management Council, and Richard Gust, chairman of the Tire and Rubber Recycling Advisory Committee, the ad hoc Civil Engineering Committee organizers.

The guidelines were developed to incorporate design and engineering criteria for two classes of tire shred fills: Class I, up to one meter in depth; and Class II, greater than one meter, up to three meters in depth. Tire chip size, cleanliness, exposed wire, fill access to water and air, geotextile covering and cover fill material are among the criteria addressed in the guidelines. Dr. Dana Humphrey, professor of civil engineering at the University of Maine and a member of the committee, served as the principal author for the guidelines.

"Although there have been no catastrophic heating events in projects up to about four meters of tire shred fill, we recommend keeping tire shred fills three meters thick or less until the causes of the reaction are fully understood," Humphrey said. In addition to using the new design guidelines, Humphrey recommends that tire shred fills be designed to meet the requirements imposed by the engineering performance of the individual projects.

The guidelines were developed with input from all committee members and were reviewed by state highway engineers with experience in utilizing tire shred fills.

Design guidelines to minimize internal heating of tire shred fills

Background

Since 1988 more than 70 tire shred fills with a thickness less than 1 m and an additional ten fills less than 4 m thick have been constructed. In 1995 three tire shred fills with a thickness greater than 8 m experienced a catastrophic internal heating reaction. These unfavorable experiences have curtailed the use of all tire shred fills on highway projects.

Possible causes of the reaction are oxidation of the exposed steel belts and oxidation of the rubber. Microbes may have played a role in both reactions. Although details of the reaction are under study, the following factors are thought to create conditions favorable for oxidation of exposed steel and/or rubber: free access to air; free access to water; retention heat caused by the high insulating value of tire shreds in combination with a large fill thickness; large amounts of exposed steel belts; smaller tire shred sizes and excessive amounts of granulated rubber particles; and the presence of inorganic and organic nutrients that would enhance microbial action.

The design guidelines given in the following sections were developed to minimize the possibility for heating of tire shred fills by minimizing the conditions favorable for this reaction. As more is learned about the causes of the reaction, it may be possible to ease some of the guidelines. In developing these guidelines, the insulating effect caused by increasing fill thickness and the favorable performance of projects with tire shred fills less than 4 m thick were considered. Thus, design guidelines are less stringent for projects with thinner tire shred layers. The guidelines are divided into two classes: Class I fills with tire shred layers less than 1 m thick and Class II fills with tire shred layers in the range of I m to 3 m thick. Although there have been no projects with less than 4 m of tire shred fill that have experienced a catastrophic heating reaction, to be conservative, tire shred layers greater than 3 m thick are not recommended. In addition to the guidelines given below, the designer must choose the maximum tire shred size, thickness of overlying soil cover, etc., to meet the requirements imposed by the engineering performance of the project. The guidelines are for use in designing tire shred monofills. Design of fills that are mixtures or alternating layers of tire shreds and mineral soil that is free from organic matter should be handled on a case by case basis.

General guidelines

The tire shreds shall be free of all contaminates such as oil, grease, gasoline, diesel fuel, etc., that could create a fire hazard. In no case shall the tire shreds contain the remains of tires that have been subjected to a fire because the heat of a fire may liberate liquid petroleum products from the tire that could create a fire hazard when the shreds are placed in a fill.

Class I fills

Material guidelines

The tire shreds shall have a maximum of 50% (by weight) passing the 38-mm sieve and a maximum of 5% (by weight) passing the 4.75 mm sieve.

Design guidelines

No design features are required to minimize heating of Class I fills.

Class II fills

Material guidelines

The tire shreds shall have a maximum of 25% (by weight) passing the 38 mm sieve and a maximum of 1% (by weight) passing the 4.75 mm sieve. The tire shreds shall be free from fragments of wood, wood chips and other fibrous organic matter. The tire shreds shall have less than 1% (by weight) of metal fragments which are not at least partially encased in rubber. Metal fragments that are partially encased in rubber shall protrude no more than 25 mm from the cut edge of the tire shred on 75% of the pieces and no more than 50 mm on 100% of the pieces.

Design guidelines

The tire shred fill shall be constructed in such a way that infiltration of water and air is minimized. Moreover, there shall be no direct contact between tire shreds and soil containing organic matter, such as topsoil. One possible way to accomplish this is to cover the top and sizes of the fill will a 0.5 m thick layer of compacted mineral soil with a minimum of 30% fines. The mineral soil should be free from organic matter and should be separated from the tire shreds with a geotextile. The top of the mineral soil layer should be sloped so that water will drain away from the tire shred fill. Additional fill may be placed on top of the mineral soil layer as needed to meet the overall design of the project. If the project will be paved, it is recommended that the pavement extend to the shoulder of the embankment or that other measures be taken to minimize infiltration at the edge of the pavement.

Use of drainage features located at the bottom of the fill that could provide free access to air should be avoided. This includes, but is not limited to, open graded drainage layers daylighting on the side of the fill and drainage holes in walls. Under some conditions, it may be possible to use a well graded granular soil as a drainage layer. The thickness of the drainage layer at the point where it daylights on the side of the fill should be minimized. For tire shred fills placed against walls, it is recommended that the drainage holes in the wall be covered with well graded granular soil. The granular soil should be separated from the tire shreds with geotextile.

German R&P machinery exports post record

Exports of German rubber and plastics machinery increased 4.3% in 1996, posting record sales of DM 4.71 billion, according to the Rubber and Plastics Machinery Division of VDMA. However, the production value of DM 6.44 billion (a 3.3% increase) still lies some ways below the peak result of DM 7.07 billion, achieved in 1990. Exports now account for 73%. Overall, the positive trend in the sector continued in 1996 for the third straight year, but with weakened growth rates.

There were above average growth rates in 1996, for the first time in many years, in blow molding machinery, which achieved its highest production value to date of DM 371 million. Extrusion machinery recorded DM 1.22 billion and a rise of 1.6%, slightly above the value of the previous year. In the injection molding machinery sector, output fell to DM 1.29 billion in 1996, after a boom in the previous year.

The most important export markets for the German plastics and rubber machinery sector were again in the past year the industrialized countries of Europe (+2.5% to DM 1.89 billion) and Asia (+10% to DM 1.28 billion). The ASEAN countries recorded a decline for the first time in 1996 (-3.6% to DM 304 million), due to a weakening of demand in Malaysia, Indonesia and Thailand. Supplies to India fell back after a powerful increase in the previous year (-28.5% to DM 113 million). The most important individual market worldwide for German manufacturers of plastics and rubber machinery is the U.S. (+20.2% to DM 740 million), which has therefore further expanded its leading position with a share of 15.7% in the total volume of exports.

Exports of blow molding machinery showed an above-average increase of 54.8%, while foreign deliveries of injection molding machinery (-.7%) and extruders (-3.7%) did not reach the previous year's level in 1996.

RELATED ARTICLE: Shredded tires present more danger

The costly fire that burned through millions of stored tire shreds on the Gila River Indian Reservation was just the latest example of the dangers presented by tire storage, said the Rubber Pavements Association.

"The longer the shreds are stored, either above ground or in landfills, the greater the fire danger they present," said Jeff Smith, RPA president. "Shredded tires, which ignited August I, are an even greater danger than whole tires. They are ideal kindling material."

Smith said the exposed steel belts in the shredded tires begin to oxidize, causing an exothermic reaction and raising temperatures to the point where ignition can occur.

"Tires should never be stored anywhere waiting for a disaster to strike when they can be put to a beneficial use through recycling," said Smith. "There are a number of products that can be created from scrap tires, especially asphalt-rubber which is gaining national popularity as a highway pavement."

To produce it, tires need to be processed right away. The tire shreds that burned at Gila River had been stored on the site for several years and were too dirty and degraded to be processed.

"We need to fully recognize both the dangers of tire and shred storage and the potential advantages of tire recycling if we are to avoid such dangers in the future," said Smith. "We must keep scrap tires out of all landfills and continue to promote their beneficial uses, such as for asphalt-rubber production and in various civil engineering projects."