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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Shredding of scrap tires produces chunks of rubber ranging in size from large shreds to smaller chips. Tire shreds and tire chips have both been used as lightweight fill materials for roadway embankments and backfills behind retaining walls. The shreds or chips can both be used by themselves or blended with soil. Tire shreds normally range in size from 305 mm (12 in) to 76 mm (3 in), while tire chips are usually sized from a maximum of 76 mm (3 in) down to a minimum of 12 mm (1/2 in). Embankments containing tire shreds or chips are constructed by completely surrounding the shreds or chips with a geotextile fabric and placing at least 0.9 m (3 ft) of natural soil between the top of the scrap tires and the roadway.
At least 15 states have utilized scrap tire shreds or chips as a lightweight fill material for the construction of embankments or backfills. Some projects have used tire shreds or chips as embankment material, while other projects have blended tire chips with soil. The states that have used tire shreds or chips in embankments are California, Colorado, Indiana, Maine, Minnesota, New Jersey, North Carolina, Oregon, Pennsylvania, South Carolina, Vermont, Virginia, Washington, Wisconsin, and Wyoming. The largest known use to date is in Oregon, where 580,000 scrap tires were shredded and used in a landslide correction project. In Colorado, between 400,000 and 450,000 scrap tires were used to construct an embankment containing tire chips on a section of interstate highway.(1) To date, more than 70 successful projects have been constructed on state, local, or private roads.(2)
Aside from problems with puncturing of rubber tires on haul vehicles by the exposed steel in tire chips or shreds, there have been no other construction-related problems on scrap tire embankment projects. Adequate compaction, which is always a prime concern on any embankment project, is of even greater concern on a tire shred or chip embankment project, where it is known that some consolidation will occur. Some cracking of the roadway above a tire shred or chip embankment is possible because of long-term settlement or differential settlement.
Although at least 15 states have had scrap tire embankment projects, only six states (North Carolina, Oregon, Vermont, Virginia, Wisconsin, and Maine) have prepared specifications or some provisions for this use.(1)
Scrap tire embankments that have been constructed to date have remained structurally stable; however in 1995, three shredded tire projects experienced combustion problems. Two of these projects were located in Washington State. The other project was located in Colorado. Preliminary assessments indicate that the combustion process may have been initiated by heat released either by the presence of organic soils and microbial degradation, the oxidation of exposed steel wires, or microbes consuming liquid petroleum products that could have been spilled on the tire shreds during construction.(2) The presence of both sufficient air and crumb rubber particles in the embankment may have played a role in the process. The crumb rubber in the presence of air may have ignited when exposed to the heat generated in the embankment, initiating the combustion process.
The use of tire shreds or tire chips for the construction of embankments provides a number of advantages. The most obvious advantage is that of reduced unit weight, which is especially beneficial in situations where an embankment is to be constructed over an area with low bearing support. In addition to being a lightweight fill material, tire shreds or tire chips offer good thermal characteristics in resisting frost penetration and have good drainage characteristics, being as permeable as a coarse granular soil. Embankments present an excellent opportunity to utilize large volumes of scrap tires from one location and, under the proper logistical circumstances, can be a very economical alternative to imported earth borrow.(3)
MATERIAL PROCESSING REQUIREMENTS
The size and shape of tire shreds or chips from tire shredding can vary depending on the type of shredding machinery used. Tire shreds have a wide range of sizes, from 76 mm (3 in) up to 305 mm (12 in), which is ordinarily the largest size recommended. Chip sizes normally range from 12 mm (1/2 in) up to 76 mm (3 in). Usually, tire shreds are irregular in shape with the smaller dimension being the size specified by the manufacturer and the larger dimension possibly being two or more times as much. The chips, on the other hand, are cubical in shape. Some shreds or chips may have pieces of steel belt exposed along the edges. To minimize potential compaction problems (i.e., to reduce void space) it may be desirable to use smaller size tire chips of 50 mm (2 in) or less.(4)
Some of the properties of tire chips or shreds that are of particular interest when they are planned for use in an embankment or backfill include particle size and shape, specific gravity, compacted unit weight, shear strength, compressibility, permeability, and combustibility. Due to the differences between tire shreds or chips and stone or soil-like embankment materials, physical characterization of tire shreds or chips represents a specific challenge to the tire user.
Particle Size and Shape: Tire chips are normally somewhat uniformly sized and will most often range in size from 25 mm (1 in) to 50 mm (2 in). Tire shreds are more well-graded, usually ranging from 101 mm (4 in) to 202 mm (8 in) in size. Some particles may be 305 mm (12 in) or even larger, including some strip-shaped pieces. The most unusual properties of tire shreds are their flat and somewhat irregular particle shape and their relatively low unit weight. The flat shreds, especially the larger sizes, tend to lay on top of one another and develop some degree of particle interlock. They also tend to be oriented parallel to the horizontal shear plane.
Particle size distribution can be determined by performing a standard sieve analysis using the procedures of ASTM D422. No modification of the standard test method is required, except that tire shreds larger than 76 mm (3 in) cannot be screened through standard sieves.
A limited amount of geotechnical analysis has been performed on different sizes of tire chips. Grain size analyses have indicated that the tire chips can be classified as a well-graded, coarse-grained material, similar to an A-1-b sand and gravel (AASHTO M145) or as an SW well-graded sand with gravel (ASTM D2487).(4)
Specific Gravity: The specific gravity of tire chips is expected to be in the 1.1 to 1.3 range, with higher specific gravity values for chips containing steel belts.(5)
Compacted Unit Weight: Depending on the size of the chips, compacted unit weights can range from as low as 322 kg/m3 (20 lb/ft3) to as high as 725 kg/m3 (45 lb/ft3).(4) Tire shreds or chips have a maximum density that is approximately one-third to one-fourth that of typical earthen fill material. The coarser the size of the scrap tire particle, the lower the compacted unit weight.
Determination of compacted density of air-dried tire chips is best made by an adaptation of ASTM D1557 (Modified Proctor test). A 254 mm (10 in) diameter by 254 mm (10 in) high mold is recommended instead of the usual 101 mm (4 in) diameter high mold. Since the level of energy applied is not critical, 60 percent of standard Proctor compaction effort is recommended.(6)
Shear Strength: Limited direct shear testing of tire chips has been performed using a specially made large-scale direct shear testing apparatus. The friction angle of tire chips from these tests ranged from 19° to 25°. Cohesion values range from 7.6 kPa (160 lb/ft2 ) to 11.5 kPa (240 lb/ft2), although significant deformation was required to develop cohesion. Tire chips with a greater amount of exposed steel belts tend to have a higher angle of internal friction.(5) Typical granular soils have friction angles between 30° and 40° with little apparent cohesion.
The shear strength of tire chips can be evaluated by performing direct shear tests using a 305 mm (12 in) square shear box. This is a modification of ASTM D3080 and is applicable to the testing of 76 mm (3 in) maximum size tire chips.(6)
Compressibility: Tire shreds or chips are much more compressible during the initial stages of loading than conventional soils. Subsequent loading cycles normally result in significantly less compressibility of the tire shreds or chips. Higher amounts of exposed steel belts appear to result in higher compressibility, especially during the first loading cycle, probably because of less rebound.(5)
Compressibility analysis of tire chips indicates that the Young's modulus of tire chips is 2 to 3 orders of magnitude less than the modulus of granular soil. The values of Young's modulus for tire chips range from 770 kPa (112 lb/in2) to 1250 kPa (181 lb/in2). Therefore, at least 0.9 m (3 ft) of conventional soil is required to be placed on top of a layer of tire chips in order to prevent or minimize surface deflections.
Compressibility can be analyzed by applying a vertical load to compacted tire chips (5 layers, 60% of standard Proctor effort) within a 305 mm (12 in) diameter by 305 mm (12 in) long schedule 40 PVC pipe equipped with horizontal and vertical strain gauges. The strain gauges record horizontal and vertical stress and strain readings at 10 second intervals. Readings are plotted on stress-strain curves. The slopes of these curves are indicative of the compressibility of tire chips and the coefficient of lateral earth pressures.
Permeability: The coefficient of permeability of tire chips was found to range from 1.5 to 15 cm/sec, depending on their void ratio. This is equivalent to the permeability of a clean gravel soil.(5)
Permeability testing can be accomplished using a 305 mm (12 in) diameter by 0.96 meter (38 in) long PVC pipe and following the constant head testing procedures of the California Department of Transportation. A 38 mm (1.5 in) diameter water inlet was fixed to the center of the end cap. A 101 mm (4 in) wide by 50 mm (2 in) deep slot was cut into the top of the PVC pipe to allow water to flow out the top of the apparatus. The initial length of the tire chip sample is about 600 mm (24 in).(5)
Combustibility: Although scrap tire particles (shreds or chips) are not in and of themselves capable of spontaneous combustion, it does appear to be possible that, under certain circumstances, an initial exothermic reaction may occur within a tire shred or tire chip embankment or backfill that could eventually raise the temperature within the fill to a point where ignition could possibly occur.
Tire chips can be mixed or blended with soil. As the percentage of soil is increased, the unit weight of the blend increases. To simplify blending in the field, mix ratios are usually prepared on a volumetric basis. A maximum 50:50 tire chip to soil ratio is suggested so that tire chip usage is not reduced too greatly. However, if the unit weight of the fill is not a concern, then even small percentages (10 to 25 percent) of tire chips can be blended into the soil. This could improve the compactibility of the fill.
Since tire chips and shreds are unlike conventional materials, special empirical design procedures must be considered. The principal design considerations include shred or chip containment, shred or chip particle size distribution, particle shape, type of belt, compacted density of the tire chips, and whether soil will be mixed with the chips.
To contain tire shreds or chips, a geotextile fabric should be placed beneath the shreds or chips and wrapped around and above them. The geotextile must completely enclose the tire shreds or chips in order to provide the necessary containment. Although smaller size tire chips have an angle of repose of around 50°, 2:1 side slopes (horizontal to vertical) are recommended.(4) At least a 0.9 m (3 ft) soil cover should be placed between the top of the enclosed tire chip fill and the base of a pavement to reduce deflections and to minimize differential settlement within the fill. If heavy wheel loadings are anticipated, an additional 0.6 m (2 ft) soil surcharge can be placed, which can be removed following appropriate settlement prior to pavement construction.
A major concern in the use of tire shreds or chips in an embankment are the comparatively large settlements (about 10 to 15 percent of the height of the tire layer) that have been observed in various field studies. There is little information available on the tolerable settlements of highway embankments. Postconstruction settlements of 0.3 to 0.6 m (1 to 2 ft) over the life of an embankment may be considered tolerable provided they are reasonably uniform, do not occur adjacent to a pile-support structure, and occur slowly over a long period of time. The detrimental effects of settlements in this range can be reduced by using flexible pavement over scrap tire fills, by inducing some of the postconstruction settlement during construction by placing a thicker soil cap or a surcharge earth loading over the embankment, or by using stage construction.(7)
Another possible means of mitigating scrap tire embankment settlements is to use a rubber-soil mix to construct the embankment, instead of using tire shreds or tire chips alone. It has been found that a ratio of about 40 percent tire chips by weight of soil may be an optimum value for the quantity of chips in a rubber-soil mix, although this may vary depending on the size of the tire chips and the type of soil. The optimum ratio of tire chips to soil is likely to yield a compacted dry unit weight of rubber-soil mix that is roughly two-thirds the dry unit weight of soil alone. Data on the stress-strain and strength behavior of rubber-soil mixtures are not widely available, but are necessary for the design and prediction of performance of scrap tire embankments that contain such mixtures.(7)
At the tire processing facility, the number of tires to be processed into shreds or chips is directly related to the intended volume of the tire chip portion of the embankment. It is estimated that every cubic yard of volume will require about 75 automobile tires that have been shredded into shreds or chips and compacted into an embankment.(5)
The site of the embankment should be prepared in essentially the same manner as though common earth were being used for fill material. If there is a high water table or swampy area that will be at the base of the embankment, it may be advisable to construct a drainage blanket. If there is a natural flow of runoff through the area where the embankment is to be constructed, provisions should be made to pipe the runoff beneath the embankment.
When tire shreds or chips are to be blended or mixed with soil, the mixing should be performed volumetrically, using bucket loads from a front end loader and blending the materials together as well as possible with the bucket. As another option to the mixing of tire shreds or chips and soil, alternate layers of the tire shreds or chips and the soil can be constructed.
Placing and Compaction
Once the base of the embankment has been prepared, the geotextile that will enclose the tire chips should be placed. A nonwoven geotextile fabric is recommended. Sufficient length should be provided to completely wrap around the tire chips once they have been placed and compacted.
Tire chips should be spread across a geotextile blanket using a tracked bulldozer. A minimum 0.6 m (2 ft) layer or lift should be spread out over the geotextile. A recommended maximum 1 m (3 ft) lift thickness can still be spread and compacted. Compaction may be achieved by at least three passes of the tracked bulldozer over the layer of tire chips.(8) The chip particles align themselves with each other and settle fairly readily. The weight of the bulldozer passing over the tire chips is enough to readily compact the layer of chips. For larger chips or thicker layers of chips, as many as 15 passes of a bulldozer may be required to achieve compaction.(9)
Once the bulldozer is able to pass over a layer of tire chips with little to no noticeable deflection or movement under the tracks of the bulldozer, the next layer or lift of tire chips can be placed. There is really no practical method at this time for performing an in-place density test on a layer of compacted tire chips. The best way to ensure that the layer has been sufficiently compacted before placement of the next layer is to continue passes of the bulldozer over the tire chips until there is no more movement of the tire chips when the bulldozer passes over them.
The top layer of a tire chip embankment should be kept at least 1 m (3 ft) below the base or subbase layer of the pavement that will be on top of the embankment. Each layer of a tire chip embankment must be fully compacted before the next layer is placed. When the top layer of tire chips has been fully compacted, the sides and top of the tire chips should be fully wrapped and enclosed by the geotextile.
A minimum of 0.9 m (3 ft) of compacted soil (preferably granular soil) should be placed on top of the geotextile and tire chips. The soil should be compacted in thinner layers 15 mm (6 in) to 305 mm (12 in) in thickness. The tire chip embankment will experience further deflection during placement and compaction of the soil cover.
At least 0.6 m (2 ft) of soil should also be placed on the side slopes of the embankment to cover the geotextile wrap. The soil on the slopes should be compacted to the extent possible, covered with topsoil, and seeded to establish erosion control protection. The additional soil cover on the side slopes will also help minimize the potential of exothermic reactions occurring within the scrap tire embankment.
There is little in the way of field quality control testing that can be done during the construction of a tire chip embankment, other than to very closely inspect the compaction of each tire chip layer to ensure that there is very little to no movement under the passage of a bulldozer before proceeding to install the next layer of chips. However, the overall settlement or deflection of a tire chip embankment can be monitored over time by the installation of settlement plates or platforms, slope indicator devices, and bench marks along the slopes of the embankment and within or adjacent to the roadway. Periodic readings should be taken using these devices in order to keep track of the extent and rate of settlement and to compare actual settlements with predicted settlements.
To minimize the potential for an exothermic reaction to occur within a portion of a tire shred or tire chip embankment or backfill, a number of preventive measures should be taken. Contact with oxygen within the scrap tire fill should be reduced as much as possible by covering the fill with at least 1.3 m (4 ft) of well-compacted natural nonorganic soil. The amount of exposed steel belts at the edges of the shred or chip particles should be limited by using magnetic separation or using large-size particles. No crumb rubber should be allowed to be used in a scrap tire fill. Tire shreds or chips that have been contaminated by liquid petroleum products should be removed from a scrap tire fill and disposed of in an environmentally acceptable manner. There should be no contact between tire shreds or chips and either topsoil or fertilizer.(2)
There are several unresolved issues pertaining to the preparation and use of shredded scrap tires in fills and embankments. The first and most pressing unresolved issue is to determine the cause or causes of the exothermic reactions that resulted in three scrap tire embankment fires that occurred during 1995. Other tire shred or tire chip embankment projects, especially the thick fills, including those that have caught on fire, should be more closely monitored, possibly by installing temperature probes and gas sampling wells. Gas from such wells should be periodically sampled and analyzed for oxygen level, hydrogen sulfide, carbon dioxide, carbon monoxide, and hydrocarbons. The pH of any water leaching from scrap tire fills should be measured. Laboratory investigations should also be undertaken under varying conditions and temperatures to pinpoint under which conditions exothermic reactions may be initiated.(2)
One of the principal questions concerning such use of shredded scrap tires is that of an optimum particle size and shape of the tire shreds or chips. More information is needed on the basic types of tire shredding machinery currently in use and their effect on particle shape and size. The effects of mixing or blending various size shreds or chips within an embankment also need to be further evaluated in terms of resultant engineering properties, optimum gradation of shreds or chips, compaction and settlement behavior, as well as potential combustibility.
Another consideration that warrants further investigation concerns the blending of soil and tire chips or shreds. Among the variables that need to be further investigated are the effect of various proportions of tire chips and soil on the engineering properties of the resultant composites, especially the bulk density and compaction characteristics. The type of soil is another variable that will influence the bulk density and compaction characteristics of the tire chip-soil blends. If possible, optimum proportions of tire chips and soil should be identified for different tire particle sizes and/or soil types.
There is currently very little in the way of field quality control testing that is now being done during the construction of a tire chip or tire shred embankment, other than visual inspection of movement or settlement of the tire chip or shred layers under compaction machinery. Some rational methods of in-place density and/or compaction percentage measurement need to be developed and field tested to help minimize settlement of tire chip or tire shred fills under traffic loading.