U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Scrap tire rubber can be incorporated into asphalt paving mixes using two different methods, which are referred to as the wet process and the dry process. In the wet process, crumb rubber acts as an asphalt cement modifier, while in the dry process, granulated or ground rubber and/or crumb rubber is used as a portion of the fine aggregate. In both cases, crumb rubber is sometimes referred to as crumb rubber modifier (CRM) because its use modifies the properties of the resultant hot mix asphalt concrete product.
The dry process can be used for hot mix asphalt paving in dense-graded, open-graded, or gap-graded mixtures. It cannot be used in other asphalt paving applications, such as cold mix and chip seals or surface treatments. In the dry process, granulated or ground rubber and/or crumb rubber is used as a substitute for a small portion of the fine aggregate (usually 1 to 3 percent by weight of the total aggregate in the mix). The rubber particles are blended with the aggregate prior to the addition of the asphalt cement. When tire rubber is used as a portion of the aggregate in hot mix asphalt concrete, the resultant product is sometimes referred to as rubber-modified asphalt concrete (RUMAC).
The dry process used most frequently in the United States was originally developed in the late 1960's in Sweden and is marketed in this country under the trade name PlusRide by EnviroTire. The PlusRide technology is a patented process. In this process, from 1 to 3 percent granulated crumb rubber by weight of the total mix is added to the paving mix. The granulated rubber consists of rubber particles ranging in size from 4.2 mm (1/4 in) to 2.0 mm (No 10 sieve). The target air voids content of the asphalt mix is 2 to 4 percent, which is usually attained at an asphalt binder content in the 7.5 to 9 percent range.(1)
A generic dry process technology was developed in the late 1980's to early 1990's to produce dense-graded hot mixtures. This concept uses both coarse and fine crumb rubber to match aggregate gradings and to achieve improved binder modification. The crumb rubber may need a prereaction or pretreatment with a catalyst to achieve optimum particle swelling. In this system, rubber content does not exceed 2 percent by weight of total mixture for surface courses. Experimental pavement sections have been placed in Florida, New York, Oregon, and Ontario.(2)
The U.S. Army Corps of Engineers Cold Regions Research Engineering Laboratory (CRREL) investigated dry process CRM mixtures for disbonding ice on pavements. This research resulted in a recommendation to place field sections with mixtures containing crumb rubber particles larger than 4.75 mm (No. 4 sieve), with a top size of 9.5 mm (3/8 in). The technology is referred to as the chunk rubber process.(2) Marshall properties, resilient modulus, and ice removal tests have been performed in the laboratory with crumb rubber concentrations of 3, 6, and 12 percent by weight of aggregate. Laboratory wheel testing indicates that the higher rubber content mixes can potentially increase the incidence of ice cracking.(3) The chunk rubber process has not as yet been field evaluated.(2)
The reported performance of rubber-modified asphalt concrete pavements has varied widely in different sections of the United States. The following paragraphs summarize the experiences of selected states with the dry process.
The California Department of Transportation (CalTrans) has constructed four projects using the PlusRide dry process technology. Some distress in the form of cracking or flushing in the wheel paths was observed in three of these projects, but overall, CalTrans has reported that two of the four dry process projects have out-performed conventional dense-graded asphalt, and a third project has performed comparably. A fourth project was not properly designed and required an overlay.(4)
The Minnesota Department of Transportation (MNDOT) has used the dry process in asphalt paving on a least two different projects, beginning in 1979. The two dry process projects were both PlusRide installations, using granulated crumb rubber and a gap-graded aggregate in an attempt to create a self de-icing pavement.(5)The two PlusRide sections have performed well, but have not shown benefits to offset the increased cost, and have not demonstrated any significant de-icing benefits.(5)
In New York, two experimental hot mix overlay projects using granulated rubber in the dry process were installed during 1989 to compare the construction characteristics and performance of rubber-modified asphalt concrete with a conventional top course paving mixture. All overlays were 37.5 mm (1-1/2 in) thick and placed over existing Portland cement concrete pavements, each with a leveling course of varying thickness. On both projects, the rubber-modified mixes consisted of PlusRide with 1, 2, or 3 percent granulated rubber aggregate.(6) After 3 years, the New York State DOT did not consider that these two overlay projects were either economical or successful.
One district in Texas has used rubber-modified hot mix asphalt (dry process). The mix raveled severely and the district was forced to place a chip seal over the mix within 3 months. (7)
Since 1977, the Washington State DOT (WSDOT) has undertaken a number of demonstrations with the dry process, using crumb rubber particles up to 6.3 mm (1/4 in) in size. The performance of the seven PlusRide sections has ranged from excellent to immediate failure. Construction problems have plagued several of these installations. WSDOT concluded that, overall, PlusRide did not appear to provide improved performance.(8)
In Ontario, Canada, eight rubber-modified, dry process asphalt demonstration projects were evaluated in terms of pavement performance. They generally exhibited poor short-term performance.(9)
Performance of rubber-modified asphalt using the dry process has been mixed, with some early failures. Installations in service for several years generally show little improvement over conventional overlays. Little to no evidence of ice disbonding has been observed, except in laboratory tests.
MATERIAL PROCESSING REQUIREMENTS
The initial step in the production of ground or granulated scrap tire rubber is shredding. Scrap tire rubber is delivered to rubber processing plants either as whole tires, cut tires (treads or sidewalls), or shredded tires, with shredded tires being the preferred alternative. As scrap the rubber is processed, the particle sizing is reduced, steel belting and fiber reinforcing are separated and removed from the tire, and further size reduction is then accomplished.
Rubber used in the dry process is ground rubber that is generally produced in a granulator process. This process further reduces shredded tire rubber and generates cubical, uniformly shaped particles ranging in size from 9.5 mm (3/8 in) down to a 0.42 mm (No. 40 sieve). However, the dry process can also use coarse crumb rubber from the crackermill process, which results in irregularly shaped particles ranging in size from 4.75 mm (No. 4 sieve) to 0.92 mm (No. 40 sieve).
Some of the engineering properties of granulated or ground rubber that are of particular interest when used in asphalt concrete (dry process) include its gradation, particle shape, and reaction time.
Gradation: RUMAC paving mixes incorporate granulated or coarse crumb rubber particles that are most often processed to meet the gradation requirements shown in Table 16-1.(10)
Table 16-1. Gradation Requirements for RUMAC Mixes
However, a chunk-rubber asphalt process developed for disbonding ice on pavements contains particles larger than a No. 4 sieve with a dominant size of 9.5 mm (3/8 in).
Particle Shape: Ground or granulated rubber particles produced from granulators, hammermill, or fine grinding machines have a cubical shape and a relatively low surface area. Coarse crumb rubber particles produced from the crackermill process have an irregularly torn shape and a relatively high surface area.
A cubical particle shape with a relatively low surface area is characteristic of conventional aggregate materials and is desirable for rubber particles that will function as a gap-graded aggregate in the dry process. Particles from the crackermill process that have an irregular shape with a relatively high surface area are more likely to react with asphalt cement at elevated temperatures and are better suited for use in the wet process. By limiting the time that the asphalt cement and crumb rubber particles are maintained at reaction temperatures and specifying a coarse granulated product with a relatively low surface area, the rubber particles can retain the physical shape and rigidity needed for use in the dry process. The smooth, sheared surfaces of ground or granulated rubber particles are also less reactive than the surfaces of the particles produced from the crackermill process.()
Reaction Time: In the Plus Ride process, there is a relatively short reaction time when the rubber particles and aggregate are mixed with the asphalt cement, so the rubber particles do not have much opportunity to blend with the binder. There is a generic dry process that was developed in New York State, which uses coarse and fine crumb rubber prereacted with a catalyst to achieve optimum particle swelling, and is added at a maximum of 2 percent by total mixture weight for surface courses.(2) In this process, the rubber particles may be able to react to a somewhat greater extent with the asphalt binder.
Some of the properties of RUMAC paving mixtures that are of interest include stability, resilient modulus, permanent deformation, and reflective cracking.
Stability: Paving mixtures produced by the dry process generally have reduced stability values, regardless of whether the Marshall or Hveem mix design procedures are used.
Resilient Modulus: Mixes containing granulated or crumb rubber typically have lower resilient modulus values than conventional hot mix asphalt. RUMAC paving mixes have been found to have resilient modulus values that are 10 to 20 percent higher than those of asphalt-rubber (wet process) paving mixes.
Permanent Deformation: Previous studies of granulated rubber paving mixtures indicate that resistance to permanent deformation of such mixes is reduced compared with that of conventional paving mixes. However, fatigue life is generally improved when crumb rubber is added by this process.(2)
Reflective Cracking: Addition of rubber aggregate can influence pavement performance in terms of reflective cracking. To achieve the benefits of delayed reflective cracking, a minimum rubber content must be added to the paving mix. This minimum rubber content is probably between 1 and 2 percent by weight of aggregate. The reaction between the rubber and the asphalt cement does not play a significant role in the enhancement of pavement performance in dry process mixes.(2)
Conventional Marshall and Hveem mix design methods have been used successfully for designing dense-graded mixtures with granulated rubber, but mixtures produced using the dry process typically do not follow the normal mix design procedures. Where stability is the primary design factor in most conventional mixes, the primary dry process design property is the percentage of air voids. The target air voids are between 2 and 4 percent.
During the laboratory mixing process, the granulated rubber is dry mixed with the aggregate before adding the asphalt cement. The asphalt concrete mixture is cooled for 1 hour after mixing. After compaction, the sample is cooled to room temperature. The air void content is determined after extrusion.(2)
Dry process paving mixes should be designed volumetrically to compensate for the lower specific gravity of the crumb rubber particles. Binder contents in dry process mixes are typically 10 to 20 percent higher than those of conventional mixes. Although the air voids content is the criterion for mix design, lower stability values and higher flow values can be expected, compared with conventional hot mix asphalt paving mixtures.(2)
The method used for the thickness design of rubber modified asphalt pavements, which incorporate between 1 and 3 percent by weight of granulated crumb rubber modifier (CRM) as fine aggregate, is essentially the same as that used for the thickness design of conventional hot mix asphalt pavements.(11) No adjustments are normally recommended in the design thickness of rubber modified asphalt pavements compared with that of conventional hot mix asphalt pavements.
When designing asphalt pavements using the structural number (SN), the resilient modulus at 20° C (68° F) is the material property that is considered. Resilient modulus values for 18 percent coarse (2.0 mm (No. 10 sieve)) and fine (0.2 mm (No. 80 sieve)) CRM by weight of asphalt binder in dense-graded mixtures were found to be lower than dense-graded control mixtures at three temperatures ranging from 5° C (41° F) to 40° C (104° F).(12) Since the structural layer coefficient of a pavement is directly proportional to resilient modulus, this would suggest that dry process CRM mixtures should have a lower structural layer coefficient and require some increase in thickness.
Material Handling and Storage
Both batch and drum-dryer plants have been used to produce RUMAC. The reclaimed granulated rubber is usually packed and stored in 110 kg (50 lb) plastic bags. Additional manual labor and conveying equipment, such as work platforms, are needed in order to introduce the granulated rubber into the paving mix, regardless of the type of mixing plant used. A batch plant has a quality control advantage over a drum-dryer plant because the number of preweighed bags of granulated rubber can be easily counted prior to their addition into each batch. The bags may be opened and the granulated rubber placed on a conveyor, or the bags may be put into the pugmill or cold feed bin if low melting point plastic bags are used.
Control of the feeding of granulated rubber is necessary because the correct rubber content is critical to the performance of the paving mix when using the dry process. Such control is more difficult to maintain in a drum-dryer system, due to the nature of the feed operation. Some drum-dryer plants have used recycled asphalt concrete hoppers to feed the granulated rubber, although a number of agencies recommend that the rubber be introduced into the mix through a center feed system. The process can be automated by the addition of a conveyor and hopper, plus scales to accurately proportion the granulated rubber.
For both batch and drum-dryer plants the addition of rubber normally requires that the mixing time and temperature be increased. Batch plants require a dry mix cycle to ensure that the heated aggregate is mixed with the crumb rubber before the asphalt cement application. Mixtures should be produced at 149° C to 177° C (300° F to 350° F).
Placing and Compacting
Laydown temperature should be at least 121° C (250° F). A finishing roller must continue to compact the mixture until it cools below 60° C (140° F). Otherwise, the continuing reaction between the asphalt and the crumb rubber at elevated temperatures will cause the mixture to swell.(2) Continued compaction until the mixture cools below 60° C (140° F) serves to contain the expansive pressure of the compressed rubber.
Parameters that must be monitored during mixing for dry process mixes include rubber gradation, rubber percent of total mixture weight, rubber prereaction or pretreatment, and time of plant mixing. Since dry process binder systems are partially reacted with the rubber, it is not possible to directly determine the properties of the binders.
It is recommended that compacted mixes be sampled according to AASHTO T168(13), and tested for specific gravity in accordance with ASTM D2726(14) and in-place density in accordance with ASTM D2950.(15)
There are several unresolved issues relative to the use of rubber as fine aggregate in asphalt concrete using the dry process. The overwhelming majority of projects and data concerning crumb rubber use in asphalt paving are from installations using the wet process. As a result, there is a lack of field data to evaluate performance.
There have been six projects in the United States where asphalt pavements with CRM have been recycled. Roughly half of these projects were wet process and the other half were dry process. Apparently, there are no physical problems with recycling reclaimed asphalt pavement containing CRM as a portion of the aggregate in a new asphalt paving mix; however, additional field trials are needed.
Although only a limited amount of air emissions data from asphalt plants producing hot mix containing CRM are currently available, there is no evidence thus far that the use of an asphalt paving mix containing recycled crumb rubber exhibits any increased environmental impact when compared with that of emissions from the production of a conventional asphalt pavement.(16) Air emission data from a project in New Jersey in 1992 where dry process CRM was recycled as 20 percent of new aggregate in a drum mix plant showed that current air quality standards were not exceeded during the recycling.(16) Nevertheless, there is a need for additional studies on recyclability and worker health and safety issues for CRM asphalt paving mixes. Some of this work is presently underway and, as data become available, they should be incorporated into what is already known concerning these two aspects of using CRM in asphalt pavements.
Because of fluctuations in the performance of CRM asphalt mixes in different locations and/or climatic conditions, there is a need for more carefully controlled experimental field sections in different climatic regions throughout the United States in order to obtain more reliable performance data. Binder and mixture properties in these different regions need to be more accurately determined and documented. Performance records of these test sections may need to be monitored over a long period of time, at least 5 years and possibly as long as 30 years.(2)
Additional research is needed to define the properties of binders produced by the dry process. Desirable properties for dry process hot mix asphalt mixtures need to be better defined.