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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 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 wet process can be used for hot mix asphalt paving mixtures, as well as chip seals or surface treatments. The wet process can also be used to prepare rubberized joint and crack sealants, which are not included in the scope of this document. In the wet process, crumb rubber is blended with asphalt cement (usually in the range of 18 to 25 percent rubber) before the binder is added to the aggregate.
When asphalt cement and CRM are blended together, the CRM reacting with the asphalt cement swells and softens. This reaction is influenced by the temperature at which the blending occurs, the length of time the temperature remains elevated, the type and amount of mechanical mixing, the size and texture of the CRM, and the aromatic component of the asphalt cement.
The reaction itself involves the absorption of aromatic oils from the asphalt cement into the polymer chains that comprise the major structural components of natural and synthetic rubber in CRM. The rate of reaction between CRM and asphalt cement can be increased by enlarging the surface area of the CRM and increasing the temperature of the reaction. The viscosity of the asphalt-CRM blend is the primary parameter that is used to monitor the reaction.(1) The specified reaction time should be the minimum time, at a prescribed temperature, that is required to stabilize the binder viscosity.
When CRM is blended with asphalt cement in the wet process, the modified binder is referred to as asphalt-rubber. To date, most of the experience with the use of CRM in asphalt paving has been with the wet process.
Asphalt-rubber binders are used in chip-seal coats as well as hot mix asphalt paving. Chip-seal coat applications using asphalt-rubber binders have become known as stress-absorbing membranes (SAM). When an asphalt-rubber chip seal or SAM is overlaid with hot mix asphalt, the chip seal is referred to as a stress-absorbing membrane interlayer (SAMI).
Early applications were batch wet processes and were based on the McDonald technology, which was developed in the early 1960's by Charles McDonald, a City of Phoenix engineer, and in the 1970's by Arizona Refining Company (ARCO). There are numerous patents related to the McDonald technology, some of which have expired and some of which have not.(1,2)
A continuous blending technology was developed in Florida in the late 1980's and is frequently referred to as the Florida wet process. In this process, a fine 0.18 mm (No. 80 sieve) CRM is blended with asphalt cement in a continuous process. The Florida technology differs from the McDonald process in several respects: lower percentages of CRM (from 8 to 10 percent rubber), smaller CRM particle size, lower mixing temperature, and shorter reaction time. The Florida wet process has not as yet been patented.(1)
Terminal blending is a wet process with the capability of blending or combining asphalt cement and CRM and holding the product for extended periods of time. This asphalt-rubber product has a shelf life and is blended at an asphalt cement terminal using either batch or continuous blending. Individual state highway agencies are now developing their own products with this technology, since it is not patented. At the present time, none of the terminal blending products have been fully evaluated in the field.(1)
The reported performance of crumb rubber in asphalt pavement has varied widely in different sections of the United States. Several states have had fairly extensive experience in using crumb rubber, some with the wet process. A summary of the experiences of selected states are presented in the next few paragraphs.
Over a 20-year period, starting in the early 1970's, over 3,000 lane-miles of streets in Phoenix, Arizona, have been seal coated using asphalt-rubber. In the early 1990's, the use of chip seals was discontinued in favor of 1-inch thick asphalt-rubber hot mix overlays. Nearly 600 lane-miles of streets have been paved using hot mix overlay. Both the chip seals and the hot mix overlays have reportedly been effective in retarding the reflection of alligator cracks and shrinkage cracks less than 6.3 mm (1/4 in) in width. Compared with the chip seals, the 25-cm (1- in) thick asphalt-rubber hot mix overlay reportedly provides a more improved riding surface and a marked decrease in traffic noise.(3)
The California Department of Transportation (CalTrans) has been using rubber-modified asphalt concrete since 1978 and has constructed 17 wet-process overlay installations. CalTrans has placed rubberized overlays over asphalt as well as concrete pavements using dense-graded, open-graded, and gap-graded asphalt concrete mixes. Since 1987, these CRM overlays have been placed at a reduced thickness compared with conventional overlays. Overall, CalTrans has reported that wet-process overlays have usually out-performed thicker, dense-graded asphalt concrete by exhibiting less distress, requiring less maintenance, and being able to tolerate higher deflections.(4)
The Florida Department of Transportation (DOT) constructed three asphalt-rubber demonstration projects between March 1989 and September 1990. These projects involved one dense-graded and two open-graded friction courses using the Florida wet-process technology. Although long-term performance has yet to be evaluated, data reviewed to date suggest that asphalt-rubber friction courses, particularly open-graded ores, will probably exhibit improved durability over conventional friction courses.(5)
The Kansas DOT constructed five projects using asphalt-rubber interlayers during the 1980's. On two of the projects, the interlayer was able to somewhat reduce reflective cracking. On the other three projects, there was little difference between the pavements with an interlayer and the control sections, and, in some cases, sections with the interlayer had more reflective cracking. On all five projects, the Kansas DOT concluded that the additional cost of an asphalt-rubber interlayer did not justify its use.(6)
The Minnesota DOT (MNDOT) has used CRM in asphalt paving on at least six different wet- process projects, beginning in 1979. The six projects involved two SAM's, three SAMI's, and one dense-graded overlay.(7) The MNDOT demonstration exhibited mixed results. Of the two stress- absorbing membranes, one was a success and one was a failure. The difference between success and failure depended on precoating of the aggregate chips that were used. Only minor problems were encountered with the installation of the three SAMI's. Reflective cracking was reduced, but not eliminated. Some improvement in crack reflection was observed in the asphalt-rubber dense-graded overlay, but the benefits were not deemed sufficient to offset the increase in cost.(7)
In Texas, crumb rubber has been used in asphalt paving mixtures in at least three different products dating back as far as 1976. The most frequent use of CRM in Texas has been in the construction of asphalt-rubber chip seals. Over 2,000 lane-miles of asphalt-rubber chip seals (SAM's) have been constructed in Texas. After many years of experience, Texas DOT personnel have concluded that SAM's exhibit improved resistance to alligator cracking and raveling, but that resistance to shrinkage cracking is not improved by the chip seals.(8)
Two other districts in Texas have experimented with the use of CRM in dense-graded asphalt-rubber hot mix overlays (wet process). Performance to date has been satisfactory for the two wet-process overlays.(8)
Since 1977, the Washington State DOT (WSDOT) has used three types of paving products from the wet process. Wet-process products include SAM's, SAMI's, and open-graded asphalt-rubber friction courses. WSDOT concluded that the performance of asphalt-rubber SAM's and SAMI's did not justify the added expense of their construction.(9) All five of the open-graded friction course installations are exhibiting good to very good performance, with the exception of one bridge deck overlay, which is showing some distress in the wheel path areas.(9)
In Ontario, Canada, three rubber-modified asphalt demonstration projects were evaluated in terms of pavement performance. The performance of the asphalt-rubber (wet process) projects was promising, insofar as the durability of these asphalt mixes appeared to be enhanced by the use of crumb rubber modifier.(10)
There have reportedly 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.
To summarize, the overall results of these performance investigations suggest the following:
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.
Grinding and Granulation
Crumb rubber can be produced by one of three processes. The granulator process produces cubical, uniformly shaped particles ranging in size from 9.5 mm (3/8 in) down to 0.4 mm (No. 40 sieve), which is called granulated CRM. The crackermill process, which is the most commonly used, produces irregularly shaped torn particles sized from 4.75 mm (No. 4 sieve) to 0.42 mm (No. 40 sieve), referred to as ground CRM. The micro-mill process produces a very fine ground CRM, usually ranging from 0.42 mm (No. 40 sieve) down to as small as 0.075 mm (No. 200 sieve).(1) In the wet process, ground CRM is normally used with the McDonald technology and very fine ground CRM is used with the Florida technology.
Some of the engineering properties that are of particular interest when rubber is incorporated in asphalt concrete (wet process) include asphalt viscosity, asphalt softening point, resilient modulus, permanent deformation, thermal cracking, and resistance to aging.
Viscosity: Adding crumb rubber to asphalt cement can dramatically increase the viscosity of the resultant asphalt-rubber binder. Various quantities of kerosene or other diluents can be used to adjust the viscosity. Viscosity increases can occur after the addition of diluents, but higher percentages of diluent usually result in lowered viscosity increases. Reaction temperatures also affect these relationships.(1) The benefit of increased viscosity of the asphalt-rubber binder is that additional binder can be used in the asphalt mix to reduce reflective cracking, stripping, and rutting, while improving the binder's response to temperature change and long-term durability, as well as its ability to adhere to the aggregate particles in the mix and to resist aging.
Softening Point: In addition to modifying binder viscosity, asphalt-rubber binders used in seal coats and hot mix asphalt show an increase in the softening point of the binder by 11°C (20°F) to 14°C (25°F), resulting in reduced rutting or shoving of the asphalt-rubber products at elevated temperatures. Modification of asphalt cement with ground tire rubber greatly increases binder elasticity compared with unmodified asphalt cement, thus providing asphalt-rubber pavement systems with increased resistance to deformation and cracking.
Resilient Modulus: Resilient modulus values for mixtures containing conventional aggregate and asphalt-rubber binder are generally lower than the resilient modulus values for similar mixtures in which conventional asphalt cement are used. The higher the temperature, the greater the difference between the resilient modulus of the conventional mix and the asphalt-rubber mix.(1)
Permanent Deformation: The permanent deformation properties of dense-granded asphalt-rubber mixtures are within the range of properties normally associated with conventional hot mix asphalt paving mixtures, although asphalt-rubber mixtures may be somewhat less resistant to permanent deformation.(1)
Thermal Cracking: Asphalt-rubber binders also exhibit reduced fracture temperatures compared with conventional asphalt cement, usually 5.5°C (10°F) to 8.3°C (15°F) lower, meaning that asphalt-rubber products are less brittle and more resistant to cracking at lower temperatures than conventional chip seals or hot mix asphalt paving. Isolated fatigue studies have also indicated greater resistance to low temperature thermal cracking. In summary, asphalt-rubber is more elastic than asphalt cement and remains elastic at lower temperatures.
Resistance to Aging: Laboratory data also indicate that asphalt-rubber mixtures are somewhat more resistant to aging than normal asphalt mixtures. Aging studies performed in asphalt-rubber binders placed in northern and central Arizona pavements indicate that asphalt-rubber binders have an increased resistance to hardening.(1) When crumb rubber is added to asphalt cement, fatigue life is improved.
Hot Mix Asphalt
Variations of standard Marshall and Hveem mix design procedures for hot mix asphalt have been used to design dense-graded hot mixes using crumb rubber. Marshall or Hveem stability tests and weight-volume parameters are the basis for each of these designs. Lower unit weight and Marshall or Hveem stability values are obtained using CRM asphalt mixtures, while flow and voids in mineral aggregate (VMA) values are increased compared with conventional mixes.(1) Mixing and compaction temperatures for CRM mixtures are often higher than those for conventional paving mixes. Depending on the mix design method, samples should be heated to 149°C (300°F) to 190°C (375°F) before compaction. Design air voids and aggregate gradation depend on the CRM content. Low CRM content in the wet process has little or no effect on the mix design. As a rule of thumb, if 20 percent crumb rubber is used in the binder, then the CRM binder content will be 20 percent greater than a conventional binder.(1)
Most wet processes use CRM particles ranging in size from 0.6 mm (No. 30 sieve) to 0.15 mm (No. 100 sieve). The CRM percentage by weight can range from 5 to 25 percent of the binder, but is typically 18 percent. The CRM and asphalt cement are blended at temperatures from 166°C (330°F) to 204°C (400°F). Reaction times may vary from 10 to 15 minutes up to 2 hours or more, with rubber type and gradation being the two most important variables.
When asphalt-rubber is used in chip seals, the majority of SAM's and SAMI's have been designed and placed without predetermining the binder or the aggregate application rate. The most common approach has been to specify a fixed rate of asphalt-rubber binder, and then vary the aggregate application rate to achieve the desired product. The amount of asphalt-rubber binder suggested for use in chip seals is about 15 to 20 percent higher than that required for a typical asphalt cement binder without a temperature correction. The amount of asphalt-rubber binder suggested for use in interlayers is about 45 percent higher than that typically used in asphalt cement binder without a temperature correction.(1)
Conventional AASHTO design procedures for flexible pavements are typically used for pavements containing wet process CRM.(11) Most agencies elect to use the same thickness of asphalt-rubber pavement as the design thickness of a conventional hot mix asphalt pavement.
Since 1987, CalTrans has placed and evaluated the performance of at least eight different projects in which all, or at least a portion, of the asphalt-rubber pavement was constructed at a reduced thickness, compared with the conventional pavement. Thickness reductions ranged from 20 to 50 percent in dense-graded asphalt mixes. In the majority of these projects, the thinner asphalt-rubber mixes reportedly performed at least as well as the thicker, conventional dense-graded asphalt mixes.(4)
There are a number of special construction procedures for hot mix asphalt pavements containing scrap tire rubber, as well as both chip-seal coats (SAM's) and interlayers (SAMI's).
Hot Mix Asphalt Pavements
The construction process normally used for hot mix asphalt pavements must be modified in order to produce a quality CRM hot mix. When using asphalt-rubber binders in either dense-graded, open-graded, or gap-graded mixtures, several changes in the normal construction process must be recognized.Material Handling and Storage
Crumb rubber is most often shipped in 110 kg (50 lb) bags. The bags can be emptied directly into a reaction vessel for mixing with asphalt cement. The different viscosity of wet-process binders, particularly at the higher rubber content (in the 18 to 25 percent range), can cause problems with storage and/or pumping of the binder. Such problems are most likely to occur if the hot mix plant has been shut down for an extended time period.Mixing
A blending and reacting unit should be added to ensure proper proportioning of the crumb rubber, base asphalt cement, and any other modifiers. Most wet processes use CRM particles ranging in size from 0.6 mm (No. 30 sieve) to 0.15 mm (No. 100 sieve). The CRM and asphalt cement are blended at temperatures from 166°C to 204°C (330° F to 400°F). Reaction times may vary from 10 to 15 minutes up to 2 hours or more, with rubber type and gradation being the two most important variables. The target temperature should be higher to allow for the greater viscosity of the binder at construction temperatures. Typical mixing temperatures for hot mix asphalt paving are from 149°C (300°F) to 177°C (350°F).Placing and Compacting
Placement of hot mix asphalt paving material with wet-process CRM binder can be accomplished using standard paving machinery. Laydown temperature should be at least 121°C (250°F). Compaction must be completed as soon as possible.(1) Pneumatic tire rollers cannot be used because asphalt-rubber will build up on roller tires.(1)Quality Control
To ensure proper quality control of the CRM binder, the crumb rubber particle size, the rate of addition of crumb rubber, the mixing temperature and the time of blending and reaction must all be carefully monitored.
It is recommended that compacted mixes be sampled according to AASHTO T168,(12) and tested for specific gravity in accordance with ASTM D2726(13) and in-place density in accordance with ASTM D2950.(14)
Chip-Seal Coats (SAM's) and Interlayers (SAMI's)
Construction of asphalt-rubber chip seals and interlayers (SAMs and SAMIs) is nearly identical to the construction of conventional chip seals. The major differences include the preparation of the asphalt-rubber binder and the use of specialized spray equipment.Material Handling and Storage
Crumb rubber is most often shipped in 110 kg (50 lb) bags. The bags can be emptied directly into a reaction vessel for mixing with asphalt cement.Mixing
Preparation of asphalt-rubber binder is done in blending vessels, which are often either special tanks or specialized binder distributors. They must be capable of heating the base asphalt cement, mixing the crumb rubber and the asphalt cement, and keeping the crumb rubber in suspension to avoid separation. When the crumb rubber is introduced into the asphalt cement, it swells and physical-chemical reactions occur that alter the properties of the base asphalt. Diluents of various types (such as kerosene) may be introduced to adjust the viscosity for spraying purposes.(1)Placing and Compacting
Binder distribution equipment must be able to maintain the temperature of the binder at the desired level, circulate the binder to avoid separation of the crumb rubber and base asphalt, and discharge the binder in a uniform manner. Special pumps and nozzles are required to handle some asphalt-rubber binders.(1)
For chip-seal or interlayer applications, typical asphalt-rubber spray quantities are 2.5 L/m2 (0.55 gal/yd2) to 3.2 L/m2 (0.70 gal/yd2) for SAM's and 2.7 L/m2 (0.60 gal/yd2) to 3.6 L/m2 (0.80 gal/yd2) for SAMI's, compared with 1.6 L/m2 (0.35 gal/yd2) to 2.3 L/m2 (0.50 gal/yd2) for conventional chip seals. Typical aggregate application rates are in the range of 16 kg/m2 (30 lb/yd2) to 22 kg/m2 (40 lb/yd2) for SAM's and 8 kg/m2 (15 lb/yd2) to 14 kg/m2 (25 lb/yd2) for SAMI's, compared with conventional chip-seal application rates of 11 kg/m2 (20 lb/yd2) to 14 kg/m2 (25 lb/yd2).(1) As with all chip-seal construction, application of the chips should immediately follow application of the binder to ensure proper adhesion.Quality Control
To ensure proper quality control of the CRM binder, the crumb rubber particle size, the rate of addition of crumb rubber, the mixing temperature, and the time of blending and reaction must all be carefully monitored.
Quality assurance of the chip seal will require that the particle sizing and application rate of the stone chips be closely inspected to ensure compliance with applicable specifications.
There are several unresolved issues relative to the use of crumb rubber as an asphalt cement modifier in asphalt concrete using the wet process.
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.(10) 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.(1)
Additional research is needed to define the properties of binders produced by the wet process. Desirable properties for chip seals, interlayers, and hot mix asphalt containing CRM need to be better defined using either existing or newly developed test methods.
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling