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ASPHALT RUBBER MAINTENANCE ABSTRACT: This paper addresses specific asphalt rubber maintenance strategies in California. California has been using asphalt rubber in many pavement applications because of its successful field and laboratory performance. This paper focuses on asphalt rubber chip seals, asphalt rubber hot mix - open graded and asphalt rubber hot mix - gap graded surface treatments with the emphasis on specifications, mix design, material requirements, job selection criteria, application parameters and performance history. 1. INTRODUCTION The California Department of Transportation (Caltrans) began using asphalt rubber in chip seals in the 1970s and in hot mixes in the 1980s. The first asphalt rubber test was placed in Ravendale, California in 1982. This was followed by a series of field projects, laboratory performance studies and accelerated pavement-testing research. This led to the development of specifications and design guidelines. In 1992, a design guide was developed for asphalt rubber hot mix - gap graded overlays. This guide called for a reduction in overlay thickness by as much as 50% when asphalt rubber hot mixes were used in place of conventional dense graded asphalt concrete mixes [SHA 00]. During this period, specifications and guidelines were being developed for the wide usage of asphalt rubber in various maintenance strategies. Asphalt rubber has been used in a number of different maintenance and rehabilitation applications including joint and crack seals, chip seals, stress-absorbing membrane interlayers (SAMI), asphalt rubber hot mixes (dense-, gap- and open-graded). Asphalt rubber binder has been used in both corrective and preventive pavement preservation strategies. The strategies used with these binders are valuable tools in the toolbox. They are viable options when choosing the right strategy at the right time. The specifications used in the early projects were provided by industry as patented processes until the expiration of those patents occurred in 1992. The first generic specification was developed in 1994 in the form of a "recipe" specification. This recipe specification was followed by a series of performance studies aimed at developing performance-based specification. These studies are still on going. This paper will focus on specific maintenance strategies that have been used in California including the following:
2. DESIRABLE PROPERTIES OF ASPHALT RUBBER BINDER The performance of the asphalt rubber binder depends on its elastomeric properties. The manufacturing process influences these properties. Therefore, it is important to achieve the required digestion through adequate dispersion to create a rubber network or matrix within the asphalt. The physical aspect of mixing creates a physio-chemical interaction between the asphalt and the rubber. The softer and tackier components of the binder such as natural rubber improve adhesion with the aggregate. The asphalt rubber binder consists of 18-20% rubber (80% ground tire rubber of a suitable grading with 20% natural crumb (passing a 1.16mm sieve). Heavy aromatic extender oil is often used at levels of up to 3%. The extender oil acts as a compatibilizing agent and promotes rubber swelling and interaction. The binder is blended in a low shear system with a mixing auger or a propeller mixer. It is "reacted" with the bitumen for approximately one hour and used within a few hours of manufacturing. Figure 1 shows a micrograph of the finished asphalt rubber binder. It can be seen that relatively large chunks of rubber are present. It is suggested that the large chunks act as crack blunting sites (stopping cracks from further propagation), analogous to butyl rubber in high impact polystyrene.  Figure 1. Micrograph of "GOOD" Asphalt Rubber (Largest Particle is 0.01mm) Asphalt rubber binder improves the fatigue performance in hot mixes over mixes made with conventional asphalt binders [SHA 00]. This can be seen in Figure 2, which shows a fatigue life comparison between an asphalt rubber hot mix and a conventional dense graded mix. The fatigue behavior in hot mixes is significantly influenced by the binder properties. Compatible systems prepared at low shear give better fatigue characteristics [VAN 00]. The critical parameters in the performance of asphalt rubber binder include mixing time, mixing temperature, rubber type and rubber grading. There are two types of asphalt rubber binder used in California. They are referred to as Type I and type II.  Figure 2. Flexural Fatigue for Asphalt Rubber and Dense Graded Asphalt Concrete 3. MAINTENANCE SURFACE TREATMENT STRATEGIES
3.1a Job Selection Chip seals can be referred to as stress absorbing membranes (SAM). Caltrans uses type II asphalt rubber binders in chip seals. The application is limited to state highways to control aggregate loss and windshield damage. Asphalt rubber chip seals have been used to address raveling, aging, poor skid resistance, minor bleeding, minor load associated cracking, water proofing, thermal cracking, and to provide stone retention on high traffic roads. 3.1b Material Requirements Type II asphalt rubber binder is included in Caltrans specifications (Standard Special Provisions - SSP 37-030). The asphalt rubber specification is a recipe based on extensive experience. There are two rubber types used, one is a ground tire rubber (42-65% rubber) and the other is a natural rubber rich waste (40-48% natural rubber). They are added at a ratio of 3:1 by weight into the asphalt. High aromatic- napthenic extender oil is added at about 3%, depending on the compatibility requirements of the blend. Mixing temperatures are 190-226oC (3750F- 4400F). The key requirement of the rubber solids is the grading; this is controlled to about a 40-mesh material, although finer gradings (80 mesh) have been used. Grading controls the digestion rate and reaction time of the binder. Table 1 shows this grading.
The quality of the binder is controlled in the field by viscosity measurement. The binder must be used within a few hours of manufacturing to avoid excessive viscosity increase as this may compromise the even application of the binder causing streaking. The required range is 1500-3500 centipoises at 1900C (3750F) The aggregate must be pre-coated with hot asphalt. Precoating is done by heating the aggregate to 126-163 0C (260-325 0F) and coating it with asphalt. The application level is 0.7-1.0% and AR 4000 conventional grade asphalt is used. This is to ensure a good bond. The aggregate requirements for asphalt rubber seals are generally more stringent than those for conventional chip seals. The aggregate grading is close to a single size as shown in Table 2. The aggregate must be clean, free of clay and be as close as possible to a cubical shape. The aggregate must have at least two crushed faces to promote aggregate interlock and strength. Only 12.5mm (1/2 inch) and 9.5mm (3/8 inch) aggregates are used. The larger stone is used for heavier traffic. 3.1c Mix Design Parameters Chip seal design is not like hot mix design, in that film thickness is not as applicable a concept. Binder application rates are determined based on the average least dimension (ALD) of the aggregate, as well as other aggregate properties such as shape, density, absorption and grading. The optimum binder content also depends on how much binder flows in existing voids in the pavement and how much binder is already present at or near the pavement surface.
3.1d Application Parameters The asphalt rubber is applied at 2.5-3.0 lt/m2 (0.55-0.65 gallons/yd 2). This application rate is based on experience. Some adjustment may be made by the engineer to take field conditions into account. A calibrated spray distributor performs the application for binder at a temperature of 195-213 0C (385-415 0 F). The road temperature must be a minimum of 130C (550F). The ambient temperature must be in the range of 15-41 0C (60-105 0F). The spray pattern must be even, covering the road evenly. The operations should not be conducted during rain or when rain is imminent. The aggregate is spread at a temperature of 107- 163 0C (225-325 0F) and at a rate of 15.2 -21.6 kg/m 2 (28-40lb/yd 2). This is based on experience. A self-propelled chip spreader is used to spread aggregate. Spreading should be done within 2 minutes of spray application of binder. Rolling is carried out using a pneumatic tired roller(s) within 90 seconds of the application of the chips. This should be a one complete coverage for initial rolling and three more coverages for completion. Sweeping is carried out initially using a self-propelled power broom. This is done before traffic is allowed. Extra sweeping may be carried out for loose aggregate. A fog seal and sand coat are carried out after the initial sweeping. Fog seal is carried out using a slow set emulsion (CSS-1, CSS-1h or CQS-1) diluted at a rate of 50:50 with water. The application rate based on undiluted emulsion is 0.14- 0.28 Lt/m 2 (0.03-0.06 gallons/yd2). Sand is applied immediately after the emulsion application at a rate of 1.1-2.2 kg/m 2 (2-4 lb/yd 2). This prevents pick up of the fog seal. Note that the road is not trafficked during fog sealing or prior to sand application. 3.2 Asphalt Rubber Hot Mix-Open graded 3.2a Job Selection Asphalt rubber hot mix - open graded (RAC-O) is a surface course with an aggregate gradation that provides an open void structure with air voids of 15-25%. This produces a permeable surface that can drain water reducing splash and spray. The void structure appears to also be able to attenuate noise making such pavements quieter. The relatively rough surface increases skid resistance. The flexibility of these mixes gives them reflective cracking resistance characteristics. The asphalt rubber binder is much higher in viscosity than conventional asphalt grades and allows thicker films and better aging resistance. Another form of asphalt hot mix open graded is RAC-O-HB, which has more binder content than the RAC-O. The high binder RAC-O-HB sacrifices some drainage characteristics for much improved crack alleviation properties and higher durability. All pavements selected should be structurally sound. RAC-O and RAC-O-HB mixes have been used to address raveling, aging, reflective cracking, minor surface irregularities, spray and splash, skid resistance, surface reflection (at night and in the wet), bleeding surfaces, minor load associated cracking and to lower noise level. 3.2b Material Requirements Type II asphalt rubber binder is included in Caltrans specifications (Standard Special Provisions - SSP39-480). The asphalt rubber specification is a recipe based on extensive experience and it is the same as the recipe used for chip seals. The quality of the binder is controlled in the field by viscosity measurements as for chip seal applications. The aggregate used is 12.5mm top size. The grading requirements are shown in Table 3.
3.2c Mix Design Parameters The design method used by Caltrans for the RAC-O and the high binder RAC-O-HB is based on an aggregate grading designed to give a minimum voids content of 18% (California test CT 367). A drain down test is used to determine the optimum binder content (California test CT 368). This test determines how much a conventional AR-4000 binder can be used without excessive draindown during the transport or placement. In RAC-O, the binder content is taken as the conventional AR-4000 binder content multiplied by 1.2. For high binder RAC-O-HB, the binder content is taken as the conventional AR-4000 binder content multiplied by 1.65. 3.2d Application Parameters Careful considerations when transporting the mix to the job site are important to avoid draindown and to ensure that the mix can be compacted. This is especially the case for nighttime applications where careful considerations of the haul distance are required. This determines the need to use a tarp over the truck. Currently, there are no set maximum distances for haulage. The tack coating practice is important. Often a heavier tack coat than normal is used with open graded mixtures. This is thought by some to assist in waterproofing the underlying pavement. Open graded mixes are placed using conventional pavers. Pick-up machines may be used if ambient temperatures are higher than 20 0C. At the lower temperatures, end dumps should be used. The surface temperature is critical for asphalt rubber placement; minimum air and pavement temperature should be 13 0C (550F). If the pavement and air temperatures are above 18 0 C (65 0 F), the mix shall be spread at no less than 138 0 C (2800F) or higher than 1630C (3250F) as measured in the mat directly behind the paver. If the pavement is less than 180C (650F), then tarps must be used during hauling and the mix is spread at 143-1630C (290-3250F). Rolling is carried out only with steel wheeled rollers operating in static mode at a dead weight of 7.2-9 tonnes (8-10 tons). Two roller coverages are usual. The minimum temperature is 1350C (275 0F) for breakdown and 1200C (2500F) for finishing when pavement and air is above 180C (650F). When pavement is less than 180C (650F), then breakdown is done at no less than 1380C (2800F) and finishing at not less than 1260C (2600F). Sand is applied at a rate of 0.5-1kg/m2 (approximately 1-2 lbs/yd2). Traffic is not allowed until final rolling and sanding has been done. 3.3 Asphalt Rubber Hot Mix - Gap Graded 3.3a Job Selection Asphalt rubber gap graded hot mix is a surface course with an aggregate gradation that has a gap in the continuous grading. This mix is referred to as ARHM-GG or RAC-G in California. The gap in the gradation, together with the high viscosity of the binder, allows a high level of binder (7-9%). This creates a very flexible mix that is highly resistant to reflective cracking. All pavements selected should be structurally sound. RAC-G mixes have been used to address raveling, aging, reflective cracking, minor surface irregularities, bleeding and load associated cracking. 3.3b Material Requirements Type II asphalt rubber binder is included in Caltrans specifications (Standard Special Provisions - SSP39-400). The asphalt rubber specification is a recipe based on extensive experience, and it is the same as the recipe used for chip seals and open graded mixes. The aggregate used is 12.5mm top size and has the grading shown in table 4.
3.3c Mix Design Parameters The mix design method used by Caltrans for RAC-G is based on California Test CT 367 modified to allow for different void contents in different climatic areas as shown in Table 5. A calculation is used to determine the optimum binder content to achieve the voids required. The optimum binder content is used as the upper value in a range where the lower value is 0.3% less than the optimum. If the determined optimum binder content is 7.0% there is no allowed range. The specimens are compacted according to California Test CT 304 with the mixing done at 149-163 0C and kneading compaction at 143-1490C. The requirements call for minimum Hveem stability of 23 and voids in mineral aggregate (VMA) of 18.
3.3d Application Parameters Transporting the mix to the job site is critical to ensure the required application temperatures. The application temperatures are the same as those described in the previous section for open graded mixes. Sanding is used for gap-graded materials at 0.5-1.0kg/m2. The tack coating practice is conducted according to standard practice and a conventional paving machine is used. Traffic is not allowed until final rolling is complete and the sand has been applied to the surface. 4. PERFORMANCE There has been several laboratory and field studies as well as accelerated pavement tests conducted on various asphalt rubber mixes and strategies. These studies were reported elsewhere [SHA 00], [VAN 00], [SHA 97], [EPP 97], [SOU 96], [SHA 96]. The findings from these studies have shown that the asphalt rubber mixes to be superior to conventional dense graded mixes. Performance improvements have been shown in terms of fatigue, reflective cracking and thermal cracking properties. There has been extensive field performance reviews in California.
In 1995, a field review of 88 asphalt rubber projects throughout
the state of California was conducted. Most of those projects
were constructed with reduced thicknesses based on the 1992 asphalt
rubber design guide. The review revealed good performance in most
of the projects. A similar review of 113 projects was conducted
in 1999 that confirmed the previous findings. Recent reviews were
conducted in 2002 on test roads with various test sections that
contained a variety of surface treatments. In those test sections,
several asphalt rubber strategies were compared with conventional
dense graded strategies along with other variables. The reviews
confirmed the general trend that has been observed statewide with
the good performance of the asphalt rubber strategies. One example
of some of the test sections in California is the Esparto Test
Road discussed below. 5.CASE HISTORY - ESPARTO TEST ROAD test road project with several test sections that were constructed side-by-side for comparison. The project was placed in September 1993 on Route 116 near the town of Esparto in Yolo County, near Sacramento, California. The total length of this project is 9 miles on a two-lane highway, one lane in each direction. This road experiences approximately 6,000 ADTs with truck and farming vehicles. The test road included asphalt rubber strategies and conventional dense graded strategies with various features as shown below. 5.1 Test Sections There were 11 test sections in the Esparto Test Road with each section being 0.5 mile long and placed on both directions. The maintenance strategies that were chosen for the various test sections are listed below. Asphalt Rubber Chip Seal
6.0 CONCLUSIONS Based on the performance of strategies that utilize asphalt rubber, specifications and guidelines were developed by Caltrans. Generally, the performance of asphalt rubber chip seals, asphalt rubber open graded hot mixes and asphalt rubber gap graded hot mixes have had good performance. Maintenance and rehabilitation strategies with asphalt rubber binder have been shown to be superior to treatments with conventional asphalt binder in terms of their performance. Performance improvements have been shown in terms of fatigue, reflective cracking and thermal cracking properties. 7.0 DISCLAIMER The information presented in this paper does not necessarily represent the official views of Caltrans nor is Caltrans responsible for its contents. It only reflects the experiences and the personal judgments of the authors as professional engineers. 8.0 REFERENCES [SHA 00] SHATNAWI, S. AND LONG B., "Performance of Asphalt Rubber as Thin Overlays", Proceedings of the Asphalt Rubber 2000 Conference, Vilamoura, Portugal, November 2000. [VAN 00] Van Kirk, J. and Holleran, G., "Reduced Thickness Asphalt Concrete Leads to Cost Effective Pavement Rehabilitation," Proceedings of the Asphalt Rubber 2000 Conference, Vilamoura, Portugal, November 2000. [SHA 97] SHATNAWI, S., "Fatigue Performance of Asphalt Concrete Mixes Using a New Repetitive Direct Tension Test", California Department of Transportation, 5900 Folsom Boulevard, Sacramento, California 95819, 1997. [EPP 97] EPPS, A., "Thermal Behavior of Crumb-Rubber Modified Asphalt Concrete Mixtures," Ph.D. Dissertation, University of California, Berkeley, 1997. [SOU 96] SOUSA, J., SHATNAWI, S. and COX, J., "An Approach for Investigating Reflective Fatigue Cracking in Asphalt-Aggregate Overlays," Third International RILEM Conference, Maastricht, The Netherlands, October 1996. [SHA 96] SHATNAWI, S., "A Procedure for Evaluating Reflective Cracking," Proceedings of the Fourth Materials Engineering Conference, ASCE, Washington, D.C., November 1996. Back to VSS Technology Papers Library Copyright © 2000 Valley Slurry Seal Co All Rights Reserved. |
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