Featured Story from the January/February 2006 Today's Tire Industry
Making Better Roads
by Kevin Rohlwing
Editor and TIA Senior Vice President of Education and Technical Services
The New England Rubber Modified Asphalt & Civil Engineering Workshop, held in Providence, Rhode Island, October 20-21, 2005, drew officials from the Federal Highway Administration and the Departments of Transportation from six northeastern states in a first-ever event focused on the use of crumb rubber in pavement and road construction applications in the northeast region. Attendees and panelists gave the workshop high marks for opening "a much needed technical and educational dialogue" on the uses of recycled rubber in transportation applications.
TIA's Tire and Rubber Recycling Advisory Council (TRRAC) was one of six sponsors of the event and TIA President Dick Gust, TIA President-Elect Bob Malerba, TIA Executive Vice President Roy Littlefield and TIA Director of Business Development Paul Fiore attended the workshop. Highlights of the workshop covered the gamut of subjects ranging from recycled materials in highway applications to new approaches for marketing rubber modified asphalt to civil engineering officials in cold climates.
Recent interest in the long-term engineering and environmental performance of recycled materials in the highway environment can be traced to the establishment of the Recycled Materials Resource Center (RMRC) in 1998. Located at the University of New Hampshire in Durham, the RMRC is a national center in partnership with the Federal Highway Administration (FHWA) dedicated to overcoming barriers to the appropriate use of recycled materials in the highway environment. "The Center advances this mission through research and outreach activities aimed at a range of recycled materials," said RMRC Outreach Director Dr. Jeffery Melton. These include traditional recyclables such as paper, plastic, bottles and cans; asphalt and concrete pavements; incinerator ash and coal combustion products; shingles and C&D debris; slag and foundry sands, compost, mine tailings and quarry materials and, of course, tires.
Since 1998, RMRC has undertaken two research projects involving the use of recycled tire materials. For the first project, RMRC partnered with the Colorado and Texas Departments of Transportation, Federal Highway Administration Central Lands, Front Range Tire Recycling, Inc. and the University of Sherbrooke to conduct full-scale monitoring of mechanical and exothermal response of tire-soil embankments. The 18-month (October 2003-March 2005) research effort involved the creation of full-scale embankments from soil, soil-tire shred mix and soil-tire shred layers. The embankments were outfitted with instrumentation to measure thermal and mechanical response with plate load tests being used to determine mechanical response. "Monitoring for long-term thermal response is ongoing," remarked Dr. Melton.
RMRC's second project involved a test of recycled tire spacer blocks at the University of Nebraska's Midwest Roadside Safety Facility, a ten week project (October 2002- December 2002) designed to get FHWA approval for new spacer blocks. RMRC, along with its project partners (Iowa Department of Transportation, Iowa Department of Natural Resources, the University of Nebraska, FHWA and Welch Products, Inc.), also hoped to gain experience installing the tire blocks and working with recycled tire materials. According to Dr. Melton, the way forward for recycled rubber is to highlight successful uses of rubber in the highway environment and communicate the results of those projects so the industry can share ideas for advancing new uses.
FHWA's Jason Harrington agrees that spreading the word about successful asphalt rubber and civil engineering projects is one of the best ways to increase the use of recycled tire rubber in these applications. Harrington advises states with an interest in using asphalt rubber should look at those states that are currently using the material, learn from them, and apply existing knowledge to their own states. "This workshop is a good example of how states can learn from each other," he said. "Being able to show other state representatives in New England successful asphalt rubber pavements or tire derived aggregate highway construction applications is a big help," Harrington said.
The use of crumb rubber aggregate for hot mix asphalt paving has come a long way in recent years boosted by better performance, noise reduction and safety benefits. According to Harrington, asphalt rubber also has the capability to consume large quantities of scrap rubber which would improve tire recycling efforts. Current and planned users of crumb rubber aggregate for hot mix asphalt paving include Alaska, Arizona, California, Florida, Illinois, Louisiana, Maryland, Nebraska, New York, Rhode Island, Pennsylvania, South Carolina, and Texas.
Jim Gilbert (left), NY Empire Development, and Don Larson (right), CT DOT, discuss states' use of asphalt rubber at the workshop.
To sustain these benefits, FHWA is conducting research on crumb rubber modifiers (CRM). "Research is one way to achieve measurable improvements in expanding the use of recycling technology and materials in the transportation industry," said Harrington. Most recently, FHWA has been conducting laboratory evaluations comparing Arizona (AZ) crumb rubber modifier with Terminal Blend and comparisons of crumb rubber aggregates with a range of control materials. FHWA's testing plans for CRM include French Rut test, Hamburg load weight test and a simple performance test.
According to Harrington, FHWA is primarily interested in asphalt rubber's performance properties including its cost effectiveness and any added benefits. In preliminary results, laboratory and field evaluations have already shown that crumb rubber aggregates provided equal or better performance compared to the known modifiers of similar grade. User demands such as noise abatement and the sustainability of the material also play an important role in FHWA's research efforts.
Among the next steps, FHWA plans to explore the use of Terminal Blend with Arizona-gradation aggregates and the use of AZ binder with Terminal Blend gradation aggregates. "The Arizona mixture has great crack pinning features," remarked Harrington. "In testing, we can switch aggregates and design to see what happens." Harrington also sees a role for asphalt rubber in FHWA's newest program – the Green Highway Initiative. It brings together wetland management, solid waste management and industrial by-products groups to champion Green Highway concepts such as watershed-driven storm water management, recycling and reuse of industrial by-products and solid waste, and conservation and ecosystem management. "The Green Highway Initiative will help raise the awareness of the American public about transportation's contributions to recycling and greening the environment," concluded Harrington.
New Marketing Approaches
"Promoting the noise reduction benefits of asphalt rubber could generate enough public support to convince public officials in your state to use the material," said Cliff Ashcroft, Vice President of FNF Construction. The same is true for asphalt rubber's safety and cost savings features. "Playing up these features is one of the best ways to market rubber modified asphalt," he remarked. Ashcroft also pointed out that research results supporting the performance, safety, noise reduction and cost savings benefits of asphalt rubber is well established and continues to grow as more communities report how asphalt rubber pavements are performing on their roads.
For example, interest in the noise reduction benefits of asphalt rubber has spawned quiet pavement programs in Arizona and California and provided the impetus for the Federal Highway Administration's Quiet Pavement Pilot Program. Under FHWA's Pilot Program, the Arizona and California Departments of Transportation have created noise profiles of roads using a close proximity noise-measuring device. Measurements show that the residents in the Phoenix, AZ area have benefited the most where the existing concrete pavements were the loudest (105 dBA) and then became the quietest ever measured (92 dBA) following a thin asphalt rubber overlay (a 13 dBA drop in tire pavement noise). While reducing the noise by just three decibels is like cutting the traffic in half or like doubling the distance from the source of noise, the dBA level is only half the story. For every 1 dBA in noise reduction desired, 2 feet of sound wall is required. Noise walls cost about $20 per square foot, only work in line of sight, and need two feet in height to reduce road noise by 1 dBA. The 8 dBA reduction experience in the Arizona DOT system is equivalent to a 16-foot reduction in noise walls. According to Ashcroft, the savings on a one-mile stretch of road is approximately $1,689,600.
Asphalt rubber's safety benefits are also making headline news. As the nation's road infrastructure ages, many concrete pavements – although still serviceable – exhibit very poor ride quality and unsafe driving conditions. Traditional asphalt is a dense graded material that creates an ultra-smooth surface making it prone to puddles and slick conditions in wet weather. In contrast, asphalt rubber mixes are "open graded" or "permeable" friction courses which remove the water from the surface. By adding granulated tire rubber to these open mixes, better skid resistance and durability is the result. Asphalt-rubber surfaces are now being selected more frequently as the best choice to offer the driver better traction or skid resistance, better visibility in wet weather, reduced hydroplaning and most of all, less traffic accidents that result in injury. In Texas, rubber asphalt concrete has boosted road safety conditions by a significant margin. One project that Ashcroft cited in San Antonio, Texas, compared a stretch of road that in 2002 had 85 major accidents, 39 of which occurred on rainy days. After paving it with a porous friction course asphalt overlay that included a rubber mix, only 19 of the 48 major accidents on that stretch of road occurred on days with precipitation in the 12 months after application.
In a second example of asphalt rubber safety, Ashcroft described a two-mile stretch of road, also in Texas, with a series of "S" curves. From 2001 to 2003, the road had 124 accidents, including 68 wet-weather crashes which took six lives and caused 62 injuries. After rehabilitating the road with asphalt rubber, the road had 17 accidents in 2004, a decrease of 58.9 percent from the annual average of the prior three years. More importantly, there were no fatalities or injuries. This occurred while rainfall was 55.5 percent higher in 2004 than the average of the previous three years.
Asphalt rubber pavements may last up to twice as long as conventional materials before needing maintenance or replacement. Another asphalt rubber cost advantage is that some applications can be placed at half the thickness of conventional pavement, saving on material and installation costs as well as construction time. "While asphalt rubber costs about 25 percent more than conventional asphalt on a per ton basis, it requires half the material," Ashcroft said. Also, numerous case studies have proven again and again that using an asphalt-rubber binder in a pavement provides better resistance to cracking and fatigue caused by heavy traffic which leads to a smoother road and lower operating costs. Ashcroft concluded with a 1999 U.S. study on pavement design completed at the WesTrack pavement testing facility near Reno, Nevada, where researchers showed that for identical conditions, cracked pavements cost 4.5% more in fuel.
Though asphalt rubber is gaining momentum in a few northeastern states, it still faces many obstacles to its implementation in the region according to several state officials attending the workshop. These range from perception factors to political, technical, economic and environmental concerns.
"The New York Department of Transportation (NYDOT) supports the use of crumb rubber modifiers in a variety of applications, although we have only limited experience with asphalt rubber pavements in New York," said James Klotz of the Technical Services Division. Since 1989, seven projects using asphalt rubber were constructed using 1.5 million pounds of crumb rubber. These included four crumb rubber projects using the dry process, which failed within two to five years. One section of wet process pavement was also removed, but the remaining wet process sections are performing as good as or slightly better than conventional pavement. New York's standard specifications allow crumb rubber in any of the performance grade binders used in the state, however there are issues that stand in the way of increased use of rubber modified asphalt in hot mix asphalt. Both cost and effectiveness are key considerations, as are supply and market issues. "Crumb rubber can certainly enhance the performance of hot mix asphalt but at a cost," remarked Klotz. "And, it must compete with other modifiers in a very competitive market."
NYDOT recently joined in a pooled fund research project with other northeastern states to advance the use of chemically modified crumb rubber aggregates in Superpave applications.
Tire Shred Initiative
From a civil engineering perspective, NYDOT is gearing up to begin a project in 2006 which will use 18 million tires over a two-year period. The project is part of the Tire Shred Initiative established under a provision of the state's Waste Tire Bill. The Initiative is funded through a $2.50 per tire fee collected on the sale of new tires in the state. Under the Tire Shred Initiative, tire shreds are primarily used for embankment in place. Other uses include Backfill for T-Wall; Select Structure Fill; Lightweight Embankment in Place; Mechanically Stabilized Earth System; Geosynthetic Reinforced Earth System; Mechanically Stabilized Segmental Block Retaining Wall System; Segmental Block Retaining Wall System and Roadway Foundations.
A pilot project completed in 1999 used 270,000 tires in a ramp embankment in Occanum, New York, and tire shreds from about 1 million tires were used as roadway foundation in a section of I-87 near Albany in 2004. One of two projects scheduled for placement in 2005 used tire shreds from 350,000 tires in an embankment fill project on the I-240 Expressway.
Extending Pavement Life
In Rhode Island, DOT engineers first placed a demonstration project using the Plusride asphalt rubber technology on Rt. 2 in East Greenwich in 1987. During the 1990s, research conducted at the University of Rhode Island focused on crumb rubber asphalt binder using the SuperPave Binder Specification. In 1999, RIDOT established a Pavement Preservation Program to provide and maintain serviceable roadways in the state. From the start, the program was designed to use crumb rubber in three major preservation treatments – crack seal, chip seal and modified asphalt overlays. "Based on our research, we opted to use a chemically modified crumb rubber asphalt (CMCRA)," remarked Colin Franco, RIDOT Research and Technology. Franco worked with Hudson/All States Asphalt to incorporate a low viscosity CMCRA with fibers into crack seal for use on both limited access highways and general road applications in the state. "For chip seal applications, we used CMCRA in demo sections with chip seal. Rubberized asphalt chip seals require slightly less rubber than some of the other treatments," Franco said. The agency also used CMCRA binder to produce a crack resistant elastomeric mix for paver-place elastomeric surface treatments.
To date, the state has used a total of 2,457 tons of crumb rubber in its pavement preservation program. This amount is expected to increase in future years as RIDOT expands the Pavement Preservation Program throughout the state. Also, rubber-modified binders are increasingly being used in typical overlays on rehabilitation and reconstruction projects. "Extending pavement life maximizes the return on Rhode Island's existing infrastructure," Franco concluded. "Pavements represent a billion-dollar investment in the state, so protecting them saves dollars and resources."
Vermont Focuses on Construction Uses
Historically, Vermont's usage of scrap tires in transportation projects has been driven by the desire to recycle in-state tires and that resulted in a focus on tire shreds and not rubber asphalt, according to Jim Surwilo of the Vermont Department of Natural Resources. These projects include the use of 200 cubic yards of tire chips in the base course of a local road in Georgia, Vermont in 1990. Prior to using the tire chips, the unpaved road was impassable in late winter and early spring due to poor drainage and spring thaw weakening. Within a year of constructing the base course with tire chips, the road was visibly dry and free of rutting within 300 feet of the test segment. Over the next two years, tire chips were installed in the base course of another 800 feet of the roadway, and a half-inch thick chip seal was placed on the road in 1997, which continues to perform today.
In another project, Vermont's Transportation Agency used tire chips and a native timber bin retaining wall to stabilize a steep eroding slope. Tire shreds were also placed in a highway embankment project in Middlesex, Vermont in 1999, where approximately 2,738 cubic yards of tire shred were used as lightweight fill to reduce the embankment side slope and eliminate the need for a guardrail.
From the state's first use of rubber modified asphalt in 1991 until 1995, only two state highway projects were placed using asphalt rubber hot mix (ARHM). Approximately 17,800 scrap tires were used in a wet process application on a 4.3-mile section of Route 2. To date, Vermont Department of Roads officials report that the performance of the ARHM section has been comparable to the standard mix section. However, the cost of the ARHM was significantly higher. In a second project, terminal blend ARHM placed on a section of road in Westfield in 1994 remains the wearing surface of the road today. According to Surwilo, the cost of the ARHM for this project was only slightly higher than the convention mix.
In July 2005, an asphalt rubber chip seal prepared by All States Asphalt using 10% crumb rubber was placed on a local road in Manchester, Vermont. The project saved $40,000 over the price of conventional hot mix asphalt and as a result of the small increase in height (0.5 in), no shoulder or driveway work was needed. Local road department engineers gave the projects high marks saying they were "very impressed …the material set up quickly and had very good skid resistance." And although local road officials say the true test will be "how the road performs this winter," plans are underway to resurface an additional 2.6 miles with a 20% rubber binder chip seal next summer.
In 2004, the Vermont legislature formed a study group to assess cost effective uses of shredded tires in Vtrans projects, to identify at least one project for construction by 2007, and to institutionalize uses of scrap tires as feasible. The study group concluded that Vtrans will develop scrap tire shred specifications (now in draft) and identified two town highway bridge projects where tire shreds will be used as lightweight fill. The study group also concluded that Vtrans would continue to fairly assess recycled materials, including tire shreds.
Taking a Second Look
Connecticut's first asphalt rubber pavement was placed in 1950 in Rocky Hill, Connecticut. Don Larsen, Transportation Supervising Engineer with the Connecticut Department of Transportation (ConnDOT) described more than eight projects using both wet process and the dry method asphalt rubber in thick and thin overlays, stress-relieving interlayers, rubberized crack/joint sealants and rubberized chip seals that were completed over the next 30 years including a 40 test-site multi-town dry process initiative. This project, involving roads in 16 towns throughout the state, was conducted as a joint study between ConnDOT and the University of Connecticut and remains one of the largest studies of dry process asphalt rubber ever undertaken.
ConnDOT continued to evaluate all the rubberized asphalt pavement projects in the state during the 1980s and early 1990s with mixed results, according to Larsen. Overall, the results showed no significant pavement performance increases between the rubberized pavements and conventional asphalt pavements and the cost for the asphalt rubber pavements was 25-75% higher than conventional pavements. In 1991, Connecticut highway officials, along with officials in many other states, opposed the federal mandate to use asphalt rubber promulgated by the passage of the federal Intermodal Surface Transportation and Efficiency Act (ISTEA).
Briefly, Section 1038 of ISTEA directed state departments of transportation to spend a portion of their federal highway money on asphalt rubber. ISTEA also required FHWA to transfer asphalt rubber technology to the states to ensure that the product would be used successfully. Congress repealed the ISTEA funding provisions pertaining to asphalt rubber in 1993. Despite its failure, debates over ISTEA did bring forth important information about asphalt rubber that has helped states and the paving industry address questions regarding the product's emissions, safety, recyclability, economics and more.
In 1995, ConnDOT launched an initiative to focus on pavement preservation. As defined by the American Association of State Highway and Transportation Officials, pavement preservation is "a planned strategy of cost effective treatments to an existing roadway system and its appurtenances." Under the program, ConnDOT engineers began to test asphalt modifiers to extend pavement life and preserve the state's roadway system. They include Nova Chip for enhanced friction; HM3 (hot mix maintenance membrane) for extending the performance life of the old road surface; and asphalt rubber polymerized chip seal on low volume (< 3000 vehicle average daily traffic) roads.
"These special mixes are designed to address specific maintenance or performance requirements in select situations," Larson said. Five asphalt rubber polymerized chip seals have been placed in the towns of Eastford, Colebrook, Plainfield and Winchester since 1998. In 2003, a dense graded hot mix maintenance membrane was placed on Route 9 in Cromwell to cover existing deterioration on the road and extend pavement life. To date results show that reflected cracks on the HM3 pavement are narrower than on the control section. Highway engineers are also monitoring friction levels to gauge performance.
Pavement noise reduction from open graded friction courses and rubber may be the mechanism for renewed interest in asphalt-rubber pavements in Connecticut. While no official research has been performed on pavement noise in the state, a Quiet Pavement Task force was established in August 2005 to address research needs and options for Connecticut. The Task Force plans to compile a research needs statement and report its findings to the Chief Engineer.
Maintenance and Prevention Play Key Role for Maine
Maine's use of crumb rubber in pavement applications was first recorded in several 1975 projects that incorporated crumb rubber in bituminous pavements and utilized a crumb rubber slurry in a stress relieving interlayer placed over a thin leveling course to reduce reflective cracking. In 1983, Maine DOT used Ultrapave rubber asphalt bituminous mixtures in an overlay application on Route 6. "The agency's 1986 final report on these early projects found no significant difference in performance between the test and control sections,"said Dale Peabody, Maine DOT.
Between 1991 and 1995, Maine highway engineers placed a dense graded asphalt rubber overlay on I-95 near Old Town, Maine. Over 4,000 tons of wet process asphalt rubber (16% crumb rubber and 3% natural rubber) were used for the project. While the performance of the asphalt rubber sections was not significantly different than the control, the cost for the asphalt rubber blend was nearly three times higher than the conventional asphalt mix.
A two-inch rubber modified hot mix asphalt overlay placed on Route 3 in Augusta, Maine in 1994 also marginally outperformed the standard control mix but was not cost effective, a 1996 report concluded. The report also concluded that another test section on Route 3 in Augusta overlaid with dry process hot mix asphalt with one-percent crumb rubber by weight of the total mix did not outperform the standard mix and cost about $5 more per ton.
Starting in the early 1990s the state has routinely used rubberized crack sealants as part of its road maintenance program. In the last two years, 700,000 pounds of rubber crack sealant was bid in four regions, and another five projects bid 13,500 gallons. Any application of rubberized material that improves the cost effectiveness of the state's preventative maintenance program is on deck for use on Maine roadways, according to Peabody. Current top choices include thin hot mix asphalt overlays, chip seals, stress absorbing interlayers and crack sealants. "States that have implemented pavement preventative maintenance programs using similar products have experienced significant cost savings over road construction," Peabody remarked. "We hope to capitalize on similar savings with our pavement program."
When it comes to using scrap tires in civil engineering applications, Maine is at the forefront. One of the reasons is savings. A 1997 project that used 1.2 million tires for a highway embankment at the Maine Jetport in Portland saved the state $600,000 for the project. Another reason is that tire derived aggregate (TDA) has properties needed for cold climates. "From an engineering perspective, TDA offers an economic advantage and has outstanding performance in cold climates compared with conventional materials," commented Dr. Dana Humphrey, civil engineering professor at the University of Maine. These include good thermal insulation (up to 8 times better than soil) and good drainage (10 times better than most soils). It's also lightweight (a third lighter than soil), exerts low earth pressure (half that of soil) and is compressible.
TDA's insulation properties make it an excellent material for limiting frost penetration beneath roads. It can also be used as an insulation backfill for retaining walls, as an insulating fill for highway edge drains and as lighweight fill over peat bogs and poor soil areas. "Overall, TDA is the cheapest alternative if you need their properties," said Dr. Humphrey. For example, a road reconstruction project in Tewksbury, Massachusetts in 2003 saved the town $220,000, used 200,000 tires and solved a long-standing problem with road settlement. Prior to reconstruction, 13 feet of fill had compressed the peat layer to 6 feet thick on a 790-foot section of Livingston Road. This resulted in up to 3 feet of settlement over 24 years. To solve the problem, engineers reconstructed the section with Type B-lightweight fill TDA. Three years later, long-term settlement has been eliminated and no cracking or rutting has appeared in the reconstructed section. The Massachusetts Consulting Engineers Council named it the project of the year for 2003.
In conclusion, Dr. Humphrey posed several questions. As engineers and officials continue to monitor and compile performance and cost data on existing TDA projects, will results show that TDA's insulating properties make it very effective for septic tank leachfield application in cold climates? Will TDA gain wide acceptance as backfill for residential foundations? Will TDA as lightweight fill for construction of roads on weak soils become routine in New England? "It takes years to build a market for these applications," Dr. Humphrey acknowledged. "But TDA has significant potential for growth in cold regions."