RESEARCH AND DEVELOPMENT OF FLEXIBLE PAVEMENT INFRASTRUCTURE

TECHNOLOGY

The Department of Energy has reported an estimated 2-3 billion tires exist in stockpiles, landfills, etc., in the United States. The stockpile grows by 279 million annually.

The wasted tire is not biodegradable. Past uses of tires have not created enough economic demand for their reuse.

The heating value of tires is 20% higher than that of coal. Burning of the tire for energy has been used in western Europe and is beginning to develop in the U.S. The enthalpy of combustion (energy) from burning tires has been used to produce Portland cement. Several problems with air quality have developed because of tire burning.

The 1972 oil embargo created a pyrolysis to extract the oil from tires for energy. This proved to be uneconomical. The availability of crude oil has dropped since 1972.

Asphalt rubber was pioneered in the United States by Charles McDonald for the City of Phoenix, Arizona.

Reclaimed rubber has been added to the asphalt in a paving mix to improve the physical properties and durability of the asphaltic (flexible) pavements. The Arizona Department of Transportation and the city of Phoenix have successfully used this product extensively over the past 25 years.

The asphalt-rubber process involves heating 80% conventional asphalt with 20% reclaimed ground rubber (old tires) for about 45 minutes at 375^° F. This heating process causes the rubber particles to swell and produce a softer and flexible sticky binder. during the process a small amount of oil extender is added to the mix to reduce the binder's viscosity. This binder can be used for several asphalt-rubber applications to include SAM, SAMI, Seal coat, Plus-Ride, Open Graded and Dense Graded systems. A description of these systems include:

Stress Absorbing Membrane (SAM)   also known as chip seal coat consists of a thin layer of asphalt rubber (less than 0.2 of an inch in thickness) is sprayed over existing pavement and small rock chips are then added.

Stress Absorbing Membrane Interlayer (SAMI)   is used to help prevent reflective cracking. This phenomenon inhibits further cracking and prevents any footprint of old cracks into the new surface. The major objective of this system is to place a crack arresting layer between the old and new surface. This is done by spraying a thin layer of asphalt rubber over the old pavement surface prior to surfacing with the conventional asphalt. This process can be repeated as necessary to reach the desired elevation.

Seal Coat   is another method which is used to seal existing cracking (commonly called alligator cracking). This type of cracking is very common in cold climates due to reduction in flexibility (ductility) of the traditional pavement materials. As a general rule asphalt concrete pavement should be designed with a binder which behaves flexibly in the cold climate and is semi-rigid in hot climates.

Plus-Ride   a portion of aggregates (crushed rock) is replaced with rubber granules from recycled tires.

Open Graded and Dense Graded   hot mixed asphalt rubber pavements are composed of conventional open and dense graded aggregates with asphalt rubber binder rather than plain asphalt. Evaluation of dense graded asphalt rubber by the Iowa Department of Transportation (MI.R-89-15) indicates that asphalt rubber hot mix outperforms the conventional asphalt. This year 95% of all asphalt in Mid-America will be hot mix asphalt pavement (which can be produced by all hot mix plants with conventional equipment). For example, within a 50-mile radius of Kansas City over 50,000 tons of rubber asphalt hot mix will be bid for Interstate 29, US 71, US 24, Kansas City International Airport, and others.

A national survey conducted by researchers at UNI documented that these systems have been used and field tested across the U.S. The frequency and popularity of asphalt-rubber concrete in various state across the U.S. is exhibited in Table 1.

In regard to the over 300 articles collected there exists the need to further research and field test rubber asphalt concrete systems. Documented benefits of rubber asphalt concrete include:

It is important to mention that the majority of state Departments of Transportation across the U.S. support research into the inclusion of rubber into asphalt pavement.

Asphalt rubber has been in use for over 25 years and in some cases received overwhelming positive support while others have received mixed reviews. For asphalt rubber to be a viable construction material, there needs to be clarity in regard to actual costs, equipment, as well as the thickness needed. Also there is the need to have a comparison of costs between this material and conventional asphalt mixtures.

In order to evaluate the potential usage of Asphalt rubber concrete on Iowa roads researchers at UNI have become involved with state legislators, the Iowa Department of Natural Resources and the Iowa Department of Transportation. In recent years two major grants in polymer and elastomer studies amounting to almost $230,000 worth of research monies. These grants represent involvement and interaction with Iowa legislators, the Iowa Department of Natural Resources and the Iowa Department of Transportation. The most recent grant ($98,000) with the IDOT will be presented in this paper.

The following includes portions of the research proposal which was submitted to the IDOT Highway Research Board. The result was a joint venture and partnership between the construction faculty and representatives of IDOT.

Iowa has 3,840 lane miles of interstate, 20,300 miles of primary roads and 89,500 lane miles of secondary and county roads. Ninety percent of interstate road costs and 75% of primary and secondary road costs are financed by federal aid. County roads are financed by county funds. Additionally, Iowa has extensive miles of urban streets that are supported by cities and counties. It has been estimated that Iowa wastes 3.5 million tires each year. Presently, waste tires are disposed of in designated landfill sites under city/county control. A large percentage of the waste tires are illegally dumped.

The whole tires which are stockpiled represent not only a waste of resources but also a health hazard because of mosquitoes, fires and general contamination of the environment. The mosquito is an encephalitis borne disease carrier that has migrated to the Midwest. Tire fires are an environmental risk in the form of liquid and gaseous emission. The best way of eliminating the environmental and health hazards associated with tire piles is to minimize and ultimately eliminate the stockpiling of tires.

Adequate technology exists to significantly reduce tire stockpiles. Waste tires are under-utilized because of adverse economics. It is cheaper to throw a tire away than to recycle it. Until economic forces are reversed, and environmental concerns addressed, tire stockpiling will be the option of choice. Producers and importers of tires must become involved and need to assure that such tires are ultimately managed in a responsible fashion.

ASPHALT-RUBBER EVALUATION FOR THE STATE OF IOWA:

In order to evaluate Asphalt-Rubber on Iowa roads, a demonstration project has been established. The proposed research will be incorporated into Muscatine County construction project F-61-4(49)--20-70 on US 61 from Muscatine to Blue Grass. The existing pavement is a 10 inch by 24 foot jointed portland cement concrete pavement constructed in 1957. The Muscatine project is a 14 mile, 4 inch asphalt concrete overlay that will probably consist of 2 inches of binder and 2 inches of surface. The 1988 traffic value was 6490 vehicles per day with 17% trucks. The project was scheduled for the October 2, 1990, letting. The construction will be during the summer of 1991.

The research would consist of three one-half mile experimental sections and two 500 foot Plus-Ride

sections. Plus-Ride experimental sections would use approximately 5% recycled rubber granules of a suggested gradation as follows:

These two sections will depend on the availability of satisfactory rubber granules. The two 500 foot sections would be placed directly across the road from each other to avoid Friction Number variations from lane to lane. The rubber granules will be used in the surface, but not in the binder.

The three one-half mile, 12 foot wide sections with asphalt-rubber binder will be separated by one-half mile sections of the conventional mix designed for the project. The other two comparative conventional mix sections will be adjacent on either end of the experimental sections. Mix designs for the conventional mix and the asphalt-rubber binder mixes will be developed at the Iowa Department of Transportation Materials Laboratory at Ames. Joints will be sawed over the joints in the existing pavement in 1/4 mile of all asphalt-rubber binder sections and all conventional binder sections. The asphalt-rubber binder will be used in both the binder and surface on one section and only in the surface of the other two sections.

The University of Northern Iowa (UNI) participation in the research will be predominately in determining the aging and changing of the conventional asphalt cement binder and the asphalt-rubber binder over the five year period. Some of this testing will be in the laboratory and some will be on binder extracted from cores taken from the construction project. The procedures for these tests are described in the evaluation section.

EVALUATION

The Iowa Department of Transportation and researchers at the University of Northern Iowa will evaluate the research for a minimum of five years after construction. The Iowa Department of Transportation will drill all cores needed for UNI evaluations. The UNI evaluation will include the following:

DUCTILITY TEST

ASTM D113 Ductility Test Method will be utilized. These test specimens will be tested at below freezing (intend 20°F), 32°F, 40°F, 70° F, and 100°F.

AGING TEST

ASTM 113 Ductility Test will be used to determine the effects of aging on (asphalt-rubber) and conventional asphalt binders. Ample specimens will be produced and exposed to some environmental elements as the test section. It is assumed that when asphalt is exposed to sunlight, it will lose its ductile properties and become more brittle. A comparison between ductility of control group (plain asphalt) and test group (asphalt-rubber) will be made. Specimens will be tested at 32°F, 40°F, 70^°F and 100°F. Scanning electron microscopy will also be used to evaluate each specimen after failure.

TENSILE CREEP TEST

The stress-strain properties of cast binder (asphalt-rubber) will be measured in a tensile creep test under various constant loads at 32°F, 70^°F, and 100°F. The test specimens will be cast in a mold which consists of the end pieces of the ASTM D113 ductility test mold, with 150 mm long straight side pieces instead of the wedge shape side pieces instead of the wedge shape side pieces of ductility mold.

The tensile creep tests will be run in a modified ductility bath, with the free ends of the specimen supported on a 'raft' made of wood and polystyrene foam. Specimens will be supported in the same plane as the fixed end. Dead weight loads will be applied through a thread fastened to the raft and passing over a pulley. Prior to testing, specimens will be kept in the constant temperature bath for 60 minutes.

FATIGUE TEST

A beam fatigue test will be utilized to evaluate the fatigue characteristics of the asphalt concrete mixes. The fatigue test will be performed by placing the beams on a flexible sub-grade and applying a repeated load at the center. Results will be used to determine the relative fatigue life of the mix, and to estimate the fatigue life of a test section.

Iowa Department of Transportation testing will include all standard density determinations on all sections at time of construction. In addition to the standard project testing of the mix, creep (creep resistance factor under 200 psi compression) and resilient modules will be determined for all three mixes. Friction testing by ASTM #274 will be conducted annually for five years on all nine sections. The Iowa DOT Road Rater will determine "deflections" (compressibility) annually for five years.

Annual crack and rut depth surveys will be conducted on all sections. The time until reflection cracks occur will be determined for each test section. The percentage of reflected cracks will also be determined. Rut depths under a four-foot straight edge will be determined for both wheel paths.

SUMMARY

Researchers at the University are examining, reexamining, and in some cases duplicating previously done research. Additionally, new research areas and perspectives are also being explored in regard to the

potential use of rubber asphalt concrete. Special attention is being directed at the specific tests of Penetration, Marshall Stability and Flow and Ductility. A major thrust of the rubber asphalt is to identify a mix that would offset the negative conditions (freeze­thaw cycle, heat of summer months, etc.) experienced by road pavements throughout the state of Iowa. There is a growing awareness for the need to examine the use of recycled materials such as rubber tire and their incorporation into viable construction materials. This necessitates that university faculty redirect some

of their efforts to applied research projects dealing with the solution of these environmental problems. Currently, there exists unique opportunities and research monies to work in joint ventures and/or partnerships with state agencies, such as the Department of Natural Resources and the Department of Transportation.