CWC Technology Brief

EVALUATION OF CRM ASPHALT USING SHRP TESTS

A project to judge the effectiveness of using newly developed testing procedures in evaluating the use of Crumb Rubber Modifier (CRM) from scrap tires in asphalt pavements was conducted by Terrel Research of Edmonds, Washington and Oregon State University (OSU). The project used performance based testing equipment and protocols developed under the Strategic Highway Research Program (SHRP). Two methodologies of including CRM in asphalt concrete were compared to conventional pavement mixtures as part of the 1993 resurfacing program by the Seattle Engineering Department (SED).

Background

Disposing of scrap tires as CRM in asphalt mixtures has been used on a limited basis for more than 15 years. Several competing technological approaches have been used with successful projects being offset by costly failures that were not expected or predicted using conventional engineering design and testing methods.

Conventional mixture design methods used by highway agencies are usually based on either the Hveem or Marshall procedures. In these procedures, the air voids, stability, and other factors are used to help determine the optimum asphalt content. However, a shortcoming is that the testing procedures and criteria used to judge mixture quality are empirical and do not apply to special mixtures that include modifiers such as CRM. The results of conventional tests cannot be extrapolated to evaluate expected performance with any degree of confidence. Hence, the SHRP performance based testing procedures are used with the expectation of obtaining more useful results.

Technical Problems

Most of the previous CRM projects were designed using proprietary technology without the user agencies being directly involved in the design process. Also, the empirical design procedures provided only limited data upon which to base judgments on expected performance prior to construction, so the user agency had only limited ability to evaluate the technical reasons for success or failure.

The SHRP developed tests and related tests on mixtures follow procedures that are aimed at minimizing specific types of pavement distress, especially:

· thermal cracking (resistance at low temperatures is evaluated using tension tests),
· rutting (resistance is evaluated using shear strain and wheel tracking tests),
· fatigue cracking (resistance is tested using flexural beam specimens),
· aging (potential is evaluated for both short and long term oxidation), and
· water damage (potential is evaluated in an environmental conditioning test).

The SHRP asphalt mixture tests were recently developed at OSU and the University of California, Berkley (UCB) to evaluate properties that could be used to better resist these types of distress.

Mix designs were proposed by the CRM technology suppliers, EnviroTire, Inc. (dry process known as PlusRide™ II) and Eagle Crest Construction, Inc. (wet process known as ARHM-GG) and compared to a conventional paving mixture developed by the Washington State DOT (WSDOT). At OSU, slabs of asphalt concrete were constructed following the designs provided. Various specimens were cut from the slabs for laboratory testing. In addition, core samples were obtained from each test pavement in Seattle.

Performance Test Results

The testing program was successful in demonstrating significant differences among the paving mixtures and it was concluded that the SHRP based tests are appropriate for evaluating CRM mixtures.

Specifically, all the CRM mixtures showed superior characteristics in resisting thermal and fatigue cracking. The PlusRide™ II mixtures did not fare well in resisting rutting, but better mix design procedures should improve this quality. All of the CRM mixtures showed excellent behavior in aging and water sensitivity.

Cost Impact

The cost effectiveness of this CRM project will not be known until the projected performance of the pavements is validated over the next several years. The unit cost per ton of CRM asphalt paving mixtures is higher, but the life cycle cost is expected to be lower. In addition, there is added benefit of recycling scrap tires, approximately 6,000 for this project.

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: October 1994

Fact Sheet Update: October 1994