Crumb Rubber Modifier in Asphalt Pavement

Chapter 1

Introduction

Background

In June 1993, the Federal Highway Administration (FHWA) published a congressional report summarizing the state of the practice for incorporating recycled materials, including crumb rubber modifier (CRM), into highway pavements. The report, called the Phase I study, concluded that further research is necessary to assess the performance characteristics and recycling potential of CRM asphalt concrete (AC). In September 1994, the FHWA contracted with Oregon State University to resolve many of the outstanding technical issues identified in the congressional report. The research program is extensive in scope and designed to produce guidelines that can be used by State highway agencies (SHA's) to incorporate CRM into asphalt pavements. The goal of this study (termed the Phase II study) is to answer the following questions:

What existing pavement design method is most suited for use with CRM?
Can existing performance prediction models be applied to CRM technologies?
What binder testing techniques are most suitable for use with CRM?
How can mix design procedures be adapted to this technology?
What construction equipment and production procedures are most effective for each of the CRM technologies?
What are the expected performance characteristics of these materials? Does performance vary with construction process and environmental region?
To what degree can these materials be recycled?

The overall objectives are to evaluate CRM technology as it relates to design, construction, recycling and performance of asphalt-concrete pavements. Specifically, the project will develop guidelines for the mix and structural design and construction of CRM-asphalt pavements. Guidance will also be provided for the production control and quality assurance of CRM mixes. The project will also establish the long-term performance of CRM pavements and determine recyclability.

If the Intermodal Surface Transportation Efficiency Act (ISTEA) mandate is implemented in October 1995, all State highway agencies must use 15 percent crumb-rubber-modified hot mix asphalt (CRM-HMA). At present, very few SHA's approach this level of utilization. The States that presently satisfy the ISTEA mandate include Arizona, California and Florida. Each of these agencies has developed, over several years, a CRM infrastructure (tire recyclers, blenders, and contractors) that is capable of meeting the maximum mandate (20 percent utilization).

Objectives of Report

The purpose of this report is to clearly document the use of CRM in the States of Arizona, California and Florida to allow others to benefit from their experiences. The report addresses the following: thickness design, materials/mix design, construction procedures/costs, quality control/quality assurance (QC/QA) considerations, and performance. Also included are a glossary of terms (appendix A) as well as typical specifications for the HMA applications used in these States.

History of Use

The uses of CRM in HMA in the three States has evolved over the years. The Arizona Department of Transportation (ADOT)/City of Phoenix initiated the use of CRM in HMA in the 1970's using test sections. While most of these test sections used the wet process, ADOT placed two sections using the dry process. Prior to that, Arizona DOT's CRM use had been limited to stress-absorbing membranes (SAM's), stress-absorbing membrane interlayers (SAMI's), and sealants.

More extensive use of CRM in HMA began in 1985 using open- and gap-graded mixes. Only the wet process is used now. The open-graded mixes have been used in thicknesses less than 40mm (1.6 in) over both asphalt and portland cement concrete pavements. The gap-graded mixes thicknesses less than 60mm (2.4 in) are primarily used over asphalt-concrete pavements. The CRM in both mixes provides thicker films of binder and reportedly improves the durability and cracking resistance.

California has experimented with both wet and dry CRM processes for hot mixes since the 1970's. Currently, they primarily use the wet process in a gap-graded mix. In some instances (mountain passes), they have used a dense-graded wet process. They no longer utilize the dry process because of erratic pavement performance with these systems.

Florida Department of Transportation (Florida DOT) initiated CRM-HMA work in 1988-1989. Their interest was sparked by State legislative interest. They primarily use CRM (5 percent by weight of binder) in dense-graded friction courses of 25mm (1 in) thickness to improve the resistance to shoving/rutting, particularly at intersections. On their freeways, they use a thin 15mm (0.6 in) open-graded friction course (containing 12 percent CRM by weight of binder) to improve the durability. Florida has developed a wet process that provides for the introduction of very fine rubber into the hot asphalt cement. The dry process is not used by Florida DOT.

Limitations

The agency practices documented here have been developed specifically for use in the States of Arizona, California and Florida. The information contained in this report was obtained through interviews with agency and industry personnel in each State. Currently, all three agencies use the "wet" process exclusively. Much of this technology appears to be transferable to other public agencies in the United States.