Crumb
Rubber Modifier in Asphalt Pavement
Summary
of Practices in Arizona, California and Florida
Preface
Highway agencies have been evaluating crumb rubber modifier (CRM)
technologies in hot mix asphalt (HMA) since the 1970's. Numerous
technologies have been studied, with varying degrees of success.
Three agencies have used CRM-HMA in quantities that meet or exceed
the mandate specified in Section 1038 of the Intermodal Surface
Transportation Efficiency Act (ISTEA) of 1991. These agencies,
which include Arizona, California, and Florida, use the wet process
on dense-, gap-, or open-graded mixes. To provide
other agencies with information on the use of CRM-HMA, the Federal
Highway Administration (FHWA) undertook a 5-year study titled
"Crumb Rubber Modifiers (CRM) in Asphalt Pavements"
in September 1994. The contract was awarded to a team headed
by Oregon State University and includes the National Center for
Asphalt Technology at Auburn University, University of California-Berkeley,
University of Nevada-Reno, and Nichols Consulting Engineers.
The overall objectives of the study are to: Develop
Guidelines for pavement structural and mix design and for pavement
construction, including mix production and field quality control
tests. Establish guidelines for long-term
pavement performance of CRM mixes as well as the ability to recycle
these mixes. While there is also a need to consider
all conceivable variables, there is also a need to focus the
work early on to consider higher payoff issues. This will result
in a better definition, reduction, or elimination of process
applications, materials or other variables which, based on available
data, are not important or significant. This report is one
of the early products of the study. The document summarizes the
CRM-HMA practices in the States of Arizona, California and Florida.
The information was collected through visits with agency and
industry personnel in each of the States. It is intended to provide
information on pavement design, materials, mix design, construction
(including costs and quality control), and pavement performance.
Significant findings include the following: Thickness
design considerations. Much of the use of CRM-HMA is as a
functional overlays. Only California utilizes a layer equivalency
to reduce the thickness of the CRM-HMA. Materials.
The different agencies generally use ambient-ground CRM with
asphalts that will produce an asphalt-rubber binder with a certain
viscosity. Dense-, open-, and gap-graded aggregates have been
used. Mix Design Considerations. Arizona
and Florida have modified the Marshall procedure to obtain their
mix designs. California has modified the Hveem procedure. All
have produced successful mixes. Construction
Process. The addition of CRM in HMA makes use of much of
the equipment used for conventional mixes. However, there is
an additional requirement for a blending/reaction vessel and
some increases in mixing temperatures. Costs.
The increased cost of the CRM-HMA (compared with conventional
HMA) ranges from as little as 15 percent (in Florida) to 70±
percent in Arizona/California, depending on the mix type.
Quality Control. This issue is an important one. The
uniformity of the asphalt rubber is generally checked using viscosity
tests; however, there is a definite need for improved procedures
to evaluate the binder uniformity as well as the mix properties.
Performance. All agencies feel they are obtaining
an added value. However, inadequate data are available to quantify
these benefits.
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