Although some form of pavement recycling was practiced as
early as 1915, the first sustained efforts to recover and reuse old asphalt
paving materials were conducted in the mid 1970s. With financial support of the
Federal Highway Administration (FHWA) and technical assistance from trade
associations such as the National Asphalt Pavement Association and the Asphalt
Institute, more than 40 states placed demonstration reclaimed asphalt pavement
(RAP projects by 1982. RAP, is now routinely used in nearly all 50 states.
Though reliable figures for the generation of RAP are not
readily available from all state highway agencies or local jurisdictions, the
FHWA estimates that nearly 30 million tons are recycled into hot-mix asphalt
(HMA) pavements each year. Saving taxpayers more than $300 million annually by
reducing material and disposal costs, RAP has become an increasingly valuable
asset.
This will work
Equally important is the well-documented fact that pavements
incorporating RAP can provide the same safe, smooth, durable ride as
conventional HMA. Agency and industry personnel, as well as researchers, seem
to agree that as long as RAP is treated as an engineered construction material,
it will work in Superpave. A product of the Strategic Highway Research Program,
Superpave (SUperior PERforming Asphalt PAVEments) is a performance-based system
for designing pavements to accommodate project-specific conditions including
climate, materials and traffic. As originally configured, however, Superpave
did not provide guidelines for the inclusion of RAP. Still, experience to date
indicates that RAP can be used in Superpave mixes predictably and reliably, as
long as the unique properties of the RAP are known. Recently completed
research, conducted under the auspices of the National Cooperative Highway
Research Program (NCHRP) and led by Rebecca McDaniel of the North Central
Superpave Center and Mike Anderson of the Asphalt Institute, confirms that
Superpave can easily and effectively accommodate the use of RAP.
As highway agencies transition from viscosity- or
penetration-graded asphalt cements to Superpave Performance Grade (PG) binders,
a key question is, "When the aged binder from RAP is combined with the
new binder, what effect does it have on the resultant binder grade?"
Similarly, as highway agencies move from traditional
Marshall and Hveem methods of mix design they ask, "What effect does the
RAP have on mix volumetric properties and performance?"
Intuitively, and correctly, they reasoned that at low RAP
percentages the changes would be negligible, but at higher percentages the
effect would be significant. With RAP routinely used in HMA at substitution
rates of 10- 50%, these questions were addressed in NCHRP Project 9-12,
"Incorporation of Reclaimed Asphalt Pavement in the Superpave
System." Briefly, the research objectives were two-fold: to determine the
extent of binder blending (Does RAP act like a black rock, in which no blending
of the old and new binder occurs, or does some blending occur between the old,
hardened RAP binder and the added, virgin binder?) and the effect of RAP on the
blended binder and resulting mix properties.
Being partial
Based on a comprehensive laboratory testing program using
RAP materials from field projects in Arizona, Connecticut and Florida, the
researchers concluded that RAP does not act like a black rock, nor does total
blending of the RAP and virgin binders ever occur.
Instead, partial blending occurs to a significant extent.
This partial blending means that at high RAP contents the hardened RAP binder
must be accounted for in the selection of the virgin binder.
The research findings largely confirm current practice. Low
amounts of RAP, typically 10-20%, can be used without testing the recovered
binder. With higher RAP contents, Superpave binder tests can be used to
determine how much RAP may be added or which virgin PG binder is needed. The recommended
percentage of RAP within each tier is dependent upon the grade of the extracted
RAP binder. The softer the extracted RAP binder, the greater the percentage of
RAP allowed.
For low RAP contents of 10-20%, no testing is required
because there is not enough of the old, hardened RAP binder present to change
the combined binder properties.
For intermediate ranges of RAP, the recommended virgin
binder is simply one grade softer than that used for conventional HMA.
At higher RAP contents, the RAP binder will have a
noticeable effect and must be accounted for by using a softer grade of virgin
binder. The RAP binder must be extracted, recovered and tested to develop
blending charts. To construct a blending chart, the desired final binder grade
and critical temperatures of the recovered RAP binder are needed. Once the
physical properties and critical temperatures of the recovered RAP binder are
known, the materials engineer has two approaches to consider. In the first
approach, the percentage of RAP is specified and the blending chart is used to
determine the appropriate virgin binder grade. In the second approach, the
virgin binder grade is specified and the blending chart is used to determine
the maximum percentage of RAP that may be used. The step-by-step procedures for
constructing blending charts are included in NCHRP Report 452.
The overall Superpave mix design process with RAP is
virtually identical to that described in AASHTO MP2 with only minor
differences.
Although RAP is treated like any other stockpile for
blending and weighing, it must be heated gently to avoid changing the RAP
binder properties. Also, the RAP aggregate specific gravity must be estimated.
The weight of the binder in the RAP must be accounted for when batching
aggregates, and the total asphalt content reduced to compensate for the RAP
binder. The virgin binder grade may need to be changed depending upon the RAP
percentage and binder grade and the desired blend.
Though not Superpave-specific, other factors to consider
when using RAP are those related to production and quality control testing.
Higher plant temperatures are necessary if the ambient temperature is low or
the moisture content of the materials is high. Greater energy consumption may
affect plant production. Stricter stockpile management and more frequent
sampling and testing of the RAP may be necessary to ensure consistency and
quality.
Findings of the recently completed NCHRP 9-12 project
confirm that Superpave can effectively accommodate the use of RAP.
Specifically, low amounts of RAP, typically 10-20%, can be used without testing
the recovered binder. With higher RAP contents, Superpave binder tests can be
used to determine how much RAP may be added or which virgin PG binder is needed.
