RAP and Superpave: An Excellent Blend
Did you drive on a recycled pavement today? Reclaimed asphalt pavement
(RAP), the term used to describe reprocessed pavement materials containing
asphalt and aggregates, is now routinely used in nearly all 50 States.
The Federal Highway Administration (FHWA) estimates that nearly 30 million
tons of RAP are recycled into hot-mix asphalt (HMA) pavements each year,
saving taxpayers more than $300 million annually by reducing material
and disposal costs. But can RAP be incorporated into pavements built
using the Superpave mix design system? As originally developed by the
Strategic Highway Research Program, the Superpave system did not contain
guidelines for the inclusion of RAP. However, State experiences to date
have indicated that RAP can be used in Superpave mixes both predictably
and reliably, as long as the unique properties of the RAP are known.
The recently completed National Cooperative Highway Research Program
(NCHRP) project 9-12, led by Rebecca McDaniel of the North Central Superpave
Center and Mike Anderson of the Asphalt Institute, confirms that the
Superpave system can easily and effectively accommodate the use of RAP.
"While some States had been using RAP in Superpave, many were
waiting for guidelines to come out. This is really going to open it
up in terms of use," says McDaniel.
The primary objective of NCHRP Project 9-12, "Incorporation of
Reclaimed Asphalt Pavement in the Superpave System," was to answer
several key questions:
- When the aged binder from the RAP is combined with the new binder,
what effect does it have on the resultant binder
grade? To what extent does binder blending occur?
- What effect does the RAP have on mix volumetric properties and performance?
After conducting a comprehensive laboratory testing program using RAP
materials from field projects in Arizona, Connecticut, and Florida,
the researchers concluded that partial binder blending occurs to a significant
extent when using RAP. Low amounts of RAP, typically up to 15 percent,
can be used without performing any binder tests or making changes to
the binder grade. For intermediate amounts of RAP in the 15- to 25-percent
range, the recommended virgin asphalt binder is one grade "softer"
than that used for conventional HMA. In mixes containing more than 25
percent RAP, the RAP binder will have a noticeable effect and must also
be accounted for by using a softer grade of virgin asphalt binder.
To determine the correct grade of virgin asphalt binder for mixes containing
more than 25-percent RAP, the RAP binder must be extracted, recovered,
and tested to develop blending charts. The desired final binder grade
and critical temperatures of the RAP binder are needed to construct
a blending chart. Once the physical properties and critical temperatures
of the RAP binder are known, a materials engineer can take one of two
approaches: (1) The percentage of RAP is specified and the blending
chart is used to determine the appropriate virgin binder grade; or (2)
The virgin binder grade is specified and the blending chart is used
to determine the maximum percentage of RAP that may be used. Step-by-step
procedures for constructing blending charts can be found in NCHRP Report
452, Recommended Use of Reclaimed Asphalt Pavement in the Superpave
Mix Design Method: Technician's Manual, which is available on the
Web at www.trb.org/trb/publications/nchrp/nchrp_rpt_452.pdf.
The researchers also determined that only minor changes need to be
made to the overall Superpave mix design process when using RAP. Although
RAP can be treated like any other aggregate 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.
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. Stricter material stockpile management and more frequent sampling
and testing of the RAP may also be necessary to ensure consistency and
quality.
For more information on the NCHRP project or on using RAP in Superpave
mixes, contact Rebecca McDaniel at the North Central Superpave Center,
765-463-2317, x. 226 (email: rsmcdani@purdue.edu).
The project report, Recommended Use of Reclaimed Asphalt Pavement
in the Superpave Mix Design Method, can be found on the Web at trb.org/trb/publications/nchrp/nchrp_w30-a.pdf.
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| Research has confirmed that Superpave pavements
can easily accommodate the use of RAP. |
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Articles in this issue:
RAP and Superpave: An Excellent Blend
Now is the Time for Work Zone Safety
A High-Performance Steel Scorecard
Professor Training Workshop Showcases DataPave
3.0
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