Asphalt-Rubber Systems In Road
by Joe Cano, P.E., Technical
Director, International Surfacing, Inc., Chandler, AZ
Joe Cano is a graduate of Arizona
State University with a Bachelor of Science degree in Civil Engineering.
He retired from the City of Phoenix in January, 1992 after 32
years of service. Prior to joining International Surfacing Systems
in March of 1992, Mr. Cano served as the Engineering Supervisor
of the Materials Laboratory for Phoenix Street Transportation
Department, responsible for the testing and quality assurance
of all construction materials used by the city. He developed
the gap-graded Asphalt-Rubber hot mix used by the city to resurface
badly cracked streets. Mr. Cano has authored several papers on
the use of Asphalt-Rubber including the Twenty-Year Study
of AsphaltRubber Pavements in the City of Phoenix, Arizona. He
is a member of the RPA Board of Directors and its Technical Advisory
Asphalt-Rubber is a blend of asphalt cement, reclaimed
tire rubber, and certain additives in which the rubber component
is at least 15%, by weight, of the total blend. It is mixed at
a temperature of 250'F to react the rubber and cause swelling
of the rubber particles.
In the mid 1960's, the City of Phoenix pioneered the use
of Asphalt-Rubber through the efforts of Charles McDonald, then
Materials Engineer with the City of Phoenix. Asphalt-Rubber,
for road rehabilitation, has been in use by cities and counties
in Arizona and California since 1969. It is currently in use
in many states across the country.
Asphalt-Rubber is utilized in two application processes:
(1) Chip Seals - an Asphalt-Rubber membrane spray is applied
over the road surface and covered with one-size aggregate.
(2) Asphalt-Rubber Hot Mixes - Asphalt-Rubber material
is substituted for conventional asphalt binder in the hot mix.
Asphalt-Rubber has superior engineering properties over
conventional asphalt. It is more flexible and more resistant
to cracking caused by wheel loading pressure, and it is very
effective in controlling reflective cracking in overlays.
Asphalt-Rubber paving material is more resistant to rutting
or pavement deformation because its viscosity is much less temperature
dependent compared to that of asphalt cement. Asphalt-Rubber
is cost effective as it will last longer than conventional asphalt.
It has been demonstrated that the thickness of AsphaltRubber
concrete pavement can be reduced to half the required overlay
thickness of conventional asphalt concrete pavements.
Surface treatment. Asphalt-Rubber
is spray applied to pavement surfaces at 0.5 to 0.7 gallons per
square yard (gpsy) and then covered with clean, nominal 3/8"
single-sized chips. The A-R application rate is adjusted for
the existing road surface characteristics, and the aggregate
is sized according to the membrane thickness. A tack coat of
emulsified asphalt, diluted 1:1 with water, may be used on oxidized
existing pavement surface. A 0. 1 to 0. 15 gpsy fog seal application
is recommended to maximize chip retention The fog seal should
also consist of emulsified asphalt, diluted 1:1 with water.
Provides a lasting waterproof, skid
resistant and durable surface which resists oxidation and cracking,
and flexes to conform with movements of the underlying pavement
Asphalt-Rubber SAM improves and extends
the serviceability of asphalt concrete pavements which are experiencing
distresses, such as alligator and block cracking. The pavement
life is extended by (a) decreasing the amount of surface water
intrusion into the base and subgrade by providing a waterproof
membrane to obtain maximum stability of the structure, (b) reducing
the oxidation of the existing surface, (c) binding the existing
surface together, and (d) reducing the spalling around reflective
4. Appropriate Uses:
There are a variety of situations
where Asphalt-Rubber SAMs provide highly effective alternates
to existing conventional strategies, methods, and/or materials.
The following list identifies some of the appropriate uses.
For rehabilitation / maintenance:
· over a pavement
with fatigue cracking.
· over a pavement with oxidative or block cracking.
· over pavements with raveling.
· surface treatment for low volume roads.
· to extend the life of severely
distressed pavements (basket cases) that require reconstruction
for which funds are not yet available.
· to improve fractional characteristics
· to protect new pavements
from aging and the effects of moisture.
· waterproof membrane over expansive soils
· waterproof membrane for pond liners, sewage sludge
· to prevent reflective cracking from stabilized
It should be noted that SAMs can not improve ride quality because
they are very thin layers which follow the profile of the existing
pavement. They may improve structural capacity by allowing subgrade
moisture content to stabilize and retard entry of additional
Stress Absorbing Membrane Interlayer (SAMI)
A SAM followed by the placement of one or more courses of asphalt
concrete or Asphalt-Rubber concrete (ARC).
The use of Asphalt-Rubber SAMIs extends
the serviceable life of overlays by significantly retarding the
rate of reflective crack formation. The SAMI also waterproofs
the underlying pavement and significantly retards age hardening
of underlying asphalt concrete.
Many pavements that are in need of
rehabilitation because of extensive cracking also require improvement
of rideability and/or structure. A SAMI placed as an interlayer
is more effective than as a surface course in reducing reflective
cracking. It also provides significant reduction in tensile stresses
transmitted to layers above.
4. Appropriate Uses:
Appropriate uses for SAMIs include
most of those previously listed for SAMs. SAMIs can be effectively
used in cases where it is necessary to improve ride quality or
structural capacity by constructing an overlay. An additional
function for which SAMIs are appropriate is to control reflective
cracking over cement-treated and lime-stabilized bases.
Because the low-modules SAMI significantly
improves resistance to reflective cracking, asphalt concrete
overlay thickness can be generally reduced. Use of a SAMI also
reduces the amount of surface preparation, particularly crack
sealing, required prior to overlaying. These factors reduce the
cost of the overlay.
A standard asphalt concrete opengraded
friction course (OGFC), except that the binder used is a reacted
AsphaltRubber material with substantially higher binder content.
To provide a more durable friction
course which has increased stability and greater resistance to
reflective cracking, oxidation, stripping, and chain wear. To
significantly reduce pavement noise levels. To improve safety
by reducing spray from the passage of trucks and improving visibility
as well as reducing hydroplaning by preventing accumulation of
surface water. To improve rideability and skid resistance.
The use of Asphalt-Rubber as a binder
for asphalt concrete open-graded friction courses solves many
of the durability problems associated with opengraded mixtures.
Use of A-R in OGFC permits higher binder contents, thus thicker
binder films, due to the high viscosity of the A-R, without excessive
This results in increased durability
due to the greater oxidation resistance of the Asphalt-Rubber
binder and thicker binder films. The antioxidants and carbon
black in the tire rubber retard aging of the A-R material.
Asphalt-Rubber is more flexible at
lower temperatures and stiffer at higher temperatures than the
base asphalt cement. These characteristics, combined with greater
film thickness, result in a mix which is highly resistant to
reflective and thermal cracking. The combination of stiffer binder,
at high temperatures, with the aggregate to aggregate contact
of OGFC mixes results in a mix which is highly resistant to permanent
4. Appropriate Uses:
Open-graded pavements are typically free -draining. Therefore,
if the pro posed pavement is to be bordered by curb-and-gutter,
proper grade alignment for adequate drainage of surface water
from the open-graded layer can be a critical factor in design.
Open-graded ARC overlays are highly
resistant to reflective and thermal cracking. In moderate climate
areas, thin (1 ") open-graded overlays placed directly over
short-jointed (<20' spacing) plain PCC pavements have proven
very ef. fective in resisting reflective cracking The high binder
content of these mixes also provides increased flexibility and
resilience to resist permanent deforma. tion, as well as reducing
problems with aging and reveling. When an open. graded friction
course is to be placed over pavements with moisture susceptibility,
a SAMI should be considered in the design.
In areas with significant wet weather
accident rates, open-graded pavements may reduce accident occurrence.
The porous nature of such mixes prevents accumulation of water
on the surface, reducing spray, nighttime glare, and hydroplaning.
Thus, both visibility and frictional characteristics are improved.
Where traffic noise is a problem.
thin lifts of open-graded ARC mixes car economically provide
significant noise abatement. Documented noise reductions of 50
to 70% can be readily achieved.
ARC gap-graded aggregate gradings
are much cleaner, generally having a target of 20% passing the
#8 and a maximum 7% passing the #200 sieve. Aggregate grading
limits for gap gradations fall in between the respective limits
for dense and open gradations. When compacted, the crushed coarse
aggregate forms a structural skeleton (matrix) with minimum voids
in the mineral aggregate (VMA) of up to 19%.
To provide a more durable and flexible
asphalt concrete pavement which has increased resistance to reflective
cracking, rutting, and oxidation. To reduce tire noise and provide
excellent structure support.
The use of Asphalt-Rubber binder
in gap-graded mixes results in a flexible material which can
be used as an overlay to extend pavement life, to provide structure
support, improve ride quality and skid resistance, and decrease
traffic noise level. Gap-graded mixes demonstrate an increased
ability to accommodate deflections and provide superior structure
support, thereby decreasing required overlay thickness when compared
to conventional mixes. Higher film thickness and binder content
reduces problems with raveling and chain wear as well. Air void
contents are generally similar to those of standard densegraded
asphalt concrete. These mixes are relatively impermeable, as
interconnection of voids is limited. ARC gapgraded mixes are
very similar, in fact virtually identical, to the "split-
mastic" mix being used in Europe.
4. Appropriate Uses:
ARC gap-graded mixes may be used
as overlays to extend pavement life or as the surface course
of new pavement construction. As overlays, ARC pavements are
highly resistant to reflective cracking and rutting. They reduce
tire noise and improve ride quality and frictional characteristics.
These mixes can be used to decrease required thickness of pavements.
The mix has high flow (deformation) values as a result of the
large amount of Asphalt-Rubber binder. This characteristic makes
the mix very flexible and enables it to prevent reflective cracking.
Three Layer System
Construction of an asphalt concrete
or ARC leveling course, followed by an application of an Asphalt-Rubber
SAMI, which is then covered with a gap or opengraded ARC or asphalt
To provide a means of restoring rise
quality and smoothness to deteriorated concrete pavements as
an alternative to grinding and grooving, thick overlay or reconstruction.
To provide increased structural capacity for distressed PCC or
asphalt concrete pavements exhibiting cracking, faulting and/or
rutting. Also to extend pavement life by providing a durable
and long-lived surface without reconstruction.
Asphalt-Rubber three-layer systems
have provided an effective means of rehabilitating deteriorated
concrete pavements at up to half the cost of grinding and grooving.
The stress absorbing characteristics of the sandwiched AsphaltRubber
layer reduce reflection of cracks through the system. When an
ARC surface course is used in conjunction with the SAMI, resistance
to reflective cracking is further increased along with a significant
reduction in tire noise. Noise reduction is an important added
benefit, particularly in urban areas and residential areas.
Two Layer System
4. Appropriate Uses:
A number of combinations of types
of asphalt and/or Asphalt-Rubber concrete pavements (such as
dense, opengraded, and gap-graded) can be included in three layer
systems. The table below presents the primary combinations recommended
for design consideration. For the surface courses, use of Asphalt-Rubber
concrete is strongly recommended to optimize the performance
and serviceable life of the three layer system due to enhanced
resistance to cracking.
Selection of the gradation of the
surface mix depends heavily on anticipated traffic and site conditions.
Appropriate uses of the respective types of ARC have already
been described. All properly designed ARC mixes are highly resistant
to permanent deformation (rutting) and reduce noise. Open- graded
ARC mixes can provide superior performance and safety in many
cases, but adequate drainage is necessary. Gap-graded mixes are
relatively impermeable and can provide higher stability without
sacrificing resistance to reflective and thermal cracking.
Application of an Asphalt-Rubber
SAMI, which is then overlaid with an AR( surface course.
The SAMI provides a waterproof seal
and minimizes and retards reflective cracking. The overlay restores
ride quality , reduces tire noise, extends pavement life, and
provides a durable pavemenat surface. The overlay may be designed
to increase structural capacity as well.
Prolongs the serviceable life of
both the existing pavement structure and the new overlay. Provides
all of the previously described benefits of a SAMI plus those
of ARC overlays.
4. Appropriate Uses:
Open-graded ARC or gap-graded ARC
are recommended to overlay to improve ride quality and where
resistance to reflective cracking is considered critical to performance
and serviceable life. They are effective over both asphalt and
The contents of this paper do
not necessarily reflect the official views or policies of either
the Federal Highway Administration or the State of California.
They reflect the views of the author, who is responsible for
the facts and accuracy of the data presented herein. Also, neither
the State of California nor the United States Government endorse
products or manufacturers. Trade and manufacturers names are
presented herein because they ire considered essential to the
objective of this paper.