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SCRAP
TIRES - Material Description
Asphalt
Concrete (Wet Process)
Asphalt-Rubber - Glossary of Terms
Asphalt Concrete (Dry Process)
INTRODUCTION
Scrap tire rubber can be incorporated into asphalt paving mixes using two
different methods, which are referred to as the wet process and the dry process.
In the wet process, crumb rubber acts as an asphalt cement modifier, while in
the dry process, granulated or ground rubber and/or crumb rubber is used as a
portion of the fine aggregate. In both cases, crumb rubber is sometimes referred
to as crumb rubber modifier (CRM) because its use modifies the properties of the
resultant hot mix asphalt concrete product.
The dry process can be used for hot mix asphalt
paving in dense-graded, open-graded, or gap-graded mixtures. It cannot be used
in other asphalt paving applications, such as cold mix and chip seals or surface
treatments. In the dry process, granulated or ground rubber and/or crumb rubber
is used as a substitute for a small portion of the fine aggregate (usually 1 to
3 percent by weight of the total aggregate in the mix). The rubber particles are
blended with the aggregate prior to the addition of the asphalt cement. When
tire rubber is used as a portion of the aggregate in hot mix asphalt concrete,
the resultant product is sometimes referred to as rubber-modified asphalt
concrete (RUMAC).
The dry process used most frequently in the United States was originally
developed in the late 1960's in Sweden and is marketed in this country under the
trade name PlusRide by EnviroTire. The PlusRide technology is a patented
process. In this process, from 1 to 3 percent granulated crumb rubber by weight
of the total mix is added to the paving mix. The granulated rubber consists of
rubber particles ranging in size from 4.2 mm (1/4 in) to 2.0 mm (No 10 sieve).
The target air voids content of the asphalt mix is 2 to 4 percent, which is
usually attained at an asphalt binder content in the 7.5 to 9 percent range.
A generic dry process technology was developed in the late 1980's to early
1990's to produce dense-graded hot mixtures. This concept uses both coarse and
fine crumb rubber to match aggregate gradings and to achieve improved binder
modification. The crumb rubber may need a prereaction or pretreatment with a
catalyst to achieve optimum particle swelling. In this system, rubber content
does not exceed 2 percent by weight of total mixture for surface courses.
Experimental pavement sections have been placed in Florida, New York, Oregon,
and Ontario.
The U.S. Army Corps of Engineers Cold Regions Research Engineering Laboratory (CRREL)
investigated dry process CRM mixtures for disbonding ice on pavements. This
research resulted in a recommendation to place field sections with mixtures
containing crumb rubber particles larger than 4.75 mm (No. 4 sieve), with a top
size of 9.5 mm (3/8 in). The technology is referred to as the chunk rubber
process. Marshall properties, resilient modulus, and ice removal tests have been
performed in the laboratory with crumb rubber concentrations of 3, 6, and 12
percent by weight of aggregate. Laboratory wheel testing indicates that the
higher rubber content mixes can potentially increase the incidence of ice
cracking. The chunk rubber process has not as yet been field evaluated.
PERFORMANCE RECORD
The reported performance of rubber-modified asphalt concrete pavements has
varied widely in different sections of the United States. The following
paragraphs summarize the experiences of selected states with the dry process.
The California Department of Transportation (CalTrans) has constructed four
projects using the PlusRide dry process technology. Some distress in the form of
cracking or flushing in the wheel paths was observed in three of these projects,
but overall, CalTrans has reported that two of the four dry process projects
have out-performed conventional dense-graded asphalt, and a third project has
performed comparably. A fourth project was not properly designed and required an
overlay.
The Minnesota Department of Transportation (MNDOT) has used the dry process in
asphalt paving on a least two different projects, beginning in 1979. The two dry
process projects were both PlusRide installations, using granulated crumb rubber
and a gap-graded aggregate in an attempt to create a self de-icing
pavement.(5)The two PlusRide sections have performed well, but have not shown
benefits to offset the increased cost, and have not demonstrated any significant
de-icing benefits.
In New York, two experimental hot mix overlay projects using granulated rubber
in the dry process were installed during 1989 to compare the construction
characteristics and performance of rubber-modified asphalt concrete with a
conventional top course paving mixture. All overlays were 37.5 mm (1-1/2 in)
thick and placed over existing Portland cement concrete pavements, each with a
leveling course of varying thickness. On both projects, the rubber-modified
mixes consisted of PlusRide with 1, 2, or 3 percent granulated rubber
aggregate.(6) After 3 years, the New York State DOT did not consider that these
two overlay projects were either economical or successful.
One district in Texas has used rubber-modified hot mix asphalt (dry process).
The mix raveled severely and the district was forced to place a chip seal over
the mix within 3 months.
Since 1977, the Washington State DOT (WSDOT) has undertaken a number of
demonstrations with the dry process, using crumb rubber particles up to 6.3 mm
(1/4 in) in size. The performance of the seven PlusRide sections has ranged from
excellent to immediate failure. Construction problems have plagued several of
these installations. WSDOT concluded that, overall, PlusRide did not appear to
provide improved performance.
In Ontario, Canada, eight rubber-modified, dry process asphalt demonstration
projects were evaluated in terms of pavement performance. They generally
exhibited poor short-term performance.
Performance of rubber-modified asphalt using the dry process has been mixed,
with some early failures. Installations in service for several years generally
show little improvement over conventional overlays. Little to no evidence of ice
disbonding has been observed, except in laboratory tests.
MATERIAL PROCESSING REQUIREMENTS
Shredding
The initial step in the production of ground or
granulated scrap tire rubber is shredding. Scrap tire rubber is delivered to
rubber processing plants either as whole tires, cut tires (treads or sidewalls),
or shredded tires, with shredded tires being the preferred alternative. As scrap
the rubber is processed, the particle sizing is reduced, steel belting and fiber
reinforcing are separated and removed from the tire, and further size reduction
is then accomplished.
Grinding
Rubber used in the dry process is ground rubber that is generally produced in a
granulator process. This process further reduces shredded tire rubber and
generates cubical, uniformly shaped particles ranging in size from 9.5 mm (3/8
in) down to a 0.42 mm (No. 40 sieve). However, the dry process can also use
coarse crumb rubber from the crackermill process, which results in irregularly
shaped particles ranging in size from 4.75 mm (No. 4 sieve) to 0.92 mm (No. 40
sieve).
ENGINEERING PROPERTIES
Some of the engineering properties of granulated or ground rubber that are of
particular interest when used in asphalt concrete (dry process) include its
gradation, particle shape, and reaction time.
Gradation: RUMAC paving mixes incorporate granulated
or coarse crumb rubber particles that are most often processed to meet the
gradation requirements shown in Table 16-1.
Gradation Requirements for RUMAC Mixes
Sieve Size Percent Passing by Weight
6.3 mm (1/4 in) 100
4.75 mm (No. 4) 76 - 100
2.0 mm (No. 10) 28 - 42
0.85 mm (No. 20) 16 - 24
However, a chunk-rubber asphalt process developed for disbonding ice on
pavements contains particles larger than a No. 4 sieve with a dominant size of
9.5 mm (3/8 in).
Particle Shape: Ground or granulated rubber particles produced from granulators,
hammermill, or fine grinding machines have a cubical shape and a relatively low
surface area. Coarse crumb rubber particles produced from the crackermill
process have an irregularly torn shape and a relatively high surface area.
A cubical particle shape with a relatively low surface area is characteristic of
conventional aggregate materials and is desirable for rubber particles that will
function as a gap-graded aggregate in the dry process. Particles from the
crackermill process that have an irregular shape with a relatively high surface
area are more likely to react with asphalt cement at elevated temperatures and
are better suited for use in the wet process. By limiting the time that the
asphalt cement and crumb rubber particles are maintained at reaction
temperatures and specifying a coarse granulated product with a relatively low
surface area, the rubber particles can retain the physical shape and rigidity
needed for use in the dry process. The smooth, sheared surfaces of ground or
granulated rubber particles are also less reactive than the surfaces of the
particles produced from the crackermill process.
Reaction Time: In the Plus Ride process, there is a relatively short reaction
time when the rubber particles and aggregate are mixed with the asphalt cement,
so the rubber particles do not have much opportunity to blend with the binder.
There is a generic dry process that was developed in New York State, which uses
coarse and fine crumb rubber prereacted with a catalyst to achieve optimum
particle swelling, and is added at a maximum of 2 percent by total mixture
weight for surface courses. In this process, the rubber particles may be able
to react to a somewhat greater extent with the asphalt binder.
Some of the properties of RUMAC paving mixtures that are of interest include
stability, resilient modulus, permanent deformation, and reflective cracking.
Stability: Paving mixtures produced by the dry process generally have reduced
stability values, regardless of whether the Marshall or Hveem mix design
procedures are used.
Resilient Modulus: Mixes containing granulated or crumb rubber typically have
lower resilient modulus values than conventional hot mix asphalt. RUMAC paving
mixes have been found to have resilient modulus values that are 10 to 20 percent
higher than those of asphalt-rubber (wet process) paving mixes.
Permanent Deformation: Previous studies of granulated rubber paving mixtures
indicate that resistance to permanent deformation of such mixes is reduced
compared with that of conventional paving mixes. However, fatigue life is
generally improved when crumb rubber is added by this process.
Reflective Cracking: Addition of rubber aggregate can influence pavement
performance in terms of reflective cracking. To achieve the benefits of delayed
reflective cracking, a minimum rubber content must be added to the paving mix.
This minimum rubber content is probably between 1 and 2 percent by weight of
aggregate. The reaction between the rubber and the asphalt cement does not play
a significant role in the enhancement of pavement performance in dry process
mixes.
DESIGN CONSIDERATIONS
Mix Design
Conventional Marshall and Hveem mix design methods
have been used successfully for designing dense-graded mixtures with granulated
rubber, but mixtures produced using the dry process typically do not follow the
normal mix design procedures. Where stability is the primary design factor in
most conventional mixes, the primary dry process design property is the
percentage of air voids. The target air voids are between 2 and 4 percent.
During the laboratory mixing process, the granulated rubber is dry mixed with
the aggregate before adding the asphalt cement. The asphalt concrete mixture is
cooled for 1 hour after mixing. After compaction, the sample is cooled to room
temperature. The air void content is determined after extrusion.
Dry process paving mixes should be designed volumetrically to compensate for the
lower specific gravity of the crumb rubber particles. Binder contents in dry
process mixes are typically 10 to 20 percent higher than those of conventional
mixes. Although the air voids content is the criterion for mix design, lower
stability values and higher flow values can be expected, compared with
conventional hot mix asphalt paving mixtures.
Structural Design
The method used for the thickness design of rubber modified asphalt pavements,
which incorporate between 1 and 3 percent by weight of granulated crumb rubber
modifier (CRM) as fine aggregate, is essentially the same as that used for the
thickness design of conventional hot mix asphalt pavements. No adjustments
are normally recommended in the design thickness of rubber modified asphalt
pavements compared with that of conventional hot mix asphalt pavements.
When designing asphalt pavements using the structural number (SN), the resilient
modulus at 20° C (68° F) is the material property that is considered. Resilient
modulus values for 18 percent coarse (2.0 mm (No. 10 sieve)) and fine (0.2 mm
(No. 80 sieve)) CRM by weight of asphalt binder in dense-graded mixtures were
found to be lower than dense-graded control mixtures at three temperatures
ranging from 5° C (41° F) to 40° C (104° F). Since the structural layer
coefficient of a pavement is directly proportional to resilient modulus, this
would suggest that dry process CRM mixtures should have a lower structural layer
coefficient and require some increase in thickness.
CONSTRUCTION PROCEDURES
Material Handling and Storage
Both batch and drum-dryer plants have been used to
produce RUMAC. The reclaimed granulated rubber is usually packed and stored in
110 kg (50 lb) plastic bags. Additional manual labor and conveying equipment,
such as work platforms, are needed in order to introduce the granulated rubber
into the paving mix, regardless of the type of mixing plant used. A batch plant
has a quality control advantage over a drum-dryer plant because the number of preweighed bags of granulated rubber can be easily counted prior to their
addition into each batch. The bags may be opened and the granulated rubber
placed on a conveyor, or the bags may be put into the pugmill or cold feed bin
if low melting point plastic bags are used.
Control of the feeding of granulated rubber is necessary because the correct
rubber content is critical to the performance of the paving mix when using the
dry process. Such control is more difficult to maintain in a drum-dryer system,
due to the nature of the feed operation. Some drum-dryer plants have used
recycled asphalt concrete hoppers to feed the granulated rubber, although a
number of agencies recommend that the rubber be introduced into the mix through
a center feed system. The process can be automated by the addition of a conveyor
and hopper, plus scales to accurately proportion the granulated rubber.
Mixing
For both batch and drum-dryer plants the addition of rubber normally requires
that the mixing time and temperature be increased. Batch plants require a dry
mix cycle to ensure that the heated aggregate is mixed with the crumb rubber
before the asphalt cement application. Mixtures should be produced at 149° C to
177° C (300° F to 350° F).
Placing and Compacting
Laydown temperature should be at least 121° C (250° F). A finishing roller must
continue to compact the mixture until it cools below 60° C (140° F). Otherwise,
the continuing reaction between the asphalt and the crumb rubber at elevated
temperatures will cause the mixture to swell. Continued compaction until the
mixture cools below 60° C (140° F) serves to contain the expansive pressure of
the compressed rubber.
Quality Control
Parameters that must be monitored during mixing for dry process mixes include
rubber gradation, rubber percent of total mixture weight, rubber prereaction or
pretreatment, and time of plant mixing. Since dry process binder systems are
partially reacted with the rubber, it is not possible to directly determine the
properties of the binders.
It is recommended that compacted mixes be sampled according to AASHTO T168,
and tested for specific gravity in accordance with ASTM D2726 and in-place
density in accordance with ASTM D2950.
UNRESOLVED ISSUES
There are several unresolved issues relative to the use of rubber as fine
aggregate in asphalt concrete using the dry process. The overwhelming majority
of projects and data concerning crumb rubber use in asphalt paving are from
installations using the wet process. As a result, there is a lack of field data
to evaluate performance.
There have been six projects in the United States where asphalt pavements with
CRM have been recycled. Roughly half of these projects were wet process and the
other half were dry process. Apparently, there are no physical problems with
recycling reclaimed asphalt pavement containing CRM as a portion of the
aggregate in a new asphalt paving mix; however, additional field trials are
needed.
Although only a limited amount of air emissions data from asphalt plants
producing hot mix containing CRM are currently available, there is no evidence
thus far that the use of an asphalt paving mix containing recycled crumb rubber
exhibits any increased environmental impact when compared with that of emissions
from the production of a conventional asphalt pavement.(16) Air emission data
from a project in New Jersey in 1992 where dry process CRM was recycled as 20
percent of new aggregate in a drum mix plant showed that current air quality
standards were not exceeded during the recycling.
Nevertheless, there is a
need for additional studies on recyclability and worker health and safety issues
for CRM asphalt paving mixes. Some of this work is presently underway and, as
data become available, they should be incorporated into what is already known
concerning these two aspects of using CRM in asphalt pavements.
Because of fluctuations in the performance of CRM asphalt mixes in different
locations and/or climatic conditions, there is a need for more carefully
controlled experimental field sections in different climatic regions throughout
the United States in order to obtain more reliable performance data. Binder and
mixture properties in these different regions need to be more accurately
determined and documented. Performance records of these test sections may need
to be monitored over a long period of time, at least 5 years and possibly as
long as 30 years.
Additional research is needed to define the properties of binders produced by
the dry process. Desirable properties for dry process hot mix asphalt mixtures
need to be better defined.
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