Crumb
Rubber Modifier in Asphalt Pavement
Chapter 4
Chapter 4
Construction
Process/Costs
Although the construction procedures used by the various agencies are similar, there are distinct differences. This chapter discusses the procedures used in terms of handling the CRM, the construction process, including plant operations, placement, and compaction. The costs for the asphalt-rubber binders and asphalt-rubber-modified HMA mixes are summarized.
Shipping,
Storage, and Handling of CRM Materials
The CRM is produced using one of the following processes or combination
of processes: ambient grinding, ambient granulating, cryogenic
grinding, or wet grinding. The feed stock may be buffings, whole
truck or automobile tires and, in one case, reject tennis balls.
The key for the contractor or agency purchasing the material
is that it meets the specifications, be uniform, be free of metal,
contain very little fiber, and be relatively dry. Uniformity
is required because if the gradation or chemical makeup of the
rubber changes, it may change the properties of the resultant
asphalt-rubber binder, such as viscosity, softening point, reaction
time, etc. The material must be free of metal particles because
they can, and have, damaged the contractor's pumping equipment.
An excessive amount of fibers may cause nozzles to clog. The
material is shipped to the asphalt-rubber blending operations
in either 453-kg to 907-kg (997-lb to 1,995-lb) gaylord boxes,
supersacks (approximately 1 metric ton [2,200 lbs]), bulk tanker
trucks (similar to cement or lime bulk trucks), or in bags weighing
11 kg to 27 kg (24 lb to 59 lb). Figure 4 shows the supersack
storage area on a project in California. After shipment to the
project site, the rubber must be kept dry because moisture in
the rubber can cause foaming of the asphalt cement. Once the
material is at the project site, it can be loaded directly into
a crumb rubber hopper on the blending equipment or it can be
blown from the bulk tanker truck into a silo from which it can
be augured into the asphalt-rubber blending unit. There have
been reported problems with the material clumping in the large
containers such as the gaylord boxes or super-sacks. Some rubber
suppliers (generally at the request of the asphalt-rubber blender)
have added up to 2 to 4 percent calcium carbonate or talc to
the CRM to prevent the CRM from sticking together and thus to
improve its free flow characteristics.
Construction Process: Blending
The
asphalt cement and the CRM are blended into a homogeneous asphalt-rubber
system, which is reacted at elevated temperatures. Since the
expiration of the patents on the asphalt-rubber process, there
have been a number of different processes developed for blending
the asphalt cement and the CRM. The time required to disperse,
blend and react, or melt the CRM into the asphalt cement is dependent
on a number of factors in the chemistry of the asphalt cement
and CRM, as well as the particle size and texture of the rubber
and the temperature of the blended material. The finer the material,
the quicker it will "react." Basically, for a given
weight of CRM, the reaction time is directly proportional to
the diameter squared of the CRM particles. Also, the reaction
time is inversely proportional to the temperature of the material.
The reaction time will double with every 10°C (18°F)
decrease in asphalt-cement temperature.(12) Adding CRM to the
asphalt drops the temperature of the asphalt cement due to the
ambient temperature of the CRM. For example, the addition of
20 percent CRM material to an asphalt cement at about 204°C
(400°F) will cause the combined temperature to drop to about
177°C (350°F).
The following paragraphs describe a multitude of blending systems
that are now being used. Systems in use that were in the early
stages of development or systems for which information was not
available to the team are not discussed. Any system used for
blending asphalt rubber should be evaluated for its ability to
produce a uniform, fully "reacted" product.
Arizona
The
CRM-HMA pavements built in Arizona require 17 to 23 percent of
a 600 -181-m (30 mesh) CRM, and the specifications call for 30
to 60 minutes' reaction time after the rubber is blended with
the asphalt cement. Therefore, the equipment consists of a mixing
chamber and a reaction vessel to hold the material.
FNF Construction in Tempe uses a CRM blending system manufactured
by CE Enterprises in Albuquerque, New Mexico. The system consists
of a portable asphalt-rubber mixing system and an agitation tank
that is mounted on a trailer. It uses a mass flow asphalt-cement
meter and a crumb rubber hopper that is equipped with a load
cell. The load cell provides feedback to the computer to control
the flow rate for the crumb rubber. The system is computer controlled
to provide precise batch ratios. The computer also provides constant
monitoring. The mixing tank is 1,135 liters (300 gallons) and
is equipped with a twin-bladed shaft driven by a 3-hp vertical
mixer. One mixing blade is positioned at the bottom, and the
other blade is at the middle of the unit. The asphalt cement
is introduced at 204°C (400°F). During the blending process
the temperature of the blended material will drop to approximately
177°C (350°F). After the material is blended it is fed
into a double-compartment, hot-oil-heated tank. The tank has
a 56,781-liter (15,000-gallon) capacity, and each compartment
is 28,390 liters (7,500 gallons). A turbine mixer is located
in each compartment to keep the crumb rubber in suspension. Each
tank compartment is equipped with independent heat coils. The
asphalt rubber blend is held in the agitation tank for 45 minutes
to an hour prior to use. This system can produce approximately
25 metric tons (28 tons) of asphalt-rubber binder per hour, or
sufficient binder to allow a contractor to produce approximately
357 metric tons (400 tons) of rubber-modified HMA per hour.
International Surfacing Incorporated (ISI) in Chandler uses a
blending system that they developed based on the original McDonald
process. (See figure 5 on page 4-4 for a schematic of the system.)
With the ISI system, the asphalt cement and CRM are combined
and mixed together in a blender unit and pumped into an agitated
storage/reaction tank, where it is reacted for 30 to 60 minutes.
It is a batch process. The amount of CRM is determined by weight
for each batch. The temperature of the asphalt cement at the
time the CRM is added should be between 190°C (375°F)
and 232°C (450°F). This allows for the temperature drop
caused by adding the ambient temperature rubber. The asphalt-rubber
blend is held at 162°C (325°F) to 190°C (375°F)
during the reaction period. The asphalt rubber may be allowed
to cool to between 149°C (300°F) and 176°C (350°F).
After the material has been reacted for a period of 30 to 60
minutes, the asphalt rubber can be metered into the HMA mixing
facility or pumped into an agitated holding tank. The material
is blended in 15,142-liter (4,000-gallon) batches. This system
can produce approximately 15.2 to 31.2 metric tons (17 to 35
tons) of asphalt-rubber per hour, or sufficient binder to allow
a contractor to produce approximately 226 to 454 metric tons
(250 to 500 tons) of rubber-modified HMA per hour.
California
The
asphalt-rubber HMA pavements built in California are similar
to those built in Arizona. They use about 17 to 22 percent 600
181-m (30 mesh) CRM and require a reaction time of 30 to 60 minutes.
Thus, there is a cross utilization of equipment and contractors
between the two States.
Manhole Adjusting Contractors, Inc., of Monterey Park has developed
a rubber blending system where the asphalt-CRM is generally blended
at the HMA plant. This process is similar to the original Arizona
Refining Co. process. They use a combination of scrap tires and
natural rubber (from ground tennis balls), ground rubber buffings,
and 1 - to 6-percent asphalt modifier. The modifier is high resin,
high flash point, aromatic hydrocarbon or extender oil. The rubber
(1 part natural rubber and 3 parts rubber buffings) is fed into
a mixing chamber where it is mixed at 90 gallons per minute.
The mixing process is continuous. The asphalt cement and modifier
are preblended and are between 190°C and 232°C (375°F
and 450°F) when the CRM is added. The CRM is added using
27-kg (60-lb) bags. The asphalt-rubber mix is fed into a reaction
trailer, where it is held a minimum of 30 minutes prior to use.
After reaction the asphalt-rubber binder is fed into an agitated
storage tank, where it is held until used. (See figure 6 for
a photograph of the blending system.) The system produces sufficient
asphalt-rubber binder to produce approximately 2,267 metric tons
(2,500 tons) of HMA per day.
Granite Construction, Inc., of Palm Desert operates the same
system that FNF Construction in Phoenix does. It is also built
by CEI Industries, Albuquerque, New Mexico.
Florida
Florida
specifications call for either a 180 181-m or 425 181-m (80 mesh
or 40 mesh) CRM at 5 and 12 percent concentration rates. With
the smaller material and lower concentrations the "reaction"
time can be shorter and is generally about 15 minutes. Thus,
much of the equipment consists of a wetting chamber and a mixing
chamber from which the material is pumped into a holding tank
prior to being used in the HMA mixing facility. Anderson-Columbia
Construction Co. in Lake City has established a terminal-type
operation at their HMA plant site. It is shown schematically
in figure 7. The CRM is delivered to the plant in bulk tanker
trucks and then blown into one of two 71-metric-ton (80-ton)
storage silos. The asphalt cement is metered into a 45;425-liter
(12,000-gallon) blending tank equipped with two propellers. The
ground tire rubber (GTR) is then weighed into the tank, where
it is blended for a minimum of 15 minutes at about 1710C (3400F).
The blended asphalt rubber is pumped into 22,712-liter (6,000-gallon)
transport trucks for delivery to an HMA mixing plant. These trucks
are equipped with heating systems and high viscosity pumps for
circulation.
HEATEC (a Division
of Astec Industries) has built a trailer-mounted system. It is
used by Martin Paving in Daytona Beach and by Bitcom in Coral
Springs. The CRM is loaded into a crumb rubber hopper. (See figure
8 for a schematic.) It can be loaded from supersacks or from
an auger. It is then augured into a 1,892-liter (500-gallon)
prewet mixing tank. From there, the wetted material is pumped
into another 1,892-liter (500-gallon) mixing tank for further
blending. Heatec also manufactures a unit with one 3,028-liter
(800-gallon) mixing tank rather than two 1,892-liter (500-gallon)
tanks. From these mixing tanks, it is then pumped into an 11,356-liter
(3,000-gallon) mixing tank where it is held until it is used
The tanks are vertical with two mixing paddles, one near the
bottom and one near the middle of the tanks. The asphalt cement
is heated to 204°C to 232°C (400°F to 450°F)
prior to mixing with the CRM. Care must be taken to make sure
the final temperature of the blend (asphalt cement plus crumb
rubber) is not exceeded.
Blacklidge Emulsions has three different systems, one in Mobile,
Alabama, one in Gulfport, Mississippi, and one in Tampa, Florida.
The system in Tampa consists of two 26,497-liter (7,000-gallon)
tanks that are trailer mounted. The tank is filled with asphalt
cement, and then the CRM is weighed into the tank from a storage
silo located on scales. The system in Mobile is a portable rig
that was built as a coventure with Etinourer. This unit is fed
from 22.6-kg (50-lb) sacks that are dumped into a hopper, and
the CRM is augured into the blending unit.
Rouse Rubber Industries, Inc., of Vicksburg, Mississippi, has
developed a portable blending and metering unit mounted on a
trailer. (See figure 9 for a photograph of the system and figure
10 for a schematic of the system. Note in the photo the bag delivery
system.) In this system the rubber is augured into the primary
tank, where it is mixed with an asphalt cement that is supplied
at 162°C (325°F). A secondary tank is used to increase
the reaction time for the process. The primary and secondary
tanks are 1,892-liter (500-gallon) tanks with an active space
of about 1,514 liters (400 gallons). The pump from the heat tank
feeds the asphalt cement at a rate of 264 L/min (70 gal/mm).
The percent rubber is controlled by adjusting the rate of the
auger that feeds the rubber. The residence time for the asphalt-rubber
blend in the mixing unit is approximately 15 minutes. The equipment
can produce 22,704 liters (6,000 gallons) of reacted asphalt
rubber in 1.5 hours.
Summary
Regardless
of the type of blending operation used, the plant should be operated
in such a way as to obtain a thorough and uniform mixture of
the materials. This takes care and attention to details by the
asphalt-rubber blender and the HMA contractor.
When an extended delay (greater than 6 hours) occurs before the
binder can be used after the reaction has been achieved, the
asphalt-rubber mix is normally allowed to cool to 110°C to
121°C (225°F to 250°F). It can be reheated slowly
prior to its use to a temperature of between 150°C (300°F)
and 190°C (375°F). It must be thoroughly mixed before
pumping and metering into the HMA plant. The viscosity of the
asphalt-rubber binder should be checked. If it is out of the
desired range, the asphalt-rubber blend is adjusted by the addition
of asphalt cement and/or ground tire rubber to provide the proper
viscosity. The effect of extended delays (i.e., prolonged storage
at elevated temperature) on performance is not well documented.
If the asphalt-rubber blender uses a bin-type feed system for
the CRM, steps must be taken to ensure that the material is free
flowing. As was discussed earlier with regard to shipping, the
rubber can clump and the chunks can plug up the gates and auger
systems used to feed the mixing operation. Possible remedies
include a vibrator on the bin or an individual who periodically
sticks the material in the bin to keep it free flowing.
Operation
of HMA Mixing Facility
The
operation of the HMA mixing facility for the construction of
wet-processed asphalt-rubber HMA mixes is unchanged from that
used for conventional mixes, with the exception that the mix
is produced at higher temperatures. The blending equipment or
agitated nurse tanks can easily be hooked up to both the drum
and batch plants. When a drum plant is used, a two- or three-way
valve is installed in the existing feed line on the output side
of the asphalt pump. The asphalt-rubber metering equipment is
then attached to the valve to feed the asphalt-rubber accurately.
When a batch plant is used, the valve is installed directly onto
the supply line leading to the weigh bucket. Many times a separate
supply line to the weigh bucket is installed. Separate pumps
are used by the blending contractor to prevent damage to the
HMA contractor's pumps. The asphalt rubber is generally more
viscous than the asphalt cement; thus, these pumps are generally
larger than the standard pumps used on an HMA facility. Also,
the pipes and supply lines need to be of sufficient diameter
to allow the movement of a more viscous fluid. It may be necessary
to employ jacketed and heated lines. There appears to be no problem
with the asphalt-rubber material building up in lines or any
requirements for unusual cleaning or flushing of equipment when
asphalt-rubber HMA mixes are produced.
The transportation of the asphalt-rubber HMA mix can be accomplished
in any truck typically used for the transportation of conventional
HMA. During the visits, there was no indication of sticking or
other problems associated with the trucking. Wetting agents for
the truck beds should be either soapy water or silicone emulsions.
Solvent-based wetting agents should not be used.
The
handling and placement of the CRM-HMA mix must be accomplished
to minimize segregation. It should be placed only when the surface
temperature and weather conditions are optimum. Since the material
is more viscous than conventional HMA, the temperature of the
surface on which it is placed should be warm. Arizona DOT requires
that the surface be at least 26°C (80°F). Even if the
surface temperature requirement is met, it may be necessary to
cease work due to existing or expected weather conditions that
could have an adverse effect upon the CRM-HMA mix.
The CRM-HMA mix delivered to the screed unit must be a free flowing,
homogeneous mass in which there is no segregation, crusts, lumps,
or migration of the asphalt rubber. It may be necessary to cover
the hauling units with tarpaulins, and/or dump the material directly
into the paver rather than using pick-up devices. Pick-up devices
have been used; but because of the temperature sensitivity of
the material, they should be used with caution. One contractor
suggested that the screed unit may need to be heated periodically
to prevent buildup of the mix on the screed. There can be pick-up
of the HMA mix when the roadway is turned over to traffic. This
can be addressed by lightly sanding the mix or by applying lime
water to the surface.
The
viscosity and amount of asphalt binder in an HMA mix will affect
the compactibility of the mix. The higher the viscosity of the
binder, the stiffer the mix at a given temperature. Generally,
the binder in an asphalt-rubber mix is stiffer than what the
agency or the contractor is accustomed to. Asphalt-rubber mixes
must be compacted while they are hot. Compaction is generally
not a problem if the temperature is maintained. The criticality
of the temperature increases as the rubber content increases.
Arizona DOT requires that the temperature of the rubber-modified
HMA be at least 135°C (275°F) just prior to compaction
and that compaction be completed prior to the mix temperature
reaching 104°C (220°F). A cautionary note: The presence
of CRM may generate fumes and smoke at typical compaction temperatures.
The compaction can be accomplished with either vibratory or steel-wheel
rollers. Pneumatic rollers should not be used, as the rubber-modified
binder can pick up on the pneumatic wheels.
price
of CRM asphalt cement and HMA varies from region to region. It
will generally consist of the cost of the CRM material, the asphalt
cement, and the costs of blending the material. As the CRM material
is ground finer, the cost of the CRM will increase; and as the
percentage of CRM in the asphalt cement increases, the cost of
the asphalt-rubber binder will increase.
A survey of the suppliers of CRM material indicates that the
approximate cost of the CRM at the CRM supplier's grinding plant
varies with particle size. Typical costs are shown in table 7.
Table 8 presents the typical cost of asphalt cement and asphalt
rubber for each of the surveyed States. The prices described
in table 8 reflect the reduction in the cost of asphalt-rubber
binder since the patents expired. In Arizona in 1991 (the year
before the patents expired), the average bid price was $400 per
metric ton ($448 per ton). Florida reports that the cost of blending,
etc., by the asphalt-rubber supplier is approximately $4.50 per
tonne ($5.00 per ton).
Table 9 presents the cost of the various asphalt-rubber mixes
used in each of the three surveyed States. The cost figures presented
in table 9 should be used only as a guide for large quantities
of asphalt-rubber binder. On projects that contain small amounts
of asphalt-rubber binder (which reflects many of the projects
being built), the prices can, and will, probably be considerably
higher due to the cost of mobilization of the blending equipment.
Also, it should be noted that the potential decrease in cost
of CRM-HMA due to the use of thinner sections has not yet been
addressed.
This chapter presented procedures used to blend the CRM with the asphalt, as well as mixing and placement procedures. Also presented are typical costs for the CRM, the asphalt-rubber binder, and the finished mix.
Chapter 4 | Chapter 5 | Chapter 6 | Chapter 7 | Glossary
CRM Suppliers | CRM Blenders | Typical Specifications
Local Specifications | References
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