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Foamed
asphalt base recycling
gains momentum in Quebec
By Rober L.
Consedine, Editor
Wirtgen WR 2500
applying foamed asphalt to a depth of 300 mm on a section of Antonio-Barrette
Blvd. in Notre-Dame-des-Prairies, Que. A total of 30 000 m2 of the existing
road base was pulverized and stabilized in six days.
A
newly-acquired reclaiming/stabilizing machine forms a key part of a
Quebec road builder's efforts to establish cold-in-place foamed asphalt
base recycling as the principal method for economically rebuilding deteriorated
roadways in the province.
Since commissioning the Wirtgen WR 2500 road
reclaimer and soil stabilizer in late spring 2002, Joliette-based Action
Construction Infrastructure (ACI), has employed the versatile machine
to complete four foamed asphalt base recycling jobs in Quebec, totalling
over 120 000 m2, and a 58 000 m2 project in Southern Ontario.
Founded in 1997 by Michel Bellerose, ACI initially
began operations as a milling contractor and has grown rapidly to become
a major player in the municipal road rehabilitation market. In addition
to the WR 2500, the ISO 9002 certified company operates a fleet of cold
milling and recycling machinery that includes four other Wirtgen units
Æ two W 2100s, a W 2000, a 1900 DC, a 1000 DC and a W350.
According to Bellerose, recycling and
treating existing in-place road base materials with foamed asphalt is
a cost-effective method for producing a base course equal or superior
to the original structure at substantial savings compared to conventional
rehabilitation procedures or new construction. Another major advantage
of foamed asphalt technology is that by recycling the existing natural
aggregates in the roadway, it preserves the limited local supplies of
construction aggregates for future use.
The technology has been successfully employed
in Europe, the Middle East and Africa since the late 1980s and is being
increasingly adopted in the U.S. and Canada as its benefits become more
widely known. In the last two years, the number of foamed asphalt base
stabilization projects in Quebec has been on the rise. Currently, the
Montreal Island municipalities of Montreal North, Ville LaSalle, Lachine
and Ville St. Laurent are utilizing foamed asphalt to reconstruct road
bases within their respective jurisdictions. 
Michel Bellerose,
president of ACI, sees significant growth for foamed asphalt base recycling
across Canada due to the technology's lower cost and longer road life.
Foamed asphalt, also known as expanded asphalt,
is a recycling process created by injecting a predetermined amount of
cold water into hot penetration-grade asphalt in a mixing chamber of
a self-propelled or portable remixing unit. When injected, the water
evaporates instantly, causing an explosive expansion of the bitumen
in the saturated steam 15 to 20 times its original volume. The air bubbles
in the expanded asphalt froth carry the liquid asphalt to fines in a
reclaimed asphalt pavement (RAP) aggregate mix. In less than 15 seconds
the froth subsides and the dispersion of asphalt is completed, eliminating
the waiting or curing time required when using more expensive processed
asphalt emulsions. Although foamed asphalt doesn't completely coat all
the aggregate particles, it does bond or cement all the materials in
the roadway including particles of 75 microns or less.
The WLB 10 mobile
laboratory is used for testing existing materials to determine the most
suitable foamed asphalt mix for a specific site.
The low-viscosity foamed asphalt is subsequently
mixed in-situ with pulverized materials during the roadway reclaiming
operation to produce a highly stable base course. After the treated
base course material is graded and compacted, the road can be reopened
to normal traffic almost immediately or it can be overlaid with hot
mix asphalt pavement.
Aggregates & Roadbuilding recently
travelled to Notre-Dame-des-Prairies, 90 km northeast of Montreal, to
see the WR 2500 at work on a 30 000 m2 foamed asphalt project. The project
involved rebuilding a 1.7 km long section of the mainly two-lane Antonio-Barrette
Blvd.
PGAC 58-28 liquid asphalt was delivered to the site at 180°C
by tanker and pumped to the WR 2500 through insultated hoses.
Newly stabilized foamed asphalt base is compacted and graded prior to
reopening road to traffic.
Previously a busy commercial thoroughfare with
limited access, the street is being redesigned as a divided roadway
with one driving lane in each direction and another lane in each direction
for concrete turning islands, parking and bike paths. The intention
is to make the route safer and more user friendly with additional access
to local stores and businesses. When the reconstruction project is completed,
truck traffic will be diverted to Firestone Blvd. which runs parallel
to Antonio-Barrette.
Working as a subcontractor to the general contractor,
Sintra Lanaudi²re Inc., ACI crews handled the base recycling job in
three stages. The first stage involved milling approximately 50 mm from
the 200 mm thick existing asphalt pavement with the milled asphalt reused
by the general contractor as fill material in other areas of the project.
The remaining 150 mm of pavement was then pulverized in a single pass
by the WR 2500 and mixed with the granular sub-base materials to a depth
of approximately 250 mm to 300 mm. The goal was to obtain a 50/50 ratio
of recycled asphalt and aggregates in the sub-base foundation. The pulverizing
pass was followed by the stabilizing pass during which the foamed asphalt
was applied and mixed into the base course materials. After grading
and compaction, the new base course was surfaced by the general contractor
with two 50 mm lifts of hot mix asphalt.
The mix design for the job was developed by
consulting engineers, Laboratoire de Construction 2000 inc. following
testing and analysis of a series of core samples. The basic foamed asphalt
mix comprised 2.3 per cent PGAC 58-28 and 4 per cent water. The base
course design also specified the addition of 1 per cent Type 10 Portland
cement which was applied using an ACI-built mobile metering bin in front
of the WR 2500.
To produce the foamed asphalt mix, ACI used
a total of 141 247 kg of PGAC 58-28 and 62 640 kg of Type 10 Portland
cement. The rate of application was calculated as follows: PGAC 58-28
Æ 4.87 kg/m2; Type 10 cement Æ 2.16 kg/m2 and, water Æ 4.97 litre/m2.
ACI used Wirtgen's WLB 10 foamed asphalt laboratory
to test actual samples of the existing roadway materials to determine
the most suitable mix design under lab conditions. The WLB 10 simulates
what will happen during the foamed asphalt base recycling process to
eliminate guesswork as well as any surprises that might occur during
construction.
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Recycling
existing road bases with the WR 2500
Introduced
to North American markers in 1996, the Wirtgen WR 2500 incorporates
a 690-hp, V-12 turbocharged/aftercooled diesel engine that powers
a 2438 mm (96-in) wide by 508 mm (20-in) deep cutting rotor fitted
with 206 Type lll tungsten carbide Wirtgen bits. The 28 730 kg,
four-wheel drive machine features a unique two-frame or "floating
hood" design that allows an expanded volume of material to pass
freely through the process without slowing down the cutter or
cutter drive components. Material size is controlled by using
hydraulically adjustable, maganese-lined breaker bars that can
be remotely adjusted by the machine operator to tighten or close
down the area between the rotating cutter and the impact bars,
a crushing arrangement long used by the aggregate industry. The
Type lll holder system allows quick and convenient replacement
of broken or worn bits without a cutting torch. Drum rebuilding
can be completed in one day by two workers without the removal
of the cutter.
The multi-purpose WR 2500 employs factory-built remixing, foaming
and sprying components specifically designed for foamed asphalt
base recycling applications.
Inside the WR 2500, the foaming process
takes place in 16 expansion chambers where the water is injected
into the 180ÁC liquid asphalt cement. Two microprocessor-controlled
pumps ensure the exact proportion of water and bitumen used. The
machine also has a computerized control system that adjusts the
speed of the machine to the rate of application. Typically, the
WR 2500 operates at approximately 10-15 m per minute.
According to Wirtgen, special attention
was paid to development of the spray bars on the WR 2500. The
thermostatically electric heating unit ensures the optimum temperature
of the system prior to and during the foamed bitumen production,
thus eliminating the need to clean the system when production
is interrupted or after the work has been completed. The spray
bar directly above the milling and mixing chamber of the WR 2500
consists of 16 nozzles that evenly spray the foamed asphalt over
the complete working width of the machine.
Pneumatically actuated, pulse controlled
pistons clean each nozzle regularly during the spraying operation,
thus ensuring an undisturbed flow of the hot bitumen while avoiding
clogging by contamination. At the same time, the nozzles can be
closed in order to reduce the spraying Æ and mixing Æ width.
The bitumen nozzles can be closed from
the operator's cab for quick automatic reduction in the spraying
width. As the nozzles are switched off, the microprocessor automatically
reduces the binder quantity to ensure that the percentage of added
liquid asphalt remains constant. During the foaming operation,
foam samples can be taken from the external test nozzle in order
to check and optimise the foaming properties.
(left)
The microprocessor controlling the production of foamed asphalt
in the Wirtgen machines accurately meters and sprays bitumen,
water and air. (right) Schematic drawing showing the process and
machines used for cold recycling with foamed asphalt; the hot
liquid asphalt cement is injected in metered quantities in the
form of foamed asphalt; water for compaction is also injected
in metered quantities.
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November 2002
issue
Aggregates
and Roadbuilding Magazine
4999 St Catherine Street West. Suite 315
Westmount, Quebec H3Z 1T3
Tel: (514) 487-9868 Fax: (514) 487-9276
EMail: rocktoroad@sympatico.ca
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