|
Superpave
meets SMA, attracts
international interest
By Andy Bateman,
Engineering Editor
 |
Left and
bottom: WarrenÍs paving train on this job consisted of a Roadtec
SB2500 Shuttle Buggy which provided a consistent flow of material
to the Caterpillar AP 1055B paver. |
| The
development of asphalt technology has taken another step forward
with a municipal resurfacing contract in Mississauga, Ont. that
combines the latest in Superpave and Stone Mastic Asphalt (SMA)
technology. The contract involved the resurfacing of sections of
Southdown Road between Lakeshore Rd. and the Queen Elizabeth Way
(QEW) that are subject to traffic counts of 14 000 vehicles per
day, including an 8 per cent truck count. Completed by Warren Bitulithic
Ltd., the unique paving package used here created international
interest, with representatives of parent company Lafarge attending
from France, the U.K. and the U.S., in addition to staff from the
City of Mississauga and a number of neighbouring municipalities. |
 |
The
package that Warren presented to the owner consists of a binder lift
of Superpave mix combined with a surface SMA lift. As added refinements,
both mixes utilised limestone aggregates and Manufactured Shingle Modifier
(MSM), while the asphalt cement grade in the SMA mix was bumped to PGAC
70-28. Lafarge's Paul Lum explains: "The package was designed specifically
for this application, and our overall objective is to combine the rut
resistance of the Superpave binder mix with an SMA surface mix that
delivers rut resistance and durability as well as the economy of limestone
aggregates."
 |
What
is Manufactured Shingle Modifier and why is it used?
Manufactured
Shingle Modifier or MSM is derived from transitional and off-cut
pieces generated during asphalt shingle production. These pieces
are collected and put through a proprietary shredding and cutting
process, resulting in a stable and homogenous material sized at
9.5 mm minus. Hauled in bulk, it can be stockpiled and added to
the cold feed system in the same way as aggregates. On the Southdown
Rd. contract, the cellulose fibre in MSM helps to stabilize the
relatively coarse SMA and Superpave mixes and minimise draindown
of the asphalt cement.
|
On site, 110 mm of the existing surface was milled off and replaced
with a 60 mm lift of 19 mm SP (for Superpave) mix and a 50 mm surface
lift of 12.5 mm SMA. Mix quantities included about 700 tonnes of the
Superpave mix and 600 tonnes of SMA. Lum reports that the SMA mix is
expected to deliver good rut resistance, based on laboratory wheel rut
testing that indicated surface deformations of just 2 mm. In addition,
the mix will deliver a relatively quiet running surface, with comparative
tests showing that SMA mixes deliver a noise reduction of up to 2dBA
compared to conventional surface mixes. Mix details for both the Superpave
and SMA mixes are shown above.
The contractor's paving train on this job consisted
of a Roadtec SB-2500 Shuttle Buggy material transfer vehicle (MTV) feeding
a Caterpillar AP 1055B hot mix asphalt paver. Behind the paver, compaction
of the base mix was completed by a Bomag BW164AD making two passes in
vibratory mode, followed by a Caterpillar PS 300B pneumatic roller making
five passes. Another Bomag BW164AD followed the pneumatic machine making
two passes in static mode to finish the surface. The SMA was compacted
with the two Bomag units only to avoid any pick up by the rubber-tired
machine.
Lafarge's Andrew Pahalan explains that the
target density of the binder mix was a minimum of 91.5 per cent of the
Mass Relative Density (MRD), with the equivalent number for the SMA
being a minimum of 94 per cent of the MRD. With this particular SMA
mix, density correlation tests were completed on a test section of SMA
before the project started. These indicated that the required 94 per
cent MRD figure would be equivalent to a field reading of between 89
and 90 per cent, as measured by the nuclear density gauge assigned to
this job. On site, the actual nuclear measurements showed that compaction
achieved was at least 94 per cent for both mixtures. In terms of mix
design, Pahalan explains that the addition of MSM to the Superpave binder
mix stabilizes the relatively coarse mix; its 19 mm minus Superpave
aggregates have 37 per cent passing the 4.75 mm sieve compared to a
more typical value of about 50 per cent.
According to Lum, the overall cost of the package
is competitive, with the two lifts of hot mix, supplied and installed,
in the range of $20-$25 per square metre.
Warren Bitulithic Ltd. is part of Lafarge Canada
Inc. Paul Lum is director of New Product Commercialization and Quality
Assurance, Eastern Canada Region, Lafarge Canada Inc. Andrew Pahalan
is Quality Control manager for Warren Bitulithic Ltd.
December 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
Back to Library
|
