The study evaluated dense-graded mixtures and stone mastic asphalt (SMA) mixtures.
The dense graded evaluation included two variations of asphalt cement (85/100 and
120/150 penetration grade), three increments of shingle content (0.0%, 5.0%, and
7.5%) and three types of roofing shingles (fiberglass- backed factory scrap, felt-backed
factory scrap, and tear-off). The SMA mixtures were formulated with one grade of
asphalt, and one aggregate gradation. The mixtures incorporated either 10.0%
fiberglass-backed factory scrap shingles, or 10.0% felt-backed factory scrap shingles.
An SMA control mixture contained 0.3% cellulose fiber by weight of mix.
A commercially available RS-HMA was subjected to the same testing procedures.
Among the conclusions were:
? The use of roofing shingles in the mix required less compaction effort to densify.
? A mix using 5.0% of factory scrap shingles resulted in a substantial decrease in
cold temperature susceptibility.
? Mixtures containing greater than 5.0% shingles may have a marked decrease in
mixture stiffness without a corresponding positive influence on cold temperature
susceptibility. This may result in an unacceptable stress at high temperatures
and high traffic volumes.
? Moisture sensitivity does not appear to be influenced by the inclusion of shingles
in the mix.
? It appeared that the felt-backed shingle mixes would have an increased ability
to deform in cold temperatures before thermal cracking occurred. Neither
the tear-off or the fiberglass-backed shingle mixes exhibited such behavior.
? Creep compliance analyses led the researchers to conclude that deformation
was reduced when shingles were added to a mix prepared with softer (120/150
penetration) asphalt, but that the opposite was true when shingles were added
to mixtures using the harder (85/100 penetration) asphalt.
8
? Concurrent with the University of Minnesota bench study, the Minnesota Department of
Transportation (Mn/DOT) constructed three test sections of RS-HMA. [16]
? In 1990, Mn/DOT paved a portion of a recreational trail in St. Paul with hot
mix asphalt incorporating 6% shingle scrap and 3% scrap tire rubber, and 9%
shingle scrap, by weight of aggregate. Both sections have performed well and
were in service as of October 1996.
? IN 1991, Mn/DOT repaved a portion of a town highway in Mayer using RSHMA
made with factory scrap shingles. The road had last been paved in
1974, and exhibited severe oxidation and longitudinal cracking. The project
consisted of a 1.5" leveling course and a 1" wearing course.
Seven different sections of the road were paved with various amounts (5% and
7%) of shingles in both the binder and wearing courses. Control sections of
conventional HMA were also constructed.
After four years of service, Mn/DOT reported no discernable difference
between the shingle scrap sections and the control section.
? In 1991, Scott County reconstructed a portion of County State Aid Highway
17, and RS-HMA was used in the base course on 0.5 miles of the northbound
lane. Mn/DOT reported that as of December 1995, both the shingle section
and control section were in excellent condition.
? As a result of the laboratory and field testing, Mn/DOT has a specification for salvage
material in HMA which now includes the use of up to 5% scrap shingles, by weight of
aggregate. The shingles can be felt-backed or fiberglass-backed factory scrap; no
tear-off roofing is allowed. The manufacturer must certify that the material contains no
asbestos.
Since shingle scrap is an allowable material in HMA, it is the discretion of the
contractor to use RS-HMA, and Mn/DOT is not tracking each RS-HMA project.
Because there is only one shingle manufacturer and one major shingle processor in the
state, the use of RS-HMA is limited to the area served by that particular hot mix plant.
[17]
9
Bituminous Roadways is Minnesota’s primary shingle processor. The firm has been
processing shingles and producing RS-HMA for about three years. The firm charges
the manufacturer $15.00 per ton to accept the shingles. Processing is performed with
two Rex “Maxi-grind” rotary drum grinders. Grinding is made easier if the shingles are
allowed to age for a year. Just that amount of oxidation hardens the shingles enough to
minimize agglomeration of the shreds. RS-HMA produced by Bituminous Roadways is
used primarily for commercial and residential paving, such as driveways and parking
lots. [11]
? Ross & Associates evaluated the potential use of RS-HMA in North Carolina. [2] The
research included laboratory testing of three HMA mixes each utilizing three increments
of shingles content (0.0%, 5.0%, and 10.0%). An SMA containing 8.5% shingles and
a control SMA containing 0.3% added fiber content were also tested. The results of
the testing indicated that:
? Tensile strength decreased as the concentration of shingles increased.
? The addition of 5% or 10% shingles to the mix significantly hardened the
asphalt binder, in some cases more than two penetration grades harder.
? The RS-HMA mixes showed decreased susceptibility to rutting based on
dynamic creep tests and loaded wheel testing. The authors attribute this benefit
to the increased stiffness of the asphalt binder, and the hard, angular granules of
the shingle aggregate.
? The performance of the shingle-containing SMA was equivalent to the control
SMA.
The authors also considered the economics of scrap shingles in pavement. Based on
the average cost of asphalt binder and finished HMA in North Carolina in 1997, and a
$50.00 per ton shingle processing fee, it was estimated that $1.13 per ton of HMA
savings could be realized by incorporating 5% shingles into the mix.
The North Carolina DOT has a specification that allows the use of up to 5% factory
scrap shingles in HMA. Currently, one large hot mix producer in North Carolina has
an exclusive contract to process all 35,000-40,000 tons of scrap from the CertainTeed
Corporation plant in Oxford, NC. [18] The material is incorporated into HMA or used
as aggregate.
10
? The Texas Transportation Institute at Texas A&M University conducted a 1995
laboratory study of incorporating factory scrap and tear-off scrap roofing shingles in
HMA. [9] A dense graded mixture and a coarse matrix high-binder (CMHB) mixture
were selected as the test mixtures. The shingle material consisted of coarse-ground (-
12.5 mm to +4.75 mm) tear-off scrap, fine-ground (-4.75 mm to +180 ? m) tear-off
scrap, and (? 9.5 mm to -180 ? m) fiberglass-backed factory scrap. After preparation
of RS-HMA and control mixtures, the samples were for tested for resilient modulus,
indirect tensile strength, moisture susceptibility, and static creep.
The researchers found mixed results for many of the tests, but noted that the
incorporation of either factory scrap or tear-off roofing has a negative effect on creep
stiffness. The greater the amount of shingle scrap in the mix, the poorer the creep
stiffness results. Primarily based on these test results, the researchers do not
recommend more that 5% shingle waste be used in HMA until further research has
been performed.
The report includes detailed guidelines for shingle processing, RS-HMA mixture
designs, mixture production, and RS-HMA placement and compaction. The report
also includes an example Texas DOT Specification for “Hot Mix Asphalt Concrete
Pavement Containing Reclaimed Roofing Shingles.”
? The Georgia DOT has paved two test sections of road using RS-HMA in 1994 [7].
? The first test involved the 1994 widening and reconstruction of the Chatham
Parkway in Savannah. A 1500 foot length of the northbound lane was repaved
with a 2-inch thick RS-HMA base course, overlain by a 1.5-inch thick RSHMA
wearing course. The fiberglass-backed factory shingle scrap used was
generated by GAF, Inc., in Savannah, and shipped to Baltimore for processing.
Once processed, the shreds were returned and stored under cover at the
asphalt plant. The material was incorporated into the mixture as is convention
recycled asphalt pavement (RAP). No special techniques were used in
placement, nor were any significant problems encountered.
11
Mix sampling at the time indicated that the RS-HMA material properties were
similar, or slightly improved, as compared to the convention HMA mix. Six
core samples (two from the control section, four from the RS-HMA section)
were obtained after approximately one year after service; and four additional
RS-HMA cores were obtained after 2-1/2 years. Testing revealed that the
RS-HMA cores compared well with the job mix formulas and plant mix tests.
The only unexpected result was the greater viscosity of the RS-HMA, which
may indicate that the shingle modified mix hardens at a faster rate than
conventional HMA.
Field observations demonstrate that the RS-HMA is showing little distress, and
is performing comparably to the control sections.
? One mile of State Route 21 in Effington County was also repaved with RSHMA
in 1994. This was a simple resurfacing project using the same shingle
material and mix parameters as the Chatham Parkway project.
As with the earlier project, mix sampling at the time indicated that the RS-HMA
material properties were comparable to the conventional HMA mix. Six cores
were taken from the road after approximately two years of service. Those
results, and field observations indicate that the RS-HMA is performing well.
? Economic estimates concluded that the incorporation of 5% scrap shingles
would reduce the cost of HMA by approximately $1.70 per ton. Disposal cost
for the shingles in Georgia was $16.50 per ton; processing costs were about
$5.00 per ton, resulting in a significant economic incentive.