3. Recyclability
Before 1992, A-R pavements had been performing well
and the replacement/recycling of them was not necessary. During
the mandate era the recyclability of asphalt rubber pavements
was not validated by field experience. As some sections of asphalt
rubber pavements have met their service life span, they have
been successfully recycled. The Texas Transportation Institute
(TTI) conducted a study in 1995 on this subject where two of
the earliest crumb rubber recycling operations in the United
States have transpired. (Crockford, 1995) The study concluded
that "the material is recyclable and that the recycled material,
if properly designed and constructed, should have acceptable
long-term performance." Additionally, the report pointed
out "air quality does not seem to be any more severe a problem
than it is with conventional asphalt." He also stated "the
effect of CRM on emissions may be relatively small in comparison
to the effects of other variables." Those variables include
the fueling rate of the dryer, mix temperature, asphalt throughput
rate, and asphalt binder content.
Another recent recycle job occurred in the City of Los Angeles,
California. (Youssef, 1995) The initial placement of the asphalt
rubber pavement occurred in 1982. In 1994 the pavement was milled
and stockpiled at a nearby asphalt plant. The asphalt rubber
grindings were added to the virgin rock and oil so that the grindings
composed 15% of the final mix. At another location, the grindings
were put through a microwave process where nearly 100% of the
output was composed of recycled asphalt rubber. This project
demonstrated that asphalt rubber can be recycled using either
microwave technology or conventional mix design technology. Air
sampling during paving and recycling determined that employee
exposure to air contaminates were well below the Occupational
Safety and Health Administration (OSHA) permissible exposure
limits (PEL), and in most cases below the detection limits.
4. Environmental concerns
- Fume emissions have been studied extensively in a number
of asphalt-rubber projects since 1993 and in all cases been determined
to be below the National Institute for Occupational Safety and
Health (NIOSH) recommended exposure limits. (Gunkel, 1994) Table
3 below is an excerpt from a study conducted for the Michigan
Department of natural Resources in 1993 comparing conventional
HMA and Asphalt-Rubber Hot Mix. In this study control mix 2 contained
100% virgin aggregates and asphalt cement with a penetration
of 200-250, equivalent to an AC-2.5. The rubber mix 1 (RBR1)
also contained virgin aggregates and asphalt rubber binder manufactured
using the "wet" process which was described previously.
Table 3 Continuous Emissions Measurements and Method 18 Results
(Units m/m3) Evaluation of Exhaust Gas Emissions and Worker Exposure
from Asphalt Rubber Binders in Hot Mix Asphalt Mixtures. (Gunkel,
1994)
| Operating Data/Conditions/ Measurements | Control 2 | RBR 1 |
| HMA Production Rate (tons per hour) | 351 | 357 |
| Dry Aggregate Rate (TPH) | 330 | 333 |
| Asphalt Cement Added (%) | 5.75% | 6.84% |
| Materials moisture content | 4.17% | 5.21% |
| Fuel Consumption (gal/hr) | 655 | 690 |
| Exhaust Gas Temperature (F) | 311 | 324 |
| Mix Temperature (F) | 296 | 316 |
| Sample Volume (SCF) | 46.501 | 42.823 |
| Sample Volume (cu. m) | 1.317 | 1.213 |
| Exhaust Gas Moisture (%) | 27.0% | 29.3% |
| Stack Temperature (F) | 260 | 271 |
| Actual Exhaust Gas Flow (ACFM) | 89,540 | 95,450 |
| Dry Exhaust Gas Flow (DSCFM) | 47,076 | 47,836 |
| Dry Exhaust Gas Flow (DSCMM) | 1,333 | 1,355 |
| CO2, %, Orsat Result | 5.79% | 6.02% |
| O2, %, Orsat Result | 12.75% | 12.10% |
| N2, %, Orsat Result | 81.46% | 81.88% |
| Carbon Dioxide (CO2) | 6.00% | 6.48% |
| Oxygen (O2) | 12.87% | 12.18% |
| Carbon Monoxide (CO) | 430.5 | 259.5 |
| Nitrogen Oxides (NOx) | 139.3 | 124.4 |
| Sulfur Dioxide (SO2) | 74.4 | 76.7 |
| Non Methane Total Hydrocarbons (NMTHC) as Carbon | 225.5 | 183.0 |
| Methane (CH4) as measured | 27.7 | 10.6 |
| Methane as Carbon | 20.7 | 7.9 |
| Total Hydrocarbons (THC) as Carbon | 245.1 | 191.3 |
| NMTHC as Carbon | 225.5 | 183.0 |
The findings of this study were significant to the asphalt-rubber industry in that many of the conventional mix materials had higher, but still acceptable, emissions in certain categories than those with rubber. Very few emission studies were conducted following this report.
The equipment used to blend asphalt and rubber requires little,
if any, modification to a standard hot mix asphalt plant. The
equipment is typically trailer mounted and is transported into
the asphalt plant site as depicted in Figure 4. Dedicated mixing
and reacting tanks are used which are also mobilized to the site.
Figure 4 Asphalt rubber blending unit.
Additionally, conventional paving equipment without modifications
is used to place the material. The capital investment required
for a fully operational asphalt rubber plant is anywhere from
500,000 to 750,000 USD. To put this into perspective, a used
bulldozer (1998), can be purchased for about 800,000 USD.
The traditional formulation for asphalt-rubber developed by
Charles H. MacDonald is no longer controlled by patents nor is
it proprietary in nature. The material is now part of the public
domain. Since the expiration of the patents in 1992, more paving
contractors have become involved in the industry. Initially,
there were only two companies (1970s), now the number of contractors
with some form of asphalt rubber blending or distributing equipment
is estimated in the thirties and growing. Industry growth is
dependent upon agency use of the material. It is also important
to note that this technology was not developed to consume waste
tires; ground tire rubber was used because it added significant
engineering characteristics and qualities to asphalt pavements.
Tire rubber processed through an ambient grinding system has
proven to be most effective during the blending and reacting
stage in A-R binder production. The surface area of the particle
is a critical factor. Rubber that is first cryogenically reduced
needs to be "roughened" through a cracker mill or an
equivalent piece of equipment to ensure reaction. Research has
indicated that the greater surface area produced in ambient systems
provides greater reactivity with liquid asphalt. In most specifications
the particles must be free of metal, fabric and moisture. Common
gradations for rubber particle sizes used in A-R can be found
in Table 4. The percent passing range for each sieve size is
listed by states with common A-R usage. The gray bar indicates
an unused sieve.
Table 4 Crumb Rubber Gradations for Asphalt
Rubber
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Although prices vary, a simple survey of the crumb rubber producers in the southwest provided some pricing information in Table 5. The median price trends from 1995 to 1999 for ten and twenty mesh crumb rubber are depicted in Figure 5.
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Figure 5 — Median Crumb Rubber Prices in the Southwestern United States
The use of A-R has increased significantly since 1994. DOT figures from AZ, CA, and FLA and some municipal public works departments depict a trend of growth in Figure 6. The same agencies reported 1.6 million tons placed in 1994 and 2.5
million tons placed in 1998. Total figures for the US are not currently available.
Figure 6 — Annual tonnage of Asphalt Rubber Hot Mix (Six Agencies Reporting)
Formulations for asphalt-rubber within these agencies vary. The crumb rubber content by weight of liquid asphalt cement is generally between 10 to 20% which is reacted at 149 to 204 degrees C (300-400 degreesF) for periods of 45 minutes to one hour. The most common specifications for asphalt rubber hot mix use approximately 30 pounds of rubber per ton hot mix. Estimates for crumb rubber usage in asphalt in the US have been as high as 200,000,000 million pounds for 1999 (Recycling Research Institute, The Scrap Tire and Rubber Users Directory, 1999). Surprisingly, it is difficult to ascertain the actual figures. Many agencies do not track total figures with regard to rubber used in paving applications. Based on the preceding figures, the participating agencies in this report consumed approximately 75,000,000 pounds of crumb in 1998. (30 pounds per ton)
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Rubber
Pavements Association
1801 South Jentilly Lane, Suite A-2
Tempe, AZ 85281 USA
480.517.9944
480.517.9959 fax
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