By: David Banasiak
They say necessity is the mother of invention, and-if this is true-then sometimes she takes on an unusual form. This was the case on January 1, when a federal law went into effect banning the manufacture of 1,1,1 trichloroethane because it causes ozone depletion.
This chemical was used, along with other chlorinated solvents (such as trichloroethylene),
to aid in the determination of asphalt content and aggregate gradation of
hot-mix asphalt (HMA).
Not only are these solvents dangerous to human health and the environment,
but they are also difficult and costly to dispose of and expensive to use.
While the federal government has not banned all chlorinated solvents, some
states have.
Substitutes were sought, and new safer solvents-based on terpene and terpineol,
derived from pine and citrus products-appeared. But there were problems
with these new solvents. They were not as effective in removing asphalt
from porous aggregates, and the additional required wash-ups lengthen the
process time. They also can not be easily disposed of.
Another alternative is a nuclear asphalt content gauge (NAC); how-ever,
its speed and accuracy is offset by its expense and the special precautions
that must be taken when dealing with radioactive materials. In order to
use this equipment, a license must be received from the Nuclear Registry
Commission. Another drawback with an NAC is its inability to determine gradation
of the aggregate.
A new solution has been developed which avoids all these drawbacks: The
HMA is heated to remove the asphalt binder. While this method is not new-
Dr. Herb Busching conducted work in this area during the late 1960s (see
Mailbag, May 1995 p 24)-the greatest advances with this technology occured
during the past few years.
The ignition method, as it is referred to, is really quite simple. Through
pyrolsis, the asphalt in a furnace will be burnt-off-leaving clean aggragate
which can be analyzed for gradation.
The loss in weight resulting from the burn is directly related to asphalt
content. By simply comparing the "before" and "after"
weights of the material, the asphalt content can be assessed.
"It is a promising test that will accurately determine asphalt content
provided the calibration factors are correct," says Mark Rademaker,
bituminous field engineer for the Illinois DOT. However, he does not feel
that the method will totally replace solvent use.
Brian Prowell, research engineer at Virginia DOT, adds, "As far as determining asphalt content it is a good test, but it depends on the aggregate. The test is good for Virginia because we are blessed with good aggregate."
Glenn Shiller, bituminous engineer for lab operations at the Florida DOT,
is also concerned about their aggregate. "We found that the ignition
test can accurately determine the asphalt content, but we are concerned
if it will degrade our lime rock." The Ohio DOT will allow the test
as an option this year.
David Powers, asphalt materials engineer for ODOT, explains, "Ohio
has developed its own modified procedures with the ovens. Pending our round-
robin tests and how 1996 goes, it may be required the following year or
the next."
When contractors in Virginia were given the option of using the ignition
method, they adopted it statewide. "Contractors would rather use this
method because it is faster, more accurate and cheaper," said Ray Brown,
director of the National Center for Asphalt Technology (NCAT).
Fawaz Hamoui, product manager for Gilson, agrees, "It's a step in the
right direction. No one wants to deal with the solvents. Too many headaches
with the mess, with the fumes and they're too expensive."
When looking at a furnace, the weighing system is an important consideration.
Both the internal and external scales have their advantages and disadvantages.
An internal weighing system constantly monitors weight loss during the burn-off
and provides a paper copy readout of the results. "The advantage to
this," says Lee Nelson, marketing manager for Barnstead/ Thermolyne,
"is that the asphalt content loss can be measured minute by minute.
It is more accurate."
Ken Brown, product manager for Troxler Electronic Laboratories, agrees,
"With a scale inside, weighing all the time, you know when you reach
a constant weight." When all the asphalt binder has been burnt-off,
a constant weight will be reached and the furnace will beep like a microwave
to alert the operator.
However, there is some concern in the industry that certain aggregates will
continue to burn after the asphalt burn-off has been completed. This will
cause the scale to continue to register a weight loss, which may result
in an inaccurate reading.
An internal scale also cuts down on the handling of hot trays. "It
is really a safety issue. These samples are 1,100 F. If you have the oven
in a trailer and drop the sample, you're not going to have that trailer
any longer," says Richard Schreck, executive vice president of Virginia
Asphalt Association Inc.
The metal basket which holds the material in the furnace changes weight
when heated. When removed from the furnace, it must be cooled before weighing.
This will help reestablish the weight of the metal basket as well as prevent
damage to the scales. Rademaker adds, "There is some thinking in the
industry that the sample absorbs moisture as it is cooling. This may affect
the test results, and the precautions necessary to prevent this may increase
testing time by one to two hours."
However, placing the weight system inside the furnace increases production
costs, which-in turn-raises the price of the furnace.
"The difference in price is incredible," states Hamoui. Furnaces
that do not have an internal weighing system cost at least 50% less then
those that have one.
The accuracy issue can be addressed by observing the binder burn-off at
different burn times. From this experimentation the ideal burn time can be achieved.
However, burn times may differ from batch to batch. But Hamoui does not
feel there is an accuracy problem with external weighing systems. "Several
DOTs and contractors have been able to achieve the correct numbers with
an external balance. And users of internal balance units must weight the
material on an external balance before putting it in the oven."
The Ohio DOT has been conducting round-robin tests using both the internal
and external scales for the past year. Powers comments, "They are comparable
ovens, provided the proper procedures are followed." He goes on to
add, "The NCAT model is the Cadillac, while the Gilson takes the Chevrolet
approach."
To provide furnaces for use with the ignition method, a number of laboratories
and manufactures have designed and built specialty furnaces. Gilson offers
a programmable burn-off furnace with an oversized main chamber and an afterburner
chamber. The main chamber is digitally controlled with memory to store 10
binder-ignition programs to permit warm up as well as the burn-off of binder
at temperatures below the deterioration point of mineral aggregates, about
550 C.
The unit's stainless steel case has a drop door with interlock and lockout
switches. The chamber holds a 17x17 in. sample tray with support screen
and lid. Ignition of most samples is complete in about 30 minutes, with
full results in less than an hour, after cooling and weighing.
Troxler Electronic Laboratories' Model 4155 asphalt analyzer incorporates
an internal weight scale mechanism, front panel display, separate control
console and a printer, which provides a hard copy of test results. It can
hold a sample size up to 3,000 g and can reach 550 C in 20 minutes.
Air is drawn into the chamber to aid combustion. The volatiles from combustion
are discharged through an 800 C afterburner.
Troxler also plans to offer a model with all the features of the 4155 except
the internal weight scale.
In March 1995, NCAT completed a round-robin study of an asphalt-content
tester designed and manufactured by Barnstead/Thermolyne. This furnace comes
with an internal weighing system and hard copy printout.
It also features a 24-hour timer, filter to reduce exhaust smoke and safety
door lock. It has a temperature range of 200-650 C and can accommodate sample
sizes up to 2,400 g.
Tempyrox has developed ovens for cleaning asphalt from laboratory glassware,
which may also be used in the ignition method to test asphalt content and
aggregate gradation. However, because their ovens operate at a lower temperature,
ignition never takes place and the burn-off of asphalt will be much slower.
This is done in order to prevent a fire in the unit and damage to the glassware.
Ken Mainord, president of Tempyrox, explains, "The last thing we want
is a fire inside our cleaning system. Our units use proprietary technology
to inert the oven chamber during the pyrolysis portion of the cleaning cycle
so that no fire or ignition occurs."
Ignition-method advances are continually being sought. Construction Technologies
Laboratories has been involved in numerous testings of the method, and Alan
Todres, senior principal engineer, and Sankar Battacharja, research scientist,
have published papers on their ignition-method test results.
The lab will continue active investigation of this method and plans to introduce
new technologies which should be available in time for the construction
season this year.
