Testing Asphalt With Ignition Burn-off Ovens

Dec. 28, 2000
They say necessity is the mother of invention, and-if this is true-then sometimes she takes on an unusual form.
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.