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Recycling sulphur-asphalt pavements

Harold J. Fromm
Senior Research Officer
Research and Development Branch
Ministry of Transportation and Communications
Ontario, Canada

Current shortages of asphalt cement and aggregates demand that pavements be recycled when roads are being rebuilt or resurfaced.  If sulphur is to become a replacement for part of the asphalt in paving mixtures, then sulphur-asphalt mixes will also have to be capable of being recycled.

Sulphur-asphalt mixes have been tested in Ontario since 1975. The results obtained from four test roads, show that sulphur can replace up to 50% by weight of the asphalt cement and produce a pavement as god as, or better than, conventional mixes.  If more sulphur-asphalt pavements are to be constructed, it is most important that they be capable of being recycled.  There was some fear that excessive amounts of hydrogen sulphide (H2S) or sulphur dioxide (SO2) gases might be evolved in the recycling process when the cold sulphur-asphalt recycle material met the superheated virgin aggregates.  In order to determine if recycling was possible, both laboratory and field tests were carried out.

For the laboratory test, some material was taken from Section D of Test Road One.   This section was made with a general purpose mix (Ontario's HL-4 specification), which used an evenly graded aggregate, 5/8 inch maximum size, and a 50/50 sulphur-asphalt binder. This material was crushed for use in the test, which was conducted as follows.

Virgin aggregates (1900g) were heated to 260°C and placed in a large, stainless steel mixing bowl. An equal weight of recycle material was then added to the bowl and the stirring was started.  A puff of dust and a slight sulphurous odor was noticed at the start of mixing. The odor subsided quickly as mixing continued and the asphalt-cement was added.  The experiment was repeated, with the same results.  Portable H2S and SO2 meters with their sampling tubes placed over the mixing bowl gave the results in Table 1.

For the field trial, 20 tons of material were roto-milled from Section D of Test Road One, and transported to a batch plant where a recycling contract was in operation. No problems were experienced during the roto-milling operation. The heat generated in this operation caused some H2S and SO2 gas to form inside the skirt of the roto-mill.  The gas concentrations outside the skirt and at the feet of the mill operator, were well below occupational health limits, and posed no discomfort to the crew.

The plant at the recycling contract was a batch plant, and the contract called for 30% recycle of regular pavement to 70% of virgin aggregates. For the sulphur-asphalt recycle test, it was requested that the operation be run at a 50% sulphur/asphalt - 50% virgin aggregate ratio. The plant operator was afraid this might damage his drier, since it would be necessary to heat the virgin aggregates to 302° C. He agreed to a 40/60 recycle ratio, which required a temperature of 275° C. The composition and properties of the mix are shown in Table 2.

Recording meters were placed at the chute leading to the pugmill to determine the concentration of the gaseous emissions. The concentrations of H2S, SO2, and hydrocarbon vapors were recorded (Table 1).

Prior to beginning the sulphur-asphalt recycle, gaseous emissions were recorded for the normal recycle operation. These values are shown in the Table.

The conveyor belt for adding the recycle material to the weigh batcher is shown in Figure 1.  The chute leading to the pubmill with the gas sampling tubes at 0.3 m from its opeing is shown in Figure 2. Figure 3 shows the position of the sampling gauges at the foot of the batch plant. For this experimental recycling operation, the plant operator heated the stone (retained #4 sieve) and the sand (pass #4 sieve) to 275°C, and placed them in the hot bins. He then placed the required amount of stone in the pugmill, added the sulphur-asphalt recycle via the conveyor belt, and then added the sand. When the sand was added, a puff of dust came from the chute to the pugmill. A dry mixing cycle followed, then the asphalt cement was added, and mixing completed.

At the start of the recycling operation, no SO2 or H2S could be detected 0.3 m from the chute. The sampling tubes were then thrust down into the chute to the entrance to the pugmill for the remaining recycling operation. The results are shown in Table 1.  Several observers were present and could detect occassionally, a faint sulphurous odor at the beginning of a mixing cycle, but no further odor developed.

The portable H2S and SO2 meters were carried around the plant during the operation, no traces of gas were detected.

The portable meters were lowered into a loaded truck box containing the recycle-virgin blend. Some SO2 was detected but no H2S. These values are also shown in Table 1. The recycled material was then taken to the paving site and place on the roads. The material handled like the normal mix. No problems were experienced here, and there were virtually no emissions to bother the paving crew.

These test have shown that sulphur-asphalt pavements can be recycled with no problems in either the roto-milling, the hot mixing, or the paving.