Field Manufacture of Asphalt Cutbacks
Various types and grades of asphalt cutbacks can be manufactured in the field with standard equipment. Thinner cutbacks can be produced from the more viscous grades. Field manufacture of SC and MC, however, is more practicable than field manufacture of RC because of the rapid evaporation of gasoline from RC. Also, a greater danger of fire or explosion from the gasoline exists.
Equipment and Production Rate
The rate of production is usually controlled by the speed that AC can be emptied from the drums and heated in the asphalt melters to suitable temperatures for pumping. The production rate for the asphalt melter is about 750 gallons per hour. Figure 5-1 shows the suggested arrangements of equipment uses in the manufacture of asphalt cutbacks. For small-scale production, use a truck- or trailer-mounted distributor or a 5,000-gallon, trailer-mounted asphalt tank with heating coils, instead of a 4,000-gallon mixing and storage tank. Arrange piping and pumping to fit each particular installation.
At least one safety inspector should be assigned to each operation to help personnel stay focused on their particular job. He ensures that personnel observe safety precautions within his area of responsibility. Personnel must strictly observe safety precautions when heating bitumens. They must—
Keep foam-type fire extinguishers available at all times.
Maintain oil heaters, storage tanks, asphalt melters, and distributors in a level position before heating.
Never heat bitumens near buildings or flammable materials.
Control ventilation of melters, heaters, and distributors to prevent escape of flammable vapors near flames or electrical equipment.
Stay to the windward side of equipment to avoid excessive exposure to fumes.
Reduce heat when foaming might cause overflow.
Extinguish burners after bitumen has reached the temperature given in Table 2-1 .
Extinguish burners and evacuate personnel if a dense, yellow vapor rises from the asphalt melter, distributor, or storage tanks. (This indicates overheating to the extent that a spark could cause an explosion.)
Extinguish burners before spraying bitumen from a distributor.
Never smoke within 50 feet of any equipment. A designated smoking area should be at least 100 feet upwind of the equipment during heating operations.
Examine all hoisting equipment daily.
Never fill buckets or containers to the top if they will be hoisted.
Never allow the asphalt level to fall below the fire tubes while the burners are in operation.
Consult appropriate TMs for clean-out operations.
Wear long-sleeve shirts, cuffless pants, fireproof gloves, heavy-soled boots, and steel combat helmets or civilian safety hats. This clothing helps protect workers if hot bitumen accidentally spills on them.
The procedures for the field manufacture of asphalt cutback are outlined below. Strictly observe the safety precautions associated with the procedures to avoid the danger of fire or explosion. See Table 5-2 for the composition of asphalt cutback.
Use special axes or cutting tools to remove the heads from asphalt drums, and inspect the contents of each drum. Eliminate drums that are contaminated with water or material that could cause foaming or fire. Take care when opening the drums to avoid serious injury, which can be caused by improper use of cutting tools.
Load the drums inside the dedrumming tunnel using an electric winch or a lifting device to pick up the drums. The Army uses different types of melters. The main difference between them is the melting capacity, which ranges from 8 to 12 drums inside the dedrumming tunnel. Heat the AC to about 250°F until it is fluid enough to pump easily. Figure 5-2 shows a typical asphalt melter.
Once the AC (or cutback) is heated to a workable state, pump it to storage. The 5,000-gallon heated tanks are usually used as storage tanks. Use the oil-jacketed lines between the units to maintain the asphalt in the pipelines at a constant temperature that is high enough to keep the asphalt fluid.
Blow out all the lines (except oil-jacketed) that become plugged with cold asphalt. For oil-jacketed lines, heat uneducated elbows and keep oil in the jacket. If the original asphalt material cannot be delivered hot, pump it directly to the storage tanks.
Pump about 2,000 gallons of AC from the heated storage tanks to the 5,000-gallon mixing tanks. If using a distributor or a trailer-mounted tank for mixing small quantities, pump it about half full to leave space for cutter stock and the foaming action that may result when adding the cutter stock.
Adjust the temperature in the mixing chamber between 240° and 250°F. Pump circulation accompanied by heating or atmospheric cooling will help regulate this temperature. If the temperature drops below 240°F, the AC will not be fluid enough to pump easily or mix readily with the cutter stock. At higher temperatures, much of the cutter stock will be lost in a gaseous form and a serious fire hazard will exist.
After the original material in the mixing tank is adjusted to the specified temperature, stop circulation and estimate the quantity of material in the tank. Determine the required amount of cutter stock to place in the mixing tank, and pump the desired quantity into the tank.
For best mixing results, introduce the cutter stock near the intake pipe that leads to the circulation pump. Pump the heated original material and the cutter stock through a closed system in the mixing tank. Cover the openings in the tank with wet burlap or canvas while blending cutbacks. Ensure that fire-fighting facilities are readily available.
When manufacturing cutbacks from AC, start pumping as soon as the AC is fluid enough to pump without damaging or placing a strain on the trailer-mounted asphalt pump. Ensure that the temperature is as low as possible, not to exceed 250°F because of the fire hazard. After mixing, usually about 30 minutes, pump the newly manufactured asphalt cutback to final storage.
The following example shows how to calculate the combined quantities of AC and cutter stock needed for a specific type and grade of asphalt cutback:
Example: Calculate the quantity, in gallons, of AC and diesel oil that must be combined to produce 750 gallons of SC-800.
Solution: SC-800 is composed of 70 percent soft AC, with a preferred penetration of 200 to 300 and 30 percent diesel oil by volume ( Table 5-2 ).
(0.7) (750) = 525 gallons of AC
(0.3) (750) = 225 gallons of diesel oil
The procedure for determining the proportion of cutter stock added to asphalt cutback to produce a lower (thinner) grade is outlined below. Use the data in Table 5-2 and the following formula:
x = percent of cutter stock to be added to the cutback to be thinned
a = percent of cutter stock in desired cutback of lower grade
b = percent of cutter stock in cutback to be thinned
Example: Calculate the amount of kerosene and MC-800 cutback used to produce 1,000 gallons of MC-70 cutback.
x = 20.7 percent kerosene
x = 79.3 percent MC-800
To produce 1,000 gallons of MC-70, combine 207 gallons (1,000 x 0.207) of kerosene with 793 gallons (1,000 x 0.793) of MC-800.
Use the following example to calculate yield:
Example: The materials available in the field for bituminous construction include 1,000 gallons of 120 to 150 penetration AC, 750 gallons of MC-3,000, and 1,750 gallons of kerosene. Determine the following:
How many gallons of MC-30 can be produced by combining the AC with the kerosene.
How many gallons of MC-30 can be produced by combining the MC-3,000 with the kerosene.
How many total gallons of MC-30 can be produced.
Solution: Determine the percentage of AC and kerosene in MC-30 by referring to Table 5-2 . MC-30 contains 54 percent AC and 46 percent kerosene. If 1,000 gallons of 120 to 150 penetration AC represents 54 percent of the MC-30 to be produced, use the following formula to determine how many gallons of kerosene should be added to the AC:
x = gallons
Combining 1,000 gallons of AC and 851 gallons of kerosene produces 1,851 gallons of MC-30.
Use the following formula to determine the percent of kerosene added to MC-3,000 to produce MC-30:
x = percent of kerosene to add to MC-3,000
a = percent of kerosene in MC-30 (see Table 5-2 )
b = percent of kerosene in MC-3,000 (see Table 5-2 )
Use the following formula to determine the amount of MC-30 that can be produced by combining the MC-3,000 with the kerosene. The previous equation determined that 37.2 percent of the MC-30 is kerosene, so the remaining 62.8 percent is MC-3,000.
x = amount of kerosene, in gallons, to add to 750 gallons of MC-3,000
The amount of kerosene is as follows:
Available = 1,750 gallons.
Previously used = 851 gallons.
Remaining = 899 gallons.
Since 899 gallons of kerosene are available and only 444 gallons are needed, there is enough kerosene to thin all of the MC-3,000. Add 750 gallons of MC-3,000 to 444 gallons of kerosene to make MC-30 (makes 1,194 gallons). The total amount of MC-30 that can be produced is 3,045 gallons (1,851 + 1,194).