3 SOURCES OF ASPHALT FUME EXPOSURE

The purpose of this chapter is to describe processes involved in the installation of BUR systems and the potential sources of worker exposure to asphalt and asphalt fumes. Only three low-slope roofing membrane systems—BUR, SBS modified bitumen, and PIB single-ply systems—are installed using hot asphalt. Because the equipment and operations that may result in worker exposures to asphalt fumes are the same in all three types of work, the discussion in this section addresses BUR jobs, which are more common. The same engineering controls and work practices can be used to reduce worker exposure to asphalt fumes during the installation of SBS modified bitumen and PIB single-ply systems.

3.1 TYPICAL BUR SYSTEM

The BUR roof membrane is designed to provide an asphalt-based membrane that serves as a water-impermeable covering for the roof assembly and the building as a whole. The membrane prevents water from entering the building and protects the underlying insulation and roof deck from damage caused by moisture. A typical BUR membrane consists of three basic components: (1) waterproofing material (asphalt or coal tar), (2) reinforcement material, and (3) surfacing material [NRCA 1996]. The reinforcement material (which is critical to the longevity, durability, and stability of the membrane) consists of the ply material embedded between layers of asphalt and the waterproofing material. The reinforcement material helps hold the waterproofing asphalt in place and adds tensile strength and other physical properties to the membrane. Surfacing materials (such as aggregate or mineral granules) protect the membrane from the effects of sunlight and weather exposure and may provide other benefits such as fire resistance. Some surfacing products also improve climate control by acting as solar reflectors. Granules are usually factory-applied to a premanufactured sheet or aggregate (such as pea gravel, slag, or marble chips), or they may be field-applied in a final flood coat of asphalt. The cap or final surface layer of asphalt (sometimes coal tar pitch) is usually applied with a spreader followed by another spreader that applies a layer of gravel [NRCA 1996].

3.2 DELIVERY AND HEATING OF ASPHALT

Mopping-grade roofing asphalt used in the construction of BUR systems is often delivered to the worksite as a solid, typically in the form of 100-lb cartons or kegs. When delivered in solid form, the asphalt is then broken into smaller pieces, manually inserted into a roofing kettle, heated, and pumped to the roof for application. Although asphalt may also be delivered in a tanker as a heated liquid, this practice is increasingly unusual because of cost and product supply considerations. Asphalt delivered by tanker may be heated to the proper temperature in the tanker and then pumped to the roof, or it may first be transferred to a kettle for heating before pumping to the roof.

Figure 3-1. Kettles with 80- and 200 gal capacities.

3.2.1 Kettles

Asphalt roofing kettles come in capacities of 25 to 1,500 gal. Figure 3–1 illustrates 80- and 200-gal kettles.

Kettles typically consist of a round-bottomed steel vessel, a heating unit, a motor and pump, and a supply line (often called the hot pipe). The heating unit consists of propane-fired burners and metal heating tubes inside the vessel that distribute heat from the burners to the contained asphalt. The pump circulates the asphalt within the vessel to help maintain even heat distribution, and it is used to deliver the asphalt up the hot pipe to the roof. Kettles may also be equipped with thermometers, thermostats, automatic temperature controls, and other control devices. Figure 3–2 depicts a thermometer on a kettle.

Potential exposures to asphalt fumes related to operation of the kettle include both continuous exposure to fumes that escape from the kettle during operation and intermittent exposures related to the performance of operations such as filling or loading, which require the lid to be opened. Even with a relatively good seal between the body of the kettle and the lid, asphalt fumes can escape from the kettle lid and vents.

The kettle operator may be exposed to asphalt fumes whenever the kettle lid is opened—most frequently for loading. As asphalt is drawn from the kettle, it must be loaded with chunks of asphalt. To load the kettle, the kettle operator must lift the lid of the kettle to insert these chunks (Figure 3–3).

Several other operations require an open kettle lid. For example, the lid may be opened periodically to check the level of liquid asphalt inside the kettle. This step is necessary to ensure that the supply of asphalt is adequate to perform the job task and to maintain the fluid level above the heating tubes to avoid a fire or explosion hazard. The kettle lid is also opened periodically to skim debris from the surface of the asphalt. Removal of surface debris is necessary to avoid clogged pumps and obstructions in the pipe that transports asphalt to the rooftop, to prevent fires, and to ensure a satisfactorily installed roof. In addition, the lid must be opened when checking the temperature with a hand-held thermometer. The use of devices such as dipsticks and automatic thermostats can minimize the number of times the kettle lid needs to be opened.

3.2.2 Tankers

Like kettles, tankers contain heating tubes and pumps to circulate and maintain proper asphalt temperatures. During unloading, a pump and supply line are used to pump the material from the tanker to the point of application, kettle, or storage tank. Whenever large quantities of asphalt are pumped or drawn from the tanker, the hatch on top of the tanker must be opened for both operational and safety reasons. Tankers are typically capable of pumping about 60 gal/min, which is the same rate as most kettles manufactured today.

When a tanker is used to refill a kettle, the kettle lid must be open and the kettle operator must be in the area to monitor the fill level and avoid overflow. The kettle is usually top-loaded. Although the kettle lid must be open during filling, it is usually open for a relatively short period, since the high pumping rates of the tankers allow the operation to proceed much faster than manual filling with solid chunks of asphalt.

3.2.3 Asphalt Heating and Application Temperature

The quality of the finished roof depends greatly on the application temperature of the asphalt. The recommended application temperature for mopping-grade roofing asphalts ranges from 330 to 445 ?F (166 to 229 ?C), depending on the mopping-grade type (Type I, II, III, or IV) [NIOSH 2000]. To achieve the specified asphalt temperature at the point of application, the temperature of the asphalt in the kettle has been reported to be as high as 600 ?F (316 ?C) [Puzinauskas 1979; Hicks 1995; NIOSH 2000].

To create the proper matrix between the hot asphalt and the felt plies, the liquid asphalt must be applied within a temperature range known as the equiviscous temperature (EVT). The EVT is the temperature at which the viscosity of the asphalt, when applied, will result in a quality roofing system [NRCA 1991; ARMA 1993; NIOSH 2000]. By definition, each asphalt has two EVT values—one for hand mopping and one for mechanical spreading. If the asphalt is applied by hand mopping, the EVT is the temperature at which the viscosity of the asphalt is 125±25 centistokes. If the asphalt is applied using a mechanical spreader, the EVT is the temperature at which the viscosity of the asphalt is 75±25 centistokes. Since the desired viscosity is not a precise value, the EVT is reported as the midpoint temperature ±25 ?F (±14 ?C) that will result in the desired viscosity range. According to current practice in the industry, the asphalt temperature is measured just before application to the roof surface—that is, the temperature of the asphalt is measured in the mop cart or mechanical spreader, the last point at which temperature can practicably be measured [NRCA 1996]. Because of significant differences in typical application rates of hot asphalt to the roof surface, the EVT is generally about 25 ?F (14 ?C) higher when a mechanical spreader is used than when mops are used to apply the asphalt [NRCA 1996]. Asphalt at the EVT will be the proper viscosity, depending on application technique; so it may be spread evenly to the optimum thickness and result in the proper matrix between the asphalt and the felt plies. Hot liquid asphalt fuses with the saturation or impregnation asphalt already in the layers of ply felt, thus laminating the plies together to form a strong, waterproof membrane. Again, this result is best achieved when the asphalt is applied at the appropriate EVT [NRCA 1996].

Although EVTs for asphalts of the same classification (i.e., mopping asphalt Types I through IV) tend to be similar across the industry, each EVT is unique to the particular production run of mopping-grade asphalt made by manufacturers. Today, nearly all manufacturers and suppliers of mopping-grade asphalts provide product specifications on the packaging of each keg of solid asphalt distributed to contractors or in the bill of lading accompanying each load of bulk liquid asphalt delivered by tanker truck. The information specifics include the type of asphalt, two EVTs (one for use with the mechanical spreader and the other for use with the mop), the EVT ranges for hand mopping and mechanical spreaders, and other pertinent product characteristics such as the flash point (which is also a value unique to each asphalt product).

Application within the EVT range is also critical to assure proper film thickness of the layers of asphalt. Temperature determines the viscosity of the asphalt. An overheated asphalt will be too thin, whereas an underheated asphalt will be too thick. If the asphalt is overheated for a prolonged period, a phenomenon known as “fallback” can occur. Fallback causes a reduction in the softening point of the asphalt and can affect the quality of the roof system. Such lowered-softening-point asphalts, for example, are prone to “slippage,” which allows the bitumen and reinforcement to slide down-slope [NRCA 1996; Owens Corning 1993]. Fallback is an additional reason that kettle temperatures should be monitored closely and kept only as high as needed to compensate for heat loss during travel from the kettle to the roof.

Asphalt temperatures in kettles and tankers depend on safety and operational considerations. Since several ignition sources exist during kettle operations, safety hazards are created if the temperature is allowed to rise above the flash point or fire point of the asphalt. Flash fires can occur if the temperature of the asphalt reaches or exceeds the flash point; however, continuous combustion can occur if the temperature of the asphalt reaches or exceeds the fire point, which is usually about 5 ?F (2.8 ?C) above the flash point [NSC 1996]. Some State and local laws limit kettle temperatures for fire safety or environmental protection purposes. Potential sources of ignition during kettle operation include exposed hot metal heating tubes and exhaust stacks, open flames, and hot carbon and coke buildup inside the kettle.

In conventionally configured kettles, fires are a concern when the kettle lid is open or closed. When the lid is open, these fires can lead to very serious burns. In addition, if kettle fires are not contained and immediately extinguished, they can spread to exterior parts of the kettle, engulfing the equipment (including gasoline tanks on some models), solvent containers, and propane fuel tanks with catastrophic results. In addition to fire hazards, explosion hazards exist if the headspace fume concentration is between the lower flammable or explosive limit (LEL) and the upper flammable or explosive limit (UEL). If the kettle temperature is near the flash point, care needs to be taken when opening the kettle lid because the ambient air entering the kettle can lower the fume concentration so that it is between the explosive limits. It is therefore recommended that kettle temperatures always be maintained at least 25 ?F (14 ?C) below the flash point of the asphalt [NRCA 1996].

Operational factors also influence kettle temperatures. To ensure that the asphalt is the proper temperature at the point of application on the rooftop, the temperature in the kettle must be maintained at a temperature somewhat higher than EVT. How much higher depends on a number of factors that vary from job to job, including the following:

The range of temperature drop that may occur because of these factors generally averages from about 20 ?F (11 ?C) to more than 50 ?F (28 ?C). Many roofing contractors use a 50 ?F (28 ?C) rule of thumb to determine the appropriate temperature setting for the kettle. Thus an appropriate starting point for kettle temperature may be 50 ?F (28 ?C) above the EVT midpoint, as long as this temperature is at least 25 ?F (14 ?C) below the open cup flash point. From this starting point, the kettle temperature can be adjusted up or down to account for actual temperature loss between the kettle and point of application.

On the roof, asphalt temperatures in mop carts and mechanical spreaders can be measured using hand-held thermometers. Measuring the temperature of the asphalt in the kettle may also be accomplished by using hand-held thermometers. In addition, infrared thermometers are available to measure asphalt temperature remotely; point the infrared thermometer gun at the asphalt surface after stirring to get a true reading.

Most kettles manufactured today have built-in thermometers—typically 2.5- to 3.5-in. stem thermometers that are usually screwed into the rear of the kettle vat. However, they are not always placed in the most appropriate location and may be susceptible to damage from heat and physical stress. This is particularly true in the case of older models, which may not have built-in thermometer guards and may require the kettle operator to manually regulate the firing torch of the kettle-heating source according to the asphalt temperature readings. Kettles (particularly models introduced since the late 1970s) may have temperature regulators that automatically control the heating source. Automatic controls include self-contained thermocouple controls, thermostat hi-lo controls, and electric-battery-operated controls.

3.3 INSTALLATION OPERATIONS ON THE ROOF

Installation of a BUR membrane often begins with the application of a base sheet of medium- or heavy-weight felt, although the need for a base sheet depends on the design specifications for the job. The base sheet serves to separate the BUR membrane from the roof substrate, provides support, and cushions the membrane over rough or irregular spots. The base sheet may be attached to some roof decks using mechanical fasteners. Hot asphalt and ply felt are then applied sequentially onto the base sheet. Asphalt at its EVT is mopped or mechanically applied in a thin layer, then the ply felt or ply sheet is rolled into it. It is critical that the asphalt be spread evenly so that it forms a continuous film without gaps or voids beneath the ply felt. Felt plies are laid in an overlapping edge arrangement, and the crew must be sure to maintain adequate side, end, and head lap among the sequential layers of ply felt.

The hot asphalt used in this process is delivered to the roof through a metal supply line (the hot pipe) from the kettle or tanker. The same pump that recirculates the asphalt inside the kettle is typically used to pump the hot asphalt through the supply line to the roof. Standard pumping rates range from 35 to 60 gal/min. Hot pipes are 5- to 20-ft lengths of metal tubing that can be coupled together. Figure 3–4 shows a typical pumping and hot pipe arrangement.

Asphalt delivered through the supply line is usually emptied into a container on the roof called a “lugger” or a “hot lugger,” which comes in standard sizes of 30 and 55 gal and is top-filled directly from the supply line. Most luggers have a hatch cover that can be closed once the vessel is filled. Figure 3–5 shows a typical hot lugger and mop bucket.

After delivery into the hot lugger, asphalt may be drawn off in three different ways for use in installing the BUR. In manual application operations, asphalt is drawn off either directly into mop carts or into buckets (see Figure 3–5) that are poured into mop carts for use by workers in the mopping and felt-laying operation. Alternatively, the asphalt may be unloaded directly into mechanical asphalt spreaders or mechanical felt-laying machines, which can be used to lay down the felt and apply the interply layers of asphalt. In all cases, the asphalt is drawn off from the lugger through a spigot or valve and is top-loaded into the receiving vessel. Mechanical felt-laying machines (see Figure 3–6) typically have lids that can be closed once the vessel is full, but mop carts and simple mechanical spreaders do not.

Manual installations are done with hand-held mops in a procedure that is much like mopping a floor. The carts or buckets that hold the hot asphalt are open at the top because the mop is continually dipped into the container. Mechanical asphalt spreaders, such as felt layers, have closeable lids because there is no need to enter the container to remove the asphalt. The hot asphalt is dispensed onto the substrate through a series of valves on the bottom of the machine.