REPORT

This report is an archived publication and may contain dated technical, contact, and link information

Top
< Prev
Main
9
10
11
12
13
14
15
16
17
18
Next >
>>

Publication Number: FHWA-HRT-14-021 Date: January 2014

Publication Number: FHWA-HRT-14-021
Date: January 2014

Screening Level Assessment of Arsenic and Lead Concentrations in Glass Beads Used in Pavement Markings

Background of Bead Workflow

Table 17 identifies the different stages and potential points of release of arsenic and lead within glass bead workflow. Use of glass beads as a reflective material in roadway line markings is decades old. Glass beads used in pavement markings are applied to the roadway on top of a base material that is a paint, epoxy, or thermoplastic. The beads are dropped either by a painting machine (specially designed truck or hand-cart) or by hand spreading. The bead sizes range from a fine table salt crystal up to a small ball bearing. Bead application rates, which are on the order of 192 lb/mi for a single 6-inch-wide line, result in a significant amount of beads that are not bound to the base material and are released to the environment. Additional releases of beads from markings may occur through wear due to traffic abrasion and weather, as well as during removal of the markings, such as when preparing a roadway for re-marking.

Portions of the workflow associated with the glass beads are presented in table 17, which describes each of the steps where beads are used and potentially released to the environment. The basic elements of the bead workflow include the following:

· Manufacturing.

· Transportation.

· Storage/transfer.

· Application.

· Wear/abrasion.

· Removal/disposal.

Beads are typically manufactured by melting crushed recycled glass cullet (large glass blocks) in an up-flow furnace to form the spherical glass particles. The formed particles are then passed through sieves of various sizes to isolate beads of desired size fractions that are mixed in the product formulation. Once sieved, the glass beads may be coated for various purposes (e.g., to prevent moisture accumulation, to achieve better paint adherence). The beads are then packed in large super-sacks (2,000-lb bags), smaller 50-lb bags, or in customer-specified packaging.

Table 17. Bead workflow elements and release potential.

Bead Workflow Element	Description of Release	Release Potential	Quantity Released
1.Beads arrive palletized and packaged to prevent moisture contamination	Rupture of a 2,000-lb bag possible. Majority of contents would be recovered.	Low (localized to storage yard)	< 10 lb/day
a. Where moisture contamination has occurred, workers may use hands/tools to break apart the clumps of beads resulting in direct contact.	Minimal loss while receiving/recovering/ transferring beads	Low (localized to storage yard)	< 10 lb/day
2.Bead bags are opened and used in one of two ways:
a. Vacuum beads into painting vehicle storage containers. Worker exposure to beads may occur while manipulating the vacuum nozzle or opening/resealing the bead bags. Where vacuum transfer is not functional, bead bags may be cut and contents dumped into vehicle storage bins, resulting in potential worker contact with beads. Beads may also be spilled into the yard, increasing the potential for exposure.	Where a vacuum transfer is used, potential for release is limited to the opening and closing of bead bags. Dumping of beads from bags has greater potential for release than does vacuum transfer.	Moderate (localized to storage yard)	< 25 lb/day
b.Pour beads into a bucket for hand application. Beads may be contacted during transfer and may be spilled, requiring handling during pickup.	Minimal loss while recovering/transferring beads	Low (localized to storage yard)	< 10 lb/day
3.Vehicle application of beads may require workers to manipulate nozzles, resulting in bead contact. Excess beads (up to 15 percent) are released to the environment.	Excess beads applied to lines are released to the environment. For an application requiring six lines, a rate of 192 lb applied per 6-inch-wide line per mile results in a total of 1,150 lb/mi. For a loss rate of 15 percent of applied material, a total of 175 lb are released per mile of roadway, and assumed to be distributed 6 ft on either side of the roadway. In general, roads are re-marked every 2 to 5 years.	Moderate (large quantity, widely dispersed), most significant where roadside curbs have the potential to concentrate beads	Up to 175 lb/mi
4. Application of beads by hand (without gloves) results in direct contact with beads. Excess beads are released to the environment.	Loss rate is greater than machine application (up to 25 percent), with a bead usage of approximately 210 lb/mi. Typical applications require 300 ft of marking in a small residential neighborhood (perhaps near a school zone with many crosswalks) per year.	Moderate (small quantity, potentially localized), most significant where curbing concentrates beads	< 0.01 lb/ft
5.Worker inspecting applied lines may contact the surface and dislodge beads.	Worker contact with the lines may result in minimal bead loss.	Small (very small amount, widely dispersed)	< 1 lb/day
6.Line abrasive wear and weathering	During lifetime of line, beads are slowly released from the line as a result of abrasive wear and weathering. Assume that 85 percent of the original load (1,150 lb/mi for six 6-inch lines) adhered to the line. Further assume that 25 percent of the adhered beads are released each year due to traffic wear and weathering.	Moderate (large quantity, widely dispersed, long duration release), most significant where curbing concentrates beads	Up to 250 lb/mi per year
7.Line removal of beads and substrate may include use of hand tools or vehicle-mounted equipment. When vacuum is not used to remove material, workers may contact beads. When vacuum is used, bead contact in the field is limited; however, it may occur when removing material from storage tank. Removal methods include mechanical grinding, hydraulics, etc.	Assume that lines are repainted when remaining retroreflectivity is 25 percent of original value, equivalent to retention of 25 percent of adhered beads, which is approximated as 250 lb/mi for a roadway section with six 6-inch lines. Releases may occur when vacuum removal of debris is not used, or when removed material is incorrectly stored in the marking company storage yard.	Moderate (Potential is moderate in the field because vacuum trucks capture nearly all debris. Vacuumed material may be released in the storage yard. Mitigation measures are highly variable in practice.)	Up to 250 lb/mi

The packed beads are shipped by truck, air, rail, and train to the end users who are the pavement-marking agencies (either public entities or private corporations). Beads are completely covered during transport to prevent contamination of the beads by moisture. Although the beads may be shipped a significant distance, there are few recorded incidents of spills due to crashes.

The pavement-marking agency, either a state/local entity or a private company, receives the beads and may store the packed beads in any number of ways. The storage areas may be covered with tarps or open air (with paved or bare ground). The storage yards may or may not include catch-basins to collect any material runoff. The beads are well protected from the elements by the shipment packaging because they tend to form clumps in the presence of moisture, leading to problems when used in the pavement-marking vehicles.

The packaged beads are taken from storage and opened for use in pavement-marking applications. Beads in super-sacks are transferred to specially designed vehicles with internal bead holding tanks that are isolated from the environment. Their transfer is generally accomplished with a vacuum attachment incorporated into the pavement-marking vehicle, but occasionally the super-sacks are hoisted above the vehicle and gravity fed into the storage tank. The super-sacks may also be transported from the storage yard into the field and staged for later use. Smaller bead bags are opened and poured into buckets for jobs requiring hand application of beads.

In the field, beads are applied to the pavement markings either by vehicle or by hand. Pavement-marking vehicles apply paint, thermoplastic, or epoxy substrate from one set of nozzles, followed by a stream of beads through other sets of nozzles. The nozzles are designed to result in beads that are buried halfway in the substrate for the best combination of adhesion and retroreflectivity, a measure of the line visibility at night. To achieve uniform bead coverage on the substrate, excess beads are applied, resulting in some bead loss along the edges of the marking. For smaller applications (e.g., small intersections, crosswalks), beads may be thrown onto the substrate by hand or, in some cases, using small fertilizer spreaders.

A portion of the beads that adhere to the substrate are slowly dislodged over a period of time owing to traffic wear and weathering until a fraction of the original number of beads are left and re‑marking is required. For highways, re-marking occurs every 2 to 5 years on average, whereas the frequency is approximately 3 years for urban areas. The markings are generally re-applied when 25 percent of the original retroreflectivity remains on the roadway. When re-marking is needed, the old marking material is either covered by new markings or is removed from the roadway before laying the new markings.

Removing the remnants of old roadway markings generally involves mechanical, hydraulic, or chemical methods. Specially designed vehicles use an attachment that includes a hydraulic jet and vacuum system to remove the marking material and collect the waste. The slurry is contained within a holding tank on the vehicle for disposal, and very little material is lost to the environment. Where systems are used without vacuum recovery, the old marking material is generally left along the roadside.

Each of these steps in the bead workflow and the potential for exposure are discussed in greater detail in the following subsections.