Results of a pilot study of dust control technology for asphalt milling at Payne and Dolan, Inc. US Route 12, Project, Wisconsin, report no. CT-282-11b.
The National Institute for Occupational Safety and Health (NIOSH) is located in the Centers for Disease Control and Prevention (CDC), part of the Department of Health and Human Services (DHHS). NIOSH was established in 1970 by the Occupational Safety and Health Act, at the same time that the Occupational Safety and Health Administration (OSHA) was established in the Department of Labor (DOL). The OSH Act legislation mandated NIOSH to conduct research and education programs separate from the standard-setting and enforcement functions conducted by OSHA. An important area of NIOSH research deals with methods for controlling occupational exposure to potential chemical and physical hazards. The Engineering and Physical Hazards Branch (EPHB) of the Division of Applied Research and Technology (DART) has been given the lead within NIOSH to study and develop engineering controls and assess their impact on reducing occupational illness. Since 1976, EPHB (and its predecessor, the Engineering Control Technology Branch) has conducted a large number of studies to evaluate engineering control technology based upon industry, process, or control technique. The objective of each of these studies has been to evaluate and document control techniques and to determine their effectiveness in reducing potential health hazards in an industry or for a specific process. The primary aim of this project is to determine if the engineering controls supplied with new milling machines and operated according to the manufacturers' recommendations are adequate to control worker exposures to respirable dust and respirable crystalline silica (in the form of quartz). The long term goal of this project is to reduce worker exposures to silica by providing data to support the development of a set of best practice guidelines for the equipment if the engineering controls are adequate, or to develop a set of recommendations to improve the performance of controls if they are not adequate. Many construction tasks have been associated with overexposure to crystalline silica. Among these tasks are tuck pointing, concrete sawing, concrete grinding, and abrasive blasting. Road milling has also been shown to result in overexposures to respirable crystalline silica. However, the road-milling studies are limited because they do not provide enough information about the operating parameters and engineering controls present on the milling machines to determine if the overexposures were due to a lack of effective controls or poor work practices. This study will attempt to fill that knowledge gap. A variety of machinery and work practices are employed in asphalt pavement recycling, including cold-planers, heater planers, cold-millers, and heater-scarifiers. Cold-milling, which uses a toothed, rotating drum to grind and remove the pavement to be recycled, is primarily used to remove surface deterioration on both asphalt and Portland cement concrete road surfaces. The milling machines used in cold milling are the focus of this investigation. The cold milling work observed during this pilot study was part of the US 12 reconstruction project from CTH KP to STH 19 west in Wisconsin. This was a section of old US 12 that was rehabilitated and will be transferred to the local township. There was a 20 ft. wide concrete pavement placed in the early 1900's. Above that was a layer of crushed aggregate base course of varying thickness; about 0 inches to 10 inches. Above that was varying asphalt pavement thickness of about 8 to more than 12 inches which had been placed at various times in about the last 60 years. The asphaltic pavement was rutted and cracked to varying degrees. The portion of old US 12 from Simpson Road to the south where it meets the new US 12 was re-graded on new alignment; this pavement removal was specified by a Common Excavation item. The Common Excavation item specification enables the contractor to remove the pavement in any way they see fit. In this case, the contractor chose to mill the pavement because it was of value as a recycled product. The existing concrete was also removed and disposed of. In the area from Simpson to the north, this section was not re-graded, but the entire existing asphaltic pavement was removed under the item of Removing Asphaltic Surface. This specifies the removal of only the asphaltic pavement. Again, the contractor chose to salvage this pavement and recycle it. This study was facilitated by a partnership in cooperation with the National Asphalt Pavement Association that includes milling machine manufacturers, contractors, employee representatives, NIOSH, and other interested parties. One of the milling machine manufacturers, Manufacturer A, had initially arranged to perform the pilot study on a 2003 milling machine model with the latest "state-of-the-art" water spraying system produced by the manufacturer. The mill is a half-lane milling machine rated at 560HP with a water spray system capable of 15gpm at 200psi. This system was put into production in 2001 - before initial discussions on testing airborne dust concentration. About one week before the pilot study was to be performed, Manufacturer A learned that the 2003 mill would not be available for the test. Instead, an older machine was substituted. The replacement machine was a half-lane milling machine rated at 800HP with an eight-foot cutter and an older model water system designed for 10gpm at 50psi. Manufacturer A knew that the lower water flow at much lower pressure would result in higher airborne dust content, but Manufacturer A was still confident that the dust suppression system, even on the older model machines, should still be very effective if properly maintained and operated. Therefore, Manufacturer A agreed to perform the test (even with the last minute changes) as a means of getting at least a good baseline for this type of airborne dust test. A pilot study was performed to evaluate the effectiveness of water spray controls for a cold milling machine. The objective of this study was to quantify the exposure reduction that could be achieved through the use of higher flow water-spray nozzles during pavement milling. The effectiveness of the dust controls examined in this study was evaluated by measuring the reduction in the respirable dust and respirable quartz exposures in personal and area samples collected during a typical milling job. Use of the higher flow nozzles resulted in reductions in respirable dust and respirable quartz exposures, but the differences were not statistically significant. During this study, the higher flow nozzles were only installed on the cutter drum and the cutter drum extension.
National Institute for Occupational Safety and Health, Division of Applied Research and Technology, Engineering and Physical Hazards Branch, Mail Stop R-5, 4676 Columbia Parkway, Cincinnati, OH 45226-1998