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A recent study of ready-mixed concrete facilities found that a combination of practices may reduce drivers' exposure to silica when they chip out the inside of a mixer drum.From 1997 to 1999, five member companies of The Illinois Ready Mixed Concrete Association (IRMCA) cooperated in an Onsite Consultation Program in response to OSHA's special emphasis on silica. The program, administered by the Illinois Department of Commerce & Community Affairs, evaluated material handlers, drivers, and batch loaders and found that only drivers-when chipping out hardened concrete from a truck mixer drum-were exposed to silica levels exceeding the OSHA permissible exposure limits (PELs).

Of several practices tested, the one found to most consistently reduce drivers' exposures to silica below the PEL (permissible exposure limits) was using a modified pneumatic chipper with a water spray nozzle attached to the chipper. Use of the water spray nozzle on the chipper reduced employee exposure to total dust with silica by 85% and reduced exposure to respirable dust with silica by 70%. The study also found that using the modified chipper reduced calcium hydroxide exposure.

Researchers determined nine ways to minimize employee exposures to silica during drum chipping:

  • Open hatch
  • Horizontal box fan in the hopper
  • High speed fan exhausting out
  • Chipping hammer with water spray nozzle
  • Entire inner drum spray
  • Water spray aimed at chisel point
  • Spray whenever chipper is in operation
  • Respray when dry
  • Reduce interval between chipping procedures to avoid buildup
In particular, using a pneumatic chipper with an attached water spray nozzle while cleaning the drum has been found to keep employee exposure to air contaminants below the OSHA PEL and eliminate required use of respiratory protection.

Article also shows how to modify a chipper with easy-to-find parts. It also includes information on calcium hydroxide exposure during the chipping process.