ASA has closely followed the development of the new OSHA rules for a couple of years before they were put into effect. You can find several articles in Shotcrete magazine that specifically address the rules in consideration of shotcrete application. In the Summer 2016 issue, an article, “OSHA’s New Crystalline Silica Rule–Potential Impact on Shotcrete Operations,” addressed many of the concerns. Unfortunately, with the wide variety of shooting conditions, there are no generic values. The reason for this is that the levels can vary significantly due to a variety of factors, including:

  1. The materials used—this includes comparing wet-mix to dry-mix and the variations in variability of concrete mixture design ingredients (for example, silica fume, fly ash, and accelerator).
  2. Equipment
    1. Dry-mix gun type (rotary or chamber), using a predampener or not (type of wet-mix pump likely doesn’t make much difference)
    2. Size of air compressor (more air might result in more dust)
    3. Delivery line and hose (1.5 in. [40 mm] hose versus 2 in. [50 mm]) can change volume of flow, and then level of acceleration and nozzle stream dispersion as a function of air volume)
    4. Nozzle type can significantly affect the material stream
  3. Shooting location (inside or enclosed, or open air)

With so many variables it is difficult, if not impossible, to get any reliable “generic” number for shotcrete as a whole. Many of our shotcrete contractors are using air quality consulting firms or testing labs who have the monitoring equipment. You may want to note that silica fume is amorphous silica, not crystalline, so it is not hazardous. Most exposure to crystalline silica is through sawing, cutting, or grinding of hardened concrete. We expect that most shotcrete contractors will need to establish a reliable, accurate level by on-site testing because shotcrete is not directly covered in Table 1 of the OSHA rule.