Understanding aerosol pollutant generation in wet-mix shotcrete for tunnel construction

Wet-mix shotcrete (WMS), widely used in tunnel construction, is a major aerosol pollutant source. Aerosol pollutants pose health risks and negatively impact operations, increasing maintenance costs and construction delays. However, understanding aerosol generation and mechanisms during WMS remains inadequate. This study focuses on two key factors: concrete slump and air pressure, which directly influence the jet flow characteristics. Through detailed observation of jet formation, the breakup and atomization patterns of the jet, as well as the size and the mass concentration of the resulting aerosol pollutants, were analyzed. A lubricating layer was formed near the pipe wall during effective pulsating flow. This layer undergoes fragmentation during the primary breakup stage, followed by atomization under air shear force at the secondary breakup stage. As concrete slump and air pressure increase, the jet pattern transitions from a non-effective slug flow to a primary breakup dominated flow, eventually to a secondary breakup dominated flow. The spread angle and fragment ratio of the jet exhibited a positive correlation with the concrete slump and air pressure. Notably, the main generation of aerosol pollutants occurred at the secondary breakup stage. The total and respiration concentration of aerosol pollutants in the secondary breakup dominated flow were 4.5 % and 4 % higher, respectively, than in the primary breakup dominated flow. This study provides valuable insights into the aerosol generation during WMS, shedding light on the underlying mechanisms and facilitating a better understanding and management of the associated risks.