Wet spray concrete is an important initial support method in new Austrian tunneling method construction. However, the production of harmful substances, such as dust and chloride, during construction remains a serious threat to the health of workers. At present, much research has been on the sources of dust and dust control measures in wet spray technology, whereas scarce research was on dust generation in wet spray concrete until 2023, and there is insufficient understanding of the dust generation mechanism.

In this study, concrete with different flowabilities was prepared by changing the polycarboxylate superplasticizer (PCE) dosage in the wet spray concrete mixture. A self-constructed multidimensional test platform for jet flow was used to perform flowability and spray tests on fresh concrete with different flowabilities. This work coupled the theories of jet fragmentation and surface oscillation to examine the jet fragmentation process and dust production characteristics of wet spray concrete, investigating its dust generation mechanism. The flowability tests characterized the concrete's flowability using slump and plastic viscosity. The jet tests primarily included capturing wet spray concrete jets using a high-speed camera and computer vision tools, such as OpenCV, to process the images, including grayscale conversion, noise reduction, sharpening, binarization, morphological operations, and contour extraction. The spread angle of the jet was measured to evaluate its coherence. In addition, MATLAB was used to examine the size of concrete droplets splashed on the side plate lightbox in the experimental chamber to calculate dust concentration and particle size distribution during the wet spraying process, which was compared with the results of a laser dust analyzer.

According to the jet states, the jet flow cycle was classified into primary, intense, and ending stages. The initial and ending stages did not form stable jets, whereas the intense stage demonstrated less variation in jet morphology and high levels of jet fragmentation and atomization, considerably impacting the dust concentration. For the initial and ending stages, the PCE dosages of 1.00%, 1.10%, 1.15%, and 1.20% led to relatively stable jets with minimal fragmentation. The PCE dosages of 1.25%, 1.30%, and 1.35% mainly had primary fragmentation, and a PCE dosage of 1.40% had secondary fragmentation. In the intense stage, the PCE dosages of 1.00%, 1.10%, and 1.15% mainly showed primary fragmentation, whereas the PCE dosages of 1.20%, 1.25%, 1.30%, 1.35%, and 1.40% primarily exhibited secondary fragmentation. The average dust concentration increased by 138% in the secondary fragmentation stage compared with the primary fragmentation stage. The particle size distribution of the dust decreased and then increased with increasing PCE dosage, with the smallest dispersion observed at 1.15% PCE (10.40-19.80 μm). As the PCE dosage increased, the frequency of small particle dust decreased, and the distribution of dust frequency approached uniformity while the particle size dispersion increased. The concrete mixture with the lowest dust concentration and most concentrated dust particle size distribution was identified as cement, water, fine aggregate, coarse aggregate, PCE (1.000 0∶0.370 0∶1.837 5∶1.562 5∶0.011 5). This work examined the mechanism of concrete jet fragmentation and dust production. Intense aerodynamic effects resulting from substantial velocity differences at the nozzle produced surface waves as the jet moved further, resulting in the detachment of surface concrete droplets and causing primary or secondary fragmentation and dust formation. At higher PCE dosages, a portion of free water adhered to the surface of the mixed material, producing a lubrication layer between the filled mixture and the delivery pipe during pumping. At the nozzle, the concrete aggregates and lubrication layer underwent primary and secondary fragmentation, producing small liquid droplets that collided and bonded to form dust.

This work investigated the dust generation mechanism and characteristics of wet spray concrete jets, classified the jet spray stage, and explored the mechanism of concrete jet fragmentation and dust generation. This offers valuable insights and references to dust reduction work in wet spraying; however, exploring the microlevel aspects of jet fragmentation in concrete with different flowabilities warrants further investigation, with several questions remaining to be answered in the future.