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How the structural design of the nozzle on a wet-mix shotcrete carriage affects concrete spraying quality
Release time:
2026-11-20
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Summary:
The nozzle of a wet-mix shotcrete carriage is the actuating unit where concrete, accelerating agent, and compressed air are finally mixed to form the jet stream. The internal structural design of the nozzle—including the mixer type, nozzle geometry, and air‑ring configuration—directly determines the uniformity, density, and rebound rate of the jet stream.
I. Mixed Structure: Determining the Uniformity of Accelerator Distribution
Whether the accelerating agent can be evenly dispersed in concrete is essential for ensuring consistent setting time and early strength in sprayed layers. Uneven mixing can create weak zones with inconsistent setting times within the sprayed layer, compromising the overall integrity of the support. To address this, multi-stage mixing structures are typically incorporated inside the spray nozzle. A typical solution combines a primary mixer block with a secondary mixer housing: after concrete enters the gun base via the delivery hose, it makes preliminary contact with compressed air through inclined ports arranged circumferentially along the base’s side wall, then proceeds to the secondary mixer for further blending, ensuring more thorough adhesion to the working surface. Another design features hollow helical blades within the feed tube; the accelerating agent flows through the liquid supply line into the blade cavity and is uniformly discharged through outlet holes on the blade surface, achieving dispersed addition as the concrete flows and preventing localized concentration of the agent. Additionally, mixing devices employing vortex principles can generate swirling flow of concrete and accelerating agent inside the nozzle, increasing the contact area, enhancing mixing uniformity, and reducing dust and rebound.
II. Nozzle Geometric Parameters: Determine the Shape and Impact Energy of the Material Jet
The nozzle inner diameter, length-to-diameter ratio, and converging section contraction angle are three key geometric parameters that collectively influence the velocity, concentration, and impact force of the material jet.
The nozzle inner diameter must match the maximum aggregate size of the concrete. Empirical guidelines recommend that the minimum nozzle inner diameter be 2.5 to 3.5 times the maximum aggregate size. If the inner diameter is too large, the jet velocity at the outlet is low, and the kinetic energy is insufficient to penetrate rebar gaps and effectively compact the sprayed layer, resulting in a loose structure and excessive rebound. If the inner diameter is too small, although the outlet velocity can be increased, the rebound loss when coarse aggregate impacts the target surface rises, and pipe blockages are more likely to occur.
The length-to-diameter ratio (the ratio of nozzle length to inner diameter) determines the degree of divergence of the material jet after it exits the nozzle. When this ratio is low, the jet rapidly expands into a conical flow, reducing the impact force per unit area and resulting in poor layer uniformity. When the ratio falls within the range of 1.5 to 3.0, the jet maintains a well-defined cylindrical shape with concentrated energy, which promotes higher densification and improved interlayer bonding.
The converging section’s contraction angle affects both the acceleration performance and wear within the nozzle. When the contraction angle is less than 3°, its impact on particle jetting is limited; as the angle increases, jet velocity decreases and wear on the nozzle inner wall intensifies. When the angle exceeds 6°, exit velocity drops significantly, and the nozzle faces a risk of blockage. The study indicates that a contraction angle of 6° yields optimal jetting quality.
III. Air Ring Structure and Airflow Matching: Factors Affecting the Stability of the Material Bundle
The air ring’s function is to deliver compressed air uniformly into the nozzle, enveloping and further accelerating the concrete jet. If the airflow from the air ring is uneven, the jet will pulsate or deflect, directly compromising the smoothness of the sprayed surface. Air volume must be matched to material output: insufficient air leads to inadequate cohesion and acceleration of the jet, causing it to disperse; excessive air causes premature atomization before the jet reaches the target, increasing rebound of fine aggregates and potentially blowing off freshly applied concrete that has already adhered.
IV. Nozzle Wear: Cumulative Effects of Structural Deterioration on Spray Quality
Under the erosive action of a high‑velocity slurry containing aggregates, the nozzle gradually wears at its outlet, causing the diameter to increase and the shape to become irregular. Consequently, the divergence angle of the jet increases, the effective contact area between the material stream and the target surface decreases, some particles fail to bond effectively, and the rebound rate rises. Therefore, nozzles are typically made from highly wear‑resistant materials such as tungsten carbide or ceramics and are treated as consumable components that require regular inspection and replacement. Continuing to use a worn nozzle not only degrades spraying quality but also increases material consumption.
In summary, the design of the nozzle structure influences spray quality through three pathways: mixing uniformity, jet formation, and long-term stability. The mixing structure ensures uniform dispersion of the accelerating agent; geometric parameters determine the impact energy and morphology of the jet; wind‑ring matching maintains spraying stability; and wear‑resistant design slows performance degradation. Only through the coordinated interplay of these four elements can a sprayed layer with high density, low rebound, and uniform setting be achieved.
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