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Analysis of the Correlation Model Between Spraying Quality and Nozzle Parameters
Release time:
2026-02-02
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Summary:
In the wet spraying process for concrete, the nozzle serves as the final execution unit, and there exists a direct and systematic causal relationship between its parameter settings and the quality of the sprayed concrete (i.e., the spray quality). Understanding and establishing a correlation model between these two factors is of paramount importance for achieving precise construction, optimizing spraying performance, and controlling material costs. This model aims to elucidate how key nozzle parameters—by influencing the shape of the material jet and the mixing process—ultimately determine the compactness, smoothness, strength, and rebound rate of the sprayed concrete.
I. Analysis of Core Associated Parameters
Nozzle parameters are broadly categorized into two major types: geometric parameters and operating condition parameters. These two types of parameters work together to influence the spraying process.
1. Nozzle diameter (D)
Associated model: The nozzle diameter is the key geometric parameter that controls the cross-sectional area of the material flow and the exit velocity. Its relationship with the injection mass is not linear.
Effect on jet velocity: Under the condition of constant system flow rate, the jet exit velocity is inversely proportional to the square of the nozzle diameter (V ∝ Q/D²). As the diameter decreases, the velocity increases sharply.
Impact on spray coating quality:
Too low a speed (D is too large): The material stream lacks sufficient kinetic energy, making it difficult to penetrate the gaps between the reinforcing bars. As a result, the compaction effect is poor, leading to a loose concrete structure with low strength and a high rebound rate.
Excessive speed (D too small): Although the material stream is concentrated, high-speed aggregate impacts on the sprayed surface can cause excessive rebound, thereby increasing rebound losses—this effect is particularly pronounced for coarse aggregates. Meanwhile, a diameter that is too small can easily lead to pipeline blockages.
Selection criteria: The optimal nozzle diameter must match the maximum aggregate size (d_max) of the concrete. An empirical formula is **D ≥ (2.5 ~ 3.5) × d_max**, and the system’s rated flow rate must also be taken into account.
2. Nozzle length-to-diameter ratio (L/D)
The aspect ratio: the ratio of nozzle length (L) to diameter (D), determines the cohesiveness and stability of the material jet.
Effect on the shape of the material jet: A longer nozzle (with a large L/D ratio) provides more thorough flow rectification within its internal passage, resulting in a stable material jet that is more focused, exhibits minimal divergence, and has a longer core section.
Impact on spray coating quality:
Too small an L/D ratio: After exiting the nozzle, the material stream rapidly spreads, forming an umbrella-like flow with an excessively large cone angle. This results in a large impact area but low impact force per unit area, leading to a loose spray layer with poor surface smoothness and severe rebound.
Moderate L/D ratio: Forms a cylindrical material bundle with clear boundaries, concentrating energy effectively to impact and penetrate the previously sprayed concrete, achieving high density and excellent interlayer adhesion with the lowest rebound rate.
Scope of practice: For wet spraying processes, the optimized length-to-diameter ratio is typically set between **1.5 and 3.0**.
3. Air ring structure and air volume (Q_air)
Associated Model: The annular air ring surrounding the nozzle and the compressed air it supplies serve as the second power source for shaping and accelerating the material stream.
Mechanism of action: Compressed air, guided uniformly through the air ring, “wraps” and provides secondary acceleration to the concrete jet ejected from the nozzle’s center.
Impact on spray coating quality:
Insufficient air volume and pressure: The material bundle fails to aggregate and accelerate adequately, resulting in a dispersed shape and weak impact force—consequences similar to those caused by an excessively large nozzle diameter.
Excessive airflow and pressure can excessively impact the material stream, causing it to atomize prematurely before reaching the surface to be sprayed. This also increases the rebound of fine aggregates and may even blow away freshly applied concrete that has already adhered.
The wind ring structure is improperly designed, resulting in uneven airflow and causing the material bundle to pulsate or rotate, which severely affects the smoothness of the spray surface.
Key control points: The air volume must be matched to the material output rate and capable of precise, stepless or stepped adjustment to accommodate different spray distances and positions.
4. Point of Addition and Mixing for Accelerators
Associated Model: The addition location of the accelerating agent and the uniformity of its mixing with concrete directly determine the setting speed and early strength development of the concrete.
Impact on spray coating quality:
Uneven mixing: This leads to inconsistent setting times within the concrete, creating zones of reduced strength and compromising the overall integrity and long-term stability of the support structure.
Nozzle addition (at the spray nozzle): The advantage is that the pipeline is less likely to become clogged. The disadvantage is that the mixing time is extremely short, resulting in relatively poor uniformity and potentially affecting the consistency of strength.
Onboard addition (device side): The advantage is a longer and more uniform mixing stroke, but it places high demands on pipeline maintenance and carries the risk of pipe blockage.
Optimization direction: Integrating a highly efficient **static mixer** into the nozzle—where multiple spiral blades force the materials to undergo cutting and intersection—is a key technology for enhancing the uniformity of mixing in the gun-head addition method.
II. Integrated Correlation Model and Construction Optimization
The above-mentioned parameters do not act independently; rather, they constitute an interconnected system. A simplified integrated correlation model can be expressed as:
High-quality shotcrete quality (low rebound, high density, high smoothness) = f(optimized D, optimized L/D, optimized Q_air, efficient mixing)
Among these, each “optimization” means that the parameter is set within the optimal range that matches the concrete mix proportion, aggregate characteristics, spraying distance, and the position of the sprayed surface (top arch, sidewalls).
Construction Optimization Path:
1. Parametric Trial Spraying: Before formal construction begins, conduct parametric trial spraying tailored to a specific concrete mix design. While keeping other variables constant, systematically adjust one parameter (such as changing nozzles of different diameters), record the rebound rate, and observe the quality of the sprayed surface to identify the optimal combination.
2. Establish a parameter database: Record the optimal nozzle parameter combinations that have been verified for different engineering conditions, creating standardized operating instructions for reference in subsequent projects of the same type.
3. Dynamic Fine-Tuning: During the operation, the operator should make real-time fine adjustments to adjustable parameters such as air volume, based on the spray position (the top arch requires a thicker material stream, while the sidewalls can tolerate a more fluid stream) and the observed results.
Conclusion
There exists a clear and predictable physical correlation model between nozzle parameters and shotcrete quality. Elevating nozzle selection and adjustment from an “art based on experience” to a “parameterized science” is an inevitable trend in the development of wet-mix shotcrete technology. By gaining a deep understanding of how nozzle diameter, length-to-diameter ratio, air-ring system, and rapid-setting admixture mixing mechanisms influence the final shaping quality, construction personnel can proactively optimize their operations rather than passively addressing quality issues, thereby achieving comprehensive breakthroughs in efficiency, quality, and cost control in shotcrete construction.
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