1. Introduction
Shotcrete application is not a simple placement activity. It is a high-energy material interaction process involving impact physics, material deformation, and rapid chemical reaction.
At the moment of spraying, concrete is transformed from a pumpable suspension into a compacted structural lining within fractions of a second.
This transformation occurs entirely at the face and is governed by nozzle technique.
For this reason, the nozzle operator has more influence on final lining quality than any other individual involved in the process.
EFNARC identifies execution of spraying as a primary determinant of sprayed concrete performance.
2. Impact mechanics of sprayed concrete
When concrete exits the nozzle, it is travelling at high velocity due to compressed air acceleration.
Upon impact with the substrate, several processes occur simultaneously:
• Deceleration of aggregate particles
• Plastic deformation of cement paste
• Mechanical interlock with surface irregularities
• Expulsion of entrapped air
This rapid deceleration creates compaction without vibration.
If impact velocity is insufficient, particles rebound or fall away. If velocity is excessive or misdirected, particles ricochet rather than embed.
The objective of spraying is therefore controlled impact, not maximum force.
3. Role of compressed air in projection energy
Compressed air determines projection velocity.
Air volume influences:
• Compaction efficiency
• Adhesion to surface
• Rebound behaviour
• Surface finish
Low air volume results in sluggish discharge, leading to sagging and fall-out.
Excessive air volume increases particle scattering and rebound.
Optimal air settings produce a dense, cohesive spray stream that strikes the surface without excessive dispersion.
Air adjustment is therefore a continuous process performed during spraying rather than a fixed pre-set condition.
4. Nozzle angle and force direction
The direction of impact is critical.
When the nozzle is oriented perpendicular to the surface, the impact force acts normal to the substrate. This allows aggregate particles to penetrate the paste layer and become embedded.
As the nozzle angle deviates from perpendicular, tangential forces increase. These forces cause particles to slide or ricochet rather than embed.
Even small deviations significantly increase rebound, particularly of coarse aggregate and fibres.
EFNARC requires nozzle angles to remain as close as practicable to 90 degrees throughout spraying.
5. Stand-off distance and velocity decay
Stand-off distance refers to the distance between the nozzle tip and the spraying surface.
As the spray stream travels through air, velocity decays rapidly due to turbulence and drag.
If the nozzle is held too far from the surface:
• Impact energy decreases
• Compaction quality reduces
• Rebound increases
• Layer build-up becomes unstable
If held too close:
• Material sloughing occurs
• Surface becomes uneven
• Air cannot dissipate properly
EFNARC recommends a stand-off distance typically between 1.5 and 2.0 metres, depending on mix behaviour and equipment configuration.
6. Spraying pattern and material distribution
A controlled spraying pattern ensures uniform thickness and compaction.
Random or erratic movement produces variable impact angles and inconsistent layer development.
Effective spraying patterns are characterised by:
• Continuous motion
• Overlapping passes
• Controlled build-up
• Uniform coverage
Circular or elliptical patterns are commonly used to maintain even material distribution.
The objective is to allow each pass to support the next without overloading the fresh material.
7. Bottom-up application principle
Shotcrete should always be applied starting at the bottom and progressing upward.
This approach allows freshly applied material to support subsequent layers.
If spraying begins at the crown:
• Material falls onto lower surfaces
• Rebound accumulates
• Adhesion quality decreases
Bottom-up spraying also allows visual control of thickness development and surface response.
EFNARC identifies this principle as fundamental to stable layer build-up.
8. Layer thickness and build-up behaviour
Shotcrete thickness must be developed progressively.
Each layer provides support for the next while allowing partial setting to occur.
Typical layer thickness is approximately 50 millimetres, though this depends on:
• Accelerator performance
• Mix cohesion
• Ambient temperature
• Surface orientation
Attempting to apply excessive thickness in a single pass leads to sagging, sloughing, and internal void formation.
Layered application improves density, adhesion, and structural integrity.
9. Interaction between rebound and thickness
Rebound does not occur uniformly.
It is highest during initial contact with the substrate and reduces as the surface becomes coated.
If rebound material accumulates on the surface and is re-sprayed into the lining, it creates weak zones with reduced cement content.
For this reason, rebound must be allowed to fall clear and must never be incorporated back into the lining.
EFNARC prohibits re-spraying rebound material due to its detrimental effect on strength and durability.
10. Fibre behaviour during application
Fibre-reinforced shotcrete exhibits distinct spraying behaviour.
Fibres must align within the cement matrix and become anchored upon impact.
Incorrect nozzle angle increases fibre rebound, particularly for steel fibres.
Adequate paste volume and correct impact direction are required to embed fibres effectively.
Uneven fibre distribution reduces energy absorption capacity and compromises post-crack behaviour.
11. Visual indicators of correct spraying
Experienced operators rely on visual feedback.
Indicators of correct application include:
• Dense, cohesive surface appearance
• Minimal sloughing
• Controlled rebound fall-off
• Immediate surface stiffening
• Uniform texture
Indicators of poor application include:
• Excessive material fall-out
• Dry appearance on rock
• Uneven surface
• Visible aggregate bounce
These observations guide continuous adjustment during spraying.
12. Surface finish and compaction quality
Shotcrete surface finish reflects internal compaction.
A smooth, dense surface generally indicates good compaction.
Rough, porous, or honeycombed textures indicate insufficient impact energy or poor mix cohesion.
Surface finish is therefore an indirect quality indicator but must be supported by testing.
13. Influence of temperature and environment
Temperature affects both material behaviour and setting response.
Low temperatures slow hydration and reduce accelerator effectiveness.
High temperatures accelerate setting and may cause premature stiffening in hoses or at the nozzle.
Environmental conditions must therefore be considered when adjusting spraying parameters.
14. Role of the assistant and support crew
Effective spraying requires coordinated teamwork.
The assistant supports hose movement, monitors thickness through probing, and assists the operator in maintaining consistent application.
Shotcrete quality is rarely achieved by the nozzle operator alone.
15. Application as an engineered operation
Shotcrete application combines material science, mechanical energy, and human control.
It cannot be automated fully and cannot be corrected afterward.
For this reason, professional spraying relies on trained operators applying defined principles under engineering supervision.
EFNARC regards proper application technique as essential to achieving the designed performance of sprayed concrete linings.