1. Introduction
Rebound is an inherent characteristic of sprayed concrete application. It refers to material that fails to adhere to the substrate upon impact and falls away from the sprayed surface.
Although rebound cannot be eliminated entirely, it can be controlled, measured, and managed.
In modern underground construction, rebound is not treated merely as material loss. It is recognised as an indicator of application quality, system stability, and structural reliability.
EFNARC has therefore developed formal guidelines to standardise rebound measurement and monitoring in sprayed concrete operations.
2. Physical mechanisms causing rebound
Rebound occurs when particles fail to embed into the cement paste matrix during impact.
Several mechanisms contribute:
• Excess tangential impact forces
• Insufficient impact velocity
• Inadequate paste cohesion
• Surface irregularities
• Fibre stiffness and mass
When a particle strikes the surface, its kinetic energy must be dissipated through deformation and embedment. If this energy cannot be absorbed, the particle rebounds.
Coarse aggregate and fibres are more prone to rebound due to higher mass and stiffness.
3. Influence of nozzle angle on rebound
Nozzle angle is one of the most dominant variables influencing rebound.
At near-perpendicular impact, kinetic energy is directed normal to the surface, promoting embedment.
As nozzle angle deviates, tangential forces increase. These forces redirect particles along the surface rather than into it.
Even small angular deviations significantly increase rebound, particularly on overhead and inclined surfaces.
EFNARC identifies nozzle technique as a major contributor to rebound generation.
4. Influence of stand-off distance
Stand-off distance affects both velocity and dispersion.
Excessive distance allows velocity decay and spray stream divergence, reducing compaction energy.
Insufficient distance causes excessive turbulence and ricochet.
Both conditions increase rebound.
Maintaining consistent stand-off distance is therefore critical for rebound control.
5. Influence of mix cohesion
Paste cohesion determines whether particles remain embedded after impact.
Low cohesion allows particles to bounce free.
Cohesion is influenced by:
• Water content
• Fines content
• Silica fume presence
• Admixture compatibility
Mixes with poor cohesion typically exhibit high rebound regardless of operator skill.
6. Fibre rebound behaviour
Fibre rebound differs from aggregate rebound.
Steel fibres possess high stiffness and rebound readily if not embedded at near-perpendicular impact.
Synthetic fibres exhibit lower rebound but require sufficient paste volume for anchorage.
High fibre rebound reduces energy absorption capacity of the lining and results in uneven reinforcement distribution.
For this reason, fibre rebound must be considered in both material design and application technique.
7. Structural and economic consequences of rebound
Rebound affects both performance and cost.
From a structural perspective:
• Rebound reduces effective applied volume
• Under-thickness may occur
• Fibre content in the lining is reduced
From an economic perspective:
• Material is wasted
• Disposal costs increase
• Productivity decreases
High rebound often masks under-application of shotcrete thickness, creating false confidence in lining performance.
8. Limitations of visual rebound estimation
Historically, rebound has often been estimated visually.
This approach is fundamentally flawed.
Visual estimation is subjective and influenced by:
• Excavation geometry
• Lighting conditions
• Operator perception
• Ground profile
EFNARC explicitly states that visual estimation is inaccurate and unsuitable for performance assessment.
9. EFNARC ENC 463 approach to rebound monitoring
To address inconsistencies, EFNARC developed ENC 463 Guidelines for Measuring and Monitoring Rebound in Sprayed Concrete.
The objective is to standardise measurement and enable reliable comparison between operations.
The guideline focuses on the weighing method as the most accurate manual approach currently available.
10. Selection and preparation of test area
The rebound test area must be representative of actual spraying conditions.
Considerations include:
• Geological profile
• Surface roughness
• Orientation of excavation
• Support conditions
Testing must not be performed in unstable or unsupported areas.
EFNARC requires that primary support be in place and exclusion zones respected before testing begins.
11. Surface preparation prior to testing
Surface preparation must follow standard execution procedures.
This includes:
• Removal of loose rock
• Washing down the substrate
• Control of water ingress
• Pre-wetting where required
Poor preparation invalidates rebound measurement results.
12. Installation of collection sheeting
Industrial plastic sheeting must be placed beneath the sprayed area.
The sheeting must:
• Cover the entire test area
• Extend beyond the perimeter by at least 10 percent
• Prevent contamination of rebound
Correct placement ensures accurate collection of all rebound material.
13. Determination of sprayed volume
Accurate rebound calculation requires accurate determination of sprayed concrete mass.
EFNARC recommends:
• Spraying a known minimum thickness, typically at least 50 millimetres
• Emptying the concrete truck fully
• Deducting residual volume in hoses and hopper
Volume determination must be based on actual wet density rather than theoretical assumptions.
14. Collection and weighing procedure
Rebound must be collected only once sufficient strength has developed to allow safe access.
Procedure includes:
• Taring clean collection containers
• Collecting all rebound material
• Weighing each container
• Recording cumulative rebound mass
Clean containers must be used to prevent contamination.
15. Rebound percentage calculation
Rebound percentage is calculated using the following relationship:
Rebound percent equals rebound mass divided by mass of wet sprayed concrete.
Wet sprayed concrete mass equals wet density multiplied by sprayed volume.
This calculation provides a quantitative value suitable for comparison and trend analysis.
16. Interpretation of rebound data
Rebound values should not be viewed in isolation.
They must be evaluated in relation to:
• Mix design
• Application method
• Operator technique
• Surface condition
Sudden increases in rebound indicate deviations in process control and must trigger investigation.
17. Use of rebound data for performance improvement
When measured consistently, rebound data allows:
• Identification of training needs
• Optimisation of mix design
• Adjustment of spraying technique
• Reduction of material waste
EFNARC promotes rebound monitoring as a continuous improvement tool rather than a fault-finding exercise.
18. Rebound as a quality indicator
Rebound is not merely waste.
It is a diagnostic signal.
High rebound indicates inefficiency in energy transfer, poor embedment, or instability within the spraying system.
Low rebound indicates efficient compaction and controlled application.
19. Engineering responsibility
Rebound control requires cooperation between:
• Mix designers
• Equipment operators
• Nozzle operators
• Supervisors
It cannot be solved by adjusting a single variable.
EFNARC guidance emphasises system-level understanding and disciplined execution.
20. Conclusion
Rebound measurement transforms sprayed concrete from an empirical activity into a controlled engineering process.
By applying EFNARC ENC 463 procedures, operations gain objective insight into application quality and lining reliability.