1. Introduction
The performance of sprayed concrete is governed as much by the condition of the substrate as by the quality of the concrete itself.
Shotcrete does not act independently. Its structural contribution depends on its ability to bond effectively to the rock mass and to transfer load through that interface.
Poor substrate preparation is one of the most common root causes of lining failure, excessive rebound, and premature debonding.
EFNARC therefore treats surface preparation as a structural requirement rather than a housekeeping activity.
2. Behaviour of rock immediately after excavation
Excavation disturbs the in-situ stress field within the rock mass.
This disturbance leads to:
• Stress redistribution
• Crack propagation
• Loosening of near-surface rock
• Development of relaxation zones
The thickness of this disturbed zone depends on rock strength, joint spacing, and excavation method.
Shotcrete is applied to stabilise this zone before progressive deterioration occurs.
If the surface is not properly prepared, the lining is bonded to unstable material rather than competent rock.
3. Types of surface conditions encountered underground
Underground surfaces are rarely uniform.
Common conditions include:
• Jointed hard rock
• Blocky fractured rock
• Sheared zones
• Bedding planes
• Overbreak cavities
• Wet or dripping surfaces
Each condition influences shotcrete behaviour differently.
Smooth competent rock typically produces lower rebound and good bond. Irregular fractured rock increases rebound due to variable impact angles and surface protrusions.
EFNARC requires that rebound testing areas be representative of actual excavation conditions to ensure meaningful results.
4. Removal of loose and unstable material
Loose rock must be removed prior to spraying.
This includes:
• Scaling of fractured material
• Removal of slabs and wedges
• Clearing of drill cuttings
• Removal of blast residue
Spraying onto loose material creates a false sense of stability. The lining may appear sound while the underlying rock continues to unravel behind it.
When failure occurs, it is often sudden and extensive.
Mechanical scaling and visual inspection must therefore precede any shotcrete application.
5. Importance of rock mapping prior to spraying
Rock mapping provides critical information regarding:
• Joint orientation
• Block size
• Water ingress locations
• Required support class
Shotcrete thickness, fibre dosage, and reinforcement layout are determined based on this information.
Applying shotcrete without understanding the ground conditions risks under-support or misallocation of resources.
EFNARC considers systematic assessment of ground conditions an integral part of sprayed concrete execution.
6. Surface cleanliness and bond formation
Bond between shotcrete and rock is achieved through mechanical interlock and cement hydration at the interface.
Dust, fines, oil, or drilling residue create a separation layer that prevents this interaction.
Even a thin dust film can reduce bond strength significantly.
EFNARC requires thorough wash-down of the substrate prior to spraying to remove contaminants and expose clean rock surface.
Wash-down should always begin at the crown and progress downward, ensuring contaminants are removed from the entire spraying area.
7. Moisture condition of the substrate
Substrate moisture plays a critical role in bond development.
If the rock surface is dry, it absorbs water from the fresh shotcrete. This reduces hydration at the interface and leads to weak adhesion.
If the surface is excessively wet or flowing, cement paste is washed away.
The correct condition is a saturated surface dry state, where the rock is damp but free of running water.
Pre-wetting is therefore required when surfaces have dried prior to spraying.
8. Management of water ingress
Water ingress is one of the most challenging conditions for shotcrete application.
Uncontrolled water causes:
• Cement washout
• Loss of paste
• Increased rebound
• Reduced early strength
EFNARC requires water to be drained or sealed prior to spraying.
Common control measures include:
• Drainage pipes
• Temporary diversion channels
• Injection grouting
• Plugging of concentrated flows
Attempting to spray directly against water is ineffective and leads to poor lining performance.
9. Surface profile and rebound behaviour
Surface geometry directly influences rebound.
Protrusions, sharp edges, and irregular profiles increase tangential impact and particle ricochet.
Large cavities should be pre-sprayed in layers to establish a more uniform profile before rebound measurement or full lining application.
This practice improves compaction efficiency and reduces material loss.
10. Interaction between substrate roughness and bond
While excessive roughness increases rebound, a completely smooth surface may reduce mechanical interlock.
Moderate roughness provides optimal bond.
Shotcrete adheres most effectively to surfaces that provide micro-anchoring points without severe angular protrusions.
This balance is achieved through controlled scaling and cleaning.
11. Plastic sheeting for rebound measurement
When conducting rebound measurement, EFNARC requires placement of industrial plastic sheeting beneath the sprayed area.
The sheeting must:
• Cover the full test area
• Extend at least 10 percent beyond the sprayed zone
• Prevent contamination of rebound material
This ensures accurate collection and weighing of rebound.
Incorrect placement invalidates measurement results.
12. Timing of shotcrete application
Shotcrete should be applied as soon as reasonably practicable after excavation.
Delay allows:
• Progressive loosening
• Moisture loss
• Increased contamination
• Greater scaling requirements
Early application improves bond and reduces overall support demand.
Shotcrete is most effective when used proactively rather than reactively.
13. Consequences of poor substrate preparation
Failures associated with poor preparation include:
• Delamination
• Debonding
• Hollow sounding linings
• Reduced load transfer
• Increased maintenance
These failures often appear weeks or months after application, long after corrective action is possible.
Proper preparation is therefore the most cost-effective quality control measure available.
14. Substrate preparation as a structural operation
Surface preparation is frequently underestimated.
In reality, it is a structural operation that determines whether the sprayed concrete lining functions as designed.
EFNARC’s guidance clearly establishes that substrate preparation must follow defined execution procedures to ensure lining performance and reliability.
15. Engineering responsibility
Responsibility for substrate preparation lies with the shotcrete operation as a whole.
Nozzle operators, supervisors, and engineers must verify that surfaces are suitable before spraying begins.
Shotcrete cannot compensate for unstable ground.
It can only stabilise what it is properly bonded to.