1. Introduction
Shotcrete, formally referred to as sprayed concrete, is a construction method in which concrete is pneumatically projected at high velocity onto a surface and compacted by the energy of impact. It is used extensively in underground mining, tunnelling, and civil works where rapid support and flexible application are required.
Unlike conventional concrete, shotcrete is not placed into formwork and vibrated. Its performance is created at the moment of application. The final quality of the lining is therefore dependent not only on the material design but equally on execution, equipment configuration, and nozzle operator technique.
For this reason, shotcrete must be understood as a controlled construction process rather than simply a concrete product.
EFNARC recognises sprayed concrete linings as structural elements whose behaviour depends on systematic application, monitoring, and quality control.
2. Engineering role of shotcrete in underground excavations
When an underground excavation is created, the stress equilibrium within the surrounding rock mass is disturbed. This results in deformation, loosening, and potential failure of near-surface rock.
Shotcrete is applied immediately after excavation to perform several simultaneous engineering functions.
First, it binds fractured rock fragments together, preventing unraveling of the excavation surface. Second, it forms a continuous shell that distributes stress across the excavation profile. Third, it provides confinement for reinforcement elements such as rock bolts, cables, and mesh.
In combination with these systems, shotcrete forms part of the ground reaction system. Its purpose is not to carry load independently but to control deformation and stabilise the excavation until long-term equilibrium is achieved.
Failure to achieve adequate bond or thickness compromises this entire support mechanism.
3. Shotcrete as an active support system
Shotcrete functions as an active support. This means it interacts directly with the rock mass rather than simply resisting loads after movement has occurred.
When applied correctly, shotcrete:
• Restrains dilation of fractured rock
• Transfers load between discontinuities
• Limits convergence of excavation walls
• Reduces stress concentration around openings
This behaviour is only achieved when intimate contact exists between the rock surface and the shotcrete lining. Any contamination, dust layer, or rebound inclusion at the interface reduces load transfer efficiency.
EFNARC therefore places strong emphasis on substrate preparation and execution control.
4. Fundamental difference between shotcrete and cast concrete
Shotcrete differs from cast concrete in both placement and structural behaviour.
In cast concrete, compaction is achieved through vibration and confinement within formwork. In shotcrete, compaction occurs due to kinetic energy at impact.
Typical impact velocities exceed 20 metres per second. At this velocity, aggregate interlock, cement paste densification, and mechanical bond to the substrate are created simultaneously.
If velocity is reduced due to insufficient air supply, excessive nozzle distance, or loss of mix cohesion, compaction quality decreases and rebound increases.
This explains why identical mix designs can perform very differently depending on application technique.
5. Wet shotcrete system overview
Modern underground construction predominantly uses the wet shotcrete method.
In this system:
• All materials are mixed before pumping
• Concrete is delivered hydraulically through hoses
• Compressed air is introduced at the nozzle
• Accelerator is added immediately before placement
The role of compressed air is to increase projection velocity. The role of the accelerator is to initiate rapid stiffening so that the concrete adheres to vertical and overhead surfaces.
EFNARC identifies concrete consistency stability as a critical requirement throughout the entire spraying operation, as changes directly affect rebound, accelerator demand, and lining performance.
6. Rheology and concrete behaviour during spraying
Shotcrete must satisfy two competing requirements.
It must be fluid enough to pump continuously without blockage, yet cohesive enough to adhere to the rock surface after projection.
This balance is achieved through controlled rheology.
Key parameters include:
• Flow or slump stability
• Resistance to segregation
• Uniform fibre dispersion
• Controlled bleeding
If workability decreases during spraying, pump pulsation increases. This results in unstable flow at the nozzle, irregular thickness, and increased rebound.
Conversely, excessive fluidity increases sloughing and reduces early strength development.
For this reason, workability must be measured and verified during production spraying.
7. Substrate interaction and bond development
The bond between shotcrete and rock is fundamental to performance.
Bond is achieved through:
• Mechanical interlock
• Surface roughness
• Cement hydration at the interface
EFNARC requires that substrates be cleaned thoroughly before spraying.
Loose rock, dust, oil, and drilling residue act as bond breakers. Dry rock absorbs mix water, reducing hydration at the interface and weakening adhesion.
Where water ingress exists, it must be drained or controlled before spraying. Attempting to spray directly against flowing water leads to washout and loss of cement paste.
Proper substrate preparation is therefore not optional. It is structural.
8. Nozzle technique as a governing variable
The nozzle operator directly governs lining quality.
Key variables under operator control include:
• Nozzle angle relative to the surface
• Stand-off distance
• Spraying pattern
• Layer build-up sequence
A nozzle angle approaching perpendicular ensures normal impact and optimal compaction. Oblique angles increase tangential forces, causing aggregate and fibres to rebound rather than embed.
EFNARC identifies nozzle technique as one of the dominant contributors to rebound generation and thickness variability.
9. Thickness development and layering
Shotcrete thickness is developed progressively.
Rather than applying the full design thickness in a single pass, the lining is built in controlled layers, typically around 50 millimetres depending on mix behaviour and accelerator performance.
Layering allows:
• Heat dissipation
• Controlled setting
• Reduced sloughing
• Improved compaction
Thickness must be verified continuously through probing. Visual estimation is unreliable and routinely results in under-application.
Under-thickness reduces load capacity and energy absorption, directly affecting excavation stability.
10. Rebound as a performance indicator
Rebound consists of material that fails to adhere during spraying. It represents both material loss and quality deviation.
High rebound indicates one or more of the following:
• Poor nozzle angle
• Excessive spraying distance
• Incorrect air volume
• Loss of mix cohesion
• Poor substrate condition
EFNARC requires rebound to be measured using defined weighing procedures rather than visual estimation. Accurate rebound measurement ensures reliable volume calculations and thickness control.
11. Shotcrete as a controlled construction process
Shotcrete cannot be corrected once placed.
Its performance is created in real time through:
• Mix design
• Equipment calibration
• Operator competence
• Application discipline
• Continuous monitoring
For this reason, shotcrete quality is achieved through process control rather than post-placement repair.
Professional sprayed concrete operations require engineering oversight, trained personnel, and strict adherence to established standards.
End of Article 1
Next I will write:
Article 2
Wet Shotcrete Equipment Systems and Pumping Mechanics
This will cover:
• Piston pump operation
• Hose behaviour and pressure
• Accelerator dosing mechanics
• Air volume effects
• Pump pulsation
• Blockage mechanics
• Equipment limitations