1. Introduction
The performance of wet shotcrete is governed not only by material design and spraying technique but equally by the behaviour of the equipment system that delivers the concrete to the nozzle.
In a wet shotcrete operation, the pump, delivery line, air system, and accelerator dosing unit form a single integrated system. Any instability within this system is transferred directly to the applied lining.
Blockages, pulsation, rebound, and inconsistent thickness are rarely isolated equipment failures. They are symptoms of imbalance within the pumping system.
For this reason, an understanding of wet shotcrete equipment mechanics is essential for consistent application.
2. Overview of a wet shotcrete delivery system
A typical wet shotcrete system consists of the following primary components:
• Concrete mixer or batching plant
• Concrete pump, typically piston type
• Steel pipeline and rubber delivery hoses
• Compressed air supply
• Accelerator dosing pump
• Nozzle assembly
Each component influences flow stability, pressure behaviour, and concrete integrity.
Shotcrete should never be viewed as flowing through a passive pipe system. The concrete is constantly subjected to shear forces, pressure variations, and directional changes that alter its internal structure.
3. Piston pump operating principles
Modern underground shotcrete operations predominantly use twin-cylinder piston pumps.
These pumps operate by alternately drawing concrete into one cylinder while the second cylinder discharges material into the delivery line. The changeover is controlled by an S-tube or swing valve.
The theoretical advantage of piston pumps is their ability to deliver high output rates and pump stiff mixes over long distances.
However, this design also introduces cyclic pressure fluctuations.
Each changeover event creates a brief interruption in flow, which manifests as pulsation at the nozzle.
If not properly controlled, pulsation results in:
• Irregular material discharge
• Uneven thickness
• Increased rebound
• Operator difficulty maintaining nozzle stability
Proper calibration and mechanical condition of the pump are therefore critical.
4. Influence of concrete rheology on pump behaviour
Concrete does not behave as a simple fluid.
During pumping, concrete is subjected to shear, causing temporary breakdown of internal particle structure. This phenomenon is known as thixotropy.
When shear decreases, the structure rebuilds.
If concrete rheology is unstable, this rebuild occurs irregularly, causing fluctuating resistance within the delivery line. The pump responds by generating pressure spikes.
These spikes appear at the nozzle as surging flow.
EFNARC identifies consistency stability as essential to maintaining predictable pump behaviour throughout spraying operations.
5. Delivery pipelines and hose configuration
The delivery system typically consists of steel pipes near the pump and flexible rubber hoses near the face.
Steel pipelines provide dimensional stability and reduced friction. Rubber hoses allow movement and flexibility during spraying.
However, rubber hoses also introduce deformation under pressure.
When pressure rises, the hose expands. When pressure drops, it contracts. This elastic behaviour amplifies pump pulsation if hose lengths are excessive.
For this reason, practical shotcrete operations limit individual hose lengths, commonly to 10 metres per section.
Long continuous hoses increase cleaning difficulty and magnify pressure instability.
6. Reducers and transition points
Reducers are used to transition from larger diameter pump outlets to smaller delivery hoses.
Incorrect reducer installation is a common cause of blockages.
If a reducer is omitted, concrete impacts directly against a sudden diameter change, creating accumulation and eventual obstruction.
All reducers must be clean, correctly aligned, and securely clamped. Even small amounts of hardened material at reducer edges disrupt flow and initiate blockage formation.
Transition points are high-risk zones and must receive particular attention during inspection.
7. Clamps, pressure seals, and joint integrity
Every joint within the delivery system represents a potential failure point.
Clamps must be clean, undamaged, and fitted with intact pressure seals.
Concrete leakage at joints is not a minor inconvenience. It creates localised paste loss, increasing friction and leading to internal build-up.
This build-up progressively reduces effective diameter until flow is restricted.
EFNARC practice requires daily inspection and cleaning of all joints to maintain uninterrupted flow.
8. Compressed air function and velocity generation
Compressed air is introduced at the nozzle, not to transport concrete, but to increase projection velocity.
Air volume directly controls:
• Impact energy
• Compaction efficiency
• Adhesion
• Rebound behaviour
Insufficient air reduces velocity, causing concrete to slump or fall away from the surface.
Excessive air increases rebound by ejecting coarse particles before embedment.
Air pressure and volume must therefore be balanced relative to mix design, hose length, and nozzle configuration.
There is no universal air setting. Adjustment must be performed in response to observed material behaviour.
9. Accelerator dosing mechanics
Accelerator is introduced at the nozzle to initiate rapid stiffening of the cement paste.
Dosing systems are typically hydraulically driven peristaltic pumps or piston-type dosing units.
The dosing rate must be calibrated to match concrete flow rate.
Under-dosing leads to sloughing, delayed build-up, and loss of thickness.
Over-dosing results in:
• Reduced long-term strength
• Increased brittleness
• Increased rebound
• Excessive heat generation
EFNARC requires dosing systems to be manually calibrated and cross-checked against displayed values.
10. Interaction between pump output and accelerator demand
Pump output is rarely constant.
Minor changes in piston efficiency, material consistency, or hose pressure alter flow rate.
If accelerator dosage remains fixed while concrete flow fluctuates, the effective dosage percentage changes.
This leads to inconsistent setting behaviour across the sprayed surface.
For this reason, experienced operators continuously monitor shotcrete response rather than relying solely on numerical dosage values.
Visual feedback at the face remains critical.
11. Nozzle assembly and mixing efficiency
Within the nozzle assembly, concrete, air, and accelerator must mix uniformly.
Injector cleanliness is critical.
Blocked injector ports create uneven distribution of accelerator, resulting in zones of delayed set and zones of over-accelerated material.
O-rings must seal correctly to prevent air or accelerator loss within the nozzle body.
Any leakage reduces mixing efficiency and increases rebound.
12. Start-up behaviour and system stabilisation
During start-up, the system undergoes rapid transition from static to dynamic flow.
Hoses must be lubricated using cement slurry before introducing concrete.
This reduces friction and prevents paste stripping.
Spraying should only begin once:
• Concrete flow is continuous
• Accelerator is present at the nozzle
• Reaction is visible
Premature spraying leads to dry material impact and high rebound at the start of application.
13. Pumping distance and system limitations
Although wet shotcrete can be pumped over significant distances, performance declines as distance increases.
Increased distance results in:
• Higher pressure requirements
• Increased shear degradation
• Greater temperature rise
• Increased risk of blockage
Practical limits depend on mix design, equipment condition, and hose configuration.
Engineering judgement must be applied when extending delivery distances.
14. Blockage mechanics and root causes
Blockages rarely occur suddenly.
They develop progressively due to:
• Paste loss at joints
• Segregation under shear
• Build-up at reducers
• Inconsistent pumping
• Inadequate cleaning
When a blockage occurs, pressure energy stored within the hose is significant. Improper release can result in severe injury.
For this reason, reversal and pressure relief procedures must be followed strictly.
15. Equipment systems as quality controllers
In wet shotcrete operations, equipment behaviour directly influences final lining quality.
Stable pumping produces:
• Uniform thickness
• Reduced rebound
• Consistent compaction
• Predictable accelerator response
Unstable systems cannot be compensated for by operator skill alone.
Shotcrete quality begins at the pump.