Shotcrete Spraying Robot

Overview

A shotcrete (sprayed concrete) robot is a remotely operated tracked vehicle with a telescopic spray boom that applies wet-mix or dry-mix concrete to tunnel walls and crown with precision placement and minimal rebound loss. Shotcrete robots have revolutionized underground construction by replacing manual spray crews, improving safety, achieving consistent quality, and accelerating tunnel development in new Austrian tunneling method (NATM) and open-face mining applications.

The robot is positioned inside the tunnel near the excavation face and operated from a remote pendant outside the blast zone, eliminating exposure to dust, noise, and unstable hanging walls. Spray pattern, distance-from-surface, and nozzle angle are continuously adjustable to optimize adhesion and minimize ricochet waste.

Core Components

The Tracked Carrier Chassis is a heavy-duty tracked chassis (15–30 tonnes) with Rubber Track Belts and independent Track Drive Motors. The Chassis Articulation Joint allows pitch control (+/−20°) to orient the boom upward for crown coverage or downward for lower walls.

The Telescopic Spray Boom is a telescopic boom with multiple articulation joints powered by Telescoping Boom Tubes (hydraulic cylinders) and Intermediate Boom Links (articulated bends). Total reach is 4–8 m, sufficient to span a 10 m wide tunnel from a centerline position.

The Spray Nozzle Head Assembly is the precision business end, a small rotating or fixed head with a Tungsten-Carbide Nozzle Tip opening (typically 12–16 mm diameter for wet-mix, 8–10 mm for dry-mix) and a Dosing Proportional Valve that proportionally meters flow. The nozzle receives pressurized concrete from an external Concrete Pump Interface pump truck (wet-mix) or dry-mix hopper/auger (dry-mix).

Spray System Operation

Wet-Mix Shotcrete: The robot receives pre-mixed concrete from a pump truck via a concrete delivery hose. The Dosing Proportional Valve (proportional hydraulic spool) meters concrete flow between 0–12 m³/hour. Compressed Air Supply Line compressed air at 6–8 bar atomizes the concrete jet at the Tungsten-Carbide Nozzle Tip, accelerating particles to 40–60 m/s and improving adhesion. Rebound (ricocheted material) is 5–10% in skilled hands, lower than manual spraying (10–15%).

Dry-Mix Shotcrete: Alternative systems place dry concrete components (cement, aggregates) and add water at the nozzle. Dry-mix offers better workability in damp tunnels and reduced rebound (3–8%) but requires on-site micro-dosing controls and is less common than wet-mix.

Control and Feedback

The Main Control Unit (PLC) (PLC) manages:

• Proportional boom kinematics from the operator's Wireless Remote Control Pendant joystick
• Solenoid Valve Block directional valve switching for track, slew, boom, and dosing circuits
• Pressure feedback from pressure transducers monitoring concrete line (30–50 bar), air supply (6–8 bar), and hydraulic system (210–280 bar)
• position feedback from encoders on boom joints

Real-time operator feedback includes:

• Concrete flow rate (gallons/minute or liters/minute) displayed on pendant
• Nozzle pressure gauge warning if flow is inadequate (clogged hose)
• Audio alarm if air supply drops below minimum (compressor failure)

Placement Strategy

A typical tunnel development cycle:

1. Excavation: 3–5 m advance with conventional drilling and blasting
2. Scaling: crew removes loose rock from crown and walls manually (30–60 minutes)
3. Shotcrete application: robot sprays 300–400 mm thickness on crown and upper walls (60–90 minutes per 100 m²); base typically left unsprayed for drainage
4. Temporary support: rock bolts installed in bolted zones; mesh anchored where required
5. Next cycle: advance

Spray distance is typically 1.5–3 m from nozzle to surface; closer distances improve adhesion but increase rebound. Spray angle is 60–90° from surface (perpendicular impact preferred) to minimize slumping.

Rebound Loss and Concrete Recovery

Rebound material (ricocheted aggregate and paste) represents 5–15% of sprayed volume, depending on:

• Nozzle distance (closer = less rebound)
• Spray angle (perpendicular better than glancing)
• Concrete mix design (lower water content = better adhesion)
• Surface roughness (rough, blasted faces better than smooth)
• Operator skill (steady hand, consistent spray pattern)

Recovered rebound is sometimes crushed and recycled as aggregate, reducing waste costs.

Maintenance and Operational Issues

Concrete Hose Blockages: The concrete delivery hose (typically 50 mm diameter) can clog if concrete stiffens between applications or if excessive fines in concrete mix create drying. Regular flushing with water every 4–6 hours and slump monitoring (120–160 mm) prevent blockages.

Nozzle Wear: The Tungsten-Carbide Nozzle Tip tungsten-carbide orifice wears 3–5 mm per month of continuous operation; replacement every 1–2 months is typical. Larger orifice increases flow but reduces spray pattern control.

Pneumatic System: Air compressor must deliver 500–700 l/min at 7–8 bar; undersized compressors cause flow reduction and quality issues. Air dryer maintenance prevents moisture-induced concrete defects.

Track and Drive: Heavy spray loadings and uneven tunnel floors cause early track wear (50–100 operating hours per track replacement in hard rock). Articulation joint hydraulic seals require inspection every 500 operating hours.

Automation and Modern Features

Advanced systems integrate:

• Automated traverse control: operator sets tunnel profile (arch, bench height); robot sprays horizontally at constant angle
• Thickness monitoring: ultrasonic gauging feedback; robot pauses if thickness reaches target, resuming after section settles
• Concrete quality logging: discharge pressure, air flow, and temperature recorded for batch traceability
• Energy recovery: caterpillar wheel regeneration captures track energy during descent for boom raising (rare but emerging)

These features improve consistency, reduce operator fatigue, and provide quality documentation for inspection authorities.

Safety Considerations

• Remote operation: eliminates exposure to unstable ground and flying rebound
• Emergency stops: hardwired deadman switches on remote require continuous operator engagement
• Pressure relief: hydraulic overpressure and concrete line rupture hazards mitigated by relief valves and hose inspection
• Compressed air: hose whip hazard if couplings fail; quick-disconnects with anti-bleed design required