Shotcrete Spraying Machine

Overview

A shotcrete spraying machine (often called a sprayer, spraymobile or "wet shotcrete rig") applies pumped concrete pneumatically onto excavated rock surfaces in tunnels and underground mines. Sprayed concrete is the primary means of immediate ground support after blasting: a 50–100 mm layer, usually reinforced with steel or polymer fibres, locks loose blocks in place and develops several MPa of strength within hours thanks to set accelerators dosed at the nozzle.

Modern machines are almost universally wet-mix: ready-mixed concrete arrives by agitator truck, is pumped through a delivery line, and is dispersed and projected by compressed air added at the nozzle. This contrasts with the older dry-mix gunite process, where dry material was blown through the hose and water added at the nozzle; wet-mix gives far lower rebound (5–10 % versus 15–40 %), less dust, and consistent water/cement ratio. A typical rig sprays 4–30 m³/h with a boom reaching 17 m, so a single operator covers a full tunnel profile from a safe standoff under supported ground.

How it works

Concrete is discharged into the agitated Receiving Hopper of the Concrete Pump. Two hydraulic cylinders stroke pistons in the hard-chromed Delivery Cylinder pair in opposition: one cylinder draws mix from the hopper while the other pushes it into the line. The S-Valve (Swing Tube) swings between the cylinder mouths at each changeover, sealing against the carbide Spectacle Wear Plate, so flow into the Concrete Delivery Line is nearly continuous. Concrete pressures reach 70–100 bar when pumping stiff fibre-reinforced mixes through long boom lines.

At the Nozzle Head two injections happen within the last half metre of travel. The Air Injection Ring introduces 8–12 m³/min of compressed air, fragmenting the concrete column into a high-velocity spray that compacts itself against the rock at 20–30 m/s. Immediately downstream, the Accelerator Injection Ring meters in alkali-free accelerator, typically an aluminium sulphate solution, at 3–10 % of cement weight. The accelerator collapses the cement's dormant period so the sprayed layer stiffens in seconds and supports its own weight overhead, which is what makes spraying onto a tunnel crown possible at all.

Dose accuracy matters: under-dosing causes fallouts, over-dosing kills long-term strength. The Accelerator Dosing System therefore slaves its Peristaltic Dosing Pump to the concrete pump stroke counter, with a magnetic Accelerator Flowmeter closing the loop. A peristaltic design is used because the accelerator (pH ≈ 3) attacks conventional pump seals; only the replaceable hose touches the fluid.

Boom and operator station

The Spray Boom is a telescopic manipulator with a slewing Boom Turret, a box-section Telescopic Boom Arm and three or more Boom Hydraulic Cylinder functions. The Nozzle Oscillator nutates the nozzle in a continuous small circle, which evens out layer build-up and reduces rebound compared with a fixed aim. The nozzleman stands clear of the unsupported face and works the Radio Remote Control; the Machine PLC coordinates pump output, dosing ratio and interlocks (no accelerator without concrete flow, no concrete without nozzle air).

Carrier and power

The Carrier Chassis is an articulated four-wheel-drive carrier sized for mine drives, roughly 2.3 m wide with ±40° frame steering at the Articulated Frame joint. A Stage V Diesel Engine is used only for tramming between headings; at the face the machine plugs into mine power through the Trailing Cable Reel and runs its Electric Drive Motor, eliminating diesel particulates from the heading during the spray cycle. Four Stabiliser Outrigger jacks steady the chassis against boom reaction loads. Nozzle air comes either from the onboard Onboard Screw Compressor or from the mine ring main via the Mine Air Inlet.

Wear and maintenance

Shotcrete is among the most abrasive fluids regularly pumped. The Concrete Piston cups, Spectacle Wear Plate and cutting ring, Nozzle Tip and the delivery line elbows are all scheduled wear parts, with piston and plate lives measured in thousands of cubic metres. The accelerator circuit is flushed with water after every shift because dried accelerator crystallises and blocks the injection ring check valves. Hydraulic cleanliness is maintained by 10 µm return filtration in the Hydraulic System, and the whole concrete circuit is washed out with a sponge ball driven by water or air pressure at the end of each pour.