Shotcrete Pump

Overview

A shotcrete pump sprays concrete onto a surface instead of pouring it into formwork. The mix is pumped through a hose to a nozzle, where compressed air and a liquid set-accelerator are injected, and the stream is blown onto the rock or steel so it sticks and sets in place. It is the standard way to line tunnels, stabilise rock slopes and excavation faces, build swimming-pool shells and repair concrete structures, anywhere the work needs concrete bonded to an irregular or overhead surface without formwork.

This machine is a wet-mix pump: the concrete arrives already mixed with water, and the pump moves the finished mix to the nozzle. The alternative dry-mix process pumps dry material and adds water at the nozzle, which throws more dust and rebound. Wet-mix gives steadier output and a denser layer, which is why most modern tunnel and slope work uses it. Output is variable from about 5 to 30 cubic metres per hour, set by the operator to match how fast the nozzleman can place it.

Pumping mechanism

The heart of the machine is the Pumping Unit, a pair of large material cylinders worked in opposition by hydraulic drive cylinders. One cylinder extends and pushes its charge of concrete out to the line while the other retracts and draws a fresh charge from the hopper. When a piston reaches the end of its stroke a Stroke End Sensor tells the control to reverse the cylinders and swap the valve, so the two cylinders alternate and output is nearly continuous.

The material cylinders run about 180 mm bore and 1,000 mm stroke. The concrete is pushed by a Concrete Piston, a rubber-faced ram that seals against the honed bore. Between the drive and material cylinders sits the Water Box, kept flooded so the pistons run cool and lubricated and can be inspected and changed without draining the machine. The pistons are the main scheduled wear item on the concrete side.

Switching between the two cylinders is done by the S-Valve Unit. The S-Tube Swing Valve is a cast pipe bent like an S: its outlet end stays fixed on the delivery line while its inlet end swings between the two cylinder mouths, driven by the Swing Actuating Cylinder. As it swaps it shears the column of concrete at the cylinder face. The seal between the swing tube and the cylinders is made by a Wear Ring running against a fixed Cutting Ring; both are carbide-faced and both wear, so they are adjusted and replaced together. The swing tube and these rings are the parts that define an S-tube pump and the parts that wear fastest in abrasive mixes.

Hydraulics

All the force comes from the Hydraulic Power Pack. A variable-displacement Main Hydraulic Pump feeds high-pressure oil to the drive cylinders; the Swashplate sets pump flow, and flow sets the concrete output rate. The Directional Valve Bank switches oil between the two drive cylinders and fires the swing cylinders at each changeover. An Accumulator absorbs the pressure spike when the heavy cylinders reverse, which smooths output and spares the line. Oil pressure here translates directly into pushing force on the concrete, with peak concrete pressure reaching about 70 bar at the outlet against a long or uphill line. Long continuous pours heat the oil, so an Oil Cooler holds it in range.

The main pump is turned by the prime mover. The Electric Power Unit shown here is an electric motor of 30 to 55 kW, which is normal for tunnel and underground work because it makes no exhaust in a confined space. Surface and remote machines fit a diesel engine in the same place. The same hydraulic system also drives the Agitator in the hopper through a low-speed Orbital Hydraulic Motor.

Nozzle and accelerator

Concrete leaves the pump through the Delivery Line, a reinforced Delivery Hose of 50 to 65 mm bore that the nozzleman carries to the face. At the Spray Nozzle two more streams meet the concrete. Compressed air from the Air Supply, 7 to 12 cubic metres per minute, enters through the Air and Accelerator Ring and blows the mix onto the surface fast enough to compact it. Through the same ring the Accelerator Dosing System injects a liquid set-accelerator.

The accelerator makes the sprayed layer stiffen within seconds, so it holds on a vertical wall or overhead without sagging and a thick lining can be built up in one pass. Dose matters: too little and the concrete sloughs off, too much and the cured strength drops. The Dosing Pump is a peristaltic pump whose speed is slaved to the concrete stroke rate, so it holds the dose at a set 3 to 8 percent of cement mass as output changes. A Accelerator Flow Meter checks and logs the actual flow.

Materials and wear

The wetted parts see abrasive concrete under pressure, so wear control runs through the design. The hopper floor carries a bolt-in Wear Liner in Hardox. The swing tube, wear ring and cutting ring are carbide-faced consumables. The concrete pistons are polyurethane on a steel hub and are changed on a schedule. The delivery hose and the Rubber Tip are abrasion-resistant rubber and are treated as routine spares. Modern alkali-free accelerators have replaced the older caustic types, which is safer for the nozzleman and gives higher final strength.

Variants and use

Machines are split by how they move and how they are powered. Trailer and skid-mounted pumps with electric drive serve fixed tunnel and slope sites. Self- propelled spraying robots carry the same pump on a carrier with a boom-mounted nozzle, which keeps the operator back from the unsupported face and gives a more even layer. Smaller diesel units serve surface repair and pool work where mains power is absent. Across all of them the working parts are the same: a hopper with agitator, a twin-cylinder piston pump, an S-tube swing valve, a hydraulic pack, a nozzle with air and accelerator injection, and an accelerator dosing pump slaved to output.