Shotcrete Sprayers

A shotcrete sprayer is a machine that projects concrete at high speed onto a surface through a hose and nozzle, where it compacts on impact and forms a dense layer. The technique, known as shotcrete or sprayed concrete, is used to line tunnels, support excavations and mine openings, stabilise slopes, repair structures and build curved shapes such as swimming pools, without the need for formwork. Equipment ranges from small spraying machines handled by a nozzleman to large self-propelled spraying manipulators with a hydraulic boom, used in tunnelling and mining.

The machine usually combines a concrete pump or a pneumatic conveying system, a supply of compressed air, a dosing system for accelerators and, on mobile units, a boom that holds and moves the nozzle. The quality of the result depends as much on the operator and on the mix as on the machine.

Brief History of Shotcrete Sprayers

Sprayed concrete was developed in the United States in the early twentieth century. Carl Akeley, a naturalist and taxidermist, devised a machine around 1907 to spray a mixture of sand and cement with compressed air, adding water at the nozzle, and used it for repairs on the Field Columbian Museum in Chicago. His “cement gun” was patented in 1911, and the material became known by the trademark “gunite”. The dry-mix process spread quickly for repairs, linings and mining work, while other manufacturers used names such as “shotcrete” and “pneumatic concrete”. The term shotcrete became the generic name in North America around the middle of the century.

The wet-mix process, in which ready-mixed concrete is pumped to the nozzle and compressed air is added there, developed from the 1950s. Rotor-type dry-mix machines appeared in the following decades. The rise of the New Austrian Tunnelling Method from the 1960s made sprayed concrete a primary means of tunnel support, and demand grew for higher outputs and safer working methods. This led to hydraulic spraying manipulators, first mounted on trucks and later on dedicated carriers, which removed the nozzleman from the unsupported rock face. Liquid alkali-free accelerators, steel and synthetic fibres, and computer controlled dosing followed, making sprayed concrete a precise, highly mechanised process.

How a Shotcrete Sprayer Works

Material supply. In the wet process, ready-mixed concrete from a truck mixer is discharged into the hopper of a concrete pump. In the dry process, a dry mix of cement and aggregates is fed into a rotor or chamber machine.
Conveying. Wet-mix sprayers use piston pumps, similar to concrete pumps, to push the concrete through the hose. Dry-mix machines convey the material pneumatically, carried by compressed air.
Nozzle. In the wet process, compressed air is added at the nozzle to accelerate the concrete and form a spray jet. In the dry process, water is added at the nozzle and mixed with the dry material as it leaves.
Accelerator dosing. A dosing pump adds liquid accelerator at the nozzle, so that the concrete sets quickly and can be applied in thick layers, including overhead.
Spraying boom. On mobile units a hydraulic boom with a rotating and tilting nozzle head lets the operator move the jet at the right distance and angle to the surface.
Controls. Remote controls allow the operator to stand in a safe area under supported ground. Modern machines synchronise pump output with accelerator dosing and record data for each application.

Types of Shotcrete Sprayers

Dry-Mix Spraying Machines

Rotor or chamber machines that convey dry material pneumatically to the nozzle. They are compact, handle small quantities well and allow long conveying distances. They are used for repairs, refractory work, small tunnels and mining.

Wet-Mix Spraying Pumps

Concrete pumps adapted for spraying, often trailer or skid-mounted, with the nozzle held by hand or by a small manipulator. They achieve higher outputs than dry-mix machines with less dust and rebound.

Mobile Spraying Manipulators

Self-propelled machines with a spraying boom, pump, accelerator system and compressor on a carrier, often with four-wheel drive and articulated steering. They are the standard tool in modern tunnelling and underground mining.

Truck-Mounted Sprayers

Spraying pumps and booms mounted on a truck chassis, used for slope stabilisation, retaining walls and surface projects where road mobility is important.

Small Spraying Machines

Compact electric or pneumatic machines for mortars, plasters and repair materials, used in building maintenance and renovation.

Size Classes

Type	Typical output	Typical use
Small spraying machines	a few m³/h	Repairs, mortar, renovation
Dry-mix machines	about 2 to 10 m³/h	Repairs, mining, small tunnels
Wet-mix pumps and mobile manipulators	about 5 to 30 m³/h	Tunnels, mines, slope support

Figures are indicative. Actual spraying output depends on the mix, the hose length, the accelerator and the operator.

Accessories and Related Equipment

A shotcrete operation depends on more than the spraying machine. Compressors supply the air for the nozzle or for pneumatic conveying, and on mobile manipulators they are often built into the carrier. Accelerator tanks and dosing pumps must be sized for the shift, and many machines carry their own. Truck mixers or dedicated underground concrete transporters bring the concrete to the face in tunnels and mines. Hoses, reducers and nozzles of different sizes are chosen for the mix and the output, and nozzle heads can rotate and oscillate to spread the material evenly. Lighting, cameras and profile scanners help the operator see the surface and check the thickness applied, especially in dusty conditions underground.

Main Manufacturers

Normet, the Finnish company specialised in underground equipment, including spraying machines.
Putzmeister, with its underground and shotcrete machine range.
CIFA, which builds wet-mix spraying machines for tunnelling.
Sika, with its Aliva spraying machines.
Allentown Equipment and REED, known in North America for shotcrete and gunite equipment.

Main Uses of Shotcrete Sprayers

Primary and final linings of road, rail and utility tunnels.
Ground support in underground mines and caverns.
Stabilising slopes, cuttings and deep excavations, often with soil nails or anchors.
Repairing concrete in bridges, dams, tunnels and marine structures.
Swimming pools, skate parks and curved architectural shapes.
Refractory linings in industrial furnaces and kilns.

Wet Mix or Dry Mix?

Output. Wet-mix spraying reaches much higher outputs and is preferred for large tunnel and mining volumes.
Rebound and dust. Wet mix produces less rebound and dust, improving working conditions and reducing waste.
Flexibility. Dry mix allows long conveying distances, stop and start operation and small quantities without waste.
Water control. In dry mix the nozzleman controls the water directly, so quality depends heavily on skill. Wet mix is premixed, giving more consistent water content.
Supply. Wet mix needs a continuous supply of fresh concrete, while dry mix materials can be stored in bags or silos.

Safety

Shotcrete work combines high pressure, fast-setting materials and often confined underground spaces. The most serious hazard in tunnels and mines is working under unsupported ground, which is why mobile manipulators with remote controls are used to keep the operator back from the face. Hoses and couplings under pressure can burst or whip if blocked, so pressure must be released before clearing blockages. Rebound and spray mist contain cement and accelerators that are harmful to the skin, eyes and lungs, so operators need full protective equipment and good ventilation. Accelerators, especially older alkaline types, require careful handling. Freshly sprayed concrete should not be relied on for support until it has gained the required early strength.

Trends and Technology

Shotcrete equipment is becoming more automated and more precise. Scanners measure the tunnel profile before and after spraying, and the control system calculates the thickness applied, reducing overuse and ensuring the design lining is achieved. Semi-automatic and automatic spraying functions move the nozzle along programmed paths. Battery electric carriers and electric pumps are spreading in underground mines to reduce diesel emissions and ventilation costs. Accelerator dosing is linked to pump output for consistent quality, and fibre-reinforced shotcrete is widely used to replace mesh in ground support.

Key Terms

Gunite: an early trademark for dry-mix sprayed concrete, now used informally for the dry process.
Nozzleman: the operator who directs the spray jet.
Rebound: material that bounces off the surface and does not stay in place.
Accelerator: an admixture added at the nozzle to speed up setting.
Spraying manipulator: a mobile machine with a boom that holds and moves the nozzle.
Fibre-reinforced shotcrete: sprayed concrete containing steel or synthetic fibres.

Frequently Asked Questions

What is the difference between shotcrete and gunite? Shotcrete is the general term for sprayed concrete. Gunite usually refers to the dry-mix process, from the historic trademark.

Why is shotcrete used in tunnels? It can be applied quickly to irregular rock surfaces without formwork, sets fast with accelerators and provides immediate support to the ground.

Can shotcrete be sprayed overhead? Yes. With the right mix and accelerator, it adheres to vertical and overhead surfaces and can be built up in layers.

What is rebound, and why does it matter? Rebound is material that does not stick to the surface. It wastes material and can change the composition of the layer, so good technique and equipment aim to minimise it.