Shotcreting or sprayed concrete application is a key construction method in South Africa for stabilising slopes and repairing deteriorated structures.
Shotcreting or sprayed concrete application is a key construction method in South Africa for stabilising slopes and repairing deteriorated structures.
It offers fast concrete placement without the need for traditional formwork especially on complex or vertical surfaces. The process, while efficient, requires specialised equipment and skilled operators to ensure quality and durability. Shotcrete is a method of applying concrete pneumatically at high velocity onto a surface. It can be applied using either the dry-mix or wet-mix method:
• Dry mix: Dry cement and aggregates are mixed and conveyed through a hose to a nozzle where water is introduced just before application.
• Wet mix: Premixed concrete (with water) is pumped through a hose and sprayed using compressed air at the nozzle.
The force of application ensures dense compaction allowing the concrete to adhere to vertical walls, overhead surfaces and uneven substrates without slumping or falling off. This makes shotcrete especially useful in retaining wall stabilisation and a wide variety of other applications.
The mix design for shotcrete differs from conventional concrete. For wet-mix shotcrete, a slump of 80–120 mm is typical being sufficiently fluid for pumping but stiff enough to adhere to surfaces. Dry-mix shotcrete uses virtually zero slump material since water is only added at the nozzle.
Key performance requirements include:
• High early strength gain for rapid stabilisation (typically 20 MPa in 24 hours).
• Final strengths between 25–50 MPa depending on application.
• Low water-cement ratios (w/c ≤ 0.45) for durability and reduced rebound.
• Use of accelerators to improve setting time, especially for overhead or vertical surfaces.
Fibres usually steel or synthetic are often added to enhance ductility and reduce cracking.
Essential Equipment
Successful shotcreting requires a coordinated set of equipment:
• Concrete Pumps: Piston-driven pumps are preferred for wet-mix applications. They must deliver steady pressure and flow (typically 20–30 bar), capable of handling coarse aggregates without clogging.
• Air Compressors: For dry-mix and to propel wet-mix at the nozzle. Air pressure of 5–7 bar with a flow rate of 8–12 m³/min is required for effective spray.
• Shotcrete Nozzles: Specialised nozzles fitted with water injection (dry-mix) or air assistance (wet-mix) ensure proper atomisation and spray control.
• Mixers: Pan or drum mixers for dry-mix; transit mixers for wet-mix.
• Robotic Arms or Nozzleman platforms: In tunnels or complex environments, robotic shotcrete arms improve safety and consistency.
Access and Support Equipment
To reach high or complex surfaces contractors will need cherry pickers or scissor lifts which can be used to elevate nozzle operators safely. Telehandlers often transport bulk dry material or premixed concrete to confined access points. Scaffold systems may also be erected where surface geometry allows. Safety platforms must comply with SANS 10085 and occupational safety standards especially for work at heights.
Once applied the shotcrete requires immediate curing to prevent moisture loss. This is usually done using spray-on curing compounds, Polyethylene sheeting or continuous water misting in closed environments. Minimum curing duration is 7 days though accelerated admixtures and curing accelerators are common to speed up processes in time-sensitive builds.
Typical Applications and Local Specialists
Shotcreting is commonly used in slope stabilisation: Used in cuttings along national roads or housing developments.
• Tunnel linings: Including Gautrain tunnels and infrastructure upgrades.
• Dam and pool linings: Offering seamless, water-tight finishes.
South African specialists like Baseline Civils, Ground Breakers Construction and Gunite Construction offer turnkey shotcrete services. International brands like Normet and Putzmeister supply pumps and robotic arms locally through partners.
