What is shotcrete?

Shotcrete, also called sprayed concrete or gunite, is concrete that is conveyed under pressure through a hose and projected at high velocity onto a surface or one-sided formwork. It is a mix of cement, aggregate, water and, where needed, admixtures. Because it is usually applied in thin layers, the maximum aggregate size is smaller than in cast concrete, typically up to about 8 mm. Compaction on impact allows it to reach high density and strength. In Europe, requirements and execution are covered by the EN 14487 series of standards.

Dry-mix and wet-mix processes

• Dry-mix. A dry blend of cement and aggregate is blown through the hose; water is added only at the nozzle. The nozzle operator controls the water content. Drawbacks are heavy dust and more rebound: part of the material bounces off the surface. The process suits smaller volumes and work with frequent stops.
• Wet-mix. The concrete is fully mixed with water beforehand and pumped. This means less dust, less rebound and higher output. However, the mix is more prone to sagging and shrinkage needs more attention; fibres or a reinforcing mesh can limit it.

Uses

• Repair of damaged concrete, for example after concrete corrosion
• Fire protection linings
• Tunnelling and stabilising rock faces and slopes
• Free-form shapes and shell roofs that are hard to form
• Swimming pools with an unusual shape

Application

1. Prepare the substrate. Remove loose and deteriorated concrete, clean the reinforcement and pre-wet the substrate as specified.
2. Spray square-on. Keep the nozzle as perpendicular to the surface as possible; spraying at an angle increases rebound and leaves voids behind the reinforcement.
3. Build thick sections in several passes, especially overhead, to prevent sagging.
4. Never reuse rebound. Rebounded material has a different composition and must not go back into the work.
5. Curing is critical. Thin layers dry out quickly; keep the surface damp or use a curing compound.

Common mistake: treating shotcrete as an easy DIY material. Quality depends heavily on the nozzle operator’s skill; leave structural work to a specialist contractor.

Related terms

• Formwork
• Concrete curing
• Curing compound
• Concrete corrosion
• Concrete shrinkage
• Reinforcement
• Concrete pump
• Cementing

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