Gunite and shotcrete are two trade terms for different types of sprayed concrete. Sprayed concrete was developed as a construction solution for tunnels, underground structures, slope stabilization, structural repairs, and swimming pools. The process involves a mortar or small-aggregate concrete that is sprayed with air power onto surfaces at a high velocity. The force of the concrete spray consolidates and compacts the material and ensures that it adheres to the host surface.
Gunite Types: Dry Shotcrete or Wet Shotcrete
Gunite and shortcrete are essentially the same material, but they are applied with different processes. Gunite is the popular trade term for dry-gun concrete, while shotcrete is the common term for wet-gun concrete.
With the gunite method, the dry concrete ingredients are placed into a hopper and then pushed out pneumatically through a hose to a nozzle. The nozzle operator then controls the addition of water at the nozzle, turning the dry ingredients into concrete that is fully mixed by the time the material hits the host surface. This gives the operator control over the water content being put into the mix, allowing for a better placement process without the need for additional accelerators. It is the recommended method when the process involves frequent stops during the application process.
Wet-mix shotcrete uses fully mixed concrete and sends it through a hose and nozzle without additional water. This is the most commonly used process, as it produces less rebound (material that falls to the ground and is wasted) and dust compared to dry-mix gunite. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.
Sprayed Concrete Applications
Gunite or shotcrete can be used in the many specialty applications:
- Slope stabilization
- Dome Construction
- Retention walls
- Water tanks and pools
- Artificial ponds
- Ditches and Channels
- As structural reinforcement
- Mining applications
- Dikes and dams
Advantages of Gunite and Shotcrete
Sprayed concrete offers several advantages over traditional poured concrete:
- Placed, consolidated, and compacted at the same time
- Has a maximum aggregate size of 3/8 inch (typically), which helps improve quality and manageability
- Adheres to surfaces better than regular concrete
- Can be mixed with steel fiber as a replacement for welded wire mesh, offering better flexural strength, ductility, and toughness
- Generally less expensive than traditional concrete
- Reduced shrinkage and lower permeability
General Application Tips
Both gunite and shotcrete should be installed only by trained operators. Gunite typically requires more expertise and experience because the operator must adjust the water content for optimal results throughout the application process, and operator error can ruin the finished product.
- Concrete joints should be designed as if you were placing regular concrete.
- Shotcrete can be used to repair existing concrete without the need to have a bonding agent applied to the surface being repaired.
- Sprayed concrete must be cured just like poured concrete. Moist curing is the preferred method, but curing also can be done with membrane curing compounds.
- Success depends largely on the skill and actions of the nozzle operator. The nozzle should ACI-certified for the application.
- Sprayed concrete should be protected with a surface sealer and maintained by removing any material that might lead to surface deterioration.
- There are different textures that can be applied to the finish surface of the sprayed concrete.
- Sprayed concrete can use the same steel reinforcement as poured concrete or precast concrete.
- Typical shrinkage ranges from 0.06 to 0.10 percent after 28 days of drying. This is typically slightly higher than poured concrete of similar strength.
- Sprayed concrete must be protected from rain until it obtains its final set, usually four or five hours. Following the final set, it should be wet-cured for four to seven days.
- Gunite and shotcrete have a similar fire rating to that of regular concrete.