What is Gunite? Applications and Tips When Using Shotcrete

Types of Shotcrete and General Tips When Applying Gunite

Shotcrete
Shotcrete. Photo Getty Images/ SB-8NIHAT

What is Gunite? Shotcrete or Gunite

Gunite is a product developed as a solution for tunnels, underground structures, slope stabilization, structural repairs, and pools. Gunite is a mortar that is sprayed pneumatically onto surfaces at a high velocity. Gunite or shotcrete material is mortar or small-aggregate concrete applied using a wet or dry process. Once gunite has been applied it starts a simultaneous process of consolidation and settling.

Gunite Advantages

Gunite offers some advantages over traditional concrete such as:

  • It will be placed and consolidated at the same time.
  • Gunite maximum aggregate size is normally 3/8”, which helps improving quality and improves manageability.
  • Shotcrete will adhere to surfaces better than regular concrete.
  • Lower water-cementitious material ratio than commonly found in residential wall mixtures.
  • Shotcrete can be used with steel fiber that will be used as a replacement of welded wire mesh
  • When steel fibers are used in shotcrete, better flexural strength, ductility, and toughness are obtained.
  • It generally offers lower costs when compared to traditional concrete.
  • It offers reduced shrinkage and lower permeability.

The cost of the shotcrete is around $180 to $190 per cubic yard. This cost includes the material and forming of the concrete while the labor is separate. Remember all this might change depending on the size, depth and type of structure being completed.

Gunite Applications

Gunite or shotcrete can be used in the following applications:

  • Slope stabilization
  • Dome Construction
  • Tunneling
  • Retention walls
  • Water tanks and pools
  • Artificial ponds
  • Ditches and Channels
  • As structural reinforcement
  • Mining applications
  • Dikes and dams

Gunite Types: Dry Shotcrete or Wet Shotcrete

Shotcrete could be either dry-applied or wet-applied.

The dry shotcrete method is used when ingredients are placed into a hopper and then pushed out pneumatically through a hose to the nozzle. The nozzle man or the certified operator then controls the addition of water at the nozzle that would eventually be combined once the material hits the surface. If you are using dry shotcrete, then you will have the benefit of adjusting the water content being put into the mix instantaneously allowing for a better placement process without the needs of adding accelerators. It is the recommended method when the process involves frequent stops during the application process.

Wet-mix shotcrete is the type of application that normally uses prepared concrete or ready-mixed concrete. Compressed air is then applied at the nozzle to propel the wet mixture over the receiving surface. This is the most common used process as it produces less rebound and dust compared to a dry-mix process. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.

Shotcrete General Tips

Here are some good tips and general information that you would like to know before applying shotcrete.

  • Concrete joints should be designed as if you were placing regular concrete.
  • Shotcrete could be used to repair concrete areas without the need to have a bonding agent applied over the surface being repaired. However, be sure to remove all loose material and clean the surface before applying.
  • Gunite needs to be cured too. Moist curing is the preferred method of curing or it also can be done by using membrane curing compounds.
  • Shotcrete success depends largely on the skill and actions of the nozzle man. For this reason, it is important to require that the nozzle is ACI certified for the application.
  • Shotcrete needs to be maintained too. Remember to apply a good surface sealer and remove any material that might lead to surface deterioration.
  • There are different textures that can be applied to the finish surface of the shotcrete.
  • The same steel reinforcement requirements could be used on cast-in-place concrete, precast concrete or shotcrete.
  • Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete
  • Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following the final set, it should be wet cured for at least 4 days, preferably 7 days if possible.
  • The fire-rating of a concrete wall constructed by shotcrete or regular concrete will be the same.