The technology of shotcrete. Can we eliminate rebound problems?

Shotcrete is concrete or mortar pneumatically projected at high velocity through a nozzle. Its components are aggregates, cement and water, and it can be complemented by fine materials, chemical additives and reinforcing fibers. Shotcrete can be applied with mechanized equipment or manually, using wet-mix or dry-mix spraying. The choice of the spraying method depends on the dimensions of the project, the quantity of concrete to be applied, as well as the logistical and environmental circumstances. Some important properties of shotcrete are the appropriate consistency and early strength development in its fresh state as well as compressive strength and durability in its hardened state. Let´s discuss some basic properties and functionalities.

How is shotcrete used?

There are many applications that shotcrete can be used for and is an ever expanding field. The most common applications cover repairing damaged surfaces on existing structures, for example with bridges, buildings, marine structures, spillway surfaces. Another common application is the supplementing or replacing of conventional support materials such as lagging and steel sets, sealing rock surfaces, channelling water flows and installing permanent linings. Slope and surface protection is another useful application of shotcrete. Exposed rock surfaces or slopes that are liable to erode are often covered by shotcrete. Finally, new structures that involve large areas can be covered with shotcrete to save time. Examples of this can include pools and tanks, shotcrete walls and floors and shotcrete domes.

Implementing this technology effectively, either as a wet-mix or dry-mix shotcrete, comes with its’ own challenges. For example, employment of wet-mix spraying can lead to generally lower compressive strengths, restricted layer thickness and unadjustable moisture content. Employment of dry-mix spraying can lead to higher rebound, low air-entrainment and more dust. Both types of shotcrete also have issues with respect to pumpability and fluidity during their application.

Reduction of these adverse effects can be aided by applying various additives or admixtures into the shotcrete mix.

• Silica Fume – reduces permeability and rebound, increases compressive and flexural strength and gives thicker layers.
• Air-Entraining Admixtures – improves pumpability, adhesion and freeze-thaw durability.
• Fibers – reduces cracking, increases toughness and improves impact resistance.
• Accelerators – improves placement of shotcrete in tough conditions, allows for thicker layers and reduces vibrational impact.
• Plasticizers & Superplasticizers – increase the fluidity of the mix without resistance loss.
• Rebound Reducing Admixtures
  
  

New technology: cellulose fibrils

Today, there is very little research that has been done with the use of cellulose fibrils as an alternative to these admixtures and additives in shotcrete. The broad range of functionalities that cellulose fibrils possess and that could be utilized is of great interest to us.

• Viscosity Modifying Effect - to increase thickness of the shotcrete, reduce the amount of layers needed and reducing rebound.
• Shear-Thinning Effect - improve the pumpability and flow of the shotcrete.
• Reduction of bleeding, sedimentation and segregation - improve the shotcrete mix leading to easier spraying.
• Air-Entraining effect - increase the viscosity of the shotcrete, again reducing the layers needed and reducing rebound, decreasing the mass of the shotcrete used and giving it better freeze-thaw durability.
• Reduction of the Mud-Cracking Effect – less surface cracking, improving the durability of the concrete.

There are many more attributes that cellulose fibrils possess and have yet to be tested in a shotcrete medium, in addition, cellulose fibrils will contribute to a more sustainable and bio-based shotcrete industry.