There are two different processes for shotcrete:

1. Dry process

The dry mix system is a popular choice where smaller volume and intermittent shotcrete is required – because it requires smaller and more compact equipment.

1. The aggregate and cement are batched, mixed together and fed into a purpose-made machine.
2. The mixture is pressurized and metered into a compressed air stream, then conveyed through a pipe to a nozzle, where water is introduced as a very fine spray to wet the mixture.
3. It is then projected continuously into place.

Gunite, a proprietary name for dry-sprayed mortar used in the early 1900's, has fallen into disuse in favour of the modern ‘shotcrete’.

2. Wet process

The wet mix system is ideal for high production applications, where access allows the application equipment and delivery trucks to operate on a continuous basis.

• Cement, aggregate and water are batched and mixed together, prior to delivery, into a positive displacement pumping unit.
• This unit delivers the mix hydraulically to the nozzle where air is then added to project the material onto the surface.

Shotcrete technology has seen many developments over the years, but amorphous silica and steel fibres are by far the most significant and have made the largest impact.

Steel and macro fibres

In shotcrete, reinforcement provides ductility to a brittle material. Steel and Macro fibre reinforced shotcrete have gained world-wide acceptance as a replacement for traditional wire mesh reinforced plain shotcrete.

Rock support is only called upon to carry significant loads once the rock surrounding an underground excavation deforms. This means that unevenly distributed, non-elastic deformations may overload and lead to failure of the support system – unless that system has sufficient ductility to accommodate these deformations.

Inclusion of Microsilica SF

Microsilica SF is an extremely fine pozzolan – a cementitious material that reacts with the calcium hydroxide produced during hydration. While rebounding material is an inevitable byproduct of shotcrete’s high velocity, Microsilica SF helps to reduce it. It’s other benefits also include:

• Improved flexural strength and pumping
• Improved surface bond
• Capability to place thick layers in a single pass

Our recommendation: add Microsilica SF in quantities of 8% by weight of cement to achieve significantly higher compressive strengths than the value of plain shotcrete mixes. The end product will be an extremely strong, impermeable shotcrete.