Sprayed concrete, or its generic name of Shotcrete, is a mortar or concrete pneumatically projected at high velocity from a nozzle to produce a dense homogenous mass.
The force of the jet impacting on the surface compacts the material in place and can support itself without sagging, whether on a vertical, sloping or, within certain limits, an overhead surface.
In this process only the aggregate and cement are batched, mixed together and fed into a purpose made machine. The mixture is then pressurized and metered into a compressed air stream and conveyed through a pipe to a nozzle, where water is introduced as a very fine spray to wet the mixture. 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 more general term shotcrete.
In this technique, 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 added to project the material onto the surface.
The final product of either the dry or wet shotcrete process is very similar. The dry mix system tends to be more widely used where smaller volume and intermittent shotcrete is required. It generally uses smaller and more compact equipment which can be moved around relatively easily. The wet mix system is ideal for high production applications, where access allows the application equipment and delivery trucks to operate on a more or less continuous basis. Decisions to use the dry or wet mix shotcrete process are usually made on a site-by-site basis.
Developments of Amorphous Silica and Steel Fibres in Shotcrete
Of the many developments in shotcrete technology in recent years, two of the most significant were the introduction of amorphous silica (used as a cementitious admixture) and fibre reinforcement.
Microsilica SF is an extremely fine pozzolan. Pozzolans are cementitious materials which react with the calcium hydroxide produced during cement hydration. Microsilica SF, added in quantities of 8% by weight of cement, can allow shotcrete to achieve significantly higher compressive strengths than the value of plain shotcrete mixes.
The result is an extremely strong, impermeable and durable shotcrete.
Some rebounding material is unavoidable when concrete is projected at a relatively hard surface at high speed. This material, which consists mainly of the larger aggregate particles, is known as rebound. The inclusion of Microsilica SF will reduce the rebound due to the increased cohesion and paste bond to the coarser aggregates.
Other benefits include improved flexural strength, improved pumping, improved bond with the surface, and the capability to place thicker layers in a single pass because of the shotcrete's increased cohesiveness.
Steel and Macro fibre reinforced shotcrete has gained world-wide acceptance as a replacement for traditional wire mesh reinforced plain shotcrete. The main role that reinforcement plays in shotcrete is to provide ductility to an otherwise brittle material. 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 of significant magnitude may overload and lead to failure of the support system, unless that system has sufficient ductility to accommodate these deformations. The inclusion of Microsilica SF promotes increased bond to the fibres, therefore improving performance.