Shotcrete is pnumaticaly applied fined grained mortar i.e. usually placed in a 20 to 100 mm layer and is often reinforcement to improve tensile and shear strength. Zones and beds of closely fractured or degradable rock may be protected by applying a layer of shotcrete to the rock face. The shotcrete will control both the fall of small blocks of rock, and progressive raveling that could eventually produce unstable overhangs. Primary function of shotcrete is to provide surface protection. The two common methods of reinforcing are welded-wire mesh, or steel or polypropylene fibres (Figure 33).
Shotcrete is applied either by wet or dry applications. For dry mix shotcrete, additives and mortar are mixed on-site and pumped via compressed air to the nozzle, where the water is added. The wet mix is premixed at a central plant to specifications and then transported to the site in bulk.
An alternative to mesh reinforcement is to use steel or polypropylene fibers that are a component of the shotcrete mix and form a reinforcement mat throughout the shotcrete layer. The steel fibers are manufactured from high strength carbon steel with a length of 30–38mm and diameter of 0.5 mm. The principal function of fibers is to significantly increase the shear, tensile and post crack strengths of the shotcrete compared to non-reinforced shotcrete.
Steel fibers, when added to the mix, increase the tensile strength of the shotcrete by providing numerous bonding surfaces within a small area. The fiber reinforcement also reduces the risk that shrinkage cracks will develop during curing. In many cases, the addition of fibers can replace wire mesh as reinforcement, thus reducing the overall cost.
The addition of drain holes through the shotcrete is essential to eliminate water pressure behind the shotcrete. Small length of steel PVC pipe, inserted prior to shotcrete application into joints in the rock face where seeps have been noted or where seeps may occur, will provide partial drainage. Other drain holes should be created at regular intervals along the slope face. It is widely used technique for not only stabilization of slope but also used in underground excavations.
Figure 33: Application of shortcut in the loose rock(a) with wire mesh (6).