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Shotcrete, simply put, is spray-applied concrete. Concrete is pumped through a hose and projected at high velocity onto the desired surface. There are two methods of shotcrete application, dry (also called gunite) and wet. With the dry method, the dry ingredients and water are not mixed until they reach the nozzle of the hose, where the nozzleman controls the addition rate of the water. With the wet method, conventional concrete meeting the requirements of ACI 506 is pumped directly to the nozzle. The dry-mix method is not as common anymore, however, it is still used for pool applications and sometimes in repair applications. The dry-mix method requires a very skilled nozzleman to control the water content, and this method produces considerably more waste as compared to the wet method due to loss of dry material as it is sprayed or as rebound as it hits the contact surface. The wet method produces less waste, either as away material or rebound. A major benefit is that larger volumes/thicker layers can be placed in less time as compared to the dry method.

Traditionally, welded wire fabric (WWF) is used as temperature-shrinkage reinforcement in shotcrete applications. However, as with slabs-on-ground, steel and synthetic fiber reinforcement is an advantageous alternative, both technically and economically.

First, fibers reduce rebound, which provides a cost advantage by increasing the yield per cubic yard of concrete. This increase could exceed 10%, depending on factors such as fiber configuration, length, and dosage.

Second, WWF is rigid, so it cannot follow the contours of the receiving face, thus resulting in an uneven thickness of the shotcrete being applied. Again, this reduces the yield per cubic yard of concrete. Fiber Reinforced Shotcrete (FRS) also allows for a uniform thickness and uniform density following the contours of the receiving face. When using the WWF, there is always the issue of uniform density of the shotcrete layer because if the shotcrete is not built up correctly around the WWF, voids in the shadow of the WWF become a concern. WWF is also cumbersome and time-consuming to install, but fiber reinforcement is simply added to the concrete mixing system and pumped directly with the concrete eliminating much of the time, labor, and hazards (and consequently the costs) associated with the installation of the WWF.

Finally, thicker layers of FRS can be applied. ACI 506.1R-08 Guide to Fiber Reinforced Shotcrete gives guidance on FRS production, testing, and quality control procedures, as well as design considerations when comparing to traditional steel reinforcement such as WWF, and this guide can be referenced for more detail.

Fiber Reinforced Shotcrete Applications