Fiber Reinforced Shotcrete
September 7, 2017
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 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.
FRS is an excellent Value Engineering selection in lieu of conventional reinforcement systems providing several technical, economical, and safety benefits as compared to traditional secondary, steel reinforcement including, but not limited to, temperature-shrinkage crack resistance, crack-width control, impact and abrasion resistance, and spalling resistance. The long-term durability benefits far outweigh the often-questionable performance of wire mesh at a very competitive cost.
With the help of your FiberForce Representative, the correct product and dosage rate can be selected to meet the shotcrete project specifications for temperature-shrinkage reinforcement or as a complimentary reinforcement system.
TUNNEL LINERS: Tunnels liners derive several benefits from FRS, but the most noteworthy benefit relates to the innermost liner that is used to limit damage caused by res and/or explosions. The refractory industry learned over 20 years ago that when synthetic fibers melt and evaporate, they provide a network of voids that allow for temperatures to stabilize and vapor pressures to dissipate, thus retaining the stability of the liner and reducing the potential of spalling. Typically, monofilament microsynthetic, polypropylene fibers of 1⁄2” to 3⁄4” length and 3 to 15 deniers provide the most spalling resistance.
SLOPE STABILIZATION: Slope stabilization has been a standard for FRS. Where slopes are steep, for example, most railroad cuts, the use of WWF is nearly impossible. FRS is the chosen option. Here the fibers provide a three-dimensional reinforcement system that resists temperature and moisture related volume change, which with time, can create cracking. Furthermore, the FRS provides enhanced impact and surface abrasion resistance. When considering rock slides and other natural concerns, FRS has become a standard with railroads and transportation agencies.
WATER CONVEYANCE: Other excellent application for FRS include water diversion channels, irrigation ditches, and cast-in-place pipe. FRS resists temperature-shrinkage cracking and enhances impact and surface abrasion resistance plus the fibers yield two additional important benefits. Based on field tests of water diversion channels and cast-in-place pipes, it was determined that the fibers measurably reduced the number of cracks and the size of the cracks, which equates to less water lost to the sub-grade. The other quantifiable benefit is the increased durability of the channel. The fibers have a major impact on reducing cavitation and erosion of the concrete’s surface due to waterborne particulates. This is particularly true during storm events when the velocity of the run-o water is high and contains increased volumes of material gather by the water.