In this post we are covering the topic of fiber-reinforced shotcrete.
A combination of sprayed concrete technique and steel fibers technology gives advantages when needed to place conventional reinforcement.
The replacement of meshes by fibers generates time savings that are associated with the placement and fixation of the meshes. In addition, it allows significant savings in the amount of concrete released because the thicknesses can be reduced.
The presence of steel fibers in the layer of shotcrete will produce a tensile strength along a cracked section in the area of tension. The distribution of the resistances throughout the section depends on the type of fiber, percentage used and magnitude of the tension.
The fiber shotcrete method has many advantages in comparison to traditional mesh reinforcement. Production of SFRS shotcrete is preferred option in the tunneling construction cycle because it eliminates the time-consuming application of wire mesh. Fixing mesh to a tunnel wall is difficult, time consuming, costly, and sometimes hazardous. On an irregular tunnel surface, mesh is pinned mostly at spots that project from the surface. Mesh is pinned back inside large depressions and draped over most small ones. Filling the voids behind the mesh in these draped over areas takes extra shotcrete, more than what is required by the minimum specified thickness. Placing fiber shotcrete is easier and safer.
The application of the shotcrete with steel fibers is versatile and flexible, since it adheres and molds very easily to the surface, covering it completely without the generation of cavities or shadows (this is the case of using traditional solutions added to the possibility of an unsuitable technique).
During shotcreting operation if incorrect pressure is applied or shot-gun is not held at the correct distance from the rock face, shotcrete can build up on the face of the mesh, leaving voids and sand pockets behind it. Not bonded properly to the rock, this shotcrete will most probably deteriorate quickly, especially if exposed to groundwater or freezing conditions.
SFRS can follow the exact contours of the rock resulting in less materials consumption. Shotcreting an irregular rock surface may require up to 40 percent more mesh-reinforced shotcrete than SFRS. SFRS also takes no extra time or money to install, except for the few minutes to add the fibers to the mix.
Fibers restrict early shrinkage cracking and may also have a positive effect on the bond to the rock surface. Cracks will be evenly distributed and their widths will be smaller than in mesh-reinforced shotcrete.
The high tensile strength of steel fibers will allow the shotcrete reinforced with them (SFRS), even when cracked, to resist tensile stress. For the structural reinforcement of the shotcrete, the minimum recommended dosage of steel fibers in order to reach acceptable ductility index values is 30 kg / m3.
Recommended SPAJIC Steel fibers for Shotcrete are:
Comparative Design for Steel Fiber Reinforced vs Double Mesh Reinforced Shotcrete
Design comparison to achieve same ultimate bending moment capacity:
Flexural toughness of shotcrete shall be determined based on European Standard EN 14651 test method for steel fiber concrete. The mechanical properties obtained from the test include the residual flexural strength at the following deflections:
• fR,L = residual strength at a CMOD = 0.05 mm (LOP-Limit of Proportionality)
• fR,1 = residual strength at a CMOD = 0.5 mm.
• fR,2 = residual strength at a CMOD = 1.5 mm.
• fR,3 = residual strength at a CMOD = 2.5 mm.
• fR,4 = residual strength at a CMOD = 3.5 mm.
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