Fibres are an evenly distributed form of post-crack reinforcement that can be added to fresh concrete prior to spraying thereby eliminating the need for labour intensive installation of reinforcing bars. The fibres bridge cracks that may form in the concrete matrix and help to redistribute load to uncracked regions of a Fibre Reinforced Shotcrete (FRS) lining. Post-crack performance testing is used to determine which fibre and mix design is the most effective for a particular application. The only part of a FRS specimen that is assessed during a toughness test is the cracked section. It is therefore important to obtain a large cracked section to produce results that are truly representative of the sprayed material. The large size of a round panel specimen compared to beams means that the resulting data more accurately represents the performance of the material produced during spraying.

The ASTM C-1550 round panel test involves the imposition of a point load at the centre of a round panel measuring ∅800×75 mm centered on three symmetrically arranged pivots located on a 750 mm diameter circle (see Figure 1). The loading piston is advanced at a constant rate of 4 mm/min. The test proceeds to a total central deflection of 40 mm after which the energy absorbed by the specimen is obtained as the area under the load-deflection curve. The load-deflection response typical of a panel reinforced with Barchip fibres is shown in Figure 2. The round panel test offers designers, contractors, and owners several important advantages over alternative forms of postcrack performance assessment. The most important of these is the low within-batch variability in results due to the repeatability of the cracking pattern, but other advantages include the elimination of saw-cutting from the process of specimen production and the use of easy-to-prepare form work.

Extensive testing of FRS based on round panels for tunneling and mining projects in Australia has demonstrated a reduction in QC costs of about 40% compared to the use of beams. Performance data obtained from C-1550 panels is presently used as part of “observational design” during ground stabilization in tunnels and mines. This involves the selection of lining thickness and minimum FRS toughness by an engineer at the work face based on the level of ground instability observed during excavation. Stable ground situations typically require a FRS displaying 300 Joules of energy absorption up to 40 mm central deflection, or an energy absorption value specified at a smaller deformation. The exact figure required will depend on the ground-induced loads expected on the lining. Unstable ground has been found to require FRS with a toughness of at least 500 Joules at 40 mm central deflection. For extreme ground conditions the test may be performed to 100 mm central deflection to evaluate the performance of alternative FRS mixes. Load, kN