An analysis of flexural strength and crack width for fiber-reinforced shotcrete used in weak rock mines

The National Institute for Occupational Safety and Health (NIOSH), Office of Mine Safety and Health, Ground Control Engineering Branch is investigating the use of shotcrete in weak rock mass mines with the objective of reducing fatalities and injuries resulting from rock fall accidents. When shotcrete is used as part of a multielement ground support system, there is a need to know the support characteristics that the shotcrete contributes to the overall system. To quantify the support provided by the shotcrete, flexural strength tests were conducted with two common, commercially available fiber-reinforced shotcrete (FRS) mixes using the round determinant panel (RDP) test method, ASTM C 1550. A portable RDP test machine developed by NIOSH researchers was used to determine the peak flexural load and residual load capacity for a steel FRS and a synthetic poly FRS that were sprayed using dry mix procedures and equipment. Besides the flexural strength and loading behavior determined from these tests, a method was also developed for measuring the width of cracks exposed on the underside of a shotcrete panel during a RDP test and relating these measurements to residual load values. Quantifying the peak flexural load and residual loads of a shotcrete mix through on site testing at a mine and visually assessing the loading cycle and load-carrying capability of the shotcrete applied to underground workings will improve mine safety by providing a better assessment of the stability of shotcrete supported entries. Introduction Underground mines in the western United States often have weak, raveling ground conditions (RMR76<44), particularly the underground gold mines in Nevada. A typical drift in these mines usually has a span of less than 4 m (13 ft) and a service life of less than one month (Pakalnis, 2002). In these weak rock conditions, the surface or skin of the underground openings must be continually supported in order to adequately protect underground workers. The traditional support method for this type of ground is to install rock bolts for the primary ground support and then use wire mesh to support the loose broken material between the bolts. If the wire mesh does not provide sufficient support to maintain the integrity of the surface of the underground opening, shotcrete is applied, either instead of the mesh or in conjunction with the mesh to retain the loose, small material that would otherwise fall out. By providing a stiff flexural support that limits movement at the shotcrete/rock interface, the shotcrete securely holds the loose material in place, whereas the mesh exhibits a more flexible response to loading, stretching under load and allowing the broken material to move out of position. To provide additional support, fiber-reinforced shotcrete (FRS) is used instead of conventional shotcrete in the most extreme ground conditions (Pakalnis, 2010). The fibers hold the shotcrete together, preventing fragments of cracked shotcrete from falling out between the bolts and wire mesh. In most underground mines, shotcrete is typically used to provide surface support in a multicomponent ground control system consisting of rock bolts, wire mesh, plates and shotcrete. A more complete explanation regarding the use of shotcrete in mechanized cut-and-fill stopes in Nevada is given by Clark et al. (2010). Figures 1-3 illustrate three general types of shotcrete loads and their resultant failure modes. Pertinent ground support design parameters for these idealized loading conditions include the thickness, flexural strength and residual strength (load capacity at deflection) of the shotcrete, the span between the rock bolts and the size, mass and distribution of the rock load at the mid-span distance between the bolts. The failure shown in Fig. 1 occurs in blocky ground when a large rock mass loads the shotcrete lining to produce a moment that breaks the shotcrete at the mid span near the bolts. The weight of the block mass illustrated in Fig. 2 is held by the shear strength of the shotcrete lining around the block perimeter. This shear failure mode is more common in hard, jointed rock. A different flexural loading model, representing a shotcrete lining that supports more highly fractured ground, is illustrated in Fig. 3. The highly fractured ground conditions and uniformly distributed flexural loading shown are encountered in many underground mines in the western United States, particularly in Nevada. This type of shotcrete loading is the main focus for this paper. Furthermore, the use of fibers in the shotcrete enhances its support characteristics by giving it a post-break toughness. Toughness can be assessed in terms of either the residual load capacity or energy-absorption capacity, joules (kNmm or Nm). The former typically is used for measurements taken between the onset of loading and a specified deflection in a beam or panel test, while the latter is used for the area under the load deflection plot for the test specimen (AuSS, 2008). To find peak and residual loads, along with the corresponding toughness or energy, FRS testing is conducted using the current testing practices of round determinate panel (RDP) after Bernard, 2002 and 2006. This peak load is directly related to the shotcrete’s tensile strength and is primarily influenced by the water-to-cement ratio and the Portland cement content of the shotcrete mix. A shotcrete testing standard, ASTM 1550-05 round determinate panel (RDP) test, was designed to replicate the typical shotcrete loading conditions in a mine or tunnel and gives the best representation and test repeatability. An explanation of the theory behind the round panel test can be reviewed in Johnansen (1972). For further explanation, Tran et al. (2005) give an eloquent review of multiple findings utilizing the yield line theory for post crack use and how the RDP test results compare back to the yield line theory. Additionally, a set of support guidelines has also been developed based on FRS strength properties (Papworth, 2002; Grant et al., 2001; Barton et al., 1974; Bernard, 2002; AuSS, 2008). Another issue with a ground control system is ensuring that the level of ground support provided is safe. The installed support characteristics of FRS are difficult to determine in mine openings. There is a risk that miners will enter mine openings that no longer have sufficient strength to provide adequate ground support. To address these concerns, the researchers from the National Institute for Occupational Safety and Health (NIOSH) have developed at-mine-site testing and conducted studies on the development of test specimen cracks. These cracked shotcrete widths to applied loads have been selected for post crack analysis. The ultimate goal of this work is to better enable a mining engineer to characterize the in situ load-carrying strength properties of mine-applied shotcrete based on a visual assessment of the condition of the shotcrete back as indicated by cracks and post-break crack width and, thus, determine when the mine support system is safe for re-entry of miners and machinery. Figure 1 — Flexural resistance model for a loosened block representing a concentrated load (Diamantidis and