An analysis of flexural strength 

and crack width for fiber-reinforced 

shotcrete used in weak rock mines

L. Martin, B. Seymour, C. Clark and M. Stepan 

Research mechanical engineer, mining engineer, mechanical engineer and engineering technician, 

respectively, National Institute for Occupational Safety and Health, Spokane, Washington

R. Pakalnis, M. Roworth and C. Caceres

Mining engineers, University of British Columbia, Vancouver, British Columbia

Abstract

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 

Bernard, 2004).

Figure 2 — Shear resistance model for a loosened block 

acting as a rigid body (after Diamantidis & Bernard, 2004).

Figure 3 — Flexural resistance model for a loosened block 

representing a distributed load. (Diamantidis & Bernard, 

2004).

Background

As currently used in the underground gold mines in Nevada, 

shotcrete acts as a thin shell or lining, providing surface support 

between the primary ground support elements, the rock bolts. 

As the ground starts to unravel, the shotcrete shell between the 

bolts is loaded by the dead weight of the loose rock in the mine 

roof. The shotcrete provides a resistive force, supporting this 

dead weight load until the weight of the loose rock exceeds 

the tensile strength of the shotcrete, causing the shotcrete shell 

to break in flexure. The flexural strength of the FRS is one 

of the key physical property parameters for ground support 

design calculations. The RDP test provides design engineers 

with a measurement of the shotcrete’s peak flexural load at 

first break. As the shotcrete undergoes further displacement 

after this initial failure, the RDP test also provides important 

information about the shotcrete’s residual load capacity and 

its toughness of the fiber-reinforced shotcrete.

Some of RDP’s key factors that will be discussed in this 

paper include the peak flexural load, first break, crack-through, 

residual load carrying capacity and crack width. While observing a test on the machine, the load builds very quickly against 

the FRS panel, the panel starts to dish in the middle and then 

it pops. This is the first break with its corresponding peak 

flexural load, which usually occurs within the first 15 seconds 

of a ten-minute test (Fig. 4). From these two data, flexural 

strength is calculated. This is the maximum load applied to 

the panel before it breaks. It is evidenced as a hairline crack 

on the underside of the shotcrete. This test provides a conve-
nient standard for comparing shotcrete mixes having different 

mix constituents, including different types and densities of 

embedded fiber.

Figure 4 — Broken sample showing tension crack.

The next event to manifest visually is the crack-through 

of the panel. This is when one of the cracks reveals itself 

on the top of the panel crack-through. This is important, in 

that the load is now completely dependent on the shotcrete 

reinforcement. The reinforcement used could be either fibers, 

mesh or a combination of both of them. This now begins the 

load-carrying capacity of the shotcrete and the rock mass carrying capabilities between the bolts. Generally, the shotcrete 

residual load capacity at a given deflection is specified by 

the fiber type, gauge (diameter) and density used in the mix. 

Shotcrete residual load capacity is an important consideration 

when the material is expected to support rock mass in the mine 

roof. This is the most noticeable and observed trait for the 

major part of the test. The underside of the shotcrete panel 

opens up, increasing the crack width and exposing the fiber 

support matrix. The residual load capacity is provided by the 

fibers in the shotcrete mix, which break or are pulled out as 

the test progresses.

 A typical load versus displacement profile obtained from 

the test results is shown in Fig. 5. 

Figure 5 — RDPT Load vs. displacement graph indicating 

peak flexural load and residual load.

The area under the loaddisplacement curve from 0 to 40 mm is the energy absorption 

or toughness of the shotcrete. The peak flexural load can be 

easily identified from the graph. The residual load capacity 

is denoted by the load values after the peak flexural load, 

and represents the load that the shotcrete can still support at 

specific displacement intervals beyond the displacement at 

peak flexural load.

Figure 6 — Schematic of portable RDP test system.

Although RDP tests provide useful information, this type 

of testing has only been available in a few laboratories utilizing large stationary test machines. As a result, the shotcrete 

test panels usually have to be shipped after forming, causing 

logistical as well as testing problems, particularly for mines 

that are located in remote areas. Currently, very little information is available regarding either the flexural strength or the 

residual load capacity of FRS, especially for underground mines 

in the United States. Shotcrete toughness values have been 

related to different types of ground support requirements for 

recent Australian mining projects (Table 1). 

Table 1 — Typical toughness values specified in recent 

Australian mining projects (after AuSS, 2008).

Type of support Specified toughness

Nonstructural or low deformation 280 joules1

Moderate ground support 360 joules1

High-level ground support 450 joules1

140 mm deflection in ASTM C-1550

 However, these 

guidelines have not yet been implemented in U.S. mines; these 

listed shotcrete toughness values may not be appropriate for 

the Nevada mines, because the type of support required would 

more than likely be classified as non-structural (i.e., surface 

support) but with high deformation.

By providing load versus displacement profiles for the 

shotcrete, RDP tests can supply specific strength and physical 

behavior attributes for the shotcrete that are not typically known. 

By determining the residual load at a specific displacement 

interval (residual strength), it may be possible to relate the 

width of the crack formed on the bottom of the round panel 

specimen during the test to the residual load that the cracked 

shotcrete specimen is still able to support.
Round determinant panel (RDP) test

To more easily determine flexural and residual strength 

values, NIOSH researchers developed a portable round determinant panel test machine that can be transported and set 

up directly at the mine site (Fig. 6). The usefulness of this 

field-ready test machine has been previously documented in 

a FRS fiber dosage comparison study conducted at the Chief 

Joseph Mine in Butte, MT by Martin et al. (2007) and also at 

the Devil’s Slide Tunnel project near Pacifica, CA by Decker 

et al. (2010).

To prepare the round panel specimens for testing, FRS is 

sprayed into ring-shaped molds that are 800 mm in diameter 

and 75 mm thick. After the shotcrete is applied to the mold, 

the exposed top surface of the specimen is roughly leveled 

(screeded off) with a board and then finished flat with a trowel. 

The panels are stored and allowed to cure in an environmental 

chamber in a manner consistent with ASTM recommendations. 

The round panel specimens are tested after 7, 14 and 28 days 

of curing time, as prescribed in ASTM 1550.

The load applied to a round panel specimen during an RDP 

test (Figs. 4 and 6) roughly approximates the loading conditions 

shown in Fig. 3. Three reactant pins support the round panel 

specimen, as the test machine’s loading ram exerts a downward 

load on the center of the panel at a rate of 4 mm per minute. 

This produces a determinate set of three cracks along hinge 

lines spaced about 120° apart, as shown in Fig. 4. As the RDP 

test continues and the deflection in the panel increases, the 

cracks open on the underside of the panel, exposing the fibers 

in the shotcrete that bridge the crack. At the conclusion of 

the test, the round panel specimen is broken apart and closely 

examined along these cracks to determine both the distribution 

and quantity of the fibers in the applied shotcrete.

The RDP test results can be used to determine the peak 

flexural strength and residual load capacity for the shotcrete. 

These values and crack width properties of FRS can be used 

to better understand the in-mine sprayed shotcrete to allow 

ground control engineers to determine when mine re-entry or 

applied shotcrete rehabilitation is needed.

Test results and observations

NIOSH researchers conducted RDP tests using two commercially available FRS mixes that are commonly used for 

ground support in weak rock conditions. In addition to the RDP 

tests, unconfined compression and splitting tensile tests were 

also conducted with cored samples of these shotcrete mixes 

after 7, 14 and 28 days of curing. An average of the unconfined compression test results are listed in Table 2. 

Table 2 — Unconfined compression test results.

Unified compressive strength

Curing 

time, 

days

Shotcrete fiber type

Steel Poly

MPa psi MPa psi

7 11 1,616 10 1,450

14 13 1,858 12 1,740

28 16 2,294 17 2,465

 Splitting 

tensile strengths were generally about 12% of the measured 

compressive strength values. The compressive and tensile tests 

were primarily conducted to help evaluate the consistency and 

quality of the applied shotcrete.

 Plots of the load versus displacement measurements from 

RDP tests with steel FRS and poly FRS are shown below in 

Figs. 7 and 8, respectively. 

Figure 7 — Load versus displacement graphs for steel FRS.

Figure 8 — Load versus displacement graphs for poly FRS.

 A typical tested panel underside 

is shown in Fig. 9 with its distinctive 120 degree determinate 

sector cracks. RDP tests were conducted after 7, 14 and 28 

days of curing time.

Both of the FRS mixes evaluated in this study are dry-mix 

shotcrete that are typically used in underground mines to rehabilitate previously installed shotcrete. The steel FRS, trade 

name Superstick Shotcrete SCA-PT100, has an open-staple 

design with smooth steel fibers that are 40 mm long. This 

product is applied at a steel fiber dosage of 44.5 kg/m3. The 

poly FRS, trade name Superstick Shotcrete SCAPF, has twisted, 

smooth polyfibers that are 50 mm long. The fiber dosage for 

the synthetic polyfibers in this product is 4.15 kg/m3.

Average peak flexural loads from the RDP tests are comparable to similar values shown in the AuSS guide (2008), 

below 25 kN. These test results also compare favorably with 

a shotcrete mix that was specifically designed for the Rodeo 

Mine near Carlin, NV by D. Thibodeau in 2005. As reported 

by Martin et al. (2007), this particular shotcrete mix had an 

average peak flexural load of 35 kN at a fiber dosage of 6.53 

kg/m3 using Kudo, bar-chip I fibers.
Fiber comparison. As expected, the peak flexural load increased with shotcrete curing time (Figs. 7 and 8). However, the 

type of fiber (steel or poly) did not appear to significantly affect 

this peak load value, even though the polyfiber specimens did 

have a slightly larger average peak load compared to the steel 

fiber specimens, particularly after 28 days of curing. As the 

shotcrete cured, it increased in toughness compared to earlier 

curing times. This may indicate that the steel fibers become 

more interlocked in the shotcrete matrix as the shotcrete ages.

The steel FRS appeared to provide more uniform RDP 

test results. In contrast, the RDP test results for the poly fiber 

shotcrete were not as consistent with a couple of the tests, 

producing higher loads over the rest of the tests conducted. 

While the peak flexural load generally increased with curing 

time, the residual load capacity at given deflections of the 

shotcrete varied dramatically depending on the curing age of 

the shotcrete. As the polyfiber shotcrete cured, it appears to 

have become more brittle. Consequently, the post crack load 

that the shotcrete was able to support decreased markedly with 

an increase in curing time for a given displacement beyond the 

first break failure. These test results clearly indicate that the 

type of fiber governs to a large extent the residual load capacity 

(area under the load displacement curve) of FRS.

The difference in these two sets of curves also demonstrates 

quality control issues associated with different types of shotcrete 

mixes. Inconsistencies in the RDP test results for the poly FRS 

are at least in part caused by a higher percentage of rebound 

and a greater variability in fiber count for the applied poly fiber 

shotcrete versus the applied steel fiber shotcrete.

Crack width. The load and crack width values are shown in 

Fig. 10 and visually presented in Figs. 4 and 9. These graphs 

are the average results obtained from a series of the three round 

panel tests conducted for the times indicated.

Results from the round panel tests for loads versus crack 

width are presented in Fig. 10. 

Figure 9 — Typical round panel with three cracks.

Figure 10 — Panel load versus crack width at center of panel.

These values were obtained 

by measuring the dilation or width of the crack at post peak 

flexural loads. Further measurements were taken after a crack 

had progressed through the entire thickness of the shotcrete 

panel and was visually apparent on the upper surface of the 

panel. This stage of crack development in the round panel 

specimen during the RDP test is referred to as crack-through. 

While the RDP test was being conducted, crack width values 

were measured on the underside of the shotcrete panel using 

a caliper at specific displacements of test machine’s loading 

ram (e.g., 10, 20, 30 and 40 mm of displacement).

For the crack width measurements, the steel fiber shotcrete as 

a general rule supported higher loads with less crack width than 

the poly fiber shotcrete at a given measure of ram displacement 

(10-40 mm). As shown in Figs. 7, 8 and 10, the shotcrete still 

provides support after the first break. In contrast, a load versus 

displacement profile for a unfibered shotcrete would indicate 

that the cracked shotcrete is no longer providing support after 

the first break. The fact that the crack width expands and the 

sections still remain attached illustrates the type of in-place 

support characteristics that have been observed for applied 

shotcrete in underground mines, where the cracked FRS still 

retains the loose material and prevents the rock mass from 

unraveling between the bolts.

Crack-through. For the steel fiber shotcrete, the load at 

crack-through remained consistent with curing time, but the 

displacement at crack-through increased with shotcrete curing 

time, with 20 mm displacement after seven days of curing to 

28 mm of displacement after 28 days of curing. In contrast, 

the displacement at crack through for the polyfiber specimens 

remained fairly consistent regardless of curing time, but the 

load at crack-through increased slightly with an increase in 

shotcrete curing time.

As shown in Figs. 7, 8 and 10, round panel tests can provide 

important information for determining the available ground 

support capacity of the in-place shotcrete and can also indicate whether or not rehabilitation of the shotcrete is needed. 

Because the movement of the shotcrete is difficult to detect, 

much less determine underground, the width of the exposed 

crack on the surface of the shotcrete may be a more appropriate 

indication of the deformation of the shotcrete after first break. 

Instruments for monitoring the geomechanical behavior (stress 

change, deformation, crack dilation, etc.) of the shotcrete are 

currently not available.

Methods for conducting visual assessment of 

FRS

The RDP test results and the crack widths that are measured and plotted with residual loads in Fig. 10 may provide a 

practical means for underground miners to assess the stability 
of shotcrete applied to the surfaces of underground openings. 

Assessment should include the following questions: 

• What is the shotcrete on the mine back telling the mine 

personnel?

• What does a hairline crack represent in the shotcrete 

life cycle?

• What does a pencil-tip width represent in the shotcrete 

life cycle?

• What does a finger- or a thumb- width represent in the 

shotcrete life cycle?

These questions are answered, at least in part, through the 

RDP test results and the additional insight gained by carefully 

observing the loading behavior of the round panel specimens 

during the test. For example, the peak flexural load is the 

maximum load applied to the panel before it breaks. At peak 

flexural load or first break, a small hairline crack can be observed on the underside of the shotcrete panel. This peak load 

value determines how much rock mass load the shotcrete can 

support under similar loading conditions.

The next event to observe is the crack-through of the panel. 

This is when the crack has progressed and broken through the 

entire thickness of the panel. The load is now primarily supported by the shotcrete reinforcement, which, depending on 

the application, may consist of fibers, mesh, or a combination 

of both fibers and mesh. At this stage of loading, the support 

provided by the shotcrete is dependent on the cross sectional 

area of the panel reinforcement, fibers and/or mesh. The typical crack width observed on the bottom or underside of the 

shotcrete panel at this phase would be about a pencil-tip width.

During the next phase of the RDP test, the crack width is 

measured in terms of the shotcrete’s residual load-carrying capacity, after specific displacement ranges for the test machine’s 

loading (10 mm through 40 mm). As the RDP test continues, 

the cracks on the underside of the shotcrete panel open up and 

significantly increase in width, exposing the shotcrete’s fiber 

support matrix. This same phenomenon can be observed in 

underground mines, where the applied shotcrete is cracked 

and its fibers are exposed. An estimate of the post-crack loadcarrying capacity of the applied shotcrete at a mine may be 

obtained by determining how much residual load the round 

panel specimen can support with the same corresponding crack 

width. Using common place measures for crack width, such 

as the width of a finger or thumb, may provide a convenient 

universal standard for visually representing the shotcrete crack 

width. As shown in Fig. 7, the residual load-carrying capacity 

of the shotcrete is relatively low at this stage of loading (2-4 

kN at a 10-15 mm crack width).

Ground support load calculations

Determining the rock mass load that the applied shotcrete 

supports in an underground mine is difficult, because the load 

exerted on the shotcrete by loose material near the surface of 

the mine opening is hidden from observation. However, the 

maximum load anticipated from this loose material can be 

roughly estimated based on dead weight load calculations. 

Although these calculations are conservative, they provide 

an established method for estimating the maximum expected 

load on the shotcrete. In this multicomponent ground support 

system, the rock bolts provide the primary ground support and 

are therefore assumed to hold the active mining loads. The 

shotcrete, whether intact or cracked, is assumed to provide 

surface support for loose material in the immediate mine roof 

between the bolts.

Because the shotcrete primarily provides surface support 

between the bolts, the geometry of the bolt spacing can be 

used to determine the maximum volume of loose material 

that would be expected to load the shotcrete. Two methods 

are commonly used to predict the maximum volume of loose 

material that would be reasonably expected to load the shotcrete. 

These methods are based on the three-dimensional shape of the 

assumed volume of loose material. Using the block method, 

the volume of loose material is assumed to form a large block 

of ground that would separate from the mine roof within the 

existing bolting pattern or spacing. Using this method with a 

bolt spacing of 1.2 by 1.2 m, the volume of the largest block 

of ground that would be expected to fall out of the mine roof 

between the bolts would be 1.2 x 1.2 x 1 m high. For convenience, the rock block is assumed to have a 1-m height; therefore, the weight of the block (Wtb) is estimated to equal 1.2 t.

In weak, raveling ground conditions, a cone method provides a more accurate estimate of the expected volume of 

loose material that would load the shotcrete. As shown in 

Fig. 11, the base of the cone is a 1.2-m2 circle circumscribed 

inside of the assumed bolting pattern. 

Figure 11 — Estimated cone volume for determining dead 

weight loading between the bolts, bolt spacing = 1.2 m.

The height of the cone 

is assumed to equal to half of the 1.2-m span or 0.6 m, based 

on the volume of loose material that would reasonably be expected to separate from the mine roof at this span dimension 

(Pakalnis, 2002). As a result, the weight of the cone (Wtc) is 

estimated to equal 0.6 t.

Ground support load to panel load calculations

However, these calculations and comparisons do not account 

for the quality of the applied shotcrete or its adhesion strength, 

both of which significantly affect the ground support capability 

of the in-place shotcrete. Shotcrete adhesion strength can be 

measured at desired locations in an underground mine using 

a portable direct tensile test system, as explained by Seymour 

et al. (2010). In addition, the RDP test does not exactly replicate the loading and failure conditions of in-place shotcrete 

and a maximum expected load is estimated from the ground 

support load calculations, rather than the actual load applied to 

the shotcrete. Correcting for discrepancies in the geometrical 

length of the shotcrete panel and the assumed bolt spacing, the 

RDP test currently provides the best available method for ap-
proximating the ground support loading of applied shotcrete. 

The geometrical size has been changed to the bolt geometry 

with plates at 1.2 m, but the only repeatable test for the mining 

industry is the round panel ASTM C-1550 test, to relate loads 

as seen by the shotcrete. Although the ratio of load-to-crack 

width will remain the same from test to field even with geometry, changes are expected when measurements are converted 

to units based on (kNmm or Nm) joules.

Given that 10 kN is equivalent to 1.0 t, the 0.6-t dead weight 

of loose material in the cone volume can be converted to an 

equivalent force of 6 kN. This rock mass load can then be used 

in conjunction with the RDP test results shown in Figs. 7- 9 

to determine the residual load capacity at a given deflection 

of the shotcrete with the corresponding crack width. From the 

RDPT results, the peak flexural strength at first break was usually much greater than 6 kN, indicating that the shotcrete mix 

design is more than adequate. To further assess the stability of 

in-place shotcrete, the 6 kN rock mass load can be compared 

with the residual strength at given deflections of the shotcrete 

shown in Figs. 7, 8 and 10.

If the shotcrete has cracked, the load versus displacement 

curves for both the steel fiber and polyfiber shotcrete indicate 

that the shotcrete may not be stable after more than 10 mm of 

displacement or a corresponding crack width of 2 mm, about 

the width of a pencil tip.

Comparing the 6-kN rock mass load with the post-peak load 

values after first break that are shown in Fig. 7, it appears that 

none of shotcrete panels tested would be able to safely support 

this rock mass load for an extended period of time.

As a result, the shotcrete design methodology mentioned 

above is a good initial approach, but further testing is needed 

to quantitatively relate the crack width of the shotcrete to its 

residual strength. These residual load capacity values then 

need to be compared with some type of realistic assessment 

of shotcrete loading conditions in underground mines to more 

appropriately determine the loads that cracked shotcrete can 

safety support.

Strength characteristics of fiber-reinforced 

shotcrete and welded wire mesh as ground 

support system components in weak rock mass

Shotcrete is a brittle system with fibers to give it some 

yield ability; in other words, it is a soft mining system that 

is known to work by acting as a bag to keep loose rock from 

falling down. Both FRS and wire mesh systems are set for a 

1.2 x 1.2-m bolt system with plates.

In calculating the applied shear on the shotcrete as mentioned 

in Fig. 2, a length of 4.8 m is used with a bolt spacing of 1.2 

m2. This value multiplied by the shotcrete thickness estimates 

the shear area. This value is then multiplied by 0.25% of the 

shotcrete compressive strength to give shear strength of about 

2 MPa (Table 3). 

Table 3 — Wire bag strength, 1.2 x 1.2-m (4 x 4-ft) pattern.

Item Gauge Bag strength, t

10 x 10 cm (4 x 4 in.) welded wire mesh 4 3.6

10 x 10 cm (4 x 4 in.) welded wire mesh 6 3.3

10 x 10 cm (4 x 4 in.) welded wire mesh 9 1.9

10 x 5 cm (4 x 2 in.) welded wire mesh 12 1.4

5-cm (2-in.) chain link 11 - bare metal 2.9

5-cm (2-in.) chain link 11 - galvanized 1.7

5-cm (2-in.) chain link 9 - bare metal 3.7

5-cm (2-in.) chain link 9 - galvanized 3.2

4 gauge = 0.58 cm (0.23-in.) diameter; 6 gauge = 0.5-cm (0.2-in.) diameter; 9 gauge = 0.4-cm (0.16-in.) diameter; 11 gauge = 0.3-cm 

(0.125-in.) diameter; 12 gauge = 0.28-cm (0.11-in.) diameter

Shotcrete shear strength = 2 MPa = 200 t/m2

This value may or may not be used when 

conducting support design.

To relate the ground support capability of shotcrete to that 

of wire mesh, the peak flexural strength of the shotcrete at first 

break can be compared with the wire mesh bag strength values 

listed in Table 3. The bag strength of the screen listed in Table 

3 is developed at over 254 mm deflection (Dolinar, 2006).

The energy of the panels and the energy of the wire mesh 

would be comparable in joules, but the use of the two systems 

is completely different. The shotcrete is used to contain the 

rock mass with small deflections and high loads while producing energy, whereas the wire mesh is used for large deflections 

of unraveling rock mass while building loads that produce 

energy. One is a stiff ground support system and the other is 

flexible. The reality is that the systems are used together in 

mines and therefore need to be analyzed as such. In work 

conducted by Stacey and Ortlepp (2001), an extensive test 

series was conducted that showed wire mesh gave 10 kJ at 100 

mm of deformation, while welded wire mesh shotcrete gave 

15 kJ at 150 mm of deformation. This data set shows that the 

ground support system is optimized when the support system 

components, the wire mesh and the shotcrete, are combined.

Conclusions

A series of round determinate panel tests were conducted 

with steel and synthetic ploy fibers, using two commercially 

available weak rock mass FRS mixes. Peak load and displacementwas measured and both peak flexural strength and residual 

flexural strength were determined.

The FRS specimens enter “peak crack strength” with the 

appearance and propagation of a determinate crack. After this 

phase of the test, the specimens are in the residual load carrying 

capacity of the fiber matrix. This occurs at about 5 to 10 mm 

of vertical deflection. If the shotcrete has cracked, the load 

versus displacement curves for both the steel fiber and poly 

fiber shotcrete indicate that the shotcrete may not be stable after 

more than 10 mm of displacement or a corresponding crack 

width of 2 mm, about the width of a pencil tip.

In addition, the progression of the post-peak cracks were 
mapped. These values were compared to calculated ground 

force loadings using two popular methods. The fact that the 

crack width expanded while the sections remain attached indicates the type of in-place support characteristics seen, where 

the FRS prevents the rock mass between the supports from 

unraveling. For the crack width measurements, the steel fiber 

shotcrete as a general rule supported higher loads with less 

crack width than the poly fiber shotcrete at a given measure 

of ram displacement (10-40 mm).

A visual assessment leading to better assessment of shotcrete 

support load capability and ground support stability in mine 

FRS support characteristics is offered, based on the similarity 

of the failure process observed in the RDP test.

Finally, a qualitative assessment of the crack width at which 

rehabilitation would be necessary at 5-10 mm or 2-4 kN is offered. The load-carrying capacity at the mine can be thought 

of as how much rock mass can be applied to the panel with 

the corresponding crack with. The visual representations of 

these crack widths are the finger and thumb.

Disclaimer

The findings and conclusions presented in this document 

have not been formally disseminated by the National Institute 

for Occupational Safety and Health and should not be construed 

to represent any agency determination or policy. Mention of 

any company name or product does not constitute endorsement by NIOSH.

Acknowledgements

The authors would like to thank James Schumacher with 

Thiessen Team USA in Elko, NV and Vince Mindive with 

Thiessen Team USA in Big Timber, MT for their technical 

support concerning commercially available shotcrete mixes. 

We also thank Justin Deemer and Rocky Wegland at Westech 

Automation in Spokane, WA for their technical and mechanical 

assistance developing the portable RDPT machine.

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