1 

AN ANALYSIS OF FIBER-REINFORCED ROUND PANEL STRENGTHS AND COMPARISON TO WIRE MESH BAG 

STRENGTH 

L. Martin, NIOSH, Spokane, WA

B. Seymour, NIOSH, Spokane, WA

C. Clark, NIOSH, Spokane, WA

M. Stepan, NIOSH, Spokane, WA

R. Pakalnis, Univ. of British Columbia, Vancouver, BC, Canada

M. Roworth, Univ. of British Columbia, Vancouver, BC, Canada

C. Caceres, Univ. of British Columbia, Vancouver, BC, Canada

ABSTRACT 

Researchers at National Institute for Occupational Safety and 

Health (NIOSH) are investigating how fiber-reinforced shotcrete mix 

designs can be extrapolated from proven designs employing wire 

mesh. This paper explains the results of round determinate panel tests 

and relates the flexural strength and toughness of fiber-reinforced 

shotcrete to known energy absorbing properties of wire mesh. The 

energy absorbed with central deflection is strongly influenced by the 

amount of fibres. The higher the energy absorption the more load 

cracked shotcrete can support. Two types of fiber-reinforced shotcrete 

were investigated, one with steel fiber and another with polyfiber. The 

residual load supported in Round Determinate Panel Tests can be 

matched with the strength of wire mesh, thus providing equivalent bag 

strength values for fiber reinforced shotcrete panels. There is also 

evidence of a panel crack width comparison to the load holding 

capabilities of the f ber in the shotcrete mixes. In practice, this 

provides designers with a method for comparing the ground support 

provided by fiber-reinforced shotcrete with that supplied by wire mesh. 

INTRODUCTION 

Underground mines in Nevada, as well as other parts of the 

northwestern United States, often have weak, raveling ground 

(RMR<44). 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 ground conditions, the surface or skin of the 

underground openings must be continually supported in order to 

adequately protect underground workers. One traditional solution has 

been to use wire mesh to support loose broken material between the 

rock bolts which provide the primary ground support. The mesh serves 

to maintain the integrity of the opening’s surface by generally holding 

loose rock in position. 

 In ground support conditions where wire mesh is not suitable 

alone f ber-reinforced shotcrete (FRS) is used in addition. When FRS 

is used for surface support, a shotcrete mix having the necessary 

strength properties must be specified. This typically defines the type 

and the density of the fibers that are used in the mix. One of the 

critical safety issues involved with using FRS instead of mesh is 

insuring that the level of ground support provided is safe. The (NIOSH) 

is conducting shotcrete-related research to specifically address this 

concern. 

In underground mines, shotcrete is typically used in conjunction 

with other ground support components. Although it primarily functions 

to prevent fragments of ground from falling out between bolts or other 

structural support members, shotcrete can be used to support several 

different types of loads in a variety of ground conditions as shown in 

Figures 1-3. Regardless of whether the roof load is uniformly 

distributed or concentrated, the loosened material still must be held 

within a “bag” formed by the deformed or partially failed (cracked) 

shotcrete. Consequently, the residual strength of the shotcrete is an 

important design consideration, in addition to its flexural strength. 

For the flexural resistance model illustrated in Figure 1, the 

relevant ground support design parameters include the thickness, 

flexural strength, and residual strength of the FRS; the span between 

the rock bolts; and the size and mass of the concentrated rock load at 

the mid-span distance between the bolts. 

Figure 1. Flexural resistance model for a loosened block representing 

a concentrated load (Diamantidis & Bernard, 2004). 

A different flexural resistance model, representing a shotcrete 

lining that supports more highly fractured ground, is illustrated in Figure 

2. This uniformly distributed load is more representative of the weak 

rock mass loading conditions in the underground mines in Nevada. 

Figure 2. Flexural resistance model for a loosened block representing 

a distributed load. (Diamantidis & Bernard, 2004). 

Shear failure may occur if the span over which the bending 

stresses act is small compared to the magnitude of the shear force 

(Fig. 3). In this case, the weight of the block mass is held by the shear 

strength of the shotcrete lining around the block perimeter. This failure 

mode is more common in hard, jointed rock. 
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Figure 3. Shear resistance model for a loosened block acting as a 

rigid body (after Diamantidis & Bernard, 2004). 

The highly fractured ground conditions shown in Figure 2 are 

encountered in many underground mines in the western United States, 

particularly in Nevada. A set of support guidelines has been 

developed to address these failure conditions based on FRS strength 

properties (Papworth, 2002; Grant et al., 2001; Barton et al., 1974). 

Generally, the shotcrete residual strength is specified by the type, 

gauge (diameter), and density of the fibers used in the mix. 

BACKGROUND 

The flexural strength of the f ber-reinforced shotcrete is one of the 

key physical property parameters for ground support design in weak 

rock mass mines. The shotcrete between the bolts acts as a thin shell 

that is loaded by the dead weight of the loose rock in the mine roof. As 

the ground starts to unravel, the load builds up on the shotcrete shell 

between the bolts. The shotcrete in turn provides a resistive force until 

the weight of the loose rock exceeds the tensile strength of the 

shotcrete and breaks the shotcrete shell in flexure. This is known as 

the first break or first crack strength (Fig. 4). This first break flexural 

strength is directly related to the shotcrete’s tensile strength and is a 

primarily influenced by the Portland cement content of the shotcrete 

mix. 

Figure 4. Broken sample showing tension crack. 

A theoretical method of determining the first break load was 

developed by (Johnansen, 1972) for shotcrete used in tunneling and 

mining. This method known as the “yield line theory” is listed in 

Equation 1 and shown in Figure 5. 

Figure 5. Yield line theory diagram. 

Although the first break load can be 

determined using Equation 1, the calculated load is usually 13 pct less 

than the actual breaking load. Because the shotcrete generally does 

not break in perfect 1/3 sections, this error is thought to be due to the 

geometry of the crack formations or breaks. If the tensile strength of 

the shotcrete is known from standard laboratory splitting tensile tests, 

this value can be entered in Equation 1 to accurately estimate the first 

break load of the shotcrete for design purposes. 

R

Pcrack = 3 3m (1)

r

where Pcrack = first crack load 

 R = round panel radius 

 r = reactant radius 

and m = moment/unit length, 

σbd 2

and where m = 

6 σ = tensile strength 

b = R 

and d = thickness of panel 

The use of the yield line theory is not a practical approach to

determining the post peak strength of shotcrete containing fibers. The 

equation only determines first break flexural load and not residual 

strength of the shotcrete after this initial failure. Shotcrete residual 

strength is an important consideration when loose broken material in 

the mine roof is supported in a bag of partially failed or cracked 

shotcrete. The primary reason that f bers are used in a shotcrete mix 

is to improve the residual strength or toughness of the shotcrete. 

A shotcrete testing standard, ASTM 1550-05 Round Determinate 

Panel Test (RDPT), was designed to replicate the typical shotcrete 

loading conditions in a mine or tunnel. This test provides design 

engineers with a peak flexural load at first break and also a measure of 

the residual strength or toughness of the fiber-reinforced shotcrete as 

the shotcrete undergoes further displacement after this initial failure. 

The first break of the shotcrete occurs early in the test, usually within 

the first minute of a ten minute test. Although originally developed for 

tunneling applications (Bernard & Pireher, 2000), this test provides a 

convenient standard for comparing shotcrete mixes having different 

mix constituents including different types and densities of embedded 

fiber. 

Although round panel tests provide useful information, these tests 

are conducted in only a few laboratories throughout the world. 

Consequently, the shotcrete panels usually have to be shipped after 
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being formed, causing logistical as well as testing problems, 

particularly for mines that are located distant from urban areas. As a 

result, very little field data is available regarding either the flexural 

strength or the residual strengths of FRS, especially for underground 

mining applications. 

EXPERIMENTAL INVESTIGATION 

To provide flexural and residual strength values, NIOSH 

researchers developed a portable round panel test machine that could 

be transported and set up directly at the mine site (Fig. 6). The 

usefulness of this field-ready test machine has been previously 

demonstrated during a fiber dosage comparison study conducted at 

the Chief Joseph Mine in Butte, Montana (Martin et al., 2007). 

Figure 6. RDPT test system schematic. 

The round panel test illustrated in Figures 4 and 6 approximates 

the loading conditions shown in Figure 2 via the support pivot points 

indicated in Figure 5. A centrally- placed ram loads the shotcrete 

panel at a rate of 4 mm per minute while the three “determinate” pins 

deliver a reactive force through the three pivot points. During this test, 

three independent cracks, bridged by fibers in the shotcrete, are 

formed between the three pivot points and the center of the specimen 

(Fig. 4). Increasing the f ber dosage, (the weight of fibers per cubic 

meter of shotcrete), up to saturation, increases the residual strength or 

toughness in the shotcrete panels. 

In conducting these round panel tests, shotcrete specimens are 

prepared and cured in an environmental chamber in a manner 

consistent with ACI recommendations and then tested at 7, 14, and 28 

days as prescribed in ASTM 1550-05. A typical load versus 

displacement profile that is developed during the test is shown in 

Figure 7. 

Figure 7. RDPT Load vs. displacement graph indicating peak flexural 

strength and residual strength. 

 The first break failure generally occurs within a few mm of 

displacement, and afterwards a large percentage of the load is lost. 

Residual strength is provided by the f bers in the shotcrete mix and is 

shown by the colored area under the curve, referred to as the energy 

or toughness of the shotcrete. The fibers typically break or are pulled 

out of the shotcrete matrix as the test progresses. 

Many of the mines that are operating in weak ground conditions 

are starting to use FRS in conjunction with wire mesh. When the 

shotcrete is used in conjunction with bolts, the design capabilities of 

this integrated ground support system are not well known. By 

providing load versus displacement profiles for the shotcrete, round 

panel tests can supply specific strength and physical behavior 

attr butes for the shotcrete that are not typically known. These strength 

properties can then be used in mine design equations to develop safer 

ground control plans. By determining the residual load at specific 

displacement intervals, the residual strength of the shotcrete can be 

compared with the bag strength or support capacity of wire mesh. In 

addition, it may also be possible to relate the width or dilation 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. If this crack dilation can be related to the residual 

strength of the shotcrete, this information may provide a practical 

means for underground miners to assess the stability of shotcrete 

applied to the surfaces of underground openings. 

To determine if crack dilation could be quantitatively related to 

residual load, NIOSH researchers have conducted a series of round 

panel tests using two commercial shotcrete mixes containing either 

steel fibers (Superstick steel fiber shotcrete, SCA-PT100) or poly f bers 

(Superstick poly fiber shotcrete, SCAPF). Unconfined compression 

and splitting tensile tests were also conducted with these shotcrete 

mixes to determine the compressive and tensile strength properties for 

representative samples of the applied shotcrete. Compressive 

strength results are listed below in Table 1 for these two shotcrete 

mixes. Splitting tensile strengths were generally about 12 pct of the 

measured compressive strength values. The tensile strength values 

are input into the yield line Equation 1 for determining the first break 

strength. 

Table 1 Compressive strengths of shotcrete. 

Results of these round panel tests are listed in Tables 2-7 and 

shown graphically in Figures 9 and 10. The peak flexural load at the 

first break of the round panel specimen was recorded along with the 

displacement of the loading ram as the crack progressed through the 

entire thickness of the shotcrete panel (i.e., crack-through). 

Furthermore, the residual load was noted at 10, 20, 30, and 40 mm of 

displacement of the loading ram, and the dilation or width of the crack 

propagated on the underside of the round panel specimen was also 

measured and recorded for these respective displacements at the 

center, mid diameter, and outer diameter of the specimen (Figure 8 - 

A, B, & C, respectively). Round panel tests were conducted with three 

specimens for each of the mixes after the shotcrete had cured for 7, 

14, and 28 days. Therefore, the load, displacement, and dilation 

values listed in Tables 2-7 are average results obtained from a series 

of three round panel tests conducted with a specific shotcrete mix after 

the specimens had cured for the time indicated. 
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As expected, the first break or peak load increased with shotcrete 

curing time. However, the type of fiber, steel or poly, did not appear to 

significantly affect this peak load value, even though the poly f ber 

specimens did have a slightly larger average peak load as compared 

to the steel fiber specimens, particularly after 28 days of curing. For 

the steel fiber shotcrete, the load at crack-through remained close with 

curing time, but the displacement at crack-through increased with 

shotcrete curing time from 20 mm after 7 days of curing to 28 mm after 

28 days of curing. As the shotcrete cured, it was able to withstand 

more displacement under load than at earlier curing times. This may 

indicate that the steel fibers become more interlocked in the shotcrete 

matrix as the shotcrete ages. In contrast, the displacement at crackthrough for the poly fiber specimens remained close regardless of 

curing time, but the load at crack-through increased slightly with an 

increase in shotcrete curing time. For the dilation or crack width 

measurements, the steel fiber shotcrete as a general rule supported 

higher loads with less crack dilation than the poly fiber shotcrete at a 

given measure of ram displacement (10-40 mm). 

Figure 8. Measurement points of the round panel. 

Load versus displacement profiles for the round panel tests with 

steel fiber shotcrete are plotted in Figure 9, and similar profiles for the 

poly f ber shotcrete are plotted in Figure 10. The steel fiber shotcrete 

appeared to provide more uniform RDPT results with a noticeable 

increase in peak flexural load with curing time and a relatively constant 

residual strength regardless of curing time. In contrast, the RDPT 

results for the poly fiber shotcrete were not as consistent. While the 

peak flexural load generally increased with curing time, the residual 

strength or toughness of the shotcrete varied dramatically depending 

on the curing age of the shotcrete. As the poly fiber shotcrete cured, it 

appears to have become more brittle. Consequently, the residual 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 strength and energy absorption capability 

(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 RDPT results for the poly fiber shotcrete 

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. 

As shown in Figures 9 and 10 and also Tables 2-7, round panel 

tests can provide important information for determining the available 

ground support capacity of the in-place shotcrete and also indicate 

whether or not rehabilitation of the shotcrete is needed. 

Figure 9. Load versus displacement graphs for SCAPT-100. 

Figure 10. Load versus displacement graphs for SCAPF. 

GROUND SUPPORT LOAD CALCULATIONS 

Determining the in-situ load shotcrete is supporting in 

underground mines is difficult because the load exerted on the 

shotcrete by loose material near the surface of the mine opening is 

often hidden from observation. In contrast, the loads applied to wire 

mesh are easier to assess because the wire mesh tends to bag as it 

retains more loose material. The RDPT test provides a method for 

simulating a load on the shotcrete so strength values can be 

determined. These strength values can be compared against 

estimated loads determined through dead weight load calculations. 

Although these calculations are conservative, they provide an 

established method for estimating the maximum expected load on the 

shotcrete. 

Because the shotcrete primarily provides surface support for 

loose material between the bolts, the geometry of the bolt spacing can

be used to determine the maximum volume of loose material that will 

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. As shown in Figure 11, a block method can be 

used to calculate the volume of the largest block of ground that would 

fall between the bolts. Equation 2 can then be used to calculate the 

block’s dead weight load. 

Wtb (block 1-m slice) = b2

*h* γ (2)

where Wtb = Dead weight block 

 b = base length, 

 h = height, 
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γ = specific weight = 2.7 t/m3 

 Wtb = (1.2 m)2*1.0 m* 2.7 t/m3 = 1.2 tonnes 

Figure 11. Estimated block volume for determining dead weight 

loading between the bolts. 

For weak raveling ground conditions, a more realistic estimate of 

the expected volume of loose material that would load the shotcrete is 

depicted by the cone illustrated in Figure 12. 

Figure 12. Estimated cone volume for determining dead weight 

loading between the bolts. 

Equation 3 can be used to calculate the volume of loose material 

in this cone, and Equation 4 provides the dead weight load of this 

loose material. 

1 d 2

V = π h (3)

3 4

where V = volume of cone

 d = diameter, 

h = height 

1 1.22

 V = π 0.6 = 0.22 m3 

3 4

Wtc = V * γ (4)

where Wtc = dead weight of cone 

 V = volume of cone

γ = specific weight = 2.7 t/m3

 Wtc = 0.22 m3 * 2.7t/m3 = 0.6 tonnes 

Because 10 kN is equivalent to 1.0 tonnes, the 0.6-tonnes dead 

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

equivalent force of 6 kN. This critical load can then be used in 

conjunction with the RDPT results shown in Figures 9 and 10 and the 

values listed in Tables 2-7 to determine the required flexural strength 

of the shotcrete mix and in turn, to assess the stability of in-place 

shotcrete. For example, the peak flexural strength at first break for the 

RDPT tests, which were conducted, was usually much greater than 6 

kN indicating that the shotcrete mix design is more than adequate. To 

assess the stability of in-place shotcrete, the 6-kN critical load can be 

compared with the residual strength of the shotcrete shown in Figures 

9 and 10 and listed in Tables 2-7. If the shotcrete has cracked, the 

load versus displacement curves for both the steel fiber and poly f ber 

shotcrete indicate that the shotcrete may not be stable after more than 

10 mm of displacement. Because the movement of the shotcrete is 

difficult to detect, much less determine underground, the dilation or 

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. Comparing the 6-kN critical load with the post peak load 

values after first break that are listed in Tables 2-7, it appears that 

none of shotcrete panels, which were tested, would be able to safely 

support this critical load for an extended period of time. Furthermore, 

the tables indicate that after the round panel crack dilation has reached 

about 2 mm, the shotcrete panel will no longer support the critical load. 

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. In addition, the round panel test does not exactly replicate 

the loading and failure conditions of in-place shotcrete, and a 

maximum expected load is estimated from the critical load calculations, 

rather than the actual load applied to the shotcrete which should be 

much less. As a result, the shotcrete design methodology mentioned 

above is a good initial approach, but further research is needed to 

quantitatively relate the crack width of the shotcrete to its residual 

strength, and thus the load that it is able to safely support after the 

shotcrete has cracked. 

Comparison of Fiber Reinforced Shotcrete Strength to Welded 

Wire Mesh Bag Strength 

To relate the ground support capability of shotcrete with that of 

wire mesh, the peak flexural strength of the shotcrete at first break can 
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be compared with the wire mesh bag strength values listed in Table 8. 

Table 8. Wire bag strengths. (Pakalnis, 2002) 

After 28 days of curing, the average peak flexural load for the steel 

fiber shotcrete was approximately 19 kN (1.9 tonnes) (Table 4) and 

about 21 kN (2.1 tonnes) for the poly fiber shotcrete (Table 7). These 

peak loads are equivalent to the bag strength of 9-gauge 4x4 wire 

welded mesh and 12-gauge 4x4 welded wire mesh, respectively. 

CONCLUSIONS 

A testing program was devised and demonstrated for determining 

the residual strength of fiber reinforced shotcrete. Tests showed that a 

visible crack indicated that the test specimen had already gone beyond 

the “peak crack strength” and is approaching residual load. This 

occurs at 5~10 mm vertical panel deflection. 

A method is also demonstrated for determining ground support 

loads for shotcrete mixes that are equivalent to various grades of wire 

mesh. For example, these results show that a 9-gauge 4x4 wire mesh 

with 1.9-tonnes load capacity is equivalent to a commercial steel f ber 

reinforced shotcrete after 28 days of curing with a peak flexural load of 

1.9 tonnes at first break. 

These test results indicate that the ground support capacity of 

shotcrete varies with crack aperture during deformation. This 

information is useful for assessing the residual strength of bagged f ber 

reinforced shotcrete in situ. For example, 16 mm of crack dilation was 

shown to provide only 1.19 kN (0.1 tonnes) of residual strength, which 

is virtually null. 

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 and 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 E ko, Nevada and Vince Mendive with 

Thiessen Team USA in Big Timber, Montana 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. 

REFERENCES 

ASTM 1550-05, 2005, flexural toughness of Fiber-Reinforced Concrete

(using centrally-loaded round panel) ASTM West Conshohocken 

PA 2005. 

Barton, N., Lien, R., Lunde, J., 1974, Engineering Classification of 

Rock Masses for the Design of Tunnel Support. Rock Mechanics 

and Rock Engineering, 1974, Springer_Verlag, pp.189-236. 

Bernard, E.S. and Pireher, M., 2000, The use of Round Determinate 

Panels for the Assessment of Flexure Performance of F ber 

Reinforced Concrete. Cement, Concrete and Aggerates, ASTM 

(submitted Feb 2000) 

Diamantidis, D. and Bernard, E., 2004, “Comparison of methods for 

early age strength testing of shotcrete,” Shotcrete: More 

Engineering Developments, Proceedings of the Second 

International Conference on Engineering Developments in 

Shotcrete, 4 October 2004, Cairns, Queensland, Australia, 

Bernard ed., Taylor and Francis Group, London, 2004, pp. 109-

126. 

Grant, N.B., Ratcliffe, R.C., Papworth, F., Design Guidelines for use of 

SFRS in Ground Support, Shotcrete: Engineering Developments, 

Bernard (ed) @ 2001, Swets & Zeitlinger, Lisse, ISBN 90 

5809.176 7, pp.111-118. 

Johnansen, K.W., 1972. Yeild Line Theory, Cement and Concrete 

association, UK. 

Martin, L., Dunford, J., MacLaughlin, M., Coronoble, R., Tests of fiber -

reinforced shotcrete at the Chief Joseph Mine, Butte, Montana , 

SME, Vol. 322, 2007 Transactions, pp. 61-64. 

Pakalnis, R., 2002, Site Characterization Design of Underground Mine 

Openings, Notes taken during short course at Spokane Research 

Laboratory, May 28-29, 2002. 

Papworth, F. 2002, Design Guidelines for use of Fiber Reinforced 

Shotcrete in Ground Support, American Shotcete Association, 

Shotcrete Magazine, spring 2002, pp.16-21.