RI 9697
 REPORT OF INVESTIGATIONS/2015 

Shotcrete Design and Installation 

Compliance Testing: 

Early Strength, Load Capacity, Toughness, 

Adhesion Strength, and Applied Quality 
Report of Investigations 9697

Shotcrete Design and Installation Compliance Testing: Early Strength, 

Load Capacity, Toughness, Adhesion Strength, and Applied Quality

Lewis A. Martin, Curtis C. Clark, Joseph B. Seymour, and Michael A. Stepan

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Office of Mine Safety and Health Research

Pittsburgh, PA • Spokane, WA

March 2015
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Suggested Citation

NIOSH [2014]. Shotcrete design and installation compliance testing: early strength, load

capacity, toughness, adhesion strength, and applied quality. By Martin LA, Clark CC, Seymour

JB, and Stepan MA. Spokane WA: U.S. Department of Health and Human Services, Centers for

Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS

(NIOSH) Publication No. 2015-107, RI 9697. 

DHHS (NIOSH) Publication No. 2015–107

March 2015

Cover photo by NIOSH. 
Contents

Executive Summary ................................................................................................................... 1

Development of Portable Test Machines ........................................................................ 1

Shotcrete Characteristics and Application ...................................................................... 2

Introduction................................................................................................................................ 3

Research Impetus and Report Contents......................................................................... 3

Shotcrete and Weak Rock.............................................................................................. 3

Shotcrete Quality Monitoring for Safety .......................................................................... 3

Background................................................................................................................................ 4

Ground Control and Rockfalls......................................................................................... 4

Weak Rock Conditions ................................................................................................... 5

Weak Rock Mining Method............................................................................................. 7

Typical Top-Cut Mining Cycle for Weak Rock................................................................. 8

Application and Use of Shotcrete for Surface Control in Weak Rock .............................11

Shotcrete Parameters .............................................................................................................. 20

Shotcrete Mix Types......................................................................................................20

Shotcrete Mix Constituents............................................................................................23

Shotcrete Reinforcement Methods ................................................................................24

Shotcrete Mix Characteristics ........................................................................................26

Shotcrete Application.....................................................................................................27

Field Testing Methods for Shotcrete Application ...................................................................... 28

Slump Test ....................................................................................................................29

Compression Test .........................................................................................................31

Early Strength, Partial-beam Compression Test............................................................31

Overcoring and Direct-Tension Pull Test .......................................................................43

Round Determinate Panel Flexural Test........................................................................57

Comparison of Shotcrete Support Systems Based on RDP Tests .................................84

Quality Control Testing in the Field................................................................................92

Summary and Conclusions ...................................................................................................... 95

Early Strength, Partial-beam Compression Test ............................................................95

Overcoring and Direct-tension Pull Test ........................................................................96

Round Determinate Panel Test .....................................................................................96

Summary of Discussion.................................................................................................97

Acknowledgements.................................................................................................................. 98

References .............................................................................................................................. 99

Appendix................................................................................................................................ 105

Equipment Specifications for Portable Shotcrete Test Machines ................................. 105

Conversion of Pressure Gauge Bond Strength Test Values to Direct Tensile Force....108
Figures

Figure 1. Distribution of lost-time injuries in underground metal mines by accident class, 

2006–2010 (operator and contractor cases)............................................................................... 5

Figure 2. Poor-quality, raveling rock in an underground gold mine in Nevada. ........................... 6

Figure 3. Poor-quality, highly deformable rock in an underground gold mine in Nevada............. 6

Figure 4. Final in-cycle application of wet mix shotcrete............................................................. 7

Figure 5. Drilling the first blast round.......................................................................................... 8

Figure 6. Blasting the first round. ............................................................................................... 8

Figure 7. Applying first layer of shotcrete. .................................................................................. 8

Figure 8. Removing blasted rock or muck with loader. ............................................................... 9

Figure 9. Installing short bolts and welded wire mesh. ............................................................... 9

Figure 10. Drilling a second blast round. .................................................................................... 9

Figure 11. Blasting the second round........................................................................................10

Figure 12. Installing a second layer of shotcrete. ......................................................................10

Figure 13. Removing blasted rock or muck with loader. ............................................................10

Figure 14. Installing long primary bolts......................................................................................11

Figure 15. Installing short bolts and welded wire mesh. ............................................................11

Figure 16. Application of a thin initial layer or flash coat of shotcrete.........................................12

Figure 17. Installation of welded wire mesh, plates, and short bolts. .........................................12

Figure 18. Application of a second layer of shotcrete. ...............................................................13

Figure 19. Completed ground control system with plated, long primary bolts through the 

matrix and shorter bolts to the springline...................................................................................14

Figure 20. Multi-element ground support system for weak rock.................................................15

Figure 21. Plan view of primary bolt pattern, looking up at the back from below........................16

Figure 22. Flexural failure of shotcrete resulting from insufficient adhesion strength .................17

Figure 23. Two basic types of shotcrete failure modes: A – fallout of only shotcrete and 

B – fallout of shotcrete and rock................................................................................................17

Figure 24. Schematic of shotcrete loading and failure modes ...................................................18

Figure 25. Supermesh provides support between the primary bolts. .........................................19

Figure 26. Shotcrete failure modes that can be prevented by the addition of welded 

wire mesh. ................................................................................................................................19

Figure 27. Simplified schematic of a typical dry mix shotcrete system.......................................21

Figure 28. Operation of a dry mix shotcrete machine equipped with a rotary barrel. .................21

Figure 29. Simplified schematic of a typical wet mix shotcrete system. .....................................22

Figure 30. Operation of a wet mix shotcrete machine................................................................22

Figure 31. Consistency of wet mix shotcrete being loaded into the hopper of a 

shotcrete machine.....................................................................................................................30
Figure 32. Filling a slump cone with wet mix shotcrete prior to conducting a slump test in an 

underground mine.....................................................................................................................30

Figure 33. Slump test with wet mix shotcrete showing a 292-mm (11.5-in) slump; 

typical slump is 100 to 200 mm (4 to 8 in). ................................................................................31

Figure 34. Portable shotcrete test machine designed by NIOSH researchers. ..........................32

Figure 35. Operation of portable shotcrete test machine. ..........................................................33

Figure 36. Diagram of the compressive forces applied to a partial-beam shotcrete sample 

during an early strength test......................................................................................................33

Figure 37. Partial-beam shotcrete mold (left) and shotcrete test sample (right).........................34

Figure 38. Containment fixtures and molds for making partial-beam shotcrete samples ...........35

Figure 39. Loading of a partial-beam shotcrete sample during an eary strength test.................35

Figure 40. Shotcrete sample showing vertical shear failure.......................................................36

Figure 41. Shotcrete sample showing single side failure, loaded from top and bottom 

of specimen. .............................................................................................................................36

Figure 42. Shotcrete sample showing dual side failure, loaded from top and bottom 

of specimen. .............................................................................................................................37

Figure 43. Shotcrete sample showing exposed failure plane.....................................................37

Figure 44. Shotcrete failure modes ...........................................................................................37

Figure 45. Early-age compressive strength versus curing time for sprayed and cast samples 

of K1 shotcrete for 1 to 6 hours of curing time...........................................................................38

Figure 46. Early-age compressive strength versus curing time for sprayed partial-beam 

samples of 5 shotcrete mixes for 1 to 6 hours of curing time.....................................................39

Figure 47. Positioning partial-beam shotcrete molds and containment fixtures prior to 

shotcreting. ...............................................................................................................................40

Figure 48. Portable test machine for determining early-age strength of shotcrete. ....................41

Figure 49. Loading of partial-beam shotcrete sample during an early strength test...................41

Figure 50. Shotcrete sample showing exposed failure plane.....................................................42

Figure 51. Early-age compressive strength versus curing time for partial-beam samples 

of K1 shotcrete with 1 to 6 hours of curing time.........................................................................43

Figure 52. Schematic of overcoring and direct-tension test equipment for determining 

shotcrete adhesion strength......................................................................................................45

Figure 53. Operation of overcoring and direct-tension test equipment.......................................45

Figure 54. Tensile loading of shotcrete core sample to determine bond strength. .....................45

Figure 55. Wet drilling with diamond core bit.............................................................................47

Figure 56. Drill hole configuration with extension rod connected to epoxied stud. .....................48

Figure 57. Schematic showing a vertical cross-section of the drill hole configuration for a 

shotcrete bond strength test......................................................................................................48

Figure 58. Strap brackets centered around the epoxied stud to anchor the direct-tension 

test equipment. .........................................................................................................................49

Figure 59. Pulling unit mounted to mine rib using a three-point safety restraint.........................50
Figure 60. Typical shotcrete tensile strength test results for 0–90 day cure time (n=54)............50

Figure 61. Shotcrete test core and drill hole from a bond strength test with dry mix shotcrete 

applied to a concrete substrate. ................................................................................................52

Figure 62. Shotcrete adhesion test site with three drill stands mounted on mine rib..................53

Figure 63. Dry drilling the installation hole for the epoxied stud pull anchor...............................53

Figure 64. Wet drilling a 102-mm (4-in) diameter hole for the shotcrete adhesion test core 

(note bit change).......................................................................................................................53

Figure 65. Preparing to remove drill stand and install direct-tension test equipment. ................54

Figure 66. Adhesion test #1: core broke in shotcrete and at interface between two 

shotcrete layers.........................................................................................................................55

Figure 67. Adhesion test #2: core broke entirely in the top layer of shotcrete............................55

Figure 68. Adhesion test #3: core broke prematurely in weak underlying host rock...................55

Figure 69. Adhesion test #3: drill hole and core indicating a tensile failure located primarily 

in the host rock..........................................................................................................................56

Figure 70. Test core #3: tensile failure in weak host rock. .........................................................56

Figure 71. Adhesion test cores showing two breaks in shotcrete (left, test 1 and middle, 

test 2), and one break in host rock (right, test 3). ......................................................................56

Figure 72. Diagram of the force applied to a round-panel shotcrete sample during 

an RDP test. .............................................................................................................................58

Figure 73. Schematic of a portable round determinate panel (RDP) test machine developed 

by NIOSH researchers..............................................................................................................61

Figure 74. Operation of portable round determinate panel (RDP) test machine.........................61

Figure 75. Mold ring used to prepare a round-panel shotcrete sample......................................62

Figure 76. Sprayed round-panel shotcrete samples. .................................................................63

Figure 77. Round-panel shotcrete sample positioned in portable RDP test machine.................63

Figure 78. Broken round-panel test sample showing tension cracks. ........................................64

Figure 79. Synthetic macro fibers exposed along the failure surface of round-panel 

shotcrete sample.......................................................................................................................64

Figure 80. Broken round panel sample of fiber-reinforced shotcrete showing the 120° cracks. .65

Figure 81. Typical roof bolt pattern with a superimposed RDP test panel showing the 

assumed flexural loading of the shotcrete between the bolts.....................................................65

Figure 82. Load versus displacement curve from RDP test.......................................................66

Figure 83. Load versus displacement curve from RDP test indicating peak load and 

first break..................................................................................................................................67

Figure 84. Load versus displacement curve from RDP test indicating peak load, residual 

load, and the energy or toughness of the shotcrete...................................................................67

Figure 85. Energy versus displacement curve from RDP test....................................................68

Figure 86. Spraying round-panel shotcrete samples reinforced with wire mesh. .......................69

Figure 87. Spraying round panel forms with a dry mix shotcrete. ..............................................70
Figure 88. Comparison of load versus displacement curves for SCACR shotcrete mixed 

with well water and cured for 8 hours. .......................................................................................71

Figure 89. Comparison of load versus displacement curves for SCACR shotcrete mixed 

with mine water and cured for 8 hours. .....................................................................................71

Figure 90. Comparison of load versus displacement curves for SCACR shotcrete mixed 

with well water, reinforced with wire mesh, and cured for 28 days.............................................72

Figure 91. Spraying round panel forms with wet mix shotcrete..................................................73

Figure 92. Swing-type, positive-displacement pump used to apply wet mix shotcrete. ..............73

Figure 93. Comparison of load versus displacement curves for 295-kg (650-lb) mix with 

24-hour cure time......................................................................................................................74

Figure 94. Comparison of load versus displacement curves for 318-kg (700-lb) mix with 

24-hour cure time......................................................................................................................74

Figure 95. Load versus displacement and energy versus displacement curves for 

5-kg/m3 (8.5-lb/yd3

) Shogun fiber mix with 7-day cure time. ......................................................76

Figure 96. Shooting wet mix shotcrete into a round panel form with wire mesh reinforcement. .77

Figure 97. Comparison of load versus displacement and energy versus displacement 

curves for 1.87-kg/m3 (3.15-lb/yd3

) Shogun fiber mix with 28-day cure time. .............................81

Figure 98. Comparison of load versus displacement and energy versus displacement 

curves for 2.99-kg/m3 (5.0-lb/yd3

) Shogun fiber mix with 28-day cure time. ...............................81

Figure 99. Comparison of load versus displacement and energy versus displacement 

curves for 4.1-kg/m3 (7.0-lb/yd3

) Shogun fiber mix with 28-day cure time..................................82

Figure 100. Comparison of load versus displacement and energy versus displacement 

curves for shotcrete reinforced with six-gauge wire mesh and cured for 28 days. .....................83

Figure 101. Comparison of load versus displacement and energy versus displacement 

curves for shotcrete with a water-reducing admixture that is reinforced with six-gauge wire 

mesh and cured for 28 days......................................................................................................83

Figure 102. Comparison of load versus displacement and energy versus displacement 

curves for 1.87-kg/m3 (3.15-lb/yd3

) Performax fiber mix with 28-day cure time..........................84

Figure 103. Comparison of load versus displacement curves for round-panel shotcrete 

samples reinforced with synthetic macro fiber (polyfiber), steel fiber, or wire mesh...................87

Figure 104. Comparison of load versus displacement and energy versus displacement 

curves for round-panel shotcrete samples reinforced with synthetic macro fiber (polyfiber), 

steel fiber, or wire mesh............................................................................................................88

Figure 105. Estimated support categories based on tunneling quality index Q..........................90
Tables

Table 1. Summary of all fatalities and injuries in underground metal mines, 2006–2010 

(operator and contractor cases) ................................................................................................. 4

Table 2. Typical water-to-cement ratios for shotcrete................................................................24

Table 3. Typical mix designs for steel-fiber-reinforced shotcrete with silica fume additive .........26

Table 4. Typical range of shotcrete early strength and re-entry time by region..........................34

Table 5. Early strength test results for partial-beam shotcrete samples using the NIOSH 

portable test machine................................................................................................................42

Table 6. Typical indicators of the quality of a shotcrete application obtained through a careful 

examination of the test core and drill hole .................................................................................46

Table 7. Typical range for shotcrete bond strength with differing substrates .............................46

Table 8. Shotcrete adhesion test results ...................................................................................54

Table 9. Typical minimum load capacity values specified for shotcrete in recent mining and 

civil engineering projects in North America for given ground support conditions and RDPT 

ram displacements....................................................................................................................59

Table 10. Typical values specified for shotcrete toughness in recent mining and civil 

engineering projects in North America for given ground support conditions ..............................60

Table 11. Comparison of shotcrete toughness performance level to Q-system rock class, 

RDPT energy, ground support reinforcement category, and RMR for recent mining projects 

in North America .......................................................................................................................60

Table 12. Proportions of various mix constituents for round-panel shotcrete samples at the 

Devil’s Slide Tunnels Project.....................................................................................................75

Table 13. Proportions of various mix constitutents for round-panel samples of reinforced 

shotcrete at the Leeville Mine....................................................................................................78

Table 14. Average results from field tests with round-panel shotcrete samples.........................85

Table 15. Summary of shotcrete applications in underground mining for different rock mass 

conditions..................................................................................................................................91

Table 16. Recommended frequency of shotcrete tests for mining applications..........................93

Table 17. Typical corrective actions for shotcrete problems ......................................................94

Table A1. Early strength test machine.....................................................................................105

Table A2. RDP test machine...................................................................................................106

Table A3. Overcoring and direct-tension test system ..............................................................106
ACRONYMS AND ABBREVIATIONS

ACI American Concrete Institute

ASTM American Society for Testing and Materials

BASF Registered logo of BASF, The Chemical Company

CRF cemented rockfill

EFNARC European Federation of National Associations Representing 

Concrete

ESR Excavation support ratio

MSHA Mine Safety and Health Administration

NATM New Australian Tunneling Method

NDL no days lost

NFDL nonfatal days lost

OMSHR Office of Mine Safety and Health Research

PLC programmable logic controller

RDP Round determinate panel

RDPT Round determinate panel test

RMR Rock Mass Rating

SCA Superstick with water accelerant

SCACR Superstick water accelerant with corrosion resistance

SCAPF Superstick water accelerant and polyfiber

SCAPT-100 Superstick water accelerant and steel fiber

SCAR Superstick water accelerant with retarder

TPL toughness performance level

UCS unconfined compressive strength

w:c water-cement ratio
UNIT OF MEASURE ABBREVIATIONS

cfm cubic feet per minute

m3/m cubic meter per minute

ft feet

in inch

in/min inches per minute

kg kilogram

kg/m3 kilogram per cubic meter

kN kilonewton

L liters

m meter

mm millimeter

mm/min millimeter per minute

min minute

MPa megapascal

% percent

lb pound

lbf pound force

lb/ft3 pounds per cubic foot

lb/yd3 pounds per cubic yard

psi pounds per square inch

qt quart
Shotcrete Design and Installation Compliance Testing: 

Early Strength, Load Capacity, Toughness, Adhesion 

Strength, and Applied Quality

Lewis A. Martin,1

1 Research Mechanical Engineer, Ground Control Branch, Office of Mine Safety and Health Research

(OMSHR), NIOSH, Spokane, WA.

Curtis C. Clark,2

2 Mechanical Engineer, Ground Control Branch, Office of Mine Safety and Health Research (OMSHR),

NIOSH, Spokane, WA.

 Joseph B. Seymour,3

3 Mining Engineer, Ground Control Branch, Office of Mine Safety and Health Research (OMSHR),

NIOSH, Spokane, WA.

and Michael A. Stepan4

4 Engineering Technician, Ground Control Branch, Office of Mine Safety and Health Research (OMSHR),

NIOSH, Spokane, WA.

Executive Summary

The National Institute for Occupational Safety and Health (NIOSH) conducted a research 

study to document and develop safe practices for the use of shotcrete as ground support in 

underground mines, particularly in underground metal mines operating in weak host rock. 

Shotcrete is the generic name for a mixture of cement, sand, fine aggregate, and water that is 

applied pneumatically and compacted dynamically under high velocity. The objective of this 

research is to reduce mine worker fatalities and injuries resulting from rockfall accidents. 

Although the information, techniques, and technology covered in this publication will impact 

both the mining and construction sectors, the primary audience is the mining industry with a 

focus on underground metal mines operating in weak ground conditions. The information and 

practices covered in this publication relating to the use of shotcrete can be put to use by mining 

professionals towards improving mine design and ground control plans. The guidance and 

practices reported in this document will help safety auditors, mining companies, and shotcrete 

suppliers in improving their shotcrete product specifications and the performance of ground 

support systems, evaluating ground control plans, and assessing shotcrete quality control.

Ground control safety can be improved by providing these groups with a better understanding of 

the use of shotcrete in weak rock conditions, field test methods and equipment for measuring the 

strength properties of shotcrete directly at the mine site, and a practical means of conducting 

quality control during shotcrete applications. 

Development of Portable Test Machines

NIOSH researchers developed three portable test machines for determining shotcrete strength 

properties directly at the mine site. These portable test machines can be used to measure the 

early-age compressive strength of the shotcrete, the flexural load capacity and toughness of the 

shotcrete, and the installed quality and bond strength of the shotcrete that is applied to 

underground entries; this enables the test machines to be used to verify safe re-entry times. 

Onsite testing of as-placed shotcrete allows the mine personnel and shotcrete supplier to 

determine if the shotcrete is performing to design specifications. Using these shotcrete test 

 
2 

machines directly at a mine site allows the support capabilities of the shotcrete to be evaluated in 

terms of the specific ground conditions, support methods, mining spans, and entry dimensions at 

the mine. As a result, mine design decisions regarding the use of shotcrete can be made from a 

much more informed position. Ultimately, this enhanced onsite knowledge of shotcrete strength 

properties and the quality of shotcrete application techniques can result in better ground support 

system designs and procedures, thereby reducing the number of fatalities and injuries associated 

with groundfall accidents. 

Shotcrete Characteristics and Application

When the shotcrete is applied in-cycle to underground mine surfaces as part of the ground 

support system, it becomes important to quantify when mining can safely resume under the 

material. As part of the overall ground support system, shotcrete is typically sprayed on the 

surface of an underground opening to stabilize the ground and prevent raveling. Shotcrete is also 

applied at lower water-to-cement ratios than concrete and develops its own unique strength 

characteristics. In addition, the quality of the applied shotcrete, the competency of the underlying 

rock, and the load-carrying capability of the shotcrete once cured are of critical importance. The 

significant shotcrete characteristics examined in this report are: slump, compressive strength, 

tensile strength, early strength, adhesion strength, and flexural strength. Of these shotcrete 

characteristics the engineering strength properties that are determined using early strength, 

adhesion strength, and flexural strength testing methods are the focus of this report. The 

significant shotcrete tests to determine characteristics examined in this research study are:

• Slump Test – used to determine (wet) uncured shotcrete consistency. 

• Compression Test – used to measure cured shotcrete compression strength. 

• Tensile Test – used to measure cured shotcrete tensile strength. 

• Early Strength Partial-beam Test – used to measure shotcrete cure strength development over 

time. 

• Overcoring and Direct-tension Pull Test – used to measure cured shotcrete adhesion strength. 

• Round Determinate Panel Flexure Test – used to measure cured shotcrete load capacity and 

toughness. 
3 

Introduction

Research Impetus and Report Contents

The National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety 

and Health Research (OMSHR) Ground Control Branch conducted investigations into the use of 

shotcrete as ground support in mines that have weak rock mass with the objective of reducing 

fatalities and injuries resulting from rockfall accidents. This Report of Investigations presents the 

results of efforts to document current shotcrete ground support system techniques for weak rock

and to determine shotcrete application requirements. In addition, this report also identifies 

critical shotcrete strength parameters and presents methods for conducting mine-site 

measurements. Finally, this report describes the standards that are currently in use for the safe 

and effective application of shotcrete for support in weak rock. In reporting these findings, the 

hope is that this information will become more widely accepted and implemented by the mining 

and safety community. 

Shotcrete and Weak Rock

Shotcrete is sprayed concrete consisting of cement, sand, and fine aggregate that is applied 

pneumatically and compacted dynamically under high velocity. Shotcrete has been used to 

construct support structures in civil engineering applications for nearly 100 years and for support 

in underground mining excavations for at least 40 years [Morgan 2008; Spearing 2001]. The 

underground metal mining industry is a major user of shotcrete for engineered underground 

support and has pioneered novel applications using shotcrete. The demand for support systems 

that can accommodate the extreme underground loadings of weak rock mass, defined as having a 

rock mass rating (RMR) from 20 (very poor) to 45 (poor), is being met with the incorporation of 

shotcrete into a multi-element ground control system consisting of shotcrete, welded wire mesh, 

and plated bolts. The use of shotcrete is an essential factor in the success of this support system, 

and when combined with welded wire mesh, this composite system is superior to either welded 

wire mesh or fiber-reinforced shotcrete alone in low RMR ground mining conditions. When used

as an essential component of the ground support system, shotcrete is applied during the active

mining cycle, and as such, monitoring the application quality, early strength gain, and long-term 

support characteristics of the material is very important.

Shotcrete Quality Monitoring for Safety

In order for the mining cycle to resume safely after the application of shotcrete, under which

the miners and machinery will be working, the shotcrete must first reach a self-supporting state 

or strength. NIOSH has undertaken the study of how this shotcrete performance is defined and 

how the performance can be determined or monitored in the field. Mine staff must have a means

of quantifiably determining the shotcrete strength after application to keep their mine safe for 

their workers and equipment. 
4 

Researchers at NIOSH developed shotcrete material testing machines that are used at the 

mine site to assist mines in determining if the shotcrete used in the ground control support 

system is meeting the specified shotcrete performance requirements. These portable, mine-site 

testing machines measure early compressive strength related to re-entry time (amount of time 

that elapses before mine workers can safely re-enter an area of the mine), flexural load capacity, 

toughness, installed quality, and bond strength. 

Background

Ground Control and Rockfalls

As noted by Seymour et al. [2013], rockfalls or groundfalls are a serious hazard in 

underground metal mines. From 2006 through 2010, groundfalls caused 50% of the fatalities, 

13% of the nonfatal days lost (NFDL) injuries, and 15.9% of the no days lost (NDL) injuries that 

occurred in underground metal mines (Table 1). During this time period, groundfall accidents 

were the leading cause of fatalities in underground metal mines and also an important 

contributing factor in the injuries that were reported by the Mine Safety and Health 

Administration (MSHA) for the underground metal mining workforce. 

Table 1. Summary of all fatalities and injuries in underground metal mines, 2006–2010

(operator and contractor cases) after Seymour et al. [2013] 

Type of Accident Fatal 

injuries

Nonfatal days 

lost injuries

No days lost 

injuries

Total 

injuries

All accident classifications 14 923 579 1,516 

Groundfalls 7 120 92 219 

Groundfall percentage 50.0% 13.0% 15.9% 14.4% 

As shown in Figure 1, groundfalls were the third highest ranking source of lost-time injuries 

in underground metal mines from 2006 through 2010. These statistics indicate that ground 

control is an extremely challenging issue in underground metals mines and that further ground 

support research is needed to address this problem. Because many of these groundfalls occur 

between traditional ground support elements (e.g., bolts, trusses, timber, etc.), improvements in 

support methods that provide surface control, such as shotcrete and wire mesh, could be expected 

to reduce the number of groundfalls and thus, potentially decrease the number of fatalities and 

injuries associated with these accidents.
5 

Figure 1. Distribution of lost-time injuries in underground metal mines by accident class, 

2006–2010 (operator and contractor cases) after Seymour et al. [2013]. 

Weak Rock Conditions

Nevada surface gold mine operations started driving underground access tunnels in the early 

1990s to develop their underground gold reserve properties. These high-grade Nevada gold 

deposits are exceptional in that they are found predominantly in argillized fault and fracture 

zones; the deposits are often large and irregularly shaped and are surrounded by waste rock of 

similar characteristics. The ore-bearing rock exhibits high deformation under loading. The 

geologic conditions of the rock are unique with poor-quality raveling rock in some areas (Figure 

2) and highly deformable intrusions and foliations that can be gouged out by hand in others 

(Figure 3). In addition, the country rock outside the ore zones is localized with unpredictable 

fracturing and is susceptible to groundfalls. The common factor in all the mines operating in 

these deposits is the presence of a weak back (roof) or walls. The definition of a weak rock mass 

in this context is an RMR of under 45 or a Q value under 1.0. Rock mass values that have been 

reported vary from an RMR high of 70 to a low of 16 in the gold-bearing fault gouge [Pakalnis

2008]. These unusual ground characteristics make mine openings particularly vulnerable to 

hazardous falls of ground. 
6 

Photos by NIOSH Photos by NIOSH

Figure 2. Poor-quality, raveling rock in an underground gold mine in Nevada.

Figure 3. Poor-quality, highly deformable rock in an underground gold mine in Nevada. 
7 

Weak Rock Mining Method

Photo by NIOSH

The mining method commonly used in the weak rock underground gold mines is a modified

form of mechanized cut-and-fill that works well to achieve high ore recovery rates in deposits 

with irregular geometries. In order to mine safely in weak rock mass ground conditions, mining 

operations have developed support methodologies that use extensive ground support and backfill.

In general, the method is for production stopes to be mined under the host rock on the top cuts or 

upper levels, followed by further mining on lower levels under cemented rockfill (CRF). The 

mine openings (drifts) are typically less than 4.6-m (15-ft) wide and 3 to 4.6-m (10 to 15-ft) high 

and mined with short 3-m (10-ft) blast rounds. In some instances, the drifts need only remain 

open for as little as 3–5 days.

The ground control support used in the top-cut of weak rock mines is installed concurrently 

during the active mine cycle. The initial ground support consists of a shotcrete flash coat, 

followed by welded wire mesh, plates, installation of short bolts, and a second layer of 

encapsulating shotcrete. Deep support bolts with plates are then installed through the shotcrete 

welded wire mesh matrix to complete the support system during the final shotcrete phase as 

shown in Figure 4. After the top-cut ore has been removed, the top cut is backfilled with 

cemented rockfill. Subsequent levels are mined under the cemented rockfill (CRF), as noted by 

Seymour et al. [2013]. 

Figure 4. Final in-cycle application of wet mix shotcrete. 
8 

Typical Top-Cut Mining Cycle for Weak Rock

The top-cut mining method starts with a series of drifts that are driven into the upper level of

the orebody using a jumbo drill. The blastholes are typically drilled to a depth of 3 m (10 ft) and

then loaded for a blast round. These top-cut drifts are typically small in size because of the poor

rock quality and geologic structure present in the ore zone (Figure 5).

Figure 5. Drilling the first blast round.

The blast round is then removed. The rounds are designed to break the rock and advance the 

drift an additional 3 m (10 ft). Drill hole spacing and explosive load are carefully manipulated to 

minimize damage to the surrounding rock (Figure 6).

Figure 6. Blasting the first round. 

Following the blast, a shotcrete machine equipped with a remote-controlled manipulator arm 

is used to spray an initial layer of shotcrete (flash coat) onto the mine roof and walls to stabilize 

the ground and prevent raveling (Figure 7). 

Figure 7. Applying first layer of shotcrete.
9 

The blasted ore is then removed with a mucker operating under supported roof (Figure 8). 

This clears the drift of broken rock so that other equipment can be used to install the remaining 

ground support components, including bolts and welded wire mesh. 

Figure 8. Removing blasted rock or muck with loader. 

A roof bolting machine is then used to install short, secondary bolts and welded wire mesh 

over the first layer (flash coat) of shotcrete (Figure 9). 

Figure 9. Installing short bolts and welded wire mesh.

The next 3-m (10-ft) blast round is then drilled using a jumbo drill (Figure 10).

Figure 10. Drilling a second blast round.
10 

Following a second blast, the drift is advanced an additional 3 m (10 ft) past the secondary 

ground support (short bolts, welded wire mesh, and flash coat of shotcrete) that was installed in 

the previous advance (Figure 11). 

Figure 11. Blasting the second round. 

The shotcrete machine is then used to apply a second layer (final coat) of shotcrete over the 

short bolts and welded wire mesh installed in the previous advance as well as a flash coat of 

shotcrete over the rock surface exposed by this blast (Figure 12).

Figure 12. Installing a second layer of shotcrete.

The mucker operating under supported roof is again used to remove blasted ore from the drift, 

providing access for the roof bolting machine to install additional ground support (Figure 13).

Figure 13. Removing blasted rock or muck with loader. 
11 

Next, a roof bolting machine is used to install long bolts through the supermesh created by 

the short bolts, welded wire mesh, and shotcrete layers. These longer bolts provide the primary 

ground support by preventing large blocks of ground from falling into the drift (Figure 14).

Figure 14. Installing long primary bolts.

This completes the ground support cycle, as the roof bolting machine is once again used to 

install welded wire mesh and short bolts over a flash coat of shotcrete (Figure 15).

Figure 15. Installing short bolts and welded wire mesh.

Application and Use of Shotcrete for Surface Control in Weak Rock

Support System

The squeezing and raveling rock encountered in weak ground is not self-supporting after 

excavation and consequently, the support system must be installed concurrently with the active 

mining cycle, as previously mentioned. In the ground control system, shotcrete is used to provide 

temporary surface control for local stability before the primary support is installed. Because the 

shotcrete must become self-supporting before miners and equipment can work safely underneath, 

the curing characteristics of the shotcrete are critical to the speed of the mining cycle. 

Initially, a thin layer of shotcrete is applied as surface control to prevent the rock from 

unraveling (Figure 16). The flash coat is 19 to 25-mm (0.75 to 1-in) thick. This thin layer of 

shotcrete stabilizes the newly exposed surface and prevents very small rock and debris from 

falling, and also prevents air slacking and dehydration of the rock material exposed in the cracks. 

If this material is not confined, the small-sized rock can loosen and fall out, allowing additional 

material to displace, which compromises the geometric structure, ultimately causing massive 

failure [Bauer and Donaldson 1992].
12 

Figure 16. Application of a thin initial layer or flash coat of shotcrete. 

Next, a 100 x 100 mm (4 x 4 in) six-gauge welded wire mesh is prepositioned, and friction

bolts with plates are installed to hold the mesh in place. This prevents unraveling of the rock and 

will temporarily stabilize the ground between the future primary support locations (Figure 17). 

The plated short bolts also reduce vibration as the second shotcrete layer is applied to the welded 

wire mesh. It must be recognized that the weak rock mass will likely unravel, and the result will 

be the individual rock blocks falling from between the bolts; therefore, surface support is 

required to contain the rock mass as a single support unit [Pakalnis 2010]. 

Figure 17. Installation of welded wire mesh, plates, and short bolts. 
13 

A second layer of shotcrete is sprayed to encapsulate the welded wire mesh, the short bolts, 

plates, and the first layer of shotcrete creating a composite matrix, or supermesh (Figure 18). 

Supermesh is applied in such a manner as to form an arch around the opening. The total 

thickness of the combined supermesh matrix is 75 to 100 mm (3 to 4 in). 

Figure 18. Application of a second layer of shotcrete. 
14 

The primary ground support is then installed through the supermesh. This consists of long 

SwellexTM roof bolts (from Atlas Copco, Minova) with plates and either 0.9-m (3-ft) friction bolts

or 1.8-m (6-ft) Swellex bolts, depending on the rock mass rating. A view of the completed 

ground control system is shown in Figure 19. 

Figure 19. Completed ground control system with plated, long primary bolts through the matrix 

and shorter bolts to the springline.

Installing the primary support bolts through the supermesh provides a ground support system 

that has capabilities not provided by the use of mesh or shotcrete alone. Finally, for the ribs, 0.9-

m-long (3-ft-long) bolts or 1.8-m-long (6-ft-long) bolts with plates are installed depending on the 

RMR (if RMR is < 45, then 1.8-m (6-ft) bolts are installed to support the rib area of the 

opening). The long roof bolts are required to stabilize the opening while the supermesh serves to 

keep the small rocks from falling between the bolts.
15 

Figure 20. Multi-element ground support system for weak rock. 

The support profile of a mine opening using the supermesh multi-element ground control 

system is shown in Figure 20. The local support provided by the supermesh and short bolts is 

shown by the red load cones (small red wedges). The supermesh provides local surface control

holding the rock mass together and enabling the ground support load to be transferred back to the 

primary supports, the long bolts. Even after the shotcrete begins to develop cracks and dislodge, 

it still provides residual structural support because of the wire mesh embedded within the 

shotcrete. The global support provided by the primary bolts is represented by the diamondshaped load cones, shown in Figure 20 as dotted lines. The compression arch formed by the long 

rock bolts is the primary structural support and has the greatest influence on stabilizing the mine 

opening and preventing failure [Kaiser and Tannant 2001]. 
16 

Figure 21. Plan view of primary bolt pattern, looking up at the back from below. 

The long primary support bolts are installed on a defined bolt spacing pattern along the roof 

arch of the opening (Figure 21). The bolt length and spacing are determined by ground control 

experts using the appropriate empirical methods. In general, the bolt length and spacing are

based on the height of a wedge of the material the bolts must support. Pakalnis [2008] observed 

and reported that the depth of failure in the absence of structural support will be equal to one-half 

the opening span. 
17 

Shotcrete Failure Modes

Unsupported, unreinforced shotcrete applied to underground mine openings fails primarily 

when there is a loss of contact with the underlying substrate. This is followed by a subsequent 

failure in flexure as the shotcrete bends under the weight of additional loading from loose material 

(Figure 22) [Barrett and McCreath 1995; Holmgren 2001]. Failure of the shotcrete can be caused 

by a loss of adhesion with the rock surface or from the rock itself losing contact with adjacent 

rock. There are two basic types of shotcrete failure modes: fallout of only shotcrete indicating 

poor adhesion and fallout of shotcrete and rock, indicating zones of weak rock (Figure 23) 

[Malmgren and Svensson 1999]. 

Figure 22. Flexural failure of shotcrete resulting from insufficient adhesion strength, 

after Kuchta [2002].

Figure 23. Two basic types of shotcrete failure modes: A – fallout of only shotcrete and B – fallout 

of shotcrete and rock, after Malmgren and Svensson [1999].
18 

When shotcrete is used in conjunction with roof bolts, several other failure modes are 

possible as shown in Figure 24. 

Figure 24. Schematic of shotcrete loading and failure modes, after Morton et al. [2008], 

Barrett and McCreath [1995].

Post Deformation Strength

The inclusion of welded wire mesh reinforcement with shotcrete produces a product with 

redundant ground support capabilities in the event the rock mass loading exceeds the capacity of 

the shotcrete and cracking occurs. The combination of shotcrete and welded wire mesh creates a 

support structure with sufficient flexural stiffness to provide good rock confinement, 

displacement prevention, and load-transfer properties back to the primary supports. The welded 
19 

wire mesh stabilizes the matrix and continues to provide support through large displacements. 

The supermesh can deform while still providing surface control by retaining a sufficient loadcarrying capacity after cracking or failure of the shotcrete (Figure 25) [SRSUM 1999]. 

Figure 25. Supermesh provides support between the primary bolts. 

Supermesh is better able to withstand the types of shotcrete failure mechanisms previously 

described than shotcrete alone. Figure 26 shows the loading of a multicomponent ground control 

system in weak rock and the types of failures that can be prevented through the addition of 

welded wire mesh. The red arrow indicates the plated bolts placed through the supermesh, and 

the orange grid represents the blocky ground mass. 

Figure 26. Shotcrete failure modes that can be prevented by the addition of welded wire mesh.
20 

Shotcrete Parameters 

Shotcrete Mix Types

This section contains important information about shotcrete and brief descriptions of 

shotcrete mix types. The Australian Shotcrete Society [AuSS 2008] provides excellent in-depth 

coverage of the shotcrete topic and was used heavily in developing the content in the following 

sections of this document. Every effort was made to credit the original source material.

Shotcrete is an engineered product composed of a mixture of cement, sand, fine aggregates, 

and admixtures. Admixtures are used to influence spray ability, adhesion, curing time, and cured 

strength or toughness. These admixtures come in the form of micro silica, accelerators or 

retarders, plasticizers, and reinforcing components (fibers) [Kaiser and Tannant 2001]. Shotcrete 

is available as either a wet mix or a dry mix product with the constituent ingredients 

preformulated in the correct proportions by the supplier. Shotcrete can be batched onsite or 

delivered from the supplier via mix truck. 

Shotcrete can be sprayed using either a wet or dry process. After an initial curing time of 24

hours, the final installed product has very similar strength properties regardless of the mix type 

process. A number of factors influence the type of shotcrete that is used at a particular mine site, 

including the size of the mine opening, quantity of material that must be applied, physical layout 

and spacing of the workings, available surface support operations, and other considerations 

[Morgan 2008]. 

Dry Mix Shotcrete

The dry mix shotcrete system uses smaller equipment and requires less capital costs than a 

wet mix shotcrete system. Because the equipment is smaller and mobile, the dry system can be 

used in all areas. However, dust from dry mix shotcrete can be a problem. The shotcrete 

components, which may be slightly predampened to reduce dust, are fed into a hopper with 

continuous agitation. Compressed air is introduced through a rotating barrel or feed bowl to 

convey the materials into a continuous stream through the delivery hose. Water is added to the 

mix at the nozzle (Figure 27 and Figure 28). 
21 

Photo by NIOSH

Figure 27. Simplified schematic of a typical dry mix shotcrete system, after Mahar et al. [1975].

Figure 28. Operation of a dry mix shotcrete machine equipped with a rotary barrel. 

Wet Mix Shotcrete

The wet mix shotcrete system requires an extensive commitment in terms of capital 

expenditure and infrastructure. A batch plant for mixing shotcrete can be set up either on the 

surface or underground. For deep mines, a slick line is often used to transport the mixed 

shotcrete underground. Once the shotcrete is mixed, a shotcrete transport vehicle is used to

deliver the shotcrete to the application site. In some cases, mix trucks are employed underground 

to batch and deliver the mix. The wet mix system typically has higher production rates than the 

dry mix process. Although there is less dust in the wet mix process, fogging is significant. After 

the batched wet shotcrete mix is delivered to the application site, the shotcrete mix is poured into 

a positive displacement pumping unit. The pump then delivers the wet mix hydraulically to the 

nozzle where air and an accelerator are added to spray the shotcrete onto the rock surface (Figure 

29 and Figure 30). 
22 

Photo by NIOSH

Figure 29. Simplified schematic of a typical wet mix shotcrete system, after Vandewalle [1993].

Figure 30. Operation of a wet mix shotcrete machine. 
23 

Shotcrete Mix Constituents

Cement

The cement used for wet or dry mix shotcrete is generally a Portland type I/II variety or a 

blended cement variation. In many cases the supplier/manufacturer will incorporate 

supplementary cementitious materials into the shotcrete. 

Supplementary Cementitious Materials

One of the most frequently added materials to cement is fly ash. This material improves the

material flow characteristics and workability of the mix and reduces cost of the cement because it 

is a substitute for a portion of the cement. The main characteristic of fly ash is that it produces

good strengths when fully cured. Fly ash is supplied in two forms: (1) normal, which is finely

divided, inorganic, pozzolanic material and (2) special grade, which is a more reactive, faster 

curing, and finer variety. Silica fume with finer grade and faster early curing characteristics is also 

used. Granulated iron produced from ground furnace slag is also used as an admixture in some 

instances to alter the set time or to impart chemical resistance to the cured material [AuSS 2008]. 

Aggregates

The source aggregate component of each engineered mix is specified along with defined 

grading curves and the amount of variation permitted. These factors are the responsibility of the 

supplier in cases of proportioned bag or bulk mixes. For onsite batching using source materials, 

the mix should have a consistent grading within the allowable variation [AuSS 2008]. 

Mixing Water

The water used to batch the shotcrete mix is an important component in terms of its effect on 

mixing, application, curing, and long-term strength development. The temperature, presence of 

mineralization, and/or acidity of the water can affect the shotcrete mix performance. The purity of 

the water used to batch and mix the shotcrete should be tested along with allowable temperature 

variations. Hot water will accelerate and cold water will delay the cementitious curing process. 

The presence of mineralization in some cases will interfere with the cementitious process as will 

acidity levels outside the range with which the mix was designed to work [AuSS 2008]. 

Water-to-cement Ratio

The water-to-cement (w:c) ratio of a shotcrete mix has a strong influence on drying 

characteristics, strength development, and long-term durability of the applied material. Water-tocement ratio is obtained by dividing the total water weight by the cementitious materials’ weight 

in the shotcrete mix. For dry mix shotcrete the standard ratio is 35%, and for wet mix shotcrete, 

the standard ratio is 45% (Table 2). Water dosage, in terms of using the correct amount of water 

to maintain the specified w:c ratio for the shotcrete mix when batching, is critical as it affects the 

curing and strength development of the material from initial placement through maturity. The 

materials’ strength must increase to threshold levels in a short amount of time after application in 

order for miners and equipment to safely re-enter the heading and then, when fully cured, to reach 

a specified ultimate strength. Overwatering the mix when batching or spraying the shotcrete can 

reduce the proportion of cementitious materials, delay curing time, reduce the long-term strength 

gain, and produce a loss of adhesion at the application surface. Underwatering of the mix can 
24 

cause the delivery-application equipment to plug, resulting in a lack of adhesion of the material to 

the application surface, producing heavy rebound and segregation of the shotcrete mix, 

compromising the hydration process, and reducing the strength of the shotcrete [AuSS 2008].

Table 2. Typical water-to-cement ratios for shotcrete

Mix type and 

application process Typical range, % 

Dry mix shotcrete 35 

Wet mix shotcrete 45 

Chemical Admixtures

There are a number of chemical admixtures that can be used to change the transport, curing, 

and overall timing of the shotcrete application process. The addition of these products, though 

helpful in extending working time or improving the transport and application characteristics, can 

degrade the support performance of the shotcrete if not used properly. Water reducers may be 

used to accelerate or retard curing. Superplasticizers (high-range water reducers) allow for less 

water to be used while maintaining good transport and application characteristics. Using reducers 

results in higher strengths than a comparable mix that uses excess water to achieve the 

workability of the shotcrete. In applications where the mix must be batched and then transported 

considerable distances involving time delays, hydration control admixtures can be used to 

suspend the hydration process for a period of time. After delivery and prior to application, an 

accelerator can then be added to resume the hydration process. Shotcrete accelerators are also 

used in situations where adhesion is paramount. However, these accelerators can lower the final 

cured strength of the shotcrete [AuSS 2008]. Care should be taken to ensure that the amount of 

accelator added at the nozzle does not adversely affect the design strength of the shotcrete.

Shotcrete Reinforcement Methods

Shotcrete can be reinforced through the use of embedded steel mesh or through the addition

of fibers to the mix prior to application. Unreinforced shotcrete exhibits high strength 

characteristics in compressive loading but has very low tensile strength and ductility. Weak rock

ground conditions subject the shotcrete to high tensile loads, and therefore additional

reinforcement is often needed in the shotcrete.

Shotcrete Reinforced with Welded Wire Mesh

Shotcrete is applied to welded wire mesh that is positioned over a previously installed flash 

coat of unreinforced shotcrete. This embeds the mesh within the shotcrete matrix. The mesh also 

helps during shotcrete installation as it provides additional anchor surface area for the shotcrete 

to stick on and build up bulk. 

In weak rock when tensile stresses cause the brittle shotcrete to fail, the ductile steel provides 

toughness (post-crack ductility) that can prolong and preserve the integrity of the structure. The 

type of wire mesh reinforcement (wire gauge, grid spacing, and weave style) required for 

structural purposes is determined by ground control experts using the appropriate empirical 

methods. 
25 

 For weak rock conditions six-gauge, 100 x 100-mm (4 x 4-in) grid welded wire mesh is 

used. When used as part of the supermesh system described in the Typical Weak Rock Top-Cut 

Mining Cycle section, the welded wire mesh reinforcement is installed following an initial 

shotcrete flash coat. A second layer of shotcrete is then applied to encapsulate the wire mesh 

reinforcement, followed by the installation of primary bolts with plates through the structure. 

When this system is loaded by the rock mass, the forces are distributed into the supermesh 

matrix through the encapsulated wire mesh. The grid configuration of the mesh helps to 

redistribute the large reactant point loads that the rock bolts would otherwise introduce to the 

shotcrete. 

Toughness describes the ability of the reinforced shotcrete to sustain and redistribute load 

after cracking. Toughness is quantified in terms of post-crack, load-carrying capacity or energy 

absorption, which can be assessed using panel or beam testing specimens. Guidance on 

toughness values to specify for mining applications can be obtained from various geotechnical 

design tools. This is covered in more detail in the testing section of AuSS [2008]. 

Shotcrete Reinforced with Polypropylene or Steel Fiber

As in the use of welded wire mesh described in the Support System section of this document, 

the addition of short steel or polypropylene fibers to shotcrete can increase toughness (post-crack 

ductility) that can prolong and preserve the integrity of the structure when subjected to tensile 

stresses. The selection of welded wire mesh over fiber reinforcement is based on ground 

conditions and the mine management’s determination of the cost-benefit ratio. In the case of 

weak rock conditions, welded wire mesh is used because ground movement in excess of 25 mm 

(1 in) is expected and welded wire mesh offers superior performance.

To increase tensile or flexural strength fiber dosage can often be reduced in the unbroken 

cementitious material [AuSS 2008]. The benefits of fibers compared to the use of steel mesh 

reinforcement include a significant reduction in cost and effort because the equipment and 

personnel required for handling and installation of the wire mesh panels are eliminated. 

Compared with wire mesh panels, fiber-reinforced shotcrete has advantages in terms of ground 

support and cost when applying a uniform thickness of shotcrete over irregular surfaces. 

Shotcreting over pre-installed wire mesh panels cannot exactly follow the contour of the rock 

surface and, as a result, many bridged voids are created that require more material to be applied 

to bring the shotcrete layer up to the plane of the wire mesh and create the necessary 

encapsulation. Blinding behind the individual wires of the mesh is also a problem that requires 

considerable skill to overcome on the part of the applicator. Problems with fiber reinforcement 

are directly related to the fibers themselves. Several mining operations have relayed that they 

will not use fiber reinforcement because, in the case of the poly product, it creates serious 

problems with the mine’s water management and removal system because fibers can clog sump 

pumps. In the case of the steel fibers, a serious safety issue exists because of the potential for 

mine personnel to become injured from abrasive contact with steel fibers. 
26 

Shotcrete Mix Characteristics

Application

The batched and mixed shotcrete must have a consistency that can be transported and applied 

to the mine surfaces without bleeding (washing out constituent materials or becoming 

segregated), including sufficient adhesion and cohesion to stick and hold to overhead surfaces 

and not run down vertical surfaces [AuSS 2008]. 

Early Strength

Re-entry time is critical to the speed of the mining cycle used in weak rock. The applied 

shotcrete material must develop enough strength to be self-supporting upon application and 

develop additional strength that allows mine workers and equipment to re-enter and install incycle mining support [AuSS 2008]. 

Flexural Strength

The applied material must develop the specific load capacity and toughness within the 

variation allowed by the design criteria and ground control system [AuSS 2008; Bernard 2008]. 

Typical Formulations for Wet Mix and Dry Mix Shotcrete

A typical shotcrete mix in both wet and dry forms is summarized in Table 3. These mixes can 

be used as a starting point for reference. It should be noted that most shotcrete mixes are 

preblended by the shotcrete suppliers who have a set number of mix variations available based 

on application and cost. These mixes can be purchased as premixed bags having capacities of up 

to 1,500 kg (3,306 lb). The size of the bag for dry or wet mix applications is limited by the 

strength of the available bag as per ASTM C136 [2006]. 

Table 3. Typical mix designs for steel-fiber-reinforced shotcrete with silica fume additive, 

after Wood [1992] 

a. Dry mix

Mix constituent Amount, 

kg/m3 (lb/yd3

) 

Percentage of dry 

materials, % 

Cement 420 (708) 19 

Silica fume additive 50 (84) 2.2 

Blended aggregate 1,670 (2,815) 75.5 

Steel fiber 60 (101) 2.7 

Accelerator 13 (22) 0.6 

Water controlled at nozzle controlled at nozzle 

Total 2,213 (3,730) 100 
27 

b. Wet mix

Mix constituent Amount,

kg/m3 (lb/yd3)* 

Percentage of wet 

materials, % 

Cement 420 (708) 18.1 

Silica fume additive 40 (67) 1.7 

Blended aggregate 1,600 (2,697) 68.9 

Steel fiber 60 (101) 2.6 

Accelerator 13 (22) 0.6 

Superplasticisor 6 L (6.3 qt) 0.3 

Water reducer 2 L (2.1 qt) 0.1 

Air entraining 

admixture if required if required

Water 180 (303) 7.7 

Total 2,321 (3,912) 100 

*Unless listed otherwise as liters and quarts

Shotcrete Application

The shotcrete application process has a major effect as to the final quality of the installed 

shotcrete material. This process is impacted by onsite conditions and includes surface 

preparation and application phases. The onsite lighting and visibility available to the mine 

worker and support crew applying the material must be good enough for the operator to see the 

application area in detail and to communicate commands effectively.

The size and shape of the mine opening can dictate the application method and type of 

equipment that can be used. If the mine opening size is too small or too large, mechanized, 

manipulator-type devices cannot be used and the material will have to be applied manually. The 

mobile chassis and manipulator arm of the mobile equipment has a fixed size, range-of-spray 

motion, and various maneuverability characteristics. In some cases, manipulator-type shotcrete 

application equipment is too large to spray shotcrete and to maneuver effectively [AuSS 2008]. 

In other cases, the size of the opening can be so large that the shotcrete application process will 

have to be performed manually from a suitably sized man-lift platform [Pakalnis 2010]. 

Substrate Preparation

Shotcrete quality is influenced by the condition of the surface onto which it is applied 

[Barton and Brandis 1990]. The amount of surface preparation required depends on the condition 

and nature of the substrate against which the shotcrete is to be applied. In weak rock conditions 

only minimal surface preparation can be completed because of the loose and raveling nature of 

the host rock [Spearing 2001]. Shotcrete should not be applied directly to dry, dusty rock 

surfaces, so the work area surface is sprayed top to bottom (from the back to the floor) with 

water to predampen the surface. This predampening creates a good surface on which to bond the 

initial layer of shotcrete and prevents loss of water from the newly applied shotcrete. 
28 

For rehabilitation work over existing shotcrete sprayed on permanent structures, such as shaft 

stations and shop facilities, where adhesion to the substrate is required as part of the structural 

system, high-pressure water and scaling are used for substrate preparation in order to promote 

adhesion. In this situation, it is very important to insure that the application surfaces are free of 

loose material, dust and films (such as oils), excessive water, and other contaminants which 

might interfere with bonding of the shotcrete to the surface [Bernard 2008; Kuchta 2003]. 

Initial Adhesion

Because shotcrete is applied pneumatically and compacted dynamically, it must develop a 

sufficient cohesive interlock upon impact with the mine surface to be self-supporting [Rispin

2005]. The sprayed shotcrete material should not generate excessive rebound waste. If either the 

mix consistency in terms of water content (too runny or too dry) or the application technique of 

the nozzle operator are not correct, then the sprayed material will not adhere. If the material 

cannot perform in this manner, then the entire process cannot continue [NCA 2007]. 

Shotcrete Application Process

The shotcrete application process must be correct for the installed material to perform as 

designed [Morgan 2008]. The shotcrete nozzle operator and shotcrete support crew should be 

well trained and thoroughly understand the communication signals used, the shotcrete process,

and equipment. The same step-by-step process should be used from batch to batch for 

consistency. In general, the material is applied from bottom to top using a technique of moving 

the nozzle in small circles. The shotcrete must be applied to the thickness specified in order to 

perform as designed. Every effort should be made to avoid applying material on top of rebound. 

For manual application, the operator is usually positioned about 1 to 1.5 m (3 to 4 ft) back from 

the substrate. For mechanized application using a manipulator arm, the operator is positioned 

back from the surface, but with a good vantage point, and controls the application with a remote 

control device [Melbye 2001; Thomas 2009]. 

Field Testing Methods for Shotcrete Application

The material properties of shotcrete are most commonly referenced in terms of the 

measurable characteristics described in this section. NIOSH researchers investigated American 

Society for Testing and Materials (ASTM) test methods to determine material properties of wet 

and dry shotcrete. It was found that field-capable testing units were needed to test shotcrete in 

mine environments to determine shotcrete properties in the wet and hardened states. This is due 

to the mines being located outside typical laboratory locations, so to obtain reliable results the 

testing needed to be conducted in situ at the mine.

For the weak rock mass ground support systems, there are a number of performance 

requirements that the shotcrete must satisfy. The shotcrete is applied in-cycle and is the only 

element of the ground support system that has a time-dependent variability. The entire mining 

cycle is dependent on the material curing and the onset of this early strength support 

characteristic. The time required for the material to have acquired enough strength for workers 

and machinery to be located under the freshly applied material is the most important 

performance characteristic to quantify for ground support systems with weak rock mass. 
29 

Assurance tests should be reliable, meaningful, timely, simple, and relatively inexpensive. 

The objective is to ensure that the performance of the shotcrete in-place meets operator design 

requirements. The main targets examined in any shotcrete testing program should be design 

compliance for shotcrete strength (early compressive strength, flexural strength, and bond 

strength) and thickness. Existing methods for determining these characteristics were reviewed 

and found to be unsatisfactory in terms of the assurance test criteria mentioned above.

Therefore, researchers at NIOSH developed a set of shotcrete material property testing 

machines that can be used directly at the mine site. Prior to this research, shotcrete testing 

machines were not available for field testing at mines. As a result, mine personnel could not 

readily determine if the applied shotcrete met the original shotcrete performance requirements 

for ground support. These portable test machines measure early strength related to re-entry time; 

flexural load capacity and toughness; and installed quality, thickness, and bond strength. The 

NIOSH machines required extensive development and field testing to meet quality assurance test 

criteria. The tests described in the following sections are used to obtain specific measurable 

shotcrete characteristics.

From a longer-term ground control standpoint, the flexural load-carrying capability and 

toughness of the cured shotcrete, the quality of the applied shotcrete, and the competency of the 

underlying rock are also major performance requirements. All of these performance requirements 

must be established and monitored, first through pre-installation trials and then as part of an 

ongoing quality monitoring program. Mine staff must have a means of quantifiably determining 

the material properties for comparison to performance requirements.

Slump Test

The ASTM C143 [2010] slump test should be used for mix design changes, batch plant 

changes, mixing truck changes, and personnel changes in the shotcrete process. The typical 

concrete slump test used in the construction industry on cast concrete is used on shotcrete. The 

test apparatus base is set on a clean level surface and prewetted before each use (Figure 31 

through Figure 33). When conducting the test, shotcrete is poured into the test apparatus cone in 

three equal height layers. Each layer will be rodded 25 times by the steel rod. After rodding the 

final layer, the top surface of the shotcrete is struck off. To complete the test, the cone is lifted 

vertically from the shotcrete. The distance that the shotcrete slumps below the apparatus handle 

is measured. This is deemed as the “slump” of the shotcrete. With the use of wet additives, mixes 

may require spread tests rather than slump tests.

When design strength is determined at a given water ratio, a slump test is performed and that 

slump height is documented. If the field slump height is not comparable, overwatering is

occurring. Batched shotcrete mix can have added additives to modify the slump at the point of 

application. If the shotcrete mix batch is overwatered, producing a large slump, then it can be 

accelerated (mixed with an additive to accelerate setting) to allow it to adhere to the mine walls 

and roof. This overwatering and accelerating should be avoided as the shotcrete strength 

characteristics are changed from the original design. The nozzle operator can make the shotcrete 

stay in place on the mine opening at high slumps with accelerants, but this reduces strength and

compromises the structure [Morgan et al. 1999]. 
30 

In most cases, the overwatering occurs due to the conflicting interests of the shotcrete crew 

nozzle operator and those of the mine. The nozzle operator does not want to risk plugging the 

equipment and thus would prefer a wet mix. The mine owners who are paying for the concrete 

would not like to see this expensive component washed away or incur additional expense due to 

the necessity of adding accelerants to give a proper consistency for the sprayed material. As 

always, a balance between the conflicting interests must be achieved. This can be accomplished 

through training on how to conduct quality control testing and appropriate corrective actions 

[Spearing 2001]. 

Photo by NIOSH Photo by NIOSH

Figure 31. Consistency of wet mix shotcrete being loaded into the hopper of a shotcrete machine. 

Figure 32. Filling a slump cone with wet mix shotcrete prior to conducting a slump test 

in an underground mine. 
31 

Photo by NIOSH

Figure 33. Slump test with wet mix shotcrete showing a 292-mm (11.5-in) slump; 

typical slump is 100 to 200 mm (4 to 8 in). 

Compression Test

The primary material property specified for plain cured shotcrete is the compressive strength. 

Compressive strength is measured as the resistance of the shotcrete specimen to an axially 

applied crushing force, usually referred to as the unconfined compressive strength (UCS). This 

measure is an indicator of the quality of the shotcrete. The compressive strength of shotcrete inplace is a critical element that is used for design, and therefore needs to be determined. During 

the application process the strength of a mix as supplied can be changed due to many variables 

such as accelerators, rebound, poor application techniques, and overwatering. The drilling of 

sprayed cores from the in-situ shotcrete is the best method to use in determining the compressive 

strength. The cores should be tested in accordance with ASTM C1140 using cores secured in 

accordance with ASTM C1609 [2010], ASTM C39 [2011], and ASTM C1604 [2005]. The 

testing time at 28 days is normally taken to represent the long-term strength of this material.

Early Strength, Partial-beam Compression Test

Purpose of the Test

The goal of early strength testing is to characterize and define the mine worker and 

equipment re-entry time for a particular shotcrete mix and thus determine when the mining cycle 

can safely restart with miners and machinery. Early strength testing of in-stope, as-sprayed 

shotcrete samples can supply specific material properties for the shotcrete that are then 

comparable to known specifications. This testing can be conducted as part of pre-installation 

trials and as part of an ongoing quality monitoring program [AuSS 2008]. 

The early-age strength is the design strength of shotcrete specified at a time interval earlier 

than the conventional 28-day cure time. In the mining industry, early-age strength is usually in 

the range of 1–6 hrs before re-entry by miners and equipment after spraying [Rispin 2005]. The

safe re-entry time is the time it takes for the freshly applied shotcrete material to have developed 

enough strength to resist the normal ground pressure [Iwaki et al. 2001]. Early strength equates 
32 

to an as-placed, cured shotcrete strength that allows the material to be drilled and the remaining 

support system elements to be installed without degradation of the shotcrete or its long-term 

strength gain characteristics [O’Toole and Pope 2006; Clements 2009; Clements et al. 2004]. 

Literature and research have shown that obtaining drill cores and cylinders does not provide a 

reliable testing medium; therefore, the early strength compression beam and beam strength tests 

are used. Suitable results can be obtained by spraying the molds either by hand, or manipulator 

arm, for both wet and dry shotcrete mixes. The ASTM C-116 [1990] based tester and variations 

[Morgan et al. 1999; Clark et al. 2011] are the only early-age strength testing devices that 

involve direct compressive loading of shotcrete samples. 

Mechanical Principle

NIOSH designed a portable partial beam ASTM C116 [1990] test machine for use onsite at 

all the mines (Figure 34 and Figure 35). The unit and apparatus applies a compressive load at a 

fixed displacement rate to conduct the partial-beam strength determination test. As illustrated in 

Figure 36, the test sample is loaded in direct compression by the upper (force) and lower 

(reactant) platens of the test fixture, and compression strength is determined based on the contact 

area of the platens.

The device is comprised of a small-footprint, self-contained, servo-controlled, stiff-frame 

press with advanced load-rate and load-collection capability. The operator is presented with a 

proper scale and resolution load output display. The machine resolution is 0.45 kg (1 lb) over a 

2,268 kg (5,000 lb) operating range. A programmable-logic-controller (PLC) driven machine 

head applies a load to keep the machine at the proper ASTM test cycle displacement rate of 

1.278 mm/min (0.050 in/min) with an auto return after 6.35 mm (0.25 in) of displacement

[ASTM C116 (1990)]. The automated load cycle greatly reduces the chance of human error and 

the time required to conduct a test.

Figure 34. Portable shotcrete test machine designed by NIOSH researchers. 
33 

Photo by NIOSH

Figure 35. Operation of portable shotcrete test machine. 

Figure 36. Diagram of the compressive forces applied to a partial-beam shotcrete sample 

during an early strength test. 

Performance Standard

Early strength values were obtained and reported from testing conducted at North American 

mines for the 1 to 6-hr cure time. These tests produced strength ranges from 1.0 to 1.6 MPa (145 

to 233 psi) [O’Toole and Pope 2006], with an early compressive strength equivalent of 1.0 MPa 

(145 psi) being the norm for safe re-entry time [Rispin et al. 2003; O’Toole and Pope 2006; 

Bernard 2008]. Typical early strengths and re-entry times worldwide are listed in Table 4. The 

beam test methods are calibrated against data obtained using this test and the ASTM C1117 

[1989] and ASTM C403 [1999] test methods. 
34 

Table 4. Typical range of shotcrete early strength and re-entry time by region, after 

Rispin et al. [2003], O’Toole and Pope [2006], and Bernard [2008] 

Region Early strength, MPa (psi) Re-entry time, hr

United States 0.5–3 (75–435) 2–6 

Canada 1.6–2 (232–300) 2 

Austria 0.8–1.6 (116–232) 2–6 

Australia 0.5–3 (75–435) 1–6 

Test Procedure

The ASTM C116 [1990] testing protocol calls for samples of the shotcrete mixes to be 

sprayed into 102 x 102 x 152 mm (4 x 4 x 6 in) mold boxes as seen in Figure 37 to make the 

three test specimens for each hour test. Beam molds have been used successfully for creating test 

specimens in both the United States and Canada [Heere et al. 2002]. 

Photos by NIOSH

Figure 37. Partial-beam shotcrete mold (left) and shotcrete test sample (right). 

A mold containment system, consisting of two frames, was developed by the NIOSH 

researchers to restrain the partial-beam molds and orient them at a 45° angle to reduce the 

amount of rebound that would build up in the boxes. The smaller frame houses three partialbeam molds and can be hand-carried to the testing location to conduct 1-hr tests (Figure 38, 

upper left). The larger frame contains forklift pockets allowing the entire unit to be moved to the 

test location (Figure 38). This system allows the 2- through 6-hr samples to have the cure time 

necessary for movement while still allowing for the 1-hr tests.
35 

Photo by NIOSH Photo by NIOSH

Figure 38. Containment fixtures and molds for making partial-beam shotcrete samples 

(large, center; small, upper left). 

After the samples have been sprayed, tests are conducted at one-hr intervals over the next six

hrs (1 to 6-hr tests). The shotcrete samples are carefully demolded by disassembling the mold 

fixtures and removing the enclosed samples. Next, a shotcrete sample is placed in a specialized 

testing fixture and centered under the loading head of the test machine. The loading head is then 

lowered to a position just above the testing fixture’s upper plate via a manual toggle switch 

(Figure 39). A demolded, sprayed test sample is subjected to a direct compression load between 

the platens of the test device that induces a complex diagonal tensile failure; the compressive 

strength is determined based on the contact area of the platens. This testing is conducted on the 

hour for the time period corresponding to the 1–6 hrs from the time the samples are collected. A 

minimum of three samples should be tested for each hour interval to determine this strength 

[ASTM C116 (1990)]. 

Figure 39. Loading of a partial-beam shotcrete sample during an eary strength test. 

To conduct a test, the green start switch is depressed initiating the automated test cycle. The 

PLC-controlled press applies a fixed-rate load to the sample. The load profile is shown on a 

graphical output display, and the measured test parameters (time, displacement, and load) are 

stored on a thumb drive. The sample is tested to failure as indicated by the appearance of a crack

(Figure 40). The operator will observe a well-defined peak in the load profile curve indicating 

the test is completed. The test is then stopped by depressing the red button on the test machine’s
36 

control panel, and the test machine’s loading platen is returned to its initial starting position. 

After testing only a few samples, it is relatively easy to determine when a test has reached 

completion. This process is repeated for three specimens so that an average can be determined. Photo by NIOSH Photo by NIOSH

Figure 40. Shotcrete sample showing vertical shear failure.

Observations

Peak load is reflected by the development of large vertically oriented single or dual side 

cracks (Figure 41, Figure 42, and Figure 43) along the platen-to-sample contact edges. 

Figure 41. Shotcrete sample showing single side failure, loaded from top and bottom of specimen.
37 

 

Photo by NIOSH Photo by NIOSH

Figure 42. Shotcrete sample showing dual side failure, loaded from top and bottom of specimen.

Figure 43. Shotcrete sample showing exposed failure plane.

The vertical crack is a complex failure that exhibits elements of shear, tensile, and 

compressive failure modes. Figure 44 depicts examples of shotcrete failure modes.

Figure 44. Shotcrete failure modes, after Rose [1985].

Laboratory Tests

Early strength testing of several mixtures of shotcrete was conducted at NIOSH’s Spokane 

Research Laboratory in accordance with ASTM C116 [1990]. The shotcrete was applied to the 

molds by a competent nozzle operator. The superstick corrosive accelerant (SCA) mixes were 

supplied by a shotcrete manufacturer, different from the supplier of the K1 and P2 mixes. SCA is 

manufactured in Nevada and used in Nevada and Montana mines. K1 and P2 are manufactured 

in the state of Washington and used in Alaska and Idaho mines. The superstick corrosive 

accelerant polyfiber (SCAPF) and superstick corrosive accelerant steel fiber type-100 (SCAPT100) are fibered versions of the SCA mix. SCAPF is a polyfibered mix, and SCAPT-100 is a 

steel-fibered mix. K1 has a greater percentage of accelerants than P2, and P2 has a greater 

percentage that SCA.
38 

The merits of the shotcrete process in terms of strength gain arising from pneumatic 

application and dynamic compaction are illustrated in Figure 45. The sprayed material exhibited 

an order of magnitude strength increase for identically batched samples (five samples each) that 

were cast from the same mix. The similarity between cast concrete and cast shotcrete in terms of 

curing behavior with respect to strength gain is also shown in Figure 45. This graph illustrates 

that the strength gain arises from the application process and not from some mix formulation 

difference specific to shotcrete. The curing phase of the sprayed shotcrete also shows an 

accelerated strength gain, followed by a slow down or hump, which is typical for sprayed 

shotcrete.

Figure 45. Early-age compressive strength versus curing time for sprayed and cast samples 

of K1 shotcrete for 1 to 6 hours of curing time. 

Figure 46 shows the early-age compressive strength test results for the five commercially 

available dry shotcrete mixes tested (n = 72 samples). The shotcrete strength gain is shown over 

the time interval from 1 to 6 hrs with four of the five mixes indicating a strength plateau at the 4-

hr mark and one mix indicating a strength plateau starting at the 2-hr mark.

This initial set and then phase transfer was observed in all shotcrete samples tested in the 

NIOSH study. This characteristic has been well referenced in the literature on shotcrete [Jolin et 

al. 1999; Heere et al. 2002; Knight et al. 2006; Rispin 2005; O’Toole and Pope 2006; Bernard 

2008] and is depicted as well in Figure 46. The initial strength gain provided by the accelerant

additives appears to progress into a cementitious hydration phase. For the mixes tested, three out 

of the five reached the early re-entry strength threshold of 1 MPa (145 psi) within 3 hrs and were 

confirmed again at the 4-hr mark. All of the shotcrete mixes exceeded 1 MPa (145 psi) after 24 

hrs of curing.
39 

Figure 46. Early-age compressive strength versus curing time for sprayed partial-beam samples 

of 5 shotcrete mixes for 1 to 6 hours of curing time. 

The strength curves shown in Figure 46 are in good agreement with O’Toole and Pope 

[2006] who reported similar strengths ranging from 0.25 to 2.25 MPa (36 to 326 psi) for tests 

conducted on partial-beam shotcrete samples after 1 to 6 hours of curing. These strength curves 

are also similar to Bernard [2008] who reported strength values ranging from 0.2 to 3 MPa (29 to

435 psi) for 1-hr through 10-hr tests on partial-beam samples of shotcrete. 

After six hrs of curing, the fibered mixes exceeded the capacity of the test machine (shown as 

dotted lines in Figure 46), with compressive strengths greater than 2 MPa (290 psi). For this test 

method, the addition of fibers generally produced higher early strengths.

Field Verification of Test Method and Apparatus for Determining the Early Strength of Partial-beam 

Shotcrete Samples. 

Purpose of the Test

The early strength test machine is used to identify the onset of 1-MPa (145-psi) strength gain 

of the shotcrete from 1 to 6 hrs. The field trials consisted of collecting multiple shotcrete samples

from the mine and testing the early strength gain of the shotcrete. 

Trial Infrastructure

The Turquoise Ridge Joint Venture (TRJV) near Goldconda, Nevada supplied the shotcrete 

product, SCA (Figure 31). The mix consistency as measured by a slump test was 29.2 cm (11.5 in) 

(Figure 32 and Figure 33). Samples were collected in-stope on the 3,550 level of the mine and 

tested in the mine’s material property testing lab on the 1,250 level of the mine.

Sample Collection

A mechanical shotcrete machine was used to spray eighteen early strength shotcrete test 

samples. The wet-mix shotcrete was applied using a remotely controlled arm with an oscillating 

nozzle that allowed for a uniform deposition of the material into the sample collection boxes 

measuring 102 x 102 x 152 mm (4 x 4 x 6 in) from a distance of about 1.2 m (4 ft) (Figure 47). 

This method of collecting shotcrete test material ensures an accurate sampling of the actual mine 

mix from which a true early strength time can be developed.
40 

Photo by NIOSH

Figure 47. Positioning partial-beam shotcrete molds and containment fixtures prior to shotcreting. 

After the samples were sprayed, they were immediately hand-troweled for a uniform top 

surface. The 1-hour test samples were carefully transported by researchers riding in the tractor 

from the spray location to the test site. The remaining 15 samples were allowed to cure longer 

before being transported on foam in a tractor basket to the test site. 

Testing

Early strength tests were conducted with partial-beam shotcrete samples using the NIOSH 

portable test machine in accordance with ASTM C116 [1990]. Figure 48 and Figure 49 show a 

partial-beam shotcrete sample being tested at the field site.
41 

Photo by NIOSH Photo by NIOSH

Figure 48. Portable test machine for determining early-age strength of shotcrete. 

Figure 49. Loading of partial-beam shotcrete sample during an early strength test. 
42 

Photo by NIOSH

Figure 50. Shotcrete sample showing exposed failure plane. 

Trial Results

The peak strength values for the six hourly tests are listed in Table 5. For a wet mix shotcrete 

with a 292.1-mm (11.5-in) slump, the re-entry time was determined to be 5 hrs at an average

early strength of 1.18 MPa (171.6 psi). As mentioned by Melbye [2001], an earlier re-entry time 

may be achieved by using a shotcrete mix with a lower slump.

Table 5. Early strength test results for partial-beam shotcrete samples using the 

NIOSH portable test machine

Curing 

time, 

hr

Sample 1

Ultimate stress, 

MPa (psi)

Sample 2

Ultimate stress, 

MPa (psi)

Sample 3

Ultimate stress, 

MPa (psi)

Average 

Ultimate stress, 

MPa (psi)

1 0.07 (9.9) 0.15 (21.1) 0.13 (18.4) 0.12 (19.8) 

2 0.21 (30.0) 0.19 (27.6) 0.19 (27.7) 0.2 (28.5) 

3 0.33 (48.2) 0.41 (58.7) 0.41 (59.2) 0.38 (55.1) 

4 0.53 (76.5) 0.57 (82.4) 0.68 (98.8) 0.59 (86.0) 

5 1.24 (179.5) 1.19 (173.3) 1.12 (162.8) 1.18 (171.6) 

6 1.69 (245.1) 1.50 (218.0) nr* 1.60 (231.9) 

*nr = not reported

Figure 51 shows the early-age compressive strength versus curing time for partial-beam

shotcrete samples tested at the mine site. Of particular note is the shallow slope of the strength 

curve up to 4 hrs, followed by an accelerated strength gain as the mix cured further in the 4 to 6-

hr period. As noted by Clark et al. [2011], a similar strength gain or change in the slope of the 

early strength curve has been observed during other tests conducted at the OMSHR facility in 

Spokane. Furthermore, this strength gain trend is comparable to the results of other early strength 

tests reported by the mining industry [Clark et al. 2011]. However, test results can vary, and they 

are highly dependent on the characteristics of the specific shotcrete mix and also on the sampling 

and testing procedures. For example, the results of these early strength field tests showed a much 
43 

more uniform and consistent trend in strength gain than laboratory tests conducted with a similar 

K1 shotcrete mix (Figure 46). As a result, shotcrete early strength tests should be conducted 

routinely at the mine site to verify safe re-entry times.

Figure 51. Early-age compressive strength versus curing time for partial-beam samples 

of K1 shotcrete with 1 to 6 hours of curing time. 

Overcoring and Direct-Tension Pull Test

Purpose of the Test

The goal of the overcoring and direct-tension pull testing is to characterize and define the 

quality and bond strength of the applied shotcrete and thus, determine if the application protocol 

has been followed and what, if any, load-carrying support is provided by the shotcrete to rock 

bond. This inspection and testing is conducted in-stope at the shotcrete application site as part of 

an ongoing quality monitoring program. 

The installed quality is assessed by comparing the observable core characteristics to known 

standards set forth in the quality monitoring program. Upon retrieving the core samples, host 

rock quality and strength can be compared to that of the shotcrete and an assessment made as to 

whether the rock is physically stronger or weaker than the shotcrete based on where the break 

occurred. The core cross section can be visually examined to reveal the structure of the shotcrete 

application with regard to defects and the presence of structural reinforcement in the form of 

welded wire mesh or fiber.

Bond strength, the adhesion of the shotcrete to the host rock, provides additional loadcarrying support over and above the ground support supplied by the system’s conventional 

support components—primary and secondary bolts with plates. Bond strength is reported in 

terms of peak tensile load values for the shotcrete to underlying rock bond using direct tensile 

pull test methods. Core inspection and direct tensile testing can supply specific information 

regarding the quality of the applied shotcrete, the competency of the underlying rock, and the 

bond strength of the shotcrete to the rock [Hahn and Holmgren 1979]. Bond strength is a very 

difficult property to measure. The measurement of bond strength is accomplished using a drilled 

shotcrete core pulling apparatus. Although there are nondestructive tests like hammer sounding 

and chain dragging, these do not determine the strength but only detect voids through sound 

differences. To determine if the shotcrete has bonded correctly to the host rock or underlying 

shotcrete layer, a pull test is required.
44 

Several methods, however, have been put forth for use, including the ASTM C1583/C1583M 

[2004], and internationally, the European Federation of National Associations Representing 

Concrete (EFNARC) bond test and the Swedish Standard test [AuSS 2008]. Most existing test 

methods that attempt to establish an actual bond strength value involve proprietary equipment for 

the extraction of a core from in-place shotcrete. These test methods are difficult to perform, 

cumbersome, and unreliable, and the required equipment is generally unavailable. 

For this reason, NIOSH researchers developed a bond strength testing method and equipment 

[Seymour et al. 2011]. This system provides a practical method of determining bond strength 

through a direct tensile test conducted in underground mines. This measurement of shotcrete 

bond strength can be accomplished in as soon as one day following application [Malmgren et al. 

2005]. This helps confirm when the shotcrete strength (bond) is sufficient to support its own 

weight. It is important to measure shotcrete bond strength on test sections during the initial phase 

of curing in order to help identify when it is safe to work under the recently sprayed shotcrete.

Mechanical Principle

To determine shotcrete bond strength, NIOSH researchers designed a portable test system for 

determining shotcrete bond strength in underground mines [Seymour et al. 2011]. Through this 

test system, the bond strength can be determined by conducting a pull test on the applied 

shotcrete. The system enables in-place (in situ) coring of the cured shotcrete matrix followed by 

controlled direct-tensile loading of the cored sample to failure. The basic test configuration 

consists of a core-shaped sample produced through diamond drilling, and then through the use of 

an installed stud pull anchor, a direct tensile force can be applied with a loading ram on the core 

to induce a tensile failure. The failures occur in the shotcrete matrix, and also in the shotcrete-toshotcrete and shotcrete-to-rock interfaces.

This method of determining bond strength involves the generation of three concentric core 

holes from a single drill setup. A 102-mm (4-in) diameter core is then extracted in direct tension 

through the use of an epoxied pull-stud and an extraction device that ensures concentric loading. 

Bond strength is given in terms of the stress required to fail the core specimen in tension. The 

system primarily consists of a small, stand-mounted core drill, and a pulling-unit equipped with a 

precision digital pressure gauge (Figure 52 and Figure 53). As illustrated in Figure 54, the cored 

test sample is loaded in direct tension by the pulling cylinder (force) and base fixture ring 

(reactants). Bond strength is determined based on the cross-sectional area of the test sample. The 

pulling unit load is produced by manually pumping a hydraulic power pack connected to a 

hollow ram cylinder that is capable of exerting 5,443.1 kg (12,000 lb). The cylinder applies a 

direct axial load to the cored sample via an embedded pulling stud. The loading rate is controlled 

by the cycle rate of the hand pump. The load rate is adjusted by the operator to coincide with a 

test duration of approximately 1 to 2 min. A dial pressure gauge is used with a scale and 

resolution of 0.007 MPa (1 psi) over a 0 to 60-MPa (0 to 8,700-psi) operating range. 
45 

Photo by NIOSH

Figure 52. Schematic of overcoring and direct-tension test equipment for determining 

shotcrete adhesion strength. 

Figure 53. Operation of overcoring and direct-tension test equipment.

Figure 54. Tensile loading of shotcrete core sample to determine bond strength. 
46 

Performance Standard

Typical indicators of application quality with regard to the cored sample include: the applied 

thickness, the presence of voids or lenses of noncompacted material, the presence of 

reinforcement (type, location, and density), the quality or tensile strength of the underlying host 

rock, the bond strength of the shotcrete to the host rock substrate or an underlying layer of 

shotcrete, and the relative location of the failure surface on the test core (Table 6). During the 

pull test, the core sample fails in tension. This tensile failure can occur in the shotcrete, at the 

bond surface (the interface between the shotcrete and an underlying layer of shotcrete or the 

interface between the shotcrete and the rock), in the rock, or at some combination of these 

locations. Typical values for the bond strength of shotcrete applied to different substrates are 

listed in Table 7. 

Table 6. Typical indicators of the quality of a shotcrete application 

obtained through a careful examination of the test core and drill hole

Parameter Typical value 

Matrix thickness 38–51 mm (1.5–2 in) 

Presence of voids (air to solids ratio) yes (%), no 

Presence of lensing (air to solids ratio) yes (%), no 

Presence of reinforcement yes (#), no 

Type of reinforcement Wire mesh, steel fiber, synthetic fiber

Density of reinforcement Sparse, average, extreme

Host rock quality RMR* or Q†

Bond to underlying host rock 0.2–1.5 MPa (29–218 psi) 

Failure surface location In shotcrete (%), at interface (%), in host rock (%)

*RMR = Rock Mass Rating

†

Q = Rock Mass Quality

Table 7. Typical range for shotcrete bond strength with differing substrates* 

*after Brennan [2005]; Nordström and Grändås [2005]; Barrett and McCreath [1995]; Malmgren and 

Svensson [1999]; Kuchta [2003]; Malmgren et al. [2005]; Saiang et al. [2005]; Morton et al. [2008] 

†

Austria, Australia, Canada

‡

nr = not reported

Interface United States,

MPa (psi)

Other Countries,†

MPa (psi)

Sweden,

MPa (psi)

Shotcrete to shotcrete nr‡ nr 1.0 (145) 

Shotcrete to rock nr 0.2–1.5(29–218) 0.5 (73) 

Shotcrete to concrete 0.69–1(100–145) 0.5–3 (73–435) 1.5 (218) 
47 

Test Procedure

The shotcrete bond strength test starts with the selection of a test site. Because this is an insitu test, the location of the test site can be either predetermined or random. Sites could be 

randomly selected for quality control purposes, whereas specific areas of shotcrete fallout could 

be preselected to verify that shotcrete rehabilitation measures had corrected application 

problems. At least three tests should be performed at each general location to develop a statistical 

set of bond strength values. Once the desired location for a test has been determined, a handoperated rotary percussive drill is used to drill a 16-mm x 51-mm (0.625-in x 2-in) hole for 

anchoring the drill stand. The hole is cleared of debris, and a threaded expansion anchor is 

installed, followed by a 13-mm-diameter (0.5-in-diameter) stud onto which the drill stand and 

core drill are mounted in position (Figure 55). Next, an 11.1-mm-diameter (0.4375-in-diameter) 

hole is drilled dry into the shotcrete, using a rotary percussive bit, to a depth of about 60 mm 

(2.375 in), assuming a shotcrete thickness of 75 mm (3 in). The hole is cleaned, filled with a 

quick-setting, two-part epoxy adhesive, and a 9.5-mm (0.375-in) diameter pull anchor is inserted. 

After the epoxy has initially set or gelled (approximately 15 min), a 102-mm-diameter (4-indiameter) diamond core bit is used to wet-drill a second hole through the shotcrete to a depth of 

about 25 to 50 mm (1 to 2 in) into the underlying rock. Finally, a 127-mm-diameter (5-indiameter) diamond core bit is used to wet-drill a shallow kerf for seating the base of the pulling 

fixture (Figure 56).

Photo by NIOSH

Figure 55. Wet drilling with diamond core bit. 

Figure 56 shows the test site after the pull anchor has been epoxied into place, the hole for 

the test core has been drilled into the underlying rock, and a kerf has been drilled for the base of 

the pulling fixture. Figure 57 shows a vertical cross-section schematic of the drill hole 

configuration for the pull anchor, shotcrete test core, and pulling fixture kerf. Using these 

procedures, three parallel, concentric drill holes are produced. This drill hole configuration 

reduces eccentric loading during the direct-tension pull test and ensures that a uniform tensile 

load is applied to the shotcrete test core.
48 

Photo by NIOSH

Figure 56. Drill hole configuration with extension rod connected to epoxied stud. 

Figure 57. Schematic showing a vertical cross-section of the drill hole configuration 

for a shotcrete bond strength test. 

After the the core drilling is completed, the drill stand is removed, and a hand-operated rotary 

percussive drill is again used to drill two additional 16-mm x 51-mm (0.625-in x 2-in) holes. 

These holes are located about 508 mm (20 in) from the epoxied stud and spaced on a 120° 

pattern with the first hole that was original used for mounting the drill stand (Figure 58). Next, 

the holes are cleared of debris, and threaded expansion anchors are installed. Three aluminum 

strap brackets that will be used to anchor the pulling fixture are then bolted to the shotcreted 

surface of the underground entry using 13-mm-diameter (0.5-in-diameter) fasteners (Figure 58).
49 

Photo by NIOSH

Figure 58. Strap brackets centered around the epoxied stud to anchor the

direct-tension test equipment. 

After the epoxy has fully set (30–60 min), a threaded extension rod is connected to the pull 

anchor with a coupling nut, and the pulling fixture is carefully placed over the core sample with 

the base of its reaction ring positioned in the kerf of the outer drill hole. Safety straps are then 

installed through the eyelets on the strap brackets bolted to the housing of the pulling fixture and

the strap brackets mounted on the surface of the mine entry. The safety straps are tensioned to 

secure the pulling fixture in position. The hydraulic hose from the hand pump is then connected 

to the hydraulic cylinder, and the pump and cylinder are cycled several times to purge extraneous 

air from the hydraulic system. Next, a collet and a slip-on, quick-threading locknut are connected 

to the threaded extension rod to serve as a mechanical stop for the pulling fixture’s ram (Figure 

59). Prior to conducting the test, the pressure gauge should be zeroed by pressing the reset 

button. To conduct a direct-tension test, an increasing tensile load is applied to the core sample 

through a slow and steady movement of the pump handle until the core breaks. Test duration 

varies depending on the tensile strength of the test core (typically 30 sec to 2 min). After testing 

only a few samples, it is relatively easy to determine the required pumping rate. This test process 

is repeated with as many core samples as needed until the shotcrete bond strength is 

characterized for the selected test area. An explanation of the procedures and calculations that 

are used to determine the tensile strength of the test core from the maximum hydraulic pressure 

obtained from the pressure gauge are provided in the Appendix. 
50 

Photo by NIOSH

Figure 59. Pulling unit mounted to mine rib using a three-point safety restraint.

Laboratory Testing and Results

Six test panels with nine core test locations were sprayed at the Spokane Research 

Laboratory to provide for tester commissioning and to develop a series of data points for 

comparison to field results. The cores were tested from 4 hrs to 90 days of cure time, and a total 

of 54 samples were tested. 

Results from spray panel tests conducted at the Spokane facility indicate that the shotcrete

average adhesion strength increased markedly from 1 to 3 days of curing, from 0.44 to 1.13 MPa 

(64 to 164 psi) or, in other words, from 28% to 72% of the average 90-day adhesion strength 

(Figure 60).

Figure 60. Typical shotcrete tensile strength test results for 0–90 day cure time (n=54).

The average bond strength test values increased with shotcrete curing age and ranged from 

0.44 MPa (64 psi) after one day of curing to 1.58 MPa (229 psi) after 90 days of curing. The 

range of these results are comparable to previously published values for the adhesion of shotcrete

to concrete test panels [Malmgren and Svensson 1999; Kuchta 2002] and are within the normal 

range of bond strengths specified for shotcrete applied to concrete substrates [Brennan 2005; 

Nordström and Grändås 2005]. 
51 

Observations During the Coring and Tensile Test

As wet core drilling is conducted, a series of operational conditions should be monitored 

continuously throughout the process. These include, but are not limited to, the return water from

the drill, color of the cuttings coming out with the drill water, torque reaction through the drill 

stand, drill pressure necessary for penetration, and core breakage (Figure 55). With regard to 

these conditions, normal operation would consist of a continuous flow of drill water with a 

component of shotcrete cuttings followed by cuttings containing the host rock. The drill torque 

through the drill stand can also provide an indication of the quality of the shotcrete. In competent

shotcrete, the drill will have a constant harmonic vibration and sound and will rotate and drill 

smoothly. Furthermore, the drill pressure required for penetration, as indicated through the stand 

handle, will be constant and steady if no voids or loose shotcrete lenses are present in the 

shotcrete. The driller can feel and see core breakage due to lensing. The drill bit will advance 

rapidly and no cuttings will be present in the drill return water. Another indicator is that the drill 

speed can increase and then stall as it enters and then exits a void. 

An indication of a proper pull test is that a consistent pressure increase on the gauge develops 

with each pump stroke. The core sample breaks in an abrupt manner when the ultimate tensile 

load has been reached. Testing of core samples that have lensing, low adhesion, and voids

produces no buildup on the pressure gauge and little or no resistance on the pump handle. Another 

core failure that provides small load is the case of fiber lensing, where there is high deflection but 

little gauge pressure or resistance on the pump handle as the core sample pulls apart. 

To aid in the interpretation of the bond strength test results, the failure surface on the test 

core should be examined along with the bottom of the drill hole. The failure location should be 

recorded as a percentage of the shotcrete, interface, and substrate that are exposed on the tensile 

failure surface (Figure 61). In addition, the overall depth of the 102-mm-diameter (4-in-diameter) 

drill hole should be noted along with the length of the test core and the thickness of the shotcrete 

layer so that drilling depth for the pull anchor and test core can be adjusted if necessary.

Following the direct tensile test, the broken core is inspected and the key elements of quality

are noted [Neville 2009]. Any lenses or voids or their absence in the core can be observed. Also, 

the presence and location of any reinforcement in the core profile can be determined from the 

extracted core. Furthermore, the thickness of the shotcrete application can be measured, and the 

shotcrete-to-shotcrete application profile can be evaluated. Finally, the host-rock-to-shotcrete 

interface and the structure of the host rock can be observed and noted (Figure 61).
52 

Photo by NIOSH

Figure 61. Shotcrete test core and drill hole from a bond strength test with dry mix shotcrete

applied to a concrete substrate. 

Field Verification of Test Method and Apparatus for Determining the Bond Strength of Applied Shotcrete

Purpose of the Test

The overcoring and direct-tension pull test equipment are used to determine the bond strength

and quality of applied shotcrete. Field trials were conducted with this portable test system in an 

underground mine to verify test procedures, confirm that the test equipment was suitable for use 

in underground mining conditions, and evaluate the system’s ease of use by mine personnel.

Testing Location

The Turquoise Ridge Joint Venture (TRJV) near Goldconda, Nevada provided a test site

along a shotcreted rib of an underground entry on the 1,250 level of the mine. A wet mix 

shotcrete had been previously sprayed on the mine rib in two applications at a design thickness

of 75 to 100 mm (3 to 4 in). 

Test Procedure

As shown in Figure 62, three test sites were selected on fairly even surfaces of the shotcreted 

rib about 1.2 to 1.5 m (4 to 5 ft) above the mine floor. Each drill stand was secured to the mine 

rib using the procedures described previously in the Overcoring and Direct-Tension Pull Test

section of this document. 
53 

Photo by NIOSH Photo by NIOSH Photo by NIOSH

Figure 62. Shotcrete adhesion test site with three drill stands mounted on mine rib. 

Once the drill stand was mounted to the mine rib, the testing protocol was followed as 

described in the previous Test Procedure section and as shown in Figure 63 through Figure 65. 

Figure 63. Dry drilling the installation hole for the epoxied stud pull anchor.

Figure 64. Wet drilling a 102-mm (4-in) diameter hole for the shotcrete adhesion test core 

(note bit change). 
54 

Photo by NIOSH

Figure 65. Preparing to remove drill stand and install direct-tension test equipment.

Trial Results

The results of the three shotcrete bond strength tests conducted at the mine are shown in 

Table 8. The tensile strength of the shotcrete test cores varied from 1.3 to 1.7 MPa (189 to 246 

psi) and are within the typical range of shotcrete bond strengths reported by other researchers 

(Table 7). The location of the tensile failure surface on the test core varied depending on the test 

as noted in Table 8. The shotcrete test cores failed in the shotcrete, at a shotcrete-to-shotcrete

interface, at the shotcrete-to-host-rock interface, and in the host rock. The relative location of 

these tensile failures is given as a percentage of the total area exposed along the failure surface. 

Table 8. Shotcrete adhesion test results

Test

number

Maximum tensile strength,

MPa (psi)

Failure surface location, 

(%)

1 1.7 (246) Shotcrete (70), interface (30) 

2 1.3 (189) Shotcrete (100) 

3 Core broke prematurely in rock Rock (95), interface (5)

Photographs of the drill holes and test cores after the shotcrete bond strength tests were

conducted are shown in Figure 66 through Figure 71. A visual examination of the test cores 

indicated that at least two layers of shotcrete were applied in the test area. As shown by the 

photographs in Figure 70 and Figure 71, the measured overall thickness of the shotcrete layers 

varied from 114 to 127 mm (4.5 to 5 in) over the 5-m (15-ft) lateral spacing of the three test sites. 

Furthermore, there was no indication that fiber or wire mesh were used at this location to 

reinforce the shotcrete. For the first two tests, the cores broke primarily in the shotcrete. The core 

from the first test broke at or near the interface between two shotcrete layers. In the second test, 

the core broke in the top shotcrete layer at a bond strength exceeding 1.0 MPa (145 psi), the 

typical required bond strength for shotcrete applied to shotcrete as noted in Table 7. The third test

core broke prematurely at the beginning of the test, more than likely due to the weak underlying 

host rock (Figure 68 and Figure 69). Although a limited number of shotcrete bond strength tests 

were conducted at this field site, these tests confirm that the test cores will normally break in the 

shotcrete matrix unless the underlying rock is exceptionally weak or highly fractured. 
55 

Photo by NIOSH Photo by NIOSH Photo by NIOSH

Figure 66. Adhesion test #1: core broke in shotcrete and at interface between 

two shotcrete layers. 

Figure 67. Adhesion test #2: core broke entirely in the top layer of shotcrete.

Figure 68. Adhesion test #3: core broke prematurely in weak underlying host rock. 
56 

Photo by NIOSH

Photo by NIOSH Photo by NIOSH

Figure 69. Adhesion test #3: drill hole and core indicating a tensile failure located 

primarily in the host rock. 

Figure 70. Test core #3: tensile failure in weak host rock.

Figure 71. Adhesion test cores showing two breaks in shotcrete (left, test 1 and middle, test 2),

and one break in host rock (right, test 3). 
57 

Round Determinate Panel Flexural Test

Purpose of the Test

The goal of round determinate panel testing (RDPT) is to characterize and define the peak 

flexural load, the post-crack load-carrying capacity at a given displacement, and the toughness of 

a particular shotcrete mix and thus determine if the material can withstand the anticipated mine

opening support loadings. Peak load capacity represents the maximum load the material can 

withstand prior to cracking. Residual load capacity is reported in terms of resistive load at 

subsequent increasing deflections. Toughness is derived from the aggregate area under the load 

capacity versus displacement curve and is indicative of the energy required to displace the 

specimen through a given distance. Both flexural load capacity and toughness represent the 

ability of shotcrete reinforced with welded wire or fiber to carry load following cracking, and 

therefore continue to offer structural support. Flexural load capacity and toughness testing can 

supply specific material properties for the shotcrete that are then comparable to known 

specifications. This testing can be conducted as part of pre-installation trials and as part of an 

ongoing quality monitoring program. 

Flexural load capacity is the peak or breaking load when the shotcrete is subjected to bending 

forces and the force on the shotcrete specimen post-peak is usually up to 40 mm (1.6 in) of 

deflection. The flexural capacity is given in terms of the load at failure or at a given post-peak 

displacement for the specimen. The flexural load capacity can serve as an indirect indication of 

the strength of a member when subjected to bending. The residual flexural load capacity is 

determined as a post-crack load resistance at a specified deflection. 

Toughness is a measure of the amount of energy that the shotcrete can absorb for a given 

displacement and still retain some load capacity. It is an important property where post-crack 

displacement and deformation are expected. Toughness and absorbed energy can be assessed 

from load and displacement that are measured while conducting a round determinate panel test. 

The total absorbed energy is affected primarily by adding reinforcement (welded wire mesh or 

fiber). In the case of welded wire mesh, the wire spacing and gauge which correlates to the wire 

cross-sectional area will affect the toughness. In the case of fiber, the toughness changes by the 

type and density of fibers. Toughness can also be strongly influenced by the strength and quality 

of the shotcrete matrix [Papworth 2002]. 

The flexural load capacity and toughness of shotcrete that is encapsulated with welded wire 

mesh or fiber-reinforced shotcrete can be determined using an adaptation of the ASTM C-1550 

[2010] test employing sprayed 75-mm-thick x 800-mm-diameter (3-in-thick x 31.5-in-diameter) 

round panel samples. 

As determined by the ASTM C1550 standard [2010], the test panel that sits on three pivots 

symmetrically arranged around its circumference is subjected to direct loading at its center by a 

spherically shaped ram. During the test, the load and controlled displacement of the center of the 

test specimen is recorded. The toughness or energy of the panel (measured in Joules) is derived 

from the aggregate area under the load versus displacement curve. The energy is normally

designated at a specified central deflection, usually 40 mm (1.6 in). The energy absorbed

represents the ability of welded-wire-reinforced or fiber-reinforced shotcrete to redistribute stress 

following cracking, and therefore continue to offer structural support. 
58 

Mechanical Principle

As illustrated in Figure 72, the test sample is loaded in direct compression by the upper 

(force) platen and the lower three (reactant) pins of the test fixture. Flexural load capacity is 

determined based on sample first-break or post-first-break resistance (support capacity) at a 

given displacement. 

Figure 72. Diagram of the force applied to a round-panel shotcrete sample during an RDP test. 

Performance Standard

Specified minimum values for load capacity are listed in Table 9 [AuSS 2008]. Typical 

specified minimum values for North American projects using the panel test for flexural load 

capacity first-break are 20 to 35 kN (4496.2 to 7,868.3 lbf) with residual load capacity of 6 kN 

(1,348.9 lbf) at 40-mm (1.6-in) displacement. Specified minimum values for toughness in mining 

and civil tunneling applications are listed in Table 10 and Table 11 [Morgan 1999; Papworth 

2002; Pakalnis 2008]. A sample must break with a minimum toughness of 280 Joules as 

recommended by the New Austrian Tunneling Method (NATM) to be considered applicable for 

weak rock mass support [Barton and Morgan 1999]. The peak strength range also corresponds to 

7%–15% of the compressive strength values obtained from traditional testing of cored samples 

[AuSS 2008; Bernard 2006]. 
59 

Table 9. Typical minimum load capacity values specified for shotcrete in recent mining 

and civil engineering projects in North America for given ground support conditions and 

RDPT ram displacements, after AuSS [2008] 

a. General ground support

RDPT ram displacement, 

mm (in)

Shotcrete load capacity, 

kN (lbf) 

First break 20–25 (4,496.2–5,620.2) 

10 (0.4) 11 (2,472.9) 

20 (0.8) 9 (2,023.3) 

30 (1.2) 7.5 (1,686.1) 

40 (1.6) 6 (1,348.9) 

b. Low-level ground support (nonstructural or low deformation)

RDPT ram displacement, 

mm (in)

Shotcrete load capacity, 

kN (lbf) 

First break 20–25 (4,496.2–5,620.2) 

10 (0.4) 7.5 (1,686.1) 

20 (0.8) 6 (1,348.9) 

30 (1.2) 4 (899.2) 

40 (1.6) 3 (674.4) 

c. Moderate-level ground support (moderate deformation)

RDPT ram displacement, 

mm (in)

Shotcrete load capacity, 

kN (lbf) 

First break 25–30 (4,496.2–6,744.3) 

10 (0.4) 17 (3,821.8) 

20 (0.8) 12 (2,697.7) 

30 (1.2) 9 (2,023.3) 

40 (1.6) 6 (1,348.9) 
60 

d. High-level ground support (high deformation requiring wire mesh)

RDPT ram displacement, 

mm (in)

Shotcrete load capacity, 

kN (lbf) 

First break 35 (7,868.3) 

10 (0.4) 15 (3,372.1) 

20 (0.8) 17 (3,821.8) 

30 (1.2) 10 (2,248.1) 

40 (1.6) 8 (1,798.5) 

Table 10. Typical values specified for shotcrete toughness in recent mining and civil 

engineering projects in North America for given ground support conditions* 

Level of ground support Specified toughness (Joules)†

Low 280 

Moderate 320 

High 450 

*Excerpts from modified Barton chart by Morgan [1999], Papworth [2002], 

and Pakalnis [2008]

†

RDPT energy determined at 40 mm (1.6 in) of deflection using ASTM C1550 [2010] 

Table 11. Comparison of shotcrete toughness performance level to Q-system rock class, 

RDPT energy, ground support reinforcement category, and RMR for recent mining 

projects in North America* 

*Excerpts and modified Barton chart from Morgan [1999], Papworth [2002], 

and Pakalnis [2008]

†

TPL = Toughness Performance Level

‡

RDPT energy determined at 40 mm (1.6 in) of deflection using ASTM C1550

TPL† Rock

class

Energy,

Joules‡

Ground

support RMR

IV F 560 8+ 0–10 

IV E 400 7 10–25 

III D 280 6 25–45 

II C 200 5 45–55 

I B 200 5 45–55 

0 A 0 4 55–80 
61 

Description of Apparatus

Round panels must be tested in a suitable servo-controlled machine in order to comply with 

the test standard listed in ASTM C1550 [2010]. NIOSH researchers designed a portable round 

panel test machine for use onsite at mines (Figure 73 and Figure 74). The unit and apparatus 

applies a compressive load at a fixed displacement rate. The device is comprised of a portable, 

self-contained, servo-controlled, stiff-frame press with advanced load-rate and load-collection 

capability. There is a load output display with an appropriate scale and resolution, along with a 

current state of energy output. The machine resolution is 0.45 kg (1 lb) over a 10,886 kg (24,000 

lb) operating range. A programmable-logic-controller (PLC) driven machine head applies a 

displacement rate of 4.0 mm/min (0.16 in/min) with an auto return after 45 mm (1.8 in) of 

displacement [ASTM 1550 (2010)]. The automated load cycle greatly reduces the chance of 

human error and the time to conduct a standard test.

Figure 73. Schematic of a portable round determinate panel (RDP) test machine developed by 

NIOSH researchers.

Photo by NIOSH

Figure 74. Operation of portable round determinate panel (RDP) test machine.
62 

Test Procedure

The ASTM 1550 [2010] testing protocol calls for sample preparation and testing of shotcrete 

mixes to be sprayed into 75 x 800-mm (3 x 31.5-in) mold rings to create round-panel test 

specimens (Figure 75). The samples must be carefully prepared to meet the required 

specifications concerning flatness, thickness, and diameter. For example, the top surface of the 

sprayed panel must be screeded to a flatness tolerance of at least 5 mm (0.2 in). The sample must 

also have a thickness of 70 to 90 mm (2.75 to 3.5 in) and a diameter of 750 to 850 mm (29.5 to 

33.5 in). Correction factors are available when variances in the thickness and diameter occur. 

The round-panel mold rings should be secured to a stable base of sufficient thickness to prevent 

vibration, and the overall form (mold ring and base) should be oriented at a 45° angle to reduce 

the amount of rebound that will build up in the mold (Figure 76). Measures should be taken to 

achieve a dense and homogeneous shotcrete without segregation, sloughing, collapsing, 

excessive rebound, or other visible imperfections. 

Based on ASTM C1550 [2010], a minimum of two round panel samples should be tested. 

However, at least three round panel samples should be sprayed in case one is damaged prior to 

testing. RDP tests are normally conducted after the round panel samples have cured for 7, 14, or 

28 days. After the tests are conducted, the round panel samples should be inspected to ensure that 

the tolerances for specimen thickness have been achieved and that the exposed cross section of 

the sample is free of defects such as voids, lenses, and poorly consolidated regions. 

If a comparative compressive strength of the fiber-reinforced shotcrete is required, a 1.2 x 1.2 

x 0.24-m (4 x 4 x 0.8-ft) panel-shaped sample can be sprayed and 76.2-mm (3-in) diameter x 

152.4-mm (6-in) long cores extracted in accordance with ASTM C1140 [2003] and tested in 

accordance with ASTM C1609 [2010]. 

Photo by NIOSH

Figure 75. Mold ring used to prepare a round-panel shotcrete sample. 
63 

Photo by NIOSH Photo by NIOSH

Figure 76. Sprayed round-panel shotcrete samples.

After the round panel samples have been sprayed, the specimens are allowed to cure. The 

shotcrete test panels are then carefully demolded by disassembling the mold ring fixtures and

removing the enclosed samples. After curing for a designated period of time at ASTM 

specifications, the sample is positioned on a three-pin test fixture and centered under the loading 

head of the RDP test machine. The loading head is then lowered to a position just above the 

upper surface of the round panel sample using a manual toggle switch (Figure 77).

Figure 77. Round-panel shotcrete sample positioned in portable RDP test machine.

To conduct a test, the green start switch is depressed initiating the automated test cycle, 

whereupon a PLC-controlled press loads the sample at a fixed displacement rate. The load 

profile is shown on a graphical output display, and the measured test parameters (time, 

displacement, and load) are stored on a thumb drive. The sample is tested to failure as indicated 

by the appearance and development of three distinct tension cracks in the sample. The RDP test 

continues until the loading ram has moved a minimum of 40 mm (1.6 in) from its initial starting 

position (Figure 78). These tension cracks usually develop at less than 5 mm (0.2 in) of ram 

displacement. The operator will typically observe a well-defined peak in the load profile curve, 

indicating the first break load capacity of the sample. 
64 

After the 40-mm (1.6-in) test cycle is completed, the loading ram automatically retracts to

unload the broken sample. Next, the round panel sample is removed from the RDP test machine, 

and it is broken apart to expose the failure surfaces along the tensile cracks. The thickness of the 

panel sections are then measured at ten locations. The failure surfaces exposed on the panel 

sections are closely examined to determine the distribution and quantity of the fibers in the 

applied shotcrete, and a fiber count is conducted along each broken section of the panel and 

recorded (Figure 79). Using this procedure, RDP tests are conducted with three round panel 

samples, ensuring that the results from at least two of the tests can be used for analysis. Photo by NIOSH Photo by NIOSH

Figure 78. Broken round-panel test sample showing tension cracks.

Figure 79. Synthetic macro fibers exposed along the failure surface of 

round-panel shotcrete sample.
65 

Observations, Anomalies, and Influencing Factors During RDP Tests

The RDP test produces a determinate set of three tensile cracks along hinge lines spaced 

about 120° apart as shown in Figure 80. As the test progesses and the deflection in the panel 

increases, these tensile cracks open on the underside of the panel, exposing the fibers in the 

shotcrete which bridge the crack. Note the nonuniform surface in the lower left section of the 

shotcrete panel in Figure 80. The presence of voids in the sample usually indicates that a nozzle 

operation problem has occurred during the spraying of the shotcrete sample. Any shotcrete 

application problem needs to be corrected to ensure that valid test results are obtained from the 

round panel samples. It is important that representative test samples are prepared in order for the 

RDP test results to realistically reflect the in-place strength properties of the bulk shotcrete 

product. As an example, a drawing of a typical roof bolt pattern for an underground opening is 

shown in Figure 81 along with a superimposed RDP test panel illustrating the flexural loading of 

the shotcrete that is assumed to occur between the bolts.

Photo by NIOSH

Figure 80. Broken round panel sample of fiber-reinforced shotcrete showing the 120° cracks.

Figure 81. Typical roof bolt pattern with a superimposed RDP test panel showing the assumed 

flexural loading of the shotcrete between the bolts.
66 

Fiber Count and Distribution

The fiber count or fiber dosing of the reinforced shotcrete can be determined by counting the 

fibers in a cross section of the post-tested panel. This method is used only when determining 

fiber content for steel and macro-synthetic fibers. The cross section of the panel should be 

inspected for even dispersion of the fibers. 

Analysis of Round Determinate Panel (RDP) Test Results

After conducting an RDP test, a load versus displacement profile is obtained from the test as 

shown in Figure 82. 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. These are separate from the 

displacement at peak flexural load.

Figure 82. Load versus displacement curve from RDP test. 

After the peak flexural load has been developed, the panel undergoes a first break or an 

initial failure of the cemented material. Once this first break occurs, the cementitious matrix is no 

longer the primary load-carrying component of the shotcrete. Instead, the cross-sectional area of 

the reinforcement component in the shotcrete (e.g., steel fiber, synthetic macro fiber, or wire 

mesh) begins to carry the applied load. Although the first break usually occurs immediately after 

the peak flexural load, the timing of this event is somewhat variable as indicated by the first 

break bandwidth shown in Figure 83. 
67 

Figure 83. Load versus displacement curve from RDP test indicating peak load and first break. 

The area under the load-displacement curve from 0 to 40 mm (0 to 1.6 in) of displacement is 

referred to as the energy absorption or toughness of the shotcrete. This shotcrete property is 

indicated by the green shaded area beneath the load-displacement curve shown in Figure 84. 

Figure 84. Load versus displacement curve from RDP test indicating peak load, residual load, and 

the energy or toughness of the shotcrete. 
68 

With displacement, the energy capacity or toughness increases as the residual load decreases

through 40 mm (1.6 in) of displacement (Figure 85).

Figure 85. Energy versus displacement curve from RDP test.

Field Verification of Test Method and Apparatus for Determining the Flexural Strength of Shotcrete Using

the Round Determinate Panel Test (RDPT) Method

Background

The RDP test machine commissioning trials consisted of flexural strength testing of onsite, 

as-sprayed shotcrete samples over a range of 8-hrs to 28-days curing time. All collection of 

samples and testing of shotcrete material properties were conducted in accordance with the 

requirements of the ASTM C1550 test [2010]. The usefulness of this field-ready test machine 

had been previously documented during a shotcrete fiber dosage comparison study conducted at 

the Chief Joseph Mine in Butte, Montana, [Martin et al. 2007] and Devil’s Slide Tunnels Project

near Pacifica, California [Decker et al. 2010]. 

Specific Trials and Locations

The RDP test machine commissioning trials consisted of a series of field tests that were 

conducted at multiple mine sites, one tunnel site, and one shotcrete producer’s site. A first set of 

tests was conducted at the Rockwell Mine in Greybull, Wyoming to determine the effect of batch 

source water on peak load using either mine or well-water sources, and additional tests were 

conducted to determine the effect of welded wire mesh on flexural load capacity. A second set of 

tests was conducted on a mix intended for use at the Kensington Mine near Juneau, Alaska, and 

the Pogo Mine located near Delta Junction, Alaska. These tests were conducted at Central PreMix, Spokane Valley, Washington to determine if a target shotcrete design criteria, an optimum 

design energy for safe ground support, could be achieved in 24 hrs through the manipulation of 

the cement dosage rate. A third set of tests was conducted on civil engineering shotcrete mix 

blends intended for use in extreme ground conditions. Engineers for the Kiewit Corporation, 

working under contract for the California Department of Transportation at the Devil’s Slide 

Tunnels Project, contacted NIOSH personnel about using the newly developed, portable RDPT 
69 

machine to validate mix designs for shotcrete reinforced with synthetic fiber. RDP tests were

conducted at the Devil’s Slide Tunnels site near Pacifica, California to determine the effect of 

synthetic fiber dosage on total energy absorption. A final set of tests was conducted at Newmont 

Mining Corporation’s Leeville Mine near Carlin, Nevada to determine the effect of different 

reinforcement, welded wire mesh and fiber (both type and dosage), on the flexural load capacity 

for weak rock mass shotcrete. 

Rockwell Mine, Greybull, Wyoming

The goal of this set of tests was to identify if the batching source water, either well water or 

mine waste water, significantly affects the shotcrete load capacity. The secondary goal of this set 

of tests was to determine the flexural load capacity for shotcrete reinforced with welded wire 

mesh using either type of mix water.

The mix designs were blended by Thiessen Team USA and consisted of a typical dry mix 

design superstick with water additive (SCA) and superstick water accelerant with corrosion 

resistance (SCACR). The specific mixes tested were SCACR/well water and SCACR/well water

with wire mesh, and SCACR/mine water mix. These mixes were blended at the Big Timber, 

Montana, shotcrete plant and shipped to the Greybull, Wyoming, mine site for testing.

The research team from NIOSH prepared 15 panels at the mine facility with five panels for 

each of the three mix designs. All of the previously mentioned panels were tested for 8-hr 

strengths. In addition, 12 test panels were shot for 28-day strength testing (Figure 86). These 

round panels were stored underground at the mine. 

Photo by NIOSH

Figure 86. Spraying round-panel shotcrete samples reinforced with wire mesh.
70 

The shotcrete round panels were mounted on pallets for ease of handling, then set at 40°–45° 

to the horizontal against blocking outside the mine. The panels were washed and degreased, then 

the shotcrete was shot into the panels by a certified nozzle operator using an Aliva 242.5 machine 

(Figure 87). The water was added using a special wetting nozzle at the clamp intersection instead 

of the nozzle hose intersection. Compressed air was supplied from an 18.4-m3/m (650-cfm) twostage compressor. The water was distributed using a water pump on a water truck. The shot panels 

varied up to 3 mm (0.12 in) in thickness. This is typical for stiff shotcrete mix with a 127-mm (5-

in) slump. After each mixture was shot, the pot and hose were cleaned.

Temperature has an effect on the curing of the shotcrete due to the additives used; this in turn 

has a direct impact on the strength of the shotcrete over the curing time. Shotcrete with additives 

are thermally dependent for their manufacturing listed cure times. Therefore, concrete insulation 

blankets were applied at the completion of shooting to keep the panels from freezing, and were 

left on the panels for 8 hrs. 

The curing of the panels was performed at in-mine conditions located in a laydown area. The 

shotcrete was applied at an ambient air temperature of between 0°C (32°F) and 7.2°C (45°F), 

and a round panel form temperature of 26.6°C (80oF). The final shotcrete temperature as applied 

with water and compressed air to the forms was measured at 21.1oC (70oF). Photo by NIOSH

Figure 87. Spraying round panel forms with a dry mix shotcrete.

Trial Results

Mine staff were interested in duplicating the flexural load capacity documented through 

previous NIOSH tests with polyfiber shotcrete [Martin et al. 2007]. Figure 88 and Figure 89 

show the results of the 8-hr round panel tests with the two types of water. The shotcrete using 

mine water developed a higher peak load than the well-water shotcrete.
71 

Figure 88. Comparison of load versus displacement curves for SCACR shotcrete mixed 

with well water and cured for 8 hours. 

Figure 89. Comparison of load versus displacement curves for SCACR shotcrete mixed 

with mine water and cured for 8 hours. 

Mine ground control staff determined that SCACR batched with well-water shotcrete panels 

containing the welded wire mesh would be tested due to the abundance and ease of acquiring the 

well water. The other panels with mine water were not tested. The results of the 28-day round 

panel tests using this mix design and welded wire mesh reinforcement are shown in Figure 90. 
72 

Figure 90. Comparison of load versus displacement curves for SCACR shotcrete mixed 

with well water, reinforced with wire mesh, and cured for 28 days. 

The panels in this test series have a somewhat lower first-break load and an erratic loading 

pattern. This low load could be due to the cold temperatures in which the panels were shot and 

cured. The usual deflection of 40 mm (1.6 in) as run by the industry is to allow for the polyfibers 

to develop an energy requirement before fiber pullout. However, the mine ground control staff 

determined that 25.4 mm (1.0 in) of deflection instead of the typical 40 mm (1.6 in) would be 

their allowable deflection with wire mesh before rehabilitation of the ground support would 

occur. Therefore, the tests were carried out to only 35 mm (1.4 in) to show loading trends beyond 

the mine’s design requirements. The average peak load is 13.4 kN (3,012.4 lbf), and the average 

residual is around 8.9 kN (2,000.8 lbf) at 35 mm (1.4 inches) of deflection. The combination of 

SCACR shotcrete mix, well water, and welded wire mesh reinforcement can achieve the load of 

6 kN (1,348.9 lbf) at 35 mm (1.4 inches) of deflection. This support value meets typical load 

capacity specifications for weak rock mass ground support. 

Pogo and Kensington Mines

Purpose of the Tests

The goal of this set of tests was to determine the effect of cement dosage on flexural load 

capacity. Two wet mixes specifically designed for use in the Pogo and Kensington mines in 

Alaska were compared for initial flexural strength. The wet mixes were batched using 295 kg

(650 lb) of Portland cement per ton for Pogo and 318 kg (700 lb) for Kensington. Due to the 

nature of the mix and the competitive bid market, the mix design is not published, but is

available from Central Pre-Mix, Spokane Valley, Washington. 

Sample Preparation and Testing

The test panels were shot by a certified nozzle person (Figure 91) at the Central Pre-Mix site 

with a swing-type positive displacement pump (Figure 92). Five samples of each blend were 

sprayed into molds and then blanketed to retain heat to simulate the in-mine conditions typical of

Alaskan applications for the 24-hour curing process. 
73 

Photo by NIOSH Photo by NIOSH

Figure 91. Spraying round panel forms with wet mix shotcrete. 

Figure 92. Swing-type, positive-displacement pump used to apply wet mix shotcrete. 

The testing was conducted to determine peak load and residual load capability at 80 mm (3

in) of displacement rather than the standard displacement of 40 mm (1.6 in). Toughness 

calculations were not considered due to the freshness of the Portland cement in the 24-hr tests.

Trial Results

The results of the 24-hr tests show that the 318-kg (700-lb) mix with an average peak load of 

8.9 kN (2,000.8 lbf) was significantly stronger than the 295-kg (650-lb) mix with an average 

peak load of 1.8 kN (404.7 lbf) (see Figure 93 and Figure 94). This strength differential was also 

reflected in the residual strength characteristics. Further, the 295-kg (650-lb) mix samples broke 

under their own weight after testing. The 318-kg (700-lb) mix samples had to be broken with a 

hammer to be removed from the test frame.
74 

Figure 93. Comparison of load versus displacement curves for 295-kg (650-lb) mix 

with 24-hour cure time. 

Figure 94. Comparison of load versus displacement curves for 318-kg (700-lb) mix 

with 24-hour cure time. 

Devil’s Slide Tunnels Project, Pacifica, California

Purpose of the Tests

The goal of this set of tests was to determine the effect of synthetic fiber dosage on total

energy absorption. Tests were conducted on civil engineering shotcrete mix blends that were 

developed from the mining industry’s Shogun® fiber mix intended for use in extreme ground 

conditions [Martin et al. 2011]. Testing was conducted in cooperation with Kiewit engineering 

staff at the Devil’s Slide Tunnels Project near Pacifica, California. 
75 

Sample Preparation and Testing

The panels for the Devil’s Slide Tunnels Project were shot onsite by a certified nozzle 

operator using a swing-style, positive-displacement pump and a wet-mix shotcrete reinforced 

with Barchip Shogun fiber. The shotcrete was mixed in an onsite batch plant using the blend of 

constituents listed in Table 12. This polyfiber shotcrete mix design was developed to address the 

weak ground conditions at the Devil’s Slide Tunnels site. A synthetic fiber dosage of 5 kg/m3

(8.5 lb/yd3) was used to achieve a recommended shotcrete toughness of 320 Joules at 40-mm 

(1.6-in) of displacement as required for class III and higher ground conditions by the Norwegian 

Concrete Association [NCA 2007] and the New Austrian Tunneling Method [NATM 1962]. The 

Kiewit tests were conducted in compliance with ASTM C1550 [2010]. The samples were cured 

for 7 days under ASTM-recommended conditions before being tested. A total of three roundpanel samples were prepared. The first two tests exceeded the shotcrete toughness requirements; 

therefore, the third panel was not tested. 

Table 12. Proportions of various mix constituents for round-panel shotcrete samples at 

the Devil’s Slide Tunnels Project

Mix constituent Amount,

kg/m3 (lb/yd3

)* Product, supplier, or comment

Synthetic macro fiber 5 (8.5) Barchip Shogun 

10-mm aggregate 478 (806) Vulcan

Quarry sand 1,235 (2,082) Sechelt

Cement 446 (752) Hanson type II-V 

Water 178 (300) na†

Water-to-cement ratio na 0.40 

Water reducer 2.9 L (3.2 qt) BASF Rheobuild 1000 

Set-retarding admixture 0.58 L (0.6 qt) BASF Delvo Stabilizer

Slump of mixes na 38 mm (1.5 in) 

*Unless listed otherwise as liters and quarts

†

na = not applicable

Trial Results

The typical results of the tested panels are shown in Figure 95. The Shogun synthetic-fiberdosed shotcrete formulated for use in extreme ground conditions achieved a peak load of 25 kN 

(5,620.2 lbf) which has an energy capacity of 400 Joules at 40-mm (1.6-in) displacement (Figure 

95). This exceeds the project design requirements of 320 Joules of toughness at only 7-days 

curing time. For comparison, it should also be noted that the slump of the mix was at a relatively 

stiff mix at 38 mm (1.5 in), as compared to a typical 127 to 178-mm (5 to 7-in) slump. 
76 

Figure 95. Load versus displacement and energy versus displacement curves for 

5-kg/m3 (8.5-lb/yd3

) Shogun fiber mix with 7-day cure time. 

Leeville Mine, Carlin, Nevada

Purpose of the Tests

The goal of this set of tests was to determine the effect of different reinforcement types,

including welded wire mesh and fiber (both type and dosage), on flexural load capacity for weak

rock mass shotcrete. The testing was conducted at Newmont Corporation’s Leeville Mine near

Carlin, Nevada.

Sample Preparation and Testing

The mine provided a wet mix shotcrete that was designed by Modern Concrete in Carlin, 

Nevada to conform with the American Concrete Institute (ACI) specifications for shotcrete 

blends [ACI 506R-05]. The mix was reinforced with either welded wire mesh or fiber in 

accordance with ACI 506.1R-98 [1998]. Round-panel forms were sprayed on the surface of the 

mine site using a wet-mix shotcrete machine equipped with a robotic manipulator-arm (Figure 

96). Samples of the welded wire mesh were cut to fit into the round panel forms, and the legs of 

the mesh wires were bent down to position the mesh in the middle of the panel, similar to its use 

in underground mines. The samples were collected and cured under wet burlap in a sample 

storage room at saturated conditions in accordance with ASTM standards. 
77 

Photo by NIOSH

Figure 96. Shooting wet mix shotcrete into a round panel form with wire mesh reinforcement.

Shotcrete Constituents

The blends that were used in the panel tests are shown in Table 13. Two round panel samples 

were prepared and tested for all Shogun fiber mix designs, including 1.87 kg/m3 (3.15 lb/yd3),

2.99 kg/m3 (5.0 lb/yd3), 4.12 kg/m3 (7.0 lb/yd3), six samples, plus the welded wire mesh and the 

Performax® fiber mix of 2 samples each. In some cases, additive mixtures were also incorporated

and are documented in the tables and graphs. 

The samples were tested in conformance with the requirements of ASTM C1550 [2010] for 

toughness over 40 mm (1.6 in) of displacement. Testing was initially proposed on samples that 

had cured for 8 and 24 hours. However, the 8-hr and 24-hr samples were not strong enough to be 

tested. As a result, only the 28-day tests were conducted. 
78 

Table 13. Proportions of various mix constitutents for round-panel samples of reinforced 

shotcrete at the Leeville Mine

a. 1.87 kg/m3 (3.15 lb/yd3) of Shogun fiber

Mix constituent Amount,

kg/m3 (lb/yd3

)* Product, supplier, or comment

Synthetic macro fiber 1.87 (3.15) Barchip Shogun 

10-mm aggregate 146 (247) Modern Concrete

Quarry sand 831 (1,402) Modern Concrete

Cement 236 (398) Ashgrove Corporation 

Water 143.3 (242) na†

Water-to-cement ratio na 0.54 

Water reducer 1.5 L (1.6 qt) BASF Glenium 3030 

Accelerator 11.4 L (12.0 qt) BASF MEYCO SA 430 

Slump of mixes na 171 mm (6.75 in) 

*Unless listed otherwise as liters and quarts

†

na = not applicable

b. 2.99 kg/m3 (5.0 lb/yd3

) of Shogun fiber

Mix constituent Amount,

kg/m3 (lb/yd3)* Product, supplier, or comment

Synthetic macro fiber 2.99 (5.0) Barchip Shogun 

10-mm aggregate 146.75 (247.4) Modern Concrete

Quarry sand 831.57 (1,402) Modern Concrete 

Cement 263.13 (444) Ashgrove Corporation 

Water 135.5 (228.4) na†

Water-to-cement ratio na 0.52 

Water reducer 1.5 L (1.6 qt) BASF Glenium 3030 

Accelerator 11.4 L (12.0 qt) BASF MEYCO SA 430 

Slump of mixes na 165 mm (6.5 in) 

*Unless listed otherwise as liters and quarts

†

na = not applicable
79 

c. 4.12 kg/m3 (7.0 lb/yd3) of Shogun fiber

Mix constituent Amount,

kg/m3 (lb/yd3)* Product, supplier, or comment

Synthetic macro fiber 4.12 (7.0) Barchip Shogun 

10-mm aggregate 146.75 (247.4) Modern Concrete 

Quarry sand 831.57 (1,402) Modern Concrete 

Cement 263.13 (444) Ashgrove Corporation 

Water 139.9 (235.8) na†

Water-to-cement ratio na 0.53 

Water reducer 1.5 L (1.6 qt) BASF Glenium 3030 

Accelerator 11.4 L (12.0 qt) BASF MEYCO SA 430 

Slump of mixes na 140 mm (5.5 in) 

*

†

Unless listed otherwise as liters and quarts

na = not applicable

d. Six-gauge wire mesh

Mix constituent Amount,

kg/m3 (lb/yd3

)* Product, supplier, or comment

10-mm aggregate 141.31 (698.5) Modern Concrete

Quarry sand 800.75 (1,349.8) Modern Concrete

Cement 280.67 (473.2) Ashgrove Corporation 

Water 139.6 (235.4) na†

Water-to-cement ratio na 0.50 

Accelerator 11.4 L (12.0 qt) BASF MEYCO SA 430 

Slump of mixes na 127 mm (5 in) 

*Unless listed otherwise as liters and quarts

†

na = not applicable
80 

e. Six-gauge wire mesh plus water-reducing admixture

Mix constituent Amount,

kg/m3 (lb/yd3)* Product, supplier, or comment

10-mm aggregate 146.75 (247.4) Modern Concrete

Quarry sand 831.57 (1,402) Modern Concrete

Cement 263.13 (443.6) Ashgrove Corporation 

Water 151.6 (255.6) na†

Water-to-cement ratio na 0.58 

Water reducer 1.5 L (1.6 qt) BASF Glenium 3030 

Accelerator 11.4 L (12.0 qt) BASF MEYCO SA 430 

Slump of mixes na 171 mm (6.75 in)

*Unless listed otherwise as liters and quarts

†

na = not applicable

f. 1.87 kg/m3 (3.15 lb/yd3

) of Performax fiber

Mix constituent Amount,

kg/m3 (lb/yd3)* Product, supplier, or comment

Synthetic macro fiber 1.87 (3.15) Performax

10-mm aggregate 146.75 (2,474) Modern Concrete 

Quarry sand 831.57 (1,402) Modern Concrete 

Cement 263.13 (443.6) Ashgrove Corporation 

Water 148.5 (250.4) na†

Water-to-cement ratio na 0.56 

Water reducer 1.5 L (1.6 qt) BASF Glenium 3030 

Accelerator 11.4 L (12.0 qt) BASF MEYCO SA 430 

Slump of mixes na 159 mm (6.25 in) 

*Unless listed otherwise as liters and quarts

†

na = not applicable

Trial Results

The results of both round panels tested for each of the given mix designs are included in the 

following set of figures. Figure 97 through Figure 99 are the load-displacement and energy 

curves for the mix designs containing the Shogun fiber at dosage rates of 1.87 kg/m3 (3.15 

lb/yd3), 2.99 kg/m3 (5.0 lb/yd3), and 4.12 kg/m3 (7.0 lb/yd3). 
81 

Figure 97. Comparison of load versus displacement and energy versus displacement curves for

1.87-kg/m3 (3.15-lb/yd3

) Shogun fiber mix with 28-day cure time. 

In Figure 97 the load and energy curves for the Shogun fiber dosage at 1.87-kg/m3 (3.15-

lb/yd3) mix indicate an average first break load of 17 kN (3,821.8 lbf), though there is some 

variability between the strengths. Both samples maintained some residual load throughout the 40 

mm (1.6 in) of displacement. The toughness, however, never reached the 280 Joules 

recommended by NATM for weak rock mass support. When comparing the results of the two 

samples, a small difference in the residual load can result in a large difference in the energy at 40 

mm (1.6 in). It should also be noted that this mix had a slump of 171.4 mm (6.75 in).

Figure 98. Comparison of load versus displacement and energy versus displacement curves for

2.99-kg/m3 (5.0-lb/yd3

) Shogun fiber mix with 28-day cure time. 
82 

For the 2.99-kg/m3 (5.0-lb/yd3) Shogun mix, the peak load and energy was superior to the 

1.87-kg/m3 (3.15-lb/yd3) mix at 28 days. The first break load averaged about 20 kN (4,496.2 lbf), 

which is just slightly higher than the lower fiber dose samples. However, the residual loads 

through 40 mm (1.6 in) of displacement are much higher, thus resulting in an average of 280 

Joules of energy. This toughness meets the recommended values for weak rock mass ground 

control. Also, a load greater than 6 kN (1,348.9 lbf) was achieved at 25 mm (1 in) of 

displacement, which corresponds to design criteria for weak rock mass suggested by Pakalnis 

[2010]. The slump at 165.1 mm (6.5 in) was stiffer by 6.3 mm (0.25 in) than the lower dose mix.

Figure 99. Comparison of load versus displacement and energy versus displacement curves for 

4.1-kg/m3 (7.0-lb/yd3

) Shogun fiber mix with 28-day cure time. 

The results of tests at 28 days for shotcrete with a fiber dosage of 4.1-kg/m3 (7.0-lb/yd3) 

Shogun fiber mix indicate that this mix and dosage formulation exceeded the toughness 

requirements of 280 Joules energy recommended for weak rock mass support [Pakalnis 2010]. 

The first-break strength was 20 kN (4,496.2 lbf), with both samples having similar load profiles. 

Additionally, at 40 mm (1.6 in) of displacement the residual load is 6 kN (1,348.9 lbf). There 

was only a small gain in the peak load, residual load, and energy at 40 mm (1.6 in) over that of 

the 2.99-kg/m3 (5.0-lb/yd3

) fiber dose, which suggests that there is a limit to the effect of the 

fiber dosage. It should also be noted that this mix had a slump of 139.7 mm (5.5 in), which was 

the stiffest of the three mixes. 
83 

Figure 100. Comparison of load versus displacement and energy versus displacement curves for 

shotcrete reinforced with six-gauge wire mesh and cured for 28 days. 

In Figure 100 the load-displacement and energy curves are shown for the two panels with the 

welded wire mesh with no additives. The measured flexural load capacity for tests conducted 

with welded wire mesh is of particular interest to underground mines because this supermesh 

support system is currently being used in weak rock conditions. The shape of the loaddisplacement and energy curves after the peak load is significantly different than the curves 

previously shown for the fiber-reinforced panels. Both welded wire mesh panels without the 

additive have similar load-displacement curves. Further, the developed average toughness at 450 

Joules far exceeds the 280 Joules recommended for weak rock mass support [Pakalnis 2010]. 

The panels were capable of holding 6 kN (1,348.9 lbf) at 40 mm (1.6 in) for a mix with a slump 

of 127 mm (5 in).

Figure 101. Comparison of load versus displacement and energy versus displacement curves for 

shotcrete with a water-reducing admixture that is reinforced with six-gauge wire mesh 

and cured for 28 days. 
84 

As shown by the load-displacement and energy curves in Figure 101, 700 Joules of 

toughness was measured for the two panels with welded wire mesh reinforcement and additives. 

This toughness level exceeds the 280 Joules of energy recommended by Pakalnis [2010] for 

ground support in weak rock mass mines. There was an increase in the total energy of almost 200 

Joules when compared to the wire mesh panels without the admixture I accelerant. Additionally, 

the residual load at 40 mm (1.6 in) exceeded the 6 kN (1,348.9 lbf) as required for dead weight 

loads in weak rock mass mines [Martin et al. 2010]. This mix design had a slump of 171.5 mm 

(6.75 in) as compared to 127 mm (5 in) for the welded wire mesh panels without the admixtures, 

thus showing the effects of admixtures. 

Furthermore, Figure 102 shows the load-displacement and energy curves for the two panels 

with the Performax fiber mix. The Performax fiber mix panels made with a fiber dose of 1.87 

kg/m3 (3.15 lb/yd3) showed a very similar performance as the Shogun fiber mix panels with the 

same fiber dosage in terms of the peak load, and the energy and residual load at 40 mm of 

displacement (Figure 97). The Performax fiber mix, however, has a more consistent load and 

energy profile than the Shogun fiber mix with the same dosage. The slump at 158.8 mm (6.25 in) 

was a little stiffer than the corresponding Shogun mix by 13 mm (0.5 in). 

Figure 102. Comparison of load versus displacement and energy versus displacement curves for 

1.87-kg/m3 (3.15-lb/yd3

) Performax fiber mix with 28-day cure time. 

Comparison of Shotcrete Support Systems Based on RDP Tests

A summary of the test results are presented in Table 14. There is a wide range of peak 

flexural loads and energies for shotcrete. The highest energy capacities are achieved with the 

welded wire mesh.
85 

Table 14. Average results from field tests with round-panel shotcrete samples

a. Wet-mix shotcrete for the Pogo and Kensington mines

Sample Mix type Cement content,

kg/tonne (lb/ton)

Cure 

time, 

days 

Brand of 

fiber

Peak 

flexural 

load, 

kN (lbf) 

Residual 

load 

capacity, 

kN (lbf)*

P2 Wet 325 (650) 1 Forta 1.7 (382.2) 0.25 (56.2) 

K1 Wet 350 (700) 1 Forta 9 (2,023.3) 2 (449.6) 

*Residual load capacity determined at 40 mm (1.6 in) of displacement

b. Dry mix shotcrete at Rockwell Mine

Sample Mix type 

Source 

of mix 

water

Slump, 

mm (in)

Cure 

time, 

days 

Type of 

mesh

Peak 

flexural 

load, 

kN (lbf)

Residual 

load 

capacity, 

kN (lbf)*

SCACR-1 Dry Well nm† 28 none 8 (1,798.5) nm 

SCACR-2 Dry Mine 127 (5) 28 none 

15 

(3,372.1) nm 

SCACR-3 Dry Well 127 (5) 28 Wire mesh 16 

(3,596.9) 

11 

(2,472.9) 

*Residual load capacity determined at 35 mm (1.4 in) of displacement

†

nm = not measured

c. Wet mix shotcrete reinforced with synthetic macro fiber at Devil’s Slide Tunnels Project

Sample Mix type Slump, 

mm (in)

Cure 

time, 

days 

Brand 

of fiber

Average 

energy, 

Joules*

Peak 

flexural 

load, 

kN (lbf) 

Residual 

load 

capacity, 

kN (lbf)

†

Kiewit-1 Wet 38 (1.5) 7 Shogun 400 25 

(5,620.2) 4 (899.2) 

*RDPT energy determined at 35 mm (1.4 in) of displacement

†

Residual load capacity determined at 40 mm (1.6 in) of displacement
86 

d. Wet mix shotcrete reinforced with synthetic macro fiber at Leeville Mine

Sample Mix 

type

Slump, 

mm (in)

Cure 

time, 

days 

Brand of 

fiber

Fiber 

content, 

kg/m3

(lb/yd3) 

Average 

energy, 

Joules*

Peak

flexural 

load, 

kN (lbf) 

Residual 

load 

capacity, 

kN (lbf)

†

LF-1 Wet 171 

(6.75) 28 Shogun 1.87 (3.15) 160 18 (4,046.6) 2 (449.6) 

LF-2 Wet 165 

(6.5) 28 Shogun 2.99 (5.0) 276 21 (4,721.0) 

5 

(1,124.0) 

LF-3 Wet 140 

(5.5) 28 Shogun 4.12 (7.0) 302 23 (5,170.6) 

6 

(1,348.9) 

LF-4 Wet 159 

(6.25) 28 Performax 1.87 (3.15) 175 20 (4,496.2) 2 (449.6) 

*RDPT energy determined at 40 mm (1.6 in) of displacement

†

Residual load capacity determined at 40 mm (1.6 in) of displacement

e. Wet mix shotcrete reinforced with wire mesh at Leeville Mine

Sample Mix 

type

Slump, 

mm (in)

Cure 

time, 

days 

Type of 

wire 

mesh

Water 

reducer, 

kg/m3

(lb/yd3

) 

Average 

energy, 

Joules* 

Peak 

flexural 

load, 

kN (lbf) 

Residual 

load 

capacity, 

kN (lbf)

†

LM-1 Wet 171 

(6.75) 28 6 gauge None 500 23 (5,170.6) 

8 

(1,798.5) 

LM-2 Wet 127 

(5.0) 28 6 gauge 1.6 (2.7) 710 22 (4,945.8) 

11 

(2,472.9) 

*RDPT energy determined at 40 mm (1.6 in) of displacement

†

Residual load capacity determined at 40 mm (1.6 in) of displacement
87 

Figure 103. Comparison of load versus displacement curves for round-panel shotcrete samples 

reinforced with synthetic macro fiber (polyfiber), steel fiber, or wire mesh.

A comparison of the load displacement for selected panels reinforced with welded wire mesh 

or fibers (polyester and steel) is shown in Figure 103. For the panels reinforced with welded wire 

mesh, a six-gauge wire mesh was used with a spacing of 100 x 100 mm (4 x 4 in). For the 

shotcrete panels reinforced with polyfiber, a fiber dosage of 4.15 kg/m3 (7.0 lb/yd3) was used. 

The shotcrete panels reinforced with steel fiber were cast and tested at SRL in Spokane, 

Washington with a steel fiber dosage of 44.5 kg/m3 (75.1 lb/yd3). Both the steel and polyfiber 

dosages are typically recommended for shotcrete used in underground mines operating in weak 

ground conditions.

Peak flexural loads occur at displacements less than 6.3 mm (0.25 in). With the welded wire 

mesh, the peak flexural and residual loads are significantly higher than for either of the panels 

with fiber (Figure 103). The first-break load for the wire mesh is about 23 kN (5,170.6 lbf) as 

compared to 19 kN (4,271.4 lbf) for steel fiber and 17 kN (3,821.8 lbf) for polyfiber 

reinforcement. 

At 40 mm (1.6 in), the welded wire mesh panel had a residual load of about 6 kN (1,348.9 

lbf), though this load could only be maintained up to 12 mm (0.5 in) of displacement with the 

fiber reinforcement. The wire mesh curve shows that it has substantially greater post-peak, loadcarrying capabilities even beyond the 40-mm (1.6-in) test standard. For use in a high 

displacement environment between 40–50 mm (1.6–1.9 in), the welded wire mesh panel can still 

carry a residual load much larger than that of the fiber reinforcement.

The energy capacity or toughness for each of the panels is also shown in Figure 104. The 

welded wire has a superior energy capacity or toughness not only at 40 mm (1.6 in), but up to 50 

mm (1.9 in) as well. This implies that shotcrete with welded wire mesh will resist crack growth 

and have a much higher residual load for a given displacement than fiber-reinforced shotcrete. 
88 

Figure 104. Comparison of load versus displacement and energy versus displacement

curves for round-panel shotcrete samples reinforced with synthetic macro fiber (polyfiber), 

steel fiber, or wire mesh.

Design Requirements of Shotcrete as a Component of Support System Based on RDP Tests

Shotcrete is one of the support components in a multi-element ground support system that is 

typically used in weak rock conditions. Because shotcrete is applied in combination with a number 

of primary and secondary ground control devices (long and short bolts, mesh, strapping, etc.) the 

actual support provided by the shotcrete is difficult to determine. Because the support provided by 

the shotcrete is not well defined, analytical approaches for the design of shotcrete have not been 

developed. Instead, empirical designs based on type of ground (RMR or Q), span of the 

underground entry, and rock bolt spacing are commonly used, particularly for weak rock conditions.

Empirical Systems

Following are examples of some commonly used empirical systems that are currently available:

• The Q Classification System from Grimstad et al. [1993] 

• The Rock Mass Rating (RMR) system from Bieniawski [1973] 

• The New Austrian Tunneling Method [NATM 1962] 

Both the Q and RMR classification systems are based on a rating of the following three 

principal properties of a rock mass:

• Intact rock strength

• Discontinuities and their strength properties

• Geometry of intact block of rock defined by the discontinuities 
89

Characteristics of Empirical Systems

The main advantage of the Q classification system over the RMR system is that it is able to 

account for minor variations in rock properties. The disadvantage is that more knowledge and 

experience are required to use the Q system. The definitions used to describe joint conditions are 

complicated making the system difficult for inexperienced users to use in mines [Milne et al.

1998]. In actual application, the rock mass characterization systems are usually paired, analyzed 

simultaneously, and the results compared in order to give proper consideration to any parameters 

that one system might not include [Pakalnis 2008; Papworth 2002; Morgan et al. 1999; Bernard 

2002; Grant 2001 et al.; Hoek et al. 2005; Barton et al. 1974].

The empirical systems have continued to evolve over time as new data becomes available 

and added to the existing characterization systems. Barton et al. [1974] originally developed a 

tunnel quality index (Q system) based on a rating of the principle properties of a rock mass for 

the Norwegian Geotechnical Institute. Grimstad and Barton [1993] then combined this Q rating 

with a number of geomechanical properties to produce a composite chart that provides guidance 

on the appropriate level of support required for the ground conditions.

Grimstad and Barton [1993] updated the chart adding fiber-reinforced shotcrete support 

systems. Papworth [2002] and Morgan et al. [1990] used a 10-m (30-ft) span opening to further 

refine the chart by adding recommendations on shotcrete thickness (in conjunction with rock 

bolts or steel sets) for different RMR values that had been developed by Bieniawski [1973], and 

a toughness correlation that had been developed by Grant et al. [2001]. However, the shotcrete 

with welded wire mesh was not included in the chart. Based on the round panel tests conducted 

in this study, the shotcrete reinforced with welded wire mesh that is used in the weak rock 

conditions of Nevada can now be incorporated into the design chart. Depending on the shotcrete 

design, the energy capacity or toughness of the shotcrete reinforced with the six-gauge welded 

wire mesh ranged from 500 to 700 joules at 40 mm (1.6 in) of displacement.

Empirical Design Chart

The composite chart developed through these efforts including the results of the testing on 

the panels with shotcrete reinforced with welded wire mesh, shown as “WM” in the 

reinforcement categories, is shown in Figure 105. Shotcrete reinforced with welded wire mesh is 

used in very poor to extremely poor rock where support systems with 400 joules of energy 

capacity or higher are recommended.

The chart in Figure 105 can be used as a guide for underground support system design by first 

assessing the rock mass quality, Q. This value is then applied along the x-axis of the chart. An 

estimate of the opening size (span or height) is made based on the ground conditions. The opening 

duration or excavation support ratio (ESR) value is used to indicate the time an excavation is 

expected to remain open in the mining cycle. In underground gold mines in Nevada, production 

stopes usually remain open for 3 to 5 days, and then they are backfilled. In other cases, such as an 

access or haulage way, the mine opening will have a long-term design life. Based on the geologic 

structure, an estimate of the span opening and duration can be made. The span or height is divided 

by the duration or ESR to obtain a value, which is then applied to the y-axis of the chart. The 

intercept of these x and y values on the chart yields the reinforcement category or level of support 

required for the ground conditions. The reinforcement categories, listed below the chart and 

illustrated between the dark bands on the chart, are used to identify the components and 

characteristics of the support system, including bolt length, bolt spacing, reinforcement, shotcrete 
90 

thickness, and shotcrete support capacity in terms of energy. The region of the chart highlighted 

with a red box most closely resembles the types of rock mass, rock mass ground conditions, 

support requirements, and type of ground support incorporating shotcrete that is used in weak 

rock mass mines (low RMR ground conditions). This is the intended subject of this document. 

Using this empirical design approach, the required length and spacing of the rock bolts can be 

determined, and the recommended energy of the shotcrete can be identified. These ground support 

criteria can then be used to establish an applicable testing and quality control program. 

Figure 105. Estimated support categories based on tunneling quality index Q, after Grimstad et al.

[1993], Papworth [2002], Pakalnis [2010], and modified by NIOSH.
91 

Empirical Design Table

Hoek et al. [2005] produced a compilation of current shotcrete practices, combining many of 

these empirical guidelines along with their practical experience. The information presented in

Table 15 is a summary of the types of rock mass, rock mass ground conditions, support 

requirements, and type of support incorporating shotcrete to use in low RMR or weak rock 

conditions. This table can only be used as an approximate guide when deciding upon the type 

and thickness of shotcrete to be applied in a specific application. Modifications will almost 

certainly be required to deal with local variations in rock conditions and shotcrete quality. 

Table 15. Summary of shotcrete applications in underground mining for different rock 

mass conditions, after Hoek et al. [2005] 

Rock mass 

description

Rock mass 

behavior 

Support 

requirements Shotcrete application

Highly 

jointed and 

bedded weak 

sedimentary 

rock. 

Surface 

slabbing, 

spalling, and 

potential for 

wedges or 

blocks to fall or 

slide due to 

bedding plane 

exposures. 

Provision of 

support in 

addition to that 

available from 

rock bolts or 

cables. Sealing 

of weak bedding 

plane exposures. 

Apply 75 mm (3 in) of shotcrete on rock, 

install rock bolts with faceplates over 

wire mesh to springline, and then apply a 

second 25-mm (1-in) layer of shotcrete. 

Install a second round of longer rock 

bolts with faceplates in mine back.

Metamorphic 

or igneous 

rock with 

shear zone 

planes, 

joints, or 

faults with 

gouge. 

Raveling of 

small rock 

wedges and 

blocks defined 

by intersecting 

joints along 

with squeezing 

and ‘plastic’ 

flow of rock 

mass opening. 

Prevention of 

progressive 

raveling, 

retention of 

broken rock, 

control of rock 

mass dilation, 

and control of 

squeezing. 

Rock bolts or dowels are required to 

control bed separation. Apply 75 mm (3 

in) of plain shotcrete over welded wire 

mesh anchored behind bolt faceplates or 

apply 75 mm (3 in) of steel-fiberreinforced shotcrete on rock. Extend 

shotcrete application down sidewalls 

where required. Install rock bolts with 

faceplates and then apply a second 25-

mm (1-in) layer of shotcrete. Install 

second round of longer rock bolts with 

faceplates in mine back. Thicker 

shotcrete layers may be required at high 

stress concentrations.
92 

Table 16 (continued). Summary of shotcrete applications in underground mining for 

different rock mass conditions, after Hoek et al. [2005] 

Rock mass 

description

Rock mass 

behavior 

Support 

requirements Shotcrete application

Heavily 

jointed 

sedimentary 

rock with 

clay coated 

surfaces with 

metamorphic 

intrusion. 

Bed separation 

in wide-span 

excavations 

with potential 

for wedges or 

blocks to fall or 

slide and 

raveling of 

bedding in 

inclined 

hanging and 

footwalls. 

Control of rock 

mass failure and 

dilation along 

with retention of 

broken rock and 

control of 

propagation in 

addition, sealing 

of weak bedding 

plane exposures. 

Apply 50-mm (2-in) shotcrete over 

welded wire mesh anchored behind bolt 

faceplates, or apply 50 mm (2 mm) of 

steel-fiber-reinforced shotcrete on rock 

and install rock bolts with faceplates. 

Extend shotcrete application down 

sidewalls where required. Install second 

round longer rock bolts with faceplates in 

mine back apply a second 25 to 50-mm 

(1 to 2-in) layer of shotcrete. Install rebar 

or cable lacing where applicable and then 

apply another 25 to 50-mm (1 to 2-in) 

layer of shotcrete in critical areas. This 

develops 125 to 150 mm (5 to 6 in) of 

shotcrete over mesh or cable lacing that is 

firmly attached to the rock surface by 

means of rock bolts or cable bolts.

Quality Control Testing in the Field

The objective of quality control testing is to ensure that the shotcrete meets design 

requirements. The tests and methods used should be reliable and timely, simple to perform, and 

relatively inexpensive. The type of information gathered from testing should provide an 

assurance of design compliance or identify a discrepancy that leads to a specific course of 

corrective action. As discussed in previous sections, the elements that make up the shotcrete 

process affect the quality of the final installed product and its ability to contribute to the entire 

support system. In addition, you must allow sufficient curing time following the application of 

the shotcrete before mining activities resume. Before underground mine workers are allowed 

access to recently sprayed entries, the shotcrete must gain enough strength to safely support the 

ground. Furthermore, the installation of rock bolts through freshly applied shotcrete can

compromise the integrity of the shotcrete. 

The focus of quality control testing in the context of this report are four main shotcrete 

design and installation compliance requirements that consist of early strength, bond strength, 

load-capacity, toughness, and thickness. Two of these tests, early strength and load capacity, are 

performed on samples of material collected just prior to application and allowed to cure. The 

remaining tests involve adhesion strength testing and installed thickness determination on in-situ 

cured shotcrete material. The quality testing presented in this section assumes that the shotcrete 

program at the particular mine location is at a point in development where suitably qualified 

professionals have determined a mix design, constituent materials, and shotcrete application 

equipment; have trained qualified personnel; and are ready to proceed with premining trials 

followed by ongoing performance tests with shotcrete during the mining cycle [Spearing 2001; 

Morgan 2008]. 
93 

Premining-cycle Trials

Premining-cycle trials are used to validate the process and determine expected values and 

allowable variability for ongoing mining-cycle quality control design and installation compliance 

tests. Premining-cycle trials should be conducted using the same equipment, methods, and 

personnel that will be used for installing the shotcrete once the mining cycle starts. Test samples 

for early strength, flexural load capacity, and toughness should be collected, tested, and the 

results assessed in terms of the reliability and repeatability of the values obtained and the 

soundness of the methods. If the procedures and methods produce reliable values to the standards 

set by qualified professionals administering the quality control program, the performance of the 

mix design proposed for mine cycle use can then be used as a benchmark. At this point, the 

averaged results of early-strength testing of sprayed samples for the period of 1–6 hrs after 

spraying and that for the averaged results of round panel flexural and toughness testing should 

have been determined [AuSS 2008]. 

Mine Cycle Testing

The frequency of mining cycle testing of the shotcrete will depend on the severity of the 

support application demands and if the installed product is not performing up to the design 

requirements. There is also a timing element, whereas, the sooner a problem is detected, the 

sooner corrective action can be taken. The frequency of testing can be specified on the basis of 

volume of shotcrete applied, area of shotcrete sprayed, or per shift. An example of recommended 

frequencies for mining applications is shown in Table 16 [AuSS 2008]. 

Table 17. Recommended frequency of shotcrete tests for mining applications, 

after AuSS [2008] 

Characteristic analyzed Location Test method Minimum 

frequency

Shotcrete quality, thickness, 

tensile strength, and bond 

strength

In-stope after 

application

NIOSH 

Shotcrete

Adhesion Test 

System

One hole for each 50 

m

2 (65 yd2

) or part 

thereof 

Shotcrete early strength

From batched 

and sprayed 

shotcrete

ASTM C116 

One set per week or 

per 250 m3 (325 yd3

) 

or part thereof, 

whichever is more 

frequent 

Shotcrete peak flexural load, 

residual load capacity, and 

energy or toughness 

From batched 

and sprayed 

shotcrete

ASTM C1550 

One set per week or 

per 250 m3 (325 yd3) 

or part thereof, 

whichever is more 

frequent 

A well-designed shotcrete program requires a detailed and comprehensive quality-management 

plan that checks all aspects of the application process and takes effective, appropriate, and 

immediate action when a problem is identified. There are numerous examples in the literature of 

these kinds of well-defined plans and checklists. In the case of the tests described in the preceding
94 

sections, the accurate documentation of the areas where shotcrete is applied each day that can then 

be indexed to test results is of particular importance because with this information the area where 

corrective action must be taken can be located. Should a situation arise where obvious operational, 

procedural, equipment, safety, or post-installation testing of cured samples reveals test values for

the shotcrete performance that are not in compliance with the allowable limits, then a feasible and 

safe contingency action plan of specific corrective measures should be available [Spearing 2001]. 

An example of a simple corrective action plan that covers preshotcreting, during and shortly after 

shotcreting, and a period of time after shotcreting is shown in Table 17. 

Table 18. Typical corrective actions for shotcrete problems

Time period Characteristics Comments

Preshotcreting

Mix composition, 

aggregate grading, cement, 

available material, storage 

conditions, equipment 

condition, services (power, 

water, lighting), and safety

Correct discrepencies in the preshotcreting 

phase prior to begining the batch process. 

During and after 

shotcreting Substrate condition Properly prepare substrate prior to applying shotcrete.

During and after 

shotcreting Accelerator level Do not apply shotcrete if it is too hot and 

beginning to flash. 

During and after 

shotcreting

Early strength, visual 

assessment

Allow shotcrete to cure sufficiently to a selfsupporting state before permitting entry.

During and after 

shotcreting

Early strength, partialbeam test sample

If the early strength of the test sample is 

inadequate, modify the mix, or allow longer 

cure time.

During and after 

shotcreting Thickness Apply more shotcrete, if thickness is less than design criteria.

During and after 

shotcreting Rebound Modify mix process or shotcrete spraying 

application to minimize rebound. 

During and after 

shotcreting Surface finish Modify application technique to minimize 

variations in thickness and eliminate lensing.

During and after 

shotcreting Curing Modify accelerator or water-to-cement ratio, 

or allow longer curing time for strength gain. 

Days after 

shotcreting 

Test samples and in-place 

ground support 

Monitor shotcrete process through testing and 

visual inspection, monitor ground stability, 

and install additional ground support (bolts 

and shotcrete) as needed. 
95 

Summary and Conclusions

NIOSH researchers successfully conducted tests onsite at mine sites, using the three portable 

test machines they developed, to determine shotcrete early strengths, adhesion strengths, and 

toughness on field-specified shotcrete mixes. With the ability to test shotcrete strength at the 

mine site or field location researchers and mine personnel are now able to analyze the data from 

the tests and make corrections immediately if needed; they no longer have to wait weeks for the 

data to be analyzed by an outside company. This type of testing and immediate analyzing of the 

data obtained from the tests can provide a safer workplace by informing mine personnel, in a 

timely manner, of the strength properties of the in-place ground control at the mine. All of the 

test systems introduced by NIOSH researchers were well-received at the locations where the 

tests were conducted; some locations even had machines commissioned to carry on the testing 

after the intial research was completed. 

Field trials conducted with the portable shotcrete test machines have demonstrated that the 

test equipment can operate reliably in underground mine environments and that mine personnel 

can easily conduct the tests. These unique machines produce timely, meaningful, repeatable, and 

accurate test results that are consistent with the results obtained from fixed laboratory-based test 

equipment. 

Early Strength, Partial-beam Compression Test

Safe re-entry time can be determined by conducting an early strength test to characterize and 

define the strength development of freshly sprayed shotcrete. Determining safe re-entry can 

improve mine safety by identifying an appropriate re-entry time for mine workers and machinery 

in the mining cycle. A field-expedient test method and portable onsite test equipment have been 

developed to measure shotcrete strength in the first six hours after application. Using this test 

method and equipment, underground mine personnel can measure the early compressive strength 

properties of as-placed shotcrete and clearly identify the shotcrete’s early strength threshold in 

real time. This information allows more informed decisions to be made regarding safe re-entry 

times and can also improve mine safety by supplying pertinent site-specific shotcrete 

information. A typical shotcrete mix used for ground control would exhibit the development of a 

compressive strength of 1 MPa (145 psi) within 1 to 6 hours after placement when measured, 

using the partial-beam test standard, ASTM C 116 [1990]. Another safe re-entry definition is: the

time shotcrete takes to develop enough strength to resist the normal ground pressure and allow 

drilling and the remaining support system elements to be installed without degradation of the 

shotcrete or its long-term strength gain characteristics [Iwaki et al. 2001, O’Toole and Pope 

2006; Clements 2009]. NIOSH researchers conducted early strength tests on commercially 

available shotcrete for weak rock conditions and found that the majority of the samples reached 1 

MPa (145 psi) after three hours, which corresponds to the findings of Morgan et al. [1995] and 

Clements [2004]. 
96

Overcoring and Direct-tension Pull Test

The installed quality of shotcrete in terms of thickness and adhesion are important indicators 

of the available load-carrying support of the shotcrete and its adherence to shotcrete application 

criteria. Mine safety can be improved by determining if the shotcrete application criteria have

been followed and if any load-carrying support is available from the adhesion of the shotcrete to

the underlying rock. Shotcrete bond strength adds to the load-carrying capability provided by the 

host rock over and above the system’s intrinsic support (i.e. plated primary and secondary bolts). 

The direct-tension pull test provides a method for measuring the bond strength as well as 

evaluating the quality of the shotcrete application. Installed quality is assessed by comparing the 

observable core characteristics obtained during the pull test to known standards set forth in the 

quality monitoring program. Additionally, the host rock quality and strength can be directly 

compared to the shotcrete’s strength and a determination made on whether the host rocks’

geological structure is either physically stronger or weaker than the shotcrete. NIOSH 

researchers developed a wet core drilling method used to extract core samples, allowing for 

scrutiny of the core. Typical application quality indicators with regard to the wet core drilled 

sample that are examined and scrutinized include the applied thickness, the presence of voids 

and lenses, the presence of reinforcement, type and location and/or density of the reinforcement, 

underlying host rock quality, the relative bond to the host rock substrate, and relative failure 

location. Researchers conducted core inspections from direct tensile pull testing of as-placed 

shotcrete to characterize and define the quality and bond strength at multiple sites. Results from 

the tests conducted on sprayed test panels at NIOSH’s Spokane facility indicate that the 

shotcrete’s average adhesion strength increased markedly between one and three days of curing, 

from 0.44 to 1.13 MPa (64 to 164 psi).

Round Determinate Panel Test

This research has shown that the round determinate panel test (RDPT) of as-placed shotcrete 

to characterize and define the peak flexural load, the post-crack load-carrying capacity at a given 

displacement, and the toughness of a particular shotcrete mix can improve mine safety by 

allowing for a better understanding of the support provided by the shotcrete component. Residual 

load capacity and toughness are indicators of the effects of the reinforcement added to the 

shotcrete to increase strength after cracks have formed and represent the capacity of a welded

wire mesh or fiber-reinforced shotcrete to carry load following cracking and therefore continue 

to offer structural support. When these values are known, the ground control system can then be 

compared to the anticipated mine opening support loads and displacements. Multiple series of 

round determinate panel tests were conducted using either synthetic polyfiber, welded wire 

mesh, or steel fiber. During the tests the 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 reinforcement placed in the panels; this in turn 

determines the total energy of the panels from the load over a given displacement, usually 40

mm (1.6 in). Peak load and displacements were measured on all tests along with energy where 

applicable. The flexural load capacity first break values for RDPT on the tested weak rock 

shotcrete mixes with reinforcement typically ranged from 20–35 kN (4,496.2–7,868.3 lbf) with a 

residual load capacity of 6 kN (1,348.9 lbf) at 40-mm (1.6-in) displacement and 280 Joules of 

energy or toughness, which is in agreement with high-displacement ground control as suggested 
97 

by AuSS [2008], Bernard [2002], and Papworth [2002]. Although test results with welded wire 

mesh provided some of the highest energy values during testing, it should be noted that the 

welded wire mesh was placed in the middle of the test panels. Panel strengths and energies with 

welded wire mesh are affected by the placement of the welded wire mesh. Typical alternative 

placement locations are either in the top or bottom layer of the panel, and testing has shown that 

these welded wire mesh locations produce different load and energy values. Therefore, 

placement location of the welded wire mesh should match the in-mine construction to properly 

determine the welded wire mesh test panel loads and energies for a particular mine.

Summary of Discussion

By using NIOSH-developed portable, mine-site shotcrete testing machines, mine personnel 

can measure early compressive strength development, bond strength, and flexural load capacity. 

Establishing and monitoring these strength characteristics can improve safety by insuring that the 

shotcrete used within the ground support system meets the ground support design specifications 

as shown in this study. 

Field trials conducted with the portable shotcrete test machines have demonstrated that the 

test equipment can operate reliably in underground mine environments and that mine personnel 

can easily conduct the tests. These unique machines produce timely, meaningful, repeatable, and 

accurate test results that are consistent with the results obtained from fixed laboratory-based test 

equipment. 

Furthermore, an empirical design chart can be used to assist in determining the appropriate 

design requirements for shotcrete in weak rock mass conditions. This design chart was modified 

based on the results from NIOSH tests conducted with shotcrete round panels with and without 

welded wire mesh and observed ground control practices in U.S. underground mines. 
98 

Acknowledgements

The authors thank James Schumacher and Paul Meyer with Thiessen Team USA, Elko, 

Nevada; Vince Mendive with Thiessen Team USA, Big Timber, Montana; and Mark Mudlin 

with BASF, Master Builders, Elko, Nevada, for their technical support regarding commercial 

shotcrete mixes and application practices. We also thank Justin Deemer and Rocky Wegland 

with Westech Automation in Spokane, Washington, for their assistance developing the testing 

machines. In addition, we thank Louis Sandbak and Rory Upshaw, Barrick Gold Corporation, 

Turquoise Ridge Joint Venture, Golconda, Nevada; Dr. Changshou Sun and Bob Riley, 

Newmont Mining Corporation, Leeville Mine, Carlin, Nevada; and Dr. Jeramy Decker, Kiewit 

Corporation, Devil’s Slide Tunnels Project, Pacifica, California for their cooperation and 

assistance conducting shotcrete tests on site. We especially thank Dr. Rimas Pakalnis, Pakalnis 

and Associates, Vancouver, B.C., Canada, for his guidance and collaboration in this research. 
99

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105 

Appendix 

Equipment Specifications for Portable Shotcrete Test Machines

Table A1. Early strength test machine

a. Press specifications

Capacity 2,268 kg (5,000 lb) 

Height 70 cm (27.5 in) 

Width 30.5 cm (12 in) 

Depth 25.5 cm (10 in) 

Speed, test 1.27 mm/min (0.50 in/min) 

Speed, jog 91.4 mm/min (3.6 in/min) 

Drive system Ball screw

Control AutomationDirect closed 

loop servo

Height 36 cm (14.25 in) 

Width 17 cm (6.95 in) 

Depth 35.5 cm (14 in) 

Power 110 VAC 

Operating system Eaton controls 

Data system Eaton controls 

Output ± 0–5 Volt signal 

b. Control box specifications

Height 36 cm (14.25 in) 

Width 17 cm (6.95 in) 

Depth 35.5 cm (14 in) 

Power 110 VAC 

Operating system Eaton controls 

Data system Eaton controls 

Output ± 0–5 Volt signal
106 

Table A2. RDP test machine

a. Press specifications

Capacity 10,886 kg (24,000 lb) 

Height 185 cm (73 in) 

Width 150 cm (59 in) 

Depth 84 cm (33 in) 

Speed, test 4.0 mm/min (0.16 in/min) 

Speed, jog 45.7 mm/min (1.8 in/min) 

Drive system Ball screw

Control AutomationDirect closed loop servo 

b. Control box specifications

Height 41 cm (16 in) 

Width 23 cm (9 in) 

Depth 41 cm (16 in) 

Power 110 VAC 

Operating system Eaton controls 

Data system Eaton controls 

Output ± 0–5 Volt signal 

Table A3. Overcoring and direct-tension test system

a. Drilling unit specifications

Component Specification

Drill Hilti DD-130 core drill (wet/dry) 

Drive system Three-speed forward and reverse

Drill stand Hilti DD-ST-130, 11-in width x 22-in depth x 43-in height 

Diamond core bit, test core Hilti DD-BI, 4-in OD* x 17-in length

Diamond core bit, kerf Hilti DD-BI, 5-in OD x 17-in length

Drill bit, pulling stud Hilti TE-C3X, 16

7 -in OD x 6-in length

Custom drill chuck Hilti DD-BI drill bit chuck welded to Bosch spline to SDS 

adaptor 
107 

b. Pulling unit specifications

Component Specification

Extension rod 

Threaded rod, 3⁄8-16 UNC† x 30 in 

Hex coupling nut, 3⁄8-16 UNC x 11⁄8 in

Hex nut, 3⁄8-16 UNC

Step collet, 3⁄8-in ID‡ x 1-in overall height with 11⁄4-in OD x 3⁄8-in 

height and 3⁄4-in OD x 5⁄8-in height 

Locknut, slip-on quick-threading, 3⁄8-16 UNC

Pulling fixture, 

aluminum weldament

Aluminum tubing, 6-in OD x 61⁄2-in height x 3⁄8-in wall thickness

Aluminum top plate, 6-in OD x 3⁄4-in thickness with 1-in-ID 

center bore and bottom seating flange, 51⁄4-in OD x 1⁄4-in height 

Pulling fixture, steelbase insert

Steel tubing, 53⁄4-in OD x 2-in height x 1⁄2-in wall thickness with 

upper seating flange, 51⁄4-in OD x 3⁄8-in height and bottom kerf 

flange, 51⁄8-in OD x 5-in ID x 1-in height 

Hydraulic pump Enerpac P-142 two-speed: 1st stage 200 psi, 2nd stage 10,000 psi 

Hydraulic ram Enerpac RCH-123, Holl-O-Ram: 10,000 psi, 12 tons 

Digital pressure gauge Parker SCJR 8700-2 PD with min/max display, 0–8,700 psi 

c. Safety restraint specifications

Component Specification

Safety straps (3) NRS HD tie-down strap, 1-in width x 3-ft length with heavyduty 1-in cam buckle 

Strap brackets (6) 

Wall mount (3), 2-in x 2-in aluminum angle, 1⁄4-in thickness x 

2-in length

Pulling fixture (3), aluminum flat bar, 1⁄4-in thickness x 11⁄2-in 

width x 2-in length
108 

d. Anchors specifications

Component Specification

Shotcrete anchors (3) Drill stand (1) and wall-mount strap brackets (2): Hilti HDI 

drop-in anchor, 5⁄8-in OD x 2-in length with 1⁄2-13 UNC x 1 in

Drill stand anchor system 

Hilti Kwik-LOK spindle, 71⁄2-in overall length with 1⁄2-13 

UNC x 11⁄2 in and 5⁄8-in rope thread x 6 in 

Hilti Kwik-LOK nut, 5⁄8-in rope thread 

Pulling stud Hilti HIT-TZ threaded rod, 3⁄8-in OD x 43⁄8-in overall length 

with 3⁄8-16 UNC x 11⁄2 in

Epoxy Hilti HIT-HY 150 MAX

Strap bracket anchors (6) Wall mount (3), hex bolt, 1⁄2-13UNC x 11⁄2 in

Pulling fixture (3), hex bolt, 1⁄4-20 UNC x 1 in

*OD = outside diameter

†

UNC = Unified National Coarse thread

‡

ID = inside diameter

Conversion of Pressure Gauge Bond Strength Test Values to Direct Tensile Force

The ultimate tensile force applied to the test core is determined by converting the maximum 

hydraulic pressure obtained from the pressure gauge to the maximum tensile force acting normal 

to the core’s failure surface. The tensile force is assumed to act in a direction parallel with the 

longitudinal axis of the test core, based on the configuration of the drill holes and the design of the 

direct-tension pulling equipment used for the shotcrete bond strength test. The area of the failure 

surface is assumed to be equivalent to the cross-sectional area of the test core. The relationship 

between the hydraulic pressure measured at the pressure gauge and the applied tensile load can be 

established using a calibrated load cell and the hydraulic components of the pulling system. As 

the hydraulic ram is cycled through an expected loading range, a series of load cell values are 

recorded along with the corresponding pressure gauge readings. This data is then plotted, and a 

trend line representing the data is used to determine the tensile force applied to the test core from 

the hydraulic pressure gauge reading. Alternatively, a linear regression equation can be calculated 

from the plotted data and used to calculate the applied tensile force. A typical equation 

representing this linear relationship is listed below (Equation 1).

 Ft = (2.7181) p – 10.058 (1)

where Ft = tensile force, kN (lbf)

p = hydraulic pressure, MPa (psi) 

The maximum tensile stress at failure is then calculated using Equation 2.

 σT = FT / (πd2 / 4) (2)

where σT = ultimate tensile stress, MPa (psi)

FT = ultimate tensile force, kN (lbf) 

d = diameter of test core, m (in)

π = 3.14159 
 

 

 

 

 

 

 

 

 

 

 

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