A shotcrete adhesion test system for 

mining applications

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

Mining engineer, research mechanical engineer, mechanical engineer, engineering technician and 

elctronics technician, National Institute for Occupational Safety and Health, Spokane, WA

R. Pakalnis, M. Roworth and C. Caceres

Mining engineers, University of British Columbia, Vancouver, BC

Abstract

The National Institute for Occupational Safety and Health (NIOSH) is conducting research to develop 

safe practices for the use of shotcrete as ground support in underground mines, particularly mines 

operating in weak host rock. As part of this research, a rugged, portable direct tensile test system was 

developed for measuring shotcrete adhesion strength in underground mines. During the development 

of this test system, more than 185 direct tensile tests were conducted with a common, commercially available macro-synthetic fiber-reinforced shotcrete that was applied to concrete test panels using a dry mix 

process and machinery. The average bond strength of the shotcrete to the concrete substrate typically 

increased as a function of the shotcrete’s curing age, ranging from 0.44 MPa (64 psi) after one day of 

curing to 1.58 MPa (229 psi) after 90 days of curing. Adhesion strength increased markedly between 

one and three days of curing, reflecting a similar trend of increasing shotcrete tensile strength with curing time. This robust direct tensile test system can improve mine safety by providing a reliable means of 

measuring shotcrete adhesion strength and also supplying important information about the quality of 

the applied shotcrete and the competency of the underlying rock.

Introduction

Historically, a significant percentage of the injuries and 

fatalities that occur in underground mines are caused by falls 

of ground (Fig. 1). To protect mine personnel from ground 

fall hazards, particularly in underground mines where the 

host rock is weak (RMR<40), research is being conducted 

by the National Institute for Occupational Safety and Health 

(NIOSH) to develop safe practices for the use of shotcrete as 

ground support.

Shotcrete is a specially blended, cement-based product that 

is pneumatically sprayed at a high velocity on the exposed 

surfaces of underground openings to provide ground support. 

In underground hard rock mines in the western U.S., shotcrete 

is generally used as an integral part of a ground support system 

consisting of multiple components. When ground conditions are 

poor and the host rock is weak, as in many of the underground 

gold mines in Nevada, extensive ground support is required. 

In these situations, shotcrete is typically applied in conjunction with other ground support elements, such as bolts and 

mesh, but it is also used with spiling or cemented rockfill for 

extremely weak ground. A more complete explanation concerning the use of shotcrete in mechanized cut-and-fill stopes 

in Nevada is provided by Clark et al. (2010). In raveling and 

highly fractured ground, shotcrete is mainly used to provide 

surface support or skin control between the roof bolts, which 

serve as the primary ground support elements. By supporting 

the rock near the surface of the mine opening, shotcrete helps 

prevent degradation of the anchorage for the other ground support components and bridges the span between the rock bolts, 

thereby supporting the loose material that typically causes many 

of the small ground falls (Bernard, 2008). Shotcrete holds by 

adhesion, strengthens the rock by preventing relative movement 

at the shotcrete/rock interface and acts as a “super mesh” by 

providing a stiff retaining component with substantial bending 

or flexural capacity (Kaiser and Tannant, 2001).

When shotcrete is used as an integral part of a mine’s ground 

support system, it is important to know the strength properties of the in-place shotcrete. Besides conventional strength 

parameters, such as the shotcrete’s flexural, compressive or 

tensile strength, the adhesion or bond strength of the shotcrete 

to the host rock must also be known, in order to adequately 

determine the shotcrete’s ability to support the immediate 

ground near the surface of the mine opening. Consequently, 

the adhesion strength of the shotcrete is a necessary parameter 

for ground support design.

Researchers have found that shotcrete applied in underground mines primarily fails in adhesion and that this initial 

debonding of the shotcrete from the underlying substrate is 

followed by a subsequent failure in flexure, as the shotcrete 

bends under the weight of additional loading from loose material (Fig. 2) (Barrett and McCreath, 1995; Holmgren, 2001). 

In another study, two basic types of shotcrete failure modes 

were identified through a mapping program at the Kiirunavaara 

Mine in Kiruna, Sweden (Fig. 3).
Laboratory tests in Sweden have indicated that the primary 

mode of failure of a good-quality shotcrete lining on hard rock 

is governed by adhesion (Holmgren, 2001). As discussed by 

Thomas (2009), the crown of an underground opening presents the worst condition for shotcrete stability, because the 

shotcrete is loaded by its own weight from the moment it is 

sprayed. Further research has identified a more complete listing of possible shotcrete failure modes, particularly for cases 

where shotcrete is used in conjunction with roof bolts (Fig. 4).

Figure 1 — Underground metal mining injuries by accident 

class, 2004-2008 (MSHA).

Figure 2 — Flexural failure of shotcrete resulting from 

insufficient adhesion strength (after Kuchta, 2002).

Figure 3 — Two basic types of shotcrete failure modes. 

A: Fallout of only shotcrete, indicating poor adhesion; B: 

fallout of shotcrete and rock ,indicating zones of weak rock 

(after Malmgren & Svensson, 1999).

Figure 4 — Schematic of shotcrete loading and failure 

modes (after Morton et al., 2008; Barrett and McCreath, 

1995).

Good bond strength depends on a number of factors, including proper surface preparation (Kuchta, 2002; Malmgren 

et al., 2005), adequate compaction between the shotcrete and 

substrate (Brennan, 2005) and also compatibility of the shotcrete 

with the host rock. Studies have indicated that the type of rock 

mineralogy can affect the bond strength (Hahn and Holmgren, 

1979). For example, the adhesion of shotcrete to weak geologic 

formations, such as shale and mudstone, is frequently poor 

(Spearing, 2001). Experience has also shown that shotcrete 

bond strength can be poor in rock that is structurally weak in 

tension or, in other words, rock that is highly foliated, closely 

bedded or spalling (Kaiser and Tannant, 2001; Kuchta, 2002).

As mentioned by Spearing (2001), key elements of a shotcrete quality control program should include design compliance 

for bond, strength and thickness of the sprayed shotcrete. As a 

result, the ability to determine the bond strength or adhesion of 

the shotcrete is a key component of mine design and ground 

control methodology (Norwegian Concrete Association, 2007). 

A more thorough understanding of the in-place strength properties of shotcrete, particularly the bond strength of shotcrete 

to the host rock, will lead to improvements in ground support 

practices, thereby preventing falls of ground and reducing 

mine roof fall accidents.

Background

Adhesion strength of sprayed shotcrete is generally determined by a simple pull test known as the tensile bond strength 

test (Fig. 5). During this test, a direct tensile load is applied 

to a core drilled through the shotcrete into the underlying sub-
strate. As this load is gradually increased, the test core typically 

breaks or fails in tension. This tensile failure can occur in the 

shotcrete, at the bond surface (interface), in the substrate or at 

some combination of these locations. The adhesion or bond 

strength of the shotcrete to the substrate material is determined 

by measuring the maximum tensile load at failure. Depending 

on where the core breaks, the tensile strength at failure can 

represent either the actual adhesion strength of the shotcrete or 

an assumed lower limit of the adhesion strength. For example, 

if the core fails predominantly at the bond surface, then the 

bond strength or adhesion of the shotcrete to the substrate can 

be determined directly. However, if the core fails primarily in 

the shotcrete or substrate rather than at the bond surface (interface), then the adhesion strength of the shotcrete is inferred to 

at least exceed the measured tensile strength of the failed core.

Figure 5 — Simplified schematic of a tensile bond strength 

test (after ACI 506.4R, 2004).

Various test methods have been developed using this basic 

test configuration. For example, a standard test was established 

for the U.S. concrete industry, in which the direct tensile load 

is applied through a steel disk that is glued to the top of the 

shotcrete core (American Society for Testing and Materials, 

2004; American Concrete Institute, 2007); an adhesion test 

method was developed in Sweden (SS 13 72 43), whereby a 

direct tensile load is applied to a test core using a friction grip 

or core sleeve (Nordström and Grändås, 2005) and Canadian 

experiments have been reported in which shotcrete is applied 

over drilled pucks to determine adhesion strengths (Archibald 

et al., 1992). In the U.S., commonly specified direct tensile 

strength values for shotcrete applied to properly prepared concrete substrates range from 0.69 to 1.00 MPa (100 to 145 psi) 

(Brennan, 2005). In Sweden, the bond strength is commonly 

required to be a minimum of 0.5 MPa (73 psi) between shotcrete and rock, 1.0 MPa (145 psi) between different shotcrete 

layers and 1.5 MPa (218 psi) for shotcrete applied to repair 

concrete (Nordström and Grändås, 2005). Values that have 

been typically reported for the adhesion strength of shotcrete 

applied to host rock in underground mines range from about 

0.2 to 1.5 MPa (29 to 218 psi) (Barrett and McCreath, 1995; 

Malmgren and Svensson, 1999; Kuchta, 2003; Malmgren et 

al., 2005; Saiang et al., 2005; Morton et al., 2008).

Although a number of adhesion test methods have been 

developed, no universal procedure has been adopted or used 

extensively by the mining industry. Usually, the adhesion test 

equipment is unavailable or too expensive, complicated or 

fragile for extended use underground. Some of the test methods 

are not practical for typical mining conditions, because they 

require special surface preparation, gluing or curing time to 

allow the shotcrete to gain sufficient strength before a test can 

be conducted. As a result, the adhesion strength of shotcrete 

is seldom measured in underground mines, even though this 

strength parameter plays a major role in the stability of the 

shotcrete, particularly during the initial phase of curing.

As explained by Clark et al. (2010), it is important to measure 

the early-age strength characteristics of shotcrete in order to 

conservatively determine when the applied shotcrete is capable 

of providing ground support and, thus, when it is safe to reenter a sprayed area. Adhesion strength is especially important 

during this initial phase of curing, because the shotcrete must 

be securely bonded to the host rock and held in place until the 

applied material can gain sufficient internal strength to resist 

further loading. Before an integrated, comprehensive design 

approach can be developed for the use of shotcrete as ground 

support, more complete information is needed regarding the 

shotcrete’s in-place strength properties, including the bond 

strength of the shotcrete to the rock.

Research and development

To develop a practical method of measuring shotcrete 

adhesion strength in underground mines, NIOSH researchers 

conducted several series of direct tensile tests with shotcrete 

applied to concrete test panels. The focus of this investigation 

was to develop a suitable test method incorporating lightweight, 

portable, robust equipment, along with simple test procedures, 

so that mine personnel can measure the bond strength of the 

shotcrete that is used to support their underground workings. 

Concrete was used for these tests, to provide a substrate 

material of known quality and consistent strength properties, 

thus eliminating the influence of many confounding factors 

that would normally be present underground, such as varying 

rock type, geologic structure, discontinuities, blasting-induced 

fracturing, loose material, mud, dust and oil (Fig. 6).

Figure 6 — Average unconfined compression and splitting 

tensile strengths for the concrete substrate (n=36).

A commercial shotcrete mix commonly used in western 

hard rock mines was applied to the panels using a dry mix 

process and an Aliva-252.1 shotcrete machine equipped with 

a hopper and pre-dampener. For consistency, the same brand 

of macro-synthetic fiber-reinforced shotcrete (Superstick Shotcrete SCAPF) was used for all of the tests, along with similar 
preparation methods and spraying procedures. Water was 

sprayed on the test panels to clean and moisten the concrete 

prior to applying shotcrete.

As explained by Seymour et al. (2010), five separate series 

of direct tension tests were conducted using the adhesion test 

fixtures or pull anchors shown in Fig. 7. 

Figure 7 — Pull anchors investigated during the development 

of a direct tensile test system for determining shotcrete 

adhesion strength.

The first four series 

of tests were conducted with pull anchors that were embedded 

in the shotcrete as it was sprayed on the concrete test panels. 

These embedded test fixtures consisted of an expanded metal 

anchor and two different-sized metal washer anchors, which 

were held in place either manually or with a mounting grid 

while the shotcrete was applied.

To address problems encountered while using these embedded test fixtures, an alternative pull anchor was developed, in 

which a threaded metal stud was glued in the shotcrete after 

it was applied to the concrete substrate. To center the pull 

anchor in the test core and align the longitudinal axes of the 

pull anchor, test core and pulling fixture, a drilling method 

was adapted from the Swedish friction grip test, whereby three 

parallel concentric holes are drilled from a single drill setup 

(Fig. 8). 

Figure 8 — Schematic showing a vertical cross-section of 

the drill holes for a tensile bond strength test.

The base of the pulling fixture was also redesigned 

to seat in the kerf of the outer drill hole.

Utilizing these modifications, a fifth series of adhesion tests 

was conducted. The improved drilling method and revised pulling fixture automatically centered and aligned the pull anchor 

with the test core and pulling fixture, thereby limiting eccentric 

loading. Because the pull anchor is installed after the shotcrete 

is applied, the shotcrete application process is not disrupted, 

and mine personnel are not exposed to unsupported ground 

while establishing the position of the anchor. Furthermore, 

because the adhesion test site is not predetermined, the location 

for conducting a test can be specifically selected by the mine 

staff, such as in a problem area where poor shotcrete adhesion 

strength is suspected. The final version of the shotcrete adhesion test system that was developed for use in underground 

mines is shown in Fig. 9.

Figure 9 — Field-expedient direct tensile test system.

A total of almost 200 adhesion tests were conducted with 

only five test cores failing prematurely, either during core 

drilling or while setting up the pulling apparatus. All of these 

failures occurred during the first three series of tests, before 

centering of the anchor fixture and eccentricity of the pulling 

axis were controlled more closely. As a general rule, the direct tensile test measurements became more consistent as the 

testing progressed and the equipment and procedures were 

improved. As indicated in Fig. 10, the shotcrete adhesion 

strengths determined from these developmental tests ranged 

from 0.15 to 2.22 MPa (22 to 322 psi).

One of the primary factors that must be controlled in any 

shotcrete investigation is the quality of the applied product, 

which is governed to a large extent by the application technique and the skill of the operator (Melbye, 2001; Thomas, 

2009). Although measures were taken during these tests to 

control the quality of the applied shotcrete, obvious defects 

were observed in some of the panels and test cores. However, 

all of the results from the completed tests have been plotted in 

Fig. 10, including anomalously low strength values that were 
obviously caused by poor shotcrete quality.

Table 1 provides a statistical summary of the average test 

results organized in terms of test series and curing time. 

Table 1 — Average adhesion test results.

Test 

series

Curing 

time, 

days

Number 

of tests

Adhesion strength, MPa

Standard 

deviation, 

MPa

Coefficient 

of variation, 

percent

Failure surface predominant 

location, %

MinimumMaximum Range Avg Concrete Interface Shotcrete

1 7 8 0.97 1.96 0.99 1.56 0.31 20.0 0.0 12.5 87.5

1 14 4 0.74 2.22 1.48 1.28 0.65 50.9 0.0 0.0 100.0

2 7 46 0.18 1.67 1.49 0.88 0.34 38.6 4.3 34.8 60.9

3 3 28 0.15 1.41 1.25 0.79 0.29 36.7 14.3 21.4 64.3

4 3 15 0.80 1.59 0.78 1.21 0.25 20.6 13.3 26.7 60.0

4 7 15 0.94 1.69 0.74 1.41 0.19 13.8 53.3 6.7 40.0

4 14 15 1.07 1.71 0.64 1.40 0.19 13.7 53.3 26.7 20.0

5 1 9 0.19 0.91 0.72 0.44 0.28 64.0 0.0 22.2 77.8

5 3 9 0.67 1.43 0.76 1.13 0.28 25.1 0.0 33.3 66.7

5 7 9 0.95 1.53 0.58 1.22 0.20 16.0 0.0 11.1 88.9

5 14 9 0.32 1.96 1.64 1.30 0.49 38.0 0.0 33.3 66.7

5 28 9 0.75 1.67 0.92 1.46 0.31 21.0 0.0 11.1 88.9

5 90 9 1.25 1.99 0.75 1.58 0.22 14.2 0.0 0.0 100.0

Because 

direct tensile tests with a composite material are influenced by 

a number of factors, there is an inherent variability in adhesion 

test results, as indicated by the coefficient of variation values 

listed in Table 1. This innate variability demonstrates the importance of quality control measures, not only for casting the 

concrete panels, preparing the interface surface and applying 

the shotcrete, but also for operating the adhesion test equipment and consistently following well-defined test procedures.

As mentioned earlier, typical values reported for the adhesion 

strength of shotcrete in underground mines range from about 

0.2 to 1.5 MPa (29 to 218 psi). Direct tensile tests conducted 

using the epoxy stud pull anchor gave adhesion strengths that 

were within a similar range, 0.19 to 1.99 MPa (28 to 289 psi). 

Therefore, the developed test method appears to provide a 

credible means of measuring shotcrete adhesion strength.

Conducting a shotcrete adhesion test

This shotcrete adhesion test system consists of readily available and relatively inexpensive components, primarily a small 

stand-mounted core drill and a pulling unit equipped with a 

precision pressure gage (Figs. 9 and 11). The components are 

rugged and portable and they can be reliably used to measure 

the adhesion strength of shotcrete applied to the surface of an 

underground opening.

Figure 10 — Adhesion strength versus shotcrete curing 

time for various anchor configurations (n=185).

Figure 11 — Schematic of direct tensile test system for 

determining shotcrete adhesion strength.

Once a desired test site has been selected, a hand-operated 

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. After installing 

a 13-mm- (0.5-in.-) diameter threaded stud and expansion 

anchor in this hole, the drill stand is leveled and secured in 
position. Three holes are then drilled from this single drill 

setup, ensuring that all of the holes are parallel and concentric 

(Figs. 8 and 12).

First, an 11.1-mm- (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.). Next, 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 (approx. 15 min), a 102-mm- (4-in.-) diameter 

diamond core bit is used to wet-drill a second hole through 

the shotcrete, to a depth of about 25-50 mm (1-2 in.) into the 

underlying substrate. Finally, a 127-mm- (5-in.-) diameter 

diamond core bit is used to wet drill a shallow kerf for seating 

the base of the pulling fixture, typically to a depth of about 

3-6 mm (0.125-0.25 in.) depending on the irregularity of the 

shotcrete surface (Fig. 12).

Figure 12 — Parallel and concentric drill holes with 

extension rod connected to epoxied stud.

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. The hydraulic hose from the hand pump is 

then connected to the loading ram and the ram is cycled a few 

times to remove any extraneous air from the 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. To conduct a test, the pressure gage 

is zeroed and 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).

The ultimate tensile force applied to the test core is determined by converting the maximum hydraulic pressure value, 

saved on the pressure gage’s digital display, to the maximum 

tensile force acting normal to the core’s failure surface. To 

simplify analysis of the test results, the tensile force is assumed 

to act in a direction parallel to the longitudinal axis of the 

test core and the area of the failure surface is assumed to be 

equivalent to the cross-sectional area of the test core.

Prior to conducting these tests, the hydraulic pump and loading ram were calibrated in a laboratory test machine equipped 

with a certified load cell. The hydraulic components were tested 

at several load values over the range of the rated capacity of 

the loading ram, thus providing a direct comparison between 

the hydraulic pressure reading on the pump’s digital pressure 

gauge and the corresponding load or force reading measured 

by the test machine’s load cell. Using a simple linear regression equation obtained from this calibration procedure, the 

actual force exerted by the hydraulic ram can be accurately 

determined from the measured hydraulic pressure. Equation 

1 shows the simple linear relationship between measured 

hydraulic pressure and applied tensile load for the hydraulic 

components used in these tests.

Ft = (2.7181) p – 10.058 (1)

where Ft = tensile force (lbf)

and p = hydraulic pressure (psi)

The maximum tensile stress at failure is then calculated 

using Eq. (2).

σT = FT / (πd2 / 4) (2)

where σT = ultimate tensile stress (psi)

 FT = ultimate tensile force (lbf)

and d = diameter of test core (in.)

To determine whether the maximum tensile stress calculated 

above is a direct measure of the shotcrete’s adhesion strength 

or an assumed lower limit of its bond strength, the test core 

should be examined along with the bottom of the drill hole to 

identify the location of the tensile failure surface. The failure 

location is typically recorded as a percentage of the shotcrete, 

interface and substrate that are exposed on the tensile failure 

surface (Fig. 13). 

Figure 13 — Adhesion test specimen showing a tensile 

failure surface located predominantly in the concrete 

substrate.

The overall depth of the 102-mm- (4-in.-) 

diameter drill hole should also be noted, along with the length 

of the test core and the thickness of the shotcrete layer so that 
drilling depth(s) for the pull anchor and/or test core can be 

adjusted, if necessary, for further tests.

Discussion of test results

Results of direct tensile tests using the epoxy-stud pull anchor are shown in Fig. 14 for macro-synthetic fiber-reinforced 

shotcrete applied to concrete test panels. 

Figure 14 — Adhesion strength versus curing time for direct 

tensile tests using the epoxy stud fixture (n=54).

 The adhesion tests 

were conducted after the shotcrete had cured for 1, 3, 7, 14, 

28 and 90 days. Average adhesion strength 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).

Measuring adhesion strength in terms of shotcrete curing 

time helps identify when the bond strength of the shotcrete is 

sufficient to support more than its own weight. In contrast with 

other methods that require a relatively stiff shotcrete in order 

to conduct adhesion tests (Malmgren et al., 2005), adhesion 

strengths were successfully measured using the epoxy stud 

pull anchor after only one day of shotcrete curing time. These 

tests indicated 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); or, in other words, from 28 to 

72% of the average 90-day adhesion strength (Fig. 14 and Table 

1). As previously noted, it is important to measure shotcrete 

adhesion strength during the initial phase of curing, in order 

to identify safe re-entry times for underground workings that 

have been recently sprayed with shotcrete.

Because it is difficult to sample and test a weakly consolidated material such as freshly sprayed shotcrete, the strength 

properties of early-age shotcrete are usually related to some 

direct or indirect measure of its compressive strength. Adhesion 

strength gain with shotcrete curing time has been identified in 

other studies, and this trend has been related to an increase in 

the shotcrete’s compressive strength over time (Malmgren et 

al., 2005; Bernard, 2008).

Rather than comparing the bond strength of the shotcrete to 

some measure of its compressive strength, it is more appropriate 

to compare adhesion with tensile strength, particularly when 

a direct tensile test is used to determine adhesion strength. 

In Fig. 15, the results of the epoxy-stud adhesion test series 

are plotted with shotcrete tensile strengths estimated from 

unconfined compression and splitting tensile tests with cored 

samples of similar shotcrete. 

Figure 15 — Comparison of adhesion strength and estimated shotcrete tensile strength.

The concrete test panels had 

generally cured for over 28 days before they were sprayed 

with shotcrete, so the average tensile strength of the concrete 

ranged from at least 4.8 to 6.2 MPa (700 to 900 psi) when the 

adhesion tests were conducted (Fig. 6). Although adhesion 

strength closely follows the strength gain trend estimated for 

the tensile strength of the shotcrete, the results of the adhesion 

tests are much lower in magnitude (Fig. 15).

These lower strength values are at least partially caused 

by inherent differences between the two types of tests and 

the complexity of testing composite samples. During a direct 

tensile test, failure occurs at the weakest element in the test 

specimen. Unlike tests such as the flexural test, the entire 

volume of a direct tensile test specimen is subjected to the 

maximum stress; therefore, the probability of a weak element 

occurring in the test specimen and influencing the test results 

is relatively high (Neville, 2009). For a direct tensile test with 

a composite material, this issue is compounded because the 

weakest element can occur in any of the individual materials 

or at their interfaces. Therefore, in a shotcrete adhesion test, a 

tensile failure can occur in the shotcrete, at the bond interface 

(contact surface), in the substrate, or in a combination of these 

locations. For concrete, splitting tensile tests provide more 

uniform results and give strengths that are 5 to 12% higher 

than those obtained from direct tensile tests (Neville, 2009). 

Further testing is needed to clearly define the relationship between the adhesion strength at the bond surface and the tensile 

strength of the shotcrete.

If a high-quality shotcrete is applied using correct procedures 

and the bond surface is prepared properly (i.e., clean and free 

from loose materials, mud, dust or oil, with sufficient roughness and moisture to permit a good bond), the location of the 

tensile failure should depend on the relative difference between 

the tensile strength of the shotcrete and that of the substrate. 

Results reported for shotcrete adhesion tests in underground 
mines indicate that the majority of the tensile failures occurred in the host rock or at the contact surface with the host 

rock (Kuchta, 2002; Clements et al., 2004; Malmgren et al., 

2005). In contrast, the majority of the tensile failures in these 

tests occurred in the shotcrete, more than likely because the 

shotcrete had not yet developed sufficient strength with curing 

time to match the tensile strength of the concrete (Figs. 6 and 

15). As the test series progressed, the concrete, shotcrete and 

test procedures were controlled more closely, improving the 

consistency of the test results, as shown in part by the values 

listed in Table 1. This was also reflected in the predominant 

locations of the tensile failure surfaces, which averaged 64% 

in the shotcrete, 23% at the interface and 13% in the concrete 

for the entire test series and 81%, 19% and 0%, respectively, 

for the final epoxy stud test series (Fig. 16 and Table 1).

According to Swedish Standard SS 13 72 43, the result of a 

direct tensile test is reported as adhesion strength if more than 

80% of the tensile failure surface is located at the interface 

(bond surface). Otherwise, the test result represents a lower 

limit for adhesion strength. In other words, the actual adhesion 

strength at the bond surface is larger than a tensile failure that 

occurs at some other location (shotcrete, host rock or some 

combination of locations). Using this criterion, only 4% of 

these tests failed at the bond surface (eight of the 185 total 

tests). This restriction produced significantly lower adhesion 

strengths than the average adhesion strength curves shown in 

Figs. 10 and 14-16. By averaging all the test results, a more 

consistent and representative value was reported for adhesion 

strength, because anomalously low values for tensile failures 

at the bond surface were not given an undue weight in the 

data analysis.

Figure 16 — Predominant location of the tensile failure 

surface for shotcrete adhesion tests with concrete panels 

(n=185).

Conclusions

NIOSH researchers have developed a rugged, portable direct 

tensile test system, consisting of readily available, inexpensive 

components that can be used in conjunction with simple test 

procedures to determine the adhesion strength of shotcrete 

in underground mines. Several series of direct tensile tests 

were conducted with this equipment to measure the adhesion 

strength of macro-synthetic fiber-reinforced shotcrete applied 

to concrete test panels using a dry mix process and machinery.

The average bond strength of the shotcrete to the concrete 

substrate typically increased as a function of the shotcrete’s 

curing age, ranging from 0.44 MPa (64 psi) after one day of 

curing to 1.58 MPa (229 psi) after 90 days of curing. A substantial increase in adhesion strength was noted between one 

and three days of shotcrete curing time (approximately a 44% 

increase in terms of the shotcrete’s average 90-day adhesion 

strength). The increase in adhesion strength as a function of 

curing time appeared to reflect a similar trend of increasing 

tensile strength in the shotcrete with curing time.

This robust direct tensile test system provides a reliable 

means of measuring the adhesion strength of freshly applied 

shotcrete that has cured for as little as one day. Because the 

epoxy stud pull anchor is installed after the shotcrete has been 

applied, the test procedures do not interfere with the shotcrete 

application process, and the adhesion test can be conducted at 

a desired location rather than at a site which is predetermined 

by the test method. As a result, this adhesion test system can 

improve mine safety by providing a viable means of evaluating 

the in-place strength and quality of the applied shotcrete, as well 

as the tensile strength and competency of the underlying rock.

Disclaimer

The findings and conclusions presented in this document 

have not been formally disseminated by the National Institute 

for Occupational Safety and Health and should not be construed 

to represent any agency determination or policy. Mention of 

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

Acknowledgments

The authors thank James Schumacher and Paul Meyer with 

Thiessen Team USA, Elko, NV; Vince Mendive with Thiessen 

Team USA, Big Timber, MT; and Mark Mudlin with BASF, 

Master Builders, Elko, NV for their technical support regarding commercial shotcrete mixes and application practices. We 

also thank Justin Deemer and Rocky Wegland with Westech 

Automation in Spokane, WA, for their assistance developing 

a base for the adhesion test pulling fixture.

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