1 

A PRACTICAL METHOD OF MEASURING SHOTCRETE ADHESION STRENGTH 

B. Seymour, NIOSH, Spokane, WA 

L. Martin, NIOSH, Spokane, WA 

C. Clark, NIOSH, Spokane, WA 

M. Stepan, NIOSH, Spokane, WA 

R. Jacksha, NIOSH, Spokane, WA 

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

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

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

ABSTRACT 

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, tests were 

conducted with a commercial poly-fiber reinforced shotcrete mix to 

develop a practical means of measuring shotcrete adhesion strength. 

Full-scale tests were conducted in a test frame equipped with concrete 

panels having three distinct surface roughness profiles. Adhesion test 

fixtures either embedded or epoxied in the shotcrete were overcored 

into the underlying concrete, and direct tensile tests were conducted to 

determine the bond strength of the shotcrete cores to the substrate 

after a selected shotcrete curing interval (1, 3, 7, 14, 28, or 90 days). 

Measured adhesion strengths typically ranged from 0.5 to 2.0 MPa 

depending on the curing age of the shotcrete samples. This paper 

presents the results of these tests, describes the portable test system 

components, and addresses some of the design issues encountered in 

the development of a rugged and reliable method for determining 

shotcrete adhesion strength in underground mines. 

INTRODUCTION 

Historically, a significant percentage of the injuries and fatalities 

that occur in underground mines are caused by falls of ground (Fig. 1). 

Figure 1. 2004-2008 Underground metal mining injuries by accident 

class, MSHA. 

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 may also be used with spiling or cemented 

rockfill for extremely weak ground. 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 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 [1]. Shotcrete holds by adhesion, strengthens the rock by 

preventing relative movements at the shotcrete/rock interface, and acts 

as a “super mesh” by providing a stiff retaining component with 

substantial bending or flexural capacity [2]. 

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 inplace 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) [3,4]. 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 failure 

of a good quality shotcrete lining on hard rock is adhesion failure [3]. 

As discussed by Thomas [7], the crown of an underground opening 

presents the worst condition for shotcrete stability because the 

shotcrete is loaded by its own self 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). 

Good bond strength depends on a number of factors including 

proper surface preparation [5,9], adequate compaction between the 

shotcrete and substrate [10], and also compatibility of the shotcrete 
2 

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

mineralogy can affect the bond strength [11]. For example, the 

adhesion of shotcrete to weak geologic formations such as shales and 

mudstones is frequently poor [12]. 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 [2,5]. 

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). 

As mentioned by Spearing [12], key elements of a shotcrete 

quality control program should include design compliance for bond and 

strength as well as sprayed thickness. 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 [13]. A 

more thorough understanding of the in-situ 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. As 

shown in Figure 5, a direct tensile load is applied to a core drilled 

through the shotcrete into the underlying substrate. 

Figure 5. Simplified schematic of a tensile bond strength test (after 

ACI 506.4R, 2004). 

The tensile load at 

failure is then measured to determine the adhesion or bond strength of 

the shotcrete to the underlying substrate material. Depending on 

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

actual adhesion strength of the shotcrete or an assumed lower limit of 

this adhesion strength. If the core fails in the shotcrete or substrate 

rather than at the bond surface or interface, the adhesion strength of 

the shotcrete is then known to at least exceed the measured tensile 

strength of the failed core. 

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 

[14,15]; 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 [16]; and Canadian experiments have been 

reported in which shotcrete is applied over drilled pucks to determine 

adhesion strengths [17]. 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) [10]. 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 [16]. 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) 

[4,6,8,9,18,19]. 
3 

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 early stages of curing. 

To develop a more integrated design approach for the use of shotcrete 

as ground support, further information is needed regarding the 

shotcrete’s in-place strength properties (e.g., magnitude, parameter 

relationships, strength gain with curing time, etc.). Therefore, a 

practical method needs to be developed so that a typical underground 

miner or foreman can use a standard set of rugged and reliable tools in 

conjunction with simple test procedures to measure the adhesion 

strength of the in-place shotcrete. 

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 these experimental tests was to select light 

weight, portable, and robust equipment that would be suitable for use 

underground and to develop a simple set of test procedures so that 

shotcrete adhesion strengths could be measured by mine personnel. 

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). 

To investigate the influence of substrate surface roughness, the 

concrete was cast in special forms to produce test panels with three 

visually distinct surface textures or surface roughness profiles (Fig. 7). 

After the concrete panels had cured for several days, they were 

attached to a test frame that simulated the dimensions of a typical 

underground opening, and 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 (Fig. 8). 

Figure 7. Concrete panels cast with three distinct surface roughness 

profiles. 

Figure 8. Simulated Underground Mine Test Frame. 

 For consistency, the same brand of polyfiber reinforced shotcrete (SCAPF) was used for all of the tests along 

with similar preparation methods and spraying procedures. 

During the development of a mine-worthy adhesion test system, 

five separate series of direct tension tests were conducted using a 

logical progression of adhesion test fixtures or pull anchors (Fig. 9). 

For an initial series of tests, an expanded metal anchor was manually 

held in place with a hollow rod (preformed aluminum conduit) as the 

shotcrete was applied to the test panel. Not only was it difficult to core 

drill around this particular anchor and center the fixture in the test core, 

but it also appeared that flexing of the expanded metal during the pull 

test may have caused some of the test cores to break near the fixture. 

As a result, the next two series of tests were conducted with pull 

anchors consisting of a 44.5-mm- (1.75-in-) or 38-mm- (1.5-in-) 

diameter metal washer welded to a coupling nut. Although it was 

easier to core drill around this fixture and center the metal washer in 

the test core, it was difficult to maintain the position of the fixture 

perpendicular to the concrete surface while the shotcrete was applied. 

This tended to produce voids in the shotcrete near the anchor and also 

caused the pulling axis angle to deviate from the longitudinal axis of 

the test core. 

The first three series of adhesion tests were conducted with 

concrete panels mounted either vertically to a side wall or overhead in 

an arch position on the test frame (Fig. 10). Extra precautions were 

taken during the next two series of tests to more closely control the 

spatial orientation of the pull anchor in relation to the test core and 

substrate surface. Concrete panels for the fourth and fifth series of 

tests were sprayed with shotcrete while they were propped against the 

test frame at approximately a 45° angle. Adhesion tests were then 

conducted with the panels after they were placed in a horizontal 

position so that the core drill and pulling fixture could be positioned 

more precisely. To hold the adhesion fixture perpendicular to the 

substrate surface, the fourth series of adhesion tests were conducted 
4 

with a pull anchor consisting of a 35-mm- (1.375-in-) diameter metal 

washer welded to a coupling nut and connected to a long section of allthread that was in turn secured to a rigid metal grid bolted to the panel. 

Although the diameter of the metal washer did not appear to affect the 

adhesion strength measurements, more closely controlling the 

orientation of the pull anchor reduced eccentric loading and thus, 

produced more consistent and repeatable test results. 

Figure 9. Pull anchors investigated during the development of a direct 

tensile test system for determining shotcrete adhesion strength. 

Figure 10. Drilling an adhesion test core on a vertical surface. 

One of the main disadvantages of the pull anchors used in the 

first four series of adhesion tests was that the anchor had to be held in 

place either manually or with a mounting grid while the shotcrete was 

sprayed on the substrate. This interfered with the shotcrete application 

process, produced voids in the shotcrete at the location of the test 

fixture, and caused subsequent problems with eccentric loading if the 

test fixture was not held perpendicular to the surface of the substrate. 

Further problems with eccentric loading were encountered if the core 

drill and/or the pulling fixture were not positioned and oriented correctly 

over the pull anchor. To address these issues, experiments were 

conducted to develop an alternative technique in which a threaded 

metal stud was glued in the applied shotcrete and then used as a pull 

anchor during a direct tensile test. 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 set-up (Fig. 11). The base of the pulling fixture was also 

redesigned to seat in the kerf of the outer drill hole. 

Figure 11. Schematic showing a vertical cross-section of the drill 

holes for a tensile bond strength test. 

Utilizing these modifications, a fifth and final series of adhesion 

tests was conducted with another set of horizontal test panels. 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, a 

desired location for conducting the 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 through these tests is shown 

in Figure 12. Further tests will be conducted using this system in 

underground mines to determine if the equipment can withstand typical 

mining conditions and to obtain suggested improvements from industry 

personnel. 

Overall, the results of these developmental tests were fairly 

consistent with adhesion strengths ranging from 0.15 to 2.22 MPa (22 

to 322 psi). 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 the test series progressed and the experimental development 

continued, there was generally less variability in the measured 

adhesion strengths (Fig. 13). The spread in the test results illustrates 

the variability inherent in adhesion testing and demonstrates the 

importance of quality control measures not only for casting the 

concrete panels, preparing the interface surface, and applying the 
5 

shotcrete, but also for operating the adhesion test equipment and 

consistently following well defined test procedures. 

Figure 12. Field expedient direct tensile test system. 

Figure 13. Adhesion strength versus shotcrete curing time for various 

anchor configurations (n=185). 

As mentioned earlier, typical values that have been 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 that were 

conducted using the epoxy stud method gave adhesion strengths that 

were within a similar range, about 0.2 to 2.0 MPa (29 to 290 psi). 

Consequently, this test method appears to provide a credible means of 

measuring shotcrete adhesion strength. 

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 [7,20]. For the adhesion tests conducted in test series 1-

5, water was sprayed on the test panels to clean and moisten the 

concrete prior to applying shotcrete. While further measures were 

taken to control the quality of the applied shotcrete, obvious defects 

were observed in some of the panels and test cores. No attempt has 

been made here to discard anomalously low strength values caused by 

poor shotcrete quality; instead all of the results from the completed 

tests have been reported. 

CONDUCTING A SHOTCRETE ADHESION TEST 

Unlike other methods, 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. 12 & 14). These robust and 

reliable components are portable and can be used to measure 

shotcrete adhesion strength in either the rib or back of an underground 

opening (Fig. 10). 

Figure 14. 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 set-up ensuring that all of the holes are parallel 

and concentric (Fig. 11). 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 2-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 and 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. 15). 

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 split-nut 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 with 

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 
6 

core. Prior to conducting our 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 our tests. 

Figure 15. Parallel and concentric drill holes with extension rod 

connected to epoxied stud. 

Ft

 = (2.7181) p – 10.058 (1) 

where Ft

 = tensile force 

and p = hydraulic pressure 

The maximum tensile stress at failure is then calculated using 

Equation 2. 

σT = FT / ( d2 / 4) (2) 

where σT = ultimate tensile stress 

 FT = ultimate tensile force 

and d = diameter of test core 

To aid in the interpretation of the test results, the failure surface 

on the test core should be examined along with the bottom of the drill 

hole, and the failure location should be recorded as a percentage of 

the shotcrete, interface, and substrate that are exposed on the tensile 

failure surface (Fig. 16). In addition, the overall depth of the 102-mm- 

(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(s) for the pull anchor and/or test core can be adjusted, if 

necessary for further tests. 

Figure 16. Adhesion test specimen showing a tensile failure surface 

located predominantly in the concrete substrate. 

DISCUSSION OF TEST RESULTS 

Results of direct tensile tests using the epoxy-stud pull anchor are 

shown in Figure 17 for poly-fiber reinforced shotcrete applied to 

concrete test panels. The substrate panels were positioned at about a 

45° angle when the shotcrete was applied. Later, they were placed in 

a horizontal position, and a sequential series of adhesion tests were 

conducted after the shotcrete had cured for 1, 3, 7, 14, 28, and 90 

days. 

Figure 17. Adhesion strength versus curing time for direct tensile tests 

using the epoxy stud fixture (n=54). 

Average adhesion strength values increased with shotcrete curing 

age and ranged from 0.44 MPa (64 psi) after 1 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 [5,6] and are within the normal range 

of bond strengths specified for shotcrete applied to concrete substrates 

[10,16]. In contrast with other methods that require a relatively stiff 

shotcrete in order to conduct adhesion tests [9], shotcrete adhesion 

strengths were successfully measured after only 1 day of curing using 

the epoxy stud anchor. Average adhesion strength increased 

markedly between 1 and 3 days of curing, from 0.44 to 1.13 MPa (64 

to 164 psi) or in other words, from 28 to 72 pct of the average 90-day 

adhesion strength (Fig. 17). These test results may have important 

ramifications in terms of safe re-entry times for underground openings 

that have been recently sprayed with shotcrete and warrant further 

testing to determine if adhesion strengths can be measured at earlier 

shotcrete curing times using this method. 

It is important to know 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 re-enter a sprayed area. Adhesion strength is especially 

important during the early stages of curing because the shotcrete must 
7 

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

can gain sufficient internal strength to resist further loading. Measuring 

adhesion strength in terms of curing time helps identify when the bond 

strength of the shotcrete is sufficient to support more than its own self 

weight. However, 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 [1,9]. 

Rather than comparing the bond strength of the shotcrete to some 

measure of its compressive strength, it may be more appropriate to 

compare adhesion with tensile strength, particularly if a direct tensile 

test is used to determine adhesion strength. In Figure 18, the results 

of the epoxy-stud adhesion test series are plotted together with 

average concrete tensile strengths obtained from splitting tensile tests 

with cast concrete samples from the substrate test panels and also 

with shotcrete tensile strengths estimated from unconfined 

compression and splitting tensile tests with cored samples of similar 

shotcrete. 

Figure 18. Comparison of adhesion and splitting tensile test results. 

 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 should have ranged from at least 4.8 to 6.2 

MPa (700 to 900 psi) when the adhesion tests were conducted. 

Although adhesion strength closely follows the strength gain trend 

estimated for tensile strength of the shotcrete, the results of the 

adhesion tests are much lower in magnitude (Fig. 19). 

Figure 19. Comparison of adhesion strength and estimated shotcrete 

tensile strength. 

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 other tests 

such as a 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 [21]. 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 pct higher than those obtained from direct tensile tests 

[21]. 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 [5,9,22]. In contrast, the majority of the tensile failures in our 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. 18 & 20). As our test series 

progressed, the consistency of the concrete, shotcrete, and test 

procedures were controlled more closely. This was reflected some 

what in the predominant locations of the tensile failure surfaces, which 

averaged 64 pct in the shotcrete, 23 pct at the interface, and 13 pct in 

the concrete for the entire test series and 81 pct, 19 pct, and 0 pct 

respectively, for the final epoxy stud test series. 

Figure 20. Predominant location of the tensile failure surface for 

shotcrete adhesion tests with concrete panels (n=185). 

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

direct tensile test is reported as adhesion strength if more than 80 pct 

of the tensile failure surface is located at the interface (bond surface). 

Otherwise, the test result represents a lower limit for adhesion 
8 

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 pct of our tests failed at the bond surface (8 

of the 185 total tests). This restriction produced significantly lower 

adhesion strengths than the average adhesion strength curves shown 

in Figures 13 and 17-21. By averaging the entire 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 21. Shotcrete adhesion test results indicating the substrate 

surface roughness of the concrete panels (n=185). 

To investigate the effect of substrate surface roughness on 

adhesion strength, the concrete test panels were cast to produce three 

visually distinct surface textures, referred to as smooth, medium, and 

coarse (Fig. 7). A representative profile of each surface was measured 

and compared to a joint roughness coefficient chart [23] to obtain the 

following JRC values, respectively: 6-8, 16-18, and 14-16. Because 

the surface profile of the panels was measured over a distance of 

approximately 30 cm (12 in), the JRC value for the medium panel was 

larger than that of the coarse panel. As shown in Figure 21, the 

surface roughness of the concrete panels did not appear to 

significantly affect the adhesion strength of the test cores. Neglecting 

curing time, the average adhesion strength for all of the tests 

conducted on the smooth, medium, and coarse panels were 

respectively, 1.07 MPa (155 psi), 1.09 MPa (158 psi), and 1.15 MPa 

(167 psi). These limited results indicate the complexity of relating large 

scale differences in surface roughness to adhesion strength 

measurements collected over a much smaller surface area. Further 

tests need to be conducted with test panels having a more uniform 

surface roughness that is scaled in a consistent manner with respect to 

the cross-sectional area of the test cores. 

CONCLUSIONS 

NIOSH researchers have developed a 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 

determine the adhesion strength of poly-fiber reinforced shotcrete 

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

Results of these full-scale field tests suggest the following conclusions: 

The components of the adhesion test system are robust, light 

weight, and reliable, and when used in accordance with simple test 

procedures produce consistent and credible results. Further shotcrete 

adhesion strength testing is merited using this equipment in vertical as 

well as overhead positions under actual mining conditions. 

Depending on shotcrete curing age, average adhesion strength 

values derived from these direct tensile tests ranged from 0.44 to 1.58 

MPa (64 to 229 psi). These results are comparable to previously 

published adhesion strengths from similar studies and are within the 

normal range of bond strengths specified for shotcrete applied to 

concrete substrates. 

Using epoxy stud pull anchors, direct tensile tests were 

successfully conducted after the shotcrete had only cured for 1 day. 

Test results indicated a substantial increase in shotcrete adhesion 

strength between 1 and 3 days of curing (approximately a 44-pct 

increase in terms of the average 90-day adhesion strength). This gain 

in adhesion strength appeared to closely reflect a similar trend for 

tensile strength gain with curing time. 

During a direct tensile test, the test core fails at its weakest 

location. As a result, these tests can provide important information 

about the quality and tensile strength of the applied shotcrete and the 

strength and competency of the underlying host rock, as well as the 

adhesion or bond strength of the shotcrete to the rock. 

This paper is not a guideline just a statement regarding shotcrete 

adhesion strengths measured by NIOSH researchers. Mine staff 

should conduct their own site specific tests using due diligence and 

available standards to determine the strength properties of their 

shotcrete. This information can then be used to design a ground 

control plan that specifically addresses the conditions at their mine. 

The findings and conclusions presented in this document have 

not been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed to 

represent any agency determination or policy and mention of any 

company name or product does not constitute endorsement by NIOSH. 

ACKNOWLEDGEMENTS 

The authors 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, WA for their assistance developing a base for the adhesion 

test pulling fixture. 

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