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REPORT ON EARLY STRENGTH PERFORMANCE OF MODERN DAY WEAK ROCK MASS SHOTCRETE MIXES

C. C. Clark, NIOSH, Spokane, WA 

M. A. Stepan, NIOSH, Spokane, WA 

J. B. Seymour, NIOSH, Spokane, WA 

L. A. Martin, NIOSH, Spokane, WA 

ABSTRACT 

The National Institute for Occupational Safety and Health 

(NIOSH), Ground Control Engineering Branch is investigating the use 

of shotcrete in weak rock mass mines with the objective of reducing 

fatalities and injuries resulting from rock fall accidents. When shotcrete 

is used as part of a multi-element ground support system there is a 

need to know when the material has developed a threshold early 

compressive strength of approximately 1 MPa (145 psi) for safe reentry of miner and machine into shotcreted mine workings. This 

equates to the material having developed enough strength to be selfsupporting and allow for emplacement of the remaining support 

elements that require drilling of the shotcrete layer without degradation. 

NIOSH researchers have developed methods and portable test 

equipment to measure shotcrete strength on-site in the first six hours 

after application using the ASTM C 116-90 (1990) standard. These 

advances were demonstrated in tests on five commercially available 

shotcrete mixes, sprayed as dry shotcrete using the field expedient 

methods and equipment. The strength values from these tests allowed 

for real-time identification of the early strength threshold and were 

consistent with strengths reported using laboratory-type equipment. 

The findings of this NIOSH research for determining shotcrete early 

strength can improve mine safety by providing a method for 

determining initial site specific material property values to be used for 

design and as a means to conduct on-site quality control and 

monitoring during production. 

INTRODUCTION 

Fall of rock is a major hazard in underground mining with thirty-six 

percent of all underground metal/nonmetal mines fatalities (1996-2004) 

being attributed to this cause. MSHA statistics show that of these 

unplanned falls of rock 95% weighed less than one ton with the 

majority 59% weighing 11 kgs (25 lbs) or less (Fig. 1. and Table 1). 

Figure 1. 1996 U.S. metal mine rockfalls (after Zipf 2002). 

Table 1. MSHA rockfall review for Nevada Mines (after Lacerda, 

2004). 

These small rocks can develop 106 N (23.8 lbf) of force over a 3-m 

(10-ft) fall (Lacerda, 2004). The falls often occur between traditional 

ground control support components (e.g., bolts, trusses, timber, etc.) 

and can only be prevented by skin control components such as 

shotcrete and wire mesh. 

When mining operations move into weak rock mass conditions 

(Rock Mass Rating of less than 45), such as those found in Nevada 

gold mines, rock falls are a constant threat. This is due in part to 

unraveling of the poor quality, broken rock. 

The mining method commonly used in these mines is a modified 

form of mechanized cut and fill amenable to high ore recovery rates in 

irregular deposit geometries. Shotcrete is a key ground support 

element in this mining method. In addition, the time required for 

shotcrete to set sufficiently for safe re-entry is a key factor in the 

mining cycle. However, the early strength properties of shotcrete 

mixes used in these mining applications are not well defined and 

difficult to measure in situ. 

Thus, there is a risk that miners will prematurely enter workings, 

before safety support is obtained. Because of this, the National 

Institute for Occupational Safety and Health (NIOSH) developed an inmine test protocol and applied it to the study of early-age shotcrete 

strength for five dry shotcrete mixes currently in use in weak rock mass 

mines. 

The ultimate goal of this work was to enable a mining engineer to 

characterize and define the strength properties of a particular shotcrete 

mix and thus determine when the mining cycle could safely restart with 

miners and machinery. This would be determined by knowing when 

the shotcrete applied as a surface ground support skin control has 

obtained or built the minimum strength required to resist the normal 

ground pressure (Iwaki et al., 2001) and upon development of 

sufficient bond strength or initial skin control interlock such that 

shotcrete-rock matrix does not fall off or apart within the first 20 

minutes after application (Rispin, 2003). 
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BACKGROUND 

Ground control systems used in mechanized cut-and-fill stopes in 

Nevada require extensive ground support. Typical support includes a 

shotcrete flash coat 19-to 25- mm (3/4- to 1-in) thick, followed by 

screen, plates, bolts, and a second layer of shotcrete bringing the 

combined thickness to 75- to 100-mm (3- to 4-in). In areas requiring 

rehabilitation, the second layer of shotcrete is plated and bolted as 

well. 

Shotcrete is an integral and vital component of this ground 

support system. In the initial application, a remote controlled shotcrete 

machine sprays shotcrete to form a skin or shell. This thin skin 

prevents very small rock debris from falling and fouling the machinery 

used for remote application of the mesh and bolting. The second 

application of shotcrete ties together the plated and bolted mesh and 

prevents unraveling of the small-in-size rock which if left unchecked 

can propagate to allow for substantial slabbing and ultimately massive 

failure. 

Most observed ground control failures involve broken rock within 

a half-meter of the excavation perimeter (Bauer and Donaldson, 1992). 

The failure mode is illustrated in figure 2. 

Figure 2. Flexural resistance model for a loosened block representing 

a distributed load (after Diamantidis and Bernard, 2004). 

Setting of shotcrete in this support system is critical to the mining 

cycle. Re-entry time is the minimum curing time required for shotcrete 

to develop enough strength to protect miners. In other words (quoting 

Rispin, 2003), "the re-entry time is defined as when work can safely 

resume in an advancing underground heading.” What this means in 

practical terms is that shotcrete can be drilled and support system 

components emplaced, without damaging long-term strength

properties (O’Toole and Pope, 2006 and Clemets, 2009). 

Development of early strength is a characteristic of shotcrete and 

occurs considerably faster than ordinary concrete, especially with 

lower water to cement ratios, Figs. 3 and 4. In addition, fiber additives 

have been developed that can greatly increase the toughness and 

tensile strength of shotcrete. This greatly benefits the structural 

properties of the shotcrete in terms of failure strength after onset of 

initial cracking. 

Figure 3. Typical early strength partial beam test values, for weak 

rock mass dry shotcrete and concrete, n=54 samples. 

Figure 4. Strength gain of different blends of shotcrete (Brummer and 

Swan, 2001; after Tannant, 1994). 

Early strength is typically taken to mean the strength values the 

shotcrete material obtains in the time period from 0- to 6-hrs following 

being sprayed. Early strength values using unconfined compressive 

tests as low as 0.5A MPa (73 psi) have been used as a benchmark to 

identify conditions under which re-entry can safely and acceptably be 

permitted on International Mining Properties (Rispin, 2005). Typically 

however, for safe re-entry practices for North American mines, the 

shotcrete should develop a compressive strength of 1- to 1.6-MPa 

(145- to 233-psi) to be competent for drilling operations on it (O’Toole 

and Pope, 2006). The time reported for the safe re-entry has been as 

soon as 2 hrs (Knight et al., 2006) with 4 hrs and compressive strength 

equivalent to 1 MPa (145 psi) being the norm (Rispin, 2003 and 2005; 

O’Toole and Pope, 2006). 

TEST PROTOCOL 

Testing of the early strength of shotcrete introduces two issues 

not normally faced when testing of concretes. First, samples must be 

obtained “as shot” rather than poured into test cylinders. Moreover, it 

is not realistic to extract green samples, via a coring process, from a 

shotcrete panel for testing (Heere et al., 2002; Clemets, 2004). 

Second, samples must be tested very quickly after collection. 

Researchers trying to determine the early age strength (0- to 6-

hrs) of green shotcrete have had to resort to indirect test methods due 

to the difficulty in handling the material. Methods have been 

demonstrated using a Meyco penetrometer ASTM C 1117-89 (1989) 

(Jolin et al., 1999; Heere et al., 2002), and ASTM C 403-99 (1999) 

(Rispin, 2003, Clemets, 2004; Knight et al., 2006; O’Toole and Pope, 

2006; Bernard, 2009), pneumatic pin (Iwaki et al., 2001), long partial 

beam, ASTM C 116-90 (1990) (Morgan, 1998; Heere et al., 2002) and 

partial beam ASTM C 116-90 (O’Toole and Pope, 2006; Bernard, 

2009). 

An example of early compressive strengths values obtained for 

five shotcrete mixes using penetrometer and partial beam methods as 

reported by O’Toole are shown in Fig. 5. 

NIOSH researchers chose the partial beam ASTM C 116-90 

(1990) standard in the 1- through 6-hr time frame because it allowed 

the early-age shotcrete specimen to be sprayed, de-molded, and 

emplaced in the test machine without degradation (due to its low 

strength state) prior to testing. The 0- to 1-hr time frame utilizing 

penetrometer technology was not used because of the non-linearity in 

data (Fig. 5) and the requirement for a second measuring protocol. 

Beam molds have been used successfully for creating test 

specimens in the USA and Canada (Heere et al., 2002). The molds 

can be either hand, or manipulator arm, sprayed for both wet and dry 

shotcrete and provide suitable results. The partial beam ASTM C 116-

90 strength determination test and apparatus used by researchers 

applies a compressive load that induces a diagonal tensile failure. 
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Figure 5. Penetrometer and sprayed beam compression test results 

for shotcrete mixes (O’Toole and Pope 2006). 

The shotcrete machine used to spray the fibered and non-fibered 

dry mixes was a hopper and pre-dampener design (Aliva 252.1 series) 

used on dry mine shotcrete mixes in Nevada and Alaska for production 

and rehabilitation. The use of a hopper and pre-dampener allows for a 

consistent mix of the shotcrete with an even distribution of material at 

the nozzle. The dry mix was sprayed at an average water to cement 

ratio of 35%. The water content of the mix as it left the pre-dampener 

was 5% with the nozzleman adding an additional 2.5% for a final water 

content of 7.75% for the spray. The same task trained nozzleman was 

used throughout the testing program to insure application consistency. 

A number of initial partial beam tests were conducted with a 

traditional Carver, dual-gage read-out, manually actuated, hydraulic 

press. The 0- to 5,000-psi and 0- to 25,000-psi analog scales on the 

unit proved to be too large to reflect the resolution necessary for the 

low strengths of the green mixes. In addition the manual operation of 

the machine introduced error with respect to uniformly loading the 

samples over the specified time interval and extended the time interval 

for conducting each test. In addition to the difficulty in obtaining 

meaningful strength numbers for the green shotcrete from the test 

machine, staff also encountered difficulty in shooting the material into 

the partial beam molds with respect to rebound and maintaining the 

position of the molds. It became apparent that developing a sound 

method for properly orienting the molds and restraining them would be 

necessary. 

A mold containment system consisting of two frames was 

developed to restrain the partial beam boxes 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 box molds and can be hand 

carried to the testing location to coincide with the 1-hr tests (Fig. 6 

upper-left). The larger frame was constructed with forklift pockets to 

allow the entire unit to be moved to the test location (Fig. 6). This 

system allows the 2- thru 6-hr samples, the cure time necessary for 

movement while still allowing for the 1-hr tests. 

Figure 6. Partial Beam Box Shooting Frame. 

A partial beam testing machine was designed, that had the

capability to automatically load the samples at a fixed rate and collect 

the load versus displacement data (Figs. 7 and 8). 

Figure 7. Early strength shotcrete test machine design. 

Upon delivery the unit was commissioned and pre-tested using 

cast and sprayed samples (Fig. 9). The unit was found to be capable 

of accurately measuring load rate and the automated load cycle greatly 

reduced the chance of human error and reduced the time to conduct a 

proper test. The field worthy design incorporates a small-footprint, 

self-contained, servo-controlled, stiff-frame press configuration 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 servo loop controlled 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 C 116-90, 1990). Load versus

displacement values are collected and stored using a Campbell 

Scientific 850 data logger. 

The ASTM testing protocol used called 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 Figs. 6 and 10 to make the test specimens. 

Tests are conducted at one hour intervals on the de-molded 

specimens for the 1- through 6-hr testing. The shotcrete sample is 

carefully taken from the beam box mold and positioned with-in a 

specialized test fixture (Fig. 9). 

To conduct a test, the ram is lowered to a position just above the 

specimen and the automated test cycle initiated by depressing the 

green start button along with the data acquisition start. The operator 

observes the specimen for development of single or dual side tension 

cracks (Fig. 11 and Fig. 12) along one of the platen-to-sample contact 

edges. 
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Figure 8. Early strength shotcrete test machine in operation. 

Figure 9. Shotcrete partial beam test specimen undergoing loading.

Figure 10. Partial beam box mold and test specimen. 

Figure 11. Shotcrete partial beam test specimen showing single side 

failure crack. 

Figure 12. Shotcrete Partial Beam Test Specimen Showing Dual 

Sided Failure Cracks. 

Upon on-set of a crack, the red button on the test machine control 

panel is depressed stopping the test. The shotcrete failure mode 

depicted in the diagonal tension failure illustration of Fig. 13 represents 

the failure cracks observed. 

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. 

Figure 13. Shotcrete failure modes (Rose, 1985). 

EXPERIMENTAL RESULTS 

Figure 14 shows the rate of early strength development for the 

five commercially available weak rock mass dry shotcrete mixes tested 

by NIOSH researchers using a sprayed mold system subjected to 

ASTM C 116-90 partial beam methods. 

Figure 14. Summary graph of early age sprayed partial beam 

compression tests on five commercially available Nevada weak rock 

mass mine shotcrete mixes, n=72 samples. 

Examination of the data indicates a consistent early strength rise 

followed by a plateau at the 3- to 4-hr curing time. This initial set and 

then phase transfer was observed in all shotcrete samples tested in 
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the NIOSH study. This characteristic has been well referenced in the 

literature on shotcrete (Jolin et al., 1999; Heere et al., 2001; Knight et 

al., 2006; Rispin, 2005, O’Toole and Pope, 2006; Bernard, 2008) and 

is depicted as well in Figs. 15 and 16. This characteristic has been 

attributed to the initial added accelerants giving way to the 

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 (Note: all of the 

mixes tested exceeded 1 MPa (145 psi) after 24 hrs of curing). 

These early compressive strength values are in good agreement 

with O’Toole who reported trends between 0.25 and 2.25 MPa for tests 

on 1- through 6-hr partial beams (Fig. 15) and Bernard who reported 

trends between 0.2- and 3-MPa for tests on 1- through 10-hr partial 

beams (Fig. 16). The method of display used by NIOSH (Figs. 14 and 

15) makes it easier to distinguish when the shotcrete goes into a 

holding state at 3- to 4-hrs of curing time. 

Figure 15. Summary graph of early age sprayed beam compression 

tests from surface trials of shotcrete mixes (After O’Toole and Pope, 

2006). 

Figure 16. Compressive strength as a function of time after spraying 

for a typical trial (Bernard, 2008). 

The SCAPF and SCAPT-100 are fibered versions of the SCA mix. 

SCAPF is a poly-fibered mix and SCAPT-100 is a steel-fibered mix. 

After six hours of curing the fibered mixes exceeded the capacity of the 

test machine (shown as dotted lines in Fig. 14). The compressive 

strength exceeded 2 Mpa (290 psi). The addition of fibers seems to 

produce higher early strengths. Additional testing will be conducted at 

mine sites to examine and confirm this characteristic. 

CONCLUSIONS 

A field expedient test method and portable on-site test equipment 

have been developed. This test method allows for real-time early 

compressive strength testing of shotcrete “as-placed” underground. 

This is important because there are a variety of conditions that 

can influence early development of shotcrete strength. These include 

the mine environment, application quality, application quantity, ambient 

temperature of the applied shotcrete and host rock, mix design of the 

commercial shotcrete used and its characteristics and the water to 

cement ratio at which the mix was actually sprayed. These factors are 

difficult to replicate in tests conducted elsewhere, and delays in 

obtaining results negate their usefulness in controlling quality and 

preserving safety. 

The method was demonstrated in tests on five commercially 

available shotcrete mixes marketed for use in mining of weak rock. 

Three out of the five mixes developed the accepted minimum standard 

for shotcrete early compressive strength of 1 MPa (145 psi) within 4 

hrs of set time Fig. 16 (Morgan, 1991; Clemets, 2004 and 2009). 

The key consideration, though, is the actual shotcrete strength 

realized at the mining face as a function of time. Safe re-entry 

depends on assuring that adequate shotcrete strength has been 

obtained. 

The findings and conclusions presented in this document have 

not been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed to 

represent any agency determination or policy and mention of any 

company name or product does not constitute endorsement by NIOSH. 

ACKNOWLEDGEMENTS 

The authors would like to thank James Schumacher at Thiessen 

Team USA, Elko, Nevada, Vince Mendive at Thiessen Team USA, Big 

Timber, Montana and Mark Mudlin, BASF, Master Builders, Elko, 

Nevada for their technical support on commercially available mixes. 

Also Justin Deemer and Rocky Wegland at Westech Automation 

Spokane, WA for their technical and mechanical assistance in 

development of the early strength testing machine. 

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